Water dynamics in human blood via combined measurements of T2 relaxation and diffusion in the presence of gadolinium

Detection of Treatment Response in Triple-Negative Breast Tumors to Paclitaxel Using MRI Cell Size Imaging

BACKGROUND

Breast cancer treatment response evaluation using the response evaluation criteria in solid tumors (RECIST) guidelines, based on tumor volume changes, has limitations, prompting interest in novel imaging markers for accurate therapeutic effect determination.

PURPOSE

To use MRI-measured cell size as a new imaging biomarker for assessing chemotherapy response in breast cancer.

STUDY TYPE

Longitudinal; animal model.

STUDY POPULATION

Triple-negative human breast cancer cell (MDA-MB-231) pellets (4 groups, n = 7) treated with dimethyl sulfoxide (DMSO) or 10 nM of paclitaxel for 24, 48, and 96 hours, and 29 mice with MDA-MB-231 tumors in right hind limbs treated with paclitaxel (n = 16) or DMSO (n = 13) twice weekly for 3 weeks.

FIELD STRENGTH/SEQUENCE

Oscillating gradient spin echo and pulsed gradient spin echo sequences at 4.7 T.

ASSESSMENT

MDA-MB-231 cells were analyzed using flowcytometry and light microscopy to assess cell cycle phases and cell size distribution. MDA-MB-231 cell pellets were MR imaged. Mice were imaged weekly, with 9, 6, and 14 being sacrificed for histology after MRI at weeks 1, 2, and 3, respectively. Microstructural parameters of tumors/cell pellets were derived by fitting diffusion MRI data to a biophysical model.

STATISTICAL TESTS

One-way ANOVA compared cell sizes and MR-derived parameters between treated and control samples. Repeated measures 2-way ANOVA with Bonferroni post-tests compared temporal changes in MR-derived parameters. A P-value <0.05 was considered statistically significant.

RESULTS

In vitro experiments showed that the mean MR-derived cell sizes of paclitaxel-treated cells increased significantly with a 24-hours treatment and decreased (P = 0.06) with a 96-hour treatment. For in vivo xenograft experiments, the paclitaxel-treated tumors showed significant decreases in cell size at later weeks. MRI observations were supported by flowcytometry, light microscopy, and histology.

DATA CONCLUSIONS

MR-derived cell size may characterize the cell shrinkage during treatment-induced apoptosis, and may potentially provide new insights into the assessment of therapeutic response.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY STAGE: 4.

Improving MR cell size imaging by inclusion of transcytolemmal water exchange

The goal of the current study is to include transcytolemmal water exchange in MR cell size imaging using the IMPULSED model for more accurate characterization of tissue cellular properties (e.g., apparent volume fraction of intracellular space v in ) and quantification of indicators of transcytolemmal water exchange. We propose a heuristic model that incorporates transcytolemmal water exchange into a multicompartment diffusion-based method (IMPULSED) that was developed previously to extract microstructural parameters (e.g., mean cell size d and apparent volume fraction of intracellular space v in ) assuming no water exchange. For t diff ≤ 5 ms, the water exchange can be ignored, and the signal model is the same as the IMPULSED model. For t diff ≥ 30 ms, we incorporated the modified Kärger model that includes both restricted diffusion and exchange between compartments. Using simulations and previously published in vitro cell data, we evaluated the accuracy and precision of model-derived parameters and determined how they are dependent on SNR and imaging parameters. The joint model provides more accurate d values for cell sizes ranging from 10 to 12 microns when water exchange is fast (e.g., intracellular water pre-exchange lifetime τ in ≤ 100 ms) than IMPULSED, and reduces the bias of IMPULSED-derived estimates of v in , especially when water exchange is relatively slow (e.g., τ in &gt; 200 ms). Indicators of transcytolemmal water exchange derived from the proposed joint model are linearly correlated with ground truth τ in values and can detect changes in cell membrane permeability induced by saponin treatment in murine erythroleukemia cancer cells. Our results suggest this joint model not only improves the accuracy of IMPULSED-derived microstructural parameters, but also provides indicators of water exchange that are usually ignored in diffusion models of tissues.

Precision of region of interest-based tri-exponential intravoxel incoherent motion quantification and the role of the Intervoxel spatial distribution of flow velocities

PURPOSE

The purpose of this work was to obtain precise tri-exponential intravoxel incoherent motion (IVIM) quantification in the liver using 2D (b-value and first-order motion moment [M1 ]) IVIM-DWI acquisitions and region of interest (ROI)-based fitting techniques.

METHODS

Diffusion MRI of the liver was performed in 10 healthy volunteers using three IVIM-DWI acquisitions: conventional monopolar, optimized monopolar, and optimized 2D (b-M1 ). For each acquisition, bi-exponential and tri-exponential full, segmented, and over-segmented ROI-based fitting and a newly proposed blood velocity SDdistribution (BVD) fitting technique were performed to obtain IVIM estimates in the right and left liver lobes. Fitting quality was evaluated using corrected Akaike information criterion. Precision metrics (test-retest repeatability, inter-reader reproducibility, and inter-lobar agreement) were evaluated using Bland-Altman analysis, repeatability/reproducibility coefficients (RPCs), and paired sample t-tests. Precision was compared across acquisitions and fitting methods.

RESULTS

High repeatability and reproducibility was observed in the estimations of the diffusion coefficient (Dtri  = [1.03 ± 0.11] × 10-3  mm2 /s; RPCs ≤ 1.34 × 10-4  mm2 /s), perfusion fractions (F1  = 3.19 ± 1.89% and F2  = 16.4 ± 2.07%; RPCs ≤ 2.51%), and blood velocity SDs (Vb,1  = 1.44 ± 0.14 mm/s and Vb,2  = 3.62 ± 0.13 mm/s; RPCs ≤ 0.41 mm/s) in the right liver lobe using the 2D (b-M1 ) acquisition in conjunction with BVD fitting. Using these methods, significantly larger (p < 0.01) estimates of Dtri and F1 were observed in the left lobe in comparison to the right lobe, while estimates of Vb,1 and Vb,2 demonstrated high interlobar agreement (RPCs ≤ 0.45 mm/s).

CONCLUSIONS

The 2D (b-M1 ) IVIM-DWI data acquisition in conjunction with BVD fitting enables highly precise tri-exponential IVIM quantification in the right liver lobe.

b value and first-order motion moment optimized data acquisition for repeatable quantitative intravoxel incoherent motion DWI

PURPOSE

To design a b value and first-order motion moment (M1 ) optimized data acquisition for repeatable intravoxel incoherent motion (IVIM) quantification in the liver.

METHODS

Cramer-Rao lower bound optimization was performed to determine optimal monopolar and optimal 2D samplings of the b-M1 space based on noise performance. Monte Carlo simulations were used to evaluate the bias and variability in estimates obtained using the proposed optimal samplings and conventional monopolar sampling. Diffusion MRI of the liver was performed in 10 volunteers using 3 IVIM acquisitions: conventional monopolar, optimized monopolar, and b-M1 -optimized gradient waveforms (designed based on the optimal 2D sampling). IVIM parameter maps of diffusion coefficient, perfusion fraction, and blood velocity SD were obtained using nonlinear least squares fitting. Noise performance (SDs), stability (outlier percentage), and test-retest or scan-rescan repeatability (intraclass correlation coefficients) were evaluated and compared across acquisitions.

