Musculoskeletal MRI at 3.0T and 7.0T: Relaxation Times and Image Contrast

+1Jordan, C D; 1Saranathan, M; 2Bangerter N K; 1Hargreaves B A; 1Gold G E +1Stanford University, CA, 2Brigham Young University, Provo, UT

cjordan@stanford.edu

Introduction: Magnetic resonance imaging (MRI) at higher magnetic field strengths such as 7.0T offers several advantages over imaging at lower magnetic field strengths. First, signal-to-noise ratio (SNR) is expected to increase with increasing field strength due to the increased spin polarization. The increased SNR can often be traded for higher spatial resolution and shorter scan time [1]. Improved spatial resolution allows visualization of small structures in the soft tissues. Certain cartilage imaging techniques for osteoarthritis, such as sodium imaging, are well suited for 7.0T. Knowledge of the T1 and T2 relaxation times of musculoskeletal tissues is important for obtaining image contrast similar to existing methods at lower field strengths, and the T2 relaxation time can also be used to measure biochemically significant changes in cartilage and muscle, and monitor musculoskeletal disease progression. Current 7.0T human MRI systems are research only, and many relaxation parameters at 7.0T are still unknown [2]. T1 is expected to increase with increasing field strength, while T2 is expected to decrease slightly [3-5]. The purpose of this study was to measure and compare the T1 and T2 relaxation times of musculoskeletal tissues at 3.0T and 7.0T, and to use these measurements to select appropriate parameters for musculoskeletal MRI methods at 7.0T that have similar image contrast to current 3.0T clinical protocols.

Methods: T1 and T2 relaxation times were measured in the knees of five healthy volunteers for each set of relaxation measurements at both 3.0T and 7.0T. The T1 relaxation times were measured using a spin-echo inversion recovery sequence with six inversion times. T2 relaxation times were measured using a spin-echo sequence with seven echo times. Accuracy of the T1 and T2 measurements was verified in phantoms. IRB approval was obtained for all human scans. A single axial slice with a 16cm field-of-view (FOV) was acquired through the patella and included five musculoskeletal tissue types: patellar cartilage; lateral gastrocnemius muscle; femoral bone marrow; patellofemoral joint fluid; and medial subcutaneous fat. A region of interest (ROI) was drawn within each tissue type, and the mean signal of pixel signal intensity was recorded for each echo or inversion time. The size of the ROI varied for each tissue type to minimize partial volume effects. Curves were fit using a nonlinear least squares regression to calculate T1 and T2 relaxation times, and the regression coefficient was calculated to measure the goodness of fit. Relaxation times were averaged across all subjects and the standard deviations of the measurements were calculated. The Wilcoxon signed-rank test was used to test for differences between the 3.0T and 7.0T measurements. Based on the measured T1 and T2, the fluid-cartilage image contrast ratio was estimated for the following 3.0T clinical sequences: proton density (PD) fast-spin-echo (FSE), T2-weighted FSE, and 3D-FSE-Cube. Using these ratios, we calculated the repetition time (TR) and echo time (TE) for each sequence to create similar contrast at 7.0T, as shown in Figure 1. Results: In vivo results: Table 1 shows the measured T1 relaxation times at both 3.0T and 7.0T, the percent increase, and the p-value, demonstrating a significant increase from 3.0T to 7.0T for all tissues. The regression coefficient r2 was greater than or equal to 0.97 for all T1 fits. Table 2 shows the measured T2 relaxation times at both 3.0T and 7.0T, the percent decrease, and the p-value demonstrating significant decrease for all tissues. The regression coefficient r2 was greater than or equal to 0.98 for all T2 fits. Musculoskeletal Methods Comparison: The relaxation times were used to develop examples of very high-resolution protocols at 7.0T for a PD-FSE sequence (4.8x reduction in voxel volume at 7.0T), a T2-weighted FSE sequence (3.2x reduction in voxel volume at 7.0T), and a 3D-FSE-Cube sequence (1.2x reduction in voxel volume at 7.0T) that had a similar fluid-cartilage contrast as the standard clinical protocols at 3.0T. SNR efficiency was higher at 7.0T for all tissues for all three protocols.

Table 1: T1 values (ms) Tissue At 3.0T At 7.0T Increase p Cartilage 1015.6±71.1 1568±335.0 54.4% 0.028 Subcutaneous Fat 403.8±17.7 583.1±22.1 44.4% 2.4e-5 Bone Marrow 381.2±8.0 548.7±11.8 43.9% 1.4e-5 Muscle 1255.9±57.9 1553±97.3 23.6% 0.007 Synovial Fluid 2565±269.7 4813±701.4 87.7% 0.007 Table 2: T2 values (ms) Tissue At 3.0T At 7.0T Decrease p Cartilage 39.1±1.3 31.7±3.4 18.9% 0.020 Subcutaneous Fat 48.3±0.8 46.1±1.7 4.5% 0.019 Bone Marrow 51.5±1.2 46.7±0.8 9.1% 0.001 Muscle 29.3±1.1 23.0±1.0 21.6% 0.003 Synovial Fluid 653±113.4 325±60.9 50.2% 0.016

Discussion: Knowledge of relaxation times at 7.0T can be used to design imaging protocols by using an appropriate signal equation to calculate the contrast ratios at different TEs and TRs [6]. We found T1 relaxation times were increased at 7.0T compared with 3.0T, and T2 relaxation times were decreased at 7.0T. The cartilage T1 values at 3.0T were consistent with literature T1 values from the deep zone of patellar cartilage, and these T1 values are lower than the T1 values in the superficial zone of patellar cartilage [7]. The low repetition time used (to keep scan times

reasonable) may affect the accuracy of the T1 and T2 measurements for synovial fluid, due to incomplete signal recovery. Therefore, there may be some variation and error in the relaxation measurements of fluid. Significance: T1 and T2 relaxation times in musculoskeletal tissues have been measured and compared between 3.0T and 7.0T field strengths in a single subject population. Knowledge of these relaxation parameters is important for parameter selection when designing imaging protocols at 7.0T. Acknowledgements: Richard M. Lucas Foundation, NCRR Center of Advanced MR Technology at Stanford P41RR09784. NIH EB002524, Arthritis Foundation, GE Healthcare. References: [1] Collins CM, et. al. MRM 2001;45(4):684-91. [2] Regatte RR, et. al.. JMRI 2007;25(2):262-9. [3] Vaughan JT, et al. MRM 2009;61(1):244-8. [4] Bottomley PA, et. al. Med Phys 1984;11 (4):425-48. [5] Welsch GH, et al. Eur Radiol 2011;21(6):1136-43. [6]    Bernstein  MA,  et.  al.  Handbook  of  MRI  pulse  sequences.  2004.  [7]    Watanabe  A,  et.  al.  JMRI  2007;26(1):109-­‐17.

3.0T 3D-FSE-CUBE 7.0T 3D-FSE-CUBE

7.0T PD-FSE 3.0T PD-FSE

3.0T T2w-FSE 7.0T T2w-FSE

Poster No. 0461 • ORS 2012 Annual Meeting