A review of normal tissue hydrogen NMR relaxation times and relaxation mechanisms from 1-100 MHz: dependence on tissue type, NMR frequency, temperature, species, excision, and age

Simultaneous multi-slice T1 mapping using MOLLI with blipped CAIPIRINHA bSSFP

BACKGROUND

This study evaluates the possibility for replacing conventional 3 slices, 3 breath-holds MOLLI cardiac T1 mapping with single breath-hold 3 simultaneous multi-slice (SMS3) T1 mapping using blipped-CAIPIRINHA SMS-bSSFP MOLLI sequence. As a major drawback, SMS-bSSFP presents unique artefacts arising from side-lobe slice excitations that are explained by imperfect RF modulation rendering and bSSFP low flip angle enhancement. Amplitude-only RF modulation (AM) is proposed to reduce these artefacts in SMS-MOLLI compared to conventional Wong multi-band RF modulation (WM).

MATERIALS AND METHODS

Phantoms and ten healthy volunteers were imaged at 1.5 T using a modified blipped-CAIPIRINHA SMS-bSSFP MOLLI sequence with 3 simultaneous slices. WM-SMS3 and AM-SMS3 were compared to conventional single-slice (SMS1) MOLLI. First, SNR degradation and T1 accuracy were measured in phantoms. Second, artefacts from side-lobe excitations were evaluated in a phantom designed to reproduce fat presence near the heart. Third, the occurrence of these artefacts was observed in volunteers, and their impact on T1 quantification was compared between WM-SMS3 and AM-SMS3 with conventional MOLLI as a reference.

RESULTS

In the phantom, larger slice gaps and slice thicknesses yielded higher SNR. There was no significant difference of T1 values between conventional MOLLI and SMS3-MOLLI (both WM and AM). Positive banding artefacts were identified from fat neighbouring the targeted FOV due to side-lobe excitations from WM and the unique bSSFP signal profile. AM RF pulses reduced these artefacts by 38%. In healthy volunteers, AM-SMS3-MOLLI showed similar artefact reduction compared to WM-SMS3-MOLLI (3 ± 2 vs 5 ± 3 corrupted LV segments out of 16). In-vivo native T1 values obtained from conventional MOLLI and AM-SMS3-MOLLI were equivalent in LV myocardium (SMS1-T1 = 935.5 ± 36.1 ms; AM-SMS3-T1 = 933.8 ± 50.2 ms; P = 0.436) and LV blood pool (SMS1-T1 = 1475.4 ± 35.9 ms; AM-SMS3-T1 = 1452.5 ± 70.3 ms; P = 0.515). Identically, no differences were found between SMS1 and SMS3 postcontrast T1 values in the myocardium (SMS1-T1 = 556.0 ± 19.7 ms; SMS3-T1 = 521.3 ± 28.1 ms; P = 0.626) and the blood (SMS1-T1 = 478 ± 65.1 ms; AM-SMS3-T1 = 447.8 ± 81.5; P = 0.085).

CONCLUSIONS

Compared to WM RF modulation, AM SMS-bSSFP MOLLI was able to reduce side-lobe artefacts considerably, providing promising results to image the three levels of the heart in a single breath hold. However, few artefacts remained even using AM-SMS-bSSFP due to residual RF imperfections. The proposed blipped-CAIPIRINHA MOLLI T1 mapping sequence provides accurate in vivo T1 quantification in line with those obtained with a single slice acquisition.

New Imaging Techniques

The chapter discusses the advancement of new imaging techniques, the role of imaging in CCA diagnosis, anatomical and morphological classification, ultrasound screening of CCA, ultrasound findings of MF-CCA, PI-CCA, ID-CCA, the use of CT in CCA diagnosis, staging and treatment planning, CT volumetry and estimation of future liver remnant, post-treatment follow-up and surveillance, MRI imaging, Positron Emission Tomography (PET)/CT, limitations to contrast studies and resolution, internal receivers for CCA imaging, and in vitro imaging of CCA.

Alterations of brain microstructures in a mouse model of prenatal opioid exposure detected by diffusion MRI

Growing opioid use among pregnant women is fueling a crisis of infants born with prenatal opioid exposure. A large body of research has been devoted to studying the management of opioid withdrawal during the neonatal period in these infants, but less substantive work has explored the long-term impact of prenatal opioid exposure on neurodevelopment. Using a translationally relevant mouse model of prenatal methadone exposure (PME), the aim of the study is to investigate the cerebral microstructural differences between the mice with PME and prenatal saline exposure (PSE). The brains of eight-week-old male offspring with either PME (n = 15) or PSE (n = 15) were imaged using high resolution in-vivo diffusion magnetic resonance imaging on a 9.4 Tesla small animal scanner. Brain microstructure was characterized using diffusion tensor imaging (DTI) and Bingham neurite orientation dispersion and density imaging (Bingham-NODDI). Voxel-based analysis (VBA) was performed using the calculated microstructural parametric maps. The VBA showed significant (p < 0.05) bilateral alterations in fractional anisotropy (FA), mean diffusivity (MD), axial diffusivity (AD), radial diffusivity (RD), orientation dispersion index (ODI) and dispersion anisotropy index (DAI) across several cortical and subcortical regions, compared to PSE. Particularly, in PME offspring, FA, MD and AD were significantly higher in the hippocampus, dorsal amygdala, thalamus, septal nuclei, dorsal striatum and nucleus accumbens. These DTI-based results suggest widespread bilateral microstructural alterations across cortical and subcortical regions in PME offspring. Consistent with the observations in DTI, Bingham-NODDI derived ODI exhibited significant reduction in PME offspring within the hippocampus, dorsal striatum and cortex. NODDI-based results further suggest reduction in dendritic arborization in PME offspring across multiple cortical and subcortical regions. To our best knowledge, this is the first study of prenatal opioid exposure to examine microstructural organization in vivo. Our findings demonstrate perturbed microstructural complexity in cortical and subcortical regions persisting into early adulthood which could interfere with critical neurodevelopmental processes in individuals with prenatal opioid exposure.

Primary Multiparametric Quantitative Brain MRI: State-of-the-Art Relaxometric and Proton Density Mapping Techniques

This review on brain multiparametric quantitative MRI (MP-qMRI) focuses on the primary subset of quantitative MRI (qMRI) parameters that represent the mobile ("free") and bound ("motion-restricted") proton pools. Such primary parameters are the proton densities, relaxation times, and magnetization transfer parameters. Diffusion qMRI is also included because of its wide implementation in complete clinical MP-qMRI application. MP-qMRI advances were reviewed over the past 2 decades, with substantial progress observed toward accelerating image acquisition and increasing mapping accuracy. Areas that need further investigation and refinement are identified as follows: (a) the biologic underpinnings of qMRI parameter values and their changes with age and/or disease and (b) the theoretical limitations implicitly built into most qMRI mapping algorithms that do not distinguish between the different spatial scales of voxels versus spin packets, the central physical object of the Bloch theory. With rapidly improving image processing techniques and continuous advances in computer hardware, MP-qMRI has the potential for implementation in a wide range of clinical applications. Currently, three emerging MP-qMRI applications are synthetic MRI, macrostructural qMRI, and microstructural tissue modeling.

Correlation analysis between the complex electrical permittivity and relaxation time of tissue mimicking phantoms in 7 T MRI

Dielectric relaxation theory describes the complex permittivity of a material in an alternating field; in particular, Debye theory relates the time it takes for an applied field to achieve the maximum polarization and the electrical properties of the material. Although, Debye’s equations were proposed for electrical polarization, in this study, we investigate the correlation between the magnetic longitudinal relaxation time T1 and the complex electrical permittivity of tissue-mimicking phantoms using a 7 T magnetic resonance scanner. We created phantoms that mimicked several human tissues with specific electrical properties. The electrical properties of the phantoms were measured using bench-test equipment. T1 values were acquired from phantoms using MRI. The measured values were fitted with functions based on dielectric estimations, using relaxation times of electrical polarization, and the mixture theory for dielectrics. The results show that, T1 and the real permittivity are correlated; therefore, the correlation can be approximated with a rational function in the case of water-based phantoms. The correlation between index loss and T1 was determined using a fitting function based on the Debye equation and mixture theory equation, in which the fraction of the materials was taken into account. This phantom study and analysis provide an insight into the application relaxation times used for estimating dielectric properties. Currently, the measurement of electrical properties based on dielectric relaxation theory is based on an antenna, sometimes invasive, that irradiates an electric field into a small sample; thus, it is not possible to create a map of electrical properties for a complex structure such as the human body. This study could be further used to compute the electrical properties maps of tissues by scanning images and measuring T1 maps.

