Optimization of acquisition parameters of diffusion-tensor magnetic resonance imaging in the spinal cord

Diffusion tensor imaging of fetal spinal cord: feasibility and gestational-age-related changes

OBJECTIVES

Diffusion tensor imaging (DTI) of the fetal brain is a relatively new technique that allows evaluation of white matter tracts of the central nervous system throughout pregnancy, as well as in certain pathological conditions. The objectives of this study were to evaluate the feasibility of DTI of the spinal cord in utero and to examine gestational-age (GA)-related changes in DTI parameters during pregnancy.

METHODS

This was a prospective study conducted between December 2021 and June 2022 in the LUMIERE Platform, Necker-Enfants Malades Hospital, Paris, France, as part of the LUMIERE SUR LE FETUS trial. Women with a pregnancy between 18 and 36 weeks of gestation without fetal or maternal abnormality were eligible for inclusion. Sagittal diffusion-weighted scans of the fetal spine were acquired, without sedation, using a 1.5-Tesla magnetic resonance imaging scanner. The imaging parameters were as follows: 15 non-collinear direction diffusion-weighted magnetic-pulsed gradients with a b-value 700 s/mm2 and one B0 image without diffusion-weighting; slice thickness, 3 mm; field of view (FOV), 36 mm; phase FOV, 1.00; voxel size, 4.5 × 2.8 × 3 mm3 ; number of slices, 7-10; repetition time, 2800 ms; echo time, minimum; and total acquisition time, 2.3 min. DTI parameters, including fractional anisotropy (FA) and apparent diffusion coefficient (ADC), were extracted at the cervical, upper thoracic, lower thoracic and lumbar levels of the spinal cord. Cases with motion degradation and those with aberrant reconstruction of the spinal cord on tractography were excluded. Pearson's correlation analysis was performed to evaluate GA-related changes of DTI parameters during pregnancy.

RESULTS

During the study period, 42 pregnant women were included at a median GA of 29.3 (range, 22.0-35.7) weeks. Five (11.9%) patients were not included in the analysis because of fetal movement. Two (4.8%) patients with aberrant tractography reconstruction were also excluded from analysis. Acquisition of DTI parameters was feasible in all remaining cases (35/35). Increasing GA correlated with increasing FA averaged over the entire fetal spinal cord (r, 0.37; P < 0.01), as well as at the individual cervical (r, 0.519; P < 0.01), upper thoracic (r, 0.468; P < 0.01), lower thoracic (r, 0.425; P = 0.02) and lumbar (r, 0.427; P = 0.02) levels. There was no correlation between GA and ADC averaged over the entire spinal cord (r, 0.01; P = 0.99) or at the individual cervical (r, -0.109; P = 0.56), upper thoracic (r, -0.226; P = 0.22), lower thoracic (r, -0.052; P = 0.78) or lumbar (r, -0.11; P = 0.95) levels.

CONCLUSIONS

This study shows that DTI of the spinal cord is feasible in normal fetuses in typical clinical practice and allows extraction of DTI parameters of the spinal cord. There is a significant GA-related change in FA in the fetal spinal cord during pregnancy, which may result from decreasing water content as observed during myelination of fiber tracts occurring in utero. This study may serve as a basis for further investigation of DTI in the fetus, including research into its potential in pathological conditions that impact spinal cord development. © 2023 International Society of Ultrasound in Obstetrics and Gynecology.

Cervical Spinal Cord DTI Is Improved by Reduced FOV with Specific Balance between the Number of Diffusion Gradient Directions and Averages

BACKGROUND AND PURPOSE

Reduced-FOV DTI is promising for exploring the cervical spinal cord, but the optimal set of parameters needs to be clarified. We hypothesized that the number of excitations should be favored over the number of diffusion gradient directions regarding the strong orientation of the cord in a single rostrocaudal axis.

