Purpose: This work presents the design and preliminary evaluation of a new laterally mounted phased-array MRI-guided high-intensity focused ultrasound (MRgHIFU) system with an integrated 11-channel phased-array radio frequency (RF) coil intended for breast cancer treatment. The design goals for the system included the ability to treat the majority of tumor locations, to increase the MR image's signal-to-noise ratio (SNR) throughout the treatment volume and to provide adequate comfort for the patient. Methods: In order to treat the majority of the breast volume, the device was designed such that the treated breast is suspended in a 17-cm diameter treatment cylinder. A laterally shooting 1-MHz, 256-element phased-array ultrasound transducer with flexible positioning is mounted outside the treatment cylinder. This configuration achieves a reduced water volume to minimize RF coil loading effects, to position the coils closer to the breast for increased signal sensitivity, and to reduce the MR image noise associated with using water as the coupling fluid. This design uses an 11-channel phased-array RF coil that is placed on the outer surface of the cylinder surrounding the breast. Mechanical positioning of the transducer and electronic steering of the focal spot enable placement of the ultrasound focus at arbitrary locations throughout the suspended breast. The treatment platform allows the patient to lie prone in a face-down position. The system was tested for comfort with 18 normal volunteers and SNR capabilities in one normal volunteer and for heating accuracy and stability in homogeneous phantom and inhomogeneous ex vivo porcine tissue. Results: There was a 61% increase in mean relative SNR achieved in a homogeneous phantom using the 11-channel RF coil when compared to using only a single-loop coil around the chest wall. The repeatability of the system's energy delivery in a single location was excellent, with less than 3% variability between repeated temperature measurements at the same location. The execution of a continuously sonicated, predefined 48-point, 8-min trajectory path resulted in an ablation volume of 8.17 cm 3 , with one standard deviation of 0.35 cm 3 between inhomogeneous ex vivo tissue samples. Comfort testing resulted in negligible side effects for all volunteers. Conclusions: The initial results suggest that this new device will potentially be suitable for MRgHIFU treatment in a wide range of breast sizes and tumor locations.

Purpose: This work further evaluates the functionality, efficacy, and safety of a new breast-specific magnetic resonance guided high intensity focused ultrasound (MRgFUS) system in an in vivo goat udder model. Methods: Eight female goats underwent an MRgFUS ablation procedure using the breast-specific MRgFUS system. Tissue classification was achieved through the 3D magnetic resonance imaging (MRI) acquisition of several contrasts (T1w, T2w, PDw, 3-point Dixon). The MRgFUS treatment was performed with a grid trajectory executed in one or two planes within the glandular tissue of the goat udder. Temperature was monitored using a 3D proton resonance frequency (PRF) MRI technique. Delayed contrast enhanced-MR images were acquired immediately and 14 days post MRgFUS treatment. A localized tissue excision was performed in one animal and histological analysis was performed. Animals were available for adoption at the conclusion of the study. Results: The breast-specific MRgFUS system was able to ablate regions ranging in size from 0.4 to 3.6 cm 3 in the goat udder model. Tissue damage was confirmed through the correlation of thermal dose measurements obtained with realtime 3D MR thermometry to delayed contrast enhanced-MR images immediately after the treatment and 14 days postablation. In general, lesions were longer in the ultrasound propagation direction, which is consistent with the dimensions of the ultrasound focal spot. Thermal dose volumes had better agreement with nonenhancing areas of the DCE-MRI images obtained 14 days after the MRgFUS treatment. Conclusions: The system was able to successfully ablate lesions up to 3.6 cm 3 . The thermal dose volume was found to correlate better with the 14-day postablation nonenhancing delayed contrast enhanced-MR image volumes. While the goat udder is not an ideal model for the human breast, this study has proven the feasibility of using this system on a wide variety of udder shapes and sizes, demonstrating the flexibility that would be required in order to treat human subjects.

The experimental validation of a model-based, thermal therapy control system which automatically and simultaneously achieves the specified efficacy and safety objectives of the treatment is reported. MR-thermometry measurements are used in real-time to control the power of a stationary, focused ultrasound transducer in order to achieve the desired treatment outcome in minimum time without violating the imposed safety constraints. Treatment efficacy is quantified in terms of the thermal dose delivered to the target. Normal tissue safety is ensured by automatically maintaining normal tissue temperature below the imposed limit in the user-specified locations. To reflect hardware limitations, constraints on the maximum applied power are also imposed. At the pretreatment stage, MR imaging and thermometry are used to localize the treatment target and identify thermal and actuation models. The results of phantom and canine experiments demonstrate that spatially-distributed, real-time MR temperature measurements enhance one's ability to robustly achieve the desired treatment outcome in minimum time without violating safety constraints. Post-treatment evaluation of the outcome using T2-weighted images of canine muscle showed good spatial correlation between the sonicated area and thermally damaged tissue.