Quantitative rotating frame relaxometry methods in MRI

Rotary excitation of non-sinusoidal pulsed magnetic fields: Towards non-invasive direct detection of cardiac conduction

PURPOSE

There is a need for high resolution non-invasive imaging methods of physiologic magnetic fields. The purpose of this work is to develop a MRI detection approach for non-sinusoidal magnetic fields based on the rotary excitation (REX) mechanism which was previously successfully applied for the detection of oscillating magnetic fields in the sub-nT range.

METHODS

The new detection concept was examined by means of Bloch simulations, evaluating the interaction effect of spin-locked magnetization and low-frequency pulsed magnetic fields. The REX detection approach was validated under controlled conditions in phantom experiments at 3 T. Gaussian and sinc-shaped stimuli were investigated. In addition, the detection of artificial fields resembling a cardiac QRS complex, which is the most prominent peak visible on a magnetocardiogram, was tested.

RESULTS

Bloch simulations demonstrated that the REX method has a high sensitivity to pulsed fields in the resonance case, which is met when the spin-lock frequency coincides with a non-zero Fourier component of the stimulus field. In the experiments, we found that magnetic stimuli of different durations and waveforms can be distinguished by their characteristic REX response spectrum. The detected REX amplitude was proportional to the stimulus peak amplitude (R2 > 0.98) and the lowest field detection was 1 nT. Furthermore, the detection of QRS-like fields with varying QRS durations yielded significant results in a phantom setup (p < 0.001).

CONCLUSION

REX detection can be transferred to non-sinusoidal pulsed magnetic fields and could provide a non-invasive, quantitative tool for spatially resolved assessment of cardiac biomagnetism. Potential applications include the direct detection and characterization of cardiac conduction.

Quantification of the rotating frame relaxation time T2ρ: Comparison of balanced spin-lock and continuous-wave Malcolm-Levitt preparations

For the quantification of rotating frame relaxation times, the T2ρ relaxation pathway plays an essential role. Nevertheless, T2ρ imaging has been studied only to a small extent compared with T1ρ, and preparation techniques for T2ρ have so far been adapted from T1ρ methods. In this work, two different preparation concepts are compared specifically for the use of T2ρ mapping. The first approach involves transferring the balanced spin-locking (B-SL) concept of T1ρ imaging. The second and newly proposed approach is a continuous-wave Malcolm-Levitt (CW-MLEV) pulse train with zero echo times and was motivated from T2 preparation strategies. The modules are tested in Bloch simulations for their intrinsic sensitivity to field inhomogeneities and validated in phantom experiments. In addition, myocardial T2ρ mapping was performed in mice as an exemplary application. Our results demonstrate that the CW-MLEV approach provides superior robustness and thus suggest that established methods of T1ρ imaging are not best suited for T2ρ experiments. In the presence of field inhomogeneities, the simulations indicated an increased banding compensation by a factor of 4.1 compared with B-SL. Quantification of left ventricular T2ρ time in mice yielded more consistent results, and values in the range of 59.2-61.1 ms (R2 = 0.986-0.992) were observed at 7 T.

Combining T1rho and advanced diffusion MRI for noninvasively staging liver fibrosis: an experimental study in rats

PURPOSE

To investigate the value of imaging parameters derived from T1 relaxation times in the rotating frame (T1ρ or T1rho), diffusion kurtosis imaging (DKI) and intravoxel incoherent motion (IVIM) in assessment of liver fibrosis in rats and propose an optimal diagnostic model based on multiparametric MRI.

METHODS

Thirty rats were divided into one control group and four fibrosis experimental groups (n = 6 for each group). Liver fibrosis was induced by administering thioacetamide (TAA) for 2, 4, 6, and 8 weeks. T1ρ, mean kurtosis (MK), mean diffusivity (MD), perfusion fraction (f), true diffusion coefficient (D), and pseudo-diffusion coefficient (D*) were measured and compared among different fibrosis stages. An optimal diagnostic model was established and the diagnostic efficiency was evaluated by receiver operating characteristic (ROC) curve analysis.

RESULTS

The mean AUC values, sensitivity, and specificity of T1ρ and MD derived from DKI across all liver fibrosis stages were comparable but much higher than those of other imaging parameters (0.954, 92.46, 91.85 for T1ρ; 0.949, 92.52, 91.24 for MD). The model combining T1ρ and MD exhibited better diagnostic performance with higher AUC values than any individual method for staging liver fibrosis (≥ F1: 1.000 (0.884-1.000); ≥ F2: 0.935 (0.782-0.992); ≥ F3: 0.982 (0.852-1.000); F4: 0.986 (0.859-1.000)).

CONCLUSION

Among the evaluated imaging parameters, T1ρ and MD were superior for differentiating varying liver fibrosis stages. The model combining T1ρ and MD was promising to be a credible diagnostic biomarker to detect and accurately stage liver fibrosis.

Adding quantitative T1rho-weighted imaging to conventional MRI improves specificity and sensitivity for differentiating malignant from benign breast lesions

OBJECTIVES

To investigate the feasibility of T1rho-weighted imaging in differentiating malignant from benign breast lesions and to explore the additional value of T1rho to conventional MRI.

MATERIALS AND METHODS

We prospectively enrolled consecutive women with breast lesions who underwent preoperative T1rho-weighted imaging, diffusion-weighted imaging, and dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) between November 2021 and July 2023. The T1rho, apparent diffusion coefficient (ADC), and semi-quantitative parameters from DCE-MRI were obtained and compared between benign and malignant groups. The diagnostic performance was analyzed and compared using receiver operating characteristic (ROC) curves and the Delong Test.

