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: 2.76). 03/2012; 57(6):1631-40. DOI: 10.1088/0031-9155/57/6/1631
Source: PubMed


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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Available from: Yì-Xiáng Wáng, Dec 10, 2014
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    • "In addition to the normal T 1 , T 2 and T 2 * relaxations, T 1ρ relaxation, or the spin-lattice relaxation in the rotating frame, is becoming a potential mechanism to investigate low frequency motional processes in tissues, and has the potential for various clinical applications. Many studies have been devoted to examining the merit of T 1ρ relaxation contrast for many diseases, involving various tissues of brain (Borthakur et al 2004b, 2008, Michaeli et al 2006), head and neck (Markkola et al 1998), breast (Santyr et al 1989), liver (Wang et al 2011, Halavaara et al 2003, Yuan et al 2012, Zhao et al 2012a, Deng et al 2012), cartilage (Duvvuri et al 1997, Mlynarik et al 1999, Regatte et al 2003, Li et al 2009), spine (Blumenkrantz et al 2006, Johannessen et al 2006, Nguyen et al 2008) and wrist (Akella et al 2003). T 1ρ relaxation time represents the time constant of transverse magnetization decay that occurs during the application of a continuous wave radiofrequency (RF) pulse, called the spinlock (SL) pulse, aligned with the net magnetization vector. "
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    ABSTRACT: T(1ρ) relaxation is traditionally described as a mono-exponential signal decay with spin-lock time. However, T(1ρ) quantification by fitting to the mono-exponential model can be substantially compromised in the presence of field inhomogeneities, especially for low spin-lock frequencies. The normal approach to address this issue involves the development of dedicated composite spin-lock pulses for artifact reduction while still using the mono-exponential model for T(1ρ) fitting. In this work, we propose an alternative approach for improved T(1ρ) quantification with the widely-used rotary echo spin-lock pulses in the presence of B(0) inhomogeneities by fitting to a modified theoretical model which is derived to reveal the dependence of T(1ρ)-prepared magnetization on T(1ρ), T(2ρ), spin-lock time, spin-lock frequency and off-resonance, without involving complicated spin-lock pulse design. It has potentials for T(1ρ) quantification improvement at low spin-lock frequencies. Improved T(1ρ) mapping was demonstrated on phantom and in vivo rat spin-lock imaging at 3 T compared to the mapping using the mono-exponential model.
    Full-text · Article · Jul 2012 · Physics in Medicine and Biology
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    ABSTRACT: Spin-lattice relaxation in the rotating frame, or T(1ρ) relaxation, is normally described by a mono-exponential decay model. However, compartmentation of tissues and microscopic molecular exchange may lead to bi-exponential or multi-exponential T(1ρ) relaxation behavior in certain tissues under the application of spin lock pulse field strength. To investigate the presence of bi-exponential T(1ρ) relaxations in in-vivo rat head tissues of brain and muscle. Five Sprague-Dawley rats underwent T(1ρ) imaging at 3T. A B(1)-insensitive rotary echo spin lock pulse was used for T(1ρ) preparation with a spin lock frequency of 500Hz. Twenty-five T(1ρ)-weighted images with spin lock times ranging from 1 to 60 ms were acquired using a 3D spoiled gradient echo sequence. Image intensities over different spin lock times were fitted using mono-exponential as well as bi-exponential models both on region-of-interest basis and pixel-by-pixel basis. F-test with a significance level P value of 0.01 was used to evaluate whether bi-exponential model gave a better fitting than mono-exponential model. In rat brains, only mono-exponential but no apparent bi-exponential T(1ρ) relaxation (~70-71 ms) was found. In contrast, bi-exponential T(1ρ) relaxation was observed in muscles of all five rats (P < 10(-4)). A longer and a shorter T(1ρ) relaxation component were extracted to be ~37- ~41 ms (a fraction of ~80- ~88%) and ~9- ~11 ms (~12-20%), compared to the normal single T(1ρ) relaxation of ~30- ~33 ms. Bi-exponential relaxation components were detected in rat muscles. The long and the short T(1ρ) relaxation component are thought to correspond to the restricted intracellular water population and the hydrogen exchange between amine and hydroxyl groups, respectively.
    Full-text · Article · Jul 2012 · Acta Radiologica
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    ABSTRACT: B(0) and B(1) field inhomogeneities may generate banding-like artifacts in T(1ρ)-weighted images and hence result in errors of T(1ρ) quantification. Several types of composite spin-lock pulses have been proposed to alleviate such artifacts. In this study, magnetization evolution with T(1ρ) and T(2ρ) relaxation by using these composite spin-lock pulses are theoretically derived. The effectiveness and limitation of each spin-lock pulse are explicitly illustrated in mathematical forms and phantom T(1ρ)-weighted images acquired by using each spin-lock pulse are presented. This study also provides a theoretical framework for T(1ρ) quantification from T(1ρ)-weighted images even with B(0) and B(1) inhomogeneity artifacts.
    No preview · Article · Aug 2012 · Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference
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