On the application of a non-CPMG single-shot fast spin-echo sequence to diffusion tensor MRI of the human brain

Application of Magnetic Resonance Diffusion Tensor Imaging in Diagnosis of Lumbosacral Nerve Root Compression

OBJECTIVE

The aim of this study was to assess the value of 3.0T magnetic resonance (MR) Diffusion tensor imaging (DTI) in the diagnosis of lumbosacral nerve root compression.

METHODS

The radiology reports, and clinical records of 34 patients with nerve root compression caused by lumbar disc herniation or bulging and 21 healthy volunteers who had undergone magnetic resonance imaging (MRI) and DTI scan were retrospectively reviewed. The differences in fractional anisotropy (FA) and apparent diffusion coefficient (ADC) between compressed and non-compressed nerve roots from patients and the normal nerve roots from healthy volunteers were compared. Meanwhile, the nerve root fiber bundles were observed and analyzed.

RESULTS

The average FA and ADC values of the compressed nerve roots were 0.254 ± 0.307 and 1.892 ± 0.346 10^-3mm2/s, respectively. The average FA and ADC values of the non-compressed nerve roots were 0.377 ± 0.659 and 1.353 ± 0.344 10^-3mm2/s, respectively. The FA value of compressed nerve roots was significantly lower than that of non-compressed nerve roots (P < 0.01). The ADC value of compressed nerve roots was significantly higher than that of non-compressed nerve roots. There were no significant differences between the left and right nerve roots of normal volunteers in FA and ADC values (P > 0.05). The nerve roots at different levels of L3-S1 had significantly different FA and ADC values (P < 0.01). Incomplete fiber bundles with extrusion deformation, displacement or partial defect were observed in the compressed nerve root fiber bundles. The real diagnosis of the clinical situation of the nerve can provide neuroscientists with an important computer tool to help them infer and understand the possible working mechanism from the experimental data of behavior and electrophysiology.

CONCLUSION

The compressed lumbosacral nerve roots can be accurately localized through 3.0T magnetic resonance DTI, which is instructive for accurate clinical diagnosis and preoperative localization.

Optimized flip angle schemes for the split acquisition of fast spin-echo signals (SPLICE) sequence and application to diffusion-weighted imaging

PURPOSE

The diffusion-weighted SPLICE (split acquisition of fast spin-echo signals) sequence employs split-echo rapid acquisition with relaxation enhancement (RARE) readout to provide images almost free of geometric distortions. However, due to the varying T 2 $$ {}_2 $$ -weighting during k-space traversal, SPLICE suffers from blurring. This work extends a method for controlling the spatial point spread function (PSF) while optimizing the signal-to-noise ratio (SNR) achieved by adjusting the flip angles in the refocusing pulse train of SPLICE.

METHODS

An algorithm based on extended phase graph (EPG) simulations optimizes the flip angles by maximizing SNR for a flexibly chosen predefined target PSF that describes the desired k-space density weighting and spatial resolution. An optimized flip angle scheme and a corresponding post-processing correction filter which together achieve the target PSF was tested by healthy subject brain imaging using a clinical 1.5 T scanner.

RESULTS

Brain images showed a clear and consistent improvement over those obtained with a standard constant flip angle scheme. SNR was increased and apparent diffusion coefficient estimates were more accurate. For a modified Hann k-space weighting example, considerable benefits resulted from acquisition weighting by flip angle control.

CONCLUSION

The presented flexible method for optimizing SPLICE flip angle schemes offers improved MR image quality of geometrically accurate diffusion-weighted images that makes the sequence a strong candidate for radiotherapy planning or stereotactic surgery.

Accelerating in vivo fast spin echo high angular resolution diffusion imaging with an isotropic resolution in mice through compressed sensing

PURPOSE

Echo planar imaging (EPI) is commonly used to acquire the many volumes needed for high angular resolution diffusion Imaging (HARDI), posing a higher risk for artifacts, such as distortion and deformation. An alternative to EPI is fast spin echo (FSE) imaging, which has fewer artifacts but is inherently slower. The aim is to accelerate FSE such that a HARDI data set can be acquired in a time comparable to EPI using compressed sensing.

METHODS

Compressed sensing was applied in either q-space or simultaneously in k-space and q-space, by undersampling the k-space in the phase-encoding direction or retrospectively eliminating diffusion directions for different degrees of undersampling. To test the replicability of the acquisition and reconstruction, brain data were acquired from six mice, and a numerical phantom experiment was performed. All HARDI data were analyzed individually using constrained spherical deconvolution, and the apparent fiber density and complexity metric were evaluated, together with whole-brain tractography.

RESULTS

The apparent fiber density and complexity metric showed relatively minor differences when only q-space undersampling was used, but deteriorate when k-space undersampling was applied. Likewise, the tract density weighted image showed good results when only q-space undersampling was applied using 15 directions or more, but information was lost when fewer volumes or k-space undersampling were used.

