3D diffusion tensor MRI with isotropic resolution using a steady‐state radial acquisition

Purpose

To obtain diffusion tensor images (DTI) over a large image volume rapidly with 3D isotropic spatial resolution, minimal spatial distortions, and reduced motion artifacts, a diffusion‐weighted steady‐state 3D projection (SS 3DPR) pulse sequence was developed.

Materials and Methods

A diffusion gradient was inserted in a SS 3DPR pulse sequence. The acquisition was synchronized to the cardiac cycle, linear phase errors were corrected along the readout direction, and each projection was weighted by measures of consistency with other data. A new iterative parallel imaging reconstruction method was also implemented for removing off‐resonance and undersampling artifacts simultaneously.

Results

The contrast and appearance of both the fractional anisotropy and eigenvector color maps were substantially improved after all correction techniques were applied. True 3D DTI datasets were obtained in vivo over the whole brain (240 mm field of view in all directions) with 1.87 mm isotropic spatial resolution, six diffusion encoding directions in under 19 minutes.

Conclusion

A true 3D DTI pulse sequence with high isotropic spatial resolution was developed for whole brain imaging in under 20 minutes. To minimize the effects of brain motion, a cardiac synchronized, multiecho, DW‐SSFP pulse sequence was implemented. Motion artifacts were further reduced by a combination of linear phase correction, corrupt projection detection and rejection, sampling density reweighting, and parallel imaging reconstruction. The combination of these methods greatly improved the quality of 3D DTI in the brain. J. Magn. Reson. Imaging 2009;29:1175–1184. © 2009 Wiley‐Liss, Inc.     

Evaluation of measurement uncertainties in human diffusion tensor imaging (DTI)‐derived parameters and optimization of clinical DTI protocols with a wild bootstrap analysis

Purpose

To quantify measurement uncertainties of fractional anisotropy, mean diffusivity, and principal eigenvector orientations in human diffusion tensor imaging (DTI) data acquired with common clinical protocols using a wild bootstrap analysis, and to establish optimal scan protocols for clinical DTI acquisitions.

Materials and Methods

A group of 13 healthy volunteers were scanned using three commonly used DTI protocols with similar total scan times. Two important parameters—the number of unique diffusion gradient directions (NUDG) and the ratio of the total number of diffusion‐weighted (DW) images to the total number of non‐DW images (DTIR)—were analyzed in order to investigate their combined effects on uncertainties of DTI‐derived parameters, using results from both the Monte Carlo simulation and the wild bootstrap analysis of uncertainties in human DTI data.

Results

The wild bootstrap analysis showed that uncertainties in human DTI data are significantly affected by both NUDG and DTIR in many brain regions. These results agree with previous predictions based on error‐propagations as well as results from simulations.

Conclusion

Our results demonstrate that within a clinically feasible DTI scan time of about 10 minutes, a protocol with number of diffusion gradient directions close to 30 provides nearly optimal measurement results when combined with a ratio of the total number of DW images over non‐DW images equal to six. Wild bootstrap can serve as a useful tool to quantify the measurement uncertainty from human DTI data. J. Magn. Reson. Imaging 2009;29:422–435. © 2009 Wiley‐Liss, Inc.     

Contiguous‐slice zonally oblique multislice (CO‐ZOOM) diffusion tensor imaging: Examples of in vivo spinal cord and optic nerve applications

Purpose

To describe and demonstrate a new technique that allows diffusion tensor imaging of small structures such as the spinal cord (SC) and optic nerve (ON) with contiguous slices and reduced image distortions using a narrow field of view (FOV).

Materials and Methods

Images were acquired with a modified single‐shot echo‐planar imaging (EPI) sequence that contains a refocusing radio frequency (RF) pulse in the presence of the phase‐encoding (rather than slice‐select) gradient. As a result, only a narrow volume may be both excited and refocused, removing the problem of signal aliasing for narrow FOVs. Two variants of this technique were developed: cardiac gating is included in the study of the SC to reduce pulsation artifacts, whereas inversion‐recovery (IR) cerebrospinal fluid (CSF) suppression is utilized in the study of the ON to eliminate partial volume effects. The technique was evaluated with phantoms, and mean diffusivity (MD) and fractional anisotropy (FA) measurements were made in the SC and ON of two healthy volunteers.

