Characteristics and quality assurance of a dedicated open 0.23 T MRI for radiation therapy simulation

System-dependent image distortion related to gantry positions of a 0.35 T MRgRT: Characterization and the corresponding correction

PURPOSE

MR-guided radiotherapy with high accuracy treatment planning requires addressing MR imaging artifacts that originate from system imperfections. This work presents the characterization and corresponding correction of gantry-related imaging distortions including geometric distortion and isocenter shift in a 0.35 T magnetic resonance imaging (MRI)-guided radiotherapy (MRgRT) system using distortion vector fields (DVFs).

METHODS

Two phantoms, the magnetic resonance imaging distortion in 3D (MRID^3D ) phantom and the Fluke phantom, along with a human volunteer were imaged at different gantry angles on a 0.35 T MR-Linac. The geometric distortion and isocenter shift were characterized for both phantom images. DVFs with a field of view extended beyond the physical boundary of the MRID^3D phantom were extracted from images taken at 30° gantry angle increments, with vendor-provided distortion correction turned on and off (DstOff). These extended DVFs were then applied to the relevant phantom images to correct their geometric distortions and isocenter shift at the respective gantry angles. The extended DVFs produced from the MRID^3D phantom were also applied to Fluke phantom and human MR images at their respective gantry angles. The resampled images were evaluated using structural similarity index measure (SSIM) comparison with the vendor corrected images from the MRgRT system.

RESULTS

Geometric distortion with "mean (± SD) distortion" of 3.2 ± 0.02, 2.9 ± 0.02, and 1.8 ± 0.01 mm and isocenter shift (±SD) of 0.49 ± 0.3, 0.05 ± 0.2, and 0.01 ± 0.03 mm were present in the DstOff MRID^3D phantom images in right-left (RL), anterior-posterior (AP), and superior-inferior (SI) directions, respectively. After resampling the originally acquired images by applying extended DVFs, the distortion was corrected to 0.18 ± 0.02, 0.09 ± 0.01, 0.15 ± 0.01 mm, and isocenter shift was corrected to 0.14 ± 0.05, -0.02 ± 0.04, and -0.07 ± 0.05 mm in RL, AP, and SI directions, respectively. The Fluke phantom average geometric distortion with "mean (± SD) distortion" of 2.7 ± 0.1 mm was corrected to 0.2 ± 0. 1 mm and the average isocenter shift (± SD) of 0.51 ± 0.2 mm, and 0.05 ± 0.03 was corrected to -0.08 ± 0.03 mm, and -0.05 ± 0.01 in RL and AP directions, respectively. SSIM (mean ± SD) of the original images to resampled images was increased from 0.49 ± 0.02 to 0.78 ± 0.01, 0.45 ± 0.02 to 0.75 ± 0.01, and 0.86 ± 0.25 to 0.98 ± 0.08 for MRID^3D phantom, Fluke phantom, and human MR images, respectively, for all the gantry angles compared to the vendor corrected images.

CONCLUSION

The gantry-related MR imaging distortion including geometric distortion and isocenter shift was characterized and a corresponding correction was demonstrated using extended DVFs on 0.35 T MRgRT system. The characterized gantry-related isocenter shift can be combined with geometric distortion correction to provide a technique for the correction of the full system-dependent distortion in an MRgRT system.

Development of a vendor neutral MRI distortion quality assurance workflow

With the utilization of magnetic resonance (MR) imaging in radiotherapy increasing, routine quality assurance (QA) of these systems is necessary. The assessment of geometric distortion in images used for radiotherapy treatment planning needs to be quantified and monitored over time. This work presents an adaptable methodology for performing routine QA for systematic MRI geometric distortion. A software tool and compatible protocol (designed to work with any CT and MR compatible phantom on any scanner) were developed to quantify geometric distortion via deformable image registration. The MR image is deformed to the CT, generating a deformation field, which is sampled, quantifying geometric distortion as a function of distance from scanner isocenter. Configurability of the QA tool was tested, and results compared to those provided from commercial solutions. Registration accuracy was investigated by repeating the deformable registration step on the initial deformed MR image to define regions with residual distortions. The geometric distortion of four clinical systems was quantified using the customisable QA method presented. Maximum measured distortions varied from 2.2 to 19.4 mm (image parameter and sampling volume dependent). The workflow was successfully customized for different phantom configurations and volunteer imaging studies. Comparison to a vendor supplied solution showed good agreement in regions where the two procedures were sampling the same imaging volume. On a large field of view phantom across various scanners, the QA tool accurately quantified geometric distortions within 17–22 cm from scanner isocenter. Beyond these regions, the geometric integrity of images in clinical applications should be considered with a higher degree of uncertainty due to increased gradient nonlinearity and B₀ inhomogeneity. This tool has been successfully integrated into routine QA of the MRI scanner utilized for radiotherapy within our department. It enables any low susceptibility MR‐CT compatible phantom to quantify the geometric distortion on any MRI scanner with a configurable, user friendly interface for ease of use and consistency in data collection and analysis.