CONCLUSION

It was found that acquiring 15 to 20 diffusion directions with a full k-space and reconstructed using compressed sensing could suffice for a replicable measurement of quantitative measures in mice, where areas near the sinuses and ear cavities are untainted by signal loss.

Minimizing the echo time in diffusion imaging using spiral readouts and a head gradient system

PURPOSE

Diffusion weighted imaging (DWI) is commonly limited by low signal-to-noise ratio (SNR) as well as motion artifacts. To address this limitation, a method that allows to maximize the achievable signal yield and increase the resolution in motion robust single-shot DWI is presented.

METHODS

DWI was performed on a 3T scanner equipped with a recently developed gradient insert (gradient strength: 200 mT/m, slew rate: 600 T/m/s). To further shorten the echo time, Stejskal-Tanner diffusion encoding with a single-shot spiral readout was implemented. To allow non-Cartesian image reconstruction using such strong and fast gradients, the characterization of eddy current and concomitant field effects was performed based on field-camera measurements.

RESULTS

An echo time of only 19 ms was achieved for a b-factor of 1000 s/mm² . An in-plane resolution of 0.68 mm was encoded by a single-shot spiral readout of 40.5 ms using 3-fold undersampling. The resulting images did not suffer from off-resonance artifacts and T 2 ∗ blurring that are common to single-shot images acquired with regular gradient systems.

CONCLUSION

Spiral diffusion imaging using a head gradient system, together with an accurate characterization of the encoding process allows for a substantial reduction of the echo time, and improves the achievable resolution in motion-insensitive single-shot DWI.

The quest for high spatial resolution diffusion-weighted imaging of the human brain in vivo

Diffusion-weighted imaging, a contrast unique to MRI, is used for assessment of tissue microstructure in vivo. However, this exquisite sensitivity to finer scales far above imaging resolution comes at the cost of vulnerability to errors caused by sources of motion other than diffusion motion. Addressing the issue of motion has traditionally limited diffusion-weighted imaging to a few acquisition techniques and, as a consequence, to poorer spatial resolution than other MRI applications. Advances in MRI imaging methodology have allowed diffusion-weighted MRI to push to ever higher spatial resolution. In this review we focus on the pulse sequences and associated techniques under development that have pushed the limits of image quality and spatial resolution in diffusion-weighted MRI.

Mapping water exchange across the blood-brain barrier using 3D diffusion-prepared arterial spin labeled perfusion MRI

PURPOSE

To present a novel MR pulse sequence and modeling algorithm to quantify the water exchange rate (k_w ) across the blood-brain barrier (BBB) without contrast, and to evaluate its clinical utility in a cohort of elderly subjects at risk of cerebral small vessel disease (SVD).

METHODS

A diffusion preparation module with spoiling of non-Carr-Purcell-Meiboom-Gill signals was integrated with pseudo-continuous arterial spin labeling (pCASL) and 3D gradient and spin echo (GRASE) readout. The tissue/capillary fraction of the arterial spin labeling (ASL) signal was separated by appropriate diffusion weighting (b = 50 s/mm² ). k_w was quantified using a single-pass approximation (SPA) model with total generalized variation (TGV) regularization. Nineteen elderly subjects were recruited and underwent 2 MRIs to evaluate the reproducibility of the proposed technique. Correlation analysis was performed between k_w and vascular risk factors, Clinical Dementia Rating (CDR) scale, neurocognitive assessments, and white matter hyperintensity (WMH).

RESULTS

The capillary/tissue fraction of ASL signal can be reliably differentiated with the diffusion weighting of b = 50 s/mm² , given ~100-fold difference between the (pseudo-)diffusion coefficients of the 2 compartments. Good reproducibility of k_w measurements (intraclass correlation coefficient = 0.75) was achieved. Average k_w was 105.0 ± 20.6, 109.6 ± 18.9, and 94.1 ± 19.6 min^-1 for whole brain, gray and white matter. k_w was increased by 28.2%/19.5% in subjects with diabetes/hypercholesterolemia. Significant correlations between k_w and vascular risk factors, CDR, executive/memory function, and the Fazekas scale of WMH were observed.

CONCLUSION

A diffusion prepared 3D GRASE pCASL sequence with TGV regularized SPA modeling was proposed to measure BBB water permeability noninvasively with good reproducibility. k_w may serve as an imaging marker of cerebral SVD and associated cognitive impairment.

