MR Environments Research Articles

Rapid prediction of MRI-induced RF heating of active implantable medical devices using machine learning.

The interaction between an active implantable medical device and magnetic resonance imaging (MRI) radiofrequency (RF) fields can cause excessive tissue heating. Existing methods for predicting RF heating in the presence of an implant rely on either extensive phantom experiments or electromagnetic (EM) simulations with varying degrees of approximation of the MR environment, the patient, or the implant. On the contrary, fast MR thermometry techniques can provide a reliable real-time map of temperature rise in the tissue in the vicinity of conductive implants. In this proof-of-concept study, we examined whether a machine learning (ML) based model could predict the temperature increase in the tissue near the tip of an implanted lead after several minutes of RF exposure based on only a few seconds of experimentally measured temperature values. We performed phantom experiments with a commercial deep brain stimulation (DBS) system to train a fully connected feedforward neural network (NN) to predict temperature rise after ~3 minutes of scanning at a 3 T scanner using only data from the first 5 seconds. The NN effectively predicted ΔTmax-R2 = 0.99 for predictions in the test dataset. Our model also showed potential in predicting RF heating for other various scenarios, including a DBS system at a different field strength (1.5 T MRI, R2 = 0.87), different field polarization (1.2 T vertical MRI, R2 = 0.79), and an unseen implant (cardiac leads at 1.5 T MRI, R2 = 0.91). Our results indicate great potential for the application of ML in combination with fast MR thermometry techniques for rapid prediction of RF heating for implants in various MR environments.Clinical Relevance- Machine learning-based algorithms can potentially enable rapid prediction of MRI-induced RF heating in the presence of unknown AIMDs in various MR environments.

Dream-Experiment: A MR User Interface with Natural Multi-channel Interaction for Virtual Experiments.

This paper studies a set of MR technologies for middle school experimental teaching environments and develops a multi-channel MR user interface called Dream-Experiment. The goal of Dream-Experiment is to improve the traditional MR user interface, so that users can get a real, natural 3D interactive experience like real experiments, but without danger and pollution. In terms of visual presentation, we design multi-camera collaborative registration to realize robust 6-DoF MR interactive space, and also define a complete rendering pipeline to provide improved processing of virtual-real objects' occlusion including translucent devices. In the virtual-real interaction, we provide six interaction modes that support visual interaction, tangible interaction, virtual-real gestures with touching, voice, thermal feeling, and olfactory feeling. After users' testing, we find that Dream-Experiment has better interactive efficiency and user experience than traditional MR environments.

Swedish national survey on MR safety compared with CT: a false sense of security?

ObjectivesThe objectives were to survey MR safety incidents in Sweden during a 12-month period, to assess severity scores, and to evaluate the confidence of MR personnel in incident-reporting mechanisms.MethodData were collected within a web-based questionnaire on safety in clinical MR environments with CT for comparison. Data reported MR and CT safety incidents (human injury, material damage, and close calls), incident severity, and confidence of participants in incident-reporting systems.ResultsThe study population consisted of 529 eligible participants. Participants reported 200 MR and 156 CT safety incidents. Among MR incidents, 16% were given the highest potential severity score. More MR workers (73%) than CT workers (50%) were confident in being aware of any incident occurring at their workplace. However, 69% MR workers (83% for CT) were not aware of reported incidents at their hospitals.ConclusionSafety incidents resulting in human injury, material damage, and close calls in clinical MR environments do occur. According to national risk assessment recommendations, risk level is high. Results indicated that MR personnel tend to a false sense of security, as a high proportion of staff members were sure that they would have been aware of any incident occurring in their own department, while in reality, incidents did occur without their knowledge. We conclude that false sense of security exists for MR.Key Points• Safety incidents in clinical MR environments still result in human injury and material damage.• Severity level of MR incidents—assessed using Swedish national risk assessment recommendations—is high.• Confidence of MR personnel in incident-reporting mechanisms is high, but reflects a false sense of security, as a high proportion of staff is unaware of reported incidents in the same workplace.

Help! I Need a Remote Guide in My Mixed Reality Collaborative Environment.