Task group 284 report: magnetic resonance imaging simulation in radiotherapy: considerations for clinical implementation, optimization, and quality assurance

The use of dedicated magnetic resonance simulation (MR-SIM) platforms in Radiation Oncology has expanded rapidly, introducing new equipment and functionality with the overall goal of improving the accuracy of radiation treatment planning. However, this emerging technology presents a new set of challenges that need to be addressed for safe and effective MR-SIM implementation. The major objectives of this report are to provide recommendations for commercially available MR simulators, including initial equipment selection, siting, acceptance testing, quality assurance, optimization of dedicated radiation therapy specific MR-SIM workflows, patient-specific considerations, safety, and staffing. Major contributions include guidance on motion and distortion management as well as MRI coil configurations to accommodate patients immobilized in the treatment position. Examples of optimized protocols and checklists for QA programs are provided. While the recommendations provided here are minimum requirements, emerging areas and unmet needs are also highlighted for future development.

IPEM topical report: guidance on the use of MRI for external beam radiotherapy treatment planning. Phys Med Biol 2021;66:055025. [PMID: 33450742 DOI: 10.1088/1361-6560/abdc30]

This document gives guidance for multidisciplinary teams within institutions setting up and using an MRI-guided radiotherapy (RT) treatment planning service. It has been written by a multidisciplinary working group from the Institute of Physics and Engineering in Medicine (IPEM). Guidance has come from the experience of the institutions represented in the IPEM working group, in consultation with other institutions, and where appropriate references are given for any relevant legislation, other guidance documentation and information in the literature. Guidance is only given for MRI acquired for external beam RT treatment planning in a CT-based workflow, i.e. when MRI is acquired and registered to CT with the purpose of aiding delineation of target or organ at risk volumes. MRI use for treatment response assessment, MRI-only RT and other RT treatment types such as brachytherapy and gamma radiosurgery are not considered within the scope of this document. The aim was to produce guidance that will be useful for institutions who are setting up and using a dedicated MR scanner for RT (referred to as an MR-sim) and those who will have limited time on an MR scanner potentially managed outside of the RT department, often by radiology. Although not specifically covered in this document, there is an increase in the use of hybrid MRI-linac systems worldwide and brief comments are included to highlight any crossover with the early implementation of this technology. In this document, advice is given on introducing a RT workload onto a non-RT-dedicated MR scanner, as well as planning for installation of an MR scanner dedicated for RT. Next, practical guidance is given on the following, in the context of RT planning: training and education for all staff working in and around an MR scanner; RT patient set-up on an MR scanner; MRI sequence optimisation for RT purposes; commissioning and quality assurance (QA) to be performed on an MR scanner; and MRI to CT registration, including commissioning and QA.

Characterizing MR Imaging isocenter variation in MRgRT

We characterized MRI isocenter variation at various gantry positions in two 0.35 T MRgRT systems using two independent methods. First, image center-based quantification was employed on 3D volumetric and 2D cine images of a 24 cm diameter spherical phantom at various gantry positions in the MRI QA mode. The center of the phantom images was identified to quantify the variation of the imaging center at each gantry position. Second, image registration-based quantification was used in radiotherapy mode. 3D volumetric MRIs of a cylindrical phantom were acquired and corresponding image registration from MRI to planning CT was performed. The shifts of the couch were identified to quantify the variation of the imaging center. For verification of noticeable MRI isocenter variation, star-shot pattern measurements with five beams were delivered on the radio-chromic film inserted into the phantom after the couch was shifted. The center of the star-shot pattern was identified to quantify the variation of the imaging center. The proposed methods for measuring MRI isocenter variation were demonstrated with MR-LINAC and MR-60Co systems. Both of the MRgRT systems had field inhomogeneities <5 ppm over a 24 cm diameter spherical volume (DSV) and spatial integrity distortion: <1 mm within 100 mm radius and <2 mm within 175 mm radius. The MRI isocenter of the MR-LINAC system showed noticeable 3D variation (max magnitude: 1.8 mm) compared to that of MR-60Co system (max magnitude: 0.9 mm) relative to the reference gantry positions. In addition, 2D variations (max magnitude) of the MRI isocenter from sagittal cine images were 0.9 mm for the MR-LINAC system and 0.5 mm for the MR-60Co system. Two proposed methods quantified the MRI isocenter variation for various gantry positions in two 0.35 T MRgRT systems. The results of significant isocenter variation in the MR-LINAC system requires further investigation to determine the cause.

Modern Radiation Therapy Planning and Delivery

Modern radiation therapy treatment planning and delivery is a complex process that relies on advanced imaging and computing technology as well as expertise from the medical team. The process begins with simulation imaging, in which three-dimensional computed tomography images (or magnetic resonance images in some cases) are used to characterize the patient anatomy. From there, the radiation oncologist delineates the relevant target/tumor volumes and normal tissue and communicates the goals for treatment planning. The planning process attempts to generate a radiation therapy treatment plan that will deliver a therapeutic dose of radiation to the tumor while sparing nearby normal tissue.

