Optimized efficient liver T(1ρ) mapping using limited spin lock times.

Department of Imaging and Interventional Radiology, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, People's Republic of China.
Physics in Medicine and Biology (Impact Factor: 2.92). 03/2012; 57(6):1631-40. DOI: 10.1088/0031-9155/57/6/1631
Source: PubMed

ABSTRACT T(1ρ) relaxation has recently been found to be sensitive to liver fibrosis and has potential to be used for early detection of liver fibrosis and grading. Liver T(1ρ) imaging and accurate mapping are challenging because of the long scan time, respiration motion and high specific absorption rate. Reduction and optimization of spin lock times (TSLs) are an efficient way to reduce scan time and radiofrequency energy deposition of T(1ρ) imaging, but maintain the near-optimal precision of T(1ρ) mapping. This work analyzes the precision in T(1ρ) estimation with limited, in particular two, spin lock times, and explores the feasibility of using two specific operator-selected TSLs for efficient and accurate liver T(1ρ) mapping. Two optimized TSLs were derived by theoretical analysis and numerical simulations first, and tested experimentally by in vivo rat liver T(1ρ) imaging at 3 T. The simulation showed that the TSLs of 1 and 50 ms gave optimal T(1ρ) estimation in a range of 10-100 ms. In the experiment, no significant statistical difference was found between the T(1ρ) maps generated using the optimized two-TSL combination and the maps generated using the six TSLs of [1, 10, 20, 30, 40, 50] ms according to one-way ANOVA analysis (p = 0.1364 for liver and p = 0.8708 for muscle).

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    ABSTRACT: Long scanning time greatly hinders the widespread application of spin-lattice relaxation in rotating frame ( T1ρ) in clinics. In this study, a novel method is proposed to reconstruct the T1ρ-weighted images from undersampled k-space data and hence accelerate the acquisition of T1ρ imaging. The proposed approach (PANDA- T1ρ) combined the benefit of PCA and dictionary learning when reconstructing image from undersampled data. Specifically, the PCA transform was first used to sparsify the image series along the parameter direction and then the sparsified images were reconstructed by means of dictionary learning and finally solved the images. A variation of PANDA- T1ρ was also developed for the heavy noise case. Numerical simulation and in vivo experiments were carried out with the accelerating factor from 2 to 4 to verify the performance of PANDA- T1ρ. The reconstructed T1ρ maps using the PANDA- T1ρ method were found to be comparable to the reference at all verified acceleration factors. Moreover, the variation exhibited better performance than the original version when the k-space data were contaminated by heavy noise. PANDA- T1ρ can significantly reduce the scanning time of T1ρ by integrating PCA and dictionary learning and provides better parameter estimation than the state-of-art methods for a fixed acceleration factor. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 01/2015; 73(1). DOI:10.1002/mrm.25130 · 3.40 Impact Factor
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    ABSTRACT: With biliary duct ligation and CCl4 induced rat liver fibrosis models, recent studies showed that MR T1rho imaging is able to detect liver fibrosis, and the degree of fibrosis is correlated with the degree of elevation of the T1rho measurements, suggesting liver T1rho quantification may play an important role for liver fibrosis early detection and grading. It has also been reported it is feasible to obtain consistent liver T1rho measurement for human subjects at 3 Tesla (3 T), and preliminary clinical data suggest liver T1rho is increased in patients with cirrhosis. In these previous studies, T1rho imaging was used with the rotary-echo spin-lock pulse for T1rho preparation, and number of signal averaging (NSA) was 2. Due to the presence of inhomogeneous B0 field, artifacts may occur in the acquired T1rho-weighted images. The method described by Dixon et al. (Magn Reson Med 1996;36:90-4), which is a hard RF pulse with 135° flip angle and same RF phase as the spin-locking RF pulse is inserted right before and after the spin-locking RF pulse, has been proposed to reduce sensitivity to B0 field inhomogeneity in T1rho imaging. In this study, we compared the images scanned by rotary-echo spin-lock pulse method (sequence 1) and the pulse modified according to Dixon method (sequence 2). When the artifacts occurred in T1rho images, we repeated the same scan until satisfactory. We accepted images if artifact in liver was less than 10% of liver area by visual estimation. When NSA =2, the breath-holding duration for data acquisition of one slice scanning was 8 sec due to a delay time of 6,000 ms for magnetization restoration. If NSA =1, the duration was shortened to be 2 sec. In previous studies, manual region of interest (ROI) analysis of T1rho map was used. In this current study, histogram analysis was also applied to evaluate liver T1rho value on T1rho maps. MRI data acquisition was performed on a 3 T clinical scanner. There were 29 subjects with 61 examinations obtained. Liver T1rho values obtained by sequence 1 (NSA =2) and sequence 2 (NSA =2) showed similar values, i.e., 43.1±2.1 ms (range: 38.6-48.0 ms, n=40 scans) vs. 43.5±2.5 ms (range: 39.0-47.7 ms, n=12 scans, P=0.74) respectively. For the six volunteers scanned with both sequences in one session, the intraclass correlation coefficient (ICC) was 0.939. Overall, the success rate of obtaining satisfactory images per acquisition was slightly over 50% for both sequence 1 and sequence 2. Satisfactory images can usually be obtained by asking the volunteer subjects to better hold their breath. However, sequence 2 did not increase the scan success rate. For the nine subjects scanned by sequence 2 with both NSA =2 and NSA =1 during one session, the ICC was 0.274, demonstrated poor agreement. T1rho measurement by ROI method and histogram had an ICC of 0.901 (P>0.05), demonstrated very good agreement. We conclude that by including 135° flip angle before and after the spin-locking RF pulse, the rate of artifacts occurring did not decrease. On the other hand, sequence 1 and sequence 2 measured similar T1rho value in healthy liver. While reducing the breath-holding duration significantly, NSA =1 did not offer satisfactory signal-to-noise ratio. Histogram measurement can be adopted for future studies.
    12/2013; 3(6):308-315. DOI:10.3978/j.issn.2223-4292.2013.12.10


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Dec 10, 2014

Queenie Chan