Transverse relaxation in fixed tissue: Influence of temperature and resolution on image contrast in magnetic resonance microscopy

To describe transverse relaxation of water in fixed tissue, we propose a model of transverse relaxation accelerated by diffusion and exchange (TRADE) that assumes exchange between free (visible) and bound (invisible) water, which relax by the dipole-dipole interaction, chemical exchange, and translation in the field gradient. Depending on the prevailing mechanism, transverse relaxation time (T₂ ) of water in fixed tissue could increase (when dipole-dipole interaction prevails) or decrease with temperature (when diffusion in the field gradient prevails). Chemical exchange can make T₂ even temperature independent. Also, variation of resolution from 100 to 15 μm/pxl (or less) affects effective transverse relaxation. T₂ steadily decreases with increased resolution ( T 2 ∝ ∆ x 2 , ∆ x is the read direction resolution). TRADE can describe all of these observations (semi)quantitatively. The model has been experimentally verified on water phantoms and on formalin-fixed zebrafish, mouse brain, and rabbit larynx tissues. TRADE could help predict optimal scanning parameters for high-resolution MRM from much faster measurements at lower resolution.

The effects of long-term methylphenidate administration and withdrawal on progressive ratio responding and T2 MRI in the male rhesus monkey

Methylphenidate is a frequently prescribed drug treatment for Attention-Deficit/Hyperactivity Disorder. However, methylphenidate has a mode of action similar to amphetamine and cocaine, both powerful drugs of abuse. There is lingering concern over the long-term safety of methylphenidate, especially in a pediatric population, where the drug may be used for years. We performed a long-term evaluation of the effects of chronic methylphenidate use on a behavioral measure of motivation in male rhesus monkeys. Animals were orally administered a sweetened methylphenidate solution (2.5 or 12.5 mg/kg, twice a day, Mon-Fri) or vehicle during adolescence and into adulthood. These animals were assessed on a test of motivation (progressive ratio responding), during methylphenidate treatment, and after cessation of use. Moreover, animals were evaluated with quantitative T₂ MRI about one year after cessation of use. During the administration phase of the study animals treated with a clinically relevant dose of methylphenidate generally had a higher rate of responding than the control group, while the high dose group generally had a lower rate of responding. These differences were not statistically significant. In the month after cessation of methylphenidate, responding in both experimental groups dropped compared to their previous level of performance (p = 0.19 2.5 mg/kg, p = 0.06 12.5 mg/kg), and responding in the control animals was unchanged (p = 0.81). While cessation of methylphenidate was associated with an acute reduction in responding, group differences were not observed in the following months. These data suggest that methylphenidate did not have a significant impact on responding, but withdrawal from methylphenidate did cause a temporary change in motivation. No changes in T₂ MRI values were detected when measured about one year after cessation of treatment. These data suggest that long-term methylphenidate use does not have a negative effect on a measure of motivation or brain function / microstructure as measured by quantitative T₂ MRI. However, cessation of use might be associated with temporary cognitive changes, specifically alteration in motivation. Importantly, this study modeled use in healthy individuals, and results may differ if the same work was repeated in a model of ADHD.

T2 mapping in myocardial disease: a comprehensive review

Cardiovascular magnetic resonance (CMR) is considered the gold standard imaging modality for myocardial tissue characterization. Elevated transverse relaxation time (T2) is specific for increased myocardial water content, increased free water, and is used as an index of myocardial edema. The strengths of quantitative T2 mapping lie in the accurate characterization of myocardial edema, and the early detection of reversible myocardial disease without the use of contrast agents or ionizing radiation. Quantitative T2 mapping overcomes the limitations of T2-weighted imaging for reliable assessment of diffuse myocardial edema and can be used to diagnose, stage, and monitor myocardial injury. Strong evidence supports the clinical use of T2 mapping in acute myocardial infarction, myocarditis, heart transplant rejection, and dilated cardiomyopathy. Accumulating data support the utility of T2 mapping for the assessment of other cardiomyopathies, rheumatologic conditions with cardiac involvement, and monitoring for cancer therapy-related cardiac injury. Importantly, elevated T2 relaxation time may be the first sign of myocardial injury in many diseases and oftentimes precedes symptoms, changes in ejection fraction, and irreversible myocardial remodeling. This comprehensive review discusses the technical considerations and clinical roles of myocardial T2 mapping with an emphasis on expanding the impact of this unique, noninvasive tissue parameter.

Simultaneous high-resolution T2 -weighted imaging and quantitative T2 mapping at low magnetic field strengths using a multiple TE and multi-orientation acquisition approach

PURPOSE

Low magnetic field systems provide an important opportunity to expand MRI to new and diverse clinical and research study populations. However, a fundamental limitation of low field strength systems is the reduced SNR compared to 1.5 or 3T, necessitating compromises in spatial resolution and imaging time. Most often, images are acquired with anisotropic voxels with low through-plane resolution, which provide acceptable image quality with reasonable scan times, but can impair visualization of subtle pathology.

METHODS

Here, we describe a super-resolution approach to reconstruct high-resolution isotropic T₂ -weighted images from a series of low-resolution anisotropic images acquired in orthogonal orientations. Furthermore, acquiring each image with an incremented TE allows calculations of quantitative T₂ images without time penalty.

RESULTS

Our approach is demonstrated via phantom and in vivo human brain imaging, with simultaneous 1.5 × 1.5 × 1.5 mm³ T₂ -weighted and quantitative T₂ maps acquired using a clinically feasible approach that combines three acquisition that require approximately 4-min each to collect. Calculated T₂ values agree with reference multiple TE measures with intraclass correlation values of 0.96 and 0.85 in phantom and in vivo measures, respectively, in line with previously reported brain T₂ values at 150 mT, 1.5T, and 3T.

CONCLUSION

Our multi-orientation and multi-TE approach is a time-efficient method for high-resolution T₂ -weighted images for anatomical visualization with simultaneous quantitative T₂ imaging for increased sensitivity to tissue microstructure and chemical composition.

MR relaxation times of agar-based tissue-mimicking phantoms

Agar gels were previously proven capable of accurately replicating the acoustical and thermal properties of real tissue and widely used for the construction of tissue-mimicking phantoms (TMPs) for focused ultrasound (FUS) applications. Given the current popularity of magnetic resonance-guided FUS (MRgFUS), we have investigated the MR relaxation times T1 and T2 of different mixtures of agar-based phantoms. Nine TMPs were constructed containing agar as the gelling agent and various concentrations of silicon dioxide and evaporated milk. An agar-based phantom doped with wood powder was also evaluated. A series of MR images were acquired in a 1.5 T scanner for T1 and T2 mapping. T2 was predominantly affected by varying agar concentrations. A trend toward decreasing T1 with an increasing concentration of evaporated milk was observed. The addition of silicon dioxide decreased both relaxation times of pure agar gels. The proposed phantoms have great potential for use with the continuously emerging MRgFUS technology. The MR relaxation times of several body tissues can be mimicked by adjusting the concentration of ingredients, thus enabling more accurate and realistic MRgFUS studies.

Fast Quantitative Low-Field Magnetic Resonance Imaging With OPTIMUM-Optimized Magnetic Resonance Fingerprinting Using a Stationary Steady-State Cartesian Approach and Accelerated Acquisition Schedules

OBJECTIVE

The aim of the proposed work is to develop model-based, fast multiparametric magnetic resonance imaging (MRI) in field regimes where signal-to-noise ratio is poor, such as encountered at low-field and in low γ nuclei.

MATERIALS AND METHODS

A custom, optimized MRI pipeline was developed at low field (0.1 T) that relies on the magnetic resonance fingerprinting framework, called OPTIMUM. An optimization algorithm was used to select a short acquisition schedule (n = 18 images) that favors maximal discrimination across varying magnetic properties (T1, T2) and off-resonance effects while maintaining high transverse magnetization at the steady state. In the presented study, a stationary balanced steady-state approach was investigated that allows for Cartesian (used here) and non-Cartesian acquisition schemes. Images were collected in calibrated samples containing different concentrations of manganese(II) chloride (MnCl2) in deionized water and compared with gold standard techniques (ie, inversion recovery for T1, Carr-Purcell-Meiboom-Gill for T2). Images were then collected in vivo in the human hand and wrist.

RESULTS

OPTIMUM successfully provided sets of quantified maps (T1, T2, T2*, M0, ΔB0, B1+) in calibrated samples and in vivo in the human hand and wrist in 3 dimensions, in ~8.5 minutes, with a voxel resolution of [1.5 ×1.5 × 6.5] mm3. Relaxation parameters (T1, T2) scale linearly with [MnCl2] and are in good agreement with the calibrations performed for T1, with a consistent trend to underestimate T2.

CONCLUSION

We show that low-field MRI can benefit from innovative multiparametric approaches to gain speed and become realistic in clinical environments. For the first time, we report simultaneous, multiparametric imaging (6 quantitative maps) in 3 dimensions, in vivo in the human hand and wrist, obtained in just 8.5 minutes. It is sometimes overlooked that low magnetic fields provide higher dispersion of nuclear spin relaxation rates. Rapid quantification such as offered by OPTIMUM could be an enabling technology to explore new metrics and contrasts in point-of-care MRI diagnosis, making it an important step toward broad democratization.

