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

Fully Balanced SSFP Without an Endorectal Coil for Postimplant QA of MRI-Assisted Radiosurgery (MARS) of Prostate Cancer: A Prospective Study

PURPOSE

To investigate fully balanced steady-state free precession (bSSFP) with optimized acquisition protocols for magnetic resonance imaging (MRI)-based postimplant quality assessment of low-dose-rate (LDR) prostate brachytherapy without an endorectal coil (ERC).

METHODS AND MATERIALS

Seventeen patients at a major academic cancer center who underwent MRI-assisted radiosurgery (MARS) LDR prostate cancer brachytherapy were imaged with moderate, high, or very high spatial resolution fully bSSFP MRIs without using an ERC. Between 1 and 3 signal averages (NEX) were acquired with acceleration factors (R) between 1 and 2, with the goal of keeping scan times between 4 and 6 minutes. Acquisitions with R >1 were reconstructed with parallel imaging and compressed sensing (PICS) algorithms. Radioactive seeds were identified by 3 medical dosimetrists. Additionally, some of the MRI techniques were implemented and tested at a community hospital; 3 patients underwent MARS LDR prostate brachytherapy and were imaged without an ERC.

RESULTS

Increasing the in-plane spatial resolution mitigated partial volume artifacts and improved overall seed and seed marker visualization at the expense of reduced signal-to-noise ratio (SNR). The reduced SNR as a result of imaging at higher spatial resolution and without an ERC was partially compensated for by the multi-NEX acquisitions enabled by PICS. Resultant image quality was superior to the current clinical standard. All 3 dosimetrists achieved near-perfect precision and recall for seed identification in the 17 patients. The 3 postimplant MRIs acquired at the community hospital were sufficient to identify 208 out of 211 seeds implanted without reference to computed tomography (CT).

CONCLUSIONS

Acquiring postimplant prostate brachytherapy MRI without an ERC has several advantages including better patient tolerance, lower costs, higher clinical throughput, and widespread access to precision LDR prostate brachytherapy. This prospective study confirms that the use of an ERC can be circumvented with fully bSSFP and advanced MRI scan techniques in a major academic cancer center and community hospital, potentially enabling postimplant assessment of MARS LDR prostate brachytherapy without CT.

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

PURPOSE

Dosimetric assessment following permanent prostate brachytherapy (PPB) commonly involves seed localization using CT and prostate delineation using coregistered MRI. However, pelvic CT leads to additional imaging dose and requires significant resources to acquire and process both CT and MRI. In this study, we propose an automatic postimplant dosimetry approach that retains MRI for soft-tissue contouring, but eliminates the need for CT and reduces imaging dose while overcoming the inconsistent appearance of seeds on MRI with three projection x rays acquired using a mobile C-arm.

METHODS

Implanted seeds are reconstructed using x rays by solving a combinatorial optimization problem and deformably registered to MRI. Candidate seeds are located in MR images using local hypointensity identification. X ray-based seeds are registered to these candidate seeds in three steps: (a) rigid registration using a stochastic evolutionary optimizer, (b) affine registration using an iterative closest point optimizer, and (c) deformable registration using a local feature point search and nonrigid coherent point drift. The algorithm was evaluated using 20 PPB patients with x rays acquired immediately postimplant and T2-weighted MR images acquired the next day at 1.5 T with mean 0.8 × 0.8 × 3.0 mm 3 voxel dimensions. Target registration error (TRE) was computed based on the distance from algorithm results to manually identified seed locations using coregistered CT acquired the same day as the MRI. Dosimetric accuracy was determined by comparing prostate D90 determined using the algorithm and the ground truth CT-based seed locations.

RESULTS

The mean ± standard deviation TREs across 20 patients including 1774 seeds were 2.23 ± 0.52 mm (rigid), 1.99 ± 0.49 mm (rigid + affine), and 1.76 ± 0.43 mm (rigid + affine + deformable). The corresponding mean ± standard deviation D90 errors were 5.8 ± 4.8%, 3.4 ± 3.4%, and 2.3 ± 1.9%, respectively. The mean computation time of the registration algorithm was 6.1 s.

CONCLUSION

The registration algorithm accuracy and computation time are sufficient for clinical PPB postimplant dosimetry.