MATERIALS AND METHODS

Fifteen healthy individuals underwent cervical spinal cord MR imaging at 3T, including an anatomic 3D-Multi-Echo Recombined Gradient Echo, high-resolution full-FOV DTI with a NEX of 3 and 20 diffusion gradient directions and 5 sets of reduced-FOV DTIs differently balanced in terms of NEX/number of diffusion gradient directions: (NEX/number of diffusion gradient directions = 3/20, 5/16, 7/12, 9/9, and 12/6). Each DTI sequence lasted 4 minutes 30 seconds, an acceptable duration, to cover C1-C4 in the axial plane. Fractional anisotropy maps and tractograms were reconstructed. Qualitatively, 2 radiologists rated the DTI sets blinded to the sequence. Quantitatively, we compared distortions, SNR, variance of fractional anisotropy values, and numbers of detected fibers.

RESULTS

Qualitatively, reduced-FOV DTI sequences with a NEX of ≥5 were significantly better rated than the full-FOV DTI and the reduced-FOV DTI with low NEX (N = 3) and a high number of diffusion gradient directions (D = 20). Quantitatively, the best trade-off was reached by the reduced-FOV DTI with a NEX of 9 and 9 diffusion gradient directions, which provided significantly fewer artifacts, higher SNR on trace at b = 750 s/mm² and an increased number of fibers tracked while maintaining similar fractional anisotropy values and dispersion.

CONCLUSIONS

Optimized reduced-FOV DTI improves spinal cord imaging. The best compromise was obtained with a NEX of 9 and 9 diffusion gradient directions, which emphasizes the need for increasing the NEX at the expense of the number of diffusion gradient directions for spinal cord DTI contrary to brain DTI.

Comparison of Diffusion MRI Acquisition Protocols for the In Vivo Characterization of the Mouse Spinal Cord: Variability Analysis and Application to an Amyotrophic Lateral Sclerosis Model

Diffusion-weighted Magnetic Resonance Imaging (dMRI) has relevant applications in the microstructural characterization of the spinal cord, especially in neurodegenerative diseases. Animal models have a pivotal role in the study of such diseases; however, in vivo spinal dMRI of small animals entails additional challenges that require a systematical investigation of acquisition parameters. The purpose of this study is to compare three acquisition protocols and identify the scanning parameters allowing a robust estimation of the main diffusion quantities and a good sensitivity to neurodegeneration in the mouse spinal cord. For all the protocols, the signal-to-noise and contrast-to noise ratios and the mean value and variability of Diffusion Tensor metrics were evaluated in healthy controls. For the estimation of fractional anisotropy less variability was provided by protocols with more diffusion directions, for the estimation of mean, axial and radial diffusivity by protocols with fewer diffusion directions and higher diffusion weighting. Intermediate features (12 directions, b = 1200 s/mm²) provided the overall minimum inter- and intra-subject variability in most cases. In order to test the diagnostic sensitivity of the protocols, 7 G93A-SOD1 mice (model of amyotrophic lateral sclerosis) at 10 and 17 weeks of age were scanned and the derived diffusion parameters compared with those estimated in age-matched healthy animals. The protocols with an intermediate or high number of diffusion directions provided the best differentiation between the two groups at week 17, whereas only few local significant differences were highlighted at week 10. According to our results, a dMRI protocol with an intermediate number of diffusion gradient directions and a relatively high diffusion weighting is optimal for spinal cord imaging. Further work is needed to confirm these results and for a finer tuning of acquisition parameters. Nevertheless, our findings could be important for the optimization of acquisition protocols for preclinical and clinical dMRI studies on the spinal cord.

Quantifying the impact of underlying measurement error on cervical spinal cord diffusion tensor imaging at 3T

PURPOSE

To empirically characterize and quantify the impact of gradient weighting schemes on the appearance and fidelity of diffusion tensor imaging of the human spinal cord in vivo in clinically relevant scan time equivalents (STE).

MATERIALS AND METHODS

In five healthy controls at 3T, we evaluated test-retest reproducibility and performed voxelwise analysis of diffusion tensor imaging (DTI)-derived indices (fractional anisotropy [FA], mean [MD], axial [AD], and radial [RD] diffusivity) in the cervical spinal cord to assess spatial dependencies of measurement error and differences across three different sampling schemes (6, 15, and 32 directions) at STE of 4.5, 9, and 18 minutes. A subjective assessment was also performed.