Role and future of MRI in radiation oncology

Technical innovations and developments in areas such as disease localization, dose calculation algorithms, motion management and dose delivery technologies have revolutionized radiation therapy resulting in improved patient care with superior outcomes. A consequence of the ability to design and accurately deliver complex radiation fields is the need for improved target visualization through imaging. While CT imaging has been the standard of care for more than three decades, the superior soft tissue contrast afforded by MR has resulted in the adoption of this technology in radiation therapy. With the development of real time MR imaging techniques, the problem of real time motion management is enticing. Currently, the integration of an MR imaging and megavoltage radiation therapy treatment delivery system (MR-linac or MRL) is a reality that has the potential to provide improved target localization and real time motion management during treatment. Higher magnetic field strengths provide improved image quality potentially providing the backbone for future work related to image texture analysis-a field known as Radiomics-thereby providing meaningful information on the selection of future patients for radiation dose escalation, motion-managed treatment techniques and ultimately better patient care. On-going advances in MRL technologies promise improved real time soft tissue visualization, treatment margin reductions, beam optimization, inhomogeneity corrected dose calculation, fast multileaf collimators and volumetric arc radiation therapy. This review article provides rationale, advantages and disadvantages as well as ideas for future research in MRI related to radiation therapy mainly in adoption of MRL.

Stereotactic radiosurgery planning of vestibular schwannomas: Is MRI at 3 Tesla geometrically accurate?

Purpose

MRI is a mandatory requirement to accurately plan Stereotactic Radiosurgery (SRS) for Vestibular Schwannomas. However, MRI may be distorted due not only to inhomogeneity of the static magnetic field and gradients but also due to susceptibility‐induced effects, which are more prominent at higher magnetic fields. We assess geometrical distortions around air spaces and consider MRI protocol requirements for SRS planning at 3 T.

Methods

Hardware‐related distortion and the effect of incorrect shimming were investigated with structured test objects. The magnetic field was mapped over the head on five volunteers to assess susceptibility‐related distortion in the naso‐oro‐pharyngeal cavities (NOPC) and around the internal ear canal (IAC).

Results

Hardware‐related geometric displacements were found to be less than 0.45 mm within the head volume, after distortion correction. Shimming errors can lead to displacements of up to 4 mm, but errors of this magnitude are unlikely to arise in practice. Susceptibility‐related field inhomogeneity was under 3.4 ppm, 2.8 ppm, and 2.7 ppm for the head, NOPC region and IAC region, respectively. For the SRS planning protocol (890 Hz/pixel, approximately 1 mm³ isotropic), susceptibility‐related displacements were less than 0.5 mm (head), and 0.4 mm (IAC and NOPC). Large displacements are possible in MRI examinations undertaken with lower receiver bandwidth values, commonly used in clinical MRI. Higher receiver bandwidth makes the protocol less vulnerable to sub‐optimal shimming. The shimming volume and the CT‐MR co‐registration must be considered jointly.

Conclusion

Geometric displacements can be kept under 1 mm in the vicinity of air spaces within the head at 3 T with appropriate setting of the receiver bandwidth, correct shimming and employing distortion correction.

Technical Note: Characterization and correction of gradient nonlinearity induced distortion on a 1.0 T open bore MR-SIM

PURPOSE

Distortions in magnetic resonance imaging (MRI) compromise spatial fidelity, potentially impacting delineation and dose calculation. We characterized 2D and 3D large field of view (FOV), sequence-independent distortion at various positions in a 1.0 T high-field open MR simulator (MR-SIM) to implement correction maps for MRI treatment planning.

METHODS

A 36 × 43 × 2 cm(3) phantom with 255 known landmarks (∼1 mm(3)) was scanned using 1.0 T high-field open MR-SIM at isocenter in the transverse, sagittal, and coronal axes, and a 465 × 350 × 168 mm(3) 3D phantom was scanned by stepping in the superior-inferior direction in three overlapping positions to achieve a total 465 × 350 × 400 mm(3) sampled FOV yielding >13 800 landmarks (3D Gradient-Echo, TE/TR/α = 5.54 ms/30 ms/28°, voxel size = 1 × 1 × 2 mm(3)). A binary template (reference) was generated from a phantom schematic. An automated program converted MR images to binary via masking, thresholding, and testing for connectivity to identify landmarks. Distortion maps were generated by centroid mapping. Images were corrected via warping with inverse distortion maps, and temporal stability was assessed.

RESULTS

Over the sampled FOV, non-negligible residual gradient distortions existed as close as 9.5 cm from isocenter, with a maximum distortion of 7.4 mm as close as 23 cm from isocenter. Over six months, average gradient distortions were -0.07 ± 1.10 mm and 0.10 ± 1.10 mm in the x and y directions for the transverse plane, 0.03 ± 0.64 and -0.09 ± 0.70 mm in the sagittal plane, and 0.4 ± 1.16 and 0.04 ± 0.40 mm in the coronal plane. After implementing 3D correction maps, distortions were reduced to <1 pixel width (1 mm) for all voxels up to 25 cm from magnet isocenter.

CONCLUSIONS

Inherent distortion due to gradient nonlinearity was found to be non-negligible even with vendor corrections applied, and further corrections are required to obtain 1 mm accuracy for large FOVs. Statistical analysis of temporal stability shows that sequence independent distortion maps are consistent within six months of characterization.