Results

The technique provides contiguous‐slice, reduced‐FOV images that do not suffer from aliasing and have reduced magnetic susceptibility artifacts. MD and FA values determined here lie within the ranges quoted in the literature.

Conclusion

Contiguous‐slice zonally orthogonal multislice (CO‐ZOOM‐EPI is a new technique for diffusion‐weighted imaging of small structures such as the ON and SC with high resolution and reduced distortions due to susceptibility variations. This technique is able to acquire contiguous slices that may allow further nerve‐tracking analyses. J. Magn. Reson. Imaging 2009;29:454–460. © 2009 Wiley‐Liss, Inc.     

Diffusion tensor imaging (DTI) with retrospective motion correction for large-scale pediatric imaging

Purpose:

To develop and implement a clinical DTI technique suitable for the pediatric setting that retrospectively corrects for large motion without the need for rescanning and/or reacquisition strategies, and to deliver high‐quality DTI images (both in the presence and absence of large motion) using procedures that reduce image noise and artifacts.

Materials and Methods:

We implemented an in‐house built generalized autocalibrating partially parallel acquisitions (GRAPPA)‐accelerated diffusion tensor (DT) echo‐planar imaging (EPI) sequence at 1.5T and 3T on 1600 patients between 1 month and 18 years old. To reconstruct the data, we developed a fully automated tailored reconstruction software that selects the best GRAPPA and ghost calibration weights; does 3D rigid‐body realignment with importance weighting; and employs phase correction and complex averaging to lower Rician noise and reduce phase artifacts. For select cases we investigated the use of an additional volume rejection criterion and b‐matrix correction for large motion.

Results:

The DTI image reconstruction procedures developed here were extremely robust in correcting for motion, failing on only three subjects, while providing the radiologists high‐quality data for routine evaluation.

Conclusion:

This work suggests that, apart from the rare instance of continuous motion throughout the scan, high‐quality DTI brain data can be acquired using our proposed integrated sequence and reconstruction that uses a retrospective approach to motion correction. In addition, we demonstrate a substantial improvement in overall image quality by combining phase correction with complex averaging, which reduces the Rician noise that biases noisy data. J. Magn. Reson. Imaging 2012;36:961–971. © 2012 Wiley Periodicals, Inc.     

Thoracic and lumbar spinal cord diffusion tensor imaging in dogs

Purpose:

To analyze four clinically applicable diffusion tensor imaging (DTI) protocols (two each in the transverse and sagittal planes) in the normal dog.

Materials and Methods:

Seven healthy Dachshund dogs were scanned with four DTI protocols. Within each plane, identical spatial resolution was used while the number of diffusion‐encoding directions and averages varied. Agreement of measured fractional anisotropy (FA) and apparent diffusion coefficient (ADC) was analyzed with Bland–Altman methods, subjective image quality within each plane was compared, and FA and ADC were explored as a function of anatomic location.

Results:

There was good agreement in FA and ADC values within each plane. FA had the smallest bias and most precision. No difference was detected in subjective image quality within each plane. FA and ADC were slightly higher cranial to the lumbar intumescence compared to within it.

Conclusion:

DTI is a promising tool in the assessment of spinal cord injury (SCI) in the study of dogs with intervertebral disk herniation as a preclinical model of human SCI. J. Magn. Reson. Imaging 2013;37:632–641. © 2013 Wiley Periodicals, Inc.     

Efficient fat suppression by slice‐selection gradient reversal in twice‐refocused diffusion encoding

Most diffusion imaging sequences rely on single‐shot echo‐planar imaging (EPI) for spatial encoding since it is the fastest acquisition available. However, it is sensitive to chemical‐shift artifacts due to the low bandwidth in the phase‐encoding direction, making fat suppression necessary. Often, spectral‐selective RF pulses followed by gradient spoiling are used to selectively saturate the fat signal. This lengthens the acquisition time and increases the specific absorption rate (SAR). However, in pulse sequences that contain two slice‐selective 180° refocusing pulses, the slice‐selection gradient reversal (SSGR) method of fat suppression can be implemented; i.e., using slice‐selection gradients of opposing polarity for the two refocusing pulses. We combined this method with the twice‐refocused spin‐echo sequence for diffusion encoding and tested its performance in both phantoms and in vivo. Unwanted fat signal was entirely suppressed with this method without affecting the water signal intensity or the slice profile. Magn Reson Med 60:1256–1260, 2008. © 2008 Wiley‐Liss, Inc.     