RESULTS

Cramer-Rao lower bound and Monte Carlo simulations demonstrated improved noise performance of the optimal 2D sampling in comparison to monopolar samplings. Evaluating the designed b-M1 -optimized waveforms in healthy volunteers, significant decreases (p < 0.05) in the SDs and outlier percentages were observed for measurements of diffusion coefficient, perfusion fraction, and blood velocity SD in comparison to measurements obtained using monopolar samplings. Good-to-excellent repeatability (intraclass correlation coefficients ≥ 0.77) was observed for all 3 parameters in both the right and left liver lobes using the b-M1 -optimized waveforms.

CONCLUSIONS

2D b-M1 -optimized data acquisition enables repeatable IVIM quantification with improved noise performance. 2D acquisitions may advance the establishment of IVIM quantitative biomarkers for liver diseases.

Assessing Myocardial Microstructure With Biophysical Models of Diffusion MRI

Biophysical models are a promising means for interpreting diffusion weighted magnetic resonance imaging (DW-MRI) data, as they can provide estimates of physiologically relevant parameters of microstructure including cell size, volume fraction, or dispersion. However, their application in cardiac microstructure mapping (CMM) has been limited. This study proposes seven new two-compartment models with combination of restricted cylinder models and a diffusion tensor to represent intra- and extracellular spaces, respectively. Three extended versions of the cylinder model are studied here: cylinder with elliptical cross section (ECS), cylinder with Gamma distributed radii (GDR), and cylinder with Bingham distributed axes (BDA). The proposed models were applied to data in two fixed mouse hearts, acquired with multiple diffusion times, q-shells and diffusion encoding directions. The cylinderGDR-pancake model provided the best performance in terms of root mean squared error (RMSE) reducing it by 25% compared to diffusion tensor imaging (DTI). The cylinderBDA-pancake model represented anatomical findings closest as it also allows for modelling dispersion. High-resolution 3D synchrotron X-ray imaging (SRI) data from the same specimen was utilized to evaluate the biophysical models. A novel tensor-based registration method is proposed to align SRI structure tensors to the MR diffusion tensors. The consistency between SRI and DW-MRI parameters demonstrates the potential of compartment models in assessing physiologically relevant parameters.

Quantitative theory for the transverse relaxation time of blood water

An integrative model is proposed to describe the dependence of the transverse relaxation rate of blood water protons (R_2blood = 1/T_2blood ) on hematocrit fraction and oxygenation fraction (Y). This unified model takes into account (a) the diamagnetic effects of albumin, hemoglobin and the cell membrane; (b) the paramagnetic effect of hemoglobin; (c) the effect of compartmental exchange between plasma and erythrocytes under both fast and slow exchange conditions that vary depending on field strength and compartmental relaxation rates and (d) the effect of diffusion through field gradients near the erythrocyte membrane. To validate the model, whole-blood and lysed-blood R₂ data acquired previously using Carr-Purcell-Meiboom-Gill measurements as a function of inter-echo spacing τ_cp at magnetic fields of 3.0, 7.0, 9.4 and 11.7 T were fitted to determine the lifetimes (field-independent physiological constants) for water diffusion and exchange, as well as several physical constants, some of which are field-independent (magnetic susceptibilities) and some are field-dependent (relaxation rates for water protons in solutions of albumin and oxygenated and deoxygenated hemoglobin, ie, blood plasma and erythrocytes, respectively). This combined exchange-diffusion model allowed excellent fitting of the curve of the τ_cp -dependent relaxation rate dispersion at all four fields using a single average erythrocyte water lifetime, τ_ery = 9.1 ± 1.4 ms, and an averaged diffusional correlation time, τ_D = 3.15 ± 0.43 ms. Using this model and the determined physiological time constants and relaxation parameters, blood T₂ values published by multiple groups based on measurements at magnetic field strengths of 1.5 T and higher could be predicted correctly within error. Establishment of this theory is a fundamental step for quantitative modeling of the BOLD effect underlying functional MRI.

Sensitivity of quantitative relaxometry and susceptibility mapping to microscopic iron distribution

PURPOSE

Determine the impact of the microscopic spatial distribution of iron on relaxometry and susceptibility-based estimates of iron concentration.

METHODS

Monte Carlo simulations and in vitro experiments of erythrocytes were used to create different microscopic distributions of iron. Measuring iron with intact erythrocyte cells created a heterogeneous distribution of iron, whereas lysing erythrocytes was used to create a homogeneous distribution of iron. Multi-echo spin echo and spoiled gradient echo acquisitions were then used to estimate relaxation parameters ( R 2 and R 2 * ) and susceptibility.

RESULTS

Simulations demonstrate that R 2 and R 2 * measurements depend on the spatial distribution of iron even for the same iron concentration and volume susceptibility. Similarly, in vitro experiments demonstrate that R 2 and R 2 * measurements depend on the microscopic spatial distribution of iron whereas the quantitative susceptibility mapping (QSM) susceptibility estimates reflect iron concentration without sensitivity to spatial distribution.

CONCLUSIONS

R 2 and R 2 * for iron quantification depend on the spatial distribution or iron. QSM-based estimation of iron concentration is insensitive to the microscopic spatial distribution of iron, potentially providing a distribution independent measure of iron concentration.

Evidence of the diffusion time dependence of intravoxel incoherent motion in the brain

PURPOSE

To investigate the diffusion time (T_D ) dependence of intravoxel incoherent motion (IVIM) signals in the brain.

METHODS

A 3-compartment IVIM model was proposed to characterize 2 types of microcirculatory flows in addition to tissue water in the brain: flows that cross multiple vascular segments (pseudo-diffusive) and flows that stay in 1 segment (ballistic) within T_D . The model was first evaluated using simulated flow signals. Experimentally, flow-compensated (FC) pulsed-gradient spin-echo (PGSE) and oscillating-gradient spin-echo (OGSE) sequences were tested using a flow phantom and then used to examine IVIM signals in the mouse brain with T_D ranging from ~2.5 ms to 40 ms on an 11.7T scanner.

RESULTS

By fitting the model to simulated flow signals, we demonstrated the T_D dependency of the estimated fraction of pseudo-diffusive flow and the pseudo-diffusion coefficient (D*), which were dictated by the characteristic timescale of microcirculatory flow (τ). Flow phantom experiments validated that the OGSE and FC-PGSE sequences were not susceptible to the change in flow velocity. In vivo mouse brain data showed that both the estimated fraction of pseudo-diffusive flow and D* increased significantly as T_D increased.

CONCLUSION

We demonstrated that IVIM signals measured in the brain are T_D -dependent, potentially because more microcirculatory flows approach the pseudo-diffusive limit as T_D increases with respect to τ. Measuring the T_D dependency of IVIM signals may provide additional information on microvascular flows in the brain.

Advanced Magnetic Resonance Imaging Techniques in Management of Brain Metastases

Brain metastases are the most common intracranial tumors and occur in 20–40% of all cancer patients. Lung cancer, breast cancer, and melanoma are the most frequent primary cancers to develop brain metastases. Treatment options include surgical resection, whole brain radiotherapy, stereotactic radiosurgery, and systemic treatment such as targeted or immune therapy. Anatomical magnetic resonance imaging (MRI) of the tumor (in particular post-Gadolinium T₁-weighted and T₂-weighted FLAIR) provide information about lesion morphology and structure, and are routinely used in clinical practice for both detection and treatment response evaluation for brain metastases. Advanced MRI biomarkers that characterize the cellular, biophysical, micro-structural and metabolic features of tumors have the potential to improve the management of brain metastases from early detection and diagnosis, to evaluating treatment response. Magnetic resonance spectroscopy (MRS), chemical exchange saturation transfer (CEST), quantitative magnetization transfer (qMT), diffusion-based tissue microstructure imaging, trans-membrane water exchange mapping, and magnetic susceptibility weighted imaging (SWI) are advanced MRI techniques that will be reviewed in this article as they pertain to brain metastases.

