Original contribution
Assessment of MRI issues for the Argus II Retinal Prosthesis

https://doi.org/10.1016/j.mri.2011.12.005Get rights and content

Abstract

Objective

The objective was to evaluate magnetic resonance imaging (MRI) issues (magnetic field interactions, heating, artifacts and functional alterations) at 1.5 T and 3 T for the Argus II Retinal Prosthesis (Second Sight Medical Products, Sylmar, CA, USA).

Materials and Methods

Standardized protocols were used to assess magnetic field interactions (translational attraction and torque; 3 T, worst case), MRI-related heating (1.5 and 3 T), artifacts (3 T; worst case) and functional changes (1.5 and 3 T) associated with MRI.

Results

The magnetic field interactions were acceptable. MRI-related heating, which was studied at a relatively high, MR system-reported whole body averaged specific absorption rates, will not pose a hazard to the patient under the conditions used for testing. While artifacts were “moderate” in relation to the dimensions of the Argus II Retinal Prosthesis, optimization of MRI parameters can reduce the size of the artifacts. Exposures to MRI conditions at 1.5 and 3 T did not damage or alter the functional aspects of the Argus II Retinal Prosthesis.

Conclusions

In consideration of the test results, a patient with the Argus II Retinal Prosthesis may undergo MRI at 1.5 T or 3 T when specific guidelines and MRI conditions are followed, including those advised by the manufacturer.

Introduction

Retinal prostheses have demonstrated the ability to restore partial vision in blind individuals through electrical stimulation of the retina [1], [2], [3], [4], [5], [6]. Several clinical trials have established the ability to detect light vs. dark, and more recent studies have shown improved mobility and letter reading [4], [5], [6]. This represents a significant advancement in the treatment of vision loss.

Currently, most electronically activated implants used to treat neurological and other disorders are unacceptable or contraindicated for patients referred for magnetic resonance imaging (MRI) procedures, unless comprehensive evaluations are conducted to identify specific conditions that ensure patient safety [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. This is due to concerns related to MRI-based issues that may pose risks or other problems to patients with these implants including magnetic field interactions, MRI-related heating, disturbances in the functional aspects of the devices and imaging artifacts that can impact the diagnostic use of MRI [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17].

A new retinal prosthesis, called the Argus II Retinal Prosthesis System (Second Sight Medical Products, Sylmar, CA, USA), was developed to provide electrical stimulation of the retina to induce visual perception in blind individuals. This implant has a surgically implanted intraocular electrode array placed on the ganglion side of the retina (i.e., epiretinal) that is connected to an extraocular electronic stimulator with a receiver radiofrequency (RF) coil. These intraocular components are wirelessly connected to external equipment that includes glasses with a camera and transmitter RF coil, and a belt-worn video processing unit (VPU) with battery which is connected to the glasses through a cable [3], [4], [5], [6].

After receiving a retinal implant, it is likely that an individual may need to undergo an MRI procedure for purposes of diagnostic evaluation with this important imaging modality. Therefore, to ensure the safe use of this device in a patient referred for an MRI examination, this investigation evaluated the factors that may impact this implant in 1.5-T and 3-T MRI environments (i.e., worst case for a clinical MR system), including magnetic field interactions (translational attraction and torque), MRI-related heating, artifacts and functional changes associated with different MRI conditions. The primary goal was to identify specific guidelines and conditions that would permit the safe use of MRI in patients with the Argus II Retinal Prosthesis System. Of note is that the glasses with the camera and transmitter RF coil, and the belt-worn VPU with battery are not intended to be present during an MRI procedure.

Section snippets

Argus II Retinal Prosthesis

The Argus II Retinal Prosthesis (Second Sight Medical Products, Sylmar, CA, USA) underwent evaluation in 1.5-T and 3-T MRI environments. This implant is designed to provide visual function to individuals with severe to profound vision loss due to outer retinal degeneration. It consists of implanted and external components. The Argus II Retinal Prosthesis is an epiretinal prosthesis that includes a receiving coil, electronics case and an electrode array that are surgically implanted in and

Magnetic field interactions

The results for the assessment of magnetic field interactions at 3 T for the Argus II Retinal Prosthesis indicated that the mean deflection angle was 12° and the torque was 0 (no torque).

MRI-related heating

At 1.5 T/64 MHz, the highest temperature change was +0.6°C in association with MRI performed for 15 min at an MR system-reported whole body averaged SAR of 3.5 W/kg. At 3 T/128 MHz, the highest temperature change was +2.1°C in association with MRI performed for 15 min at an MR system-reported whole body

Magnetic field interactions

For the assessment of translational attraction, the average deflection angle was 12° at 3 T for the Argus II Retinal Prosthesis. This information should be considered according to the information provided by the American Society for Testing and Materials International [19], as follows: “If the implant deflects less than 45°, then the magnetically induced deflection force is less than the force on the implant due to gravity (its weight). For this condition, it is assumed that any risk imposed by

Acknowledgments

Special thanks to Sam Valencerina, B.S., R.T. (R) (MR), for his highly professional assistance with the MRI investigations.

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Funding: Supported by Second Sight Medical Products, Sylmar, CA, USA.

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