ACPSEM position paper: the safety of magnetic resonance imaging linear accelerators

Magnetic Resonance Imaging linear-accelerator (MRI-linac) equipment has recently been introduced to multiple centres in Australia and New Zealand. MRI equipment creates hazards for staff, patients and others in the MR environment; these hazards must be well understood, and risks managed by a system of environmental controls, written procedures and a trained workforce. While MRI-linac hazards are similar to the diagnostic paradigm, the equipment, workforce and environment are sufficiently different that additional safety guidance is warranted. In 2019 the Australasian College of Physical Scientists and Engineers in Medicine (ACPSEM) formed the Magnetic Resonance Imaging Linear-Accelerator Working Group (MRILWG) to support the safe clinical introduction and optimal use of MR-guided radiation therapy treatment units. This Position Paper is intended to provide safety guidance and education for Medical Physicists and others planning for and working with MRI-linac technology. This document summarises MRI-linac hazards and describes particular effects which arise from the combination of strong magnetic fields with an external radiation treatment beam. This document also provides guidance on safety governance and training, and recommends a system of hazard management tailored to the MRI-linac environment, ancillary equipment, and workforce.

Practical Safety Considerations for Integration of Magnetic Resonance Imaging in Radiation Therapy

Interest in integrating magnetic resonance imaging (MRI) in radiation therapy (RT) practice has increased dramatically in recent years owing to its unique advantages such as excellent soft tissue contrast and capability of measuring biological properties. Continuous real-time imaging for intrafractional motion tracking without ionizing radiation serves as a particularly attractive feature for applications in RT. Despite its many advantages, the integration of MRI in RT workflows is not straightforward, with many unmet needs. MR safety remains one of the key challenges and concerns in the clinical implementation of MR simulators and MR-guided radiation therapy systems in radiation oncology. Most RT staff are not accustomed to working in an environment with a strong magnetic field. There are specific requirements in RT that are different from diagnostic applications. A large variety of implants and devices used in routine RT practice do not have clear MR safety labels. RT-specific imaging pulse sequences focusing on fast acquisition, high spatial integrity, and continuous, real-time acquisition require additional MR safety testing and evaluation. This article provides an overview of MR safety tailored toward RT staff, followed by discussions on specific requirements and challenges associated with MR safety in the RT environment. Strategies and techniques for developing an MR safety program specific to RT are presented and discussed.

Assessment of MRI safety issues for stainless steel sutures used for microtia reconstruction

INTRODUCTION

Potential magnetic resonance imaging issues for stainless steel sutures used for microtia reconstruction could be clinically significant for safety and diagnostic yield considerations. Therefore, the purpose of this investigation was to assess magnetic resonance issues (magnetic field interactions, heating, and artifacts) for different types of stainless steel sutures used for microtia reconstruction.

METHODS

Small gauge, commonly used stainless steel sutures from four different manufacturers (5/0 Steelex, Aesculap/B, Braun Medical, Inc.; Nagata 38 Gauge Microtia Wire, Bear Medical Corporation; Auricular Reco Wire, Medicon Surgical Inc.; and 5-0 B&S 35 Surgical Steel Suture, Ethicon, Inc.) were tested using standardized ex vivo techniques to assess magnetic field interactions, heating, and artifacts at 3 Tesla. Before testing, the stainless steel sutures were configured in a manner same as that for cartilage reconstruction used to treat microtia.

RESULTS

Each stainless steel suture exhibited minor magnetic field interactions at 3 Tesla (translational attraction, deflection angle <10°, and no torque). Heating associated with a whole-body averaged specific absorption rate of 2.9 W/kg was not excessive (highest temperature changes, ≤1.8 °C). Artifacts were relatively minor in relation to the size and shape of each stainless steel suture (artifact size in relation to the size and shape of each stainless steel suture extending ≤5 mm).

CONCLUSIONS

The stainless steel sutures that underwent testing do not present additional risks to patients in a 3-Tesla or less magnetic field setting (i.e., magnetic resonance conditional). Artifacts for these sutures may only be an issue within close proximity to the reconstructed ear.