INDIANA: An in-cell diffusion method to characterize the size, abundance and permeability of cells

NMR and MRI diffusion experiments contain information describing the shape, size, abundance, and membrane permeability of cells although extracting this information can be challenging. Here we present the INDIANA (IN-cell DIffusion ANAlysis) method to simultaneously and non-invasively measure cell abundance, effective radius, permeability and intrinsic relaxation rates and diffusion coefficients within the inter- and intra-cellular populations. The method couples an experimental dataset comprising stimulated-echo diffusion measurements, varying both the gradient strength and the diffusion delay, together with software to fit a model based on the Kärger equations to robustly extract the relevant parameters. A detailed error analysis is presented by comparing the results from fitting simulated data from Monte Carlo simulations, establishing its effectiveness. We note that for parameters typical of mammalian cells the approach is particularly effective, and the shape of the underlying cells does not unduly affect the results. Finally, we demonstrate the performance of the experiment on systems of suspended yeast and mammalian cells. The extracted parameters describing cell abundance, size, permeability and relaxation are independently validated.

Physical and numerical phantoms for the validation of brain microstructural MRI: A cookbook

Phantoms, both numerical (software) and physical (hardware), can serve as a gold standard for the validation of MRI methods probing the brain microstructure. This review aims to provide guidelines on how to build, implement, or choose the right phantom for a particular application, along with an overview of the current state-of-the-art of phantoms dedicated to study brain microstructure with MRI. For physical phantoms, we discuss the essential requirements and relevant characteristics of both the (NMR visible) liquid and (NMR invisible) phantom materials that induce relevant microstructural features detectable via MRI, based on diffusion, intra-voxel incoherent motion, magnetization transfer or magnetic susceptibility weighted contrast. In particular, for diffusion MRI, many useful phantoms have been proposed, ranging from simple liquids to advanced biomimetic phantoms consisting of hollow or plain microfibers and capillaries. For numerical phantoms, the focus is on Monte Carlo simulations of random walk, for which the basic principles, along with useful criteria to check and potential pitfalls are reviewed, in addition to a literature overview highlighting recent advances. While many phantoms exist already, the current review aims to stimulate further research in the field and to address remaining needs.

Characterization of the diffusion coefficient of blood

PURPOSE

To characterize the diffusion coefficient of human blood for accurate results in intravoxel incoherent motion imaging.

METHODS

Diffusion-weighted MRI of blood samples from 10 healthy volunteers was acquired with a single-shot echo-planar-imaging sequence at body temperature. Effects of gradient profile (monopolar or flow-compensated), diffusion time (40-100 ms), and echo time (60-200 ms) were investigated.

RESULTS

Although measured apparent diffusion coefficients of blood were larger for flow-compensated than for monopolar gradients, no dependence of the apparent diffusion coefficient on the diffusion time was found. Large differences between individual samples were observed, with results ranging from 1.26 to 1.66 µm2 /ms for flow-compensated and 0.94 to 1.52 µm2 /ms for monopolar gradients. Statistical analysis indicates correlations of the flow-compensated apparent diffusion coefficient with hematocrit (P = 0.007) and hemoglobin (P = 0.017), but not with mean corpuscular volume (P = 0.64). Results of Monte-Carlo simulations support the experimental observations.

CONCLUSIONS

Measured blood apparent diffusion coefficient values depend on hematocrit/hemoglobin concentration and applied gradient profile due to non-Gaussian diffusion. Because in vivo measurement is delicate, an estimation based on blood count results could be an alternative. For intravoxel incoherent motion modeling, the use of a blood self-diffusion constant Db  = 1.54 ± 0.12 µm2 /ms for flow-compensated and Db  = 1.30 ± 0.18 µm2 /ms for monopolar encoding is suggested. Magn Reson Med 79:2752-2758, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Transverse signal decay under the weak field approximation: Theory and validation

PURPOSE

To derive an expression for the transverse signal time course from systems in the motional narrowing regime, such as water diffusing in blood. This was validated in silico and experimentally with ex vivo blood samples.

METHODS

A closed-form solution (CFS) for transverse signal decay under any train of refocusing pulses was derived using the weak field approximation. The CFS was validated via simulations of water molecules diffusing in the presence of spherical perturbers, with a range of sizes and under various pulse sequences. The CFS was compared with more conventional fits assuming monoexponential decay, including chemical exchange, using ex vivo blood Carr-Purcell-Meiboom-Gill data.

RESULTS

From simulations, the CFS was shown to be valid in the motional narrowing regime and partially into the intermediate dephasing regime, with increased accuracy with increasing Carr-Purcell-Meiboom-Gill refocusing rate. In theoretical calculations of the CFS, fitting for the transverse relaxation rate (R₂ ) gave excellent agreement with the weak field approximation expression for R₂ for Carr-Purcell-Meiboom-Gill sequences, but diverged for free induction decay. These same results were confirmed in the ex vivo analysis.

CONCLUSION

Transverse signal decay in the motional narrowing regime can be accurately described analytically. This theory has applications in areas such as tissue iron imaging, relaxometry of blood, and contrast agent imaging. Magn Reson Med 80:341-350, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

NMR relaxation properties of the synthetic malaria pigment β-hematin

200 million patients suffer from malaria, a parasitic disease caused by protozoans of the genus Plasmodium. Reliable diagnosis is crucial since it allows the early detection of the disease. The development of rapid, sensitive and low-cost diagnosis tools is an important research area. Different studies focused on the detection of hemozoin, a major by-product of hemoglobin detoxification by the parasite. Hemozoin and its synthetic analog, β-hematin, form paramagnetic crystals. A new detection method of malaria takes advantage of the paramagnetism of hemozoin through the effect that such magnetic crystals have on Nuclear Magnetic Resonance (NMR) relaxation of water protons. Indeed, magnetic microparticles cause a shortening of the relaxation times. In this work, the magnetic properties of two types of β-hematin are assessed at different temperatures and magnetic fields. The pure paramagnetism of β-hematin is confirmed. The NMR relaxation of β–hematin suspensions is also studied at different magnetic fields and for different echo-times. Our results help to identify the best conditions for β–hematin detection by NMR: T₂ must be selected, at large magnetic fields and for long echo-times. However, the effect of β-hematin on relaxation does not seem large enough to achieve accurate detection of malaria without any preliminary sample preparation, as microcentrifugation.

Brain microstructure by multi-modal MRI: Is the whole greater than the sum of its parts?

