High intensity focused ultrasound (HIFU) is a non-invasive tissue ablation technique that uses specially designed transducers to focus an ultrasound beam of high energy into a small volume at specific target locations within the body. The focused beam causes for a few seconds localized high temperatures and can produce well-defined regions of necrosis. The goal of this study is to assess the precision of HIFU prostate tissue ablation with Magnetic Resonance Imaging (MRI) monitoring during HIFU treatment in canines in-vivo.

In this ongoing Institutional Animal Care and Use Committee (IUCAC) approved study, eight male canines were scanned in a clinical 3T Achieva MRI scanner (Philips Healthcare) before, during and after HIFU treatment with a Sonalleve MR-HIFU system (Philips Healthcare). The MR-HIFU system includes an MR-HIFU therapy console to plan treatment, calculate real-time temperature maps, and control HIFU delivery. A transurethral transducer with eight elements was used to ablate canine prostate tissue in-vivo.

MRI-HIFU procedure

The MR-HIFU procedure of the canine prostate in-vivo was performed in three stages:

1. A planning stage in which a 3D MRI dataset was used to plan and simulate the procedure;
2. A non-invasive treatment stage in which a high-intensity focused ultrasound energy beam penetrated through the skin and soft tissue, causing localized high temperatures to coagulate tissue only in the focus area and leaving the skin and intermediate tissue unharmed. The process was monitored and controlled by anatomical and functional MR imaging and feedback; and,
3. A therapy verification stage in which high resolution T₁– and T₂-weighted MR imaging, gadolinium contrast-enhanced and diffusion weighted MR imaging techniques were used to assess the procedure

MRI Analysis

MRI analysis was performed by a board certified radiologist with extensive experience in prostate imaging. The edge and the contour characteristics of the HIFU induced lesions in the prostate were evaluated and marked on the MRI images. Potential areas of damage to the rectal wall and the neurovascular bundle were marked and tabulated. MR-HIFU ablated prostate tissue volumes (as measured with contrast enhanced MRI) were compared to the volumes prescribed by the HIFU unit to evaluate technical effectiveness.

Intra-operative evaluation of the neurovascular bundle integrity

After seven days the canines were anesthetized and underwent a surgical procedure to visualize neurovascular bundles intra-operatively. The cavernosal nerve filaments were stimulated with low intensity electrical pulses using CaverMap Surgical Aid (Blue Torch, Rockville, MD) which measures erectile response by tumescence on an ordinal scale (Fig. 2a, b).

Figure 2 a) CaverMap Surgical Aid Device (Blue Torch, Rockville, MD) to measure erectile response by tumescence on an ordinal scale b) Intraoperative nerve stimulation with the CaverMap device c) Surgical prostate specimen after prostatectomy