Robust Self-Calibrating nCPMG Acquisition: Application to Body Diffusion-Weighted Imaging

This paper demonstrates a robust diffusion-weighted single-shot fast spin echo (SS-FSE) sequence in the presence of significant off-resonance, which includes a variable-density acquisition and a self-calibrated reconstruction as improvements. A non-Carr-Purcell-Meiboom-Gill (nCPMG) SS-FSE acquisition stabilizes both the main and parasitic echo families for each echo. This preserves both the in-phase and quadrature components of the magnetization throughout the echo train. However, nCPMG SS-FSE also promotes aliasing of the quadrature component, which complicates reconstruction. A new acquisition and reconstruction approach is presented here, where the field-of-view is effectively doubled, but a partial k-space and variable density sampling is used to improve scan efficiency. The technique is presented in phantom scans to validate SNR and robustness against rapidly varying object phase. In vivo healthy volunteer examples and the clinical cases are demonstrated in abdominal imaging. This new approach provides comparable SNR to previous nCPMG acquisition techniques as well as providing more uniform apparent diffusion coefficient maps in phantom scans. In vivo scans suggest that this method is more robust against motion than previous approaches. The proposed reconstruction is an improvement to the nCPMG sequence as it is auto-calibrating and is justified to accurately treat the signal model for the nCPMG SS-FSE sequence.

Slice profile effects on nCPMG SS-FSE

PURPOSE

To determine the effects of the RF refocusing pulse profile on the magnitude of the transverse signal smoothness throughout the echo train in non-Carr-Purcell-Meiboom-Gill (nCPMG) single-shot fast spin echo (SS-FSE) imaging and to design an RF refocusing pulse that provides improved signal stability. THEORY AND METHODS: nCPMG SS-FSE quadratic phase modulation requires sufficiently high and uniform refocusing flip angle to achieve a stable signal. Typically, refocusing pulses used in SS-FSE sequences are designed for minimum duration to minimize echo spacing and as a consequence have poor selectivity. However, delay-insensitive variable rate excitation Shinnar-Le Roux (DV-SLR) refocusing pulses can achieve both improved selectivity as well as a short duration. This class of RF pulse is compared against a traditional low time-bandwidth refocusing pulse in a nCPMG SS-FSE in simulation, phantom, and in vivo.

RESULTS

DV-SLR pulses achieve a more stable signal in simulation, phantom, and in vivo cases while maintaining an appropriately short duration as well as not dramatically increasing specific absorption rate (SAR) accumulation.

CONCLUSION

The nCPMG SS-FSE method demonstrates improved robustness when a more selective refocusing pulse is used. Refocusing pulses that use a time-varying excitation gradient can achieve this selectivity while maintaining short echo spacing. Magn Reson Med 79:430-438, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Body Diffusion Weighted Imaging Using Non-CPMG Fast Spin Echo

SS-FSE is a fast technique that does not suffer from off-resonance distortions to the degree that EPI does. Unlike EPI, SS-FSE is ill-suited to diffusion weighted imaging (DWI) due to the Carr-Purcell-Meiboom-Geill (CPMG) condition. Non-CPMG phase cycling does accommodate SS-FSE and DWI but places constraints on reconstruction, which are resolved here through parallel imaging. Additionally, improved echo stability can be achieved by using short duration and highly selective DIVERSE radiofrequency pulses. Here, signal-to-noise ratio (SNR) comparisons between EPI and nCPMG SS-FSE acquisitions and reconstruction techniques give similar values. Diffusion imaging with nCPMG SS-FSE gives similar SNR to an EPI acquisition, though apparent diffusion coefficient values are higher than seen with EPI. In vivo images have good image quality with little distortion. This method has the ability to capture distortion-free DWI images near areas of significant off-resonance as well as preserve adequate SNR. Parallel imaging and DIVERSE refocusing RF pulses allow shorter ETL compared to previous implementations and thus reduces phase encode direction blur and SAR accumulation.

3.0T MRI tractography of lumbar nerve roots in disc herniation

BACKGROUND

Diffusion tensor imaging (DTI) with fiber tracking (FT) has found clinical applications in the evaluation of the central nervous system and has been extensively used to image white matter tract. The feasibility of FT of the lumbar nerve roots in disc herniation is unclear.

PURPOSE

To demonstrate the feasibility of FT in the lumbar nerve roots, and to assess potential differences in fractional anisotropy (FA) and apparent diffusion coefficient (ADC) of L4, L5, and S1 nerves between healthy disc and disc herniation.

MATERIAL AND METHODS

Twenty patients with unilateral sciatica related to posterolateral or foraminal disc herniation and 20 healthy volunteers were enrolled in our study. Anatomical fusion with the axial T2 sequences was used to estimate the relevance of reconstructions. DTI with tractography of the L4, L5, and S1 nerves was performed. Mean FA and ADC values were calculated from tractography images.

RESULTS

Lumbosacral root compression sites could be clearly identified on the tractography images. There was no significant difference in FA or ADC between left and right nerve roots at the same level (P > 0.05) in healthy volunteers. The mean FA value of the compressed spinal nerve roots was significantly lower than that of FA of the contralateral nerve roots (P = 0.0001). ADC was significantly higher in compressed nerve roots than that in the contralateral nerve root (P = 0.0002).

