Assessment of MRI issues for the Argus II Retinal Prosthesis

Safety and Functional Integrity of Continuous Glucose Monitoring Components After Simulated Radiologic Procedures

BACKGROUND

We investigated wearable components of the Dexcom G6 continuous glucose monitoring (CGM) System in simulated therapeutic and diagnostic radiologic procedures.

METHODS

G6 transmitters were loaded with simulated glucose data and attached to sensors. Sets of sensor/transmitter pairs were exposed to x-rays to simulate a radiotherapeutic procedure and to radiofrequency (RF) and magnetic fields to simulate diagnostic magnetic resonance imaging (MRI). The x-ray simulation provided a cumulative dose of 80 Gy. The MRI simulation used RF fields oscillating at 64 or 128 MHz and magnetic fields of 1.5 or 3 T. During the MRI simulation, displacement force, induced heating, and induced currents were measured. After the simulations, bench tests were used to assess data integrity on the transmitters and responsiveness of sensors to various concentrations of aqueous glucose.

RESULTS

Glucose concentrations reported by sensor/transmitter pairs after undergoing x-irradiation or a simulated MRI exam were similar to those from control (unexposed) devices. During the 3 T MRI simulation, the devices experienced a displacement force of 306 g, which was insufficient to dislodge the sensor/transmitter from the substrate, RF-induced heating of <2°C, and an induced current of <16 pA. Data stored on the transmitters prior to the MRI simulation remained intact.

CONCLUSION

Wearable components of the G6 CGM System retain basic functionality and data integrity after exposure to simulated therapeutic and diagnostic radiologic procedures. The devices are unlikely to be affected by x-irradiation used in typical imaging studies. Simulated MRI procedures create displacement force, minimal heating, and current in sensor/transmitter pairs.

Electronic retinal prosthesis for severe loss of vision in geographic atrophy in age-related macular degeneration: First-in-human use

BACKGROUND

To date there are yet no available approved therapies for Geographic Atrophy (GA) secondary to age-related macular degeneration (AMD).

METHODS

Single site, non-randomized safety and efficacy study presenting the preliminary results in a cohort of five late stage AMD (GA) patients successfully implanted with the Argus II Retinal Prosthesis System (Second Sight Medical Products Inc., Sylmar, CA, USA). Extensive fundus imaging including retinal photographs from which the GA area was measured. A combination of custom and traditional tests designed for very low vision subjects assessed visual function in study subjects. A Functional Low-Vision Observer Rated Assessment was carried out to evaluate the impact of the system on the subject's daily life. In addition, a study to evaluate structural characteristics of the visual cortex of the brain was performed in one subject using magnetic resonance imaging.

RESULTS

Seven device-related adverse events were reported, four of which were classed as serious adverse events. Retinal detachment was reported in three patients and was successfully treated within 12 months of onset. Testing showed an improvement in visual function in three of five patients with the system turned on. Magnetic resonance imaging assessed in one patient after implantation indicates a selective increase in cortical myelin and thickness in visual brain regions 1 year post implantation.

CONCLUSIONS

Epiretinal prostheses can successfully be implanted in those affected by GA secondary to late-stage AMD and can elicit visual percepts by electrical stimulation of residual neuroretinal elements and improve basic visual function in those affected.

Towards safe operation of an active retinal prosthesis during functional MRI and diffusion tensor imaging

OBJECTIVE

To determine if the Argus II retinal prosthesis can operate during functional MRI (fMRI) and diffusion tensor imaging (DTI) acquisitions and if currents induced in the prosthesis by imaging are at safe levels.

MATERIALS AND METHODS

One Argus II retinal prosthesis was modified to enable current measurements during imaging. Active electronics were modified to enable operation during scans. Induced current was measured during diagnostic scans, which were previously shown to be safe for implanted patients, and during fMRI and DTI scans. All measurements were performed using an ASTM phantom to ensure reproducible placement.

RESULTS

The prosthesis was able to maintain communication with the external RF coil during the fMRI and DTI scans except briefly during pre-scans. Current levels induced during fMRI and DTI scans were consistently below those measured during diagnostic scans.

CONCLUSIONS

fMRI and DTI may be safely performed while the Argus II retinal prosthesis is operating.

