Reproducibility of diffusion tensor imaging in human forearm muscles at 3.0 T in a clinical setting

The aim of the present study was to evaluate a fast clinical protocol to enable diffusion tensor imaging of the human forearm and assess the reproducibility of six diffusion tensor imaging parameters, i.e., the tensor eigenvalues (λ₁, λ₂, and λ₃), mean diffusivity, fractional anisotropy, and ellipsoid eccentricity. The right forearms of 10 healthy volunteers were scanned twice, with a 1‐week interval. Reproducibility of the diffusion tensor imaging parameters was interpreted using Bland‐Altman plots, coefficient of repeatability, repeatability index, and the intraclass correlation coefficient. Analysis was done for three regions of interest: the whole muscle volume, flexor digitorum profundus, and extensor digitorum. The Bland‐Altman analysis showed that there is good agreement between the two measurements. Based on the intraclass correlation coefficients, agreement was substantial (0.59 < intraclass correlation coefficient < 0.92) for all six parameters of the whole muscle volume and flexor digitorum profundus but only fair (0.18 < intraclass correlation coefficient < 0.64) for the extensor digitorum. Using a 7 min 40 sec scan protocol, which was well tolerated by the volunteers, the reproducibility of diffusion tensor imaging parameters was demonstrated. However, repeatability varies, depending on the region of interest and diffusion tensor imaging parameters. This should be taken into account when a longitudinal study is designed. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Diffusion tensor imaging of forearm nerves in humans

Purpose:

To implement a diffusion tensor imaging (DTI) protocol for visualization of peripheral nerves in human forearm.

Materials and Methods:

This Health Insurance Portability and Accountability Act (HIPAA)‐compliant study was approved by our Institutional Review Board and written informed consent was obtained from 10 healthy participants. T₁‐ and T₂‐weighted turbo spin echo with fat saturation, short tau inversion recovery (STIR), and DTI sequences with 21 diffusion‐encoding directions were implemented to acquire images of the forearm nerves with an 8 channel knee coil on a 3T MRI scanner. Identification of the nerves was based on T₁‐weighted, T₂‐weighted, STIR, and DTI‐derived fractional anisotropy (FA) images. Maps of the DTI‐derived indices, FA, mean diffusivity (MD), longitudinal diffusivity (λ_//), and radial diffusivity (λ_?) along the length of the nerves were generated.

Results:

DTI‐derived maps delineated the forearm nerves more clearly than images acquired with other sequences. Only ulnar and median nerves were clearly visualized on the DTI‐derived FA maps. No significant differences were observed between the left and right forearms in any of the DTI‐derived measures. Significant variation in the DTI measures was observed along the length of the nerve. Significant differences in the DTI measures were also observed between the median and ulnar nerves.

Conclusion:

DTI is superior in visualizing the median and ulnar nerves in the human forearm. The normative data could potentially help distinguish normal from diseased nerves. J. Magn. Reson. Imaging 2012;36:920–927. © 2012 Wiley Periodicals, Inc.

     

High‐resolution diffusion tensor imaging with inner field‐of‐view EPI

Purpose

To demonstrate the applicability of inner field‐of‐view (FOV) echo‐planar imaging based on spatially two‐dimensional selective radiofrequency excitations to high‐resolution diffusion tensor imaging.

Materials and Methods

Diffusion tensor imaging of inner FOVs with in‐plane resolutions of 0.90 × 0.90 mm² and 0.50 × 0.50 mm² was performed in the human brain and cervical spinal cord on a 3 T whole‐body MR system.

Results

Using inner FOVs reduces geometric distortions in echo‐planar imaging and allows for an improved in‐plane resolution. Some of the crossings of transverse pontine fibers with the pyramidal tracts in the brainstem could be resolved, increased diffusion anisotropy and fiber orientation could be identified in cerebellar white matter, and the reduced diffusion anisotropy of spinal cord gray matter could be detected.

Conclusion

Inner FOV echo‐planar imaging may help to improve the spatial resolution and thus the accuracy of diffusion anisotropy and white matter fiber orientation measurements in the human central nervous system. J. Magn. Reson. Imaging 2009;29:987–993. © 2009 Wiley‐Liss, Inc.     