RESULTS

With six directions, column-specific errors are highest (effect size = 2.9%, 4.4%, 7.2% for FA in dorsal column, lateral column, and gray matter) and different than the 15-direction scheme (P < 0.05). STE sequences with 15 and 32 directions exhibited small differences in error (P > 0.05). For FA and AD, measurement errors are prevalent in gray matter, while partial volume effects with cerebrospinal fluid heavily influence RD. Measurement errors decreased with increasing scan time (P < 0.01), albeit with diminishing returns at scan times longer than 9 minutes (P < 0.05).

CONCLUSION

A 15-direction scheme of 9 minutes yields measurements of the cervical spinal cord with low error. J. Magn. Reson. Imaging 2016;44:1608-1618.

Various diffusion magnetic resonance imaging techniques for pancreatic cancer

Pancreatic cancer is one of the most common malignant tumors and remains a treatment-refractory cancer with a poor prognosis. Currently, the diagnosis of pancreatic neoplasm depends mainly on imaging and which methods are conducive to detecting small lesions. Compared to the other techniques, magnetic resonance imaging (MRI) has irreplaceable advantages and can provide valuable information unattainable with other noninvasive or minimally invasive imaging techniques. Advances in MR hardware and pulse sequence design have particularly improved the quality and robustness of MRI of the pancreas. Diffusion MR imaging serves as one of the common functional MRI techniques and is the only technique that can be used to reflect the diffusion movement of water molecules in vivo. It is generally known that diffusion properties depend on the characterization of intrinsic features of tissue microdynamics and microstructure. With the improvement of the diffusion models, diffusion MR imaging techniques are increasingly varied, from the simplest and most commonly used technique to the more complex. In this review, the various diffusion MRI techniques for pancreatic cancer are discussed, including conventional diffusion weighted imaging (DWI), multi-b DWI based on intra-voxel incoherent motion theory, diffusion tensor imaging and diffusion kurtosis imaging. The principles, main parameters, advantages and limitations of these techniques, as well as future directions for pancreatic diffusion imaging are also discussed.

Diffusion tensor magnetic resonance imaging of the pancreas

PURPOSE

To develop a diffusion-tensor-imaging (DTI) protocol that is sensitive to the complex diffusion and perfusion properties of the healthy and malignant pancreas tissues.

MATERIALS AND METHODS

Twenty-eight healthy volunteers and nine patients with pancreatic-ductal-adenocacinoma (PDAC), were scanned at 3T with T2-weighted and DTI sequences. Healthy volunteers were also scanned with multi-b diffusion-weighted-imaging (DWI), whereas a standard clinical protocol complemented the PDAC patients' scans. Image processing at pixel resolution yielded parametric maps of three directional diffusion coefficients λ1, λ2, λ3, apparent diffusion coefficient (ADC), and fractional anisotropy (FA), as well as a λ1-vector map, and a main diffusion-direction map.

RESULTS

DTI measurements of healthy pancreatic tissue at b-values 0,500 s/mm² yielded: λ1 = (2.65±0.35)×10⁻³, λ2 = (1.87±0.22)×10⁻³, λ3 = (1.20±0.18)×10⁻³, ADC = (1.91±0.22)×10⁻³ (all in mm²/s units) and FA = 0.38±0.06. Using b-values of 100,500 s/mm² led to a significant reduction in λ1, λ2, λ3 and ADC (p<.0001) and a significant increase (p<0.0001) in FA. The reduction in the diffusion coefficients suggested a contribution of a fast intra-voxel-incoherent-motion (IVIM) component at b≤100 s/mm², which was confirmed by the multi-b DWI results. In PDACs, λ1, λ2, λ3 and ADC in both 0,500 s/mm² and 100,500 s/mm² b-values sets, as well as the reduction in these diffusion coefficients between the two sets, were significantly lower in comparison to the distal normal pancreatic tissue, suggesting higher cellularity and diminution of the fast-IVIM component in the cancer tissue.

CONCLUSION

DTI using two reference b-values 0 and 100 s/mm² enabled characterization of the water diffusion and anisotropy of the healthy pancreas, taking into account a contribution of IVIM. The reduction in the diffusion coefficients of PDAC, as compared to normal pancreatic tissue, and the smaller change in these coefficients in PDAC when the reference b-value was modified from 0 to 100 s/mm², helped identifying the presence of malignancy.