Spatial Precision in Magnetic Resonance Imaging-Guided Radiation Therapy: The Role of Geometric Distortion

Because magnetic resonance imaging-guided radiation therapy (MRIgRT) offers exquisite soft tissue contrast and the ability to image tissues in arbitrary planes, the interest in this technology has increased dramatically in recent years. However, intrinsic geometric distortion stemming from both the system hardware and the magnetic properties of the patient affects MR images and compromises the spatial integrity of MRI-based radiation treatment planning, given that for real-time MRIgRT, precision within 2 mm is desired. In this article, we discuss the causes of geometric distortion, describe some well-known distortion correction algorithms, and review geometric distortion measurements from 12 studies, while taking into account relevant imaging parameters. Eleven of the studies reported phantom measurements quantifying system-dependent geometric distortion, while 2 studies reported simulation data quantifying magnetic susceptibility-induced geometric distortion. Of the 11 studies investigating system-dependent geometric distortion, 5 reported maximum measurements less than 2 mm. The simulation studies demonstrated that magnetic susceptibility-induced distortion is typically smaller than system-dependent distortion but still nonnegligible, with maximum distortion ranging from 2.1 to 2.6 mm at a field strength of 1.5 T. As expected, anatomic landmarks containing interfaces between air and soft tissue had the largest distortions. The evidence indicates that geometric distortion reduces the spatial integrity of MRI-based radiation treatment planning and likely diminishes the efficacy of MRIgRT. Better phantom measurement techniques and more effective distortion correction algorithms are needed to achieve the desired spatial precision.

Continuous table acquisition MRI for radiotherapy treatment planning: distortion assessment with a new extended 3D volumetric phantom

PURPOSE

Accurate geometry is required for radiotherapy treatment planning (RTP). When considering the use of magnetic resonance imaging (MRI) for RTP, geometric distortions observed in the acquired images should be considered. While scanner technology and vendor supplied correction algorithms provide some correction, large distortions are still present in images, even when considering considerably smaller scan lengths than those typically acquired with CT in conventional RTP. This study investigates MRI acquisition with a moving table compared with static scans for potential geometric benefits for RTP.

METHODS

A full field of view (FOV) phantom (diameter 500 mm; length 513 mm) was developed for measuring geometric distortions in MR images over volumes pertinent to RTP. The phantom consisted of layers of refined plastic within which vitamin E capsules were inserted. The phantom was scanned on CT to provide the geometric gold standard and on MRI, with differences in capsule location determining the distortion. MRI images were acquired with two techniques. For the first method, standard static table acquisitions were considered. Both 2D and 3D acquisition techniques were investigated. With the second technique, images were acquired with a moving table. The same sequence was acquired with a static table and then with table speeds of 1.1 mm/s and 2 mm/s. All of the MR images acquired were registered to the CT dataset using a deformable B-spline registration with the resulting deformation fields providing the distortion information for each acquisition.

RESULTS

MR images acquired with the moving table enabled imaging of the whole phantom length while images acquired with a static table were only able to image 50%-70% of the phantom length of 513 mm. Maximum distortion values were reduced across a larger volume when imaging with a moving table. Increased table speed resulted in a larger contribution of distortion from gradient nonlinearities in the through-plane direction and an increased blurring of capsule images, resulting in an apparent capsule volume increase by up to 170% in extreme axial FOV regions. Blurring increased with table speed and in the central regions of the phantom, geometric distortion was less for static table acquisitions compared to a table speed of 2 mm/s over the same volume. Overall, the best geometric accuracy was achieved with a table speed of 1.1 mm/s.

CONCLUSIONS

The phantom designed enables full FOV imaging for distortion assessment for the purposes of RTP. MRI acquisition with a moving table extends the imaging volume in the z direction with reduced distortions which could be useful particularly if considering MR-only planning. If utilizing MR images to provide additional soft tissue information to the planning CT, standard acquisition sequences over a smaller volume would avoid introducing additional blurring or distortions from the through-plane table movement.

Initial clinical experience with a radiation oncology dedicated open 1.0T MR-simulation

The purpose of this study was to describe our experience with 1.0T MR-SIM including characterization, quality assurance (QA) program, and features necessary for treatment planning. Staffing, safety, and patient screening procedures were developed. Utilization of an external laser positioning system (ELPS) and MR-compatible couchtop were illustrated. Spatial and volumetric analyses were conducted between CT-SIM and MR-SIM using a stereotactic QA phantom with known landmarks and volumes. Magnetic field inhomogeneity was determined using phase difference analysis. System-related, in-plane distortion was evaluated and temporal changes were assessed. 3D distortion was characterized for regions of interest (ROIs) 5-20 cm away from isocenter. American College of Radiology (ACR) recommended tests and impact of ELPS on image quality were analyzed. Combined ultrashort echotime Dixon (UTE/Dixon) sequence was evaluated. Amplitude-triggered 4D MRI was implemented using a motion phantom (2-10 phases, ~ 2 cm excursion, 3-5 s periods) and a liver cancer patient. Duty cycle, acquisition time, and excursion were evaluated between maximum intensity projection (MIP) datasets. Less than 2% difference from expected was obtained between CT-SIM and MR-SIM volumes, with a mean distance of &lt; 0.2 mm between landmarks. Magnetic field inhomogeneity was &lt; 2 ppm. 2D distortion was &lt; 2 mm over 28.6-33.6 mm of isocenter. Within 5 cm radius of isocenter, mean 3D geometric distortion was 0.59 ± 0.32 mm (maximum = 1.65 mm) and increased 10-15 cm from isocenter (mean = 1.57 ± 1.06 mm, maximum = 6.26 mm). ELPS interference was within the operating frequency of the scanner and was characterized by line patterns and a reduction in signal-to-noise ratio (4.6-12.6% for TE = 50-150 ms). Image quality checks were within ACR recommendations. UTE/Dixon sequences yielded detectability between bone and air. For 4D MRI, faster breathing periods had higher duty cycles than slow (50.4% (3 s) and 39.4% (5 s), p &lt; 0.001) and ~fourfold acquisition time increase was measured for ten-phase versus two-phase. Superior-inferior object extent was underestimated 8% (6 mm) for two-phase as compared to ten-phase MIPs, although &lt; 2% difference was obtained for ≥ 4 phases. 4D MRI for a patient demonstrated acceptable image quality in ~ 7 min. MR-SIM was integrated into our workflow and QA procedures were developed. Clinical applicability was demonstrated for 4D MRI and UTE imaging to support MR-SIM for single modality treatment planning.