Strenuous resistance exercise effects on magnetic resonance diffusion parameters and muscle–tendon function in human skeletal muscle

Purpose:

To assess the effects of strenuous exercise on magnetic resonance diffusion parameters and muscle–tendon complex function in skeletal muscle.

Materials and Methods:

Six men performed ankle plantar flexion exercises with eccentric contraction. The fractional anisotropy (FA), λ₁, λ₂, λ₃, mean diffusivity (MD), and T₂ values in the triceps surae muscles were measured by magnetic resonance diffusion tensor and spin‐echo imaging. Passive torque of plantar flexors, maximal voluntary isometric plantar flexion torques (MVIP), and Achilles tendon stiffness during MVIP were measured by combined ultrasonography and dynamometry. Plasma creatine kinase and muscle soreness were also assessed. These parameters were measured before and 1–8 days postexercise.

Results:

The medial gastrocnemius exhibited significantly decreased FA 2–5 days after, increased λ₂ 3 days after, and increased λ₃ 2 and 3 days after exercise. This muscle also showed significantly increased MD and T₂ values 3 days postexercise. MVIP significantly decreased 2 and 3 days postexercise, while passive torque significantly increased 2 days postexercise. Creatine kinase and muscle soreness increased 3–5 days and 1–5 days postexercise, respectively.

Conclusion:

Exercise‐induced muscle damage manifested as significant changes in muscle diffusion parameters with muscle–tendon complex dysfunction and delayed‐onset muscle soreness. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

Toward a practical protocol for human optic nerve DTI with EPI geometric distortion correction

Purpose

To develop a practical protocol for diffusion tensor imaging (DTI) of the human optic nerve with echo planar imaging (EPI) geometric distortion correction.

Materials and Methods

A conventional DTI protocol was modified to acquire images with fat and cerebrospinal fluid (CSF) suppression and field inhomogeneity maps of contiguous coronal slices covering the whole brain. The technique was applied to healthy volunteers and multiple sclerosis patients with and without a history of unilateral optic neuritis. DTI measures and optic nerve tractography before and after geometric distortion correction were compared. Diffusion measures from left and right or from affected and unaffected eyes in different subject cohorts were reported.

Results

The image geometry after correction closely resembled reference anatomical images. Optic nerve tractography became feasible after distortion correction. The diffusion measures from the healthy volunteers were in good agreement with the literature. Statistically significant differences were found in the fractional anisotropy and orthogonal eigenvalues between affected and unaffected eyes in optic neuritis patients with poor recovery. The diffusion measures before and after geometric distortion correction were not significantly different. For cohorts without optic neuritis, the difference between diffusion measures from left and right eyes was not statistically significant.

Conclusion

The proposed technique could provide a practical DTI protocol to study the human optic nerve. J. Magn. Reson. Imaging 2009;30:699–707. © 2009 Wiley‐Liss, Inc.     

Reproducibility analysis of diffusion tensor indices and fiber architecture of human calf muscles in vivo at 1.5 Tesla in neutral and plantarflexed ankle positions at rest

Purpose:

To investigate the reproducibility of diffusion tensor imaging (DTI) ‐derived indices and fiber architecture of calf muscles at 1.5 Tesla (T), to establish an imaging based method to confirm ankle position, and to compare fiber architecture at different ankle positions.

Materials and Methods:

Six subjects were imaged at 1.5T with the foot in neutral and plantarflexed positions. DTI indices were calculated in four muscle compartments (medial and lateral gastrocnemius [MG, LG], superficial and deep anterior tibialis [AT‐S, AT‐D]). Two subjects were scanned on 3 days to calculate the coefficient of variability (CV) and the repeatability coefficient (RC).

Results:

DTI indices were close to the values obtained in earlier 3T and 1.5T studies. Fractional anisotropy decreased significantly in the MG and increased significantly in the AT‐S and AT‐D compartments while fiber orientation with respect to the magnet Z‐axis increased significantly in the MG and decreased significantly in the AT‐S compartment with plantarflexion. The CV and RC for the DTI indices and fiber orientations were comparable to 3T studies. Fiber lengths and orientation angles in the MG matched corresponding measures from ultrasound studies.

Conclusion:

DTI at 1.5T provides reproducible measures of diffusion indices and fiber architecture of calf muscle at different muscle lengths. J. Magn. Reson. Imaging 2011;. © 2011 Wiley‐Liss, Inc.     