Optimization of the parameters for diffusion tensor magnetic resonance imaging data acquisition for breast fiber tractography at 1.5 T

INTRODUCTION

Diffusion tensor MRI has emerged as a promising tool for the analysis of the microscopic properties of tissues. Optimizing image acquisition parameters is essential for producing high-quality DTI. This study aimed to optimize the parameters for DTI data acquisition for breast fiber tractography at 1.5 T.

PATIENTS AND METHODS

A total of 21 healthy volunteers received breast DTI scanning using an ASSET-based EPI technique operated under different parameters including b value, the number of diffusion gradient directions, and spatial resolution. The images were analyzed for signal-to-noise, signal intensity ratio, mean number and length of reconstructive fiber tracts, and fractional anisotropy value.

RESULTS

The optimal acquisition parameters at 1.5 T for breast DT-MRI fiber tractography were determined as follows: axial 31 direction, b = 600 seconds per mm(2), matrix 128 × 128 with slice thickness of 3 mm.

CONCLUSION

The optimization of data acquisition parameters could improve the quality of breast DT-MRI images and assist fiber tractography at 1.5 T.

Feasibility of diffusion tensor tractography of brachial plexus injuries at 1.5 T

OBJECTIVE

The objective of this study was to examine the feasibility of diffusion tensor imaging and diffusion tensor tractography (DTT) at 1.5 T for the detection of nerve root avulsions in patients with brachial plexus injuries (BPI).

MATERIALS AND METHODS

We performed a 1.5-T magnetic resonance imaging on 28 patients (mean [SD] age, 25 [9.1]) with BPI using the following imaging protocol: (a) magnetic resonance myelography (MRM), (b) magnetic resonance neurography, and (c) diffusion tensor imaging. A reproducible tractography approach was developed to assess the myeloradicular continuity, which consists of multiple regions of interests placed on each hemicord, including the ventral and dorsal rootlets from C4 to T2 nerve roots. Two independent observers blindly evaluated DTT and MRM studies. The degree of agreement between DTT and MRM findings was estimated on a per-root basis on the 140 nerve roots (C5-T1) on the injured side by calculation of the κ coefficient (K value) and the Bland-Altman plot analysis. The diagnostic accuracy of DTT was assessed by comparing it with the MRM findings of the 140 nerve roots on the injured side on a per-root basis.

RESULTS

Diffusion tensor tractography allowed a complete visualization of the C5-T1 intact nerve roots on the normal side in 100% of studies.Complete nerve root avulsions were recognized on DTT either as a total loss of fibers or as a very short segment of incoherent fibers in apparent continuity with the spinal cord.The MRM identified 88 intact nerve roots (62.9%), 44 completely avulsed nerve roots (31.4%), and 8 partially avulsed nerve roots (5.7%). The DTT and MRM were concordant in 127 of the 140 nerve roots (90.7%) and exhibited an excellent overall agreement (K value, 80.8). The brachial plexus DTT had an 88.1% sensitivity, 98.1% positive predictive value, 98.8% specificity, 92.6 negative predictive value, and a 94.5% overall accuracy for detecting the presence of a nerve root avulsion. The κ coefficients for the interobserver reliability of DTT and MRM were 0.85 and 0.80, respectively.

CONCLUSIONS

Our results suggest that cervical nerve root avulsions can be successfully visualized at 1.5 T in patients with BPI despite the anatomical complexity and susceptibility and motion artifacts. We propose that DTT is a reliable and reproducible method for the investigation of BPI because it provides a successful anatomical and functional display of neural structures that are not otherwise attainable with conventional studies.

Improved in vivo diffusion tensor imaging of human cervical spinal cord

We describe a cardiac gated high in-plane resolution axial human cervical spinal cord diffusion tensor imaging (DTI) protocol. Multiple steps were taken to optimize both image acquisition and image processing. The former includes slice-by-slice cardiac triggering and individually tiltable slices. The latter includes (i) iterative 2D retrospective motion correction, (ii) image intensity outlier detection to minimize the influence of physiological noise, (iii) a non-linear DTI estimation procedure incorporating non-negative eigenvalue priors, and (iv) tract-specific region-of-interest (ROI) identification based on an objective geometry reference. Using these strategies in combination, radial diffusivity (λ(⊥)) was reproducibly measured in white matter (WM) tracts (adjusted mean [95% confidence interval]=0.25 [0.22, 0.29] μm(2)/ms), lower than previously reported λ(⊥) values in the in vivo human spinal cord DTI literature. Radial diffusivity and fractional anisotropy (FA) measured in WM varied from rostral to caudal as did mean translational motion, likely reflecting respiratory motion effect. Given the considerable sensitivity of DTI measurements to motion artifact, we believe outlier detection is indispensable in spinal cord diffusion imaging. We also recommend using a mixed-effects model to account for systematic measurement bias depending on cord segment.

