Spatiotemporal filtering of MR‐temperature artifacts arising from bowel motion during transurethral MR‐HIFU

Abstract

Purpose: Transurethral MR-HIFU is a minimally invasive image-guided treatment for localized prostate cancer that enables precise targeting of tissue within the gland. The treatment is performed within a clinical MRI to obtain real-time MR thermometry used as an active feedback to control the spatial heating pattern in the prostate and to monitor for potential damage to surrounding tissues. This requires that the MR thermometry measurements are an accurate representation of the true tissue temperature. The proton resonance frequency shift thermometry method used is sensitive to tissue motion and changes in the local magnetic susceptibility that can be caused by the motion of air bubbles in the rectum, which can impact the performance of transurethral MR-HIFU in these regions of the gland. Methods: A method is proposed for filtering of temperature artifacts based on the temporal variance of the temperature, using empirical and dynamic positional knowledge of the ultrasonic heating beam, and an estimation of the measurement noise. A two-step correction strategy is introduced which eliminates artifact-detected temperature variations while keeping the noise level low through spatial averaging. Results: The filter has been evaluated by postprocessing data from five human transurethral ultrasound treatments. The two-step correction process led to reduced final temperature standard deviation in the prostate and rectum areas where the artifact was located, without negatively affecting areas distal to the artifact. The performance of the filter was also found to be consistent across all six of the data sets evaluated. The evaluation of the detection criterion parameter M determined that a value of M = 3 achieves a conservative filter with minimal loss of spatial resolution during the process. Conclusions: The filter was able to remove most artifacts due to the presence of moving air bubbles in the rectum during transurethral MR-HIFU. A quantitative estimation of the filter capabilities shows a systematic improvement in the standard deviation of the corrected temperature maps in the rectum zone as well as in the entire acquired slice. C

Thermal dosimetry for bladder hyperthermia treatment. An overview

“…Second, movement of tissue interfaces leads to changes in the main magnetic (B 0 ) field of the scanner, invalidating some underlying assumptions of the reconstruction algorithm. For slow changes in the B 0 field, techniques are under development to filter artefacts [104,105] or use a silicon [106] or fat [107] reference, but fast changes require dedicated motion-robust approaches. Finally, there are motion artefacts from convective currents or externally recirculated fluids inside the bladder that induce image distortion and temperature rise calculation errors.…”

A framework for the correction of slow physiological drifts during MR‐guided HIFU therapies: Proof of concept

“…A study was recently conducted by Schmitt et al 29 in which temperature and thermal dose measurements were corrected with respect to susceptibility artifacts arising from bowel motion during MRg-HIFU interventions in the prostate. While in the current work we propose a correction strategy based on displacements provided by a motion estimation algorithm, their study focuses on developing and implementing a filter that excludes temperature information in regions that manifest a higher temperature fluctuation than the heated area.…”

Catheter-based ultrasound technology for image-guided thermal therapy: Current technology and applications

“…Several examples of successful implementation of transurethral CBUS ablation of prostate with multi-planar PRFS-based MRTI are reported in the literature [36, 46-48, 50, 52-55, 81, 82]. Ancillary studies related to diffusion-weighted imaging [80] and refinement of MR techniques for motion compensation have also resulted from similar work [76, 113, 114]. MR thermometry techniques have also been explored for monitoring endoscopic ablation of liver and pancreatic tumors [58, 65, 66].…”

“…Recent investigations described above have demonstrated that the artifacts generated from the devices themselves can be minimized with proper selection of MR compatible materials and fabrication techniques, so that regions as close as 1-3 mm from the applicator extending to the outer treatment boundary can be effectively monitored [49, 55, 82]. Tissue motion related errors can be compensated through multi-baseline or multi-reference imaging [76, 115-117]. Background or baseline phase drift errors can be compensated by tracking phase changes outside the heated region [115].…”

“…The presence of gas in organs such as bowel, rectum or lungs can also lead to changes in local magnetic susceptibility and hamper the quality of MRTI during CBUS procedures. New spatiotemporal filtering techniques are being developed to account for susceptibility changes induced by the movement of air bubbles in the rectum [76]. …”