Improving the performance of diffusion-weighted inner field-of-view echo-planar imaging based on 2D-selective radiofrequency excitations by tilting the excitation plane

Purpose:

To improve the performance and flexibility of diffusion‐weighted inner field‐of‐view (FOV) echo‐planar imaging (EPI) based on 2D‐selective radiofrequency (RF) excitations by 1) using higher gradient amplitudes for outer excitation lines, and 2) tilting the excitation plane such that the unwanted side excitations do not overlap with the current image slice or other slices to be acquired.

Materials and Methods:

Acquisitions with a conventional (untilted) and the improved setup were compared and inner FOV diffusion tensor measurements were performed in the human brain and spinal cord with voxel sizes of 1.0 × 1.0 × 5.0 mm³ and 0.6 × 0.6 × 5.0 mm³ on a 3 T whole‐body magnetic resonance imaging (MRI) system.

Results:

With the modified setup, the 2D‐selective RF excitations can be considerably shortened (e.g., from 26 msec to 6 msec) which 1) avoids profile distortions in the presence of magnetic field inhomogeneities, and 2) reduces the required echo time and increases the signal‐to‐noise ratio accordingly, e.g., by about 20% in the spinal cord.

Conclusion:

Tilting the excitation plane and applying variable gradient amplitudes improves the applicability of inner FOV EPI based on 2D‐selective RF excitations. J. Magn. Reson. Imaging 2012;35:984–992. © 2011 Wiley Periodicals, Inc.     

Optimized inversion-prepared gradient echo imaging

Purpose:

To implement a method using an extended phase graph (EPG)‐based simulation to optimize inversion‐prepared gradient echo sequences with respect to signal and contrast within the shortest acquisition time.

Materials and Methods:

A critical issue in rapid gradient‐echo imaging is the effect of residual transverse magnetization between consecutive data acquisition windows. Various spoiling schemes have been proposed to mitigate this problem, and while spoiling is often considered to be perfect, imaging can be more truthfully described using the EPG. An EPG‐based simulation is used to analyze and predict the image signal and contrast to serve as a basis for sequence optimization.

Results:

Fourteen biological phantom experiments and five brain imaging experiments on each of five healthy volunteers was performed to validate and verify the accuracy of the EPG‐based simulation. In addition, two experiments on an in‐cranial cadaver brain were performed to show the ability of the proposed method for improving overall image quality.

Conclusion:

From the experiment results, it is demonstrated that optimization of 3D magnetization‐prepared rapid gradient‐echo imaging sequences can be performed with an EPG‐based simulation to manipulate the sequence parameters for generating images with highly specific signal and contrast characteristics for quantitative T1‐weighted human brain imaging. J. Magn. Reson. Imaging 2012;36:748–755. © 2012 Wiley Periodicals, Inc.     

Quantitative BOLD: the effect of diffusion

Purpose:

To make the quantitative blood oxygenation level‐dependent (qBOLD) method more suitable for clinical application by accounting for proton diffusion and reducing acquisition times.

Materials and Methods:

Monte Carlo methods are used to simulate the signal from diffusing protons in the presence of a blood vessel network. A diffusive qBOLD model was then constructed using a lookup table of the results. Acquisition times are reduced by parallel imaging and by employing an integrated fieldmapping method, rather than running an additional sequence.

Results:

The addition of diffusion to the model is shown to have a significant impact on predicted signal formation. This is found to affect all fitted parameters when the model is applied to real data. Parallel imaging and integrated fieldmapping allowed the GESSE (gradient echo sampling of a spin echo) acquisition to be made in less than 10 minutes while maintaining high signal‐to‐noise ratio (SNR).

Conclusion:

By incorporating integrated field mapping and parallel imaging techniques, GESSE data were acquired within clinically acceptable acquisition times. These data fit closely to the diffusive qBOLD model, providing more realistic and robust measurements of T₂ and blood oxygenation than the static model. J. Magn. Reson. Imaging 2010;32:953–961. © 2010 Wiley‐Liss, Inc.     