Application of diffusion tensor imaging (DTI) in pathological changes of the spinal cord

We review the current knowledge concerning clinical applications of the advanced technique of magnetic resonance imaging (MRI): diffusion tensor imaging (DTI) of the spinal cord. Due to technical difficulties, DTI has rarely been used in spinal cord diseases. However, in our opinion it is potentially a very useful method in diagnosis of the different pathological processes of the spinal cord and spinal canal. We discuss the physical principles and technical aspects of DTI, as well as current and future applications. DTI seems to be a very promising method for assessment of spinal cord trauma, spinal canal tumors, degenerative myelopathy, as well as demyelinating and infectious diseases of the spinal cord. DTI enables both qualitative and quantitative (by measuring of the fractional anisotropy and apparent diffusion coefficient parameters) assessment of the spinal cord. The particular applications are illustrated by the examples provided in this article.

Diffusion tensor imaging and fiber tractography in cervical compressive myelopathy: preliminary results

OBJECTIVE

To assess diffusion tensor imaging (DTI) parameters in cervical compressive myelopathy (CCM) patients compared to normal volunteers, to relate them with myelopathy severity, and to relate tractography patterns with postoperative neurologic improvement.

SUBJECTS AND METHODS

Twenty patients suffering from CCM were prospectively enrolled (M:F = 13:7, mean age, 49.6 years; range 22-67 years) from September 2009 to March 2010. Sensitivity encoding (SENSE) single-shot echo-planar imaging (EPI) was used for the sagittal DTI. Twenty sex- and age-matched normal volunteers underwent the same scanning procedure. Fractional anisotropy (FA) and apparent diffusion coefficient (ADC) values in the spinal cord were compared between the patients and normal volunteers and were related to myelopathy severity based on Japanese Orthopedic Association (JOA) scores. Tractography patterns were related to myelopathy severity and postoperative improvement.

RESULTS

There were significant differences between patients and normal volunteers in terms of FA (0.498 ± 0.114 vs. 0.604 ± 0.057; p = 0.001) and ADC (1.442 ± 0.389 vs. 1.169 ± 0.098; p = 0.001). DTI parameters and tractography patterns were not related to myelopathy severity. In ten patients in the neurologically worse group, postoperative neurologic improvement was seen in four of five patients with intact fiber tracts, but only one of five patients with interrupted fiber tracts exhibited neurologic improvement.

CONCLUSION

DTI parameters in CCM patients were significantly different from those in normal volunteers but were not significantly related to myelopathy severity. The patterns of tractography appear to correlate with postoperative neurologic improvement.

Reduced field-of-view diffusion imaging of the human spinal cord: comparison with conventional single-shot echo-planar imaging

BACKGROUND AND PURPOSE

DWI of the spinal cord is challenging because of its small size and artifacts associated with the most commonly used clinical imaging method, SS-EPI. We evaluated the performance of rFOV spinal cord DWI and compared it with the routine fFOV SS-EPI in a clinical population.

MATERIALS AND METHODS

Thirty-six clinical patients underwent 1.5T MR imaging examination that included rFOV SS-EPI DWI of the cervical spinal cord as well as 2 comparison diffusion sequences: fFOV SS-EPI DWI normalized for either image readout time (low-resolution fFOV) or spatial resolution (high-resolution fFOV). ADC maps were created and compared between the methods by using single-factor analysis of variance. Two neuroradiologists blinded to sequence type rated the 3 DWI methods, based on susceptibility artifacts, perceived spatial resolution, signal intensity-to-noise ratio, anatomic detail, and clinical utility.