RESULTS

This study included 113 patients (74 malignant and 39 benign lesions). The mean T1rho value in the benign group (92.61 ± 22.10 ms) was significantly higher than that in the malignant group (72.18 ± 16.37 ms) (P < 0.001). The ADC value and time to peak (TTP) value in the malignant group (1.13 ± 0.45 and 269.06 ± 106.01, respectively) were lower than those in the benign group (1.57 ± 0.45 and 388.30 ± 81.13, respectively) (all P < 0.001). T1rho combined with ADC and TTP showed good diagnostic performance with an area under the curve (AUC) of 0.896, a sensitivity of 81.0%, and a specificity of 87.1%. The specificity and sensitivity of the combination of T1rho, ADC, and TTP were significantly higher than those of the combination of ADC and TTP (87.1% vs. 84.6%, P < 0.005; 81.0% vs. 77.0%, P < 0.001).

CONCLUSION

T1rho-weighted imaging was a feasible MRI sequence for differentiating malignant from benign breast lesions. The combination of T1rho, ADC and TTP could achieve a favorable diagnostic performance with improved specificity and sensitivity, T1rho could serve as a supplementary approach to conventional MRI.

Assessment of myocardial injuries in ischaemic and non-ischaemic cardiomyopathies using magnetic resonance T1-rho mapping

AIMS

To identify clinical correlates of myocardial T1ρ and to examine how myocardial T1ρ values change under various clinical scenarios.

METHODS AND RESULTS

A total of 66 patients (26% female, median age 57 years [Q1-Q3, 44-65 years]) with known structural heart disease and 44 controls (50% female, median age 47 years [28-57 years]) underwent cardiac magnetic resonance imaging at 1.5 T, including T1ρ mapping, T2 mapping, native T1 mapping, late gadolinium enhancement, and extracellular volume (ECV) imaging. In controls, T1ρ positively related with T2 (P = 0.038) and increased from basal to apical levels (P < 0.001). As compared with controls and remote myocardium, T1ρ significantly increased in all patients' sub-groups and all types of myocardial injuries: acute and chronic injuries, focal and diffuse tissue abnormalities, as well as ischaemic and non-ischaemic aetiologies (P < 0.05). T1ρ was independently associated with T2 in patients with acute injuries (P = 0.004) and with native T1 and ECV in patients with chronic injuries (P < 0.05). Myocardial T1ρ mapping demonstrated good intra- and inter-observer reproducibility (intraclass correlation coefficient = 0.86 and 0.83, respectively).

CONCLUSION

Myocardial T1ρ mapping appears to be reproducible and equally sensitive to acute and chronic myocardial injuries, whether of ischaemic or non-ischaemic origins. It may thus be a contrast-agent-free biomarker for gaining new and quantitative insight into myocardial structural disorders. These findings highlight the need for further studies through prospective and randomized trials.

Alternating Look-Locker for quantitative T1 , T1ρ and B1 3D MRI mapping

PURPOSE

To develop a new MRI method, entitled alternating Look-Locker (aLL), for quantitativeT 1 $$ {T}_1 $$ ,T 1 ρ $$ {T}_{1\uprho} $$ , andB 1 $$ {B}_1 $$ 3D mapping.

METHODS

A Look-Locker scheme that alternates magnetization from +Z and -Z axes of the laboratory frame is utilized in combination with a 3D Multi-Band Sweep Imaging with Fourier Transformation (MB-SWIFT) readout. The analytical solution describing the spin evolution during aLL, as well as the correction required for segmented acquisition were derived. The simultaneousB 1 $$ {B}_1 $$ andT 1 $$ {T}_1 $$ mapping are demonstrated on an agar/saline phantom and on an in-vivo rat head.T 1 ρ $$ {T}_{1\uprho} $$ relaxation was achieved by cyclically applying magnetization preparation (MP) modules consisting of two adiabatic pulses.T 1 ρ $$ {T}_{1\uprho} $$ values in the rat brain in-vivo and in a gadobenate dimeglumine (Gd-DTPA) phantom were compared to those obtained with a previously introduced steady-state (SS) method.

RESULTS

The accuracy and precision of the analytical solution was tested by Bloch simulations. With the application of MP modules, the aLL method provides simultaneousT 1 $$ {T}_1 $$ andT 1 ρ $$ {T}_{1\uprho} $$ maps. Conversely, without it, the method can be used for simultaneousT 1 $$ {T}_1 $$ andB 1 $$ {B}_1 $$ mapping.T 1 ρ $$ {T}_{1\uprho} $$ values were similar with both aLL and SS techniques. However, the aLL method resulted in more robust quantitative mapping compared to the SS method. Unlike the SS method, the aLL method does not require additional scans for generatingT 1 $$ {T}_1 $$ maps.

CONCLUSION

The proposed method offers a new flexible tool for quantitative mapping ofT 1 $$ {T}_1 $$ ,T 1 ρ $$ {T}_{1\uprho} $$ , andB 1 $$ {B}_1 $$ . The aLL method can also be used with readout schemes different from MB-SWIFT.

Quantitative MRI on MR-Linacs: Towards Biological Image-Guided Adaptive Radiotherapy

Recognizing the potential of quantitative imaging biomarkers (QIBs) in radiotherapy, many studies have investigated the prognostic value of quantitative MRI (qMRI). With the introduction of MRI-guided radiotherapy systems, the practical challenges of repeated imaging have been substantially reduced. Since patients are treated inside an MRI scanner, acquisition of qMRI can be done during each fraction with limited or no prolongation of the fraction duration. In this review paper, we identify the steps that need been taken to move from MR as an imaging technique to a useful biomarker for MRI-guided radiotherapy (MRgRT).