Evaluation of MRI Issues for a New Wirelessly Powered, Spinal Cord Stimulation Lead With Receiver

OBJECTIVE. MRI is an imaging modality frequently ordered for patients with neuromodulation systems implanted for spinal cord stimulation. The purpose of this investigation was to evaluate MRI safety issues (magnetic field interactions, MRI-related heating, functional disturbances, and artifacts) for a new wirelessly powered lead with receiver used for SCS. MATERIALS AND METHODS. Lead samples underwent in vitro evaluation for MRI safety issues using standardized techniques. Magnetic field interactions (i.e., translational attraction and torque) and artifacts were tested at 3 T. MRI-related heating was performed at 1.5 T/64 MHz and 3 T/128 MHz using two different methods: numerical simulations with analytical modeling and physical testing. Possible functional disturbances were evaluated under exposures to 1.5-T/64-MHz and 3-T/128-MHz MRI conditions. RESULTS. The lead exhibited minor magnetic field interactions (22° deflection angle, no torque) at 3 T. The highest temperature change recorded at 1.5 T/64 MHz and 3 T/128 MHz was 3.8°C and 11.3°C, respectively. Exposures to MRI conditions did not damage or alter the functional aspects of the leads. The maximum artifact size seen on a gradient-echo pulse sequence extended approximately 10 mm relative to the size of the lead. CONCLUSION. The MRI tests performed on patients with the new lead with receiver revealed no substantial concerns with respect to the conditions that we provide in the safety guidelines that were based on the results of this investigation. Therefore, MRI examinations will result in acceptable heating when conducted at appropriate whole-body-averaged specific absorption rate levels (i.e., 2.0 W/kg at 1.5 T/64 MHz and 0.3 W/kg at 3 T/128 MHz, corresponding to adjusted temperature rises of 3.6°C and 1.2°C, respectively). Therefore, patients with this wirelessly powered lead and receiver implanted can safely undergo MRI examinations under specific conditions.

MRI safety and devices: An update and expert consensus

The use of magnetic resonance imaging (MRI) is increasing globally, and MRI safety issues regarding medical devices, which are constantly being developed or upgraded, represent an ongoing challenge for MRI personnel. To assist the MRI community, a panel of 10 radiologists with expertise in MRI safety from nine high-volume academic centers formed, with the objective of providing clarity on some of the MRI safety issues for the 10 most frequently questioned devices. Ten device categories were identified. The panel reviewed the literature, including key MRI safety issues regarding screening and adverse event reports, in addition to the manufacturer's Instructions For Use. Using a Delphi-inspired method, 36 practical recommendations were generated with 100% consensus that can aid the clinical MRI community. Level of Evidence: 5 Technical Efficacy Stage: 5 J. Magn. Reson. Imaging 2020;51:657-674.

Maximizing Data Transmission Rate for Implantable Devices Over a Single Inductive Link: Methodological Review

Due to the constantly growing geriatric population and the projected increase of the prevalence of chronic diseases that are refractory to drugs, implantable medical devices (IMDs) such as neurostimulators, endoscopic capsules, artificial retinal prostheses, and brain-machine interfaces are being developed. According to many business forecast firms, the IMD market is expected to grow and they are subject to much research aiming to overcome the numerous challenges of their development. One of these challenges consists of designing a wireless power and data transmission system that has high power efficiency, high data rates, low power consumption, and high robustness against noise. This is in addition to minimal design and implementation complexity. This manuscript concerns a comprehensive survey of the latest techniques used to power up and communicate between an external base station and an IMD. Patient safety considerations related to biological, physical, electromagnetic, and electromagnetic interference concerns for wireless IMDs are also explored. The simultaneous powering and data communication techniques using a single inductive link for both power transfer and bidirectional data communication, including the various data modulation/demodulation techniques, are also reviewed. This review will hopefully contribute to the persistent efforts to implement compact reliable IMDs while lowering their cost and upsurging their benefits.

An image-processing strategy to extract important information suitable for a low-size stimulus pattern in a retinal prosthesis

BACKGROUND

A retinal prosthesis is designed to help the blind to obtain some sight. It consists of an external part and an internal part. The external part is made up of a camera, an image processor and an RF transmitter. The internal part is made up of an RF receiver, implant chip and microelectrode.

METHODS

Currently, the number of microelectrodes is in the hundreds, and we do not know the mechanism for using an electrode to stimulate the optic nerve. A simple hypothesis is that the pixels in an image correspond to the electrode. The images captured by the camera should be processed by suitable strategies to correspond to stimulation from the electrode. Thus, it is a question of how to obtain the important information from the image captured in the picture. Here, we use the region of interest (ROI), a useful algorithm for extracting the ROI, to retain the important information, and to remove the redundant information.

