MRI basics for radiation oncologists

Optimal Setup and Parameters of Diffusion-Weighted Magnetic Resonance Imaging for Translational Evaluation of a Tumor Progression Model for Soft Tissue Sarcomas

Purpose

Defining a microscopic tumor infiltration boundary is critical to the success of radiation therapy. Currently, radiation oncologists use margins to geometrically expand the visible tumor for radiation treatment planning in soft tissue sarcomas (STS). Image-based models of tumor progression would be critical to personalize the treatment radiation field to the pattern of sarcoma spread. Evaluation of these models is necessary to demonstrate feasibility in the clinical setting. This study presents an imaging protocol for the preclinical evaluation of a tumor progression model in extremity STS.

Methods and Materials

We recruited 7 healthy volunteers and acquired diffusion-weighted magnetic resonance imaging (DW-MRI) images of the thigh on a magnetic resonance imaging scanner used for imaging cancer patients in a radiation oncology department. We developed a protocol that includes positioning the patient, configuring the radiofrequency coils, and setting the DW-MRI sequence parameters. To find the optimal parameter configuration, the image signal-to-noise ratio (SNR) and the directional variability (DV) of the principal eigenvector of the diffusion tensor were calculated.

Results

The mean SNR across all trials and 12 thigh muscles was 41, with a range of 12 to 72. The mean DV was 13° and ranged from 11° to 23°. The longest scan time was 22 minutes and 58 seconds, and the shortest was 11 minutes and 46 seconds. For the high-resolution image with a voxel volume of 1.3 × 1.3 × 6 mm3 and 38 slices, the optimal parameters were found to be a repetition time of 8000 ms, 12 signal averages, and 6 gradient directions. This configuration resulted in a scan time of 11 minutes and 46 seconds, an SNR of 34, and a DV of 13°.

Conclusions

A DW-MRI scan duration acceptable for imaging cancer patients was achieved with an image quality suitable for reproducible modeling of tumor infiltration. The developed protocol can be used for preclinical evaluation in STS patients.

Multi-institutional experience treating patients with cardiac devices on a 1.5 Tesla magnetic resonance-linear accelerator and workflow development for thoracic treatments

Background and purpose

Patients with cardiac implantable electronic devices (CIED patients) are often ineligible for online magnetic resonance-guided radiotherapy (MRgRT), most likely due to the absence of established guidelines. Existing radiotherapy (RT) and magnetic resonance imaging (MRI) guidelines offer an opportunity to construct MRgRT protocols, promoting equitable access. Our objective was to present such a workflow, share multi-institutional experiences treating CIED patients with MRgRT on a 1.5 T magnetic resonance-linear accelerator (MR-linac), and investigate geometric accuracy and electrocardiogram (ECG) monitoring for thoracic treatment.

Materials and methods

A risk analysis identified strategies for safe MRgRT for CIED patients. At three institutions, 21 pelvic and abdominal patients were treated. Patient records were analyzed for adverse events. Geometric accuracy was investigated using B0-mapping with a phantom simulating moving lung and cardiac lesions near a CIED. Volunteer measurements evaluated the effects of patient positioning and MRI sequences on ECG signal distortion.

Results

MRI and RT workflows were adaptable to MRgRT. No adverse events were recorded. B0-maps showed a maximum mean difference between static and dynamic phantom configurations of 0.1 mm, increasing to 0.4 mm distortion in the presence of a CIED. ECG readings exhibited severe distortions during scanning, hampering heart rhythm detection for most MRI sequences.

Conclusions

CIED patients can safely undergo treatment on a 1.5 T MR-linac following RT and MRI guidelines. For targets near CIEDs, a B0-mapping procedure was considered accurate enough to determine MRgRT eligibility. Pulse oximetry is recommended for cardiac monitoring during MRI scanning due to ECG signal distortion.

Systematic review of MRI safety literature in relation to radiofrequency thermal injury prevention

INTRODUCTION

Magnetic resonance imaging (MRI) is a rapidly evolving modality, generally considered safe due to lack of ionising radiation. While MRI technology and techniques are improving, many of the safety concerns remain the same as when first established. Patient thermal injuries are the most frequently reported adverse event, accounting for 59% of MRI incidents to the Food and Drug Administration (FDA). Surveys indicate many incidents remain unreported. Patient thermal injuries are preventable and various methods for their mitigation have been published. However, recommendations can be variable, fragmented and confusing. The aim of this systematic review was to synthesise the evidence on MRI safety and associated skin injuries and offer comprehensive recommendations for radiographers to prevent skin thermal injuries.