Characteristics of geometric distortion correction with increasing field-of-view in open-configuration MRI

Open-configuration magnetic resonance imaging (MRI) systems are becoming increasingly desirable for musculoskeletal imaging and image-guided radiotherapy because of their non-claustrophobic configuration. However, geometric image distortion in large fields-of-view (FOV) due to field inhomogeneity and gradient nonlinearity hinders the practical applications of open-type MRI. We demonstrated the use of geometric distortion correction for increasing FOV in open MRI. Geometric distortion was modeled and corrected as a global polynomial function. The appropriate polynomial order was identified as the minimum difference between the coordinates of control points in the distorted MR image space and those predicted by polynomial modeling. The sixth order polynomial function was found to give the optimal value for geometric distortion correction. The area of maximum distortion was<1 pixel with an FOV of 285mm. The correction performance error was increased at most 1.2% and 2.9% for FOVs of 340mm and~400mm compared with the FOV of 285mm. In particular, unresolved distortion was generated by local deformation near the gradient coil center.

Commissioning of a new wide-bore MRI scanner for radiotherapy planning of head and neck cancer

OBJECTIVE

A combination of CT and MRI is recommended for radiotherapy planning of head and neck cancers, and optimal spatial co-registration is achieved by imaging in the treatment position using the necessary immobilisation devices on both occasions, something which requires wide-bore scanners. Quality assurance experiments were carried out to commission a newly installed 1.5-T wide-bore MRI scanner and a dedicated, flexible six-channel phased array head and neck coil.

METHODS

Signal-to-noise ratio (SNR) and spatial signal uniformity were quantified using a homogeneous aqueous phantom, and geometric distortion was quantified using a phantom with water-filled fiducials in a grid pattern. Volunteer scans were also used to determine the in vivo image quality. Clinically relevant T1 weighted and T2 weighted fat-suppressed sequences were assessed in multiple scan planes (both sequences fast spin echo based). The performance of two online signal uniformity correction schemes, one utilising low-resolution reference scans and the other not utilising low-resolution reference scans, was compared.

RESULTS

Geometric distortions, for a ±35-kHz bandwidth, were <1 mm for locations within 10 cm of the isocentre rising to 1.8 mm at 18 cm away. SNR was above 50, and uniformity in the axial plane was 71% and 95% before and after uniformity correction, respectively.

CONCLUSION

The combined performance of the wide-bore scanner and the dedicated coil was adjudged adequate, although superior-inferior spatial coverage was slightly limited in the lower neck.

ADVANCES IN KNOWLEDGE

These results will be of interest to the increasing number of oncology centres that are seeking to incorporate MRI into planning practice using dedicated equipment.

MRI simulation for radiotherapy treatment planning

Over the last two decades, the computed tomography simulator became the standard of the contemporary radiotherapy treatment planning (RTP) process. Along the same time, the superb soft tissue contrast of magnetic resonance imaging (MRI) was widely incorporated into RTP through the process of image coregistration. This review summarizes the efforts of incorporation of MRI data into target definition process for RTP based on gained clinical evidence so far and opens a question whether the time is up for bringing a MRI-simulator as an additional standard imaging tool into radiation oncology departments.

