Conventional approaches for MR elastography (MRE) using surface drivers have difficulty achieving sufficient shear wave propagation in the prostate gland due to attenuation. In this study we evaluate the feasibility of generating shear wave propagation in the prostate gland using a transurethral device. A novel transurethral actuator design is proposed, and the performance of this device was evaluated in gelatin phantoms and in a canine prostate gland. All MRI was performed on a 1.5T MR imager using a conventional gradient-echo MRE sequence. A piezoceramic actuator was used to vibrate the transurethral device along its length. Shear wave propagation was measured transverse and parallel to the rod at frequencies between 100 and 250 Hz in phantoms and in the prostate gland. The shear wave propagation was cylindrical, and uniform along the entire length of the rod in the gel experiments. Widespread adoption of annual digital rectal examination (DRE) and prostate-specific antigen (PSA) screening has resulted in a greater proportion of new cases of prostate cancer being diagnosed in younger men, and at an earlier stage of disease (1,2). Unfortunately, these screening tools have a low specificity for cancer, resulting in a large number of men undergoing confirmation biopsies. Although biopsies provide important histological confirmation of the presence, stage, and grade of disease, they sample only a very small percentage of the entire prostate gland. There is a pressing need for noninvasive imaging methods capable of providing greater diagnostic probability of the presence of prostate cancer and spatial localization of disease to identify suspicious regions for subsequent targeting with biopsy and to reduce unnecessary biopsies.The ability of MRI techniques such as dynamic contrastenhanced imaging (DCE-MRI), MR spectroscopy (MRS), and diffusion-weighted imaging (DWI) to identify regions for subsequent biopsy are improving (3-6) and there is intense research activity directed toward combining these techniques into an integrated imaging tool for prostate cancer diagnosis. Another promising MRI technique is MR elastography (MRE), in which images of tissue stiffness are generated (7). The spatial distribution of elastic properties can be calculated from MR images of displacement obtained by propagating shear mechanical waves through tissue from an external source of vibration. MRE is being investigated as a technique to study diseases in a variety of organs, including muscle, brain, and liver (8 -11).An imaging technique sensitive to changes in the stiffness of tissue could have an important role in prostate cancer diagnosis. Measurements of the mechanical properties of the prostate suggest that significant differences exist between malignant and benign tissues (12-14). Furthermore, DRE is an established technique that relies on the difference in mechanical properties between normal and cancerous prostate tissue. Studies performed by Dresner et al. (15,17) and Dresner and Ehman (16) investigating the shear mechanical pro...

The feasibility of using MR thermometry for temperature feedback to control a transurethral ultrasound heating applicator with planar transducers was investigated. The sensitivity of a temperature-based feedback algorithm to spatial (control point area, slice thickness, angular alignment) and non-spatial (imaging time, temperature uncertainty) parameters was evaluated through numerical simulations. The angular alignment of the control point with the ultrasound beam was an important parameter affecting the average spatial error in heat delivery. The other spatial parameters were less influential, thus providing an opportunity to reduce spatial resolution for increased SNR in the MR imaging. The update time was the most important non-spatial parameter determining the performance of the control algorithm. Combined non-spatial and spatial parameters achieved acceptable performance with a voxel size of 3 mm x 3 mm, a 10 mm slice thickness and a 5 s update time. Temperature uncertainty of up to 2 degrees C had little effect on the performance of the control algorithm but did reduce the average error slightly due to a systematic, noise-induced overestimation of the boundary temperature. These simulations imply that MR thermometry performed on clinical 1.5 T imaging systems is of sufficient quality for use as thermal feedback for conformal prostate thermal therapy with transurethral ultrasound heating applicators incorporating planar transducers.

To evaluate the feasibility of disrupting the blood-spinal cord barrier (BSCB), 31 rats were exposed to focused 1.08 MHz ultrasound at electrical powers of 0.5W and 1.0W after injection of the ultrasound contrast agent Definity. T1-weighted images were acquired before and after ultrasound exposure using a 3T MRI and the MR contrast agent Gadovist. At 0.5 and 1.0W the average relative enhancements observed were 29.1 ± 21% and 57.5 ± 34% respectively. After recovery from ultrasound exposure, the rats did not show signs of motor impairment. The results demonstrate the feasibility of BSCB disruption using ultrasound together with ultrasound contrast agents. Further studies of this method are warranted.

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