The MRI signal is dependent upon a number of sub-voxel properties of tissue, which makes it potentially able to detect changes occurring at a scale much smaller than the image resolution. This "microstructural imaging" has become one of the main branches of quantitative MRI. Despite the exciting promise of unique insight beyond the resolution of the acquired images, its widespread application is limited by the relatively modest ability of each microstructural imaging technique to distinguish between differing microscopic substrates. This is mainly due to the fact that MRI provides a very indirect measure of the tissue properties in which we are interested. A strategy to overcome this limitation lies in the combination of more than one technique, to exploit the relative contributions of differing physiological and pathological substrates to selected MRI contrasts. This forms the basis of multi-modal MRI, a broad concept that refers to many different ways of effectively combining information from more than one MRI contrast. This paper will review a range of methods that have been proposed to maximise the output of this combination, primarily falling into one of two approaches. The first one relies on data-driven methods, exploiting multivariate analysis tools able to capture overlapping and complementary information. The second approach, which we call "model-driven", aims at combining parameters extracted by existing biophysical or signal models to obtain new parameters, which are believed to be more accurate or more specific than the original ones. This paper will attempt to provide an overview of the advantages and limitations of these two philosophies.

The larmor frequency shift in magnetically heterogeneous media depends on their mesoscopic structure

PURPOSE

Recent studies have addressed the determination of the NMR precession frequency in biological tissues containing magnetic susceptibility differences between cell types. The purpose of this study is to investigate the dependence of the precession frequency on medium microstructure using a simple physical model.

THEORY

This dependence is governed by diffusion of NMR-visible molecules in magnetically heterogeneous microenvironments. In the limit of fast diffusion, the precession frequency is determined by the average susceptibility-induced magnetic field, whereas in the limit of slow diffusion it is determined by the average local phase factor of precessing spins.

METHODS

The main method used is Monte Carlo simulation of isotropic suspensions of impermeable magnetized spheres. In addition, NMR spectroscopy was performed in aqueous suspensions of polystyrene microbeads.

RESULTS

The precession frequency depends on the structural organization of magnetized objects in the medium. Monte Carlo simulations demonstrated a nonmonotonic transition between the regimes of fast and slow diffusion. NMR experiments confirmed the transition, but were unable to confirm its precise form. Results for a given pattern of structural organization obey a scaling law.

CONCLUSION

The NMR precession frequency exhibits a complex dependence on medium structure. Our results suggest that the commonly assumed limit of fast water diffusion holds for biological tissues with small cells. Magn Reson Med 79:1101-1110, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Non-invasive imaging using reporter genes altering cellular water permeability

Non-invasive imaging of gene expression in live, optically opaque animals is important for multiple applications, including monitoring of genetic circuits and tracking of cell-based therapeutics. Magnetic resonance imaging (MRI) could enable such monitoring with high spatiotemporal resolution. However, existing MRI reporter genes based on metalloproteins or chemical exchange probes are limited by their reliance on metals or relatively low sensitivity. Here we introduce a new class of MRI reporters based on the human water channel aquaporin 1. We show that aquaporin overexpression produces contrast in diffusion-weighted MRI by increasing tissue water diffusivity without affecting viability. Low aquaporin levels or mixed populations comprising as few as 10% aquaporin-expressing cells are sufficient to produce MRI contrast. We characterize this new contrast mechanism through experiments and simulations, and demonstrate its utility in vivo by imaging gene expression in tumours. Our results establish an alternative class of sensitive, metal-free reporter genes for non-invasive imaging.

Magnetic resonance imaging combined with molecular reporters can visualise cellular functions in intact organisms. Here Mukherjee et al. present a cellular imaging approach based on intracellular changes in water diffusion using human aquaporin 1 gene as a genetically encoded reporter for MRI.

Effects of iodinated contrast on various magnetic resonance imaging sequences and field strength: Implications for characterization of hemorrhagic transformation in acute stroke therapy

AIM: To characterize the effects of iodinated contrast material (ICM) on magnetic resonance imaging (MRI) comparing different sequences and magnetic fields, with emphasis to similarities/differences with well-known signal characteristics of hemorrhage in the brain.

METHODS: Aliquots of iopamidol and iodixanol mixed with normal saline were scanned at 1.5T and 3T. Signal intensity (SI) was measured using similar spin-echo (SE)-T1, SE-T2, gradient-echo (GRE) and fluid-attenuation-inversion-recovery (FLAIR) sequences at both magnets. Contrast to noise ratio (CNR) (SI contrast-SI saline/SD noise) for each aliquot were calculated and Kruskall-wallis test and graphic analysis was used to compare different pulse sequences and ICMs.

RESULTS: Both ICM showed increased SI on SE-T1 and decreased SI on SE-T2, GRE and FLAIR at both 1.5T and 3T, as the concentration was increased. By CNR measurements, SE-T2 had the greatest conspicuity at 3T with undiluted iopamidol (92.6 ± 0.3, P < 0.00) followed by iodixanol (77.5 ± 0.9, P < 0.00) as compared with other sequences (CNR range: 15-40). While SE-T2 had greatest conspicuity at 1.5T with iopamidol (49.3 ± 1, P < 0.01), SE-T1 showed similar or slightly better conspicuity (20.8 ± 4) than SE-T2 with iodixanol (23 ± 1.7). In all cases, hypo-intensity on GRE was less conspicuous than on SE-T2.

CONCLUSION: Iodixanol and iopamidol shorten T1 and T2 relaxation times at both 1.5T and 3T. Hypo-intensity due to shortened T2 relaxation time is significantly more conspicuous than signal changes on T1-WI, FLAIR or GRE. Variations in signal conspicuity according to pulse sequence and to type of ICM are exaggerated at 3T. We postulate T2 hypointensity with less GRE conspicuity differentiates ICM from hemorrhage; given the well-known GRE hypointensity of hemorrhage. Described signal changes may be relevant in the setting of recent intra-arterial or intravenous ICM administration in translational research and/or human stroke therapy.

Normal saline as a natural intravascular contrast agent for dynamic perfusion-weighted MRI of the brain: Proof of concept at 1.5T

PURPOSE

Gadolinium-based contrast agents have associated risks. Normal saline (NS) is a nontoxic sodium chloride water solution that can significantly increase the magnetic resonance imaging (MRI) relaxation times of blood via transient hemodilution (THD). The purpose of this pilot study was to test in vivo in the head the potential of normal saline as a safer, exogenous perfusion contrast agent.

MATERIALS AND METHODS

This Health Insurance Portability and Accountability Act (HIPAA)-compliant prospective study was approved by the local Institutional Review Board (IRB): 12 patients were scanned with T₁ -weighted inversion recovery turbo spin echo pulse sequence at 1.5T. The dynamic inversion recovery pulse sequence was run before, during, and after the NS injection for up to 5 minutes: 100 ml of NS was power-injected via antecubital veins at 3-4 ml/s. Images were processed to map maximum enhancement area-under-the-curve, time-to-peak, and mean-transit-time. These maps were used to identify the areas showing significant NS injection-related signal and to generate enhancement time curves. Hardware and pulse sequence stability were studied via phantom experimentation. Main features of the time curves were tested against theoretical modeling of THD signal effects using inversion recovery pulse sequences. Pearson correlation coefficient (R) mapping was used to differentiate genuine THD effects from motion confounders and noise.

RESULTS

The scans of 8 out of 12 patients showed NS injection-related effects that correlate in magnitude with tissue type (gray matter ∼15% and white matter ∼3%). Motion artifacts prevented ascertaining NS signal effects in the remaining four patients. Positive and negative time curves were observed in vivo and this dual THD signal polarity was also observed in the theoretical simulations. R-histograms that were approximately constant in the range 0.1 < |R| < 0.8 and leading to correlation fractions of F_corr (|R| > 0.5) = 0.45 and 0.59 were found to represent scans with genuine THD signal effects.

