MR-guided prostate interventions

Abstract

In this article the current issues of diagnosis and detection of prostate cancer are reviewed. The limitations for current techniques are highlighted and some possible solutions with MR imaging and MR-guided biopsy approaches are reviewed. There are several different biopsy approaches under investigation. These include transperineal open magnet approaches to closed-bore 1.5T transrectal biopsies. The imaging, image processing, and tracking methods are also discussed. In the arena of therapy, MR guidance has been used in conjunction with radiation methods, either brachytherapy or external delivery. The principles of the radiation treatment, the toxicities, and use of images are outlined. The future role of imaging and image-guided interventions lie with providing a noninvasive surrogate for cancer surveillance or monitoring treatment response. The shift to minimally invasive focal therapies has already begun and will be very exciting when MR-guided focused ultrasound surgery reaches its full potential.

Figure 1

Prostate ultrasound. Axial transrectal ultrasound image of the prostate. Arrows indicate prostate capsule boundary.

Figure 2

Normal prostate MRI. Axial T2W (a), and axial T1W (b) image of prostate taken at 1.5T using an endorectal coil. Central gland (CG, white arrows) in the T2W image shows benign prostate hyperplasia. Peripheral zone is indicated as PZ (white arrowheads). Air in the rectum (R) is due to the endorectal coil balloon. Hyperintense region of the right peripheral zone in the T1-weighted image (black arrow) indicates hemorrhage.

Figure 3

Multiplanar prostate MR. Axial (a), coronal (b), and sagittal (c) T2W prostate images taken at 1.5T using an endorectal coil in a patient with prostate cancer. Tumor in right apical region of the gland (arrows), indicated by low signal, can be seen in all three planes.

Figure 4

Advanced prostate cancer. Prostate CT and corresponding MR. a: CT image of the prostate using early-phase intravenous contrast. b: Corresponding axial T2W MR at 1.5T, obtained using a body coil but no endorectal coil. White arrows indicate a focal tumor in the left posterior region of the prostate.

Figure 5

MR of the prostate at 3T. Axial MRI of the prostate obtained prior to biopsy at 3T using a body coil. Prior rectal surgery precluded the use of an endorectal coil. Within T2W image (a), low signal region (white arrow) indicates a focal lesion in the right mid-gland. T1-weighted image (b) shows postgadolinium contrast enhancement of the lesion (white arrowhead).

Figure 6

MR-based brachytherapy planning. A screen capture of a treatment planning system used for MR-guided brachytherapy is shown. The large image (b) shows a real-time image with levels of radiation dose indicated by a color wash. Colors are used to indicate the relation between delivered dose and either the dose prescribed to the target or dose limits of the surrounding normal structures. Each structure is assigned a structure specific minimum and maximum dose. Dose summary graphs (a) show the fraction of each structure receiving below minimum (blue), between minimum and maximum (yellow), and over maximum (red). Normal structures will have a minimum dose of 0 and a maximum dose related to their tolerance of radiation. Target structures will have minimum dose that is the prescription dose and a maximum dose taken as 150% of the prescription. Due to the nature of brachytherapy, any target will have significant regions above the structure’s maximum dose. The regions of interest are delineated on a reference set of images taken at the beginning of the procedure. Dose is calculated based on needle locations as observed on images acquired during the course of the implant procedure. The planning system enables adaptive treatment planning.

Figure 7

MR-guided prostate biopsy. The MRT Signa/SP system (a) setup for MRI-guided prostate biopsy. Coronal FGR image (b) showing biopsy needle inserted on the left side. 3D Slicer (c) showing the combination of the needle on FGR image with the T2-weighted image of the prostate.

Figure 8

At top right, the 3D Slicer software platform used for MRI-guided biopsy. The 3D Slicer provides guidance and navigation during MRI-guided biopsy, allowing for multiplanar views of image volumes, target selection, and control of the MR scanner imaging plane. Here, T2W images obtained in the axial plane are shown. The control panel shown is used for slice selection to ensure that the real-time planar imaging obtained during needle insertion contains the desired target. On the left, a robotic assistant system for prostate intervention in a 3T closed-bore MRI scanner. The robot is placed between the patient’s legs and the MRI compatible mechanism (bottom right). The pneumatic actuator ensures no interference to MR images.

Figure 9

Johns Hopkins prostate biopsy manipulator. Picture of the Johns Hopkins manipulator for prostate biopsy and gold marker placements showing the different components and the needle tip, showing needle guide and sheath, positioning stage, flexible actuation shafts, and mount. Reproduced with permission, ©2005 IEEE. Courtesy of A. Krieger.

Figure 10

An example of MR to MR finite element based registration for brachytherapy treatment planning. Intraoperative axial T2-weighted imaging at 0.5T of the prostate (a) is manually contoured (b) to delineate the glandular boundary. The prostate gland, as seen in preoperative 1.5T T2W axial image obtained with an endorectal coil (c). This is registered to the contoured capsule to produce an integrated view of preoperative imaging within the intraoperative imaging space (d). The registration process here is done in three dimensions; all intraoperative images containing the prostate are manually contoured as a normal part of MRI-guided brachytherapy treatment planning. Note the significant shape change that occurs. This is due to a difference in leg position and the use of an endorectal coil during the preoperative scan.

Figure 11

MR to ultrasound registration for brachytherapy treatment planning. Similar to the case shown in Fig. 10, intraoperative axial ultrasound imaging of the prostate (a) is manually contoured (b) to delineate the glandular boundary. Preoperative 1.5T T2-weighted axial MR imaging (c), obtained with an endorectal coil, is registered to the contoured capsule to produce an integrated view of preoperative imaging within the intraoperative imaging space (d).