Diffusion-weighted whole-body MRI with background body signal suppression: technical improvements at 3.0 T

Purpose:

To improve image quality of diffusion‐weighted body magnetic resonance imaging (MRI) with background body signal suppression (DWIBS) at 3.0 T.

Materials and Methods:

In 30 patients and eight volunteers, a diffusion‐weighted spin‐echo echo‐planar imaging sequence with short TI inversion recovery (STIR) fat suppression was applied and repeated using slice‐selective gradient reversal (SSGR) and/or dual‐source parallel radiofrequency (RF) transmission (TX). The quality of diffusion‐weighted images and gray scale inverted maximum intensity projections (MIP) were visually assessed by intraindividual comparison with respect to the level of fat suppression and signal homogeneity. Moreover, the contrast between lesions/lymph nodes and background (C_lb) was analyzed in the MIP reconstructions.

Results:

By combining STIR with SSGR, fat suppression was significantly improved (P < 0.001) and C_lb was increased two times. The use of TX allowed the reduction of acquisition time and improved image quality with regard to signal homogeneity (P < 0.001) and fat suppression (P = 0.005).

Conclusion:

DWIBS at 3.0 T can be improved by using SSGR and TX. J. Magn. Reson. Imaging 2012;456‐461. © 2011 Wiley Periodicals, Inc.     

Diffusion tensor imaging of liver fibrosis in an experimental model

Purpose

To characterize changes in diffusion properties of liver using diffusion tensor imaging (DTI) in an experimental model of liver fibrosis.

Materials and Methods

Liver fibrosis was induced in Sprague–Dawley rats (n = 12) by repetitive dosing of carbon tetrachloride (CCl₄). The animals were examined with a respiratory‐gated single‐shot spin‐echo echo‐planar DTI protocol at 7 T before, 2 weeks after, and 4 weeks after CCl₄ insult. Apparent diffusion coefficient (ADC), directional diffusivities (ADC_// and ADC_?), and fractional anisotropy (FA) were measured. Liver histology was performed with hematoxylin‐eosin staining and Masson's trichrome staining.

Results

Significant decrease (P < 0.01) in ADC was found at 2 weeks (0.86 ± 0.09 × 10^?3 mm²/s) and 4 weeks (0.74 ± 0.09 × 10^?3 mm²/s) following CCl₄ insult, as compared with that before insult (0.97 ± 0.08 × 10^?3 mm²/s). Meanwhile, FA at 2 weeks (0.18 ± 0.03) after CCl₄ insult was significantly lower (P < 0.01) than that before insult (0.26 ± 0.05), and subsequently normalized at 4 weeks (0.26 ± 0.07) after the insult. Histology showed collagen deposition, presence of intracellular fat vacuoles, and cell necrosis/apoptosis in livers with CCl₄ insult.

Conclusion

DTI detected the progressive changes in water diffusivities and diffusion anisotropy of liver tissue in this liver fibrosis model. ADC and FA are potentially valuable in detecting liver fibrosis at early stages and monitoring its progression. Future human studies are warranted to further verify the applicability of DTI in characterizing liver fibrosis and to determine its role in clinical settings. J. Magn. Reson. Imaging 2010;32:1141–1148. © 2010 Wiley‐Liss, Inc.

     

Stimulated echo induced misestimates on diffusion tensor indices and its remedy

Purpose:

To report possible erroneous estimates of diffusion parameters in the twice‐refocused spin‐echo (TRSE) technique, proposed to eliminate eddy‐current‐induced geometric distortions in diffusion‐weighted echo‐planar imaging, when stimulated echo signals are inappropriately included.

Materials and Methods:

Eleven subjects were included for imaging experiments on two 1.5 Tesla systems using the TRSE sequence. Three versions, two with unbalanced crusher gradients inserted to dephase the stimulated echo from the b = 0 images and one with balanced crusher gradients, were implemented. The apparent diffusion coefficients (ADC) and fractional anisotropy (FA) were derived and compared.

Results:

The ADCs obtained with unbalanced crusher gradients were closer to values reported in the literature. Stimulated echo led to ADC over‐estimations by 34.2%, 50.4%, 54.0%, 51.5%, 24.0%, and 41.9% in the genu of corpus callosum, splenium of corpus callosum, bilateral corona radiata, internal capsule, mediofrontal gyrus, and the cuneus, respectively (P < 0.01), with concomitant reduction in FA in highly anisotropic regions. Over‐estimations of diffusion coefficients were found to be roughly equal along all directions.

