Biopsy of the prostate gland has evolved significantly over the past 4 decades. Initially, urologists performed prostate biopsy using digital guidance alone, which obviously was suboptimal. In the late 1980s and early 1990s, transrectal ultrasonography (TRUS) came into widespread use, allowing urologists to use this imaging modality to perform prostate biopsy in a more consistent and systematic manner. Transrectal ultrasonography, however, is neither a particularly sensitive nor specific imaging modality. This has led urologists and radiologists to seek other imaging modalities that may provide better visualization of the prostate, allowing us to identify tumors in the gland, resulting in more accurate biopsies.
One such modality is magnetic resonance (MR) imaging, which does allow better visualization of the prostate, particularly when a 3-T magnet is used. Image-guided biopsy within the MR scanner (so-called in-bore biopsy), however, is cumbersome, as it requires special biopsy equipment and requires that the patient spend prolonged periods of time in the MR scanner. Acknowledging this but still wanting to take advantage of the superior images obtained with MRI, some urologists have simply used the MR to cognitively guide TRUS biopsy of the prostate. This approach, although likely better than TRUS alone, is still imprecise and relies on the surgeon to effectively “fuse” the MR to the ultrasound in his/her head. To this end, there are now a number of devices that “fuse” the previously obtained MR image of the prostate to real-time TRUS imaging. These devices create three-dimensional models of the prostate using these two fused images, allowing the urologist to reliably aim the biopsy gun to any suspicious areas seen on the MR and also to perform the standard systematic templates that urologists have used for years.
Indications for MR-Fusion Prostate Biopsy
Although one could make the argument that MR-fusion prostate biopsy is indicated in any patient in whom a “traditional” TRUS biopsy is appropriate, this is not our current practice. Simply put, the added cost of the MR imaging represents a significant burden to the patient and the health care system and, as such, we do not routinely perform MR-fusion biopsy in all patients who need a prostate biopsy. Currently, we reserve MR-fusion biopsy for patients who require biopsy (either due to rising prostate-specific antigen or change on digital rectal examination) and who have had a prior negative TRUS biopsy. In addition, we believe that MR-fusion biopsy is also indicated for follow-up biopsies in patients who elect active surveillance for localized prostate cancer. In this setting, the ability to target and follow abnormal areas on the MR likely makes active surveillance a safer strategy. In both settings, it is worth noting that if the MR is normal, we do not proceed with MR-fusion biopsy as there are no areas to target.
MR Imaging, Prostate Contouring, Three-Dimensional Reconstruction of the Prostate and Identification of Regions of Interest for Biopsy
At our institution, MR imaging and prostate biopsy are performed at separate visits. Patients undergo prostate MR imaging using a 3-T MR scanner and a phased-array body coil. We do not routinely use endorectal coils when obtaining prostate MR in this setting. Prior to undergoing MR imaging, patients are instructed to use a fleet’s enema to empty their rectum. Patients do not require antibiotics before prostate MR imaging. Usual safety precautions regarding metallic implants are employed. If the patient suffers from claustrophobia, we will provide oral sedatives on a PRN basis.
Multiparametric MR imaging, including the acquisition of T1- and T2-weighted images, is critical for defining the anatomy and identifying abnormalities in the prostate. The T2-weighted images are particularly important as it allows the best visualization of prostate cancer, which often appears as an area of low intensity within the peripheral zone (which has a high signal intensity). In addition, both diffusion-weighted and dynamic contrast-enhanced images are obtained to further characterize the anatomy of the prostate and allow for PI-RADS (Prostate Imaging–Reporting and Data System) grading of any identified lesions.
After the images are obtained, the radiologist uses specialized software to perform prostate contouring and segmentation, effectively using the computer to generate a three-dimensional reconstruction of the prostate ( Fig. 75.1, A ). Although all MR phases are used for this step, the radiologist primarily focuses on the T2-weighted images, in all three planes. The radiologist then reviews all of the MR images and marks any abnormal “regions of interest” (ROIs) on the images for targeted biopsy ( Fig. 75.1, B ). The radiologist then assigns a degree of suspicion for cancer associated with each ROI (best done using the PI-PADS grading system). The entire study is then electronically transferred to the fusion biopsy platform for later use.
MR-Fusion Biopsy Platforms
The two most commonly used MR-fusion biopsy platforms in the United States use two different methods to localize and track biopsy of the abnormal areas in the prostate (co-registration). The UroNav fusion biopsy system (InVivo Corporation, Gainesville, FL) uses an external magnetic field generator that detects a signal from an electromagnetic sensor attached to the ultrasound probe. The operator maneuvers the ultrasound and obtains images using a free-hand approach. An ultrasound technician then segments the ultrasound image and performs coregistration, allowing the urologist to proceed with targeted biopsy.
In contrast, the Artemis fusion biopsy system (Eigen, Grass Valley, CA) uses a semirobotic mechanical arm to support the ultrasound probe for scanning and digitization of the prostate. Sensors built into each joint of the arm track the position of the ultrasound probe and biopsy needle relative to the prostate ( Fig. 75.2 ). This, in turn, allows localization and tracking of prostate biopsies. Because we use the Artemis device at our institution, we describe the approach to MR-fusion biopsy using this system below.