In Vitro Magnetic Resonance Imaging Evaluation of Fragmented, Open-Coil, Percutaneous Peripheral Nerve Stimulation Leads

OBJECTIVE

Percutaneous peripheral nerve stimulation (PNS) is an FDA-cleared pain treatment. Occasionally, fragments of the lead (MicroLead, SPR Therapeutics, LLC, Cleveland, OH, USA) may be retained following lead removal. Since the lead is metallic, there are associated magnetic resonance imaging (MRI) risks. Therefore, the objective of this investigation was to evaluate MRI-related issues (i.e., magnetic field interactions, heating, and artifacts) for various lead fragments.

METHODS

Testing was conducted using standardized techniques on lead fragments of different lengths (i.e., 50, 75, and 100% of maximum possible fragment length of 12.7 cm) to determine MRI-related problems. Magnetic field interactions (i.e., translational attraction and torque) and artifacts were tested for the longest lead fragment at 3 Tesla. MRI-related heating was evaluated at 1.5 Tesla/64 MHz and 3 Tesla/128 MHz with each lead fragment placed in a gelled-saline filled phantom. Temperatures were recorded on the lead fragments while using relatively high RF power levels. Artifacts were evaluated using T1-weighted, spin echo, and gradient echo (GRE) pulse sequences.

RESULTS

The longest lead fragment produced only minor magnetic field interactions. For the lead fragments evaluated, physiologically inconsequential MRI-related heating occurred at 1.5 Tesla/64 MHz while under certain 3 Tesla/128 MHz conditions, excessive temperature elevations may occur. Artifacts extended approximately 7 mm from the lead fragment on the GRE pulse sequence, suggesting that anatomy located at a position greater than this distance may be visualized on MRI.

CONCLUSIONS

MRI may be performed safely in patients with retained lead fragments at 1.5 Tesla using the specific conditions of this study (i.e., MR Conditional). Due to possible excessive temperature rises at 3 Tesla, performing MRI at that field strength is currently inadvisable.

Assessment of MRI Issues at 3 Tesla for a New Metallic Tissue Marker

Purpose. To assess the MRI issues at 3 Tesla for a metallic tissue marker used to localize removal areas of tissue abnormalities. Materials and Methods. A newly designed, metallic tissue marker (Achieve Marker, CareFusion, Vernon Hills, IL) used to mark biopsy sites, particularly in breasts, was assessed for MRI issues which included standardized tests to determine magnetic field interactions (i.e., translational attraction and torque), MRI-related heating, and artifacts at 3 Tesla. Temperature changes were determined for the marker using a gelled-saline-filled phantom. MRI was performed at a relatively high specific absorption rate (whole body averaged SAR, 2.9-W/kg). MRI artifacts were evaluated using T1-weighted, spin echo and gradient echo pulse sequences. Results. The marker displayed minimal magnetic field interactions (2-degree deflection angle and no torque). MRI-related heating was only 0.1°C above background heating (i.e., the heating without the tissue marker present). Artifacts seen as localized signal loss were relatively small in relation to the size and shape of the marker. Conclusions. Based on the findings, the new metallic tissue marker is acceptable or “MR Conditional” (using current labeling terminology) for a patient undergoing an MRI procedure at 3 Tesla or less.

Gold fiducials are a unique marker for localization in the thoracic spine: a cost comparison with percutaneous vertebroplasty

We present a unique application of the gold fiducial as a preoperative, radiographic marker placed in the thoracic spine and used for intraoperative localization. In comparison to percutaneous vertebroplasty marking of thoracic spinal levels with polymethyl methacrylate (PMMA) cement, implantation of the gold fiducial is technically facile with a minimal learning curve. The fiducial markers are also associated with significantly less financial resources. Following 2013 Current Procedural Terminology (CPT) coding, the cost of vertebroplasty under fluoroscopic guidance, $3195·43, or under computed tomography (CT) guidance, $3232·54, is more than double the cost of the gold fiducial implantation - $1237·55 and $1267·03, under similar imaging techniques, respectively. In the first description of gold fiducials in the thoracic spine, we conclude that the marker is a safe and cost-effective method for preoperative localization of the thoracic levels.