Impact of outliers on diffusion tensor and Q-ball imaging: clinical implications and correction strategies

Purpose:

To measure the impact of corrupted images often found to occur in diffusion‐weighted magnetic resonance imaging (DW‐MRI). To propose a robust method for the correction of outliers, applicable to diffusion tensor imaging (DTI) and q‐ball imaging (QBI).

Materials and Methods:

Monte Carlo simulations were carried out to measure the impact of outliers on DTI and QBI reconstruction in a single voxel. Methods to correct outliers based on q‐space interpolation and direction removal were then implemented and validated in real image data.

Results:

Corruption in a single voxel led to clear variations in DTI and QBI metrics. In real data, the method of q‐space interpolation was successful in identifying corrupted voxels and restoring them to values consistent with those of uncorrupted images.

Conclusion:

For images containing few gradient directions, where outlier removal was either impossible due to limited volumes or resulted in large changes in DTI/QBI metrics, q‐space interpolation proved to be the method of choice for image restoration. A simple decision support system is proposed to assist clinicians in the correction of their corrupted DW data. J. Magn. Reson. Imaging 2011;33:1491–1502. © 2011 Wiley‐Liss, Inc.     

Targeted single-shot methods for diffusion-weighted imaging in the kidneys

Purpose:

To investigate the feasibility of combining the inner‐volume‐imaging (IVI) technique with single‐shot diffusion‐weighted (DW) spin‐echo echo‐planar imaging (SE‐EPI) and DW‐SPLICE (split acquisition of fast spin‐echo) sequences for renal DW imaging.

Materials and Methods:

Renal DWI was performed in 10 healthy volunteers using single‐shot DW‐SE‐EPI, DW‐SPLICE, targeted‐DW‐SE‐EPI, and targeted‐DW‐SPLICE. We compared the quantitative diffusion measurement accuracy and image quality of these targeted‐DW‐SE‐EPI and targeted DW‐SPLICE methods with conventional full field of view (FOV) DW‐SE‐EPI and DW‐SPLICE measurements in phantoms and normal volunteers.

Results:

Compared with full FOV DW‐SE‐EPI and DW‐SPLICE methods, targeted‐DW‐SE‐EPI and targeted‐DW‐SPLICE approaches produced images of superior overall quality with fewer artifacts, less distortion, and reduced spatial blurring in both phantom and volunteer studies. The apparent diffusion coefficient (ADC) values measured with each of the four methods were similar and in agreement with previously published data. There were no statistically significant differences between the ADC values and intravoxel incoherent motion (IVIM) measurements in the kidney cortex and medulla using single‐shot DW‐SE‐EPI, targeted‐DW‐EPI, and targeted‐DW‐SPLICE (P > 0.05).

Conclusion:

Compared with full‐FOV DWI methods, targeted‐DW‐SE‐EPI and targeted‐DW‐SPLICE techniques reduced image distortion and artifacts observed in the single‐shot DW‐SE‐EPI images, reduced blurring in DW‐SPLICE images, and produced comparable quantitative DW and IVIM measurements to those produced with conventional full‐FOV approaches. J. Magn. Reson. Imaging 2011;33:1517–1525. © 2011 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.     