CONCLUSION

3 T magnetic resonance imaging (MRI) DTI and FT of the lumbosacral region nerve is possible. There are significant changes in FA and ADC values in the compressed L4, L5, and S1 nerves.

Interrogation of living myocardium in multiple static deformation states with diffusion tensor and diffusion spectrum imaging

Diffusion tensor magnetic resonance imaging (MRI) reveals valuable insights into tissue histo-anatomy and microstructure, and has steadily gained traction in the cardiac community. Its wider use in small animal cardiac imaging in vivo has been constrained by its extreme sensitivity to motion, exaggerated by the high heart rates usually seen in rodents. Imaging of the isolated heart eliminates respiratory motion and, if conducted on arrested hearts, cardiac pulsation. This serves as an important intermediate step for basic and translational studies. However, investigating the micro-structural basis of cardiac deformation in the same heart requires observations in different deformation states.

Here, we illustrate the imaging of isolated rat hearts in three mechanical states mimicking diastole (cardioplegic arrest), left-ventricular (LV) volume overload (cardioplegic arrest plus LV balloon inflation), and peak systole (lithium-induced contracture). An optimised MRI-compatible Langendorff perfusion setup with the radio-frequency (RF) coil integrated into the wet chamber was developed for use in a 9.4T horizontal bore scanner. Signal-to-noise ratio improved significantly, by 75% compared to a previous design with external RF coil, and stability tests showed no significant changes in mean T₁, T₂ or LV wall thickness over a 170 min period. In contracture, we observed a significant reduction in mean fractional anisotropy from 0.32 ± 0.02 to 0.28 ± 0.02, as well as a significant rightward shift in helix angles with a decrease in the proportion of left-handed fibres, as referring to the locally prevailing cell orientation in the heart, from 24.9% to 23.3%, and an increase in the proportion of right-handed fibres from 25.5% to 28.4%. LV overload, in contrast, gave rise to a decrease in the proportion of left-handed fibres from 24.9% to 21.4% and an increase in the proportion of right-handed fibres from 25.5% to 26.0%. The modified perfusion and coil setup offers better performance and control over cardiac contraction states.

We subsequently performed high-resolution diffusion spectrum imaging (DSI) and 3D whole heart fibre tracking in fixed ex vivo rat hearts in slack state and contracture. As a model-free method, DSI augmented the measurements of water diffusion by also informing on multiple intra-voxel diffusion orientations and non-Gaussian diffusion. This enabled us to identify the transition from right- to left-handed fibres from the subendocardium to the subepicardium, as well as voxels in apical regions that were traversed by multiple fibres. We observed that both the mean generalised fractional anisotropy and mean kurtosis were lower in hearts in contracture compared to the slack state, by 23% and 9.3%, respectively. While its heavy acquisition burden currently limits the application of DSI in vivo, ongoing work in acceleration techniques may enable its use in live animals and patients. This would provide access to the as yet unexplored dimension of non-Gaussian diffusion that could serve as a highly sensitive marker of cardiac micro-structural integrity.

Phase-aligned multiple spin-echo averaging: a simple way to improve signal-to-noise ratio of in vivo mouse spinal cord diffusion tensor image

PURPOSE

To improve signal-noise-ratio of in vivo mouse spinal cord diffusion tensor imaging using-phase aligned multiple spin-echo technique.

MATERIAL AND METHODS

In vivo mouse spinal cord diffusion tensor imaging maps generated by multiple spin-echo and conventional spin-echo diffusion weighting were examined to demonstrate the efficacy of multiple spin-echo diffusion sequence to improve image quality and throughput. Effects of signal averaging using complex, magnitude and phased images from multiple spin-echo diffusion weighting were also assessed. Bayesian probability theory was used to generate phased images by moving the coherent signals to the real channel to eliminate the effect of phase variation between echoes while preserving the Gaussian noise distribution. Signal averaging of phased multiple spin-echo images potentially solves both the phase incoherence problem and the bias of the elevated Rician noise distribution in magnitude image. The proposed signal averaging with Bayesian phase-aligned multiple spin-echo images approach was compared to the conventional spin-echo data acquired with doubling the scan time. The diffusion tensor imaging parameters were compared in the mouse contusion spinal cord injury. Significance level (p-value) and effect size (Cohen's d) were reported between the control and contused spinal cord to inspect the sensitivity of each approach in detecting white matter pathology.

RESULTS

Compared to the spin-echo image, the signal-noise-ratio increased to 1.84-fold using the phased image averaging and to 1.30-fold using magnitude image averaging in the spinal cord white matter. Multiple spin-echo phased image averaging showed improved image quality of the mouse spinal cord among the tested methods. Diffusion tensor imaging metrics obtained from multiple spin-echo phased images using three echoes and two averages closely agreed with those derived by spin-echo magnitude data with four averages (two times more in acquisition time). The phased image averaging correctly reflected pathological features in contusion spinal cord injury.