RESULTS

ADC values for the rFOV and both fFOV sequences were not statistically different (rFOV: 1.01 ± 0.18 × 10(-3) mm(2)/s; low-resolution fFOV: 1.12 ± 0.22 × 10(-3) mm(2)/s; high-resolution fFOV: 1.10 ± 0.21 × 10(-3) mm(2)/s; F = 2.747, P > .05). The neuroradiologist reviewers rated the rFOV diffusion images superior in terms of all assessed measures (P < 0.0001). Particular improvements were noted in patients with metal hardware, degenerative disease, or both.

CONCLUSIONS

rFOV DWI of the spinal cord overcomes many of the problems associated with conventional fFOV SS-EPI and is feasible in a clinical population. From a clinical standpoint, images were deemed superior to those created by using standard fFOV methods.

Diffusion tensor imaging and fibre tracking in cervical spondylotic myelopathy

OBJECTIVES

To (1) obtain microstructural parameters (Fractional Anisotropy: FA, Mean Diffusivity: MD) of the cervical spinal cord in patients suffering from cervical spondylotic myelopathy (CSM) using tractography, (2) to compare DTI parameters with the clinical assessment of these patients (3) and with information issued from conventional sequences.

METHODS

DTI was performed on 20 symptomatic patients with cervical spondylotic myelopathy, matched with 15 volunteers. FA and MD were calculated from tractography images at the C2-C3 level and compressed level in patients and at the C2-C3 and C4-C7 in controls. Patients were clinically evaluated using a self-administered questionnaire.

RESULTS

The FA values of patients were significantly lower at the compressed level than the FA of volunteers at the C4-C7 level. A significant positive correlation between FA at the compressed level and clinical assessment was demonstrated. Increased signal intensity on T2-weighted images did not correlate either with FA or MD values, or with any of the clinical scores.

CONCLUSION

FA values were significantly correlated with some of the patients' clinical scores. High signal intensity of the spinal cord on T2 was not correlated either with the DTI parameters or with the clinical assessment, suggesting that FA is more sensitive than T2 imaging.

White matter organization in cervical spinal cord relates differently to age and control of grip force in healthy subjects

Diffusion tensor imaging (DTI) can be used to elucidate relations between CNS structure and function. We hypothesized that the degree of spinal white matter organization relates to the accuracy of control of grip force. Healthy subjects of different age were studied using DTI and visuomotor tracking of precision grip force. The latter is a prime component of manual dexterity. A regional analysis of spinal white matter [fractional anisotropy (FA)] across multiple cervical levels (C2-C3, C4-C5, and C6-C7) and in different regions of interest (left and right lateral or medial spinal cord) was performed. FA was highest at the C2-C3 level, higher on the right than the left side, and higher in the lateral than in the medial spinal cord (p < 0.001). FA of whole cervical spinal cord (C2-C7) was lower in subjects with high tracking error (r = -0.56, p = 0.004) and decreased with age (r = -0.63, p = 0.001). A multiple regression analysis revealed an independent contribution of each predictor (semipartial correlations: age, r = -0.55, p < 0.001; tracking error, r = -0.49, p = 0.003). The closest relation between FA and tracking error was found at the C6-C7 level in the lateral spinal cord, in which the corticospinal tract innervates spinal circuitry controlling hand and digit muscles. FA of the medial spinal cord correlated consistently with age across all cervical levels, whereas FA of the lateral spinal cord did not. The results suggest (1) a functionally relevant specialization of lateral spinal cord white matter and (2) an increased sensitivity to age-related decline in medial spinal cord white matter in healthy subjects.

High-resolution diffusion tensor MR imaging for evaluating myocardial anisotropy and fiber tracking at 3T: the effect of the number of diffusion-sensitizing gradient directions

Objective

We wanted to evaluate the effect of the number of diffusion-sensitizing gradient directions on the image quality for evaluating myocardial anisotropy and fiber tracking by using in vitro diffusion tensor MR imaging (DT-MRI).

Materials and Methods

The DT-MR images, using a SENSE-based echoplanar imaging technique, were acquired from ten excised porcine hearts by using a 3T MR scanner. With a b-value of 800 s/mm², the diffusion tensor images were obtained for 6, 15 and 32 diffusion-sensitizing gradient directions at the midventricular level. The number of tracked fibers, the fractional anisotropy (FA), and the length of the tracked fibers were measured for the quantitative analysis. Two radiologists assessed the image quality of the fiber tractography for the qualitative analysis.