DTI常用扫描序列原理及比较

磁共振弥散张量成像技术是利用水分子的弥散各向异性进行成像,可用于脑白质纤维研究,常用扫描技术包括单次激发平面回波成像(EPI),线阵扫描弥散成像,导航自旋回波弥散加权成像(LSDI),半傅立叶探测单发射快速自旋回波成像等。每种成像技术各有其优缺点,EPI扫描时间短,图像信噪比高,但存在化学位移伪影、磁敏感性伪影、几何变形;LSDI精确度高,几乎无伪影及变形,但扫描时间过长;导航自旋回波弥散加权成像运动伪影少,但扫描时间长;半傅立叶探测单发射快速自旋回波成像扫描时间短,但图像模糊。综合比较,单次激发平面回波成像是用于临床研究较适宜的方法。     

Blipped‐controlled aliasing in parallel imaging for simultaneous multislice echo planar imaging with reduced g‐factor penalty

Simultaneous multislice Echo Planar Imaging (EPI) acquisition using parallel imaging can decrease the acquisition time for diffusion imaging and allow full‐brain, high‐resolution functional MRI (fMRI) acquisitions at a reduced repetition time (TR). However, the unaliasing of simultaneously acquired, closely spaced slices can be difficult, leading to a high g‐factor penalty. We introduce a method to create interslice image shifts in the phase encoding direction to increase the distance between aliasing pixels. The shift between the slices is induced using sign‐ and amplitude‐modulated slice‐select gradient blips simultaneous with the EPI phase encoding blips. This achieves the desired shifts but avoids an undesired “tilted voxel” blurring artifact associated with previous methods. We validate the method in 3× slice‐accelerated spin‐echo and gradient‐echo EPI at 3 T and 7 T using 32‐channel radio frequency (RF) coil brain arrays. The Monte‐Carlo simulated average g‐factor penalty of the 3‐fold slice‐accelerated acquisition with interslice shifts is <1% at 3 T (compared with 32% without slice shift). Combining 3× slice acceleration with 2× inplane acceleration, the g‐factor penalty becomes 19% at 3 T and 10% at 7 T (compared with 41% and 23% without slice shift). We demonstrate the potential of the method for accelerating diffusion imaging by comparing the fiber orientation uncertainty, where the 3‐fold faster acquisition showed no noticeable degradation. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Predicting survival in glioblastomas using diffusion tensor imaging metrics

Purpose

To retrospectively correlate various diffusion tensor imaging (DTI) metrics in patients with glioblastoma multiforme (GBM) with patient survival analysis and also degree of tumor proliferation index determined histologically.

Materials and Methods

Thirty‐four patients with histologically confirmed treatment naive GBMs underwent DTI on a 3.0 Tesla (T) scanner. Region‐of‐interest was placed on the whole lesion including the enhancing as well as nonenhancing component of the lesion to determine the various DTI metrics. Kaplan‐Meier estimates and Cox proportional hazards regression methods were used to assess the relationship of DTI metrics (minimum and mean values) and Ki‐67 with progression free survival (PFS). To study the relationship between DTI metrics and Ki‐67, Pearson's correlation coefficient was computed.

Results

Univariate analysis showed that patients with fractional anisotropy (FA)_mean ≤ 0.2, apparent diffusion coefficient (ADC)_min ≤ 0.6, planar anisotropy (CP)_min ≤ 0.002, spherical anisotropy (CS)_mean > 0.68 and Ki‐67 > 0.3 had lower PFS rate. The multivariate analysis demonstrated that only CP_min was the best predictor of survival in these patients, after adjusting for age, Karnofsky performance scale and extent of resection. No significant correlation between DTI metrics and Ki‐67 were observed.

Conclusion

DTI metrics can be used as a sensitive and early indicator for PFS in patients with glioblastomas. This could be useful for treatment planning as high‐grade gliomas with lower ADC_min, FA_mean, CP_min, and higher CS_mean values may be treated more aggressively. J. Magn. Reson. Imaging 2010;32:788–795. © 2010 Wiley‐Liss, Inc.     

Microstructural changes in patients with progressive supranuclear palsy: A diffusion tensor imaging study

Purpose:

To determine whether progressive supranuclear palsy (PSP) is associated with specific diffusion tensor imaging (DTI) patterns of diffusivity, anisotropy, and coherence in functionally relevant brain areas.

Materials and Methods:

In all, 17 PSP patients and 17 controls were scanned using a 3 T magnetic resonance imaging (MRI) scanner. Patients were assessed in the off‐medication condition using the Hoehn and Yahr staging and the United Parkinson's Disease Rating Scale, motor subscale (UPDRS‐III). Diffusion information were analyzed in relation to disease severity and subtypes.

