Effect of pulse sequence parameter selection on signal strength in positive‐contrast MRI markers for MRI‐based prostate postimplant assessment

Abstract

Purpose: For postimplant dosimetric assessment, computed tomography (CT) is commonly used to identify prostate brachytherapy seeds, at the expense of accurate anatomical contouring. Magnetic resonance imaging (MRI) is superior to CT for anatomical delineation, but identification of the negative-contrast seeds is challenging. Positive-contrast MRI markers were proposed to replace spacers to assist seed localization on MRI images. Visualization of these markers under varying scan parameters was investigated. Methods: To simulate a clinical scenario, a prostate phantom was implanted with 66 markers and 86 seeds, and imaged on a 3.0T MRI scanner using a 3D fast radiofrequency-spoiled gradient recalled echo acquisition with various combinations of scan parameters. Scan parameters, including flip angle, number of excitations, bandwidth, field-of-view, slice thickness, and encoding steps were systematically varied to study their effects on signal, noise, scan time, image resolution, and artifacts. Results: The effects of pulse sequence parameter selection on the marker signal strength and image noise were characterized. The authors also examined the tradeoff between signal-to-noise ratio, scan time, and image artifacts, such as the wraparound artifact, susceptibility artifact, chemical shift artifact, and partial volume averaging artifact. Given reasonable scan time and managable artifacts, the authors recommended scan parameter combinations that can provide robust visualization of the MRI markers. Conclusions: The recommended MRI pulse sequence protocol allows for consistent visualization of the markers to assist seed localization, potentially enabling MRI-only prostate postimplant dosimetry.

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.…”

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

“…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.…”

Deformable registration of x ray and MRI for postimplant dosimetry in low dose rate prostate brachytherapy

“…Metallic seeds do not generate signal intensity using conventional pulse sequences, so appear as signal voids leading to some nonseed features identified as seeds (false positives) and some missing seeds (false negatives) . Seeds designed to generate positive contrast have been demonstrated, but are not typically integrated with isotopes used for therapy . Balanced steady‐state free precession (bSSFP) pulse sequences can improve MRI quality for seed identification and susceptibility‐based pulse sequences have been developed to provide positive contrast with conventional seeds .…”

“…14,15 Seeds designed to generate positive contrast have been demonstrated, but are not typically integrated with isotopes used for therapy. 16 Balanced steady-state free precession (bSSFP) pulse sequences can improve MRI quality for seed identification 17 and susceptibility-based pulse sequences have been developed to provide positive contrast with conventional seeds. 18,19 Susceptibility-based pulse sequences have recently been combined with machine learning-based automatic seed localization enabling all implanted seeds to be localized within 0.7 mm error in phantoms.…”