Using Variable Flip Angle (VFA) and Modified Look-Locker Inversion Recovery (MOLLI) T1 mapping in clinical OE-MRI

BACKGROUND AND PURPOSE

The imaging technique known as Oxygen-Enhanced MRI is under development as a noninvasive technique for imaging hypoxia in tumours and pulmonary diseases. While promising results have been shown in preclinical experiments, clinical studies have mentioned experiencing difficulties with patient motion, image registration, and the limitations of single-slice images compared to 3D volumes. As clinical studies begin to assess feasibility of using OE-MRI in patients, it is important for researchers to communicate about the practical challenges experienced when using OE-MRI on patients to help the technique advance.

MATERIALS AND METHODS

We report on our experience with using two types of T1 mapping (MOLLI and VFA) for a recently completed OE-MRI clinical study on oropharyngeal squamous cell carcinoma.

RESULTS

We report: (1) the artefacts and practical difficulties encountered in this study; (2) the difference in estimated T1 from each method used - the VFA T1 estimation was higher than the MOLLI estimation by 27% on average; (3) the standard deviation within the tumour ROIs - there was no significant difference in the standard deviation seen within the tumour ROIs from the VFA versus MOLLI; and (4) the OE-MRI response collected from either method. Lastly, we collated the MRI acquisition details from over 45 relevant manuscripts as a convenient reference for researchers planning future studies.

CONCLUSION

We have reported our practical experience from an OE-MRI clinical study, with the aim that sharing this is helpful to researchers planning future studies. In this study, VFA was a more useful technique for using OE-MRI in tumours than MOLLI T1 mapping.

Multi-parameter quantitative mapping of R1, R2*, PD, and MTsat is reproducible when accelerated with Compressed SENSE

Multi-parameter mapping (MPM) magnetic resonance imaging (MRI) provides quantitative estimates of the longitudinal and effective transverse relaxation rates R1 and R2*, proton density (PD), and magnetization transfer saturation (MTsat). Thereby, MPM enables better comparability across sites and time than conventional weighted MRI. However, for MPM, several contrasts must be acquired, resulting in prolonged measurement durations and thus preventing MPM's application in clinical routines. State-of-the-art imaging acceleration techniques such as Compressed SENSE (CS), a combination of compressed sensing and sensitivity encoding, can be used to reduce the scan time of MPM. However, the accuracy and precision of the resulting quantitative parameter maps have not been systematically evaluated. In this study, we therefore investigated the effect of CS acceleration on the fidelity and reproducibility of MPM acquisitions. In five healthy volunteers and in a phantom, we compared MPM metrics acquired without imaging acceleration, with the standard acceleration (SENSE factor 2.5), and with Compressed SENSE with acceleration factors 4 and 6 using a 32-channel head coil. We evaluated the reproducibility and repeatability of accelerated MPM using data from three scan sessions in gray and white matter volumes-of-interest (VOIs). Accelerated MPM provided precise and accurate quantitative parameter maps. For most parameters, the results of the CS-accelerated protocols correlated more strongly with the non-accelerated protocol than the standard SENSE-accelerated protocols. Furthermore, for most VOIs and contrasts, coefficients of variation were lower when calculated from data acquired with different imaging accelerations within a single scan session than from data acquired in different scan sessions. These results suggest that MPM with Compressed SENSE acceleration factors up to at least 6 yields reproducible quantitative parameter maps that are highly comparable to those acquired without imaging acceleration. Compressed SENSE can thus be used to considerably reduce the scan duration of R1, R2*, PD, and MTsat mapping, and is highly promising for clinical applications of MPM.

3 T: the good, the bad and the ugly

It is around 20 years since the first commercial 3 T MRI systems became available. The theoretical promise of twice the signal-to-noise ratio of a 1.5 T system together with a greater sensitivity to magnetic susceptibility-related contrast mechanisms, such as the blood oxygen level dependent effect that is the basis for functional MRI, drove the initial market in neuroradiology. However, the limitations of the increased field strength soon became apparent, including the increased radiofrequency power deposition, tissue-dependent changes in relaxation times, increased artifacts, and greater safety concerns. Many of these issues are dependent upon MR physics and workarounds have had to be developed to try and mitigate their effects. This article reviews the underlying principles of the good, the bad and the ugly aspects of 3 T, discusses some of the methods used to improve image quality and explains the remaining challenges and concerns.

Quantification of brown adipose tissue in vivo using synthetic magnetic resonance imaging: an experimental study with mice model

BACKGROUND

The white adipose tissue (WAT) and brown adipose tissue (BAT) are associated with the development of several obesity-associated disorders. The use of imaging techniques to differentiate BAT from WAT and quantify BAT volume remains challenging, due to limitations such as spatial resolution and magnetic field inhomogeneity. This study aimed to investigate the feasibility for differentiating BAT from WAT, and quantify the BAT volume in vivo using synthetic magnetic resonance imaging (MRI).

METHODS

A total of 16 C57BL/6 mice were scanned using synthetic MRI. Quantitative longitudinal relaxation time (T1) and transverse relaxation time (T2) maps were obtained from the original synthetic MRI data using the synthetic MRI software offline. The T1 and T2 values of interscapular BAT (IBAT) and dorsal subcutaneous WAT were measured. The IBAT volume was calculated using synthetic MRI-derived T2-weighted images (T2WIs) based on its morphological characteristics and quantitative tissue values. The body weight of mice was measured, and the IBAT specimens were excised and weighted. The correlation between IBAT volume and the weight of IBAT gross specimen and between IBAT volume and mouse body weight was analyzed.

RESULTS

The T1 values of BAT (330.3±19.57 ms) were higher than those of WAT (304.42±4.14 ms) (P<0.001), whereas the T2 values of BAT (66.06±5.06 ms) were lower than those of WAT (88.23±7.68 ms) (P<0.001). The area under the curve (AUC) values of the T1 and T2 for differentiating BAT from WAT was 0.942 and 0.995, respectively. The AUC of the T2 values was higher than that of T1 (P=0.04) using the DeLong test. The optimal cut-off value for T2 was 76 ms for differentiating BAT from WAT (100% sensitivity, 93.7% specificity). A moderate correlation was observed between IBAT volume and the weight of the IBAT gross specimen (r=0.662, P=0.014), and between IBAT volume and mouse body weight (r=0.653, P=0.016).

CONCLUSIONS

The quantitative parameters derived using synthetic MRI may be used to detect and differentiate BAT from WAT in vivo. Synthetic MRI may help quantify BAT volume in vivo.

A method for T1 and T2 relaxation times validation and harmonization as a support to MRI mapping

We present a proof-of-concept study focusing on a method for the intra- and inter-center validation and harmonization of data obtained from MRI T₁ and T₂ maps. The method is based on a set of MnCl₂ samples that provide in-scan ground-truth reference values regardless of the details of the MRI protocol. The relaxation times of MnCl₂ aqueous solutions were first measured by means of an NMR laboratory relaxometer, as a function of concentration and temperature. The obtained T₁ and T₂ values, once renormalized at the scanner temperature, were used as reference values for the MRI mapping measurements of the MnCl₂ relaxation times. By using different clinical MRI scanners and sequences, we found a good agreement for standard and turbo sequences (limits of agreement: 5% for IR, SE, IR-TSE; 10% for TSE), while an under-estimation and an over-estimation were found respectively for MOLLI and T₂-prep TrueFISP, as already reported in the literature. The linearity of the relaxation rates with the concentration predicted by the Solomon-Bloembergen-Morgan theory was observed for every dataset at all temperatures, except for T₂-prep TrueFISP maps results. Some preliminary results of an in vivo experiment are also presented.

A low-cost and shielding-free ultra-low-field brain MRI scanner

Magnetic resonance imaging is a key diagnostic tool in modern healthcare, yet it can be cost-prohibitive given the high installation, maintenance and operation costs of the machinery. There are approximately seven scanners per million inhabitants and over 90% are concentrated in high-income countries. We describe an ultra-low-field brain MRI scanner that operates using a standard AC power outlet and is low cost to build. Using a permanent 0.055 Tesla Samarium-cobalt magnet and deep learning for cancellation of electromagnetic interference, it requires neither magnetic nor radiofrequency shielding cages. The scanner is compact, mobile, and acoustically quiet during scanning. We implement four standard clinical neuroimaging protocols (T1- and T2-weighted, fluid-attenuated inversion recovery like, and diffusion-weighted imaging) on this system, and demonstrate preliminary feasibility in diagnosing brain tumor and stroke. Such technology has the potential to meet clinical needs at point of care or in low and middle income countries.