Magnetic resonance myocardial T1ρ mapping : Technical overview, challenges, emerging developments, and clinical applications

The potential of cardiac magnetic resonance to improve cardiovascular care and patient management is considerable. Myocardial T1-rho (T1ρ) mapping, in particular, has emerged as a promising biomarker for quantifying myocardial injuries without exogenous contrast agents. Its potential as a contrast-agent-free ("needle-free") and cost-effective diagnostic marker promises high impact both in terms of clinical outcomes and patient comfort. However, myocardial T1ρ mapping is still at a nascent stage of development and the evidence supporting its diagnostic performance and clinical effectiveness is scant, though likely to change with technological improvements. The present review aims at providing a primer on the essentials of myocardial T1ρ mapping, and to describe the current range of clinical applications of the technique to detect and quantify myocardial injuries. We also delineate the important limitations and challenges for clinical deployment, including the urgent need for standardization, the evaluation of bias, and the critical importance of clinical testing. We conclude by outlining technical developments to be expected in the future. If needle-free myocardial T1ρ mapping is shown to improve patient diagnosis and prognosis, and can be effectively integrated in cardiovascular practice, it will fulfill its potential as an essential component of a cardiac magnetic resonance examination.

SPICY: a method for single scan rotating frame relaxometry

T _1ρ is an NMR relaxation mode that is sensitive to low frequency molecular motions, making it an especially valuable tool in biomolecular research. Here, we introduce a new method, SPICY, for measuring T_1ρ relaxation times. In contrast to conventional T_1ρ experiments, in which the sequence is repeated many times to determine the T_1ρ time, the SPICY sequence allows determination of T_1ρ within a single scan, shortening the experiment time remarkably. We demonstrate the method using ¹H T_1ρ relaxation dispersion experiments. Additionally, we combine the sequence with spatial encoding to produce 1D images in a single scan. We show that T_1ρ relaxation times obtained using the single scan approach are in good agreement with those obtained using the traditional experiments.

Detecting Early-Stage Liver Fibrosis Using Macromolecular Proton Fraction Mapping Based on Spin-Lock MRI: Preliminary Observations

BACKGROUND

Liver fibrosis is characterized by macromolecule depositions. Recently, a novel technology termed macromolecular proton fraction quantification based on spin-lock magnetic resonance imaging (MPF-SL) is reported to measure macromolecule levels.

HYPOTHESIS

MPF-SL can detect early-stage liver fibrosis by measuring macromolecule levels in the liver.

STUDY TYPE

Retrospective.

SUBJECTS

Fifty-five participants, including 22 with no fibrosis (F0) and 33 with early-stage fibrosis (F1-2), were recruited.

FIELD STRENGTH/SEQUENCE

3 T; two-dimensional (2D) MPF-SL turbo spin-echo sequence, 2D spin-lock T1rho turbo spin-echo sequence, and multi-slice 2D gradient echo sequence.

ASSESSMENT

Macromolecular proton fraction (MPF), T1rho, liver iron concentration (LIC), and fat fraction (FF) biomarkers were quantified within regions of interest.

STATISTICAL TESTS

Group comparison of the biomarkers using Mann-Whitney U tests; correlation between the biomarkers assessed using Spearman's rank correlation coefficient and linear regression with goodness-of-fit; fibrosis stage differentiation using receiver operating characteristic curve (ROC) analysis. P-value < 0.05 was considered statistically significant.

RESULTS

Average T1rho was 41.76 ± 2.94 msec for F0 and 41.15 ± 3.73 msec for F1-2 (P = 0.60). T1rho showed nonsignificant correlation with either liver fibrosis (ρ = -0.07; P = 0.61) or FF (ρ = -0.14; P = 0.35) but indicated a negative correlation with LIC (ρ = -0.66). MPF was 4.73 ± 0.45% and 5.65 ± 0.81% for F0 and F1-2 participants, respectively. MPF showed a positive correlation with liver fibrosis (ρ = 0.59), and no significant correlations with LIC (ρ = 0.02; P = 0.89) or FF (ρ = 0.05; P = 0.72). The area under the ROC curve was 0.85 (95% confidence interval [CI] 0.75-0.95) and 0.55 (95% CI 0.39-0.71; P = 0.55) for MPF and T1rho to discriminate between F0 and F1-2 fibrosis, respectively.

DATA CONCLUSION

MPF-SL has the potential to diagnose early-stage liver fibrosis and does not appear to be confounded by either LIC or FF.

LEVEL OF EVIDENCE

3 TECHNICAL EFFICACY STAGE: 3.

New methods for robust continuous wave T1ρ relaxation preparation

Measurement of the longitudinal relaxation time in the rotating frame of reference (T_1ρ ) is sensitive to the fidelity of the main imaging magnetic field (B₀ ) and that of the RF pulse (B₁ ). The purpose of this study was to introduce methods for producing continuous wave (CW) T_1ρ contrast with improved robustness against field inhomogeneities and to compare the sensitivities of several existing and the novel T_1ρ contrast generation methods with the B₀ and B₁ field inhomogeneities. Four hard-pulse and four adiabatic CW-T_1ρ magnetization preparations were investigated. Bloch simulations and experimental measurements at different spin-lock amplitudes under ideal and non-ideal conditions, as well as theoretical analysis of the hard-pulse preparations, were conducted to assess the sensitivity of the methods to field inhomogeneities, at low (ω₁  << ΔB₀ ) and high (ω₁ >> ΔB₀ ) spin-locking field strengths. In simulations, previously reported single-refocus and new triple-refocus hard-pulse and double-refocus adiabatic preparation schemes were found to be the most robust. The mean normalized absolute deviation between the experimentally measured relaxation times under ideal and non-ideal conditions was found to be smallest for the refocused preparation schemes and broadly in agreement with the sensitivities observed in simulations. Experimentally, all refocused preparations performed better than those that were non-refocused. The findings promote the use of the previously reported hard-pulse single-refocus ΔB₀ and B₁ insensitive T_1ρ as a robust method with minimal RF energy deposition. The double-refocus adiabatic B₁ insensitive rotation-4 CW-T_1ρ preparation offers further improved insensitivity to field variations, but because of the extra RF deposition, may be preferred for ex vivo applications.

A self-compensated spin-locking scheme for quantitative R1ρ dispersion MR imaging in ordered tissues

PURPOSE

To propose a self-compensated spin-locking (SL) method for quantitative R_1ρ dispersion imaging in ordered tissues.

