Reliable chemical exchange saturation transfer imaging of human lumbar intervertebral discs using reduced‐field‐of‐view turbo spin echo at 3.0 T

The reduced field‐of‐view (rFOV) turbo‐spin‐echo (TSE) technique, which effectively suppresses bowel movement artifacts, is developed for the purpose of chemical exchange saturation transfer (CEST) imaging of the intervertebral disc (IVD) in vivo. Attempts to quantify IVD CEST signals in a clinical setting require high reliability and accuracy, which is often compromised in the conventionally used technique. The proposed rFOV TSE CEST method demonstrated significantly superior reproducibility when compared with the conventional technique on healthy volunteers, implying it is a more reliable measurement. Phantom study revealed a linear relation between CEST signal and glycosaminoglycan (GAG) concentration. The feasibility of detecting IVD degeneration was demonstrated on a healthy volunteer, indicating that the proposed method is a promising tool to quantify disc degeneration. Copyright © 2013 John Wiley & Sons, Ltd.     

Diffusion tensor imaging and T2 relaxometry of bilateral lumbar nerve roots: feasibility of in‐plane imaging

Lower back pain is a common problem frequently encountered without specific biomarkers that correlate well with an individual patient's pain generators. MRI quantification of diffusion and T₂ relaxation properties may provide novel insight into the mechanical and inflammatory changes that occur in the lumbosacral nerve roots in patients with lower back pain. Accurate imaging of the spinal nerve roots is difficult because of their small caliber and oblique course in all three planes. Two‐dimensional in‐plane imaging of the lumbosacral nerve roots requires oblique coronal imaging with large field of view (FOV) in both dimensions, resulting in severe geometric distortions using single‐shot echo planar imaging (EPI) techniques. The present work describes initial success using a reduced‐FOV single‐shot spin‐echo EPI acquisition to obtain in‐plane diffusion tensor imaging (DTI) and T₂ mapping of the bilateral lumbar nerve roots at the L4 level of healthy subjects, minimizing partial volume effects, breathing artifacts and geometric distortions. A significant variation in DTI and T₂ mapping metrics is also reported along the course of the normal nerve root. The fractional anisotropy is statistically significantly lower in the dorsal root ganglia (0.287 ± 0.068) than in more distal regions in the spinal nerve (0.402 ± 0.040) (p < 10^–5). The T₂ relaxation value is statistically significantly higher in the dorsal root ganglia (78.0 ± 11.9 ms) than in more distal regions in the spinal nerve (59.5 ± 7.4 ms) (p < 10^–5). The quantification of nerve root DTI and T₂ properties using the proposed methodology may identify the specific site of any degenerative and inflammatory changes along the nerve roots of patients with lower back pain. Copyright © 2012 John Wiley & Sons, Ltd.     

Implementation of a reduced field-of-view method for dynamic MR imaging using navigator echoes

A new technique was designed and implemented that increases imaging speed in dynamic imaging in which change is restricted to a fraction of the full field of view (FOV). The technique is an enhancement of a reduced FOV method first reported by Hu and Parrish. This enhancement extends the use of the Hu and Parrish method to cases in which there is cyclic motion throughout the entire FOV that normally would be aliased into the reduced FOV. This method requires the initial acquisition of a number of baseline k-space data sets to characterize the background physiological motion during imaging. Projection navigator echoes along both the phase- and the frequency-encoded directions are acquired and used to correct for motion outside the reduced FOV. Automatic placement or repositioning of the updated fraction of the FOV using navigators also is investigated. With this method, when using a 32-echo rapid acquisition with relaxation enhancement (RARE) sequence, single-shot updates of T2-weighted, 128 × 128 pixel images are obtained, yielding a fourfold increase in temporal resolution compared to full k-space update methods.     

Lymphomas and glioblastomas: differences in the apparent diffusion coefficient evaluated with high b-value diffusion-weighted magnetic resonance imaging at 3T

Background and purpose

As the usefulness of the apparent diffusion coefficient (ADC) obtained from diffusion-weighted images (DWI) for the differential diagnosis between glioblastoma and primary central nervous system lymphoma is controversial, we assessed whether high b-value DWI at b 4000 s/mm² could discriminate between glioblastoma and lymphoma. We also compared the power of high- and standard b-value (b-4000, b-1000) imaging on a 3-Tesla (3 T) magnetic resonance (MR) instrument.

Materials and methods

This study was approved by our Institutional Review Board. We acquired DWI at 3 T with b = 1000 and b = 4000 s/mm² in 10 patients with lymphoma and 14 patients with glioblastoma. The ADC was measured by placing multiple regions of interest (ROI) on ADC maps of the site of enhanced lesions on contrast-enhanced T1-weighted MR images. We avoided hemorrhagic and cystic lesions by using T1-, T2-, FLAIR-, and T2* MR images. The ADC values of each tumor were determined preoperatively from several ROI and expressed as the minimum-, mean-, and maximum ADC value (ADC_MIN, ADC_MEAN, ADC_MAX). We evaluated the relationship between ADCs and histological information including tumor cellularity.

