MRI imaging has a high sensitivity for detecting prostate tumors. Unfortunately, MR imaging alone, without concurrent biopsy, suffers from low diagnostic specificity. Therefore, our primary objective was to develop a prostate biopsy system that couples superior imaging quality with accurate delivery hardware, inside a conventional MRI scanner. The challenge was three-fold:
- Conventional high-field MRI scanners apply whole-body magnets that surround the patient completely and do not allow access to the patient during imaging. The workspace inside the magnet is extremely limited, so conventional medical robots and mechanical linkages do not fit in the magnet.
- Due to the strong magnetic field, ferromagnetic materials and electronic devices are not allowed to be in the magnet, which excludes the use of traditional electro-mechanical robots and mechanical linkages.
- A real-time in-scanner guidance method is needed to operate the device.
The device is secured to the table of the scanner with an adjustable mount that allows for flexible initial positioning. The patient is positioned comfortably on the scanner’s couch in prone position with slightly elevated pelvis and the device is introduced to the rectum. A thin rigid sheath attached around a tubular obturator makes contact with the rectum, while the obturator can slide smoothly inside the sheath. The sheath prevents the obturator from causing mechanical distortion to the rectum wall and prostate while it is moving inside the rectum. After a satisfactory initial position is achieved, the mount is secured to hold this position. Using the sliding table of the scanner, the patient and device are moved into the scanner’s magnet. The MRI scanner produces signal with the patient and device in the field, at the same time. Using signal-processing tools, we determine the spatial relationship between the device and the coordinate system of the MRI scanner. The images are transferred to a computer that produces a 3D representation of the device superimposed on the anatomic images. The physician interacts with the display and selects the target point for the needle. The computer calculates the kinematic sequence to bring the needle to the selected target position. The device realizes 3-DOF motion: translation of the end-effector inside the rectum, rotation of the end-effector around the axis of translation, and the depth of needle insertion. The order of translation and rotation are interchangeable, but both must be completed before the needle is inserted. The computer can also simulate the sequence of motions by moving a 3D model of the device, so that the physician could verify that the calculated sequence of motions would indeed take the needle from its current position to the pre-selected target position. The computer displays the three motion parameters to the operator. While the actuation of the device is in progress, the MRI scanner is collecting images in continuous mode and sends them immediately to the treatment monitoring computer. The computer processes the image data and visualizes the current image, with the model of the device superimposed in the scene, allowing the physician to monitor the motion of the device toward its target. The three parameters of motion (translation, rotation, insertion depth) are recalculated in each imaging cycle, enabling real-time dynamic control of the device.
The assembly of the human-grade transrectal needle placement robot.