METHODS

Two pairs of antiphase rotary-echo SL pulses were proposed in a new scheme with each pairs sandwiching one refocusing RF pulse. This proposed SL method was evaluated by Bloch simulations and experimental studies relative to three prior schemes. Quantitative R_1ρR dispersion imaging studies with constant SL duration (TSL = 40 ms) were carried out on an agarose (1-4% w/v) phantom and one in vivo human knee at 3 T, using six SL RF strengths ranging from 50 to 1000 Hz. The performances of these SL schemes were characterized with an average coefficient of variation (CV) of the signal intensities in agarose gels and the sum of squared errors (SSE) for quantifying in vivo R_1ρ dispersion of the femoral and tibial cartilage.

RESULTS

The simulations demonstrate that the proposed SL scheme was less prone to B₀ and B₁ field inhomogeneities. This theoretical prediction was supported by fewer image banding artifacts and less signal fluctuation signified by a reduced CV (%) on the phantom without R_1ρ dispersion (i.e., 4.04 ± 1.36 vs. 18.87 ± 4.46 or 6.66 ± 2.92 or 5.71 ± 2.05 for others), and further by mostly decreased SSE (*10^-3) for characterizing R_1ρ dispersion of the femoral (i.e., 0.3 vs. 1.2 or 0.4 or 0.1) and tibial (i.e., 0.4 vs. 7.2 or 3.2 or 2.8) cartilage.

CONCLUSION

The proposed SL scheme is less sensitive to B₀ and B₁ field artifacts for a wide range of SL RF strengths and thus more suitable for quantitative R_1ρ dispersion imaging in ordered tissues.

Embedded Quantitative MRI T1ρ Mapping Using Non-Linear Primal-Dual Proximal Splitting

Quantitative MRI (qMRI) methods allow reducing the subjectivity of clinical MRI by providing numerical values on which diagnostic assessment or predictions of tissue properties can be based. However, qMRI measurements typically take more time than anatomical imaging due to requiring multiple measurements with varying contrasts for, e.g., relaxation time mapping. To reduce the scanning time, undersampled data may be combined with compressed sensing (CS) reconstruction techniques. Typical CS reconstructions first reconstruct a complex-valued set of images corresponding to the varying contrasts, followed by a non-linear signal model fit to obtain the parameter maps. We propose a direct, embedded reconstruction method for T1ρ mapping. The proposed method capitalizes on a known signal model to directly reconstruct the desired parameter map using a non-linear optimization model. The proposed reconstruction method also allows directly regularizing the parameter map of interest and greatly reduces the number of unknowns in the reconstruction, which are key factors in the performance of the reconstruction method. We test the proposed model using simulated radially sampled data from a 2D phantom and 2D cartesian ex vivo measurements of a mouse kidney specimen. We compare the embedded reconstruction model to two CS reconstruction models and in the cartesian test case also the direct inverse fast Fourier transform. The T1ρ RMSE of the embedded reconstructions was reduced by 37–76% compared to the CS reconstructions when using undersampled simulated data with the reduction growing with larger acceleration factors. The proposed, embedded model outperformed the reference methods on the experimental test case as well, especially providing robustness with higher acceleration factors.

Quantification correction for free-breathing myocardial T1ρ mapping in mice using a recursively derived description of a T1ρ* relaxation pathway

BACKGROUND

Fast and accurate T1ρ mapping in myocardium is still a major challenge, particularly in small animal models. The complex sequence design owing to electrocardiogram and respiratory gating leads to quantification errors in in vivo experiments, due to variations of the T1ρ relaxation pathway. In this study, we present an improved quantification method for T1ρ using a newly derived formalism of a T1ρ* relaxation pathway.

METHODS

The new signal equation was derived by solving a recursion problem for spin-lock prepared fast gradient echo readouts. Based on Bloch simulations, we compared quantification errors using the common monoexponential model and our corrected model. The method was validated in phantom experiments and tested in vivo for myocardial T1ρ mapping in mice. Here, the impact of the breath dependent spin recovery time Trec on the quantification results was examined in detail.

RESULTS

Simulations indicate that a correction is necessary, since systematically underestimated values are measured under in vivo conditions. In the phantom study, the mean quantification error could be reduced from - 7.4% to - 0.97%. In vivo, a correlation of uncorrected T1ρ with the respiratory cycle was observed. Using the newly derived correction method, this correlation was significantly reduced from r = 0.708 (p < 0.001) to r = 0.204 and the standard deviation of left ventricular T1ρ values in different animals was reduced by at least 39%.

CONCLUSION

The suggested quantification formalism enables fast and precise myocardial T1ρ quantification for small animals during free breathing and can improve the comparability of study results. Our new technique offers a reasonable tool for assessing myocardial diseases, since pathologies that cause a change in heart or breathing rates do not lead to systematic misinterpretations. Besides, the derived signal equation can be used for sequence optimization or for subsequent correction of prior study results.

T1ρ for Radiotherapy Treatment Response Monitoring in Rectal Cancer Patients: A Pilot Study

Quantitative MRI has the potential to produce imaging biomarkers for the prediction of early response to radiotherapy treatment. In this pilot study, a potential imaging biomarker, the T_1ρ relaxation time, is assessed for this purpose. A T_1ρ sequence was implemented on a 1.5 T MR-linac system, a system that combines an MRI with a linear accelerator for radiation treatment. An agar phantom with concentrations of 1–4% w/w was constructed for technical validation of the sequence. Phantom images were assessed in terms of short-term repeatability and signal-to-noise ratio. Twelve rectal cancer patients, who were treated with 5 × 5 Gy, were imaged on each treatment fraction. Individual changes in the T_1ρ values of the gross tumor volume (GTV) showed an increase for most patients, although a paired t-test comparing values in the GTV from the first to the last treatment fraction showed no statistically significant difference. The phantom measurements showed excellent short-term repeatability (0.5–1.5 ms), and phantom T_1ρ values corresponded to the literature values. T_1ρ imaging was implemented successfully on the MR-linac, with a repeatability comparable to diagnostic systems, although clinical benefit in terms of treatment response monitoring remains to be demonstrated.