Results:

Numerous changes in diffusion properties were identified in the subcortical areas. In the midbrain, fractional anisotropy (FA) decreased and MD (mean diffusivity) increased with disease progression. UPDRS‐III scores correlated positively with both FA in the caudate and MD in the pons. DTI analysis of disease subtypes demonstrated significant differences between PSP‐Parkinsonism and Steele‐Richardson‐Olszewski syndrome in axial diffusivity values in the putamen and globus pallidus, as well as in intervoxel diffusion coherence values in the middle cerebellar peduncle.

Conclusion:

Our findings, cautiously interpreted, demonstrate the advantage of using a functional imaging technique to aid in the specificity of defining more precisely the pathological processes taking place in white and gray matter regions in PSP. J. Magn. Reson. Imaging 2010;32:69–75. © 2010 Wiley‐Liss, Inc.     

Slab scan diffusion imaging.

For maximum robustness of a diffusion-weighted MR imaging sequence, it is desirable to use a single-shot imaging method. This article introduces a new single-shot imaging approach that combines the advantages of multiple spin-echoes with the technique of line scan diffusion imaging. A slab volume, which can be spatially encoded with fewer phase encodes than a regular field of view, is selected with 2D selective pulses. With the shorter echo train, the sensitivity to field inhomogeneities and chemical shift is thus greatly diminished. Further reduction is achieved by interleaving short gradient echo trains with refocusing spin-echo pulses. Optimized slice-selective RF pulses that produce flip angles close to 180 degrees are used to minimize the stimulated echo component. Motion-related phase shifts, which change polarity with each spin-echo excitation, will give rise to artifacts that are avoidable by processing even and odd spin-echoes separately. As with line scan diffusion imaging, the complete field of view is acquired by sequential scanning. Since with each shot several lines of data are collected, a considerable improvement over line scan diffusion imaging in terms of scanning speed is achieved. Diffusion data obtained in phantoms and normal subjects demonstrate the feasibility of this novel approach.     

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.     

Diffusion tensor MR imaging of the neurologically intact human spinal cord

BACKGROUND AND PURPOSE: The aim of this study was to characterize the diffusion properties of the entire human spinal cord in vivo. These data are essential for comparisons to pathologic conditions as well as for comparisons of different pulse sequence design parameters aimed to reduce scan time and more accurately determine diffusion coefficients.MATERIALS AND METHODS: A total of 13 neurologically intact subjects were enrolled in this study. A single-shot, twice-refocused, spin-echo, diffusion-weighted, echo-planar imaging (EPI) pulse sequence was used to obtain axial images throughout the entire spinal cord (C1–L1) in 45 minutes.RESULTS: Diffusion images indicated slight geometric distortions; however, gray and white matter contrast was observed. All measurements varied across the length of the cord. Whole cord diffusion coefficients averaged 0.5–1.3 × 10⁻³ mm²/s depending on orientation, mean diffusivity (MD) averaged 0.83 ± 0.06 × 10⁻³ mm²/s, fractional anisotropy (FA) averaged 0.49 ± 0.05, and volume ratio (VR) averaged 0.73 ± 0.05.CONCLUSION: This study provided normative diffusion values for the entire spinal cord for use in comparisons with pathologic conditions as well as improvements in pulse sequence design.