Conclusion:

Formation of stimulated echo in the TRSE technique can lead to erroneous estimations of the diffusion parameters, even if no prominent morphological artifacts are seen. J. Magn. Reson. Imaging 2010;31:1522–1529. © 2010 Wiley‐Liss, Inc.     

Ex vivo water diffusion tensor properties of the fibroid uterus at 7 T and their relation to tissue morphology

Purpose:

To investigate the water diffusion tensor properties of ex vivo tissue in the fibroid uterus, including the influence of degeneration, and the relevance of the principal eigenvector orientation to the underlying tissue structure.

Materials and Methods:

Following hysterectomy, high‐resolution structural T₂‐weighted and diffusion tensor magnetic resonance imaging (DT‐MRI) were performed on nine uteri at 7 T. Mean diffusivity (MD), fractional anisotropy (FA), and principal eigenvector orientation were measured in myometrium and in myxoid and dense tissue in fibroids. Imaging data and measurements of water diffusion parameters were compared with histopathology findings.

Results:

The nine uteri yielded 23 fibroids. MD was 50% higher in regions of myxoid degeneration compared to dense fibroid tissue (P = 0.001), while myometrium was intermediate in value (dense fibroid tissue, P = 0.15; myxoid degeneration, P = 0.23). FA was lower in dense fibroid tissue than in myometrium (P = 3 × 10^?5), but higher than in myxoid tissue (P = 0.003). Principal eigenvector orientation corresponded qualitatively with that of uterine smooth muscle fibers.

Conclusion:

The water diffusion tensor measured ex vivo in the fibroid uterus is a sensitive probe of tissue type, myxoid degeneration, and morphology. J. Magn. Reson. Imaging 2011;. © 2011 Wiley Periodicals, Inc.

     

Effect of cerebral spinal fluid suppression for diffusional kurtosis imaging

Purpose:

To evaluate the cerebral spinal fluid (CSF) partial volume effect on diffusional kurtosis imaging (DKI) metrics in white matter and cortical gray matter.

Materials and Methods:

Four healthy volunteers participated in this study. Standard DKI and fluid‐attenuated inversion recovery (FLAIR) DKI experiments were performed using a twice‐refocused‐spin‐echo diffusion sequence. The conventional diffusion tensor imaging (DTI) metrics of fractional anisotropy (FA), mean, axial, and radial diffusivity (MD, D_‖, D_?) together with DKI metrics of mean, axial, and radial kurtosis (MK, K_‖, K_?), were measured and compared. Single image slices located above the lateral ventricles, with similar anatomical features for each subject, were selected to minimize the effect of CSF from the ventricles.

Results:

In white matter, differences of less than 10% were observed between diffusion metrics measured with standard DKI and FLAIR‐DKI sequences, suggesting minimal CSF contamination. For gray matter, conventional DTI metrics differed by 19% to 52%, reflecting significant CSF partial volume effects. Kurtosis metrics, however, changed by 11% or less, indicating greater robustness with respect to CSF contamination.

Conclusion:

Kurtosis metrics are less sensitive to CSF partial voluming in cortical gray matter than conventional diffusion metrics. The kurtosis metrics may then be more specific indicators of changes in tissue microstructure, provided the effect sizes for the changes are comparable. J. Magn. Reson. Imaging 2013;37:365–371. © 2012 Wiley Periodicals, Inc.

     

Analysis of new diffusion tensor imaging anisotropy measures in the three‐phase plot

Purpose:

To evaluate diffusion anisotropy from diffusion tensor imaging using new measures derived from Hellinger divergences and from compositional data distances.

Materials and Methods:

New anisotropy measures obtained from the diffusion tensor imaging were measured and compared with classic ones such as fractional anisotropy (FA) and relative anisotropy (RA). The evaluation was done using the three‐phase plot (3P‐plot). The measures were compared with regard to their sensitivity to detect white and gray matter changes on human DTI data acquired from five normal volunteers. For each volunteer, different volumes of interest located in white matter (WM) and gray matter (GM) were considered.

