Marshall S. Sussman

Optimized spiral imaging for measurement of myocardial T2 relaxation.

Microcirculation oxygen levels and blood volumes should be reflected in measurements of myocardial T2 relaxation. This work describes the optimization of a spiral imaging strategy for robust myocardial T2 measurement to minimize the standard deviation of T2 measurement (σT2). Theoretical and experimental studies of blurring at muscle/blood interfaces enabled the derivation of parameter sets which reduce σ T2 to the level of 5%. T2 relaxation mapping within healthy volunteers provided estimation of residual σT2 within the optimized technique. The standard deviation in T2 measurement across regions of interest (ROIs) in different locations is about 9%. The standard deviation in T2 measurement in an ROI across different time points is about 5%. Magn Reson Med 49:1089–1097, 2003.

Evaluation of Diffusion Tensor Imaging and Fiber Tractography of the Median Nerve: Preliminary Results on Intrasubject Variability and Precision of Measurements

OBJECTIVE The purposes of this study were to determine the intrasubject side-to-side variability of quantitative and qualitative measures of diffusion tensor imaging (DTI) and fiber tractography of the median nerves and to determine the precision of quantitative measurements and fiber tractography. SUBJECTS AND METHODS Fifteen healthy volunteers (seven men, eight women; mean age, 31.2 years) underwent DTI of both wrists with a single-shot spin-echo-based echo-planar imaging sequence (TR/TE, 7,000/103; b value 1,025 s/mm2). Postprocessing included fiber tractography and quantitative analysis of fiber length, fiber density index, fractional anisotropy, apparent diffusion coefficient, and signal-to-noise ratio. Two readers in consensus graded the quality of fiber tract images of the two wrists as equal, slightly different, or very different. Fiber tractography and all analyses were repeated after 3 weeks, and the images from the two sessions were compared. RESULTS No statistically significant side-to-side differences in quantitative data were found (p=0.054-0.999). In all subjects, the quality of fiber tract images of the right and left median nerves was either slightly or very different. Between the initial and the second quantitative analyses, no statistically significant differences (p=0.086-0.898) were found, and the quality of fiber tract images was rated equal for nine of 15 subjects (60%) and slightly different for six of 15 subjects (40%). CONCLUSION Preliminary results indicate that quantitative evaluation of DTI of the median nerve is precise. The absence of statistically significant intrasubject side-to-side variability in quantitative data suggests that the healthy contralateral nerve can be used as an internal control. Observed side-to-side variability in the quality of fiber tract images, however, rules out side-to-side comparisons in fiber tractography.

Variable‐density adaptive imaging for high‐resolution coronary artery MRI

Variable‐density (VD) spiral k‐space acquisitions are used to acquire high‐resolution (0.78 mm), motion‐compensated images of the coronary arteries. Unlike conventional methods, information for motion compensation is obtained directly from the coronary anatomy itself. Specifically, periods of minimal coronary distortion are identified by applying the correlation coefficient template matching algorithm to real‐time images generated from the inner, high‐density portions of the VD spirals. Combining the data associated with these images together, high‐resolution, motion‐compensated coronary images are generated. Because coronary motion is visualized directly, the need for cardiac‐triggering, breath‐holding, and navigator echoes is eliminated. The motion compensation capability of the technique is determined by the inner‐spiral spatial and temporal resolution. Results indicate that the best performance is achieved using inner‐spiral images with high spatial resolution (1.6–2.9 mm), even though temporal resolution (four to six independent frames per second) suffers as a result. Image quality within the template region in healthy volunteers was found to be comparable to that achieved with cardiac‐triggered breath‐hold scans, although extended acquisition times of around 5 min were needed to overcome reduced SNR efficiency. Magn Reson Med 48:753–764, 2002.

Factors affecting the correlation coefficient template matching algorithm with application to real-time 2-D coronary artery MR imaging

This paper characterizes factors affecting the accuracy of the correlation coefficient (CC) template matching algorithm, as applied to motion tracking from two-dimensional real-time coronary artery magnetic resonance images. The performance of this algorithm is analyzed in the presence of both random and systematic error. In the presence of random error, it is shown that a necessary and sufficient condition for accurate motion tracking is a large CC difference-to-noise ratio (CCDNR). The CCDNR itself is in turn affected by five factors: image and template size, image and template structure, and the magnitude of the noise. Techniques are introduced for manipulating some of these factors in order to increase the CCDNR for greater motion tracking accuracy. In the presence of superimposed systematic error it is shown that, while large CCDNR is necessary, it alone is not sufficient to ensure accurate motion tracking. Techniques are developed for improving motion tracking accuracy that minimize the effects of systematic error, while maintaining an adequate CCDNR level. The ability of these techniques to improve motion tracking accuracy is demonstrated both in phantoms and in coronary artery images.

