MR ScanWise Implant (MROC)

As we get older, many of us have an increased chance of experiencing musculoskeletal, cardiovascular and neurological diseases as well as diabetes. We also run a higher risk of developing more than one clinical condition, either chronic or acute. Access to diagnostic imaging will be required for many patients that already suffer from another disease. For example, studies of the prevalence of comorbidity show that at age 70, having three or four conditions is not uncommon⁴. While oncology was not included in this assessment, advances in treatment and medical care is expected to soon transform several cancers into chronic diseases.

Today, treatment of many chronic conditions involves the placement of implants or use of body-worn treatment-delivery devices. Millions of patients benefit from passive implants, like hip or knee replacements, spine stabilization devices, vascular stents, clips or coils. Many others depend on active implants or devices, requiring electrical power and logic components to operate, like pacemakers, neuro stimulators, insulin pumps, spine pain-relief stimulators and cochlear implants.

For numerous medical conditions, MRI is the modality of choice^5-9 recommended by consensus guidelines for diagnosis and treatment monitoring. Current medical practice, however, excludes patients with an implant from access to MRI, based on serious and valid safety concerns. This “clash of technologies”¹⁰ results in unnecessary health inequalities for a rapidly growing group of patients.

Implants must be strong for musculoskeletal applications or flexible, for instance when used as stents, and biocompatible. Active implants must sense or stimulate electrophysiological signals from the heart or the central nervous system.¹¹ Their microcontrollers and electronics must be protected against electromagnetic interferences in everyday life. All these requirements suggest the use of metallic structures, which can be problematic when brought near or inside an MRI system. This is particularly true because the strong electromagnetic fields required for MRI far exceed the electromagnetic fields encountered in urban environments.

Scanning patients with passive or active implants requires having significant additional organizational measures in place at the hospital. Retrieval and interpretation of MR Conditional implant labeling must be organized in a multidisciplinary team. Cross-industry harmonization and standardization efforts by IEC and ISO14 and FDA guidance¹² have improved the availability of information to support the clinical decision to scan in compliance with MR Conditional labeling.

Users should understand the possible interactions between implants and the MRI system, and should be able to identify related safety concerns. The required precautions related to implants can be divided into two categories:^13,14

• The always-present static magnetic field B0 can cause attraction and device malfunction. Pacemakers may reset to programming mode, and become nonfunctional, when the magnetic field exceeds the very small value of 0.5 mT (5 Gauss). While current standards¹⁵ specify a safe limit of 1 mT (10 Gauss), older implanted devices may still be affected at 0.5 mT (5 Gauss), the value specified for delimiting the Controlled Access Area in IEC60601-2-33.
• RF pulses and switching gradients must be emitted to acquire the MR image. These outputs can be reduced by implementing and applying appropriate controls in the MRI sequences.

It is of paramount importance to perform all safety checks related to the static magnetic field before admitting a patient into the MRI suite. These checks should be done independently, and at every decision point in the workflow, by the referring physician, the radiologist and the operator.

This leads to the following typical workflow steps:¹⁴

1. Check for the presence of an active implant. Ensure that the implant will not become dysfunctional when critical for life support. Keep the patient out of the 0.5 mT (5 Gauss) line.
2. Check for the Static Field Gradient condition and verify the related exclusion zones around the magnet. Make sure that the implant does not enter these areas.
3. Check RF output (SAR or B1+rms) and gradient (slew rate or dB/dt) limitations. When defining the examination protocol, make sure that each and every sequence complies with these restrictions.