RESULTS

This paper explains the details of the principles and functions of the ROI. Because we are investigating a real-time system, we need a fast processing ROI as a useful algorithm to extract the ROI. Thus, we simplified the ROI algorithm and used it in an outside image-processing digital signal processing (DSP) system of the retinal prosthesis.

CONCLUSION

The results show that our image-processing strategies are suitable for a real-time retinal prosthesis and can eliminate redundant information and provide useful information for expression in a low-size image.

In Vitro Magnetic Resonance Imaging Evaluation of Fragmented, Open-Coil, Percutaneous Peripheral Nerve Stimulation Leads

OBJECTIVE

Percutaneous peripheral nerve stimulation (PNS) is an FDA-cleared pain treatment. Occasionally, fragments of the lead (MicroLead, SPR Therapeutics, LLC, Cleveland, OH, USA) may be retained following lead removal. Since the lead is metallic, there are associated magnetic resonance imaging (MRI) risks. Therefore, the objective of this investigation was to evaluate MRI-related issues (i.e., magnetic field interactions, heating, and artifacts) for various lead fragments.

METHODS

Testing was conducted using standardized techniques on lead fragments of different lengths (i.e., 50, 75, and 100% of maximum possible fragment length of 12.7 cm) to determine MRI-related problems. Magnetic field interactions (i.e., translational attraction and torque) and artifacts were tested for the longest lead fragment at 3 Tesla. MRI-related heating was evaluated at 1.5 Tesla/64 MHz and 3 Tesla/128 MHz with each lead fragment placed in a gelled-saline filled phantom. Temperatures were recorded on the lead fragments while using relatively high RF power levels. Artifacts were evaluated using T1-weighted, spin echo, and gradient echo (GRE) pulse sequences.

RESULTS

The longest lead fragment produced only minor magnetic field interactions. For the lead fragments evaluated, physiologically inconsequential MRI-related heating occurred at 1.5 Tesla/64 MHz while under certain 3 Tesla/128 MHz conditions, excessive temperature elevations may occur. Artifacts extended approximately 7 mm from the lead fragment on the GRE pulse sequence, suggesting that anatomy located at a position greater than this distance may be visualized on MRI.

CONCLUSIONS

MRI may be performed safely in patients with retained lead fragments at 1.5 Tesla using the specific conditions of this study (i.e., MR Conditional). Due to possible excessive temperature rises at 3 Tesla, performing MRI at that field strength is currently inadvisable.

Assessment of MRI issues for a new cerebral spinal fluid shunt, gravitational valve (GV)

PURPOSE

A gravitational valve (GV) may be used to treat hydrocephalus, offering possible advantages that include avoidance of over drainage and long-term complications. Because a GV is made from metal, there are potential safety and other problems related to the use of MRI. The objective of this investigation was to evaluate MRI-related issues (i.e., magnetic field interactions, heating, and artifacts) for a newly developed, metallic GV.

METHODS

Tests were performed on the GV (GAV 2.0) using well-accepted techniques to assess magnetic field interactions (translational attraction and torque, 3-Tesla), MRI-related heating (1.5-T/64-MH and 3-T/128-MHz, whole body averaged SAR, 2.7-W/kg and 2.9-W/kg, respectively), artifacts (3-Tesla; gradient echo and T1-weighted, spin echo sequences), and possible functional changes related to exposures to different MRI conditions (exposing six samples each to eight different pulse sequences at 1.5-T/64-MHz and 3-T/128-MHz).

RESULTS

Magnetic field interactions were not substantial (deflection angle 2°, no torque) and heating was minor (highest temperature rise, ≥1.9°C, highest background temperature rise, ≥1.7°C). Artifacts on the gradient echo pulse sequence extended approximately 10mm from the size and shape of the GV. The different exposures to 1.5-T/64-MHz and 3-T/128-MHz conditions did not alter or damage the operational aspects of the GV samples.

CONCLUSIONS

The findings demonstrated that MRI can be safely used in patients with this GV and, thus, this metallic implant is deemed acceptable or "MR Conditional" (i.e., using current labeling terminology), according to the conditions used in this study.