CONCLUSION

A measurable perfusion effect in brain tissue was demonstrated in vivo using NS as an injectable intravascular contrast agent. J. Magn. Reson. Imaging 2016;44:1580-1591.

The Effect of Microcirculatory Flow on Oscillating Gradient Diffusion MRI and Diffusion Encoding with Dual-Frequency Orthogonal Gradients (DEFOG)

PURPOSE

We investigated the effect of microcirculatory flow on oscillating gradient spin echo (OGSE) diffusion MRI at low b-values and developed a diffusion preparation method called diffusion encoding with dual-frequency orthogonal gradients (DEFOG) to suppress the effect.

METHODS

Compared to conventional OGSE sequences, DEFOG adds a pulsed gradient that is orthogonal to the oscillating gradient and has a moderate diffusion weighting (e.g., 300 s/mm² ). In vivo MRI data were acquired from adult mouse brains (n = 5) on an 11.7 Tesla scanner, with diffusion times from 23.2 to 0.83 ms and b-values from 50 to 700 s/mm² .

RESULTS

Apparent diffusion coefficients (ADCs) measured using a conventional OGSE sequence at low b-values (< 200 mm² /s) were significantly higher than those measured at moderate b-values (> 300 mm² /s), potentially due to contributions from microcirculatory flow. In comparison, OGSE ADCs measured using the DEFOG method at low b-values were comparable to those measured at moderate b-values. The effect of microcirculatory flow on diffusion signals was diffusion time-dependent, and this dependency may reflect the capillary geometry and blood flow velocity in the mouse cortex.

CONCLUSION

Microcirculatory flow affects OGSE diffusion MRI measurements at low b-values, and this effect can be suppressed using the DEFOG method. Magn Reson Med 77:1583-1592, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Exploring diffusion across permeable barriers at high gradients. II. Localization regime

We present an analytical solution of the one-dimensional Bloch-Torrey equation for diffusion across multiple semi-permeable barrier. This solution generalizes the seminal work by Stoller, Happer, and Dyson, in which the non-Gaussian stretched-exponential behavior of the pulsed-gradient spin-echo (PGSE) signal was first predicted at high gradients in the so-called localization regime. We investigate how the diffusive exchange across a semi-permeable barrier modifies this asymptotic behavior, and explore the transition between the localization regime at low permeability and the Gaussian regime at high permeability. High gradients are suitable to spatially localize the contribution of the nuclei near the barrier and to enhance the sensitivity of the PGSE signal to the barrier permeability. The emergence of the localization regime for three-dimensional domains is discussed.

Exploring diffusion across permeable barriers at high gradients. I. Narrow pulse approximation

The adaptive variation of the gradient intensity with the diffusion time at a constant optimal b-value is proposed to enhance the contribution of the nuclei diffusing across permeable barriers, to the pulsed-gradient spin-echo (PGSE) signal. An exact simple formula the PGSE signal is derived under the narrow pulse approximation in the case of one-dimensional diffusion across a single permeable barrier. The barrier contribution to the signal is shown to be maximal at a particular b-value. The exact formula is then extended to multiple permeable barriers, while the PGSE signal is shown to be sensitive to the permeability and to the inter-barrier distance. Potential applications of the protocol to survey diffusion in three-dimensional domains with permeable membranes are illustrated through numerical simulations.

Kinetic modeling in the context of cerebral blood flow quantification by H2(15)O positron emission tomography: the meaning of the permeability coefficient in Renkin-Crone׳s model revisited at capillary scale

One the one hand, capillary permeability to water is a well-defined concept in microvascular physiology, and linearly relates the net convective or diffusive mass fluxes (by unit area) to the differences in pressure or concentration, respectively, that drive them through the vessel wall. On the other hand, the permeability coefficient is a central parameter introduced when modeling diffusible tracers transfer from blood vessels to tissue in the framework of compartmental models, in such a way that it is implicitly considered as being identical to the capillary permeability. Despite their simplifying assumptions, such models are at the basis of blood flow quantification by H2(15)O Positron Emission Tomgraphy. In the present paper, we use fluid dynamic modeling to compute the transfers of H2(15)O between the blood and brain parenchyma at capillary scale. The analysis of the so-obtained kinetic data by the Renkin-Crone model, the archetypal compartmental model, demonstrates that, in this framework, the permeability coefficient is highly dependent on both flow rate and capillary radius, contrarily to the central hypothesis of the model which states that it is a physiological constant. Thus, the permeability coefficient in Renkin-Crone׳s model is not conceptually identical to the physiologic permeability as implicitly stated in the model. If a permeability coefficient is nevertheless arbitrarily chosen in the computed range, the flow rate determined by the Renkin-Crone model can take highly inaccurate quantitative values. The reasons for this failure of compartmental approaches in the framework of brain blood flow quantification are discussed, highlighting the need for a novel approach enabling to fully exploit the wealth of information available from PET data.

Potential of fluid-attenuated inversion recovery MRI as an alternative to contrast-enhanced MRI for oral and maxillofacial vascular malformations: experimental and clinical studies

OBJECTIVE

To determine the potential of fluid-attenuated inversion recovery (FLAIR) imaging of oral and maxillofacial vascular malformations as an alternative to contrast-enhanced magnetic resonance imaging (MRI), we investigated the influence of differences in T1 and T2 values on image contrast in FLAIR images and evaluated the diagnostic utility of such images.

STUDY DESIGN

FLAIR imaging and heavily T2-weighted (hT2-weighted) imaging were performed using a phantom. FLAIR and hT2-weighted images of 32 lesions (11 mucous cysts, 12 vascular malformations, and 9 tumors) were also studied retrospectively. The contrast-to-noise ratios (CNRs) and CNR change ratios were compared.

RESULTS

All aqueous solutions except those with a short T2 value were discriminated by CNR change ratio (P < .05). All 3 types of lesions were discriminated by CNR change ratio (P < .05).

CONCLUSION

FLAIR imaging has potential as an alternative to contrast-enhanced MRI in differentiating vascular malformations from other types of lesions in the oral and maxillofacial region.

Model selection in measures of vascular parameters using dynamic contrast-enhanced MRI: experimental and clinical applications

A review of the selection of models in dynamic contrast-enhanced MRI (DCE-MRI) is conducted, with emphasis on the balance between the bias and variance required to produce stable and accurate estimates of vascular parameters. The vascular parameters considered as a first-order model are the forward volume transfer constant K(trans) , the plasma volume fraction vp and the interstitial volume fraction ve . To illustrate the critical issues in model selection, a data-driven selection of models in an animal model of cerebral glioma is followed. Systematic errors and extended models are considered. Studies with nested and non-nested pharmacokinetic models are reviewed; models considering water exchange are considered.

Simultaneous estimation of T(2) and apparent diffusion coefficient in human articular cartilage in vivo with a modified three-dimensional double echo steady state (DESS) sequence at 3 T

T(2) mapping and diffusion-weighted imaging complement morphological imaging for assessing cartilage disease and injury. The double echo steady state sequence has been used for morphological imaging and generates two echoes with markedly different T(2) and diffusion weighting. Modifying the spoiler gradient area and flip angle of the double echo steady state sequence allows greater control of the diffusion weighting of both echoes. Data from two acquisitions with different spoiler gradient areas and flip angles are used to simultaneously estimate the T(2) and apparent diffusion coefficient of each voxel. This method is verified in phantoms and validated in vivo in the knee; estimates from different regions of interest in the phantoms and cartilage are compared to those obtained using standard spin-echo methods. The Pearson correlations were 0.984 for T(2) (∼2% relative difference between spin-echo and double echo steady state estimates) and 0.997 for apparent diffusion coefficient (˜1% relative difference between spin-echo and double echo steady state estimates) for the phantom study and 0.989 for T(2) and 0.987 for apparent diffusion coefficient in regions of interest in the human knee in vivo. High accuracy for simultaneous three-dimensional T(2) and apparent diffusion coefficient measurements are demonstrated, while also providing morphologic three-dimensional images without blurring or distortion in reasonable scan times.