The help of a remote expert in performing a maintenance task can be useful in many situations, and can save time as well as money. In this context, augmented reality (AR) technologies can improve remote guidance thanks to the direct overlay of 3D information onto the real world. Furthermore, virtual reality (VR) enables a remote expert to virtually share the place in which the physical maintenance is being carried out. In a traditional local collaboration, collaborators are face-to-face and are observing the same artifact, while being able to communicate verbally and use body language, such as gaze direction or facial expression. These interpersonal communication cues are usually limited in remote collaborative maintenance scenarios, in which the agent uses an AR setup while the remote expert uses VR. Providing users with adapted interaction and awareness features to compensate for the lack of essential communication signals is therefore a real challenge for remote MR collaboration. However, this context offers new opportunities for augmenting collaborative abilities, such as sharing an identical point of view, which is not possible in real life. Based on the current task of the maintenance procedure, such as navigation to the correct location or physical manipulation, the remote expert may choose to freely control his/her own viewpoint of the distant workspace, or instead may need to share the viewpoint of the agent in order to better understand the current situation. In this work, we first focus on the navigation task, which is essential to complete the diagnostic phase and to begin the maintenance task in the correct location. We then present a novel interaction paradigm, implemented in an early prototype, in which the guide can show the operator the manipulation gestures required to achieve a physical task that is necessary to perform the maintenance procedure. These concepts are evaluated, allowing us to provide guidelines for future systems targeting efficient remote collaboration in MR environments.

Text Input Performance with a Mixed Reality Head-Mounted Display (HMD)

Past research has shown that text input methods must be quick, efficient, and involve little to no learning in order for them to be accepted in a consumer market. The Microsoft HoloLens is a mixed reality head-mounted display (HMD) that requires gestures or a physical clicker for interaction, including text entry. In this study, we investigated performance, perceived usability, and workload of these text input methods. Participants were able to type only 5.41WPM with the gesture method, gave it a poor usability score. The clicker method resulted in a WPM rate of 6.58, was given an “OK” usability score, and lower perceived exertion scores in the bicep and index finger. This study demonstrates the need for further research to develop more optimal text entry methods for MR environments.

ACR guidance document on MR safe practices: Updates and critical information 2019.

The need for a guidance document on MR safe practices arose from a growing awareness of the MR environment's potential risks and adverse event reports involving patients, equipment, and personnel. Initially published in 2002, the American College of Radiology White Paper on MR Safety established de facto industry standards for safe and responsible practices in clinical and research MR environments. The most recent version addresses new sources of risk of adverse events, increases awareness of dynamic MR environments, and recommends that those responsible for MR medical director safety undergo annual MR safety training. With regular updates to these guidelines, the latest MR safety concerns can be accounted for to ensure a safer MR environment where dangers are minimized. Level of Evidence: 1 Technical Efficacy Stage: 5 J. Magn. Reson. Imaging 2020;51:331-338.

High Speed Pneumatic Stepper Motor for MRI Applications.

Presented in this paper is an exploration into the efficacy of a plastic, four-cylinder piston pneumatic motor designed for driving medical instruments inside magnetic resonance imaging (MRI) systems. Because of the increasing use of MRI for diagnostic and interventional purposes and the benefits that could be realized by operating with real-time MR image guidance, there exists a significant need for MRI compliant surgical devices. Some type of actuation mechanism is necessary to drive such devices. The motor can be controlled to operate in a "step" type motion by using pneumatic valves to sequentially apply air pressure to push the piston surfaces, meaning the motor is metal-free and does not use electricity. The stepwise nature of this piston stepper motor is ideal for the accurate, controlled movements required for MRI-guided interventions. The motor was geared down by a gearbox to increase torque. Performance indices determined include output torque and achievable rotational velocity with respect to factors such as air pressure and load conditions. The stepper motor achieved speeds of approximately 2000rpm, and maximum output torques of approximately 19Nmm. The motor represents a high speed pneumatic stepper motor design capable of actuating devices in MR environments without affecting image quality.