Quantitative magnetic resonance imaging of chronic kidney disease: an experimental in vivo study using rat chronic kidney disease models

BACKGROUND

Recent advances in magnetic resonance imaging (MRI) may allow it to be an alternative emerging tool for the non-invasive evaluation of renal parenchymal disease.

PURPOSE

To validate the usefulness of quantitative multiparametric MRI protocols and suggest the suitable quantitative MR sequence protocol to evaluate parenchymal fibrosis using an animal model of chronic kidney disease (CKD) by long-term adenine intake.

MATERIAL AND METHODS

In this prospective animal study, 16 male Wistar rats were analyzed and categorized into three groups. Rats in the CKD groups underwent 0.25% adenine administration for three or six weeks. Quantitative MRI protocols, including diffusion-weighted imaging (DWI), T1ρ (T1 rho), and T2* mapping were performed using a 9.4-T animal MR scanner. A semi-quantitative histopathologic analysis for renal fibrosis was conducted. Quantitative MR values measured from anatomic regions of kidneys underwent intergroup comparative analyses.

RESULTS

The apparent diffusion coefficient (ADC) and T1 (T1 rho) values were significantly increased in all CKD groups. Values measured from the cortex and outer medulla showed significant intergroup differences. Total ADC values tended to increase according to periods, and T1ρ values increased in three weeks and decreased in six weeks.

CONCLUSION

Quantitative MRI protocols could be a non-invasive assessment modality in the diagnosis and evaluation of CKD. Particularly, T1ρ may be a suitable MR sequence to quantitatively assess renal parenchymal fibrosis.

Fast myocardial T1ρ mapping in mice using k-space weighted image contrast and a Bloch simulation-optimized radial sampling pattern

Purpose

T_1ρ dispersion quantification can potentially be used as a cardiac magnetic resonance index for sensitive detection of myocardial fibrosis without the need of contrast agents. However, dispersion quantification is still a major challenge, because T_1ρ mapping for different spin lock amplitudes is a very time consuming process. This study aims to develop a fast and accurate T_1ρ mapping sequence, which paves the way to cardiac T_1ρ dispersion quantification within the limited measurement time of an in vivo study in small animals.

Methods

A radial spin lock sequence was developed using a Bloch simulation-optimized sampling pattern and a view-sharing method for image reconstruction. For validation, phantom measurements with a conventional sampling pattern and a gold standard sequence were compared to examine T_1ρ quantification accuracy. The in vivo validation of T_1ρ mapping was performed in N = 10 mice and in a reproduction study in a single animal, in which ten maps were acquired in direct succession. Finally, the feasibility of myocardial dispersion quantification was tested in one animal.

Results

The Bloch simulation-based sampling shows considerably higher image quality as well as improved T_1ρ quantification accuracy (+ 56%) and precision (+ 49%) compared to conventional sampling. Compared to the gold standard sequence, a mean deviation of − 0.46 ± 1.84% was observed. The in vivo measurements proved high reproducibility of myocardial T_1ρ mapping. The mean T_1ρ in the left ventricle was 39.5 ± 1.2 ms for different animals and the maximum deviation was 2.1% in the successive measurements. The myocardial T_1ρ dispersion slope, which was measured for the first time in one animal, could be determined to be 4.76 ± 0.23 ms/kHz.

Conclusion

This new and fast T_1ρ quantification technique enables high-resolution myocardial T_1ρ mapping and even dispersion quantification within the limited time of an in vivo study and could, therefore, be a reliable tool for improved tissue characterization.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10334-021-00951-y.

Balanced spin-lock preparation for B1 -insensitive and B0 -insensitive quantification of the rotating frame relaxation time T1ρ

PURPOSE

Accurate and artifact-free T_1ρ quantification is still a major challenge due to a susceptibility of the spin-locking module to B₀ and/or B₁ field inhomogeneities. In this study, we present a novel spin-lock preparation module (B-SL) that enables an almost full compensation of both types of inhomogeneities.

METHODS

The new B-SL module contains a second 180° refocusing pulse to compensate each pulse in the preparation block by a corresponding pulse with opposite phase. For evaluation and validation of B-SL, extensive simulations as well as phantom measurements were performed. Furthermore, the new module was compared to three common established compensation methods.

RESULTS

Both simulations and measurements demonstrate a much lower susceptibility to artifacts for the B-SL module, therefore providing an improved accuracy in T_1ρ quantification. In the presence of field inhomogeneities, measurements revealed an increased banding compensation by 79% compared with the frequently used composite module. The goodness of the mono-exponential T_1ρ fitting procedure was improved by 58%.

CONCLUSION

The B-SL preparation enables the generation of accurate relaxation maps with significantly reduced artifacts, even in the case of large field imperfections. Therefore, the B-SL module is suggested to be highly beneficial for in vivo T_1ρ quantification.

Musculoskeletal MR Imaging Applications at Ultra-High (7T) Field Strength

Regulatory approval of ultrahigh field (UHF) MR imaging scanners for clinical use has opened new opportunities for musculoskeletal imaging applications. UHF MR imaging has unique advantages in terms of signal-to-noise ratio, contrast-to-noise ratio, spectral resolution, and multinuclear applications, thus providing unique information not available at lower field strengths. But UHF also comes with a set of technical challenges that are yet to be resolved and may not be suitable for all imaging applications. This review focuses on the latest research in musculoskeletal MR imaging applications at UHF including morphologic imaging, T₂, T₂∗, and T_1ρ mapping, chemical exchange saturation transfer, sodium imaging, and phosphorus spectroscopy imaging applications.

