Time-dependent diffusion in skeletal muscle with the random permeable barrier model (RPBM): application to normal controls and chronic exertional compartment syndrome patients.

The purpose of this work was to carry out diffusion tensor imaging (DTI) at multiple diffusion times Td in skeletal muscle in normal subjects and chronic exertional compartment syndrome (CECS) patients and analyze the data with the random permeable barrier model (RPBM) for biophysical specificity. Using an institutional review board approved HIPAA-compliant protocol, seven patients with clinical suspicion of CECS and eight healthy volunteers underwent DTI of the calf muscle in a Siemens MAGNETOM Verio 3 T scanner at rest and after treadmill exertion at four different T(d) values. Radial diffusion values λ(rad) were computed for each of seven different muscle compartments and analyzed with RPBM to produce estimates of free diffusivity D(0), fiber diameter a, and permeability κ. Fiber diameter estimates were compared with measurements from literature autopsy reference for several compartments. Response factors (post/pre-exercise ratios) were computed and compared between normal controls and CECS patients using a mixed-model two-way analysis of variance. All subjects and muscle compartments showed nearly time-independent diffusion along and strongly time-dependent diffusion transverse to the muscle fibers. RPBM estimates of fiber diameter correlated well with corresponding autopsy reference. D(0) showed significant (p < 0.05) increases with exercise for volunteers, and a increased significantly (p < 0.05) in volunteers. At the group level, response factors of all three parameters showed trends differentiating controls from CECS patients, with patients showing smaller diameter changes (p = 0.07), and larger permeability increases (p = 0.07) than controls. Time-dependent diffusion measurements combined with appropriate tissue modeling can provide enhanced microstructural specificity for in vivo tissue characterization. In CECS patients, our results suggest that high-pressure interfiber edema elevates free diffusion and restricts exercise-induced fiber dilation. Such specificity may be useful in differentiating CECS from other disorders or in predicting its response to either physical therapy or fasciotomy.

Breast MRI at 7 Tesla with a bilateral coil and robust fat suppression.

PURPOSE: To develop a bilateral coil and fat suppressed T1-weighted sequence for 7 Tesla (T) breast MRI. MATERIALS AND METHODS: A dual-solenoid coil and three-dimensional (3D) T1w gradient echo sequence with B1+ insensitive fat suppression (FS) were developed. T1w FS image quality was characterized through image uniformity and fat-water contrast measurements in 11 subjects. Signal-to-noise ratio (SNR) and flip angle maps were acquired to assess the coil performance. Bilateral contrast-enhanced and unilateral high resolution (0.6 mm isotropic, 6.5 min acquisition time) imaging highlighted the 7T SNR advantage. RESULTS: Reliable and effective FS and high image quality was observed in all subjects at 7T, indicating that the custom coil and pulse sequence were insensitive to high-field obstacles such as variable tissue loading. 7T and 3T image uniformity was similar (P=0.24), indicating adequate 7T B1+ uniformity. High 7T SNR and fat-water contrast enabled 0.6 mm isotropic imaging and visualization of a high level of fibroglandular tissue detail. CONCLUSION: 7T T1w FS bilateral breast imaging is feasible with a custom radiofrequency (RF) coil and pulse sequence. Similar image uniformity was achieved at 7T and 3T, despite different RF field behavior and variable coil-tissue interaction due to anatomic differences that might be expected to alter magnetic field patterns.

Breast MRI at 7 Tesla with a bilateral coil and T1-weighted acquisition with robust fat suppression: image evaluation and comparison with 3 Tesla.