Quantification of Force and Torque Applied by a High-Field Magnetic Resonance Imaging System on an Ultrasonic Motor for MRI-Guided Robot-Assisted Interventions

The risk of accidental dislodgement of robot-operated surgical mechanisms can lead to morbidity or mortality. The force and torque applied by a 3.0-tesla scanner on an ultrasonic motor are not fully known. The force and torque may displace the motor, which is not fully magnetic resonance imaging (MRI)-compatible but can be safely used in MR environments. A suspension apparatus was designed to measure the angles of deflection and rotation applied to the motor by MR magnetic fields. Three orientations and two power states of the motor were assessed inside the MR bore. The displacement force and torque were measured at eight locations with respect to the bore. The displacement force on the motor from 10 cm outside the magnet bore to 20 cm inside the bore ranged from 3 to 7 gF. The experimental measurements are in agreement with the theoretical values. Running the motor altered the force by 1 gF. The force does not significantly change when the MRI scanner is on. Considerable displacement force is applied to the motor, and no deflection torque is observed. Quantified values can be used to solve dynamic equations for robotic mechanisms intended for MRI-guided operations.

WE‐A‐116‐01: MRI Safety

Medical physicists are accustomed to dealing with the possibility of biological harm to patients from procedures involving ionizing radiation, whether this is the possibility of erythema in a week or so, or an increased in the relative risk of cancer developing in a decade or two. Accidents in MRI are more concrete. Their impact is immediate and catastrophic. Still, the concept of safety in the MRI facility is not always well addressed or understood. There is confusion about staffing, screening, construction, to name a few concerns. In addition, there is a proliferation in the number of patients presenting for MRI who require clearance based on the presence of one or more implanted devices that may be relative contraindications for MRI. Management of these patients has historically been very conservative, based on limited information about the interactions of these devices with the MR environment. More recently, testing and characterization of both medical devices and MR magnets have improved, but in many cases, there remains a gap in understanding which devices are likely safe to scan in particular MR environments. This session will review MRI safety considerations, with special emphasis on the recent ACR guidance document as well as the current state of MR safety labeling for conditional devices and common conditions of operation for commercially‐available MR scanners. The physicist should gain an understanding of the hazards which must be assessed and available resources.Learning Objectives:1. Read and understand MRI conditional safety labeling for a typical medical device2. Assess the specific physical conditions created by a given MRI scanner for comparison with the safety conditions3. Advise physicians performing MRI of specific risks or concerns for patients with conditional implants that do not meet the safety conditions for a given scanner or study4. Understand the4. Zone concept and how to implement them.5. Staffing concerns, including training and dealing with emergent situations.6. Auditing of the MRI safety program.

Non-linear adaptive controllers for an over-actuated pneumatic MR-compatible stepper

Pneumatics is one of the few actuation principles that can be used in an MR environment, since it can produce high forces without affecting imaging quality. However, pneumatic control is challenging, due to the air high compliance and cylinders non-linearities. Furthermore, the system's properties may change for each subject. Here, we present novel control strategies that adapt to the subject's individual anatomy and needs while performing accurate periodic gait-like movements with an MRI compatible pneumatically driven robot. In subject-passive mode, an iterative learning controller (ILC) was implemented to reduce the system's periodic disturbances. To allow the subjects to intend the task by themselves, a zero-force controller minimized the interaction forces between subject and robot. To assist patients who may be too weak, an assist-as-needed controller that adapts the assistance based on online measurement of the subject's performance was designed. The controllers were experimentally tested. The ILC successfully learned to reduce the variability and tracking errors. The zero-force controller allowed subjects to step in a transparent environment. The assist-as-needed controller adapted the assistance based on individual needs, while still challenged the subjects to perform the task. The presented controllers can provide accurate pneumatic control in MR environments to allow assessments of brain activation.

ACR guidance document on MR safe practices: 2013

Because there are many potential risks in the MR environment and reports of adverse incidents involving patients, equipment and personnel, the need for a guidance document on MR safe practices emerged. Initially published in 2002, the ACR MR Safe Practices Guidelines established de facto industry standards for safe and responsible practices in clinical and research MR environments. As the MR industry changes the document is reviewed, modified and updated. The most recent version will reflect these changes.