Macromolecular proton fraction mapping based on spin-lock magnetic resonance imaging

PURPOSE

In MRI, the macromolecular proton fraction (MPF) is a key parameter of magnetization transfer (MT). It represents the relative amount of immobile protons associated with semi-solid macromolecules involved in MT with free water protons. We aim to quantify MPF based on spin-lock MRI and explore its advantages over the existing MPF-mapping methods.

METHODS

In the proposed method, termed MPF quantification based on spin-lock (MPF-SL), off-resonance spin-lock is used to sensitively measure the MT effect. MPF-SL is designed to measure a relaxation rate (R_mpfsl ) that is specific to the MT effect by removing the R_1ρ relaxation due to the mobile water and chemical exchange pools. A theory is derived to quantify MPF from the measured R_mpfsl . No prior knowledge of tissue relaxation parameters, including T₁ or T₂ , is needed to quantify MPF using MPF-SL. The proposed approach is validated with Bloch-McConnell simulations, phantom, and in vivo liver studies at 3.0T.

RESULTS

Both Bloch-McConnell simulations and phantom experiments show that MPF-SL is insensitive to variations of the mobile water pool and the chemical exchange pool. MPF-SL is specific to the MT effect and can measure MPF reliably. In vivo liver studies show that MPF-SL can be used to detect collagen deposition in patients with liver fibrosis.

CONCLUSION

A novel MPF imaging method based on spin-lock MRI is proposed. The confounding factors are removed, and the measurement is specific to the MT effect. It holds promise for MPF-sensitive diagnostic imaging in clinical settings.

R1ρ sensitivity to pH and other compounds at clinically accessible spin-lock fields in the presence of proteins

Numerous human diseases involve abnormal metabolism, and proton exchange is an effective source of magnetic resonance imaging (MRI) contrast for assessing metabolism. One MRI technique that capitalizes on proton exchange is R₁ relaxation in the rotating frame (R_1ρ ). Here, we investigated the sensitivity of R_1ρ to various proton-exchange mechanisms at spin-lock pulses within Food and Drug Administration (FDA) safety guidelines for radiofrequency-induced heating. We systematically varied pH known to change the rate of proton exchange as well as the glucose and lysine concentrations, thus changing the number of amide, hydroxyl and amine exchangeable sites in a series of egg-white albumin phantoms. The resulting effects on quantitative relaxation time measurements of R_1ρ , R₁ and R₂ were observed at 3 T. Using spin-lock amplitudes available for human imaging (less than 23.5 μT) at near physiologic temperatures, we found R_1ρ was more sensitive to physiologic changes in pH than to changes in glucose and lysine concentrations. In addition, R_1ρ was more sensitive to pH changes than R₁ and R₂ . Models of proton exchange fitted to the relaxation measurements suggest that amide groups were the primary source of pH sensitivity. Together, these experiments suggest an optimal spin-lock amplitude for measuring pH changes while not exceeding FDA-subject heating limitations.

T1ρ-Mapping for Musculoskeletal Pain Diagnosis: Case Series of Variation of Water Bound Glycosaminoglycans Quantification before and after Fascial Manipulation® in Subjects with Elbow Pain

Diagnosis and management of musculoskeletal pain is a major clinical challenge. Following this need, the first aim of our study was to provide an innovative magnetic resonance technique called T_1ρ to quantify possible alterations in elbow pain, a common musculoskeletal pain syndrome that has not a clear etiology. Five patients were recruited presenting chronic elbow pain (>3 months), with an age between 30 and 70 years old. Patients underwent two T_1ρ-mapping evaluations, one before and one after the series of Fascial Manipulation^® (FM) treatments. After the first MRI evaluation, a Disability of the Arm, Shoulder and Hand (DASH) questionnaire was administered to quantify the symptoms and pain intensity. Patients then received three sessions of FM, once a week for 40 min each. A statistically significant difference was found between bound and unbound water concentration before and after FM treatment. Our preliminary data suggest that the application of the manual method seems to decrease the concentration of unbound water inside the deep fascia in the most chronic patients. This could explain the change in viscosity perceived by many practitioners as well as the decrease of symptoms due to the restoration of the normal property of the loose connective tissue. Being able to identify an altered deep fascial area may better guide therapies, contributing to a more nuanced view of the mechanisms of pain.

The transformation of radiation oncology using real-time magnetic resonance guidance: A review

Radiation therapy (RT) is an essential component of effective cancer care and is used across nearly all cancer types. The delivery of RT is becoming more precise through rapid advances in both computing and imaging. The direct integration of magnetic resonance imaging (MRI) with linear accelerators represents an exciting development with the potential to dramatically impact cancer research and treatment. These impacts extend beyond improved imaging and dose deposition. Real-time MRI-guided RT is actively transforming the work flows and capabilities of virtually every aspect of RT. It has the opportunity to change entirely the delivery methods and response assessments of numerous malignancies. This review intends to approach the topic of MRI-based RT guidance from a vendor neutral and international perspective. It also aims to provide an introduction to this topic targeted towards oncologists without a speciality focus in RT. Speciality implications, areas for physician education and research opportunities are identified as they are associated with MRI-guided RT. The uniquely disruptive implications of MRI-guided RT are discussed and placed in context. We further aim to describe and outline important future changes to the speciality of radiation oncology that will occur with MRI-guided RT. The impacts on RT caused by MRI guidance include target identification, RT planning, quality assurance, treatment delivery, training, clinical workflow, tumour response assessment and treatment scheduling. In addition, entirely novel research areas that may be enabled by MRI guidance are identified for future investigation.