SNR and total acquisition time analysis of multi-echo FLASH pulse sequence for current density imaging

Magnetic Resonance Current Density Imaging (MRCDI) is an imaging modality providing cross-sectional current density (J¯) information inside the body. The clinical applicability of MRCDI is highly dependent on the sensitivity of the acquired noisy current-induced magnetic flux density (B∼_z) distributions. Here, a novel analysis is developed to investigate the combined effect of relevant parameters of the RF spoiled gradient echo (FLASH) pulse sequence on the SNR level and the total acquisition time (TAT) of the acquired B∼_z images. The proposed analysis then is expanded for a multi-echo FLASH (ME-FLASH) pulse sequence to take advantage of combining the multiple echoes to achieve B∼_z^comb distribution with a higher SNR than the one achievable with a single echo acquisition. The optimized sequence parameters to acquire a B∼_z distribution with the highest possible SNR for a given acquisition time or the desired SNR in the shortest scan time are estimated using the proposed analysis. The analysis also provides different sets of sequence parameters to acquire B∼_z distributions with the same SNR at almost the same TAT. Furthermore, the effects of intensive utilization of the gradients and the magnetohydrodynamic (MHD) flow velocity on the acquired B∼_z distribution in MRCDI experiments is investigated. The analytical results of the proposed analysis are validated experimentally using an imaging phantom having the conductivity and the relaxation parameters of the brain white matter tissue.

Temperature dependence, accuracy, and repeatability of T1 and T2 relaxation times for the ISMRM/NIST system phantom measured using MR fingerprinting

PURPOSE

Before MR fingerprinting (MRF) can be adopted clinically, the derived quantitative values must be proven accurate and repeatable over a range of T₁ and T₂ values and temperatures. Correct assessment of accuracy and precision as well as comparison between measurements can only be performed when temperature is either controlled or corrected for. The purpose of this study was to investigate the temperature dependence of T₁ and T₂ MRF values and evaluate the accuracy and repeatability of temperature-corrected relaxation values derived from a B₁ -corrected MRF-fast imaging with steady-state precession implementation using 2 different dictionary sizes.

METHODS

The International Society of MR in Medicine/National Institute of Standards and Technology phantom was scanned using an MRF sequence of 2 different lengths, a variable flip angle T₁ , and a multi-echo spin echo T₂ at 14 temperatures ranging from 15°C to 28°C and investigated with a linear regression model. Temperature-corrected accuracy was evaluated by correlating T₁ and T₂ times from each MRF dictionary with reference values. Repeatability was assessed using the coefficient of variation, with measurements taken over 30 separate sessions.

RESULTS

There was a statistically significant fit of the model for MRF-derived T₁ and T₂ and temperature (p < 0.05) for all the spheres with a T₁ > 500 ms. Both MRF methods showed a strong linear correlation with reference values for T₁ (R² = 0.996) and T₂ (R² = 0.982). MRF repeatability for T₁ values was ≤1.4% and for T₂ values was ≤3.4%.

CONCLUSION

MRF demonstrated relaxation times with a temperature dependence similar to that of conventional mapping methods. Temperature-corrected T₁ and T₂ values from both dictionaries showed adequate accuracy and excellent repeatability in this phantom study.

Post mortem brain temperature and its influence on quantitative MRI of the brain

Objective

MRI temperature sensitivity presents a major issue in in situ post mortem MRI (PMMRI), as the tissue temperatures differ from living persons due to passive cooling of the deceased. This study aims at computing brain temperature effects on the MRI parameters to correct for temperature in PMMRI, laying the foundation for future projects on post mortem validation of in vivo MRI techniques.

Materials and methods

Brain MRI parameters were assessed in vivo and in situ post mortem using a 3 T MRI scanner. Post mortem brain temperature was measured in situ transethmoidally. The temperature effect was computed by fitting a linear model to the MRI parameters and the corresponding brain temperature.

Results

Linear positive temperature correlations were observed for T₁, T₂* and mean diffusivity in all tissue types. A significant negative correlation was observed for T₂ in white matter. Fractional anisotropy revealed significant correlations in all gray matter regions except for the thalamus.

Discussion

The linear models will allow to correct for temperature in post mortem MRI. Comparing in vivo to post mortem conditions, the mean diffusivity, in contrast to T₁ and T₂, revealed additional effects besides temperature, such as cessation of perfusion and active diffusion.

Is fetal MRI ready for neuroimaging prime time? An examination of progress and remaining areas for development

A major challenge in designing large-scale, multi-site studies is developing a core, scalable protocol that retains the innovation of scientific advances while also lending itself to the variability in experience and resources across sites. In the development of a common Healthy Brain and Child Development (HBCD) protocol, one of the chief questions is "is fetal MRI ready for prime-time?" While there is agreement about the value of prenatal data obtained non-invasively through MRI, questions about practicality abound. There has been rapid progress over the past years in fetal and placental MRI methodology but there is uncertainty about whether the gains afforded outweigh the challenges in supporting fetal MRI protocols at scale. Here, we will define challenges inherent in building a common protocol across sites with variable expertise and will propose a tentative framework for evaluation of design decisions. We will compare and contrast various design considerations for both normative and high-risk populations, in the setting of the post-COVID era. We will conclude with articulation of the benefits of overcoming these challenges and would lend to the primary questions articulated in the HBCD initiative.

Measurement of sub-zero temperatures in MRI using T1 temperature sensitive soft silicone materials: Applications for MRI-guided cryosurgery

PURPOSE

One standard method, proton resonance frequency shift, for measuring temperature using magnetic resonance imaging (MRI), in MRI-guided surgeries, fails completely below the freezing point of water. Because of this, we have developed a new methodology for monitoring temperature with MRI below freezing. The purpose of this paper is to show that a strong temperature dependence of the nuclear relaxation time T₁ in soft silicone polymers can lead to temperature-dependent changes of MRI intensity acquired with T₁ weighting. We propose the use of silicone filaments inserted in tissue for measuring temperature during MRI-guided cryoablations.

METHODS

The temperature dependence of T₁ in bio-compatible soft silicone polymers was measured using nuclear magnetic resonance spectroscopy and MRI. Phantoms, made of bulk silicone materials and put in an MRI-compatible thermal container with dry ice, allowed temperature measurements ranging from -60°C to + 20°C. T₁ -weighted gradient echo images of the phantoms were acquired at spatially uniform temperatures and with a gradient in temperature to determine the efficacy of using these materials as temperature indicators in MRI. Ex vivo experiments on silicone rods, 4 mm in diameter, inserted in animal tissue were conducted to assess the practical feasibility of the method.

RESULTS

Measurements of nuclear relaxation times of protons in soft silicone polymers show a monotonic, nearly linear, change with temperature (R²  > 0.98) and have a significant correlation with temperature (Pearson's r > 0.99, p < 0.01). Similarly, the intensity of the MR images in these materials, taken with a gradient echo sequence, are also temperature dependent. There is again a monotonic change in MRI intensity that correlates well with the measured temperature (Pearson's r < -0.98 and p < 0.01). The MRI experiments show that a temperature change of 3°C can be resolved in a distance of about 2.5 mm. Based on MRI images and external sensor calibrations for a sample with a gradient in temperature, temperature maps with 3°C isotherms are created for a bulk phantom. Experiments demonstrate that these changes in MRI intensity with temperature can also be seen in 4 mm silicone rods embedded in ex vivo animal tissue.

CONCLUSIONS

We have developed a new method for measuring temperature in MRI that potentially could be used during MRI-guided cryoablation operations, reducing both procedure time and cost, and making these surgeries safer.

In vivo T1 and T2 relaxation time maps of brain tissue, skeletal muscle, and lipid measured in healthy volunteers at 50 mT

PURPOSE

Low-field (B₀ < 0.1 T) MRI has generated much interest as a means of increased accessibility via reduced cost and improved portability compared to conventional clinical systems (B₀ ≥ 1.5 Tesla). Here we measure MR relaxation times at 50 mT and compare results with commonly used models based on both in vivo and ex vivo measurements.

METHODS

Using 3D turbo spin echo readouts, T₁ and T₂ maps of the human brain and lower leg were acquired on a custom-built 50 mT MRI scanner using inversion-recovery and multi-echo-based sequences, respectively. Image segmentation was performed based on a histogram analysis of the relaxation times.

RESULTS

The average T₁ times of gray matter, white matter, and cerebrospinal fluid (CSF) were 327 ± 10 ms, 275 ± 5 ms, and 3695 ± 287 ms, respectively. Corresponding values of T₂ were 102 ± 6 ms, 102 ± 6 ms, and 1584 ± 124 ms. T₁ times in the calf muscle were measured to be 171 ± 11 ms and were 130 ± 5 ms in subcutaneous and bone marrow lipid. Corresponding T₂ times were 39 ± 2 ms in muscle and 90 ± 13 ms in lipid.

CONCLUSIONS

For tissues except for CSF, the measured T₁ times are much shorter than reported at higher fields and generally lie within the range of different models in the literature. As expected, T₂ times are similar to those seen at typical clinical field strengths. Analysis of the relaxation maps indicates that segmentation of white and gray matter based purely on T₁ or T₂ will be quite challenging at low field given the relatively small difference in relaxation times.