CONCLUSION

Our in vivo imaging results indicate that averaging the phased multiple spin-echo images yields an 84% signal-noise-ratio increase over the spin-echo images and a 41% gain over the magnitude averaged multiple spin-echo images with equal acquisition time. Current results from the animal model of spinal cord injury suggest that the phased multiple spin-echo images could be used to improve signal-noise-ratio.

Assessment of cervical spondylotic myelopathy using diffusion tensor magnetic resonance imaging parameter at 3.0 tesla

STUDY DESIGN

Cross-sectional study.

OBJECTIVE

To assess spinal cord condition in patients with cervical spondylosis (CS), using diffusion tensor imaging parameter.

SUMMARY OF BACKGROUND DATA

Although myelopathy is a common symptom after CS, clinically objective assessment for determination of surgical intervention is not straightforward.

METHODS

Twenty-six patients with CS and 30 normal control subjects were enrolled. Diffusion tensor imaging was obtained using a single-shot fast spin-echo-based sequence at 3.0 T. Mean diffusivity (MD) and fractional anisotropy (FA) were measured in the axial plane at 6 spinal levels. To evaluate MD and FA in patients with CS considering the normal variation at each spinal level and between spinal levels, MD and FA at the most compressed spinal level were transformed to normalized values with a z score. Presence of myelopathy was predicted with the MD and FA z scores. Diagnostic validity of MD and FA was compared with receiver operating characteristic analysis. More effective parameter and the optimal cutoff value for prediction were determined.

RESULTS

In normal subjects, MD and FA were significantly different between spinal levels. In patients with myelopathy, an MD increase or an FA decrease was demonstrated in most cases. Although both an MD increase and an FA decrease had diagnostic validity for myelopathy, receiver operating characteristic analysis demonstrated a higher sensitivity and specificity for prediction of an MD increase than an FA decrease (areas under the curve for MD and FA were 0.903 and 0.760, respectively). An MD z score of 1.40 was considered to be the best diagnostic cutoff value with 100% sensitivity and 75% specificity.

CONCLUSION

Myelopathy can be predicted with high accuracy with diffusion tensor imaging parameter, with the MD z score at the most compressed spinal level.

LEVEL OF EVIDENCE

3.

Normal variation of diffusion tensor parameters of the spinal cord in healthy subjects at 3.0-Tesla

AIMS

The purposes of the present study were to clarify the normal variation and to determine the normal reference values of diffusion tensor (DT) parameters (mean diffusivity [MD] and fractional anisotropy [FA]) of the spinal cord in single-shot fast spin-echo-based sequence at 3.0-Tesla (3T).

MATERIALS AND METHODS

Thirty healthy subjects (mean age = 44.2 years, range = 20-72 years) were enrolled for this study. Mean values of MD and FA in six spinal levels (C2/3, C3/4, C4/5, C5/6, C6/7, and C7/Th1) were measured. Mean values, variances, and distributions of the MD and FA in each spinal level were analyzed. Age-dependent change of MD and FA as well as correlation between MD and FA was also analyzed.

RESULTS

At all spinal levels, the values can be considered to be Gaussian distribution in MD but not in FA. A significant statistical negative correlation was observed between aging and the values of MD (r = 0.429, P = 0.018), but insignificant between the values of FA (P = 0.234). A slight significant statistical negative correlation was observed between the values of MD and FA (r = 0.156, P = 0.037). One way repeated measures analysis of variance indicated the significant difference between the spinal levels in both MD (P = 0.003) and FA (P < 0.0001).

CONCLUSIONS

The analyzed data in the present study would be helpful for comparison when investigating the spinal condition of spinal disorders.

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.

Realignment capability of the nCPMG sequence

The nCPMG sequence is based on a particular phase modulation of the refocusing pulse train, and was originally designed for rendering the spin echo amplitude insensitive to the initial magnetization phase. This pulse sequence has the peculiarity of being easily invertible, which enables perfect driven equilibrium experiments, in the absence of relaxation. This magnetization 'realignment' is effective for all three components. Hence the overall operation is transparent. Supporting theory is presented here, together with the first direct experimental proof of the claim. The experiment shows that, with the present stabilization sequence, perfect realignment is indeed made possible for a range of refocusing pulse nutation angles from 130° to 230°.