Despite the potential of diffusion tensor imaging (DTI) for providing anatomic and histologic information about the spinal cord, DTI is not yet routinely performed for identifying and characterizing pathologic changes. One important limitation to the application of DTI to spinal cord pathologic disorders is the absence of normative data for comparison. For example, diffusion changes in the spinal cord have been reported after spinal artery stroke,¹ multiple sclerosis,² cervical spondylotic myelopathy,³ spinal cord compression,⁴ acute spinal cord injury,⁵ and chronic spinal cord injury,^6,7 yet detailed baseline data with use of common imaging sequences are lacking for comparison. Some diffusion measurements have been documented in targeted regions of the neurologically intact human spinal cord,^8–12 and these values have been used for comparison to pathologic conditions; however, a comprehensive study of diffusion parameters throughout the entire spinal cord has not been reported. As a result, the primary purpose of this study was to characterize the normative diffusion values of the entire human spinal cord with use of a clinically available pulse sequence for comparison with pathologic conditions and new pulse sequence designs.Current DTI research in the human spinal cord is primarily devoted to the development of pulse sequences aimed at obtaining artifact-free diffusion measurements. Single-shot echo-planar imaging (EPI) is relatively fast but is typically not used in the spinal cord because of the small size of the cord and the perceived risk for susceptibility-related distortions. Unfortunately, the main alternative to EPI, pulsed-gradient, spin-echo DTI, is highly sensitive to motion and has very long imaging times, requiring approximately 15 minutes to image a single diffusion axis.⁹ A few pulse sequences focus on a compromise between these 2 methods, including line scan diffusion imaging,¹³ multishot echo-planar imaging,¹⁰ and fast single-shot EPI with use of sensitivitiy encoding (SENSE).¹⁴ Although these new techniques have established a reputation for accurate diffusion measurements with minimal artifacts, they typically have low signal-to-noise ratio (SNR). A novel technique, presented by Bammer et al,^12,15 uses a phase-navigated interleaved EPI method to overcome SNR challenges; however, the technique is currently not available on MR scanners and thus has limited clinical usefulness.In contrast to recently developed DTI pulse sequences, single-shot EPI is widely available on clinical MR scanners; thus, diffusion-tensor (DT) EPI could serve as a standard for comparison of new pulse sequences. Previous studies involving single-shot DT EPI of the spinal cord have demonstrated its usefulness in estimating diffusion parameters within the spinal cord,^1,4,11,16 though a systematic study of the entire spinal cord has not been conducted. To establish baseline diffusion parameters for comparing new DTI sequences, we aimed to measure the DTI parameters and SNR of the entire spinal cord by using a single-shot, twice-refocused, spin-echo EPI diffusion sequence¹⁷ in the axial plane, with no respiratory or cardiac gating to image the entire spinal cord (C1–L1). We then compared the diffusion parameters from this DT EPI sequence with reported diffusion measurements that were obtained with a variety of recently developed pulse sequences to determine the agreement in diffusion parameters.Thus, the primary aim of this study was to characterize the diffusion properties of the human spinal cord in vivo with a single-shot DT EPI sequence to establish a baseline for clinicians to compare with measurements made in pathologic conditions. The secondary goal was to characterize the diffusion measurements from the current literature and determine if differences exist in mean diffusion characteristics across various pulse sequences and imaging platforms.     

Periodically rotated overlapping parallel lines with enhanced reconstruction-based diffusion tensor imaging. Comparison with echo planar imaging-based diffusion tensor imaging

OBJECTIVE: To evaluate diffusion tensor imaging (DTI) based on periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) compared with DTI based on single-shot echo planar imaging (EPI). METHODS: Diffusion tensor data were acquired with PROPELLER (PROPELLER-DTI, 3 NEX), EPI (EPI-DTI2, 16 NEX) with the same acquisition time (11.4 minutes) and with EPI (EPI-DTI1, number of excitations = 4) with a shorter acquisition time (2.8 minutes). Regions of interest were set in the genu and splenium of the corpus callosum, as determined on T2-weighted fast spin echo images and fractional anisotropy (FA) maps, separately. Two neuroradiologists visually evaluated image distortion and quality in the supra- and infratentorial structures. RESULTS: In the genu, standard deviation determined by respective FA maps was decreased in order of PROPELLER-DTI, EPI-DTI1, and EPI-DTI2. Both EPI-DTI sequences were quantitatively superior in the splenium, but PROPELLER-DTI was less distorted. CONCLUSION: Periodically rotated overlapping parallel lines with enhanced reconstruction-DTI could become a complementary tool when qualitatively evaluating seriously distorted structures.     

Diagnostic performance of conventional diffusion weighted imaging and diffusion tensor imaging for the liver fibrosis and inflammation

Objective

To evaluate the diagnostic accuracy of liver apparent diffusion coefficient (ADC) measured with conventional diffusion-weighted imaging (CDI) and diffusion tensor imaging (DTI) for the diagnosis of liver fibrosis and inflammation.