METHODS

Four journal databases were searched for sources published January 2010-May 2023, presenting information on MRI safety and thermal injuries.

RESULTS

Of 26,801 articles returned, after careful screening and based on the eligibility criteria, only 79 articles and an additional 19 grey literature sources were included (n = 98). Included studies were examined using thematic analysis to determine if holistic recommendations can be provided to assist in preventing skin burns. This resulted in three simplified recommendations: Remove any electrically conductive items Insulate the patient to prevent any conductive loops or contact with objects Communicate regularly CONCLUSION: By implementing the above recommendations, it is estimated that 97% of skin burns could be prevented. With thermal injuries continuing to impact MRI safety, strategies to prevent skin burns and heating are essential. Assessing individual risks, rather than blanket policies, will help prevent skin thermal injuries occurring, improving patient care.

How Imaging Advances Are Defining the Future of Precision Radiation Therapy

Radiation therapy is fundamental in the treatment of cancer. Imaging has always played a central role in radiation oncology. Integrating imaging technology into irradiation devices has increased the precision and accuracy of dose delivery and decreased the toxic effects of the treatment. Although CT has become the standard imaging modality in radiation therapy, the development of recently introduced next-generation imaging techniques has improved diagnostic and therapeutic decision making in radiation oncology. Functional and molecular imaging techniques, as well as other advanced imaging modalities such as SPECT, yield information about the anatomic and biologic characteristics of tumors for the radiation therapy workflow. In clinical practice, they can be useful for characterizing tumor phenotypes, delineating volumes, planning treatment, determining patients' prognoses, predicting toxic effects, assessing responses to therapy, and detecting tumor relapse. Next-generation imaging can enable personalization of radiation therapy based on a greater understanding of tumor biologic factors. It can be used to map tumor characteristics, such as metabolic pathways, vascularity, cellular proliferation, and hypoxia, that are known to define tumor phenotype. It can also be used to consider tumor heterogeneity by highlighting areas at risk for radiation resistance for focused biologic dose escalation, which can impact the radiation planning process and patient outcomes. The authors review the possible contributions of next-generation imaging to the treatment of patients undergoing radiation therapy. In addition, the possible roles of radio(geno)mics in radiation therapy, the limitations of these techniques, and hurdles in introducing them into clinical practice are discussed. ©RSNA, 2024 Test Your Knowledge questions for this article are available in the supplemental material.

Geometric distortions in clinical MRI sequences for radiotherapy: insights gained from a multicenter investigation

BACKGROUND

As magnetic resonance imaging (MRI) becomes increasingly integrated into radiotherapy (RT) for enhanced treatment planning and adaptation, the inherent geometric distortion in acquired MR images pose a potential challenge to treatment accuracy. This study aimed to evaluate the geometric distortion levels in the clinical MRI protocols used across Danish RT centers and discuss influence of specific sequence parameters. Based on the variety in geometric performance across centers, we assess if harmonization of MRI sequences is a relevant measure.

MATERIALS AND METHODS

Nine centers participated with 12 MRI scanners and MRI-Linacs (MRL). Using a travelling phantom approach, a reference MRI sequence was used to assess variation in baseline distortion level between scanners. The phantom was also scanned with local clinical MRI sequences for brain, head/neck (H/N), abdomen, and pelvis. The influence of echo time, receiver bandwidth, image weighting, and 2D/3D acquisition was investigated.

RESULTS

We found a large variation in geometric accuracy across 93 clinical sequences examined, exceeding the baseline variation found between MRI scanners (σ = 0.22 mm), except for abdominal sequences where the variation was lower. Brain and abdominal sequences showed lowest distortion levels ([0.22, 2.26] mm), and a large variation in performance was found for H/N and pelvic sequences ([0.19, 4.07] mm). Post hoc analyses revealed that distortion levels decreased with increasing bandwidth and a less clear increase in distortion levels with increasing echo time. 3D MRI sequences had lower distortion levels than 2D (median of 1.10 and 2.10 mm, respectively), and in DWI sequences, the echo-planar imaging read-out resulted in highest distortion levels.

