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Motivation: Well established techniques for fast 3D T1 mapping with cartesian/radial trajectories are prone to respiratory artifacts.Previously established non-cartesian sequences have mitigated the influence of motion artifacts, though still suffer from long measurement times. Goal(s): Implementation of a novel 3D dual-echo rosette k-space trajectory for preclinical UTE MRI(PETALUTE) for abdominal imaging of both anatomical and quantitative T1 measurements and retrospective 4-fold acceleration. Approach: PETALUTE(resolution 0.24x0.24x0.24mm3,accelerated scan-time 2:15min) acquisition for T1 mapping via variable flip angle method and evaluation of T1 values and acceleration effects. Results: High-resolution non-gated abdominal imaging with the ability to clearly distinguish anatomy and T1 values,that did not deprecate when accelerated. Impact: Well established methods for T1 mapping using cartesian/radial trajectories suffer from motion artifacts due to long acquisition duration.PETALUTE,a novel 3D dual-echo rosette k-space trajectory for preclinical UTE-MRI,is able to generate high-resolution non-gated abdominal anatomical images and T1 mapping in ~2min.
Introduction Shen et al. 2021, 2022, and 2023 demonstrated the addition of novel rosette trajectory measurements in MRI and MRSI at ultra-high and high-field scanners (1-3). The dual-echo 3D rosette trajectories offer greater efficiency, allowing a center-out (1st echo) and center-in (2nd echo) sampling pattern that provides more outer and center k-space per petal samples than radial spokes. In addition, the rosette k-space trajectory samples center k-space in a more incoherent pattern. There, it offers the potential for further acceleration using higher under-sampling factors and the compressed sensing technique for reconstruction. This study demonstrated PETAL MR(S)I sequences for preclinical ultra-high field (7T, 9.4T) scanners. Methods Mice were scanned in a 7T and 9.4T horizontal-bore small animal MRI system (BioSpec 70/30 and BioSpec 94/20; Bruker Instruments) with a volume transmit and receive 1H 40-mm RF coil (7T) and a 2 × 2 phased array surface coil (9.4T). A custom dual-tune transceive coil for 31 P/ 1 H at 7T was used for 31P MRSI. Three experiments were conducted to implement the dual-echo novel rosette k-space trajectory for preclinical applications of 3D VFA PETALUTE, 3D PETALUTE MRSI, and MEGA-SLASER PETAL MRSI. § Abdominal Variable Flip Angle (VFA) T1 Mapping with accelerated PETALUTE (7T): VFA-PETALUTE T1 Mapping, FA: 4°,20°, DUAL-TEs = 16 µs and 2 ms, TR = 7 ms, # petals =18,192, Resolution 240 µm 3 , Acceleration Factor = 4 § Abdominal 31 P 3D PETALUTE MRSI (7T): PETALUTE 1 H, FA: 4°, Dual TEs = 0.016, 2 ms, TR = 7 ms, # petals =18,192, Resolution 175 µm 3 , Acceleration Factor = 1 Total acquisition duration =2.15 minutes. 3D PETALUTE 31 P MRSI, TE=16 µs TR = 500 ms, # petals =1444, Spectral bandwidth =6.5kHz, Resolution 1 mm 3 , Acceleration Factor = 1, Number of averages = 4, Total Acquisition duration= 48 minutes. § Neurotransmitter imaging with MEGA-SLASER PETAL MRSI (9.4T): GABA Editing, TE=68 m, TR =1000ms, #petals=256, in-plane Resolution 1 mm 2 , SLASER Localization The regular regridding applying a density-compensated adjoint nonuniform fast Fourier transform was performed using the Berkeley Advanced Reconstruction Toolbox (BART) toolbox. Results Figure 1 Abdominal Variable Flip Angle (VFA) T1 Mapping with accelerated PETALUTE: VFA measurement with an isotropic 240µm 3 resolution in ~ 2 minutes Figure 2 Abdominal 31P 3D PETALUTE MRSI Outcome: Metabolite maps with an isotropic 1mm 3 resolution in 48 minutes Figure 3 Neurotransmitter imaging with MEGA-SLASER PETAL MRSI Outcome: Neurotransmitter maps with 1mm 2 resolution in 8.5 minutes Discussion High-quality images, higher SNR, and higher resolution data were obtained with accelerated PETALUTE sequence on a preclinical scanner, resulting in (1) increased sampling density in the outer/inner k-space, (2) improved PSF and SNR compared, (3) a smooth transition (zero-delay) between the two echoes of dual-echo acquisition and (4) further acceleration using higher under-sampling factors. The novel Rosette space dual-echo Acquisition/ PETALUTE strategy could further enhance integrated and translational research studies to understand better human and non-human complex structures and physiologies.
References
1. Shen, X., Chiew, M., & Emir, U. (2021, June). Development of 3D Rosette K-Space Trajectory in Ultra-Short Echo Time MRI Applications. In MEDICAL PHYSICS (Vol. 48, No. 6). 111
2. Shen, Xin (2022). Development and Applications of 3D Ultra-short Echo Time MRI with Rosette k-Space Pattern. Purdue University Graduate School. Thesis. https://doi.org/10.25394/PGS.20323671.v1
3. Ultra-short T2 components imaging of the whole brain using 3D dual-echo UTE MRI with rosette k-space pattern. Magn Reson Med 89, 508–521 (2023).