Resolution and registration in dual-plane co-RASOR MR

Magnetic resonance imaging (MRI) has superior soft tissue contrast and lower interobserver variability compared to computed tomography and advances in equipment and pseudo-CT estimation have allowed for MR-only radiation therapy planning. Dedicated MR sequences have been used to localize paramagnetic structures with positive contrast, and most implanted seeds are gold fiducial markers (GFMs). We used a fast, dual-plane co-RASOR sequence to localize implanted GFMs with positive contrast in phantom and tissue to assess their resolution and registration accuracy of registration to CT. Off-resonant reconstructions of co-RASOR images were able to resolve GFMs down to 5 mm apart at 12 cm FOV. No systematic biases were observed by comparing registration of co-RASOR and bSSFP to CT images in an MR-compatible Lego phantom with a set of highly visible known points. The standard deviations of the MR to CT distance errors were  <0.5 mm in all directions. We separated the component due to registration by comparing the two MR sequences, which had a maximum standard deviation of 0.36 mm in the SI-direction. Registration using the positive contrast points in a porcine sample phantom showed increased errors, but co-RASOR still performs acceptably with a target registration error of  <0.75 mm. The dual-plane co-RASOR sequence could then be used for both registration and image tracking when performing MR-only radiation therapy planning.

Magnetic Resonance Imaging only Workflow for Radiotherapy Simulation and Planning in Prostate Cancer

Magnetic resonance imaging (MRI) is often combined with computed tomography (CT) in prostate radiotherapy to optimise delineation of the target and organs-at-risk (OAR) while maintaining accurate dose calculation. Such a dual-modality workflow requires two separate imaging sessions, and it has some fundamental and logistical drawbacks. Due to the availability of new MRI hardware and software solutions, CT examinations can be omitted for prostate radiotherapy simulations. All information for treatment planning, including electron density maps and bony anatomy, can nowadays be obtained with MRI. Such an MRI-only simulation workflow reduces delineation ambiguities, eases planning logistics, and improves patient comfort; however, careful validation of the complete MRI-only workflow is warranted. The first institutes are now adopting this MRI-only workflow for prostate radiotherapy. In this article, we will review technology and workflow requirements for an MRI-only prostate simulation workflow.

Evaluation of gold fiducial marker manual localisation for magnetic resonance-only prostate radiotherapy

BACKGROUND

The use of intraprostatic gold fiducial markers (FMs) ensures highly accurate and precise image-guided radiation therapy for patients diagnosed with prostate cancer thanks to the ease of localising FMs on photon-based imaging, like Computed Tomography (CT) images. Recently, Magnetic Resonance (MR)-only radiotherapy has been proposed to simplify the workflow and reduce possible systematic uncertainties. A critical, determining factor in the accuracy of such an MR-only simulation will be accurate FM localisation using solely MR images.

PURPOSE

The aim of this study is to evaluate the performances of manual MR-based FM localisation within a clinical environment.

METHODS

We designed a study in which 5 clinically involved radiation therapy technicians (RTTs) independently localised the gold FMs implanted in 16 prostate cancer patients in two scenarios: employing a single MR sequence or a combination of sequences. Inter-observer precision and accuracy were assessed for the two scenarios for localisation in terms of 95% limit of agreement on single FMs (LoA)/ centre of mass (LoA _CM) and inter-marker distances (IDs), respectively.

RESULTS

The number of precisely located FMs (LoA <2 mm) increased from 38/48 to 45/48 FMs when localisation was performed using multiple sequences instead of single one. When performing localisation on multiple sequences, imprecise localisation of the FMs (3/48 FMs) occurred for 1/3 implanted FMs in three different patients. In terms of precision, we obtained LoA _CM within 0.25 mm in all directions over the precisely located FMs. In terms of accuracy, IDs difference of manual MR-based localisation versus CT-based localisation was on average (±1 STD) 0.6 ±0.6 mm.

CONCLUSIONS

For both the investigated scenarios, the results indicate that when FM classification was correct, the precision and accuracy are high and comparable to CT-based FM localisation. We found that use of multiple sequences led to better localisation performances compared with the use of single sequence. However, we observed that, due to the presence of calcification and motion, the risk of mislocated patient positioning is still too high to allow the sole use of manual FM localisation. Finally, strategies to possibly overcome the current challenges were proposed.

Quantitative analysis of image quality for acceptance and commissioning of an MRI simulator with a semiautomatic method

Magnetic Resonance Imaging (MRI) simulation differs from diagnostic MRI in purpose, technical requirements, and implementation. We propose a semiautomatic method for image acceptance and commissioning for the scanner, the radiofrequency (RF) coils, and pulse sequences for an MRI simulator. The ACR MRI accreditation large phantom was used for image quality analysis with seven parameters. Standard ACR sequences with a split head coil were adopted to examine the scanner's basic performance. The performance of simulation RF coils were measured and compared using the standard sequence with different clinical diagnostic coils. We used simulation sequences with simulation coils to test the quality of image and advanced performance of the scanner. Codes and procedures were developed for semiautomatic image quality analysis. When using standard ACR sequences with a split head coil, image quality passed all ACR recommended criteria. The image intensity uniformity with a simulation RF coil decreased about 34% compared with the eight‐channel diagnostic head coil, while the other six image quality parameters were acceptable. Those two image quality parameters could be improved to more than 85% by built‐in intensity calibration methods. In the simulation sequences test, the contrast resolution was sensitive to the FOV and matrix settings. The geometric distortion of simulation sequences such as T1‐weighted and T2‐weighted images was well‐controlled in the isocenter and 10 cm off‐center within a range of ±1% (2 mm). We developed a semiautomatic image quality analysis method for quantitative evaluation of images and commissioning of an MRI simulator. The baseline performances of simulation RF coils and pulse sequences have been established for routine QA.