Results

By increasing the number of diffusion-sensitizing gradient directions from 6 to 15, and then to 32, the FA and standard deviation were significantly reduced (p < 0.01), and the number of tracked fibers and the length of the tracked fibers were significantly increased (p < 0.01). The image quality of the fiber tractography was significantly increased with the increased number of diffusion-sensitizing gradient directions (p < 0.01).

Conclusion

The image quality of in vitro DT-MRI is significantly improved as the number of diffusion-sensitizing gradient directions is increased.

A unified optimization approach for diffusion tensor imaging technique

An optimization approach for diffusion tensor imaging (DTI) technique is proposed, aiming to improve the estimates of tensors, fractional anisotropy (FA), and fiber directions. With the simulated annealing algorithm, the proposed approach simultaneously optimizes imaging parameters (gradient duration/separation, read-out time, and TE), b-values, and diffusion gradient directions either with or without incorporating prior knowledge of tensor fields. In addition, the method through which tensors are estimated, least squares in our study, was also considered in the optimization procedures. Monte-Carlo simulations were performed for three different scenarios of prior fiber distributions including fibers orientated in 1 (CONE1) and 3 (CONE3) cone areas (50 tensors orderly oriented within a diverging angle of 20 degrees in each cone) and a uniform fiber distribution (UNIF). In addition, three imaging acquisition schemes together with different signal-to-noise ratios were tested, including M/N=1/6, 2/12, and 5/30 for each prior fiber distribution where M and N were the number of b=0 and b>0 images, respectively. Our results show that the optimal b-value ranges between 0.7 and 1.0 x 10(9) s/m(2) for UNIF. However, the optimal b-value ranges become both higher and wider for CONE1 and CONE3 than that of UNIF. In addition, the biases and standard deviations (SD) of tensors, and SD of FA are substantially reduced and the accuracy of fiber directional estimates is improved using the proposed approach particularly in CONE1 when compared with the conventional approaches. Together, the proposed unified optimization approach may offer a direct and simultaneous means to optimize DTI experiments.

Diffusion tensor imaging in idiopathic acute transverse myelitis

OBJECTIVE

Our study was based on our hypotheses that in idiopathic acute transverse myelitis (ATM), fractional anisotropy (FA) values would be abnormal not only in the T2-hyperintense lesion but also in the surrounding normal-appearing spinal cord and that the abnormal FA values in the spinal cord could be related to clinical outcome.

SUBJECTS AND METHODS

Sagittal diffusion tensor imaging (DTI) was performed in 10 patients with idiopathic ATM (four men, six women; mean age, 45 years; age range, 20-66 years) and 10 sex- and age-matched normal volunteers. FA measurements were made in the spinal cord at three levels: lesion, proximal normal-appearing spinal cord, and distal normal-appearing spinal cord. The grade of FA decrease (mild, less than 10% decrease [(FA normal - FA pt) x 100 / FA normal]; moderate, 10-20%; severe, more than 20%) was related to the clinical outcome, which was determined by a neurologist using Paine's scale of normal, good, fair, or poor.

RESULTS

Mean FA values in patients were significantly lower than those in normal volunteers in lesions (0.5328 vs 0.7125, p = 0.002) and distal normal-appearing spinal cord (0.6676 vs 0.7720, p = 0.0137). All three patients with a mild FA decrease or increase in distal normal-appearing spinal cord showed a normal or good outcome, but all three patients with a severe FA decrease in distal normal-appearing spinal cord showed a fair outcome, among the eight patients to whom steroid treatment was given.

CONCLUSION

FA values in lesions and in distal normal-appearing spinal cord significantly decreased in patients with idiopathic ATM, and FA decrease in distal normal-appearing spinal cord might be related to clinical outcome.