The use of MR-detectable reporter molecules and ions to evaluate diffusion in normal and ischemic brain

As a result of the technical challenges associated with distinguishing the MR signals arising from intracellular and extracellular water, a variety of endogenous and exogenous MR-detectable molecules and ions have been employed as compartment-specific reporters of water motion. Although these reporter molecules and ions do not have the same apparent diffusion coefficients (ADCs) as water, their ADCs are assumed to be directly related to the ADC of the water in which they are solvated. This approach has been used to probe motion in the intra- and extracellular space of cultured cells and intact tissue. Despite potential interpretative challenges with the use of reporter molecules or ions and the wide variety used, the following conclusions are consistent considering all studies: (i) the apparent free diffusive motion in the intracellular space is approximately one-half of that in dilute aqueous solution; (ii) ADCs for intracellular and extracellular water are similar; (iii) the intracellular ADC decreases in association with brain injury. These findings provide support for the hypothesis that the overall brain water ADC decrease that accompanies brain injury is driven primarily by a decrease in the ADC of intracellular water. We review the studies supporting these conclusions, and interpret them in the context of explaining the decrease in overall brain water ADC that accompanies brain injury.

Monte Carlo study of a two-compartment exchange model of diffusion

Multisite exchange models have been applied frequently to quantify measurements of transverse relaxation and diffusion in living tissues. Although the simplicity of such models is attractive, the precise relationship of the model parameters to tissue properties may be difficult to ascertain. Here, we investigate numerically a two-compartment exchange (Kärger) model as applied to diffusion in a system of randomly packed identical parallel cylinders with permeable walls, representing cells with permeable membranes, that may serve particularly as a model for axons in the white matter of the brain. By performing Monte Carlo simulations of restricted diffusion, we show that the Kärger model may provide a reasonable coarse-grained description of the diffusion-weighted signal in the long time limit, as long as the cell membranes are sufficiently impermeable, i.e. whenever the residence time in a cell is much longer than the time it takes to diffuse across it. For larger permeabilities, the exchange time obtained from fitting to the Kärger model overestimates the actual exchange time, leading to an underestimated value of cell membrane permeability.

Detection of apoptotic cell death in vitro in the presence of Gd-DTPA-BMA

Due to variability in patient response to cancer therapy, there is a growing interest in monitoring patient progress during treatment. Apoptotic cell death is one early marker of tumor response to treatment. Using known extracellular concentrations of gadolinium diethylenetriamine pentaacetic acid bismethylamide (Gd-DTPA-BMA) to vary the exchange regime, T(1) and T(2) relaxation data for acute myeloid leukemia (AML) cell samples were obtained and then analyzed using a two-pool model of relaxation with exchange. Leukemia cells treated with cisplatin to induce apoptosis exhibited a statistically significant (P < 0.05) decrease in intracellular longitudinal relaxation time, T(1I), from 1030 ms to 940 ms, a decrease (P < 0.001) in the intracellular water fraction, M(0I), from 0.86 to 0.68 and a statistically significant increase (P < 0.01) in transmembrane water exchange rate, k(IE), from 1.4 s(-1) to 6.8 s(-1). The changes in MR parameters correlated with quantitative histology, such as cellular cross-sectional area and average nuclear area measurements. The results of this study emphasize the importance of accounting for water exchange in dynamic contrast-enhanced MRI (DCE-MRI) studies, particularly those that examine tumor response to therapies in which apoptotic changes occur.

Field strength dependence of R1 and R2* relaxivities of human whole blood to ProHance, Vasovist, and deoxyhemoglobin

This study has measured the longitudinal and transverse (T2* relaxivity curves for ProHance (Gadoteridol), Vasovist (Gadofosveset) and deoxyhemoglobin at 1.5, 3.0, and 7.0 Tesla. The plots of R(1) versus both contrast agent and deoxyhemoglobin concentration were linear. The plots of R2* versus deoxyhemoglobin concentration showed a quadratic dependence. R2* versus contrast agent concentration showed a parabolic dependence with a minimum occurring at contrast agent concentrations of approximately 1.5 mM, corresponding to an accessible concentration in vivo. Monte Carlo simulations were performed to support the hypothesis that the minimum results from the susceptibility of the red blood cells being matched to the susceptibility of the plasma. Relaxivity values (s(-1)mM(-1)) for R2* and R1 for all agents and all three field strengths are given.

Diffusion tensor magnetic resonance imaging of the human calf: comparison between 1.5 T and 3.0 T-preliminary results

OBJECTIVES

To compare diffusion tensor-magnetic resonance imaging (DT-MRI) of human calf muscles at 1.5 T and 3.0 T, and to measure a number of quantitative parameters to characterize diffusion anisotropy in organized muscle tissue using similar imaging parameters.

METHODS AND MATERIALS

After Institutional Review Board approval and informed consent, five healthy volunteers were studied. Imaging was performed on both 1.5 T and 3.0 T MR systems using the similar imaging protocol. Diffusion-sensitized single-shot spin-echo echo planar imaging pulse sequences were used to collect 2-dimensional images through the calf. Imaging was performed using b-values of 0, 300, 500, and 700 s/mm. Image analyses and tensor calculations were performed offline using DT imaging studio (Johns Hopkins University, Baltimore, MD). The eigenvalues (lambda1, lambda2, lambda3), trace of the diffusion tensor (TrD), fractional anisotropy, relative anisotropy, and volume ratio were calculated in 3 different calf muscles (medial and lateral gastrocnemius and soleus). Signal-to-noise ratios (SNRs) were compared for both field strengths (1.5 T and 3.0 T), the different muscles and all b-values. A regression analysis was performed to look at within-subject effects (linear mixed effect model).

RESULTS

No significant differences were found between all quantitative measured DT-MRI parameters, b-values, and muscle groups at 3.0 T and 1.5 T (P = 0.105; P = 0.719). The mean of SNR on the 2 different field strengths (3.0:1.5 T) was 1.64, which was significantly different (P < 0.0001). Significant differences in SNR in all 3 muscles were found between sequences using b = 300 s/mm and 700 s/mm (P < 0.001; P = 0.006) and between sequences using b = 300 s/mm and 500 s/mm (P < 0.001; P = 0.03), and 500 s/mm and 700 s/mm (P = 0.005; P = 0.03), respectively, for medial gastrocnemius and soleus muscle.

CONCLUSIONS

This study demonstrates useful parameters to perform DT-MRI at 1.5 T and 3.0 T. DT-MRI at 1.5 T and 3.0 T provide in vivo validation of quantitative structural analysis of human skeleletal muscle.

