Purpose: The goal of this study was to develop and evaluate a volumetric three-dimensional (3D) approach to improve the accuracy of ablation margin assessment following thermal ablation of hepatic tumors. Methods:The 3D margin assessment technique was developed to generate the new 3D assessment metrics: volumes of insufficient coverage (VICs) measuring volume of tissue at-risk post-ablation. VICs were computed for the tumor and tumor plus theoretical 5-and 10-mm margins. The diagnostic accuracy of the 3D assessment to predict two-year local tumor progression (LTP) was compared to that of manual 2D assessment using retrospective analysis of a patient cohort that has previously been reported as a part of an outcome-centered study. Eighty-six consecutive patients with 108 colorectal cancer liver metastases treated with radiofrequency ablation (2002 -2012) were used for evaluation. The 2-year LTP discrimination power was assessed using receiver operating characteristic area under the curve (AUC) analysis.Results: A 3D assessment of margins was successfully completed for 93 out of 108 tumors. The minimum margin size measured using the 3D method had higher discrimination power compared with the 2D method, with an AUC value of 0.893 vs. 0.790 (P = 0.01). The new 5 mm VIC metric had the highest 2-year LTP discrimination power with an AUC value of 0.923 (P = 0.004). Conclusions:Volumetric semi-automated 3D assessment of the ablation zone in the liver is feasible and can improve accuracy of 2-year LTP prediction following thermal ablation of hepatic tumors.

MR-guided focused ultrasound is a non-invasive therapy for treating various pathologies. MR-based acoustic radiation force imaging (MR-ARFI) measures tissue displacement in the focal spot due to acoustic radiation force. MR-ARFI also provides feedback for adaptive focusing algorithms that could correct for phase aberrations caused by the skull during brain treatments. This work developed a single-shot echo-planar imaging (EPI) based MR-ARFI method that reduces scan time and ultrasound energy deposition. The new method was implemented and tested in a phantom and ex vivo brain tissue. The effect of the phase aberrations on the ultrasound focusing was studied using displacement maps obtained with EPI and 2D spin-warp MR-ARFI. The results show that displacement in the focal spot can be rapidly imaged using EPI-based MR-ARFI with high SNR efficiency and without any measurable tissue heating. EPI-based displacement images also demonstrate sufficient sensitivity to phase aberrations and can serve as rapid feedback for adaptive focusing in brain treatments and other applications.

No abstract

A high resolution and high speed pulse sequence is presented for monitoring high intensity focused ultrasound (HIFU) ablations in the liver in the presence of motion. The sequence utilizes polynomial-order phase saturation bands to perform outer volume suppression, followed by spatial-spectral excitation and three readout segmented EPI interleaves. Images are processed with referenceless thermometry to create temperature rise images every frame. The sequence and reconstruction were implemented in RTHawk and used to image stationary and moving sonications in a polyacrylamide gel phantom (62.4 acoustic W, 50 sec, 550 kHz). Temperature rise images were compared between moving and stationary experiments. Heating spots and corresponding temperature rise plots matched very well. The stationary sonication had a temperature standard deviation of 0.15°C, compared to values of 0.28°C and 0.43°C measured for two manually-moved sonications at different velocities. Moving the phantom (while not heating) with respect to the transducer did not cause false temperature rises, despite susceptibility changes. The system was tested on non-heated livers of 5 normal volunteers. The mean temperature rise was −0.05°C with a standard deviation of 1.48°C. This standard deviation is acceptable for monitoring HIFU ablations, suggesting real time imaging of moving HIFU sonications can be clinically possible.

A variety of MRI acoustic radiation force imaging (MR-ARFI) pulse sequences as the means for image guidance of focused ultrasound therapy have been recently developed and tested ex vivo and in animal models. To successfully translate MR-ARFI guidance into human applications, ensuring that MR-ARFI provides satisfactory image quality in the presence of patient motion and deposits safe amount of ultrasound energy during image acquisition is necessary. The first aim of this work was to study the effect of motion on in vivo displacement images of the brain obtained with 2DFT spin-echo MR-ARFI. Repeated bipolar displacement encoding configuration was shown less sensitive to organ motion. The optimal signal-to-noise ratio of displacement images was found for the duration of encoding gradients of 12 ms. The second aim was to further optimize the displacement SNR for a particular tissue type by setting the time offset between the ultrasound emission and encoding based on the tissue response to acoustic radiation force. A method for measuring tissue response non-invasively was demonstrated. Finally, a new method for simultaneous monitoring of tissue heating during MR-ARFI acquisition was presented to enable timely adjustment of the ultrasound energy aimed at ensuring the safety of the MR-ARFI acquisition.