CONCLUSION

There is a large variation in the geometric distortion levels of clinical MRI sequences across Danish RT centers, and between anatomical sites. The large variation observed makes harmonization of MRI sequences across institutions and adoption of practices from well-performing anatomical sites, a relevant measure within RT.

Contemporary paediatric radiation oncology

Treatment with ionising radiation is a valuable component of treatment schedules for a many children and young people with cancer. While some form of radiotherapy has been in use for over 100 years, a series of innovations has revolutionised paediatric radiation oncology. Mostly, high-energy X-ray photons are used, but proton beam radiotherapy is increasingly offered, especially in children and young people. This is to reduce the radiation exposure of healthy normal tissues and so the likelihood of adverse effects. Other methods of radiotherapy delivery include brachytherapy and molecular radiotherapy. The most appropriate treatment technique should be selected for every child. Advances in computers and imaging, developments in the technology of radiation delivery and a better understanding of pathology and molecular biology of cancer, coupled with parallel improvements in surgery and systemic therapy, have led to a transformation of practice in recent decades. Initially an empirical art form, radiotherapy for children has become a technically advanced, evidence-based cornerstone of increasingly personalised cancer medicine with solid scientific foundations. Late sequelae of treatment-the adverse effects once accepted as the cost of cure-have been significantly reduced in parallel with increased survival rates. The delivery of radiotherapy to children and young people requires a specialised multiprofessional team including radiation oncologists, therapeutic radiographers, play specialists and physicists among others. This article reviews the types of radiotherapy now available and outlines the pathway of the child through treatment. It aims to demonstrate to paediatricians how contemporary paediatric radiation oncology differs from past practice.

Ensemble learning and personalized training for the improvement of unsupervised deep learning-based synthetic CT reconstruction

BACKGROUND

The growing adoption of magnetic resonance imaging (MRI)-guided radiation therapy (RT) platforms and a focus on MRI-only RT workflows have brought the technical challenge of synthetic computed tomography (sCT) reconstruction to the forefront. Unpaired-data deep learning-based approaches to the problem offer the attractive characteristic of not requiring paired training data, but the gap between paired- and unpaired-data results can be limiting.

PURPOSE

We present two distinct approaches aimed at improving unpaired-data sCT reconstruction results: a cascade ensemble that combines multiple models and a personalized training strategy originally designed for the paired-data setting.

METHODS

Comparisons are made between the following models: (1) the paired-data fully convolutional DenseNet (FCDN), (2) the FCDN with the Intentional Deep Overfit Learning (IDOL) personalized training strategy, (3) the unpaired-data CycleGAN, (4) the CycleGAN with the IDOL training strategy, and (5) the CycleGAN as an intermediate model in a cascade ensemble approach. Evaluation of the various models over 25 total patients is carried out using a five-fold cross-validation scheme, with the patient-specific IDOL models being trained for the five patients of fold 3, chosen at random.

RESULTS

In both the paired- and unpaired-data settings, adopting the IDOL training strategy led to improvements in the mean absolute error (MAE) between true CT images and sCT outputs within the body contour (mean improvement, paired- and unpaired-data approaches, respectively: 38%, 9%) and in regions of bone (52%, 5%), the peak signal-to-noise ratio (PSNR; 15%, 7%), and the structural similarity index (SSIM; 6%, <1%). The ensemble approach offered additional benefits over the IDOL approach in all three metrics (mean improvement over unpaired-data approach in fold 3; MAE: 20%; bone MAE: 16%; PSNR: 10%; SSIM: 2%), and differences in body MAE between the ensemble approach and the paired-data approach are statistically insignificant.

CONCLUSIONS

We have demonstrated that both a cascade ensemble approach and a personalized training strategy designed initially for the paired-data setting offer significant improvements in image quality metrics for the unpaired-data sCT reconstruction task. Closing the gap between paired- and unpaired-data approaches is a step toward fully enabling these powerful and attractive unpaired-data frameworks.

Possibilities and challenges when using synthetic computed tomography in an adaptive carbon-ion treatment workflow

BACKGROUND AND PURPOSE

Anatomical surveillance during ion-beam therapy is the basis for an effective tumor treatment and optimal organ at risk (OAR) sparing. Synthetic computed tomography (sCT) based on magnetic resonance imaging (MRI) can replace the X-ray based planning CT (X-rayCT) in photon radiotherapy and improve the workflow efficiency without additional imaging dose. The extension to carbon-ion radiotherapy is highly challenging; complex patient positioning, unique anatomical situations, distinct horizontal and vertical beam incidence directions, and limited training data are only few problems. This study gives insight into the possibilities and challenges of using sCTs in carbon-ion therapy.

