MR Guided Interventions in the Prostate Gland
Clare Tempany, M.D.
Associate Professor of Radiology
Brigham and Women's Hospital
Boston, Massachusetts USA
Overview:
Interventional MRI has become a practical reality and is in current use at several centers across the country. We have had this capability for several years now. The primary use of this technology was in Neurosurgery to guide and help plan neurosurgical procedures. However there have been exciting developments in prostate cancer diagnosis and treatment. We use MR guidance to obtain prostate biopsies and do brachytherapy. Both procedures are performed in the open magnet using a percutaneous approach to the placement of the biopsy guns, needles and/or seeds.
MR Imaging of the Prostate
Significant advances have been made in MR imaging of the prostate gland in the past decade. It is now clearly established that a combination of T1W or proton density (PD) and T2W images in multiple planes provides the most comprehensive set of images of the entire prostate and its substructure, including the central gland and the peripheral zone.
Focal lesions in the peripheral zone are suspicious for tumor, especially when located posteriorly in the gland. MRI can provide full visualization of the rectum, bladder, seminal vesicles, and neurovascular bundles (NVBs) (Fig. 1 & Fig. 2). The MRI technology has improved significantly, with current state-of-the-art images acquired with an endorectal coil/phased array coil.
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Figure 1:
Pre-Op 1.5T Axial |
Figure 2:
Pre-op Axial 1.5T of PZ |
Prostate Cancer Brachytherapy
We have developed an interactive comprehensive MR image-guided program for the diagnosis and treatment of localized prostate cancer, the MR-guided brachytherapy program, which utilizes the Signa SP Interventional MR system. Since the Signa SP system was introduced, there have been significant advances in MR guidance for localization, navigation and delivery of therapy, as used today in neurosurgery. The application of image guidance techniques to the diagnosis and image-guided treatment of prostate cancer has been shown to hold great promise. However in order to benefit from this technology, precise methods of localization, navigation, control, and monitoring of the intervention are required and these have not been fully developed. The principal challenge is to augment the information content of intra-operative image data by using multimodal features while preserving the interactivity and near real time capability of intra-operative imaging.
During the last few years we have developed and implemented a novel MRI based image-guidance method for prostate brachytherapy. Our original goal has been to overcome the known limitations and deficiencies of US or CT based methods. In our ongoing clinical trial (over 210 cases as of July 2001) we have established the feasibility of this new method, assembled the essential technology and obtained the necessary experience. A Radiologist, Radiation Oncologist and Urologist perform the implant procedure jointly. (Fig. 3)
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Figure 3:
Brigham & Women's Brachytherapy Team
We use 18G (E-Z-EM, Inc. Westbury NY) MR compatible needles, which are loaded with the I-125 seeds (Fig.4). Each row of I-125 seeds is implanted, by first inserting the needle through the prescribed location on the template to the desired depth, under MR guidance.
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Figure 4:
E-Z-EM needles pre-loading of I-125 seeds |
Figure 5:
Positioning of the patient and template within the Signa SP |
Figure 6:
FGR Realtime Coronal |
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For MR image guidance, a fast gradient echo (FGR) sequence is used (TR/TE = 25/12 msec, flip angle 90°, 256 x 160, 2 signal averages, 5mm thick), each image is obtained in 9-10 seconds. (Fig. 6)
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Near real-time imaging is first performed in the coronal plane during needle insertion. Subsequently, a sagittal image is obtained to verify antero-posterior positioning. Finally, an axial image is obtained near the needle tip and is sent to the treatment-planning computer, where the needle tip position is compared to the ideal target before actually deploying the seeds. Thus the needle location is monitored in the three orthogonal planes and matched with the pre-plan images. This method allows true coronal, sagittal and axial plane images of the prostate and needle to be obtained, making it easier to map the correct trajectory. The true axial images at the tip of the needle are than transferred to the treatment planning software. (Fig. 7 & 8)
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Figure 7:
Signa SP Axial |
Figure 8:
Signa SP Axial of PZ |
Figure 9:
3D SPGR Post Procedure |
If the planned and actual location match and are approved by the medical physicist, then the seeds are dropped and the needle removed. Adjustments to account for incorrect placement can be made before the seeds are placed. The causes of such may be due to prostate motion, edema, or catheter divergence. This process is repeated in an iterative fashion for all planned catheters.
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