3Tesla post-mortem MRI quantification of anatomical brain structures

Quantitative post-mortem magnetic resonance imaging (PMMR) allows for measurement of T1 and T2 relaxation times and proton density (PD) of brain tissue. Quantitative PMMR values may be used for advanced post-mortem neuro-imaging diagnostics such as computer aided diagnosis. So far, the quantitative T1, T2 and PD post-mortem values of regular anatomical brain structures were unknown for a 3 Tesla PMMR application. The goal of this basic research study was to evaluate the quantitative values of post-mortem brain structures for a 3 T post-mortem magnetic resonance application with regard to various corpse temperatures. In 50 forensic cases, a quantitative PMMR brain sequence was applied prior to autopsy. Measurements of T1 (in ms), T2 (in ms), and PD (in %) values of cerebrum (Group 1: frontal grey matter, frontal white matter, thalamus, caudate nucleus, globus pallidus, putamen, internal capsule) brainstem and cerebellum (Group 2: cerebral peduncle, substantia nigra, red nucleus, pons, middle cerebellar peduncle, cerebellar hemisphere, medulla oblongata) were conducted in synthetically calculated axial PMMR brain images. Assessed quantitative values were corrected for corpse temperature. Temperature dependence was observed mainly for T1 values. ANOVA testing resulted in significant differences of quantitative values between the investigated anatomical brain structures in both groups. It can be concluded that temperature corrected 3 Tesla PMMR T1, T2 and PD values are feasible for characterization and discrimination of regular anatomical brain structures. This may provide a base for future advanced diagnostics of forensically relevant brain lesions and pathology.

Clinical Feasibility of Multi-Acquisition Variable-Resonance Image Combination-Based T2 Mapping near Hip Arthroplasty

Background: Hip arthroplasty is increasingly prevalent, and early detection of complications can improve outcomes. Quantitative magnetic resonance imaging (qMRI) methods using multi-acquisition variable-resonance image combination (MAVRIC) may allow for the assessment of soft tissues in close proximity to hip arthroplasty devices. Question/Purposes: We sought to determine the clinical feasibility of MAVRIC-based T2 mapping as a qMRI approach for assessing synovial reactions in patients with a hip arthroplasty device. We hypothesized that there would be differences in T2 metrics by synovial type, clinical impression, and clinical findings related to synovitis. Methods: We conducted a cross-sectional study of 141 subjects with 171 hip arthroplasties with greater than 1 year post-implantation. We enrolled subjects who had had a primary total hip arthroplasty or hip resurfacing arthroplasty between May 2019 and March 2020, excluding those with a revision hip arthroplasty and those with standard safety contraindications for receiving an MRI. Institutional standard 2D fast spin echo (FSE), short-tau inversion recovery (STIR), and susceptibility-reduced MAVRIC morphological MR images were acquired for each hip and followed by a dual-echo acquisition MAVRIC T2 mapping sequence. Results: While 131 subjects (81%) were classified as having a "normal" synovial reaction, significantly longer T2 values were found for fluid synovial reactions compared with mixed reactions. In addition, subjects with synovial dehiscence and decompression present had T2 prolongation. Larger synovial volumes were found in subjects with low-signal intensity deposits. Conclusions: MAVRIC-based T2 mapping is clinically feasible and there are significant quantitative differences based on type of synovial reaction. Patients undergoing hip arthroscopy revision surgery will warrant comparison of T2 values with direct histologic assessment of a tissue sample obtained intraoperatively. The approach used in this study may be used for a quantitative evaluation and monitoring of soft tissues around metal implants.

Accessible pediatric neuroimaging using a low field strength MRI scanner

Magnetic resonance imaging (MRI) has played an increasingly relevant role in understanding infant, child, and adolescent neurodevelopment, providing new insight into developmental patterns in neurotypical development, as well as those associated with potential psychopathology, learning disorders, and other neurological conditions. In addition, studies have shown the impact of a child's physical and psychosocial environment on developing brain structure and function. A rate-limiting complication in these studies, however, is the high cost and infrastructural requirements of modern MRI systems. High costs mean many neuroimaging studies typically include fewer than 100 individuals and are performed predominately in high resource hospitals and university settings within high income countries (HICs). As a result, our knowledge of brain development, particularly in children who live in lower and middle income countries (LMICs) is relatively limited. Low field systems, with magnetic fields less than 100mT offer the promise of lower scanning costs and wide-spread global adoption, but routine low field pediatric neuroimaging has yet to be demonstrated. Here we present the first pediatric MRI data collected on a low cost and assessable 64mT scanner in children 6 weeks to 16 years of age and replicate brain volumes estimates and developmental trajectories derived from 3T MRI data. While preliminary, these results illustrate the potential of low field imaging as a viable complement to more conventional high field imaging systems, and one that may further enhance our knowledge of neurodevelopment in LMICs where malnutrition, psychosocial adversities, and other environmental exposures may profoundly affect brain maturation.

Measuring viscoelastic parameters in Magnetic Resonance Elastography: a comparison at high and low magnetic field intensity

Magnetic Resonance Elastography (MRE) is a non-invasive imaging technique which involves motion-encoding MRI for the estimation of the shear viscoelastic properties of soft tissues through the study of shear wave propagation. The technique has been found informative for disease diagnosis, as well as for monitoring of the effects of therapies. The development of MRE and its validation have been supported by the use of tissue-mimicking phantoms. In this paper we present our new MRE protocol using a low magnetic field tabletop MRI device at 0.5 T and sinusoidal uniaxial excitation in a geometrical focusing condition. Results obtained for gelatin are compared to those previously obtained using high magnetic field MRE at 11.7 T. A multi-frequency investigation is also provided via a comparison of commonly used rheological models: Maxwell, Springpot, Voigt, Zener, Jeffrey, fractional Voigt and fractional Zener. Complex shear modulus values were comparable when processed from images acquired with the tabletop low field scanner and the high field scanner. This study serves as a validation of the presented tabletop MRE protocol and paves the way for MRE experiments on ex-vivo tissue samples in both normal and pathological conditions.

Characterization of a soft tissue-mimicking agar/wood powder material for MRgFUS applications

This study describes the development and characterization of an agar-based soft tissue-mimicking material (TMM) doped with wood powder destined for fabricating MRgFUS applications. The main objective of the following work was to investigate the suitability of wood powder as an inexpensive alternative in replacing other added materials that have been suggested in previous studies for controlling the ultrasonic properties of TMMs. The characterization procedure involved a series of experiments designed to estimate the acoustic (attenuation coefficient, absorption coefficient, propagation speed, and impedance), thermal (conductivity, diffusivity, specific heat capacity), and MR properties (T1 and T2 relaxation times) of the wood-powder doped material. The developed TMM (2% w/v agar and 4% w/v wood powder) as expected demonstrated compatibility with MRI scanner following images artifacts evaluation. The acoustic attenuation coefficient of the proposed material was measured over the frequency range of 1.1-3 MHz and found to be nearly proportional to frequency. The measured attenuation coefficient was 0.48 dB/cm at 1 MHz which was well within the range of soft tissue. Temperatures over 37 °C proved to increase marginally the attenuation coefficient. Following the transient thermoelectric method, the acoustic absorption coefficient was estimated at 0.34 dB/cm-MHz. The estimated propagation speed (1487 m/s) was within the range of soft tissue at room temperature, while it significantly increased with higher temperature. The material possessed an acoustic impedance of 1.58 MRayl which was found to be comparable to the corresponding value of muscle tissue. The thermal conductivity of the material was estimated at 0.51 W/m K. The measured relaxation times T1 (844 ms) and T2 (66 ms) were within the range of values found in the literature for soft tissue. The phantom was tested for its suitability for evaluating MRgFUS thermal protocols. High acoustic energy was applied, and temperature change was recorded using thermocouples and MR thermometry. MR thermal maps were acquired using single-shot Echo Planar Imaging (EPI) gradient echo sequence. The TMM matched adequately the acoustic and thermal properties of human tissues and through a series of experiments, it was proven that wood concentration enhances acoustic absorption. Experiments using MR thermometry demonstrated the usefulness of this phantom to evaluate ultrasonic thermal protocols by monitoring peak temperatures in real-time. Thermal lesions formed above a thermal dose were observed in high-resolution MR images and visually in dissections of the proposed TMM.