Intravoxel incoherent motion imaging of tumor microenvironment in locally advanced breast cancer

Diffusion-weighted imaging plays important roles in cancer diagnosis, monitoring, and treatment. Although most applications measure restricted diffusion by tumor cellularity, diffusion-weighted imaging is also sensitive to vascularity through the intravoxel incoherent motion effect. Hypervascularity can confound apparent diffusion coefficient measurements in breast cancer. We acquired multiple b-value diffusion-weighted imaging at 3 T in a cohort of breast cancer patients and performed biexponential intravoxel incoherent motion analysis to extract tissue diffusivity (D(t)), perfusion fraction (f(p)), and pseudodiffusivity (D(p)). Results indicated significant differences between normal fibroglandular tissue and malignant lesions in apparent diffusion coefficient mean (±standard deviation) values (2.44 ± 0.30 vs. 1.34 ± 0.39 μm(2)/msec, P < 0.01) and D(t) (2.36 ± 0.38 vs. 1.15 ± 0.35 μm(2)/msec, P < 0.01). Lesion diffusion-weighted imaging signals demonstrated biexponential character in comparison to monoexponential normal tissue. There is some differentiation of lesion subtypes (invasive ductal carcinoma vs. other malignant lesions) with f(p) (10.5 ± 5.0% vs. 6.9 ± 2.9%, P = 0.06), but less so with D(t) (1.14 ± 0.32 μm(2)/msec vs. 1.18 ± 0.52 μm(2)/msec, P = 0.88) and D(p) (14.9 ± 11.4 μm(2)/msec vs. 16.1 ± 5.7 μm(2)/msec, P = 0.75). Comparison of intravoxel incoherent motion biomarkers with contrast enhancement suggests moderate correlations. These results suggest the potential of intravoxel incoherent motion vascular and cellular biomarkers for initial grading, progression monitoring, or treatment assessment of breast tumors.

High efficiency, low distortion 3D diffusion tensor imaging with variable density spiral fast spin echoes (3D DW VDS RARE)

We present an acquisition and reconstruction method designed to acquire high resolution 3D fast spin echo diffusion tensor images while mitigating the major sources of artifacts in DTI-field distortions, eddy currents and motion. The resulting images, being 3D, are of high SNR, and being fast spin echoes, exhibit greatly reduced field distortions. This sequence utilizes variable density spiral acquisition gradients, which allow for the implementation of a self-navigation scheme by which both eddy current and motion artifacts are removed. The result is that high resolution 3D DTI images are produced without the need for eddy current compensating gradients or B(0) field correction. In addition, a novel method for fast and accurate reconstruction of the non-Cartesian data is employed. Results are demonstrated in the brains of normal human volunteers.

Diffusion-weighted imaging with apparent diffusion coefficient mapping and spectroscopy in prostate cancer

Prostate cancer is a major health problem, and the exploration of noninvasive imaging methods that have the potential to improve specificity while maintaining high sensitivity is still critically needed. Tissue changes induced by tumor growth can be visualized by magnetic resonance imaging (MRI) methods. Current MRI methods include conventional T2-weighted imaging, diffusion-weighted imaging (DWI) with apparent diffusion coefficient (ADC) mapping and magnetic resonance spectroscopy (MRS). Techniques such as DWI/ADC provide functional information about the behavior of water molecules in tissue; MRS can provide biochemical information about the presence or absence of certain metabolites, such as choline, creatine, and citrate. Finally, vascular parameters can be investigated using dynamic contrast-enhanced MRI. Moreover, with whole-body MRI and DWI, metastatic disease can be evaluated in 1 session and may provide a way to monitor treatment. Therefore, when combining these various methods, a multiparametric data set can be built to assist in the detection, localization, assessment of prostate cancer aggressiveness, and tumor staging. Such a comprehensive approach offers more power to evaluate prostate disease than any single measure alone. In this article, we focus on the role of DWI/ADC and MRS in the detection and characterization using both in vivo and ex vivo imaging of prostate pathology.

Diffusion-weighted MR imaging of the kidneys and the urinary tract

There is currently a growing interest in applications of diffusion-weighted imaging (DWI) in the abdomen and pelvis. DWI provides original functional information where the signal and contrast are determined by the microscopic mobility of water. DWI can provide additional information over conventional MR sequences, and could potentially be used as an alternative to contrast-enhanced sequences in patients with chronic renal insufficiency at risk of nephrogenic systemic fibrosis. We provide an overview on basic physics background on DWI applied to the kidneys, and we summarize the current available data, including our recent experience.

Diffusion-weighted imaging of the appendicular skeleton with a non-Carr-Purcell-Meiboom-Gill single-shot fast spin-echo sequence

OBJECTIVE

The objective of our study was to prospectively evaluate the signal-to-noise ratio (SNR) improvement in diffusion-weighted imaging (DWI) of the appendicular skeleton with the use of a newly developed non-Carr-Purcell-Meiboom-Gill (non-CPMG) single-shot fast spin-echo (SSFSE) sequence and to evaluate its effect on apparent diffusion coefficient (ADC) measurements.