Quantitative MR imaging evaluation of the cartilage thickness and subchondral bone area in patients with ACL-reconstructions 7 years after surgery

OBJECTIVE To evaluate the cartilage thickness (ThC) and subchondral bone area (tAB) of the operated and contra-lateral non-operated (healthy) knees in patients with anterior cruciate ligament (ACL)-reconstruction 7 years after surgery using a quantitative and regional cartilage MR imaging (qMRI) technique. METHODS Charts of 410 patients with ACL-reconstructions were retrospectively reviewed. Fifty-two patients (male/female, 28/24; mean age, 33.3 years) were included. Patients underwent KT-1000 testing and qMRI of both knees using coronal fat-saturated 3D spoiled gradient-recalled echo (SPGR) sequences (TR/TE, 44/4 ms) at 1.5 T. Quantitative analyses of ThC and tAB in the femoro-tibial cartilage plates were performed using a subregional approach. In addition, qualitative and quantitative assessment of femoral condyle shapes was performed. t tests with Bonferroni corrections were used for statistical analysis of side-to-side differences between the operated and non-operated knees. RESULTS KT-1000 testing was abnormal in 3/52 patients (6%). Lateral femoral tAB was significantly lower (-9.2%), and medial tibial tAB was significantly larger (+2%) in the operated vs non-operated knee (P<0.001). Regional and subregional ThC side-to-side differences were less than 0.1mm and, except for the external lateral femoral subregion, they were not statistically significant. Flattened and broader shapes of medial femoral condyles (P<0.001) were found in operated knees. No significant association of presence of cartilage or meniscus lesions at surgery with ThC 7 years post-operatively was found (P=0.06-0.98). CONCLUSION There is evidence for changes in the tAB and femoral shape 7 years post-ACL-reconstruction, but no side-to-side differences in subregional ThC were found between the operated and contra-lateral non-operated knees.

T2*-weighted and arterial spin labeling MRI of calf muscles in healthy volunteers and patients with chronic exertional compartment syndrome: preliminary experience.

OBJECTIVE The purpose of our study was to assess temporal changes with exercise in T2* and arterial spin labeling signals in patients with chronic exertional compartment syndrome of the anterior compartment of the lower leg and in control subjects using T2* mapping and arterial spin labeling MRI. SUBJECTS AND METHODS This prospective study was approved by the institutional research ethics board. Ten control subjects (five women and five men; mean age, 29.0 years) and nine patients with chronic exertional compartment syndrome (three women and six men; mean age, 33.7 years) gave informed written consent and underwent MRI of the calf muscles using an axial T2*-weighted multiecho gradient-recalled echo and a flow-sensitive alternating inversion recovery sequence with echo-planar imaging readouts before (baseline) and 3, 6, 9, 12, and 15 minutes after exercise. T2* and arterial spin labeling signal changes (DeltaT2* and DeltaASL, respectively) over time were calculated relative to the baseline examination. DeltaT2* and DeltaASL between patients and control subjects were compared using the Students t test. RESULTS In both patients and control subjects, DeltaT2* and DeltaASL showed a peak at 3 minutes after exercise, followed by a decrease over time. The maximum DeltaT2* was 26% and 29% for patients and control subjects, respectively. The maximum DeltaASL was 183% and 224% for patients and control subjects, respectively. After 15 minutes, arterial spin labeling signal returned to baseline; however, T2* remained elevated (8% in patients; 10% in control subjects). No statistically significant differences between patients and control subjects in postexercise DeltaT2* and DeltaASL were found (p = 0.21-0.98). CONCLUSION After calf muscle exercise, no statistically significant differences in T2* relaxation times or arterial spin labeling signal, indicative of differences in muscle oxygenation and perfusion status, were found between patients with chronic exertional compartment syndrome and control subjects.

Delayed gadolinium-enhanced MR imaging of articular cartilage: three-dimensional T1 mapping with variable flip angles and B1 correction.

PURPOSE To develop and verify the accuracy of a rapid imaging protocol for delayed gadolinium-enhanced magnetic resonance (MR) imaging of cartilage that was based on three-dimensional (3D) spoiled gradient-recalled acquisition in the steady state (SPGR) sequences with variable flip angles (FAs) (VFAs) and where a correction method for B(1) field inhomogeneities was applied. MATERIALS AND METHODS The institutional research ethics board approved this study. Written informed consent was obtained from all subjects. A B(1) field inhomogeneity correction method was applied to a 3D SPGR pulse sequence with VFAs (repetition time msec/echo time msec, 7.1/3.3; FAs, 2 degrees , 5 degrees , 10 degrees , and 20 degrees ) and was used to perform delayed gadolinium-enhanced MR imaging of cartilage 3D T1 measurements at 1.5 T. The 3D T1 measurements were validated with the reference standard (the results of T1 mapping by using a single-section two-dimensional [2D] inversion-recovery [IR] fast spin-echo [SE] pulse sequence in vitro and in vivo) in six healthy volunteers. RESULTS T1 values calculated from 3D T1 maps were not significantly different from reference T1 values in vitro (P = .195) and in vivo (P = .52) when a B(1) field inhomogeneity correction method was applied. In vivo T1 mapping of the articular surface of the whole femoropatellar joint, including data acquisition, was performed in approximately 8 minutes of acquisition time at a spatial resolution of 0.55 x 0.55 x 3.00 mm. CONCLUSION Rapid T1 mapping by using 3D SPGR acquisitions with a VFA approach and a correction for B(1) field inhomogeneities can be used for delayed gadolinium-enhanced MR imaging of cartilage. T1 measurements performed in vitro and in vivo by using this approach are highly accurate when compared with those performed by using standard 2D IR fast SE T1 mapping as a reference.

