MRI Safe in Patients With Subcutaneous Defibrillators

Kate Johnson

March 11, 2014

VIENNA — Patients implanted with a subcutaneous implantable cardioverter defibrillator (S-ICD System, Cameron Health and Boston Scientific) can safely undergo MRI, according to a study of the first cohort of patients to be evaluated.

"The location and design of the system" make MRI safe, lead investigator Jiří Keller, MD, from Na Homolce Hospital in Prague, told Medscape Medical News.

Since 2009, more than 450 patients around the world have been implanted with the S-ICD, and 52 of them were treated at Dr. Keller's hospital. The subcutaneous system does not require electrodes in or on the heart. "The principle of this defibrillator is that it's positioned under the skin and nothing is touching the heart; from this perspective, it's safer," he explained.

The study, presented here at the European Congress of Radiology 2014, involved 12 men and 3 women with a mean body mass index of 26 kg/m².

A Siemens Avanto 1.5 Tesla MRI was used to perform the randomly assigned scans; 5 were of the brain, 4 of the heart, 6 of the L-spine, and 2 of the C-spine.

All scans were performed at least 6 weeks after S-ICD implantation, and the device was turned off during imaging.

"We didn't exceed the specific absorption rate of 2 watts/kg. For patient safety, we used pulse oximetry and electrocardiogram during the scan, with a defibrillator and cardiologist on standby," Dr. Keller said.

 
MRI scans are as important for patients with implantable cardioverter defibrillators as for any other patient.
 

During the scan, skin temperature was measured with an MRI-compatible thermistor, which was placed oblique to the surface of the S-ICD. Patients were told to immediately report any pain, torquing movement, or heating sensation in the area of either the pocket or the electrode.

S-ICD voltage, QRS signal detection, and memory function were checked before and after scanning.

The study showed no evidence of movement or rotation of the S-ICD, Dr. Keller pointed out. In addition, there were no malfunctions evident on postscan checks of the S-ICD up to 12 months later.

Imaging of the brain, C-spine, and L-spine were free of artifacts. Heart imaging showed no major artifacts in the right chamber, but there was interference in the left chamber view, he reported.

The procedure was repeated in 2 patients who complained of heating over the subcutaneous pocket during their original L-spine scan, which was caused by the thermistor. "Even though the thermistor was supposed to be MR-safe, it did produce a tiny amount of heat, which was felt by the patients," he said.

After repeat scans were modified — with reduced radiofrequency field exposure, reduced turbo factors (21 > 7), increased repetition time (>4000 ms), and reduced flip angle (170 > 120 degrees) — patients reported improved heat sensation, he said.

Although MRI is safe in patients with S-ICD, "more work on the optimization of sequences for heart examination is needed," Dr. Keller concluded.

There is an increasing number of patients with implantable cardiac devices who are indicated for MRI, so this information is important for clinicians and patients, he emphasized.

"MRI scans are as important for patients with implantable cardioverter defibrillators as for any other patient; they are not just cardiology patients, they have other illnesses," he said.

"MRI scanning in the presence of cardiac devices has been related to damage to the device, as well as to the lead systems attached to the device," said Peter Zimetbaum, MD, from the Beth Israel Deaconess Medical Center at Harvard Medical School in Boston.

"The subcutaneous device has nonintravascular components, which immediately decreases some of the concern related to conventional devices," he told Medscape Medical News.

"In addition, the evidence that there is no damage to the subcutaneous device is reassuring," noted Dr. Zimetbaum, who wrote an editorial on the subject of magnetic resonance imaging and regular implantable cardioverter defibrillators (Ann Intern Med. 2011;155:470-472).

"This is a small study, and more experience will be required before this can be done regularly, but the suggestion from this evidence is that it will be safe," he noted.

This investigator-initiated study was supported by Research Project Charles University in Prague PRVOUK P34; no support was received from the manufacturer. Dr. Keller has disclosed no relevant financial relationships.

European Congress of Radiology (ECR) 2014: Abstract B-0297. Presented March 7, 2014.

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