Lockard, C. A. | Search bioRxiv

bioRxiv2024

Ultrashort-T2* mapping at 7 tesla using an optimized pointwise encoding time reduction with radial acquisition (PETRA) sequence at standard and extended echo times

Zero echo time (ZTE) sequences capture signal from tissues with extremely short T2* and are useful for qualitative and quantitative imaging of musculoskeletal tissues ultrashort-T2* components. One such sequence is Pointwise Encoding Time Reduction with Radial Acquisition (PETRA). While this sequence has shown promising results, it has undergone only limited testing at 7 tesla (T). The purpose of this work was to evaluate PETRA at 7T in its standard form and with sequence modifications to allow extended echo times for the purpose of performing ultrashort-T2* mapping. We acquired PETRA images of MnCl2 and collagen phantoms and of the knee in eight participants (5 for optimization and 3 for ultrashort-T2* mapping assessment; 5 male/3 female, 39 {+/-} 11 years old). Images were acquired using a 1-transmit/28-receive-channel knee coil. Artifacts, signal, signal-to-noise ratio (SNR), ultrashort-T2*, the corresponding curve fit quality, and repeatability were assessed. SNR was high in knee tissues at TE = 0.07 msec compared to a conventional-TE sequence (Dual-Echo Steady State with TE = 2.55 msec), with values ranging between 68 to 337 across the assessed tissues for PETRA versus 16 to 30 for the same tissue regions of interest in the conventional-TE series. Acquisition of series for ultrashort-T2* maps was feasible at 1.50 mm isotropic acquisition resolution and TE [≤] 0.58 msec. Strong linear correlations were observed between relaxation times and MnCl2 concentration, and between signal and collagen concentration. Ultrashort-T2* signal decay curve fit R2 and repeatability were high for phantom and knee ultrashort-T2* <1 msec. PETRA imaging with minimal artifacts, high SNR, and scan time < 11 minutes was achieved at 7T at high (0.34 mm isotropic) resolution at TE = 0.07 msec and lower resolution (1.52 mm isotropic) at echo times [≤] 0.58 msec. Ultrashort-T2* mapping provided acceptable curve-fitting results for substances with sub-millisecond T2*.

bioengineering↗

bioRxiv2024

A registration strategy to characterize DTI-observed changes in skeletal muscle architecture due to passive shortening

Skeletal muscle architecture is a key determinant of muscle function. Architectural properties such as fascicle length, pennation angle, and curvature can be characterized using Diffusion Tensor Imaging (DTI), but acquiring these data during a contraction is not currently feasible. However, an image registration-based strategy may be able to convert muscle architectural properties observed at rest to their contracted state. As an initial step toward this long-term objective, the aim of this study was to determine if an image registration strategy could be used to convert the whole-muscle average architectural properties observed in the extended joint position to those of a flexed position, following passive rotation. DTI and high-resolution fat/water scans were acquired in the lower leg of seven healthy participants on a 3T MR system in +20{degrees} (plantarflexion) and -10{degrees} (dorsiflexion) foot positions. The diffusion and anatomical images from the two positions were used to propagate DTI fiber-tracts from seed points along a mesh representation of the aponeurosis of fiber insertion. The -10{degrees} and +20{degrees} anatomical images were registered and the displacement fields were used to transform the mesh and fiber-tracts from the +20{degrees} to the -10{degrees} position. Students paired t-tests were used to compare the mean architectural parameters between the original and transformed fiber-tracts. The whole-muscle average fiber-tract length, pennation angle, curvature, and physiological cross-sectional areas estimates did not differ significantly. DTI fiber-tracts in plantarflexion can be transformed to dorsiflexion position without significantly affecting the average architectural characteristics of the fiber-tracts. In the future, a similar approach could be used to evaluate muscle architecture in a contracted state.

physiology↗