Dynamics of Zeeman and dipolar states in the spin locking in a liquid entrapped in nano-cavities: Application to study of biological systems

We analyze the application of the spin locking method to study the spin dynamics and spin-lattice relaxation of nuclear spins-1/2 in liquids or gases enclosed in a nano-cavity. Two cases are considered: when the amplitude of the radio-frequency field is much greater than the local field acting the nucleus and when the amplitude of the radio-frequency field is comparable or even less than the local field. In these cases, temperatures of two spin reservoirs, the Zeeman and dipole ones, change in different ways: in the first case, temperatures of the Zeeman and dipolar reservoirs reach the common value relatively quickly, and then turn to the lattice temperature; in the second case, at the beginning of the process, these temperatures are equal, and then turn to the lattice temperature with different relaxation times. Good agreement between the obtained theoretical results and the experimental data is achieved by fitting the parameters of the distribution of the orientation of nanocavities. The parameters of this distribution can be used to characterize the fine structure of biological samples, potentially enabling the detection of degradative changes in connective tissues.

Very low field 19F MRI of perfluoro-octylbromide: Minimizing chemical shift effects and signal loss due to scalar coupling

¹⁹F images have been obtained from perflurooctylbromide (PFOB) at very low magnetic field (50 mT). The small spectral dispersion (in Hz) means that all fluorine nuclei contribute to the signal without chemical shift artifacts or the need for specialized imaging sequences. Turbo spin echo trains with short interpulse intervals and full 180° refocussing pulses suppress scalar coupling, leading to long apparent T₂ values and highly efficient data collection. Overall, the detection efficiency of PFOB is very similar that of water in tissue.

Assessing Age-Related Gray Matter Differences in Young Adults with Voxel-Based Morphometry: The Effect of Field Strengths

Knowing the patterns of brain differences with age in the young population could lead to a better understanding of the causes of certain psychiatric disorders; however, relevant information is insufficient. Here, a pattern of regional gray matter (GM) that changed with age in a young cohort aged 20-30 years was provided. Extending from previous age studies, all participants were imaged at both 1.5 T and 3 T to address the question of how far the field strength influences results. Fifty-nine young participants aged 20-30 years were scanned at both 1.5 T and 3 T. Voxel-based morphometry (VBM) was used to estimate the GM volume. Some brain regions showed a significant field strength-dependent difference in GM volume. VBM uncovered a significantly age-related increase in the GM volume in the left visual-associated area at 3 T, which was not detected at 1.5 T. In addition, voxels at 1.5 T that revealed a significant age-related reduction in the GM volume were found in the right cerebellum. In conclusion, age-related differences in human brain morphology could even be detected in a young cohort aged 20-30 years; however, the results varied across field strengths. Thus, field strength should be considered an important factor when comparing age-specific brain differences across studies.

Quantity and quality: Normative open-access neuroimaging databases

The focus of this article is to compare twenty normative and open-access neuroimaging databases based on quantitative measures of image quality, namely, signal-to-noise (SNR) and contrast-to-noise ratios (CNR). We further the analysis through discussing to what extent these databases can be used for the visualization of deeper regions of the brain, such as the subcortex, as well as provide an overview of the types of inferences that can be drawn. A quantitative comparison of contrasts including T1-weighted (T1w) and T2-weighted (T2w) images are summarized, providing evidence for the benefit of ultra-high field MRI. Our analysis suggests a decline in SNR in the caudate nuclei with increasing age, in T1w, T2w, qT1 and qT2* contrasts, potentially indicative of complex structural age-dependent changes. A similar decline was found in the corpus callosum of the T1w, qT1 and qT2* contrasts, though this relationship is not as extensive as within the caudate nuclei. These declines were accompanied by a declining CNR over age in all image contrasts. A positive correlation was found between scan time and the estimated SNR as well as a negative correlation between scan time and spatial resolution. Image quality as well as the number and types of contrasts acquired by these databases are important factors to take into account when selecting structural data for reuse. This article highlights the opportunities and pitfalls associated with sampling existing databases, and provides a quantitative backing for their usage.

Determining the effect of water temperature on the T1 and T2 relaxation times of the lung tissue at 9.4 T MRI: A drowning mouse model

Japanese individuals have a unique culture of soaking in a bathtub, and forensic pathologists have experienced fatal cases due to drowning. However, T1 and T2 relaxation times of a drowning lung are poorly documented. In the present study, we investigated the relationship between drowning water temperature and T1 and T2 relaxation times of drowning lung tissues at 9.4 T MRI (Bruker, BioSpec94/20USR). The mice used as animal drowning models were directly submerged in freshwater. Water temperature was set to 8 °C-10 °C (cold), 20 °C-22 °C (normal), 30 °C, and 45 °C. The regions of interest (ROIs) on the axial section of the third slice were set at the central and peripheral areas of each-the left and the right-lung. T1 relaxation times measured immediately after death differed by the presence or absence of soaking water, except in case of cold water temperature. In the drowning groups, T1 relaxation time showed a linear dependency on water temperature. By contrast, T2 relaxation time was almost constant regardless of the presence of drowning under the same temperature condition; when compared in the lung areas of the same individuals, the times were uniformly reduced in drowning models. To minimize the effects of hypostasis and decomposition, we performed measurements immediately after death and were able to determine the noticeable difference in drowning water temperature. These results may be useful for qualitative assessments of a drowning lung and may serve as a basis when imaging the human body during forensic autopsy cases.

Development of phantom materials with independently adjustable CT- and MR-contrast at 0.35, 1.5 and 3 T

Quality assurance in magnetic resonance (MR)-guided radiotherapy lacks anthropomorphic phantoms that represent tissue-equivalent imaging contrast in both computed tomography (CT) and MR imaging. In this study, we developed phantom materials with individually adjustable CT value as well as [Formula: see text]- and [Formula: see text]-relaxation times in MR imaging at three different magnetic field strengths. Additionally, their experimental stopping power ratio (SPR) for carbon ions was compared with predictions based on single- and dual-energy CT. Ni-DTPA doped agarose gels were used for individual adjustment of [Formula: see text] and [Formula: see text] at [Formula: see text] and 3.0 T. The CT value was varied by adding potassium chloride (KCl). By multiple linear regression, equations for the determination of agarose, Ni-DTPA and KCl concentrations for given [Formula: see text] [Formula: see text] and CT values were derived and employed to produce nine specific soft tissue samples. Experimental [Formula: see text] [Formula: see text] and CT values of these soft tissue samples were compared with predictions and additionally, carbon ion SPR obtained by range measurements were compared with predictions based on single- and dual-energy CT. The measured CT value, [Formula: see text] and [Formula: see text] of the produced soft tissue samples agreed very well with predictions based on the derived equations with mean deviations of less than [Formula: see text] While single-energy CT overestimates the measured SPR of the soft tissue samples, the dual-energy CT-based predictions showed a mean SPR deviation of only [Formula: see text] To conclude, anthropomorphic phantom materials with independently adjustable CT values as well as [Formula: see text] and [Formula: see text] relaxation times at three different magnetic field strengths were developed. The derived equations describe the material specific relaxation times and the CT value in dependence on agarose, Ni-DTPA and KCl concentrations as well as the chemical composition of the materials based on given [Formula: see text] and CT value. Dual-energy CT allows accurate prediction of the carbon ion range in these materials.

Bi-component T2 mapping correlates with articular cartilage material properties

Non-invasive estimation of cartilage material properties is useful for understanding cartilage health and creating subject-specific computational models. Bi-component T2 mapping measured using Multi-Component Driven Equilibrium Single Shot Observation of T₁ and T₂ (mcDESPOT) is sensitive for detecting cartilage degeneration within the human knee joint, but has not been correlated with cartilage composition and mechanical properties. Therefore, the purpose of this study was to investigate the relationship between bi-component T₂ parameters measured using mcDESPOT at 3.0 T and cartilage composition and mechanical properties. Ex-vivo patellar cartilage specimens harvested from five human cadaveric knees were imaged using mcDESPOT at 3.0 T. Cartilage samples were removed from the patellae, mechanically tested to determine linear modulus and dissipated energy, and chemically tested to determine proteoglycan and collagen content. Parameter maps of single-component T₂ relaxation time (T₂), the T₂ relaxation times of the fast relaxing macromolecular bound water component (T_2F) and slow relaxing bulk water component (T_2S), and the fraction of the fast relaxing macromolecular bound water component (F_F) were compared to mechanical and chemical measures using linear regression. F_F was significantly (p < 0.05) correlated with energy dissipation and linear modulus. T₂ was significantly (p ≤ 0.05) correlated with elastic modulus at 1 Hz and energy dissipated at all frequencies. There were no other significant (p = 0.13-0.97) correlations between mcDESPOT parameters and mechanical properties. F_F was significantly (p = 0.04) correlated with proteoglycan content. There were no other significant (p = 0.19-0.92) correlations between mcDESPOT parameters and proteoglycan or collagen content. This study suggests that F_F measured using mcDESPOT at 3.0 T could be used to non-invasively estimate cartilage proteoglycan content, elastic modulus, and energy dissipation.

Comparison of phase-resolved functional lung (PREFUL) MRI derived perfusion and ventilation parameters at 1.5T and 3T in healthy volunteers

Purpose

The purpose of this study is to evaluate the influence of different field strengths on perfusion and ventilation parameters, SNR and CNR derived by PREFUL MRI using predefined sequence parameters.