Results

All ADC values were statistically associated with tumor cellularity. ADC_MIN at b-4000 was associated with tumor cellularity more significantly than ADC_MIN at b-1000. All ADC values were lower for lymphoma than glioblastoma and the statistical difference was larger at b = 4000- than b = 1000 s/mm². According to the results of discriminant analysis, the log likelihood was greatest for ADC_MIN at b = 4000. At a cut-off value of ADC_MIN = 0.500 × 10⁻³ mm²/s at b-4000 it was possible to differentiate between lymphoma and glioblastoma (sensitivity 90.9%, specificity 91.7%).

Conclusions

Calculating the ADC value is useful for distinguishing lymphoma from glioblastoma. The lowest degree of overlapping and a better inverse correspondence with tumor cellularity were obtained with ADC_MIN at b-4000 s/mm² at 3 T MRI.     

Evaluation of sonoelastography in Achilles tendon of healthy volunteers and patients with symptomatic Achilles tendon

Aim and objectives: To assess the role of SEL in evaluation of Achilles tendon. Methodology: This study included 40 healthy volunteers and 40 patients with symptomatic Achilles tendon. All patients were examined by conventional B-mode ultrasound, sonoelastography and MRI. Results: Achilles tendons of the volunteers were characterized by hard texture with higher strain ratios than those of the patients with Achilles tendinopathy. No significant differences could be detected between SR of male and female volunteers yet significant differences could be detected in the volunteer group above and below forty being lower with softer tendon properties in the group above forty with a cut-off value of ≤1.84 between healthy and diseased group. Sonoelastography had the sensitivity of 89.1% and specificity of 96.1% for diagnosis of tendinopathy with results superior to those of conventional B-mode ultrasonography. Conclusion: SEL is a reliable tool in the evaluation of Achilles tendinopathy with sensitivity and specificity superior to B-mode US. Strain ratio provides excellent non-invasive diagnostic data adds strongly in more objective evaluation of Achilles tendon properties.     

Rapid MR elastography using selective excitations.

Rapid MR elastography (MRE) techniques using spatially-selective excitations to reduce acquisition times to a few seconds or less were devised and tested. The techniques included reduced field of view (rFOV) MRE and 1D MRE (beam MRE) using 2D spatially selective RF excitations for gradient-echo (GRE) applications and intersecting 90 degrees and 180 degrees slice-selective excitations for spin-echo (SE) applications. It was shown that scan times could be reduced by a factor of 8 using rFOV MRE, and by 64 using beam MRE, while still obtaining stiffness estimates comparable to full-FOV MRE. Results were shown in gel phantom experiments as well as in the case of a preserved postmortem breast tissue specimen with a stiff lesion. These methods can be used to more rapidly interrogate regions of interest (ROIs) in tissue to quickly obtain information about the viscoelastic properties of that tissue.     

Multicontrast sequences with continuous table motion: a novel acquisition technique for extended field of view imaging.

A novel acquisition technique called multicontrast imaging is presented that provides multiple datasets of different image contrasts covering an extended field of view within one measurement procedure. The technique uses a continuously moving table and is based on the repetitive acquisition of axial volume sections while the patient moves through the scanner once. To compensate for the table motion during the measurement, adaptive slice shifting is applied. Multicontrast imaging is designed to combine the comfort of a moving table examination with the high time efficiency of a multitask protocol and can be used for generating differences in both contrast and spatial parameters of the acquired data. The technique and its properties are demonstrated on healthy human volunteers.     

Creating a library of generalized Fourier sampling patterns for irregular 2D regions of support.

Multiple-region MRI (mrMRI) represents a generalization of the Shannon sampling theorem to permit sparse k-space sampling whenever the scanned object or its high-contrast edges are confined to multiple known regions. Use of an optimal mrMRI sampling pattern produces an image with root-mean-squared (RMS) noise over the supporting regions equal to the RMS noise in a conventional Fourier image with the same total area of support. Analytical solutions for such sampling patterns have been described previously for all arrangements of two or three (noncollinear) supporting regions. This work describes a robust numerical method for creating a library of optimal and near-optimal mrMRI sampling patterns for more complicated geometries. The average noise amplification over all sampling patterns in the demonstration library was only 4%, with 30% of the sampling patterns resulting in no noise amplification whatsoever.     

Super‐resolution MRI using microscopic spatial modulation of magnetization

A new super‐resolution method is presented to overcome limitations of spatial resolution in MRI. In contrast to previous attempts that are based on simple field of view shifts, the new method additionally modulates the longitudinal magnetization within the imaging plane for each shift, allowing the acquisition of new and independent k‐space data. With this approach, resolution improvements in up to three dimensions are possible, and the total acquisition time linearly scales with the improvement factor for each dimension. First super‐resolution experiments in a geometric phantom and in brain tissue of two healthy volunteers clearly demonstrate the feasibility and advantages of this new method, which has the capability to extend current resolution limits in MRI. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.