Diagnostic value and surgical implications of the magnetic resonance imaging in the management of adult patients with brachial plexus pathologies

The imaging of the brachial plexus (BP) cannot easily be achieved because of the extension of the region to be studied, the different tissue compositions of the adjacent anatomic structures and the necessity to work with thin tissue slices. Here is a description of the brachial plexus MRI (bpMRI) protocol that we use for the pre-operative evaluation of patients affected by the following brachial plexus (BP) pathologies: neoplastic, post-traumatic, inflammatory. The surgical implications are discussed. A survey was done on 115 patients (66 males; 49 females; mean age 46.4 years old), which underwent a bpMRI over a 32-month period (32 post-traumatic, 26 neoplastic, 25 affected by inflammatory diseases). Our bpMRI protocol is evaluated according to each mechanism of disease. The bpMRI provides a topographic localization of the tumour showing the nerve dislocations or infiltration by the neoplasm. In traumatic patients we suggest that the bpMRI could be useful to detect the degree of axonal damage when the nerve lesion is in continuity. We also consider the diagnosis and the possible surgical implications for the inflammatory plexopathies. In conclusion, a standardized bpMRI protocol has an invaluable diagnostic value for the surgeons and neurologists. It is only limited by its duration (at least 45 min).

In vivo DTI of the healthy and injured cat spinal cord at high spatial and angular resolution

Spinal cord diffusion tensor imaging (DTI) is challenging in many ways: the small size of the cord, physiological motion and susceptibility artifacts pose daunting obstacles to the acquisition of high-quality data. Here, we present DTI results computed from in vivo studies of the healthy and injured spinal cord of five cats. Both high spatial (1.1 mm3) and angular (55 directions) resolutions were used to optimise modelling of the diffusion process. Also, particular effort was directed towards a strategy that limits susceptibility artifacts. For validation purposes, acquisitions were repeated in two cats before and after making a spinal lesion. As a result, various axonal trajectories were identified by tractography including dorsal and ventral columns as well as lateral tracts. Also, fibre bundles showed robust disruption at the site of spinal cord injuries (partial and complete) via tractography, accompanied with significantly lower fractional anisotropy values at the site of lesions. Important outcomes of this work are (i) tractography-based localisation of anatomical tracts in the thoraco-lumbar spinal cord and (ii) in vivo assessment of axonal integrity following experimental spinal cord injury.

[Diffusion tensor imaging and tractography of the brain and spinal cord]

Diffusion tensor imaging is a magnetic resonance imaging technique that provides details on tissue microstructure and organization well beyond the usual image resolution. With diffusion tensor imaging, diffusion anisotropy can be quantified and subtle white matter changes not normally seen on conventional MRI can be detected. The aim of this article is to review the principles of diffusion tensor imaging and fiber tracking and their applications to the study of the brain, including Alzheimer disease, neuropsychiatric disorders, strokes, multiple sclerosis, brain tumors, and intractable seizures. Emerging applications to spinal cord disorders are also presented.

Quantitative Metrics for Evaluating Parallel Acquisition Techniques in Diffusion Tensor Imaging at 3 Tesla

OBJECTIVES

Single-shot echo-planar based diffusion tensor imaging is prone to geometric and intensity distortions. Parallel imaging is a means of reducing these distortions while preserving spatial resolution. A quantitative comparison at 3 T of parallel imaging for diffusion tensor images (DTI) using k-space (generalized auto-calibrating partially parallel acquisitions; GRAPPA) and image domain (sensitivity encoding; SENSE) reconstructions at different acceleration factors, R, is reported here.

MATERIALS AND METHODS

Images were evaluated using 8 human subjects with repeated scans for 2 subjects to estimate reproducibility. Mutual information (MI) was used to assess the global changes in geometric distortions. The effects of parallel imaging techniques on random noise and reconstruction artifacts were evaluated by placing 26 regions of interest and computing the standard deviation of apparent diffusion coefficient and fractional anisotropy along with the error of fitting the data to the diffusion model (residual error).

RESULTS

The larger positive values in mutual information index with increasing R values confirmed the anticipated decrease in distortions. Further, the MI index of GRAPPA sequences for a given R factor was larger than the corresponding mSENSE images. The residual error was lowest in the images acquired without parallel imaging and among the parallel reconstruction methods, the R = 2 acquisitions had the least error. The standard deviation, accuracy, and reproducibility of the apparent diffusion coefficient and fractional anisotropy in homogenous tissue regions showed that GRAPPA acquired with R = 2 had the least amount of systematic and random noise and of these, significant differences with mSENSE, R = 2 were found only for the fractional anisotropy index.

CONCLUSION

Evaluation of the current implementation of parallel reconstruction algorithms identified GRAPPA acquired with R = 2 as optimal for diffusion tensor imaging.