Emilee Minalga

Design and characterization of a laterally mounted phased-array transducer breast-specific MRgHIFU device with integrated 11-channel receiver array

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 images 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 systems 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.

An 11-Channel Radio Frequency Phased Array Coil for Magnetic Resonance Guided High Intensity Focused Ultrasound of the Breast

In this study, a radio frequency phased array coil was built to image the breast in conjunction with a magnetic resonance guided high‐intensity focused ultrasound (MRgHIFU) device designed specifically to treat the breast in a treatment cylinder with reduced water volume. The MRgHIFU breast coil was comprised of a 10‐channel phased array coil placed around an MRgHIFU treatment cylinder where nearest‐neighbor decoupling was achieved with capacitive decoupling in a shared leg. In addition a single loop coil was placed at the chest wall making a total of 11 channels. The radio frequency coil array design presented in this work was chosen based on ease of implementation, increased visualization into the treatment cylinder, image reconstruction speed, temporal resolution, and resulting signal‐to‐noise ratio profiles. This work presents a dedicated 11‐channel coil for imaging of the breast tissue in the MRgHIFU setup without obstruction of the ultrasound beam and, specifically, compares its performance in signal‐to‐noise, overall imaging time, and temperature measurement accuracy to that of the standard single chest‐loop coil typically used in breast MRgHIFU. Magn Reson Med, 2013.

In vivo evaluation of a breast‐specific magnetic resonance guided focused ultrasound system in a goat udder model

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.

Reducing morphological variability of the cervical carotid artery in serial magnetic resonance imaging using a head and neck immobilization device

To evaluate how well a head and neck immobilization device performed in reducing lumen morphology variability in repeated MR imaging of the carotid artery.

Improving signal-to-noise ratio in transcranial magnetic resonance guided focused ultrasound

Signal-to-Noise Ratio (SNR) can be increased in Magnetic Resonance Imaging (MRI) using Radio Frequency (RF) coils. Of all the options for increasing SNR, coils provide the greatest gains for the dollars spent. Coils for 1.5 and 3 Tesla MRI systems consist of conductive loops that are tuned to resonate at the fundamental frequency associated with the field strength of the MRI scanner. These coils are the transducers between the MR signal and the system electronics and are sensitive to the magnetic fields of the MR signal via Faraday’s Law of Maxwell’s equations. Sensitivity of a coil to the signal in the imaging sample depends primarily on the geometry and position of the loop with respect to the sample. To achieve the highest SNR requires the coil to be in close proximity to the sample of interest, small enough that it is not sensitive to regions outside the region of interest and large enough to pick up the signal at the depth of interest. Other factors such as dielectric or conductive material loading of the coils can significantly affect the tuning and function of a coil. Custom coils designed for a specific application typically provide much higher SNR than commercial coils that are designed for general purpose imaging of a broad range body habitus. Coils that provide even small gains in SNR provide significant imaging improvement. For example, a specific-purpose (SP) coil that can provide a 40% SNR improvement over a general-purpose (GP) coil, can achieve the same image quality as the GP coil in half the imaging time. Similarly, an SP coil that can provide a factor of 2 improvement in SNR over a GP coil can achieve the same image quality 4 times faster than the GP coil. SNR can be used to improve image quality, temporal and spatial resolution, and enable or improve imaging functionality such as temperature measurement accuracy, Diffusion Tensor Imaging (DTI), and MR Acoustic Radiation Force Imaging (MR-ARFI).