Methods

Data sets of free breathing 2d FLASH lung MRI were acquired from 15 healthy subjects at 1.5T and 3T (Magnetom Avanto and Skyra, Siemens Healthcare, Erlangen, Germany) with a maximum period of 3 days in between. The processed functional parameters regional ventilation (RVent), perfusion (Q), quantified perfusion (Q_Quant), perfusion defect percentage (QDP), ventilation defect percentage (VDP) and ventilation-perfusion match (VQM) were compared for systematic differences. Signal- and contrast-to-noise ratio (SNR and CNR) of both acquisitions were analyzed.

Results

RVent, Q, VDP, SNR and CNR presented no significant differences between 1.5T and 3T. Q_Quant (1.5T vs. 3T, P = 0.04), and QDP (1.5T vs. 3T, P≤0.01) decreased significantly at 3T. Consequently, VQM increased significantly (1.5T vs. 3T, P≤0.01). Skewness and kurtosis of the Q-values increased significantly at 3T (P≤0.01). The mean Sørensen-Dice coefficients between both series were 0.91 for QDP and 0.94 for VDP. The Bland-Altman analysis of both series showed mean differences of 4.29% for QDP, 1.23% for VDP and -5.15% for VQM. Using the above-mentioned parameters for three-day repeatability at two different scanners and field strengths, the retrospective power calculation showed, that a sample size of 15 can detect differences of 3.7% for QDP, of 2.9% for VDP and differences of 2.6% for VQM.

Conclusion

Significant differences in QDP may be related to field inhomogeneities, which is expressed by increasing skewness and kurtosis at 3T. Q_Quant reveals only poor reproducibility between 1.5T and 3T. RVent, Q, VDP, SNR and CNR were not altered significantly at the used sequence parameters. Healthy participants with minimal defects present high spatial agreement of QDP and VDP.

Technical Note: ADAM PETer - An anthropomorphic, deformable and multimodality pelvis phantom with positron emission tomography extension for radiotherapy

OBJECTIVE

To develop an anthropomorphic, deformable and multimodal pelvis phantom with positron emission tomography extension for radiotherapy (ADAM PETer).

METHODS

The design of ADAM PETer was based on our previous pelvis phantom (ADAM) and extended for compatibility with PET and use in 3T magnetic resonance imaging (MRI). The formerly manually manufactured silicon organ surrogates were replaced by three-dimensional (3D) printed organ shells. Two intraprostatic lesions, four iliac lymph node metastases and two pelvic bone metastases were added to simulate prostate cancer as multifocal and metastatic disease. Radiological properties [computed tomography (CT) and 3T MRI] of cortical bone, bone marrow and adipose tissue were simulated by heavy gypsum, a mixture of Vaseline and K₂ HPO₄ and peanut oil, respectively. For soft tissues, agarose gels with varying concentrations of agarose, gadolinium (Gd) and sodium fluoride (NaF) were developed. The agarose gels were doped with patient-specific activity concentrations of a Fluorine-18 labelled compound and then filled into the 3D printed organ shells of prostate lesions, lymph node and bone metastases. The phantom was imaged at a dual energy CT and a 3T PET/MRI scanner.

RESULTS

The compositions of the soft tissue surrogates are the following (given as mass fractions of agarose[w%]/NaF[w%]/Gd[w%]): Muscle (4/1/0.027), prostate (1.35/4.2/0.011), prostate lesions (2.25/4.2/0.0085), lymph node and bone metastases (1.4/4.2/0.025). In all imaging modalities, the phantom simulates human contrast. Intraprostatic lesions appear hypointense as compared to the surrounding normal prostate tissue in T2-weighted MRI. The PET signal of all tumors can be localized as focal spots at their respective site. Activity concentrations of 12.0 kBq/mL (prostate lesion), 12.4 kBq/mL (lymph nodes) and 39.5 kBq/mL (bone metastases) were measured.

CONCLUSION

The ADAM PETer pelvis phantom can be used as multimodal, anthropomorphic model for CT, 3T-MRI and PET measurements. It will be central to simulate and optimize the technical workflow for the integration of PET/MRI-based radiation treatment planning of prostate cancer patients.

Proton nuclear magnetic resonance J-spectroscopy of phantoms containing brain metabolites on a portable 0.05 T MRI scanner

We examined approaches for obtaining ¹H NMR spectra of brain metabolites on a low-field (B₀ = 0.05 T) portable MRI scanner, which was developed in our laboratory with the aim of bringing cost-effective radiological services to populations in underserved, remote regions. The low static magnetic field B₀ dictates low signal to noise ratio for metabolites in the mM concentration range, and results in an overall spectral region for the ¹H resonances of these metabolites narrower than the linewidth obtainable in our scanner. The narrow spectral range also precludes the possibility of suppressing the large contribution of the water resonance at the acquisition stage. We used a spectroscopic Carr-Purcell-Meiboom-Gill (CPMG) sequence to acquire multiecho data from solutions of J-coupled brain metabolites, focusing on lactic acid, a metabolite whose concentration is negligible in the healthy brain and increases significantly in several disease conditions. The J spectra we obtained for lactate from the Fourier transformation of the multiecho data are spectrally well-resolved for a range of echo spacing values. We show that the J spectra at different echo spacings fit well with simulations of the evolution of echo train signal of the lactate under the same conditions. Applying a J-refocused variant of the CPMG sequence, the J modulation of the echo decay is removed, providing a way for subtracting the large contribution of the non-modulated component in the J spectrum in conditions where notching it using post-processing methods is impossible. We also demonstrate by means of experimental data and simulations that in our experimental conditions, J-spectra of other prominent brain metabolites, such as the neurotransmitter glutamate, do not yield discernible peaks and only contribute to a broad peak at zero frequency.

Texture analysis using T1-weighted images for muscles in Charcot-Marie-Tooth disease patients and volunteers

OBJECTIVES

To explore whether texture features using T1-weighted images correlate with fat fraction, and whether they differ between Charcot-Marie-Tooth (CMT) disease patients and volunteers.

METHODS

The institutional review board approved this retrospective study, and the requirement for informed consent was waived; data of eighteen CMT patients and eighteen healthy volunteers from a previous study was used. Texture features of the muscles including mean, standard deviation (SD), skewness, kurtosis, and entropy of the signal intensity were derived from T1-weighted images. Spearman's correlation analysis was used to assess the relationship between texture features and fat fraction measured by 3D multiple gradient echo Dixon-based sequence. Mann-Whitney U test was used to compare the texture features between CMT patients and volunteers. Intraobserver and interobserver agreements for the texture features were assessed using the intraclass correlation coefficient.

RESULTS

The SD (ρ = 0.256, p < 0.001) and entropy (ρ = 0.263, p < 0.001) were significantly and positively correlated with fat fraction; skewness (ρ = - 0.110, p = 0.027) and kurtosis (ρ = - 0.149, p = 0.003) were significantly and inversely correlated with fat fraction. The CMT patients showed a significantly higher SD (63.45 vs. 49.26; p < 0.001), skewness (1.06 vs. 0.56; p < 0.001), kurtosis (4.00 vs. 1.81; p < 0.001), and entropy (3.20 vs. 3.02; p < 0.001) than did the volunteers. Intraobserver and interobserver agreements were almost perfect for mean, SD, and entropy.

CONCLUSIONS

Texture features using T1-weighted images correlated with fat fraction and differed between CMT patients and volunteers.

KEY POINTS

• Standard deviation and entropy of muscles derived from T1-weighted images were significantly and positively correlated with the muscle fat fraction. • Mean, standard deviation, and entropy were considered highly reliable in muscle analyses. • Texture features may have the potential to diagnose early stage of intramuscular fatty infiltration.

Contribution of Magnetic Resonance Imaging in Determining Lumpectomy Cavity in Breast Radiotherapy

BACKGROUND

Accurate localization of the lumpectomy cavity is important for breast cancer radiotherapy after breast-conserving surgery (BCS), but the LC localization based on CT is often difficult to delineate accurately. The study aimed to compare CT-defined LC planning to MRI-defined findings in the supine position for higher soft-tissue resolution of MRI.

METHODS

Fifty-nine breast cancer patients underwent radiotherapy CT planning in supine position followed by MR imaging on the same day. LC was contoured by the radiologist and radiation oncologist together by CT and MRI separately. T2 weighted MR images and tomography findings were combined and the LC volume, mean diameter and the longest axis length were measured after contouring. Subsequently, patients were divided into two groups according to seroma in LC and the above-mentioned parameters were compared.

RESULTS

We did not find any statistically significant difference in the LC volume, mean diameter and length at the longest axis between CT and MRI but based on the presence or absence of seroma, statistically significant differences were found in the LC volumes and the length at the longest axis of LC volumes.

CONCLUSION

We believe that the supine MRI in the same position with CT will be more effective for radiotherapy planning, particularly in patients without a seroma in the surgical cavity.