SUBJECTS AND METHODS

DWI of the bone was performed in 32 patients with an echo-planar imaging (EPI)-based sequence followed by a non-CPMG SSFSE technique. SNR and ADC values were measured over a lesion-free right femoral head. A score was assigned for each set of images to assess image quality. When a bone lesion was present, contrast-to-noise ratio (CNR) and ADC were also measured. Paired Student's t tests were used for statistical analysis.

RESULTS

The mean (+/- SD) SNR values were 9.89 +/- 2.20 and 81.68 +/- 4.87 for EPI and non-CPMG SSFSE DWI, respectively. SNR values associated with the non-CPMG SSFSE technique were found to be significantly higher than those measured with the EPI-based DWI technique (p < 0.01). Mean ADCs of the bone were 0.57 +/- 0.20 and 0.29 +/- 0.15 x 10(-3) mm2/s, respectively, for EPI and non-CPMG SSFSE DWI. Image quality scores were higher for the non-CPMG SSFSE DWI technique (p < 0.05) than for the EPI-based DWI technique. Overall lesion CNR was found to be higher in DWI performed with the non-CPMG SSFSE technique.

CONCLUSION

The non-CPMG SSFSE technique provides a significant improvement over the currently used EPI-based DWI technique and has the potential to be a powerful tool in imaging the appendicular skeleton.

Mean diffusivity, fractional anisotropy maps, and three-dimensional white-matter tractography by diffusion tensor imaging. Comparison between single-shot fast spin-echo and single-shot echo-planar sequences at 1.5 Tesla

Single-shot fast spin-echo (SSFSE)-based magnetic resonance imaging (MRI) has been introduced as a technique with less distortion and fewer artifacts for diffusion tensor imaging (DTI). The purpose of this study was to compare mean diffusivity maps, fractional anisotropy (FA) maps, and three-dimensional white-matter tractography using data obtained with SSFSE diffusion-tensor MRI technique and the much more common DTI method, echo-planar imaging (EPI), in the brain using a 1.5-Tesla clinical MR imager. Thirty patients with neurological disorders were scanned with both SSFSE-DTI and EPI-DTI using comparable scan times. Mean diffusivity and FA maps were calculated from the SSFSE-DTI and EPI-DTI data and qualitatively compared using two criteria. Three-dimensional fiber tracking was also performed on each data set. SSFSE-DTI produced image artifacts less frequently than EPI-DTI. However, demonstration of three-dimensional fiber-tracking of white matter on SSFSE-DTI was inferior to that on EPI-DTI. In conclusion, SSFSE-DTI is a promising alternative to conventional EPI-DTI imaging, producing fewer image artifacts and geometric distortions. However, for 3D streamline fiber-tracking, EPI data produced more consistent and reliable results.

Diffusion-weighted MRI in prostate cancer -- comparison between single-shot fast spin echo and echo planar imaging sequences

Diffusion-weighted (DW) MRI at 1.5 T was carried out in two groups of patients. MRI data were correlated with the biopsy and histopathology (where available). The performance of two sequences -- a single-shot FSE (14 patients) and a single-shot EPI (15 patients) -- was compared. Average ADC values from the normal peripheral zone (PZ), central gland (CG) and the tumour [prostate carcinoma (PCa)] were calculated from b values of 0 and 600. Tukey-Kramer test was used for statistical analysis. EPI produced higher values of ADC (10(-3) mm(2)/s) than FSE sequence: 1.992+/-0.208 vs. 1.573+/-0.270 in PZ (P<.001), 1.518+/-0.126 vs. 1.373+/-0.179 in CG and 1.214+/-0.254 vs. 0.993+/-0.158 in PCa (P<.01). In conclusion, both EPI and FSE sequences showed differences in ADC between normal PZ, CG and PCa; however, EPI produced significantly higher ADC values than FSE.

Correlated displacement-T2 MRI by means of a Pulsed Field Gradient-Multi Spin Echo Method

A method for correlated displacement-T2 imaging is presented. A Pulsed Field Gradient-Multi Spin Echo (PFG-MSE) sequence is used to record T2 resolved propagators on a voxel-by-voxel basis, making it possible to perform single voxel correlated displacement-T2 analyses. In spatially heterogeneous media the method thus gives access to sub-voxel information about displacement and T2 relaxation. The sequence is demonstrated using a number of flow conducting model systems: a tube with flowing water of variable intrinsic T2's, mixing fluids of different T2's in an "X"-shaped connector, and an intact living plant. PFG-MSE can be applied to yield information about the relation between flow, pore size and exchange behavior, and can aid volume flow quantification by making it possible to correct for T2 relaxation during the displacement labeling period Delta in PFG displacement imaging methods. Correlated displacement-T2 imaging can be of special interest for a number of research subjects, such as the flow of liquids and mixtures of liquids or liquids and solids moving through microscopic conduits of different sizes (e.g., plants, porous media, bioreactors, biomats).