Setting up MR compatibility of a commercial stereo-localization system for low-field open MR interventional procedures

Surgical and interventional procedures increasingly rely on spatial measurement systems. One of the most popular is NDI's Polaris Vicra. Unfortunately, this system cannot be used for magnetic resonance imaging (MRI) procedures because of the image artifacts that it produces. Here, a simple method for hardware electrical shielding is presented which makes the Polaris Vicra MR-compatible for a low-field open-magnet MR scanner. The shielding is achieved by placing the cable linking the sensor to the USB converter inside a copper braid tube. Tests performed with a variety of MR sequences show the absence of image artifacts, image distortion or grey-level uniformity degradation. In the same way, 3D localization accuracy of the spatial measurement system is not modified by the MR environment. A rough safety check shows that the system does not pose hazards with normal condition of use. This minor adaptation makes the system suitable for various applications in low field MR environments, such as intraoperative and interventional MRI.

Clinical Tip: Development of an Ankle Distraction Device Compatible With MRI and Radiography

Measurements of cartilage thickness in human ankles are important in evaluating the development of osteoarthritis, particularly following articular fractures. Unlike the knee, cartilage in the ankle is relatively thin (on the order of 1.5 mm), so it is difficult to image clearly using either MRI or CT techniques. Part of the problem is the lack of separation between the surfaces of the tibial and talar cartilage, making it essentially impossible to determine their respective thicknesses. Ankle distraction has been achieved in the operating room by applying tension to the foot using a windlass attached to the operating table, while the patient’s upper body is restrained by straps. Trauma units use devices like the Hare Traction splint, which applies force to the foot while bracing against the posterior ischium, to temporarily distract fractures of the femur. Neither of these systems is suitable for use with MRI, and significant modifications would be required to use them during CT scans. We have built a novel device that distracts the ankle (as well as the knee) using a specially modified brace above the knee to grip the distal surfaces of the thigh musculature where it narrows above the knee as anchor points (Figure 1) and a screw to apply tension to the foot strap. The distance between the tensioning mechanism and the brace is adjustable for patient size. The parts of the distractor adjacent to the ankle are radiolucent and the entire device is compatible with the MR and CT environments (Figure 2). In use, the device has been found to be minimally uncomfortable for the patient. Fig 1 Arrows indicate anchor points for the knee brace on the musculature of the distal thigh. Fig 2 The distractor in place on a subject leg To facilitate application of consistent tension to the ankle, a simple proving ring-based gauge made entirely of plastic was added to the system (Figure 3). It is placed in the load train between the tensioning screw and the ankle strap. As the force increases, the circular ring distorts and the rack attached to one side of the hoop moves relative to the pinion on the other. This action turns the attached calibrated scale, and the force is read against a mark on the rim of the hoop. Figure 3 The tension gauge is connected between the foot strap (left) and the tensioning screw (right). The gauge ensures that consistent loads are applied to the ankle and allows the technician to monitor any slippage that would cause the force to diminish. As a matter of patient safety, forces are not allowed to exceed 30 pounds (130N). Using the device, we have greatly improved the quality of our MRI scans. Distraction of the ankle joint introduces enough separation between the cartilage surfaces for accurate segmentation of the digitized image for finite element modeling, as well as for direct visual analysis (Figure 4). CT scans also benefit from use of the device (Figure 5). Figure 4 Coronal plane MRI scans showing distraction of the ankle mortise. Anatomical (left), and distracted (right). Figure 5 Sagittal plane CT scans showing distraction between the tibia and talus. Anatomical (left), and distracted (right).

Current Clinical Issues for MRI Scanning of Pacemaker and Defibrillator Patients

Dramatic increases in both magnetic resonance imaging (MRI) usage and cardiac device-based therapy have resulted in an estimated 50-75% probability of a patient being indicated for an MRI over the lifetime of their device. Some recent studies have demonstrated "safe procedures" and "no adverse events" in the limited populations, clinical situations, and specific devices and lead orientations tested. While these investigations are useful to help ascertain the hazards for patients with cardiac devices, they do not demonstrate clear freedom from risk. All components of active implantable systems must be engineered during the design stage to provide safety in current and evolving MR environments. Device manufacturers need to secure regulatory approval to confirm their products' safety under multiple clinical and technical variables.

PREGNANCY AND THE SAFETY OF MAGNETIC RESONANCE IMAGING

Many data exist regarding the safety of the static and time-varying magnetic and electromagnetic fields used in MR environments for the developing fetus. Many of these data, however, are confusing and nebulous. The literature in this area is reviewed, and the issues related to MR imaging of the pregnant patient and MR and the pregnant health care practitioner are discussed. Practice guidelines also are suggested.