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

PURPOSE

Permanent prostate brachytherapy dosimetry using computed tomography-magnetic resonance imaging (CT-MRI) fusion combines the anatomic detail of MRI with seed localization on CT but requires multimodality imaging acquisition and fusion. The purpose of this study was to compare the utility of MRI only postimplant dosimetry to standard CT-MRI fusion-based dosimetry.

METHODS AND MATERIALS

Twenty-three patients undergoing permanent prostate brachytherapy with use of positive contrast MRI markers were included in this study. Dose calculation to the whole prostate, apex, mid-gland, and base was performed via standard CT-MRI fusion and MRI only dosimetry with prostate delineated on the same T2 MRI sequence. The 3-dimensional (3D) distances between seed positions of these two methods were also evaluated. Wilcoxon-matched-pair signed-rank test compared the D90 and V100 of the prostate and its sectors between methods.

RESULTS

The day 0 D90 and V100 for the prostate were 98% versus 94% and 88% versus 86% for CT-MRI fusion and MRI only dosimetry. There were no differences in the D90 or V100 of the whole prostate, mid-gland, or base between dosimetric methods (p > 0.19), but prostate apex D90 was high by 13% with MRI dosimetry (p = 0.034). The average distance between seeds on CT-MRI fusion and MRI alone was 5.5 mm. After additional automated rigid registration of 3D seed positions, the average distance between seeds was 0.3 mm, and the previously observed differences in apex dose between methods was eliminated (p > 0.11).

CONCLUSIONS

Permanent prostate brachytherapy dosimetry based only on MRI using positive contrast MRI markers is feasible, accurate, and reduces the uncertainties arising from CT-MRI fusion abating the need for postimplant multimodality imaging.

Magnetic resonance imaging basics for the prostate brachytherapist

Magnetic resonance imaging (MRI) is increasingly being used in radiation therapy, and integration of MRI into brachytherapy in particular is becoming more common. We present here a systematic review of the basic physics and technical aspects of incorporating MRI into prostate brachytherapy. Terminology and MRI system components are reviewed along with typical work flows in prostate high-dose-rate and low-dose-rate brachytherapy. In general, the brachytherapy workflow consists of five key components: diagnosis, implantation, treatment planning (scan + plan), implant verification, and delivery. MRI integration is discussed for diagnosis; treatment planning; and MRI-guided brachytherapy implants, in which MRI is used to guide the physical insertion of the brachytherapy applicator or needles. Considerations and challenges for establishing an MRI brachytherapy program are also discussed.

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

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

PURPOSE

The purpose of this work is to present a brief review of MRI physics principles pertinent to prostate brachytherapy, and a summary of our experience in optimizing protocols for prostate brachytherapy applications.

METHODS AND MATERIALS

We summarized essential MR imaging characteristics and their interplays that need to be considered for prostate brachytherapy applications. These include spatial resolution, signal-to-noise ratio, image contrast, artifacts, geometric distortion, specific absorption rate, and total scan time. We further described the optimization of the protocols for three pulse sequences: three-dimensional (3D) fast-spoiled gradient echo sequence for T1-weighted imaging, 3D fast-spin echo sequence for T2-weighted imaging, and 3D fast imaging in steady-state precession sequence for combined T1 and T2-weighed imaging. The utilization of an endorectal coil was also described.

RESULTS

Using the optimized protocols, we acquired high-quality images of the entire prostate within 3-5 minutes for each sequence. These images display the desired image contrasts and a spatial resolution that is equal to or better than 0.59 mm × 0.73 mm × 1.2 mm. While 3D fast-spoiled gradient echo sequence and 3D fast-spin echo sequence depict radioactive seed markers and anatomic structures separately, 3D fast imaging in steady-state precession sequence demonstrates great promise for imaging both seed markers and prostate anatomy simultaneously in a single acquisition.

CONCLUSIONS

We have optimized current MRI protocols and demonstrated that the anatomic structures and positive contrast radioactive seed markers for prostate post-implant dosimetry can be adequately imaged either separately or simultaneously using different pulse sequences within a total scan time of 3-5 minutes each.

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

Purpose

Computed 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 methods

We 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.

Results

The 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.

Conclusions

We have demonstrated an MRI protocol for use with hyperintense encapsulated contrast agent markers to assist in the identification of hypointense seeds.