Diagnostic quality assessment of IR-prepared 3D magnetic resonance neuroimaging accelerated using compressed sensing and k-space sampling order optimization

PURPOSE

To evaluate the clinical diagnostic efficacy of accelerated 3D magnetic resonance (MR) neuroimaging by radiological assessment for image quality and artefacts.

STUDY TYPE

Prospective healthy volunteer study.

SUBJECTS

Eight healthy subjects.

FIELD STRENGTH/SEQUENCE

Inversion Recovery (IR) prepared 3D Gradient Echo (GRE) sequence on a 1.5 T GE Signa HDx scanner.

ASSESSMENT

Independent radiological diagnostic quality assessments of accelerated 3D MR brain datasets were carried out by four experienced neuro-radiologists who were blinded to the acceleration factor and to the subject. The radiological grading was based on a previously reported radiological scoring key that was used for image quality assessment of human brains.

STATISTICAL TESTS

Bland-Altman analysis.

RESULTS

Optimization of the k-space sampling order was important for preserving contrast in accelerated scans. Despite having lower scores than fully sampled datasets, the majority of the compressed sensing (CS) accelerated brain datasets with k-space sampling order optimization (19/24 datasets by Radiologist 1, 24/24 datasets by Radiologist 2 and 16/24 datasets by Radiologist 3) were graded to be fully diagnostic indicating that there was adequate confidence for performing gross structural assessment of the brain.

CONCLUSION

Optimization of k-space acquisition order improves the clinical utility of CS accelerated 3D neuroimaging. This method may be appropriate for routine radiological assessment of the brain.

Observation of Reduced Homeostatic Metabolic Activity and/or Coupling in White Matter Aging

BACKGROUND AND PURPOSE

Transvascular water exchange plays a key role in the functional integrity of the blood-brain barrier (BBB). In white matter (WM), a variety of imaging modalities have demonstrated age-related changes in structure and metabolism, but the extent to which water exchange is altered remains unclear. Here, we investigated the cumulative effects of healthy aging on WM capillary water exchange.

METHODS

A total of 38 healthy adults (aged 36-80 years) were studied using 7T dynamic contrast enhanced MRI. Blood volume fraction (v_b ) and capillary water efflux rate constant (k_po ) were determined by fitting changes in the ¹ H₂ O longitudinal relaxation rate constant (R₁ ) during contrast agent bolus passage to a two-compartment exchange model. WM volume was determined by morphometric analysis of structural images.

RESULTS

R₁ values and WM volume showed similar trajectories of age-related decline. Among all subjects, v_b and k_po averaged 1.7 (±0.5) mL/100 g of tissue and 2.1 (±1.1) s^-1 , respectively. While v_b showed minimal changes over the 40-year-age span of participants, k_po declined 0.06 s^-1 (ca. 3%) per year (r = -.66; P < .0005), from near 4 s^-1 at age 30 to ca. 2 s^-1 at age 70. The association remained significant after controlling for WM volume.

CONCLUSIONS

Previous studies have shown that k_po tracks Na⁺ , K⁺ -ATPase activity-dependent water exchange at the BBB and likely reflects neurogliovascular unit (NGVU) coupled metabolic activity. The age-related decline in k_po observed here is consistent with compromised NGVU metabolism in older individuals and the dysregulated cellular bioenergetics that accompany normal brain aging.

Real-time magnetic resonance imaging-guided cryoablation of the pulmonary veins with acute freeze-zone and chronic lesion assessment

Aims

The goals of this study were to develop a method that combines cryoablation with real-time magnetic resonance imaging (MRI) guidance for pulmonary vein isolation (PVI) and to further quantify the lesion formation by imaging both acute and chronic cryolesions.

Methods and results

Investigational MRI-compatible cryoablation devices were created by modifying cryoballoons and cryocatheters. These devices were used in canines (n = 8) and a complete series of lesions (PVI: n = 5, superior vena cava: n = 4, focal: n = 13) were made under real-time MRI guidance. Late gadolinium enhancement (LGE) magnetic resonance imaging was acquired at acute and chronic time points. Late gadolinium enhancement magnetic resonance imagings show a significant amount of acute tissue injury immediately following cryoablation which subsides over time. In the pulmonary veins, scar covered 100% of the perimeter of the ostium of the veins acutely, which subsided to 95.6 ± 4.3% after 3 months. Focal point lesions showed significantly larger acute enhancement volumes compared to the volumes estimated from gross pathology measurements (0.4392 ± 0.28 cm3 vs. 0.1657 ± 0.08 cm3, P = 0.0043). Additionally, our results with focal point ablations indicate that freeze-zone formation reached a maximum area after 120 s.

Conclusion

This study reports on the development of an MRI-based cryoablation system and shows that with acute cryolesions there is a large area of reversible injury. Real-time MRI provides the ability to visualize the freeze-zone formation during the freeze cycle and for focal lesions reaches a maximum after 120 s suggesting that for maximizing lesion size 120 s might be the lower limit for dosing duration.

Imaging tools for assessment of myocardial fibrosis in humans: the need for greater detail

Myocardial fibrosis is recognized as a key pathological process in the development of cardiac disease and a target for future therapeutics. Despite this recognition, the assessment of fibrosis is not a part of routine clinical practice. This is primarily due to the difficulties in obtaining an accurate assessment of fibrosis non-invasively. Moreover, there is a clear discrepancy between the understandings of myocardial fibrosis clinically where fibrosis is predominately studied with comparatively low-resolution medical imaging technologies like MRI compared with the basic science laboratories where fibrosis can be visualized invasively with high resolution using molecularly specific fluorescence microscopes at the microscopic and nanoscopic scales. In this article, we will first review current medical imaging technologies for assessing fibrosis including echo and MRI. We will then highlight the need for greater microscopic and nanoscopic analysis of human tissue and how this can be addressed through greater utilization of human tissue available through endomyocardial biopsies and cardiac surgeries. We will then describe the relatively new field of molecular imaging that promises to translate research findings to the clinical practice by non-invasively monitoring the molecular signature of fibrosis in patients.

In Vitro Intraductal MRI and T2 Mapping of Cholangiocarcinoma Using Catheter Coils

AIM

Diagnostic imaging of early-stage cholangiocarcinoma is challenging. A previous in vitro study of fixed-tissue liver resection specimens investigated T2 mapping as a method of exploiting the locally increased signal-to-noise ratio (SNR) of duodenoscope coils for improved quantitative magnetic resonance imaging (MRI), despite their non-uniform sensitivity. This work applies similar methods to unfixed liver specimens using catheter-based receivers.

METHODS

Ex vivo intraductal MRI and T2 mapping were carried out at 3T on unfixed resection specimens obtained from cholangiocarcinoma patients immediately after surgery using a catheter coil based on a thin-film magneto-inductive waveguide, inserted directly into an intrahepatic duct.

RESULTS

Polypoid intraductal cholangiocarcinoma was imaged using fast spin-echo sequences. High-resolution T2 maps were extracted by fitting of data obtained at different echo times to mono-exponential models, and disease-induced changes were correlated with histopathology. An increase in T2 was found compared with fixed specimens and differences in T2 allowed the resolution of tumour tissue and malignant features such as polypoid morphology.

CONCLUSION

Despite their limited field of view, useful data can be obtained using catheter coils, and T2 mapping offers an effective method of exploiting their local SNR advantage without the need for image correction.

In vivo 3D brain and extremity MRI at 50 mT using a permanent magnet Halbach array

Purpose

To design a low‐cost, portable permanent magnet‐based MRI system capable of obtaining in vivo MR images within a reasonable scan time.

Methods

A discretized Halbach permanent magnet array with a clear bore diameter of 27 cm was designed for operation at 50 mT. Custom‐built gradient coils, RF coil, gradient amplifiers, and RF amplifier were integrated and tested on both phantoms and in vivo.

Results

Phantom results showed that the gradient nonlinearity in the y‐direction and z‐direction was less than 5% over a 15‐cm FOV and did not need correcting. For the x‐direction, it was significantly greater, but could be partially corrected in postprocessing. Three‐dimensional in vivo scans of the brain of a healthy volunteer using a turbo spin‐echo sequence were acquired at a spatial resolution of 4 × 4 × 4 mm in a time of about 2 minutes. The T₁‐weighted and T₂‐weighted scans showed a good degree of tissue contrast. In addition, in vivo scans of the knee of a healthy volunteer were acquired at a spatial resolution of about 3 × 2 × 2 mm within 12 minutes to show the applicability of the system to extremity imaging.

Conclusion

This work has shown that it is possible to construct a low‐field MRI unit with hardware components costing less than 10 000 Euros, which is able to acquire human images in vivo within a reasonable data‐acquisition time. The system has a high degree of portability with magnet weight of approximately 75 kg, gradient and RF amplifiers each 15 kg, gradient coils 10 kg, and spectrometer 5 kg.