Quantifying muscle glycosaminoglycan levels in patients with post-stroke muscle stiffness using T1ρ MRI

The purpose of this study was to provide imaging evidence of increased glycosaminoglycan (GAG) content in patients with post-stroke muscle stiffness; and to determine the effect of hyaluronidase treatment on intramuscular GAG content. In this prospective study, we used 3D-T1ρ (T1rho) magnetic resonance (MR) mapping of the upper arm muscles to quantify GAG content in patients with post-stroke muscle stiffness before and after hyaluronidase injection treatment. For this study, healthy controls (n = 5), and patients with post-stroke muscle stiffness (n = 5) were recruited (March 2017-April 2018). T1ρ MR imaging and Dixon water-fat MR imaging of the affected upper arms were performed before and after off-label treatment with hyaluronidase injections. T1ρ mapping was done using a three-parameter non-linear mono-exponential fit. Wilcoxon Mann-Whitney test was used to compare patients' vs controls and pre- vs post-treatment conditions. The T1ρ values in the biceps were significantly higher in patients before treatment (34.04 ± 4.39 ms) compared with controls (26.70 ± 0.54 ms; P = 0.006). Significant improvement was seen in the biceps of patients before (35.48 ± 3.38 ms) and after treatment (29.45 ± 1.23 ms; P = 0.077). Dixon water-fat distribution was not significantly different in the patients compared to the controls (biceps P = 0.063; triceps P = 0.190). These results suggest that T1ρ mapping can be used to quantify GAG content in the muscles of patients with post-stroke muscle stiffness, and that muscle hyaluronan content is increased in stiff muscles compared with controls, providing imaging corroboration for the hyaluronan hypothesis of muscle stiffness.

Quantitative MRI Helps to Detect Hip Ischemia: Preclinical Model of Legg-Calvé-Perthes Disease

Purpose To determine whether quantitative MRI relaxation time mapping techniques can help to detect ischemic injury to the developing femoral head. Materials and Methods For this prospective animal study conducted from November 2015 to February 2018, 10 male 6-week-old piglets underwent an operation to induce complete right femoral head ischemia. Animals were humanely killed at 48 hours (n = 2) or 4 weeks (n = 8) after the operation, and the operated and contralateral-control femoral heads were harvested and frozen. Thawed specimens were imaged at 9.4-T MRI by using T1, T2, T1 in the rotating frame (T1ρ), adiabatic T1ρ, relaxation along a fictitious field (RAFF), and T2* mapping and evaluated with histologic analysis. Paired relaxation time differences between the operated and control femoral heads were measured in the secondary ossification center (SOC), epiphyseal cartilage, articular cartilage, and metaphysis and were analyzed by using a paired t test. Results In the SOC, T1ρ and RAFF had the greatest percent increases in the operated versus control femoral heads at both 48 hours (112% and 72%, respectively) and 4 weeks (74% and 70%, respectively). In the epiphyseal and articular cartilage, T2, T1ρ, and RAFF were similarly increased at both points (range, 24%-49%). At 4 weeks, T2, T1ρ, adiabatic T1ρ, and RAFF were increased in the SOC (P = .004, .018, < .001, and .001, respectively), epiphyseal cartilage (P = .009, .008, .011, and .007, respectively), and articular cartilage (P = .005, .016, .033, and .018, respectively). Histologic assessment identified necrosis in SOC and deep layer of the epiphyseal cartilage at both points. Conclusion T2, T1 in the rotating frame, adiabatic T1 in the rotating frame, and relaxation along a fictitious field maps are sensitive in helping to detect ischemic injury to the developing femoral head. © RSNA, 2018 Online supplemental material is available for this article.

Liver Fibrosis Quantification by Magnetic Resonance Imaging

Liver fibrosis is a hallmark of chronic liver disease characterized by the excessive accumulation of extracellular matrix proteins. Although liver biopsy is the reference standard for diagnosis and staging of liver fibrosis, it has some limitations, including potential pain, sampling variability, and low patient acceptance. Hence, there has been an effort to develop noninvasive imaging techniques for diagnosis, staging, and monitoring of liver fibrosis. Many quantitative techniques have been implemented on magnetic resonance imaging (MRI) for this indication. The most widely validated technique is magnetic resonance elastography, which aims to measure viscoelastic properties of the liver and relate them to fibrosis stage. Several additional MRI methods have been developed or adapted to liver fibrosis quantification. Diffusion-weighted imaging measures the Brownian motion of water molecules which is restricted by collagen fibers. Texture analysis assesses the changes in the texture of liver parenchyma associated with fibrosis. Perfusion imaging relies on signal intensity and pharmacokinetic models to extract quantitative perfusion parameters. Hepatocellular function, which decreases with increasing fibrosis stage, can be estimated by the uptake of hepatobiliary contrast agents. Strain imaging measures liver deformation in response to physiological motion such as cardiac contraction. T1ρ quantification is an investigational technique, which measures the spin-lattice relaxation time in the rotating frame. This article will review the MRI techniques used in liver fibrosis staging, their advantages and limitations, and diagnostic performance. We will briefly discuss future directions, such as longitudinal monitoring of disease, prediction of portal hypertension, and risk stratification of hepatocellular carcinoma.

Breath-hold black-blood T1rho mapping improves liver T1rho quantification in healthy volunteers

Background Recent researches suggest that T1rho may be a non-invasive and quantitative technique for detecting and grading liver fibrosis. Purpose To compare a multi-breath-hold bright-blood fast gradient echo (GRE) imaging and a single breath-hold single-shot fast spin echo (FSE) imaging with black-blood effect for liver parenchyma T1rho measurement and to study liver physiological T1rho value in healthy volunteers. Material and Methods The institutional Ethics Committee approved this study. 28 healthy participants (18 men, 10 women; age = 29.6 ± 5.1 years) underwent GRE liver T1rho imaging, and 20 healthy participants (10 men, 10 women; age = 36.9 ± 10.3 years) underwent novel black-blood FSE liver T1rho imaging, both at 3T with spin-lock frequency of 500 Hz. The FSE technique allows simultaneous acquisition of four spin lock times (TSLs; 1 ms, 10 ms, 30 ms, 50msec) in 10 s. Results For FSE technique the intra-scan repeatability intraclass correlation coefficient (ICC) was 0.98; while the inter-scan reproducibility ICC was 0.82 which is better than GRE technique's 0.76. Liver T1rho value in women tended to have a higher value than T1rho values in men (FSE: 42.28 ± 4.06 ms for women and 39.13 ± 2.12 ms for men; GRE: 44.44 ± 1.62 ms for women and 42.36 ± 2.00 ms for men) and FSE technique showed liver T1rho value decreased slightly as age increased. Conclusion Single breath-hold black-blood FSE sequence has better scan-rescan reproducibility than multi-breath-hold bright-blood GRE sequence. Gender and age dependence of liver T1rho in healthy participants is observed, with young women tending to have a higher T1rho measurement.