Results:

The proposed Compositional Kullback‐Leibler (KLA) and the classic FA had a similar behavior, although KLA detected better the transitions between white and gray matter. Moreover, KLA showed a better discrimination in areas with great confluence of fibers.

Conclusion:

KLA detects better than FA the difference between WM and GM. This leads KLA to be a good measure for segmenting WM from GM. J. Magn. Reson. Imaging 2010;31:1435–1444. © 2010 Wiley‐Liss, Inc.     

Human cervical spinal cord funiculi: Investigation with magnetic resonance diffusion tensor imaging

Purpose:

To use spinal cord diffusion tensor imaging (DTI) for investigating human cervical funiculi, acquire axial diffusion magnetic resonance imaging (MRI) data with an in‐plane resolution sufficient to delineate subquadrants within the spinal cord, obtain corresponding DTI metrics, and assess potential regional differences.

Materials and Methods:

Healthy volunteers were studied with a 3 T Siemens Trio MRI scanner. DTI data were acquired using a single‐shot spin echo EPI sequence. The spatial resolution allowed for the delineation of regions of interest (ROIs) in the ventral, dorsal, and lateral spinal cord funiculi. ROI‐based and tractography‐based analyses were performed.

Results:

Significant fractional anisotropy (FA) differences were found between ROIs in the dorsal and ventral funiculi (P = 0.0001), dorsal and lateral funiculi (P = 0.015), and lateral and ventral funiculi (P = 0.0002). Transverse diffusivity was significantly different between ROIs in the ventral and dorsal funiculi (P = 0.003) and the ventral and lateral funiculi (P = 0.004). Tractography‐based quantifications revealed DTI parameter regional differences that were generally consistent with the ROI‐based analysis.

Conclusion:

Original contributions are: 1) the use of a tractography‐based method to quantify DTI metrics in the human cervical spinal cord, and 2) reported DTI values in various funiculi at 3 T. J. Magn. Reson. Imaging 2010;31:829–837. ©2010 Wiley‐Liss, Inc.     

Liver fibrosis: an intravoxel incoherent motion (IVIM) study

Purpose:

To characterize longitudinal changes in molecular water diffusion, blood microcirculation, and their contributions to the apparent diffusion changes using intravoxel incoherent motion (IVIM) analysis in an experimental mouse model of liver fibrosis.

Materials and Methods:

Liver fibrosis was induced in male adult C57BL/6N mice (22–25 g; n = 12) by repetitive dosing of carbon tetrachloride (CCl₄). The respiratory‐gated diffusion‐weighted (DW) images were acquired using single‐shot spin‐echo EPI (SE‐EPI) with 8 b‐values and single diffusion gradient direction. True diffusion coefficient (D_true), blood pseudodiffusion coefficient (D_pseudo), and perfusion fraction (P_fraction) were measured. Diffusion tensor imaging (DTI) was also performed for comparison. Histology was performed with hematoxylin‐eosin and Masson's trichrome staining.

Results:

A significant decrease in D_true was found at 2 weeks and 4 weeks following CCl₄ insult, as compared with that before insult. Similarly, D_pseudo values before injury was significantly higher than those at 2 weeks and 4 weeks after CCl₄ insult. Meanwhile, P_fraction values showed no significant differences over different timepoints. For DTI, significant decrease in ADC was observed following CCl₄ administration. Fractional anisotropy at 2 weeks after CCl₄ insult was significantly lower than that before insult, and subsequently normalized at 4 weeks after the insult. Liver histology showed collagen deposition, the presence of intracellular fat vacuoles, and cell necrosis/apoptosis in livers with CCl₄ insult.

Conclusion:

Both molecular water diffusion and blood microcirculation contribute to the alteration in apparent diffusion changes in liver fibrosis. Reduction in D_true and D_pseudo values resulted from diffusion and perfusion changes, respectively, during the progression of liver fibrosis. IVIM analysis may serve as valuable and robust tool in detecting and characterizing liver fibrosis at early stages, monitoring its progression in a noninvasive manner. J. Magn. Reson. Imaging 2012;36:159–167. © 2012 Wiley Periodicals, Inc.     

Early (72-hour) detection of radiotherapy-induced changes in an experimental tumor model using diffusion-weighted imaging, diffusion tensor imaging, and Q-space imaging parameters: a comparative study

Purpose:

To assess and compare the potential of various diffusion‐related magnetic resonance imaging (MRI) parameters to detect early radiotherapy (RT)‐induced changes in tumors.

