Development of a magnetic resonance imaging protocol to visualize encapsulated contrast agent markers in prostate brachytherapy recipients: initial patient experience

Abstract

PurposeComputed tomography (CT)-based prostate post-implant dosimetry allows for definitive seed localization but is associated with high interobserver variation in prostate contouring. Currently, magnetic resonance imaging (MRI)-based post-implant dosimetry allows for accurate anatomical delineation but is limited due to inconsistent seed localization. Encapsulated contrast agent markers were previously proposed to overcome the seed localization limitation on MRI images by placing hyperintense markers adjacent to hypointense seeds. The aim of this study was to assess the appearance of these markers in prostatic tissue, and develop an MRI protocol to enable marker visualization.Material and methodsWe acquired MRI scans in prostate implant patients (n = 10) on day 0 (day of implant) and day 30 (month after implant). Before implantation of the markers, the routine post-implant MRI protocol included a 3D T2-weighted fast-spin-echo (FSE) sequence with which markers and seeds could not be clearly visualized. To visualize the MRI markers, a 3D fast radiofrequency-spoiled gradient-recalled echo (FSPGR) sequence was evaluated for marker and seed visibility, as well as prostate boundary definitions.ResultsThe 3D FSPGR sequence allowed for the visualization of markers in the prostate, enabling the distinction of signal voids as seeds versus needle tracks. The updated post-implant MRI protocol consists of this 3D FSPGR scan and an optional 3D T2-weighted FSE scan. The optional 3D T2-weighted FSE sequence may be employed to better visualize intraprostatic detail. We also described the observed image artifacts, including seed susceptibility, marker chemical shift, partial volume averaging, motion, and wraparound artifacts.ConclusionsWe have demonstrated an MRI protocol for use with hyperintense encapsulated contrast agent markers to assist in the identification of hypointense seeds.

Clinical use of magnetic resonance imaging across the prostate brachytherapy workflow

“…In addition to reports from the Dana-Farber Cancer Institute (11–17), we also evaluated five other reports, two on focal salvage brachytherapy (7, 8), two on MRI spectroscopy (9, 10), and one a preliminary report of the MD Anderson experience (6). The major drawback of these studies is that even though they used MRI at the treatment-planning stage and three of them (7–9) involved adequate fusion with TRUS during the implantation, none seem to have confirmed the quality of the implant on MR images.…”

“…These 23 reports included 12 clinical reports on LDR brachytherapy (including 7 from a Dana-Farber study) (6–17) and 11 clinical reports on HDR (18–28). The 34 excluded reports consisted of 11 reports on contouring/registration, 12 planning studies, and 11 studies on post-planning dosimetry.…”

Permanent prostate brachytherapy postimplant magnetic resonance imaging dosimetry using positive contrast magnetic resonance imaging markers

“…Focusing only on the ¼ most posterior seeds, we also found no difference in these seeds' locations after registration when identified on CT vs. MRI, suggesting that the use of endorectal devices (either inflatable or rigid) during MRI does not likely significantly impact dosimetric calculations. It should be noted that these devices may cause increased difficulty in the fusion process between CT and MRI or cause other prostate organ deformations not appreciated by brachytherapy seed locations, yet endorectal devices are currently necessary to obtain adequate MRI images sufficient for MRI only post-implant dosimetry using this approach and potentially limits the generalizability of these results to CT-MRI fusion uncertainties without an endorectal coil (20). Further analysis to precisely quantify the prostate deformation caused by these endorectal devices is also currently underway by our group.…”

“…Throughout the course of this investigation two different MRI sequences were used for MRI specific brachytherapy strand marker identification depending on the type of MRI scanner used. These were a 3D fast spoiled gradient echo (FSPGR) sequence used with a 3T General Electric Signa HDxT scanner and a 3D axial fast low angle shot (FLASH) sequence used with a 1.5T Siemens MAGNETOM Aera scanner (20, 21). The MRI parameters for the FSPGR sequence were repetition time (TR) = 6.18, echo time (TE) = ∼3.3 ms, flip angle =20, number of excitations (NEX) = 8, field of view (FOV) = 14 cm, imaging matrix = 256×256, and slice thickness = 2 mm, while MRI parameters for the FLASH sequence were TR = 6, TE = 2.38, flip angle = 25, FOV = 15 cm, imaging matrix = 256×256, and slice thickness = 1-2 mm.…”

Pulse sequence considerations for simulation and postimplant dosimetry of prostate brachytherapy

“…Without the markers, the seeds’ appearance may be difficult to distinguish from other dark signal regions, such as needle tracks, spacers, and vasculature. Presently, the concentration of solution in the C4 markers has been optimized for T1-weighted imaging (26,37,38). It is possible that additional optimization of the MR pulse sequences, concentration of the seed markers, or different types of markers will further improve the signals in the different types of images (such as CISS images), providing even better localization of the seed markers and visualization of the anatomy.…”