OBJECTIVES: To evaluate the image quality of T1-weighted fat-suppressed breast MRI at 7 T and to compare 7-T and 3-T images. METHODS: Seventeen subjects were imaged using a 7-T bilateral transmit-receive coil and 3D gradient echo sequence with adiabatic inversion-based fat suppression (FS). Images were graded on a five-point scale and quantitatively assessed through signal-to-noise ratio (SNR), fibroglandular/fat contrast and signal uniformity measurements. RESULTS: Image scores at 7 and 3 T were similar on standard-resolution images (1.1 × 1.1 × 1.1-1.6 mm(3)), indicating that high-quality breast imaging with clinical parameters can be performed at 7 T. The 7-T SNR advantage was underscored on 0.6-mm isotropic images, where image quality was significantly greater than at 3 T (4.2 versus 3.1, P ≤ 0.0001). Fibroglandular/fat contrast was more than two times higher at 7 T than at 3 T, owing to effective adiabatic inversion-based FS and the inherent 7-T signal advantage. Signal uniformity was comparable at 7 and 3 T (P < 0.05). Similar 7-T image quality was observed in all subjects, indicating robustness against anatomical variation. CONCLUSION: The 7-T bilateral transmit-receive coil and adiabatic inversion-based FS technique produce image quality that is as good as or better than at 3 T. KEY POINTS: • High image quality bilateral breast MRI is achievable with clinical parameters at 7 T. • 7-T high-resolution imaging improves delineation of subtle soft tissue structures. • Adiabatic-based fat suppression provides excellent fibroglandular/fat contrast at 7 T. • 7- and 3-T 3D T1-weighted gradient-echo images have similar signal uniformity. • The 7-T dual solenoid coil enables bilateral imaging without compromising uniformity.

Stimulated echo diffusion tensor imaging and SPAIR T2 -weighted imaging in chronic exertional compartment syndrome of the lower leg muscles.

PURPOSE: To evaluate the performance of diffusion tensor imaging (DTI) in the evaluation of chronic exertional compartment syndrome (CECS) as compared to T2 -weighted (T2w) imaging. MATERIALS AND METHODS: Using an Institutional Review Board (IRB)-approved, Health Insurance Portability and Accountability Act (HIPAA)-compliant protocol, spectral adiabatic inversion recovery (SPAIR) T2w imaging and stimulated echo DTI were applied to eight healthy volunteers and 14 suspected CECS patients before and after exertion. Longitudinal and transverse diffusion eigenvalues, mean diffusivity (MD), and fractional anisotropy (FA) were measured in seven calf muscle compartments, which in patients were classified by their response on T2w: normal (<20% change), and CECS (>20% change). Mixed model analysis of variance compared subject groups and compartments in terms of response factors (post/pre-exercise ratios) of DTI parameters. RESULTS: All diffusivities significantly increased (P < 0.0001) and FA decreased (P = 0.0014) with exercise. Longitudinal diffusion responses were significantly smaller than transversal diffusion responses (P < 0.0001). Nineteen of 98 patient compartments were classified as CECS on T2w. MD increased by 3.8 ± 3.4% (volunteer), 7.4 ± 4.2% (normal), and 9.1 ± 7.0% (CECS) with exercise. CONCLUSION: DTI shows promise as an ancillary imaging method in the diagnosis and understanding of the pathophysiology in CECS. Future studies may explore its utility in predicting response to treatment.

Highly accelerated single breath-hold noncontrast thoracic MRA: evaluation in a clinical population.