Robust prescan calibration for multiple spin-echo sequences: application to FSE and b-SSFP

The collection of fast imaging techniques that use multiple spin-echo (MSE) sequences relies on a precise phase relationship between spin echoes and stimulated echoes that form along the radiofrequency refocusing pulse train. Failure to achieve this condition produces dark banding artifacts that result from destructive interference between signal coherence pathways. Satisfying this condition on the microsecond timescale required is technically challenging for conditions involving strong diffusion-weighted gradients, for arbitrary orientation acquisitions and at large field strengths with high-resolution acquisitions. Two clinically significant MSE sequences, fast spin echo (FSE) and balanced steady-state free precession (b-SSFP), are investigated in this work using a 4-T whole-body scanner. We developed a readout-projection-based prescan technique that ensures coherent signal formation by utilizing banding artifacts to automatically adjust gradient balance. Subsequent image acquisition using the results of this prescan permits the formation of coherent-echo images, which are robust under challenging imaging conditions. The robustness of this approach is demonstrated for FSE and b-SSFP images obtained from the knees of human volunteers. We believe that the use of this prescan calibration technique for the alignment of signal pools in MSE sequences is critical at high fields and will facilitate the implementation of high-quality clinically significant sequences such as FSE and b-SSFP.

Diffusion tensor imaging and fiber tractography in acute stroke

Diffusion tensor imaging (DTI) permits the quantitative evaluation of white matter pathology using measures of diffusion anisotropy. Fiber tractography based on DTI can reveal the three-dimensional white matter connectivity of the human brain. DTI fiber tractography is used to localize stroke lesions in relation to functionally important pathways and to assess wallerian degeneration, which may allow more accurate prognosis of long-term recovery or disability. DTI also improves the evaluation of hypoxic-ischemic injury to the developing brain of newborns and infants. DTI and fiber tractography may prove useful in elucidating alterations in brain connectivity resulting from neuroplasticity after stroke.

Diffusion tensor imaging of the human optic nerve using a non-CPMG fast spin echo sequence

PURPOSE

To investigate the diffusion tensor properties of the human optic nerve in vivo using a non-Carr-Purcell-Meiboom-Gill (CPMG) fast spin echo (FSE) sequence.

MATERIALS AND METHODS

This non-CPMG FSE sequence, which is based on a quadratic phase modulation of the refocusing pulses, allows diffusion measures to be acquired with full signal and without artifacts from geometric distortions due to magnetic field inhomogeneities, which are among the main problems encountered in the orbital area.

RESULTS

Good-quality images were obtained at a resolution of 0.94 x 0.94 x 3 mm. The mean diffusivity (MD) and fractional anisotropy (FA) were respectively 1.1 +/- 0.2 x 10(-3) mm(2)/second and 0.49 +/- 0.06, reflecting the optic nerve anisotropy.

CONCLUSION

This non-CPMG-FSE sequence provides reliable diffusion-weighted images of the human optic nerve. This approach could potentially improve the diagnosis and management of optic nerve diseases or compression, such as optic neuritis, orbit tumors, and muscle hypertrophy.

Single-shot fast spin-echo diffusion tensor imaging of the brain and spine with head and phased array coils at 1.5 T and 3.0 T

In this study, we investigated the use of a single-shot fast spin-echo-based sequence to perform diffusion tensor imaging (DTI) with improved anatomic fidelity through the entire brain and the cervical spine. Traditionally, diffusion tensor images have been acquired by single-shot echo-planar imaging (EPI) methods in which large distortions result from magnetic susceptibility effects, especially near air-tissue interfaces. These distortions can be problematic, especially in anterior and inferior portions of the brain, and they also can severely limit applications in the spine. At higher magnetic fields these magnetic susceptibility artifacts are increased. The single-shot fast spin-echo (SSFSE) method used in this study utilizes radiofrequency rephasing in the transverse plane and thus provides diffusion images with negligible distortion even at 3 Tesla. In addition, the SSFSE sequence does not require multiple fast-receivers, which are not available on many magnetic resonance (MR) systems. Phased array coils were used to increase the signal-to-noise ratio of the images, offering a major inherent advantage in diffusion tensor imaging of the spine and brain. The mean diffusion measurements obtained with the SSFSE acquisition were not statistically different (p > 0.05) from EPI-based acquisitions. Compared to routine T(2)-weighted MR images, the DTI-EPI sequence showed up to 20% in elongation of the brain in the anterior-posterior direction on a sagittal image due to magnetic susceptibility distortions, whereas in the DTI-SSFSE, the image distortions were negligible. The diffusion tensor SSFSE method was also able to assess diffusion abnormalities in a brain stem hemorrhage, unaffected by the spatial distortions that limited conventional EPI acquisition.