Ischemia-induced changes of intracellular water diffusion in rat glioma cell cultures

Diffusion-weighted MRI is commonly used in the diagnosis and evaluation of ischemic stroke because of the rapid decrease observed in the apparent diffusion coefficient (ADC) of tissue water following ischemia. Although this observation has been clinically useful for many years, the biophysical mechanisms underlying the reduction of tissue ADC are still unknown. To help elucidate these mechanisms, we have employed a novel three-dimensional (3D) hollow-fiber bioreactor (HFBR) perfused cell culture system that enables cells to be grown to high density and studied via MRI and MRS. By infusing contrast media into the HFBR, signals from intracellular water and extracellular water are spectroscopically resolved and can be investigated individually. Diffusion measurements carried out on C6 glioma HFBR cell cultures indicate that ischemia-induced cellular swelling results in an increase in the ADC of intracellular water from 0.35 microm(2)/ms to approximately 0.5 microm(2)/ms (diffusion time = 25 ms).

Cardiac MRI of ischemic heart disease at 3 T: potential and challenges

Cardiac MRI has become a routinely used imaging modality in the diagnosis of cardiovascular disease and is considered the clinically accepted gold standard modality for the assessment of cardiac function and myocardial viability. In recent years, commercially available clinical scanners with a higher magnetic field strength (3.0 T) and dedicated multi-element coils have become available. The superior signal-to-noise ratio (SNR) of these systems has lead to their rapid acceptance in cranial and musculoskeletal MRI while the adoption of 3.0 T for cardiovascular imaging has been somewhat slower. This review article describes the benefits and pitfalls of magnetic resonance imaging of ischemic heart disease at higher field strengths. The fundamental changes in parameters such as SNR, transversal and longitudinal relaxation times, susceptibility artifacts, RF (B1) inhomogeneity, and specific absorption rate are discussed. We also review approaches to avoid compromised image quality such as banding artifacts and inconsistent or suboptimal flip angles. Imaging sequences for the assessment of cardiac function with CINE balanced SSFP imaging and MR tagging, myocardial perfusion, and delayed enhancement and their adjustments for higher field imaging are explained in detail along with several clinical examples. We also explore the use of parallel imaging at 3.0 T to improve cardiac imaging by trading the SNR gain for higher field strengths for acquisition speed with increased coverage or improved spatial and temporal resolution. This approach is particularly useful for dynamic applications that are usually limited to the duration of a single breath-hold.

Intracellular water specific MR of microbead-adherent cells: HeLa cell intracellular water diffusion

The (1)H MR signal arising from flowing extracellular media in a perfused, microbead-adherent cultured cell system can be suppressed with a slice-selective, spin-echo pulse sequence. The signal from intracellular water can, thus, be selectively monitored. Herein, this technique was combined with pulsed field gradients (PFGs) to quantify intracellular water diffusion in HeLa cells. The intracellular water MR diffusion-signal attenuation at various diffusion times was well described by a biophysical model that characterizes the incoherent displacement of intracellular water as a truncated Gaussian distribution of apparent diffusion coefficients (ADCs). At short diffusion times, the water "free" diffusion coefficient and the surface-to-volume ratio of HeLa cells were estimated and were, 2.0 +/- 0.3 microm(2)/ms and 0.48 +/- 0.1 microm(-1) (mean +/- SD), respectively. At long diffusion times, the cell radius of 10.1 +/- 0.4 microm was inferred and was consistent with that measured by optical microscopy. In summary: 1) intracellular water "free" diffusion in HeLa cells was rapid, two-thirds that of pure water; and 2) the cell radius inferred from modeling the incoherent displacement of intracellular water by a truncated Gaussian distribution of ADCs was confirmed by independent optical microscopy measures.

Study of diffusion in erythrocyte suspension using internal magnetic field inhomogeneity

Transport of water and ions through cell membranes plays an important role in cell metabolism. We demonstrate a novel technique to measure water transport dynamics using erythrocyte suspensions as an example. This technique takes advantage of inhomogeneous internal magnetic field created by the magnetic susceptibility contrast between the erythrocytes and plasma. The decay of longitudinal magnetization due to diffusion in this internal field reveals multi-exponential behavior, with one component corresponding to the diffusive exchange of water across erythrocyte membrane. The membrane permeability is obtained from the exchange time constant and is in good agreement with the literature values. As compared to the other methods, this technique does not require strong gradients of magnetic field or contrast agents and, potentially, can be applied in vivo.

Water exchange across the erythrocyte plasma membrane studied by HR-MAS NMR spectroscopy

Water exchange across the plasma membrane of erythrocytes (red blood cells (RBCs)) was studied by means of high-resolution magic angle spinning (HR-MAS) NMR spectroscopy. Under HR-MAS conditions, the centrifugal force causes the splitting of RBC suspensions into a two-phase system composed of a central core of cell free water and an outer layer of tightly packed cells. Water belonging to each of these phases gives rise to two separated resonances. Chemical exchange between them is not detectable on the chemical shift or saturation transfer (ST) NMR time scale because of the physical separation between the phases. When the RBCs are dispersed and immobilized within a matrix made of cross-linked albumin, the splitting into a two-phase system is prevented and a single exchange-averaged peak for water is detected in (1)H HR-MAS NMR spectra. The lineshape of this peak is dependent on transmembrane exchange kinetics, since MAS averages out all the anisotropic magnetic interactions that are responsible for additional line-broadening under conventional liquid conditions. Line-shape analysis according to a two-site exchange model yielded a residence lifetime on the order of about 10 ms (at 37 degrees C) for a water molecule within the intracellular compartment, which is not too far from the generally accepted value of 9.6-14.8 ms.

Detecting microcirculatory changes in blood oxygen state with steady-state free precession imaging

Recently, it has been demonstrated that oxygen-weighted images of whole blood can be obtained with steady-state methods. In this article, based on computational and experimental models, we investigate the potential for employing this technique to monitor oxygen changes in microcirculation. Results show that oxygen-sensitive images of rabbit kidney and muscle may be obtained at high signal-to-noise ratio within a few seconds. The results also show that in steady-state free precession imaging, in addition to the exchange mechanism that generates oxygen contrast in blood, there are additional mechanisms that provide oxygen-sensitive contrast in microcirculation.

MR microscopy of rat hippocampal slice cultures: a novel model for studying cellular processes and chronic perturbations to tissue microstructure

Brain slices provide a useful nervous tissue model to investigate the relationships between magnetic resonance imaging (MRI) contrast mechanisms and tissue microstructure; yet, these acutely isolated tissues remain viable for only 10-12 h. To study slower biological processes, this work describes the first MRI microscopy characterization of organotypic rat hippocampal slice cultures that can be maintained for several weeks. Diffusion-weighted images of slice cultures acquired with a 14.1-T magnet demonstrated the laminar anatomy of the hippocampus with relatively high signal-to-noise ratios. Diffusion data analyzed using a two-compartment model with exchange indicated that cultured slices had a comparable microstructure to acute brain slices and to in vivo brain. Immunohistochemistry indicated that slice cultures tolerated the conditions required for MRI study well. MRI of cultured tissue slices is highly amenable to correlative microscopy techniques and offers great promise for future MRI investigations of pathological tissue reorganization, molecular imaging and stem cell therapies.

A novel microbubble construct for intracardiac or intravascular MR manometry: a theoretical study

It has been demonstrated that gas-filled microbubble contrast agents, based on their volume changes, can serve as pressure probes in an MR field. It was recently reported that such an MR-based pressure measurement with microbubbles at 1.5 T must make use of microbubbles that have a volumetric magnetic susceptibility difference with the blood of at least 34 ppm in SI units. In this work, we show through analytical approximations and numerical simulations that such a microbubble formulation can be achieved by coating typical lipid-shelled microbubbles with particles of high dipole moment. Through finite-element simulations we demonstrate that the effective volumetric magnetic susceptibility of a coated microbubble is dependent on the radius, the shell volume fraction and the magnetic susceptibility of the particulates on the shell. Our calculations suggest that a suitable microbubble formulation which will be MR-sensitive to small pressure changes at 1.5 T must be 2-3 microm in radius and be uniformly coated with single-domain magnetic nanoparticles, such as magnetite, at shell volume fractions below 5%.