OBJECTIVES: The objective of this study was to evaluate the performance of a highly accelerated breath-hold 3-dimensional noncontrast-enhanced steady-state free precession thoracic magnetic resonance angiography (NC-MRA) technique in a clinical population, including assessment of image quality, aortic dimensions, and aortic pathology, compared with electrocardiographically gated gadolinium-enhanced MRA (Gd-MRA). MATERIALS AND METHODS: After approval from the institution board and informed consent were obtained, 30 patients (22 men; mean age, 53.4 years) with known or suspected aortic pathology were imaged with NC-MRA followed by Gd-MRA at a single examination at 1.5 T. Images were made anonymous and reviewed by 2 readers for aortic pathology and diagnostic confidence on a 5-point scale (1, worst; 5, best) on a patient basis. Image quality and artifacts were also evaluated in 10 vascular segments: aortic annulus, sinuses of Valsalva, sinotubular junction, ascending aorta, aortic arch, descending aorta, diaphragmatic aorta, great vessel origins, and the left main and right coronary artery origins. Finally, aortic dimensions were measured in each of the 7 aortic segments. The Wilcoxon signed rank test was used to compare diagnostic confidence, image quality, and artifact scores between NC-MRA and Gd-MRA. The paired Student t test and Bland-Altman analysis were used for comparison of aortic dimensions. RESULTS: All patients completed NC-MRA and Gd-MRA successfully. Vascular pathologic findings were concordant with Gd-MRA in 29 of 30 (96.7%) patients and 28 of 30 (93.3%) patients for readers 1 and 2, respectively, with high diagnostic confidence (mean [SD], 4.35 [0.77]) not significantly different from Gd-MRA (4.38 [0.64]; P = 0.74). The image quality and artifact scores were comparable with Gd-MRA in most vascular segments. Notable differences were observed at the ascending aorta, where Gd-MRA had superior image quality (4.13 [0.73]) compared with NC-MRA (3.80 [0.88]; P = 0.028), and at the coronary artery origins where NC-MRA was considered superior (NC-MRA vs Gd-MRA, 3.38 [1.47] vs 2.78 [1.21] for the left main artery and NC-MRA vs Gd-MRA, 3.55 [1.40] vs 2.32 [1.16] for the right coronary artery; P < 0.05, both comparisons). The aortic dimensions were comparable, with the only significant difference observed at the ascending aorta, where NC-MRA dimension (4.05 [0.76]) was less than 1 mm smaller than that of Gd-MRA (4.12 [0.7]; P = 0.043). CONCLUSIONS: Breath-hold NC-MRA of the thoracic aorta yields good image quality, comparable to Gd-MRA, with high accuracy for aortic dimension and pathology. It can be considered as an alternative to Gd-MRA in patients with relative contraindications to gadolinium contrast or problems with intravenous access.

Hippocampal blood flow in normal aging measured with arterial spin labeling at 3T.

Due to methodological difficulties related to the small size, variable distribution of hippocampal arteries, and the location of the hippocampus in the proximity of middle cranial fossa, little is known about hippocampal blood flow (HBF). We have tested the utility of a pulsed arterial spin labeling sequence based on multi-shot true fast imaging in steady precession to measure HBF in 34 normal volunteers (17 women, 17 men, 26-92 years old). Flow sensitivity to a mild hypercapnic challenge was also examined. Coregistered 3D MPRAGE sequence was used to eliminate from hippocampal and cortical regions of interest all voxel with <75% of gray matter. Large blood vessels were also excluded. HBF in normal volunteers averaged 61.2 ± 9.0 mL/(100 g min). There was no statistically significant age or gender effect. Under a mild hypercapnia challenge (end tidal CO(2) pressure increase of 6.8 ± 1.9 mmHg over the baseline), HBF response was 14.1 ± 10.8 mL/(100 g min), whereas cortical gray matter flow increased by 18.0 ± 12.2 mL/(100 g min). Flow response among women was significantly larger than in the men. The average absolute difference between two successive HBF measures was 3.6 mL/(100 g min) or 5.4%. The 3T true fast imaging in steady precession arterial spin labeling method offers a HBF measurement strategy that combines good spatial resolution, sensitivity, and minimal image distortions.

Image-guided radio-frequency gain calibration for high-field MRI.