Magnetic resonance imaging for adaptive cobalt tomotherapy: A proposal

Magnetic resonance imaging (MRI) provides excellent soft tissue contrast for oncology applications. We propose to combine a MRI scanner with a helical tomotherapy (HT) system to enable daily target imaging for improved conformal radiation dose delivery to a patient. HT uses an intensity-modulated fan-beam that revolves around a patient, while the patient slowly advances through the plane of rotation, yielding a helical beam trajectory. Since the use of a linear accelerator to produce radiation may be incompatible with the pulsed radiofrequency and the high and pulsed magnetic fields required for MRI, it is proposed that a radioactive Cobalt-60 ((60)Co) source be used instead to provide the radiation. An open low field (0.25 T) MRI system is proposed where the tomotherapy ring gantry is located between two sets of Helmholtz coils that can generate a sufficiently homogenous main magnetic field.It is shown that the two major challenges with the design, namely acceptable radiation dose rate (and therefore treatment duration) and moving parts in strong magnetic field, can be addressed. The high dose rate desired for helical tomotherapy delivery can be achieved using two radiation sources of 220TBq (6000Ci) each on a ring gantry with a source to axis-of-rotation distance of 75 cm. In addition to this, a dual row multi-leaf collimator (MLC) system with 15 mm leaf width at isocentre and relatively large fan beam widths between 15 and 30 mm per row shall be employed. In this configuration, the unit would be well-suited for most pelvic radiotherapy applications where the soft tissue contrast of MRI will be particularly beneficial. Non-magnetic MRI compatible materials must be used for the rotating gantry. Tungsten, which is non-magnetic, can be used for primary collimation of the fan-beam as well as for the MLC, which allows intensity modulated radiation delivery. We propose to employ a low magnetic Cobalt compound, sycoporite (CoS) for the Cobalt source material itself.Rotational delivery is less susceptible to problems related to the use of a low energy megavoltage photon source while the helical delivery reduces the negative impact of the relatively large penumbra inherent in the use of Cobalt sources for radiotherapy. On the other hand, the use of a (60)Co source ensures constant dose rate with gantry rotation and makes dose calculation in a magnetic field as easy as the range of secondary electrons is limited.The MR-integrated Cobalt tomotherapy unit, dubbed 'MiCoTo,' uses two independent physical principles for image acquisition and treatment delivery. It would offer excellent target definition and will allow following target motion during treatment using fast imaging techniques thus providing the best possible input for adaptive radiotherapy. As an additional bonus, quality assurance of the radiation delivery can be performed in situ using radiation sensitive gels imaged by MRI.

MRI-based Treatment Planning with Electron Density Information Mapped from CT Images: A Preliminary Study

Nowadays magnetic resonance imaging (MRI) has been profoundly used in radiotherapy (RT) planning to aid the contouring of targets and critical organs in brain and intracranial cases, which is attributable to its excellent soft tissue contrast and multi-planar imaging capability. However, the lack of electron density information in MRI, together with the image distortion issues, precludes its use as the sole image set for RT planning and dose calculation. The purpose of this preliminary study is to probe the feasibility and evaluate an MRI-based radiation dose calculation process by providing MR images the necessary electron density (ED) information from a patient's readily available diagnostic/staging computed tomography (CT) images using an image registration model. To evaluate the dosimetric accuracy of the proposed approach, three brain and three intracranial cases were selected retrospectively for this study. For each patient, the MR images were registered to the CT images, and the ED information was then mapped onto the MR images by in-house developed software generating a modified set of MR images. Another set of MR images with voxel values assigned with the density of water was also generated. The original intensity modulated radiation treatment (IMRT) plan was then applied to the two sets of MR images and the doses were calculated. The dose distributions from the MRI-based calculations were compared to that of the original CT-based calculation. In all cases, the MRI-based calculations with mapped ED yielded dose values very close (within 2%) to that of the CT-based calculations. The MRI-based calculations with voxel values assigned with water density indicated a dosimetric error of 3–5%, depending on the treatment site. The present approach offers a means of utilizing MR images for accurate dose calculation and affords a potential to eliminate the redundant simulation CT by planning a patient's treatment with only simulation MRI and any available diagnostic/staging CT data.

Characterization and 3D correction of geometric distortion in low-field open-magnet MRI

In this paper, we present a method to characterize and correct geometric image distortion in 0.2 T MR images. A large 3D phantom with spherical balls was used to characterize geometric distortion on an AIRIS Mate 0.2 T MR Scanner (Hitachi). MR images of the phantom were acquired in axial, sagittal and coronal planes using 2D Fast Spin Echo (FSE) sequence and distortions were measured at each control point. Two piecewise interpolation methods were then applied to correct geometric distortion. Distortion was characterized and corrected in any axial, sagittal or coronal slice within an effective FOV of 330(LR) x 180(AP) x 210(HF) mm(3). The distortion was reduced from 16 mm to 1.2 mm at 180 mm from the magnet center. A fast and accurate method for correction of geometric distortion was performed within large distances from the magnet isocenter.

Automated analysis of multi site MRI phantom data for the NIHPD project

In large multi-center studies it is important to quantify data variations due to differences between sites and over time. Here we investigate inter-site variability in signal to noise ratio (SNR), percent integral uniformity (PIU), width and height using the American College of Radiology (ACR) phantom scans from the NIHPD project. Longitudinal variations are also analyzed. All measurements are fully automated. Our results show that the mean SNR, PIU and the 2 metric values were statistically different across sites. The maximum mean difference in diameter across sites was 2 mm (1.1%), and the maximum mean difference in height was 2.5 mm (1.7%). Over time, an average drift of 0.4 mm per year was observed for the diameter while a drift of 0.5 mm per year was observed for the height. Trends observed over time often depended not only on site, but also on modality and scanner manufacturer.