Materials and Methods:

Nineteen tumors in a rat model were imaged on a clinical 3T system before and 72 hours after a single RT session. Diffusion imaging was performed using an echo planar sequence containing 16 b‐factors and six gradient directions. This allowed us to perform a tensor analysis of mono‐ and biexponential decays and a q‐space analysis. Parametric maps (both trace and fractional anisotropy) were reconstructed for: 2‐point apparent diffusion coefficient (ADC), 16‐point ADC, biexponential amplitudes and ADCs, and height, width, and kurtosis of the probability density function (PDF). A texture analysis yielded quantities such as average and contrast. The sensitivity of diffusion‐related parameters was quantified in terms of the mean relative difference (when comparing pre‐ and post‐RT status).

Results:

Traces and anisotropies display differences in response to RT. Average traces are most sensitive for ADCs and kurtosis. Average anisotropies are all very sensitive except the slow biexponential component. The best contrast (traces) was found for the ADCs and the width of the PDF.

Conclusion:

ADC performed well, but high b‐values analysis added extra sensitive parameters for monitoring early RT‐induced changes. J. Magn. Reson. Imaging 2012;409‐417. © 2011 Wiley Periodicals, Inc.     

High B‐value apparent diffusion‐weighted images from CURVE‐ball DTI

Purpose

To investigate the utility of a proposed clinical diffusion imaging scheme for rapidly generating multiple b‐value diffusion contrast in brain magnetic resonance imaging (MRI) with high signal‐to‐noise ratio (SNR).

Materials and Methods

Our strategy for efficient image acquisition relies on the invariance property of the diffusion tensor eigenvectors to b‐value. A simple addition to the conventional diffusion tensor MR imaging (DTI) data acquisition scheme used for tractography yields diffusion‐weighted images at twice and three times the conventional b‐value. An example from a neurosurgical brain tumor is shown. Apparent diffusion‐weighted (ADW) images were calculated for b‐values 800, 1600, and 2400 s/mm², and a map of excess diffusive kurtosis was computed from the three ADWs.

Results

High b‐value ADW images demonstrated decreased contrast between normal gray and white matter, while the heterogeneity and contrast of the lesion was emphasized relative to conventional b‐value data. Kurtosis maps indicated the deviation from Gaussian diffusive behavior.

Conclusion

DTI data with multiple b‐values and good SNR can be acquired in clinically reasonable times. High b‐value ADW images show increased contrast and add information to conventional DWI. Ambiguity in conventional b‐value images over whether hyperintense signal results from abnormally low diffusion, or abnormally long T₂, is better resolved in high b‐value images. J. Magn. Reson. Imaging 2009;30:243–248. © 2009 Wiley‐Liss, Inc.     

Intercenter differences in diffusion tensor MRI acquisition

Purpose:

To assess the effect on diffusion tensor (DT) magnetic resonance imaging (MRI) of acquiring data with different scanners.

Materials and Methods:

Forty‐four healthy controls and 36 multiple sclerosis patients with low disability were studied using eight MR scanners with acquisition protocols that were as close to a standard protocol as possible. Between 7 and 13 subjects were studied in each center. Region‐of‐interest (ROI) and histogram‐based analyses of fractional anisotropy (FA), axial (D_ax), radial (D_rad), and mean diffusivity (MD) were performed. The influence of variables such as the acquisition center and the control/patient group was determined with an analysis of variance (ANOVA) test.

Results:

The patient/control group explained ≈25% of data variability of FA and D_rad from midsagittal corpus callosum (CC) ROIs. Global FA, MD, and D_rad in the white matter differentiated patients from controls, but with lower discriminatory power than for the CC. In the gray matter, MD discriminated patients from controls (30% of variability explained by group vs. 17% by center).

Conclusion:

Significant variability of DT‐MRI data can be attributed to the acquisition center, even when a standardized protocol is used. The use of appropriate segmentation methods and statistical models allows DT‐derived metrics to differentiate patients from healthy controls. J. Magn. Reson. Imaging 2010;31:1458–1468. © 2010 Wiley‐Liss, Inc.