High-field (≥ 3T) MRI provides a means to increase the signal-to-noise ratio, due to its higher tissue magnetization compared with 1.5T. However, both the static magnetic field (B(0)) and the transmit radio-frequency (RF) field (B 1+) inhomogeneities are comparatively higher at higher field strengths than those at 1.5T. These challenging factors at high-field strengths make it more difficult to accurately calibrate the transmit RF gain using standard RF calibration procedures. An image-based RF calibration procedure was therefore developed, in order to accurately calibrate the transmit RF gain within a specific region-of-interest (ROI). Using a turbo fast low-angle shot (TurboFLASH) pulse sequence with centric k-space reordering, a series of 'saturation-no-recovery' images was acquired by varying the flip angle of the preconditioning pulse. In the resulting images, the signal null occurs in regions where the flip angle of the preconditioning pulse is 90°. For a given ROI, the mean signal can be plotted as a function of the nominal flip angle, and the resulting curve can be used to quantitatively identify the signal null. This image-guided RF calibration procedure was evaluated through phantom and volunteer imaging experiments at 3T and 7T. The image-guided RF calibration results in vitro were consistent with standard B(0) and B 1+ maps. The standard automated RF calibration procedure produced approximately 20% and 15-30% relative error in the transmit RF gain in the left kidney at 3T and brain at 7T, respectively. For initial application, a T(2) mapping pulse sequence was applied at 7T. The T(2) measurements in the thalamus at 7T were 60.6 ms and 48.2 ms using the standard and image-guided RF calibration procedures, respectively. This rapid, image-guided RF calibration procedure can be used to optimally calibrate the flip angle for a given ROI and thus minimize measurement errors for quantitative MRI and MR spectroscopy.

Hybrid adiabatic-rectangular pulse train for effective saturation of magnetization within the whole heart at 3 T.

Uniform T(1)-weighting is a major challenge for first-pass cardiac perfusion MRI at 3 T. Previously proposed adiabatic amplitude of radiofrequency field (B(1))-insensitive rotation (BIR-4) pulse and standard and tailored pulse trains of three nonselective pulses have been important developments but each pulse has limitations at 3 T. As an extension of the tailored pulse train, we developed a hybrid pulse train by synergistically combining two nonselective rectangular radiofrequency pulses and an adiabatic half-passage pulse, in order to achieve effective saturation of magnetization within the heart, while remaining within clinically acceptable specific absorption rate limits. The standard pulse train, tailored pulse train, hybrid pulse train, and BIR-4 pulse train were evaluated through numerical, phantom, and in vivo experiments. Among the four saturation pulses, only the hybrid pulse train yielded residual magnetization <2% of equilibrium magnetization in the heart while remaining within clinically acceptable specific absorption rate limits for multislice first-pass cardiac perfusion MRI at 3 T.

Three-dimensional electrocardiographically gated variable flip angle FSE imaging for MR angiography of the hands at 3.0 T: initial experience.

After institutional review board approval and informed consent were obtained for this HIPAA-compliant investigation, a three-dimensional electrocardiographically gated variable flip angle (VFA) fast spin-echo magnetic resonance (MR) angiography technique was evaluated as an unenhanced method for imaging hand arteries in 13 subjects (including four patients) at 3.0 T; this included evaluation of vessel visualization with warming and cooling in seven subjects. Examinations were evaluated for image quality and vessel conspicuity. Clear separation of arteries from veins was achieved in all subjects, with excellent vessel conspicuity and depiction of stenoses. Warming improved vessel visualization in healthy volunteers. VFA MR angiography is a high-spatial-resolution technique that enables the assessment of vascular reactivity in response to temperature challenge.

Imaging the female pelvis at 3.0 T.

Three-Tesla whole body imaging is rapidly becoming part of routine clinical practice. Although it is generally thought that pelvic imaging at 3.0 T will be beneficial because of increased signal to noise and greater spectral separation, adjustments in protocol and sequence parameters are necessary to optimize image quality. The question remains as to whether 3.0-T imaging will offer further benefits beyond 1.5 T in terms of lesion characterization and functional imaging. This article aims to address safety concerns and to illustrate the potential benefits and technical challenges of imaging the female pelvis at 3.0 T. Imaging protocols and sequence parameters for routine gynecologic indications are suggested, and potential clinical applications at 3.0 T are discussed such as magnetic resonance spectroscopy, perfusion, diffusion weighted imaging, and the use of alternate contrast agents.