Room: Track 1
Purpose: purpose of this work was to characterize image quality of a novel MR-compatible, e4D ultrasound probe (IGRT) designed for ultrasound-based image guided radiotherapy in the liver and to compare it to that of two commercial probes (4V cardiac probe and C1-6 curvilinear abdominal probe, GE Healthcare, Milwaukee, WI). This work also assessed the impact of the ultrasound probe on MR imaging including susceptibility artifacts.
Methods: quality measurements in phantoms were performed on a GE Vivid E95 scanner for the IGRT and 4V and on a GE Logiq E9 scanner for the C1-6. Lateral resolution, depth of penetration (DOP), and 4 mm-lesion detectability vs depth (LSNR) were assessed for each probe. We also analyzed the pixel intensity artifact in the ultrasound images during MR acquisition. Field maps were acquired in phantoms on a 3T MRI (Premier, GE Healthcare, Waukesha, WI) using a multi-echo gradient echo sequence (IDEAL) to assess the field inhomogeneity due to the IGRT probe. This impact was further evaluated in healthy volunteers.
Results: lateral ultrasound resolution was comparable for all three probes within 100 mm. Beyond this depth, the performance of the 4V and IGRT probes decreased more dramatically when compared to the C1-6. The IGRT and 4V probes achieved comparable DOP: 145 mm and 160 mm, respectively. However the C1-6 displayed superior DOP of 180 mm. Overall, LSNR vs depth was comparable across probes, though the IGRT showed greater LSNR within 35 mm and the C1-6 had the best LSNR from 35 mm to 105 mm. Modest MR susceptibility was observed and no ultrasound artifact was measured. Ultimately image utility is not significantly impacted.
Conclusion: IGRT probe is capable of high quality imaging and introduces minor artifacts during simultaneous acquisition. This probe will allow MRI visualization of intra-fraction anatomical motion during radiotherapy in the liver.