Geometric distortion in clinical MRI systems Part I: evaluation using a 3D phantom

Recently, a 3D phantom that can provide a comprehensive and accurate measurement of the geometric distortion in MRI has been developed. Using this phantom, a full assessment of the geometric distortion in a number of clinical MRI systems (GE and Siemens) has been carried out and detailed results are presented in this paper. As expected, the main source of geometric distortion in modern superconducting MRI systems arises from the gradient field nonlinearity. Significantly large distortions with maximum absolute geometric errors ranged between 10 and 25 mm within a volume of 240 x 240 x 240 mm(3) were observed when imaging with the new generation of gradient systems that employs shorter coils. By comparison, the geometric distortion was much less in the older-generation gradient systems. With the vendor's correction method, the geometric distortion measured was significantly reduced but only within the plane in which these 2D correction methods were applied. Distortion along the axis normal to the plane was, as expected, virtually unchanged. Two-dimensional correction methods are a convenient approach and in principle they are the only methods that can be applied to correct geometric distortion in a single slice or in multiple noncontiguous slices. However, these methods only provide an incomplete solution to the problem and their value can be significantly reduced if the distortion along the normal of the correction plane is not small.

Dosimetric evaluation of MRI-based treatment planning for prostate cancer

The purpose of this study is to evaluate the dosimetric accuracy of MRI-based treatment planning for prostate cancer using a commercial radiotherapy treatment planning system. Three-dimensional conformal plans for 15 prostate patients were generated using the AcQPlan system. For each patient, dose distributions were calculated using patient CT data with and without heterogeneity correction, and using patient MRI data without heterogeneity correction. MR images were post-processed using the gradient distortion correction (GDC) software. The distortion corrected MR images were fused to the corresponding CT for each patient for target and structure delineation. The femoral heads were delineated based on CT. Other anatomic structures relevant to the treatment (i.e., prostate, seminal vesicles, lymph notes, rectum and bladder) were delineated based on MRI. The external contours were drawn separately on CT and MRI. The same internal contours were used in the dose calculation using CT- and MRI-based geometries by directly transferring them between MRI and CT as needed. Treatment plans were evaluated based on maximum dose, isodose distributions and dose-volume histograms. The results confirm previous investigations that there is no clinically significant dose difference between CT-based prostate plans with and without heterogeneity correction. The difference in the target dose between CT- and MRI-based plans using homogeneous geometry was within 2.5%. Our results suggest that MRI-based treatment planning is suitable for radiotherapy of prostate cancer.

A proposed scheme for comprehensive characterization of the measured geometric distortion in magnetic resonance imaging using a three-dimensional phantom

Recently, a 3-dimensional phantom that can provide a comprehensive, accurate and complete measurement of the geometric distortion in MRI has been developed. In this paper, a scheme for characterizing the measured geometric distortion using the 3-D phantom is described. In the proposed scheme, a number of quantitative measures are developed and used to characterize the geometric distortion. These measures encompass the overall and spatial aspects of the geometric distortion. Two specific types of volume of interest, rectangular parallelepipeds (including cubes) and spheres are considered in the proposed scheme. As an illustration, characterization of the geometric distortion in a Siemens 1.5T Sonata MRI system using the proposed scheme is presented. As shown, the proposed scheme provides a comprehensive assessment of the geometric distortion. The scheme can be potentially used as a standard procedure for the assessment of geometric distortion in MRI.

MRI-based treatment planning for radiotherapy: Dosimetric verification for prostate IMRT

PURPOSE

Magnetic resonance (MR) and computed tomography (CT) image fusion with CT-based dose calculation is the gold standard for prostate treatment planning. MR and CT fusion with CT-based dose calculation has become a routine procedure for intensity-modulated radiation therapy (IMRT) treatment planning at Fox Chase Cancer Center. The use of MRI alone for treatment planning (or MRI simulation) will remove any errors associated with image fusion. Furthermore, it will reduce treatment cost by avoiding redundant CT scans and save patient, staff, and machine time. The purpose of this study is to investigate the dosimetric accuracy of MRI-based treatment planning for prostate IMRT.

METHODS AND MATERIALS

A total of 30 IMRT plans for 15 patients were generated using both MRI and CT data. The MRI distortion was corrected using gradient distortion correction (GDC) software provided by the vendor (Philips Medical System, Cleveland, OH). The same internal contours were used for the paired plans. The external contours were drawn separately between CT-based and MR imaging-based plans to evaluate the effect of any residual distortions on dosimetric accuracy. The same energy, beam angles, dose constrains, and optimization parameters were used for dose calculations for each paired plans using a treatment optimization system. The resulting plans were compared in terms of isodose distributions and dose-volume histograms (DVHs). Hybrid phantom plans were generated for both the CT-based plans and the MR-based plans using the same leaf sequences and associated monitor units (MU). The physical phantom was then irradiated using the same leaf sequences to verify the dosimetry accuracy of the treatment plans.

RESULTS

Our results show that dose distributions between CT-based and MRI-based plans were equally acceptable based on our clinical criteria. The absolute dose agreement for the planning target volume was within 2% between CT-based and MR-based plans and 3% between measured dose and dose predicted by the planning system in the physical phantom.

CONCLUSIONS

Magnetic resonance imaging is a useful tool for radiotherapy simulation. Compared with CT-based treatment planning, MR imaging-based treatment planning meets the accuracy for dose calculation and provides consistent treatment plans for prostate IMRT. Because MR imaging-based digitally reconstructed radiographs do not provide adequate bony structure information, a technique is suggested for producing a wire-frame image that is intended to replace the traditional digitally reconstructed radiographs that are made from CT information.

