Pulse sequence for multislice T_1ρ‐weighted MRI

Abstract

A 2D multislice spin-lock (MS-SL) MR pulse sequence is presented for rapid volumetric T 1 -weighted imaging. Image quality is compared with T 1 -weighted data collected using a singleslice (SS) SL sequence and T 2 -weighted data from a standard MS spin-echo (SE) sequence. Saturation of longitudinal magnetization by the application of nonselective SL pulses is experimentally measured and theoretically modeled as T 2 decay. The saturation data is used to correct the image data as a function of the SL pulse duration to make quantitative measurements of T 1 . Measurements of T 1 using the saturation-corrected MS-SL data are nearly identical to those measured using an SS-SL sequence. The MS-SL sequence produces quantitative T 1 maps of an entire sample volume with the high-SNR advantages conferred by SE-based sequences.Magn In this study, spin-lock (SL) MRI was used to generate an alternative contrast to conventional proton density, T 1 -, or T 2 -weighted MRI methods. In SL-MRI, a long-duration, low-power SL pulse is applied to "lock" the spins in the transverse plane. During the SL pulse, the transverse magnetization decays according to T 1 , the spin-lattice relaxation in the rotating frame of reference. The amplitude of the SL pulse is commonly referenced in terms of the nutation frequency (␥B 1 ), which is typically in the range of a few hundred hertz to a few kilohertz. T 1 relaxation phenomena are sensitive to physicochemical processes with inverse correlation times on the order of the nutation frequency of the SL pulse. By setting the amplitude of the SL pulse to coincide with the frequency of the molecular processes of interest, the signal from the SL-MRI sequence becomes heavily weighted by the T 1 parameter according to Eq. [1], where TSL is the SL pulse duration, and S is the signal intensity as a function of TSL.By acquiring a series of T 1 -weighted images at varying TSL durations with constant SL pulse amplitude, and using Eq.[1], the T 1 parameter can be measured on a pixelby-pixel basis using linear regression to create a quantitative spatial map of T 1 values. SL pulse sequences may be applicable for imaging multiple systems, such as muscle (1,2), breast (3), liver (4), brain (5,6), and tumors (7,8). T 1 relaxation has been used as an indicator of proteoglycan content in articular cartilage, and is being developed as a diagnostic tool for the detection of osteoarthritis (9 -12). In addition, T 1 -weighted MRI of H 2 17O was recently used to measure cerebral and tumor perfusion (13,14). The sensitivity of T 1 to a variety of physiological parameters has also been demonstrated (15).T 1 weighting can be added to most MRI sequences by including an SL pulse cluster at the beginning of the pulse sequence. The magnetization thereby becomes "T 1 -prepared" and results in a T 1 -weighted signal. A 2D singleslice (SS) SL sequence based on a spin-echo (SE) sequence has been successfully implemented to produce a single 2D T 1 -weighted slice (16). The schematic for the SS-SL sequence is shown in Fig. 1. The nut...

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Quantitative MRI of cartilage and bone: degenerative changes in osteoarthritis

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