Marcelo Victor Wust Zibetti^{1}, Azadeh Sharafi^{1}, Mahesh Bharath Keerthivasan^{2}, and Ravinder Regatte^{1}

^{1}Radiology, NYU Langone Health, New York, NY, United States, ^{2}Siemens Healthineers, New York, NY, United States

We modified the Cartesian 3D-fast spoiled gradient-echo sequence with T1rho magnetization preparation for prospective acceleration of knee-joint mapping using optimized sampling patterns (SPs) and compressed sensing (CS) reconstructions. In this sequence, after each T1rho preparation module, several k-space lines are captured, partially filling the 3D k-space. However, the ordering of the k-space filling is very important to maintain consistent T1rho contrast and to obtain stable quantitative mapping. This is even more challenging when arbitrary SPs are used for accelerated MRI. We investigate different k-space ordering schemes considering optimized SPs and Poisson disk SPs in prospective 3D-T1rho acquisition with CS reconstructions.

The 3D-T1rho pulse sequence used in [1]–[3], [6], [7] was modified to accept any externally-defined SP. This implemented 3D-T1rho sequence is based on fast gradient-echo [8] and collects several 3D k-space lines after each T1rho-magnetization preparation [9], partially filling the k-space (see Fig. 1a) at each block. The main block of the sequence is repeated until enough data is captured.

Final T1rho contrast is affected by the order of k-space filling, particularly when the center is captured. Following [10]–[12], we investigated sequences that capture k-space center first, just after the preparation module. The reasoning is that T1rho magnetization reduces over time, affecting the later echoes in the block.

Our modified T1rho sequence has more flexibility regarding the selection of the 2D phase-encoding positions (defined by the SP). In this study, we have investigated two SPs: Poisson disk and optimized SP [obtained using bias-accelerated subset selection (BASS)[5]], as shown in Fig. 1c-1d. However, the best order of data collection is unknown. Phase-encoding points need to be grouped into blocks of 64 and ordered. There is no significant reduction of time if less than 256 samples per readout or 64 points per block are captured. The order of the 64 readouts in each block is important because the T1rho contrast is gradually reduced over time, after T1rho preparation. This way, phase-encoding positions related to the central area of the k-space have to be acquired right after each preparation module. We investigated two customized orderings schemes: balanced center-out (BCO) and center-random (CR), and two standard machine-provided ordering: linear alternated center-out (LACO) and linear side-to-side (LSS); see Figure 2a-b.

In the BCO scheme, the phase-encoding positions are ordered in the following manner: the centermost k-space positions are assigned one to each block. After that, one block at a time receives one unselected position that is simultaneously closest to the center and closest to the previously selected point of the block. The CR scheme selects first the centermost point for each block (same way as BCO), but after it assigns the points at random. Figures 2c-f illustrate the ordering schemes.

We first evaluated the T1rho sequence on model phantoms [3%,4%,5% and 6% agar gel and 15% cross-linked bovine serum albumin (BSA)], to measure the stability of the T1rho maps for various ordering schemes [fully-sampled (FS) and accelerated 4 times (AF=4)]. Later, we also evaluated knee joint imaging on three healthy volunteers (n=3, mean age=26.6±1.5) using TSL=4ms,7ms,13ms,25ms,45ms.

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