Bi-exponential 3D-T1ρ mapping of whole brain at 3 T

Detection of multiple relaxation pools using MRI is useful in a number of neuro-pathologies including multiple sclerosis (MS), Alzheimer's, and stroke. In this study we evaluate the feasibility of using T1ρ imaging for the detection of bi-exponential decays in the human brain. A prospective T1ρ imaging study was performed on model relaxation phantoms (eggs) and 7 healthy volunteers. The data was fitted using a single pool and a 2-pool model to estimate mono- and bi-exponential T1ρ maps, respectively. Bi-exponential decays were identified in the gray matter (GM) and white matter (WM) of the brain with 40.5% of GM, and 65.1% of WM pixels showing two T1ρ relaxation pools (significance level P < 0.05). Detection of T1ρ based bi-exponential decays in the brain provides complimentary information to T₂ based contrast regarding the in vivo micro-environment in the brain.

Age- and Gender-Associated Liver Physiological T1rho Dynamics Demonstrated with a Clinically Applicable Single-Breathhold Acquisition

To understand women's and men's physiological ranges of liver T1rho relaxation time measured with a single breathhold black blood sequence, this healthy volunteer study was conducted in 62 women (mean age, 38.9 y; range, 18-75 y) and 34 men (mean age, 44.7 y; range, 24-80 y). Approval from the institutional ethics committee was obtained. Magnetic resonance imaging was performed with a 3.0T scanner with six spin-lock times of 0, 10, 20, 25, 35, and 50 ms and a single breathhold of 12 s per slice acquisition. Six slices were acquired for each examination. The results demonstrated that the female liver T1rho value ranged between 35.07 and 51.97 ms and showed an age-dependent decrease, with younger women having a higher measurement. The male liver T1rho value ranged between 34.94 and 43.39 ms, with no evidential age dependence. Postmenopausal women had similar liver T1rho values as men. For women, there was a trend that the liver T1rho value could be 4% to 5% lower during the menstrual phase than during the nonmenstrual phase. For both women and men, no evidential association was seen between body mass index and liver T1rho.

A rapid magnetization preparation for MRI measurements of sprays

The process of spray atomization, i.e., the transformation of a continuous liquid jet into μm-sub-mm sized droplets, is ubiquitous in industry yet quite complex to analyze theoretically and study experimentally. One of the main strengths of MRI is its ability to sensitize spatially-resolved NMR signal to a wide variety of physical and chemical parameters. However, standard preparation schemes are difficult to employ for studies of sprays due to sprays' fast speeds (>10-20m/s). In addition, sprays are usually low-density systems, leading to a poor SNR and a need for massive signal averaging and long acquisition time. In this paper, we reduced the interval between the preparation and the readout stages by performing SPI encoding on the rising gradients. This also enabled the use of 90-degree flip angles to maximize the spray signal and saturate the stationary water signal while avoiding unwanted slice-selection. The use of gradients during preparation stage was eliminated due to their time-consuming rise and stabilization times limiting possible preparation schemes to a combination of RF pulses and delays. The two preparation schemes presented here are Time-of-Flight (TOF) and T1ρ-weighting schemes. The total duration of the sequence (without TR) was 240-1100μs for the TOF and 410μs for T1ρ. The T1ρ prepared images of the near-atomization region (11 spin-locking frequencies, 0-15kHz) showed a strong signal attenuation at higher frequencies. In series of TOF images the clearly noticeable displacement of the liquid parcel can be utilized to measure spray speeds.

Liver fibrosis: Review of current imaging and MRI quantification techniques

Liver fibrosis is characterized by the accumulation of extracellular matrix proteins such as collagen in the liver interstitial space. All causes of chronic liver disease may lead to fibrosis and cirrhosis. The severity of liver fibrosis influences the decision to treat or the need to monitor hepatic or extrahepatic complications. The traditional reference standard for diagnosis of liver fibrosis is liver biopsy. However, this technique is invasive, associated with a risk of sampling error, and has low patient acceptance. Imaging techniques offer the potential for noninvasive diagnosis, staging, and monitoring of liver fibrosis. Recently, several of these have been implemented on ultrasound (US), computed tomography, or magnetic resonance imaging (MRI). Techniques that assess changes in liver morphology, texture, or perfusion that accompany liver fibrosis have been implemented on all three imaging modalities. Elastography, which measures changes in mechanical properties associated with liver fibrosis-such as strain, stiffness, or viscoelasticity-is available on US and MRI. Some techniques assessing liver shear stiffness have been adopted clinically, whereas others assessing strain or viscoelasticity remain investigational. Further, some techniques are only available on MRI-such as spin-lattice relaxation time in the rotating frame (T₁ ρ), diffusion of water molecules, and hepatocellular function based on the uptake of a liver-specific contrast agent-remain investigational in the setting of liver fibrosis staging. In this review, we summarize the key concepts, advantages and limitations, and diagnostic performance of each technique. The use of multiparametric MRI techniques offers the potential for comprehensive assessment of chronic liver disease severity.

LEVEL OF EVIDENCE

5 J. MAGN. RESON. IMAGING 2017;45:1276-1295.