MATERIALS AND METHODS

For head and neck patients immobilised with thermoplastic masks 30 clinically applied actively scanned carbon-ion treatment plans on 15 CTs comprising 60 beams were analyzed. Those treatment plans were re-calculated on MRI based sCTs which were created employing a 3D U-Net. Dose differences and carbon-ion spot displacements between sCT and X-rayCT were evaluated on a patient specific basis.

RESULTS

Spot displacement analysis showed a peak displacement by 0.2 cm caused by the immobilisation mask not measurable with the MRI. 95.7% of all spot displacements were located within 1 cm. For the clinical target volume (CTV) the median D_50% agreed within -0.2% (-1.3 to 1.4%), while the median D_0.01cc differed up to 4.2% (-1.3 to 25.3%) comparing the dose distribution on the X-rayCT and the sCT. OAR deviations depended strongly on the position and the dose gradient. For three patients no deterioration of the OAR parameters was observed. Other patients showed large deteriorations, e.g. for one patient D_2% of the chiasm differed by 28.1%.

CONCLUSION

The usage of sCTs opens several new questions, concluding that we are not ready yet for an MR-only workflow in carbon-ion therapy, as envisaged in photon therapy. Although omitting the X-rayCT seems unfavourable in the case of carbon-ion therapy, an sCT could be advantageous for monitoring, re-planning, and adaptation.

Rates of MRI simulator utilisation in a tertiary cancer therapy centre

Magnetic resonance imaging (MRI) is increasingly being integrated into the radiation oncology workflow, due to its improved soft tissue contrast without additional exposure to ionising radiation. A review of MRI utilisation according to evidence based departmental guidelines was performed. Guideline utilisation rates were calculated to be 50% (true utilisation rate was 46%) of all new cancer patients treated with adjuvant or curative intent, excluding simple skin and breast cancer patients. Guideline utilisation rates were highest in the lower gastrointestinal and gynaecological subsites, with the lowest being in the upper gastrointestinal and thorax subsites. Head and neck (38% vs 45%) and CNS (46% vs 67%) cancers had the largest discrepancy between true and guideline utilisation rates due to unnamed reasons and non-contemporaneous diagnostic imaging respectively. This report outlines approximate MRI utilisation rates in a tertiary radiation oncology service and may help guide planning for future departments contemplating installation of an MRI simulator.

Additively manufactured, solid object structures for adjustable image contrast in Magnetic Resonance Imaging

The choice of materials challenges the development of Magnetic Resonance Imaging (MRI) phantoms and, to date, is mainly limited to water-filled compartments or gel-based components. Recently, solid materials have been introduced through additive manufacturing (AM) to mimic complex geometrical structures. Nonetheless, no such manufactured solid materials are available with controllable MRI contrast to mimic organ substructures or lesion heterogeneities. Here, we present a novel AM design that allows MRI contrast manipulation by varying the partial volume contribution to a ROI/voxel of MRI-visible material within an imaging object. Two sets of 11 cubes and three replicates of a spherical tumour model were designed and printed using AM. Most samples presented varying MRI-contrast in standard MRI sequences, based mainly on spin density and partial volume signal variation. A smooth and continuous MRI-contrast gradient could be generated in a single-compartment tumour model. This concept supports the development of more complex MRI phantoms that mimic the appearance of heterogeneous tumour tissues.

