Quantitative assessment of patient suitability for magnetic resonance-guided High Intensity Focused Ultrasound Therapy

Abstract

Clinical assessment of a patient's suitability for magnetic resonance-guided high intensity focused ultrasound (MRgHIFU) therapy currently involves subjective judgements based on available diagnostic images and prior clinical experience. The presence of organs at risk and acoustic obstructions, such as bone and air, and target depth are taken into account. A quantitative method of assessing suitability from images available at referral may minimise the number of patients incorrectly offered, or denied, treatment. A workflow for this assessment is developed herein for pelvic tumour patients. Novel workflow components include identification of each patient's ideal treatment angle, assessment of the percentage tumour volume that can be covered using standard 'treatment cells' defined in the MRgHIFU control software, and assessment of the percentage tumour volume that can be treated (ie. receive a cytotoxic thermal dose). Volunteer and patient image datasets, with the subjects lying both supine ('referral imaging') and in an oblique supine decubitis (treatment) position, were used for methodology development and testing. A method of identifying a subject's ideal treatment angle using predicted tumour coverage was developed. These angles were compared with clinically-used treatment angles. Practical methods for assessment of tumour coverage from referral imaging have been developed and their predictive capability quantified. Tumour treatability in treatment image datasets was analysed using the k-Wave acousto-thermal simulation package. Calculated ideal treatment angles were within 5+2(o) of clinical treatment angles. Predictions of tumour coverage derived from referral images agreed with those from treatment images within 12+7% (range: 4-21%). Refinements to the 3 tumour coverage method improved computational speed by factor of 7 on average (from 19.7+8.8 to 2.8+2.0 hours). Tumour treatability was 32+14% (range: 15-50%) less than tumour coverage with ablated tissue volumes lying 9.3+1.6mm shallower than the geometric focus, suggesting tumour coverage overestimation. Despite limitations, the developed methods show significant promise.