Extremity MR System. In 1993, a specially-designed, low-field-strength (0.2 T MR system, Artoscan, Lunar Corp., Madison, WI/Esaote, Genoa, Italy) MR system became available for MR imaging of extremities. This MR system uses a small-bore permanent magnet to image feet, ankles, knees, hands, wrists, and elbows.
The ergonomic design of the extremity MR system is such that the body part of interest is placed inside the magnet bore, with the patient positioned in a seated or supine position (i.e., depending on the body part that is imaged).
The entire extremity MR system weighs approximately 800 kg, has a built-in radiofrequency shield, multiple body-part-specific extremity coils, and 10 mT/m magnetic gradients.
A major advantage of this particular extremity MR system is that it can be sited in a relatively small space (e.g., approximately 100 square feet) without the need for a special power source, magnetic field shielding, or radiofrequency shielding.
Of note is that MR imaging using the extremity MR system has been demonstrated to provide a sensitive, accurate, and reliable assessment of various forms of musculoskeletal pathology.
Because of the unique design features of the extremity MR system (which includes a low-field-strength static magnetic field with a relatively small fringe field) and in consideration of how patients are positioned for MR procedures using this device (i.e., only the body part imaged is placed within the magnet bore while the rest of the body remains outside), it was suggested that it may be possible to safely image patients with aneurysm clips, even if they are made from ferromagnetic materials.
Furthermore, it may be possible to perform extremity MR imaging in patients with cardiac pacemakers or implantable cardioverter defibrillators (ICDs). Therefore, investigations were conducted to specifically evaluate these safety issues.
Patients with Ferromagnetic Aneurysm Clips
A study was performed to assess the magnetic field interaction for a variety of different aneurysm clips exposed to the 0.2 T extremity MR system. Twenty-two different types of aneurysm clips were evaluated including those made from nonferromagnetic, weakly ferromagnetic, and ferromagnetic materials (i.e., a Heifetz aneurysm clip made from 17-7PH and a Yasargil, Model FD aneurysm clip). The results indicated that none of the aneurysm clips tested displayed substantial magnetic field interaction in association with the 0.2 T extremity MR system.
Because of unique design features of the extremity MR system and in consideration of how patients are positioned for MR procedures using this device (i.e., the head does not enter the magnet bore), it is considered safe to perform MR imaging in patients with the specific aneurysm clips that have been evaluated.
These findings effectively permit an important diagnostic imaging modality to be used to evaluate the extemities of patients with suspected musculoskeletal abnormalities using the Artoscan MR system.
By comparison, various studies have reported that patients with Heifetz (17-7PH) and Yasargil, Model FD aneurysm clips (i.e., two of the clips evaluated in the study using the Artoscan) should not undergo MR imaging using MR systems with conventional designs because of the strong attraction shown by these aneurysm clips, which would pose a potential hazard to patients.
Patients with Cardiac Pacemakers and Implantable Cardioverter Defibrillators
As previously indicated, patients with cardiac pacemakers and implantable cardioverter defibrillators (ICDs) are generally not permitted to undergo MR procedures. However, due to the design of the Artoscan extremity MR system it may be possible to safely perform MRI in patients with these devices.
Since the magnetic fringe field of the extremity MR system is contained in close proximity to the 0.2-T magnet and this system has an integrated Faraday cage, only the patient's extremity is predominantly exposed to the MR-related electromagnetic fields when a procedure is performed. Of note is that it would not be possible for the MR system's gradient or RF electromagnetic fields to induce currents in a pacemaker or ICD because the patient's thorax (i.e., where the pacemaker or ICD is typically placed) remains outside of the MR system.
Therefore, ex vivo experiments were conducted on seven different cardiac pacemakers and seven different implantable cardioverter defibrillators manufactured by Medtronic, Inc. (Minneapolis, MN). The following devices were tested:
Magnetic field attraction was assessed relative to the 0.2-T static magnetic field of the extremity MR system. Additionally, the cardiac pacemakers and implantable cardioverter defibrillators were operated with various lead systems attached while immersed in a tank containing physiologic saline. This apparatus was used to simulate the thorax and was oriented in parallel and perpendicular positions relative to the closest part of the MR system to which a patient undergoing an MR procedure would be positioned.
MR studies were performed on a phantom using T1-weighted spin echo and gradient echo sequences. Various functions of the pacemakers and ICDs were evaluated before, during, and after MR imaging.
The results of these tests indicated that magnetic field attraction did not present problems for the devices. The activation of the pacemakers and cardioverter defibrillators did not substantially affect image quality during MR imaging. Most importantly, the operation of the extremity MR system produced no alterations in the function of the cardiac pacemakers and implantable cardioverter defibrillators.
Therefore, in consideration of these data and in view of how patients are positioned during MRI using the extremity MR system (i.e., the thorax does not enter the magnet bore), it should be safe to perform MRI in patients with the specific cardiac pacemakers and implantable cardioverter defibrillators evaluated in this study.
Other types of dedicated extremity MR systems. In 1998, another dedicated extremity MR system was developed jointly by Esaote and Siemens Medical Systems. This new device allows MR studies to be conducted on each of the afore-mentioned body parts as well as the shoulder.
Understandably, this particular type of MR system does not have the same inherent design features as the previously-described dedicated extremity system and, as such, this new system should not used to conduct MR procedures in patients with ferromagnetic aneurysm clips or cardiac pacemakers and implantable cardioverter defibrillators.
Studies are required to determine if this particular dedicated extremity system may be used to perform MR examinations safely in patients with ferromagnetic implants or other similar typically contraindicated devices.
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