Magnetic Fields and Cancer: Epidemiology, Cellular Biology, and Theranostics

Humans are exposed to a complex mix of man-made electric and magnetic fields (MFs) at many different frequencies, at home and at work. Epidemiological studies indicate that there is a positive relationship between residential/domestic and occupational exposure to extremely low frequency electromagnetic fields and some types of cancer, although some other studies indicate no relationship. In this review, after an introduction on the MF definition and a description of natural/anthropogenic sources, the epidemiology of residential/domestic and occupational exposure to MFs and cancer is reviewed, with reference to leukemia, brain, and breast cancer. The in vivo and in vitro effects of MFs on cancer are reviewed considering both human and animal cells, with particular reference to the involvement of reactive oxygen species (ROS). MF application on cancer diagnostic and therapy (theranostic) are also reviewed by describing the use of different magnetic resonance imaging (MRI) applications for the detection of several cancers. Finally, the use of magnetic nanoparticles is described in terms of treatment of cancer by nanomedical applications for the precise delivery of anticancer drugs, nanosurgery by magnetomechanic methods, and selective killing of cancer cells by magnetic hyperthermia. The supplementary tables provide quantitative data and methodologies in epidemiological and cell biology studies. Although scientists do not generally agree that there is a cause-effect relationship between exposure to MF and cancer, MFs might not be the direct cause of cancer but may contribute to produce ROS and generate oxidative stress, which could trigger or enhance the expression of oncogenes.

ESTRO-ACROP recommendations on the clinical implementation of hybrid MR-linac systems in radiation oncology

Online magnetic resonance-guided radiotherapy (oMRgRT) represents one of the most innovative applications of current image-guided radiation therapy (IGRT). The revolutionary concept of oMRgRT systems is the ability to acquire MR images for adaptive treatment planning and also online imaging during treatment delivery. The daily adaptive planning strategies allow to improve targeting accuracy while avoiding critical structures. This ESTRO-ACROP recommendation aims to provide an overview of available systems and guidance for best practice in the implementation phase of hybrid MR-linac systems. Unlike the implementation of other radiotherapy techniques, oMRgRT adds the MR environment to the daily practice of radiotherapy, which might be a new experience for many centers. New issues and challenges that need to be thoroughly explored before starting clinical treatments will be highlighted.

[Modern radiation therapy planning and image-guided radiotherapy using the example of prostate cancer]

CLINICAL/METHODICAL ISSUE

Optimizing radiotherapy demands precise delineation of the target structure, not only before but also during the course of radiotherapy.

STANDARD RADIOLOGICAL METHODS

For many years, planning of external radiation treatment planning has been based on computer tomography data.

METHODOLOGICAL INNOVATIONS

With the advent of image-guided radiotherapy (IGRT), magnetic resonance imaging (MRI) and functional hybrid imaging are increasingly being integrated into radiation treatment planning. The development of the MR-linac can be seen as an innovation.

PERFORMANCE

The integration of MRI and hybrid imaging (positron emission tomography [PET]/CT, PET/MRI) in the treatment planning process enables more precise treatment planning due to the better morphological and functional information. The integration of MRI data on the MR-linac in daily position control enables adaptation of the irradiation plan to the current conditions.

ACHIEVEMENTS

Technical innovation such as the MR-linac as well as increasing use of hybrid imaging contribute to the objective of further individualization within (radio)oncology.

PRACTICAL RECOMMENDATIONS

Using the example of prostate cancer, the application of prostate-specific membrane antigen (PSMA) ligands and hybrid imaging offers great potential for individualized strategic treatment decisions. The MR-linac appears to be particularly suitable for radiation therapy of prostate cancer. Special attention must be paid to the technical aspects of positioning and data acquisition for the purpose of radiation treatment planning.

Clinical workflow for treating patients with a metallic hip prosthesis using magnetic resonance imaging-guided radiotherapy

Background & purpose

Metallic prostheses distort the magnetic field during magnetic resonance imaging (MRI), leading to geometric distortions and signal loss. The purpose of this work was to develop a method to determine eligibility for MRI-guided radiotherapy (MRIgRT) on a per patient basis by estimating the magnitude of geometric distortions inside the clinical target volume (CTV).

Materials & methods

Three patients with prostate cancer and hip prosthesis, treated using MRIgRT, were included. Eligibility for MRIgRT was based on computed tomography and associated CTV delineations, together with a field-distortion (B0) map and anatomical images acquired during MR simulation. To verify the method, B0 maps made during MR simulation and each MRIgRT treatment fraction were compared.

Results

Estimates made during MR simulation of the magnitude of distortions inside the CTV were 0.43 mm, 0.19 mm and 2.79 mm compared to the average over all treatment fractions of 1.40 mm, 0.32 mm and 1.81 mm, per patient respectively.

Conclusions

B0 map acquisitions prior to treatment can be used to estimate the magnitude of distortions during MRIgRT to guide the decision on eligibility for MRIgRT of prostate cancer patients with metallic hip implants.