Effects of geometric distortion in 0.2T MRI on radiotherapy treatment planning of prostate cancer

BACKGROUND AND PURPOSE

To evaluate the impact of two different methods of geometric distortion correction of MR images from a Siemens Magnetom Open Viva 0.2T resistive MR unit on the process of external beam radiotherapy treatment planning for prostate cancer.

PATIENTS AND METHODS

A method for correction of system related and object induced distortions and one for correction of purely system related distortions have been evaluated. The latter used information extracted from MR images of a 3D phantom specifically designed for geometric distortion evaluation. An active shim procedure was performed prior to all phantom and patient scans. For each of five patients five standard treatment plans were compared using uncorrected and corrected MR images alone (density=water) and CT images alone. Finally internal anatomical landmarks were used for image registration between MR images (corrected and uncorrected) and CT images to evaluate the impact of distortion correction on the image registration process.

RESULTS

Maximum distortions of 28 mm (mean 2.2 mm) were found within the FOV in frequency encode direction. Maximum distortions could be reduced by a factor of two (mean factor four) by our phantom measurement based technique. Distortion patterns were found to be stable and reproducible over several weeks with this MR unit. For 4/5 patients, relative doses at the normalization point as calculated on the distortion corrected MR images only (all tissues taken water equivalent) were all within 1% of the corresponding value from the standard CT-based plan (actual Hounsfield units). The largest differences in isocentric dose found in one case were 3.1% MR uncorrected vs. CT and 2.6% MR corrected vs. CT. Typical sites of internal anatomical landmarks chosen for image registration show distortions up to 3 mm.

CONCLUSIONS

Object induced distortions are negligible at such low field strengths compared to system related distortions. Treatment plans for prostate cancer do not seem to differ significantly from "standard" plans calculated on CT images when calculated on distortion corrected MR images, even if all tissues are assigned the electron density of water. Distortion correction of MR images can theoretically improve the starting point for image registration of MR and CT images.

Dose response in prostate cancer with 8–12 years’ follow-up

PURPOSE

This communication reports the long-term results of the original group of prostate cancer patients who participated in the first prospective Fox Chase Cancer Center radiation dose escalation study for which 8-12 years of follow-up is now available.

METHODS AND MATERIALS

Between March 1, 1989 and October 31, 1992, 232 patients with clinically localized prostate cancer received three-dimensional conformal radiotherapy only at Fox Chase Cancer Center in a prospective dose-escalation study. Of these patients, 229 were assessable. The 8-, 10-, and 12-year actuarial rates of biochemical control (biochemically no evidence of disease [bNED]), freedom from distant metastasis (FDM), and morbidity were calculated. The Cox proportional hazards model was used to assess multivariately the predictors of bNED control and FDM, including pretreatment prostate-specific antigen (PSA) level (continuous), tumor stage (T1/T2a vs. T2b/T3), Gleason score (2-6 vs. 7-10), and radiation dose (continuous). The median total dose for all patients was 74 Gy (range 67-81). The median follow-up for living patients was 110 months (range 89-147). bNED control was defined using the American Society for Therapeutic Radiology and Oncology consensus definition.

RESULTS

The actuarial bNED control for all patients included in this series was 55% at 5 years, 48% at 10 years, and 48% at 12 years. Patients with pretreatment PSA levels of 10-20 ng/mL had statistically significant differences (19% vs. 31% vs. 84%, p = 0.0003) in bNED control when stratified by dose (<71.5, 71.5-75.6, and > 75.6 Gy, respectively) on univariate analysis. For the 229 patients with follow-up, 124 (54%) were clinically and biochemically without evidence of disease. Sixty-nine patients were alive at the time of last follow-up, and 55 patients were dead of intercurrent disease. On multivariate analysis, radiation dose was a statistically significant predictor of bNED control for all patients and for unfavorable patients with a pretreatment PSA <10 ng/mL. For the patients with a pretreatment PSA level of 10-20 ng/mL, the radiation dose was a statistically significant predictor across all groups. No radiation dose response was seen for those patients with a pretreatment PSA level >20 ng/mL, although large numbers of patients are required to demonstrate a difference. The radiation dose, Gleason score, and palpation T stage were significant predictors for the entire patient set, as well as for those with pretreatment PSA levels between 10 and 20 ng/mL. The FDM rate for all patients included in this series was 89%, 83%, and 83% at 5, 10, and 12 years, respectively. For patients with pretreatment PSA levels <10 ng/mL, all four covariates (radiation dose, Gleason score, pretreatment PSA, and palpation T stage) were significant predictors of distance metastasis. Using the Radiation Therapy Oncology Group morbidity scale, no difference was noted in the frequency of Grade 2 and 3 genitourinary and Grade 3 gastrointestinal morbidity when patients in this data set were stratified by radiation dose. However, a significant increase occurred in Grade 2 gastrointestinal complications as the radiation dose increased.

CONCLUSION

The long-term results of the original Fox Chase radiation dose escalation study with >9 years of median follow-up confirm the existence of a dose response for both bNED control and FDM. The dose response in prostate cancer is real, and the absence of biochemical recurrence after 8 years demonstrates the lack of late failure and suggests cure.