Diffusion Tensor Imaging and Fiber Tractography of Skeletal Muscle: Optimization of b Value for Imaging at 1.5 T

OBJECTIVE The aim of our study was to determine the optimal b value for 1.5-T diffusion tensor (DT) MRI and fiber tractography of in vivo human skeletal muscle. SUBJECTS AND METHODS Five healthy volunteers were included in this prospective study. DT MRI of the proximal calf was performed with 15 directions of diffusion sensitization and parallel imaging with an acceleration factor of 2. Each single-shot spin-echo echo-planar imaging sequence was performed in each volunteer with eight b values ranging from 125 s/mm(2) to 1,000 s/mm(2). Fiber tractography of two calf muscles (anterior tibialis and lateral gastrocnemius) was performed, and image quality was ranked 1-8. Fractional anisotropy, signal-to-noise ratio, and fiber density index were assessed for both muscles at each b value. Statistical analysis was performed with repeated measurement analysis of variance. RESULTS Among all fiber tractographic images assessed, those obtained at a b value of 625 s/mm(2) were qualitatively ranked first (best image quality) or second (second-best image quality). Signal-to-noise ratio was highest for both muscles at a b value of 125 s/mm(2) (anterior tibialis, 66.2; lateral gastrocnemius, 60.1). The highest fiber density indexes were found for both muscles at a b value of 625 s/mm(2) (anterior tibialis index, 7.82; lateral gastrocnemius index, 5.75). In the analysis of variance no significant differences were found between fractional anisotropy and b values for either muscle (p = 0.359 and p = 0.078). CONCLUSION The optimal b value for in vivo DT MRI and tractographic assessment of human skeletal muscle in the calf at 1.5 T MRI was found to be 625 s/mm(2).

Impact of motion on T1 mapping acquired with inversion recovery fast spin echo and rapid spoiled gradient recalled-echo pulse sequences for delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) in volunteers.

To evaluate the impact of motion on T1 values acquired by using either inversion‐recovery fast spin echo (IR‐FSE) or three‐dimensional (3D) spoiled gradient recalled‐echo (SPGR) sequences for delayed gadolinium‐enhanced magnetic resonance imaging of cartilage (dGEMRIC) in volunteers.

Diffusion tensor magnetic resonance imaging of the human calf: comparison between 1.5 T and 3.0 T-preliminary results.

Objectives:To compare diffusion tensor-magnetic resonance imaging (DT-MRI) of human calf muscles at 1.5 T and 3.0 T, and to measure a number of quantitative parameters to characterize diffusion anisotropy in organized muscle tissue using similar imaging parameters. Methods and Materials:After Institutional Review Board approval and informed consent, five healthy volunteers were studied. Imaging was performed on both 1.5 T and 3.0 T MR systems using the similar imaging protocol. Diffusion-sensitized single-shot spin-echo echo planar imaging pulse sequences were used to collect 2-dimensional images through the calf. Imaging was performed using b-values of 0, 300, 500, and 700 s/mm2. Image analyses and tensor calculations were performed offline using DT imaging studio (Johns Hopkins University, Baltimore, MD). The eigenvalues (λ1, λ2, λ3), trace of the diffusion tensor (Tr⟨D⟩), fractional anisotropy, relative anisotropy, and volume ratio were calculated in 3 different calf muscles (medial and lateral gastrocnemius and soleus). Signal-to-noise ratios (SNRs) were compared for both field strengths (1.5 T and 3.0 T), the different muscles and all b-values. A regression analysis was performed to look at within-subject effects (linear mixed effect model). Results:No significant differences were found between all quantitative measured DT-MRI parameters, b-values, and muscle groups at 3.0 T and 1.5 T (P = 0.105; P = 0.719). The mean of SNR on the 2 different field strengths (3.0:1.5 T) was 1.64, which was significantly different (P < 0.0001). Significant differences in SNR in all 3 muscles were found between sequences using b = 300 s/mm2 and 700 s/mm2 (P < 0.001; P = 0.006) and between sequences using b = 300 s/mm2 and 500 s/mm2 (P < 0.001; P = 0.03), and 500 s/mm2 and 700 s/mm2 (P = 0.005; P = 0.03), respectively, for medial gastrocnemius and soleus muscle. Conclusions:This study demonstrates useful parameters to perform DT-MRI at 1.5 T and 3.0 T. DT-MRI at 1.5 T and 3.0 T provide in vivo validation of quantitative structural analysis of human skleletal muscle.