Magnetic resonance guided focused ultrasound (MRgFUS) has shown significant promise as a non-invasive therapy for treatment of focal bone lesions such as bone metastases and osteoid osteomas (1-3). One of the challenges in clinical treatments of bone lesions is extending the depth of ablation sufficiently deep to the cortical surface to permit ablation of the entire intramedullary extent of a lesion. When sound waves reach a bone-soft tissue interface, there is rapid attenuation secondary to reflection, scattering, and mode conversion, in addition to absorption with approximately 60-80% loss of acoustic energy (4). We have previously demonstrated that repetition of sonications within a given volume can significantly increase the depth of an ablation beyond the cortical surface (5). Anecdotally, in clinical practice, many MRgFUS interventionalists additionally use strategies such as decreasing the sonication frequency or increasing the duration of the sonication (while lowering the acoustic power) to achieve a deeper ablation. The purpose of the current study was to investigate whether or not for a specific energy dose there was a difference in ablation depth beyond the cortical surface with long versus short sonications.

All experimental procedures were done in accordance with National Institutes of Health guidelines for humane handling of animals and received prior approval from the local Institutional Animal Care and Use Committee. We performed MRgHIFU using an ExAblate® 2000 system (InSightec, Haifa, Israel) integrated with a 3.0 Tesla MR scanner (GE Healthcare, Waukesha, WI, USA) in a swine model. Multiplanar T2-weighted fast spine echo (FSE) and LAVA 3D spoiled gradient echo images were acquired for treatment planning. Two discrete ovoid treatment targets were prescribed at each proximal diaphysis and distal diaphysis of the left femur. Six sonications were used to create each lesion at 1.0 MHz with a phased array transducer of 208 elements embedded within the scanner table. During the course of the treatment, the energy level was maintained at 400 J for all sonications. For the proximal sonications, the acoustic power was set at 10 W with sonication duration of 40 seconds. For the distal sonications, the acoustic power was set at 20 W with sonication duration of 20 seconds. Average increased temperature at the bone-soft tissue interface was approximately 76 °C for the shorter sonications and 66 °C for the longer sonications. Post-treatment MR imaging was performed immediately following the treatment with delayed contrast enhanced segmented inversion recovery fast gradient echo (TR/TE/flip angle=1.5ms/15.2ms/15), LAVA 3D spoiled gradient echo sequences. A dose of 0.1mmol/kg Gd-DTPA was delivered 10min before imaging to assess the ablation size. MRgFUS was used to create two focal ablation lesions within the left femur. Delayed contrast enhanced MR images demonstrated ovoid regions of hypoenhancement, which correlated with the bone targets. The depth of ablation was greater for the shorter high power sonications (7 mm), compared to the longer lower power sonications (5 mm) (Figure 1). The other two dimensions of the ablation were also greater for the shorter sonications (26 x 16 mm, craniocaudal by anteroposterior), compared to the longer sonications (11 x 11 mm).Interestingly, this basic science study suggests that during MRgFUS of bone, increasing the duration of the sonication while maintaining the same total acoustic energy did not result in deeper ablation volumes. In fact, the shorter high power sonications demonstrated relatively increased depth and overall ablation size. Increased depth of bone ablation during clinical treatments which is anecdotally attributed to the use of longer sonications is possibly instead related to simultaneous modifications of additional technical parameters (e.g. an uncompensated decrease in acoustic power, for overall higher energy sonication; or decreased sonication frequency). This study importantly helps us to better understand the factors influencing the size of the ablation zone during MRgFUS bone treatments.