Materials and methods

Thirty-seven patients with histologic diagnosis of chronic viral hepatitis and 34 healthy volunteers were included in this prospective study. All patients and healthy volunteers were examined by 3 T MRI. CDI and DTI were performed using a breath-hold single-shot echo-planar spin echo sequence with b factors of 0 and 1000 s/mm². ADCs were obtained with CDI and DTI. Histopathologically, fibrosis of the liver parenchyma was classified with the use of a 5-point scale (0–4) and inflammation was classified with use of a 4-point scale (0–3) in accordance with the METAVIR score. Quantitatively, signal intensity and the ADCs of the liver parenchyma were compared between patients stratified by fibrosis stage and inflammation grade.

Results

With a b factor of 1000 s/mm², the signal intensity of the cirrhotic livers was significantly higher than those of the normal volunteers. In addition, ADCs reconstructed from CDI and DTI of the patients were significantly lower than those of the normal volunteers. Liver ADC values inversely correlated with fibrosis and inflammation but there was only statistically significant for inflammatory grading. CDI performed better than DTI for the diagnosis of fibrosis and inflammation.

Conclusion

ADC values measured with CDI and DTI may help in the detection of liver fibrosis. They may also give contributory to the inflammatory grading, particularly in distinguishing high from low grade.     

Diffusion-weighted imaging of the abdomen at 3.0 Tesla: image quality and apparent diffusion coefficient reproducibility compared with 1.5 Tesla

Purpose

To compare single‐shot echo‐planar imaging (SS EPI) diffusion‐weighted MRI (DWI) of abdominal organs between 1.5 Tesla (T) and 3.0T in healthy volunteers in terms of image quality, apparent diffusion coefficient (ADC) values, and ADC reproducibility.

Materials and Methods

Eight healthy volunteers were prospectively imaged in this HIPAA‐compliant IRB‐approved study. Each subject underwent two consecutive scans at both 1.5 and 3.0T, which included breathhold and free‐breathing DWI using a wide range of b‐values (0 to 800 s/mm²). A blinded observer rated subjective image quality (maximum score= 8), and a separate observer placed regions of interest within the liver, renal cortices, pancreas, and spleen to measure ADC at each field strength. Paired Wilcoxon tests were used to compare abdominal DWI between 1.5T and 3.0T for specific combinations of organs, b‐values, and acquisition techniques.

Results

Subjective image quality was significantly lower at 3.0T for all comparisons (P = 0.0078– 0.0156). ADC values were similar at 1.5T and 3.0T for all assessed organs, except for lower liver ADC at 3.0T using b0‐500‐600 and breathhold technique. ADC reproducibility was moderate at both 1.5T and 3.0T, with no significant difference in coefficient of variation of ADC between field strengths.

Conclusion

Compared with 1.5T, SS EPI at 3.0T provided generally similar ADC values, however, with worse image quality. Further optimization of abdominal DWI at 3.0T is needed. J. Magn. Reson. Imaging 2011;33:128–135. © 2010 Wiley‐Liss, Inc.     

PROPELLER EPI: an MRI technique suitable for diffusion tensor imaging at high field strength with reduced geometric distortions.

A technique suitable for diffusion tensor imaging (DTI) at high field strengths is presented in this work. The method is based on a periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) k-space trajectory using EPI as the signal readout module, and hence is dubbed PROPELLER EPI. The implementation of PROPELLER EPI included a series of correction schemes to reduce possible errors associated with the intrinsically higher sensitivity of EPI to off-resonance effects. Experimental results on a 3.0 Tesla MR system showed that the PROPELLER EPI images exhibit substantially reduced geometric distortions compared with single-shot EPI, at a much lower RF specific absorption rate (SAR) than the original version of the PROPELLER fast spin-echo (FSE) technique. For DTI, the self-navigated phase-correction capability of the PROPELLER EPI sequence was shown to be effective for in vivo imaging. A higher signal-to-noise ratio (SNR) compared to single-shot EPI at an identical total scan time was achieved, which is advantageous for routine DTI applications in clinical practice.