MR properties of excised neural tissue following experimentally induced demyelination

Changes in the magnetic resonance (MR) parameters of demyelinated neural tissue were measured in vitro using an experimental animal model. A tellurium (Te) diet was applied to weanling rats to induce the demyelination process in the sciatic nerve. The quantitative MR parameters, such as T(1), T(2) relaxation time constants and magnetization transfer (MT) were measured each day after applying the Te diet (up to 7 days) and were found to be substantially different from those of normal nerves. An increase in the average T(1) and T(2) was observed along with a decrease in the MT ratio (MTR) and the quantitative MT parameter M(0B), which describes the semisolid pool of protons. Most of the MR parameters correlated very well with the myelin fraction of neural tissue evaluated by quantitative histopathology. The T(2) relaxation spectrum provided the most efficient quantitative assessment of changes in neural tissue microstructure and its analysis resulted in a powerful tool to distinguish the processes of demyelination and inflammation. In comparison, the MT measurements were less successful.

Uncovering of intracellular water in cultured cells

The complexity of biologic tissues, with multiple compartments each with its own diffusion and relaxation properties, requires complex formalisms to model water signal in most magnetic resonance imaging or magnetic resonance spectroscopy experiments. In this article, we describe a magnetic susceptibility-induced shift in the resonance frequency of extracellular water by the introduction of a gadolinium contrast agent to medium perfusing a hollow fiber bioreactor. The frequency shift of the extracellular water (+185 Hz at 9.4 T) uncovers the intracellular water and allows direct measurement of motional and relaxation properties of the intracellular space. The proposed method provides a unique tool for understanding the mechanisms underlining diffusion and relaxation in the intracellular space.

Oxygen-sensitive contrast in blood for steady-state free precession imaging

Steady-state free precession (SSFP) methods have gained widespread recognition for their ability to provide fast scans at high signal-to-noise ratio. This paper demonstrates that such methods are also capable of reflecting functional information, particularly blood oxygenation state. It is well known that SSFP signals show substantial sensitivity to small off-resonance frequency variations. However, that mechanism cannot explain the oxygen-sensitive contrast in blood that was observed with steady-state methods using phase-cycled radiofrequency pulses. From theoretical and experimental models it is demonstrated that the mechanism responsible for such contrast originates from the motion of spins through local field inhomogeneities in and around deoxygenated red blood cells. In addition, this work shows that it is critical to choose the scan parameters carefully for robust oxygen-sensitive contrast. Finally, it is demonstrated that it is possible to build a quantitative model that incorporates the Luz-Meiboom model, which had been used in the past to estimate quantitative measures of vascular blood oxygen levels. It is envisioned that this method could be instrumental in real-time imaging focused on detecting diseases where the oxygen state of blood is impaired.

T1, T2 relaxation and magnetization transfer in tissue at 3T

T1, T2, and magnetization transfer (MT) measurements were performed in vitro at 3 T and 37 degrees C on a variety of tissues: mouse liver, muscle, and heart; rat spinal cord and kidney; bovine optic nerve, cartilage, and white and gray matter; and human blood. The MR parameters were compared to those at 1.5 T. As expected, the T2 relaxation time constants and quantitative MT parameters (MT exchange rate, R, macromolecular pool fraction, M0B, and macromolecular T2 relaxation time, T2B) at 3 T were similar to those at 1.5 T. The T1 relaxation time values, however, for all measured tissues increased significantly with field strength. Consequently, the phenomenological MT parameter, magnetization transfer ratio, MTR, was lower by approximately 2 to 10%. Collectively, these results provide a useful reference for optimization of pulse sequence parameters for MRI at 3 T.

Generalized diffusion tensor imaging and analytical relationships between diffusion tensor imaging and high angular resolution diffusion imaging

A new method for mapping diffusivity profiles in tissue is presented. The Bloch-Torrey equation is modified to include a diffusion term with an arbitrary rank Cartesian tensor. This equation is solved to give the expression for the generalized Stejskal-Tanner formula quantifying diffusive attenuation in complicated geometries. This makes it possible to calculate the components of higher-rank tensors without using the computationally-difficult spherical harmonic transform. General theoretical relations between the diffusion tensor (DT) components measured by traditional (rank-2) DT imaging (DTI) and 3D distribution of diffusivities, as measured by high angular resolution diffusion imaging (HARDI) methods, are derived. Also, the spherical tensor components from HARDI are related to the rank-2 DT. The relationships between higher- and lower-rank Cartesian DTs are also presented. The inadequacy of the traditional rank-2 tensor model is demonstrated with simulations, and the method is applied to excised rat brain data collected in a spin-echo HARDI experiment.

MR properties of excised neural tissue following experimentally induced inflammation

Changes in the MR parameters of inflamed neural tissue were measured in vitro. Tumor necrosis factor-alpha (TNF-alpha) was injected into rat sciatic nerves to induce inflammation with negligible axonal loss and demyelination. The MR parameters, such as T1/T2 relaxation and magnetization transfer (MT), were measured 2 days after TNF-alpha injection and were found to be substantially different from those of normal nerves. The average T1/T2 relaxation times increased, whereas the MT ratio (MTR) and the quantitative MT parameter M0B (which describes the semisolid pool of protons) decreased. The MR parameters correlated very well with the extracellular volume fraction (EM) of neural tissue evaluated by quantitative histopathology. The multicomponent T2 relaxation was shown to provide the best quantitative assessment of changes in neural tissue microstructure, and allowed us to distinguish between the processes of inflammation and demyelination. In comparison, the MT measurements were less successful due to competing contributions of demyelination and pH-sensitive changes in the MT effect.

Diffusion magnetic resonance imaging study of a rat hippocampal slice model for acute brain injury

Diffusion magnetic resonance imaging (MRI) provides a surrogate marker of acute brain pathology, yet few studies have resolved the evolution of water diffusion changes during the first 8 hours after acute injury, a critical period for therapeutic intervention. To characterize this early period, this study used a 17.6-T wide-bore magnet to measure multicomponent water diffusion at high b-values (7 to 8,080 s/mm(2)) for rat hippocampal slices at baseline and serially for 8 hours after treatment with the calcium ionophore A23187. The mean fast diffusing water fraction (Ffast) progressively decreased for slices treated with 10-microM/L A23187 (-20.9 +/- 6.3% at 8 hours). Slices treated with 50-micromol/L A23187 had significantly reduced Ffast 80 minutes earlier than slices treated with 10-microM/L A23187 (P < 0.05), but otherwise, the two doses had equivalent effects on the diffusion properties of tissue water. Correlative histologic analysis showed dose-related selective vulnerability of hippocampal pyramidal neurons (CA1 > CA3) to pathologic swelling induced by A23187, confirming that particular intravoxel cell populations may contribute disproportionately to water diffusion changes observed by MRI after acute brain injury. These data suggest diffusion-weighted images at high b-values and the diffusion parameter Ffast may be highly sensitive correlates of cell swelling in nervous issue after acute injury.