Visual BOLD Response in Late Blind Subjects with Argus II Retinal Prosthesis

Retinal prosthesis technologies require that the visual system downstream of the retinal circuitry be capable of transmitting and elaborating visual signals. We studied the capability of plastic remodeling in late blind subjects implanted with the Argus II Retinal Prosthesis with psychophysics and functional MRI (fMRI). After surgery, six out of seven retinitis pigmentosa (RP) blind subjects were able to detect high-contrast stimuli using the prosthetic implant. However, direction discrimination to contrast modulated stimuli remained at chance level in all of them. No subject showed any improvement of contrast sensitivity in either eye when not using the Argus II. Before the implant, the Blood Oxygenation Level Dependent (BOLD) activity in V1 and the lateral geniculate nucleus (LGN) was very weak or absent. Surprisingly, after prolonged use of Argus II, BOLD responses to visual input were enhanced. This is, to our knowledge, the first study tracking the neural changes of visual areas in patients after retinal implant, revealing a capacity to respond to restored visual input even after years of deprivation.

The potential of the second sight system bionic eye implant for partial sight restoration

INTRODUCTION

Second Sight System bionic eye implant, a commercially available visual prosthesis developed by Second Sight Medical Products, has been implanted in over 125 patients with outer retinal dystrophies such as retinitis pigmentosa. The system has gained regulatory approval in both the USA and Europe, and aims to restore vision by electrical stimulation of the nerve cells of the inner retina.

AREAS COVERED

In this review, we present the safety profile of this implant from the international clinical trial and discuss the nature and levels of improvement in visual function achieved by patients implanted with the system. Expert commentary: Future developments for the system will be explored following the discussion of the current usefulness of the device, its limitation as and the areas in which further development is necessary.

Feasibility of Structural and Functional MRI Acquisition with Unpowered Implants in Argus II Retinal Prosthesis Patients: A Case Study

PURPOSE

Magnetic resonance imaging (MRI) can measure the effects of vision loss and recovery on brain function and structure. In this case study, we sought to determine the feasibility of acquiring anatomical and functional MRI data in recipients of the Argus II epiretinal prosthesis system.

METHODS

Following successful implantation with the Argus II device, two retinitis pigmentosa (RP) patients completed MRI scans with their implant unpowered to measure primary visual cortex (V1) functional responses to a tactile task, whole-brain morphometry, V1 cortical thickness, and diffusion properties of the optic tract and optic radiation. Measurements in the subjects with the Argus II implant were compared to measurements obtained previously from RP patients and sighted individuals.

RESULTS

The presence of the Argus II implant resulted in artifacts that were localized around the patient's implanted eye and did not extend into cortical regions or white matter tracts associated with the visual system. Structural data on V1 cortical thickness and the retinofugal tract obtained from the two Argus II subjects fell within the ranges of sighted and RP groups. When compared to the RP and sighted subjects, Argus II patients' tactile-evoked cross-modal functional MRI (fMRI) blood oxygen level-dependent (BOLD) responses in V1 also fell within the range of either sighted or RP groups, apparently depending on time since implantation.

CONCLUSIONS

This study demonstrates that successful acquisition and quantification of structural and functional MR images are feasible in the presence of the inactive implant and provides preliminary information on functional changes in the brain that may follow sight restoration treatments.

TRANSITIONAL RELEVANCE

Successful MRI and fMRI acquisition in Argus II recipients demonstrates feasibility of using MRI to study the effect of retinal prosthesis use on brain structure and function.

Retinal prosthesis

Retinal prosthesis has been translated from the laboratory to the clinic over the past two decades. Currently, two devices have regulatory approval for the treatment of retinitis pigmentosa. These devices provide partial sight restoration and patients use this improved vision in their everyday lives. Improved mobility and object detection are some of the more notable findings from the clinical trials. However, significant vision restoration will require both better technology and improved understanding of the interaction between electrical stimulation and the retina. This paper reviews the recent clinical trials and highlights technology breakthroughs that will contribute to next generation of retinal prostheses.

Assessment of safety and interference issues of radio frequency identification devices in 0.3 Tesla magnetic resonance imaging and computed tomography

The objective of this study was to evaluate two issues regarding magnetic resonance imaging (MRI) including device functionality and image artifacts for the presence of radio frequency identification devices (RFID) in association with 0.3 Tesla at 12.7 MHz MRI and computed tomography (CT) scanning. Fifteen samples of RFID tags with two different sizes (wristband and ID card types) were tested. The tags were exposed to several MR-imaging conditions during MRI examination and X-rays of CT scan. Throughout the test, the tags were oriented in three different directions (axial, coronal, and sagittal) relative to MRI system in order to cover all possible situations with respect to the patient undergoing MRI and CT scanning, wearing a RFID tag on wrist. We observed that the tags did not sustain physical damage with their functionality remaining unaffected even after MRI and CT scanning, and there was no alternation in previously stored data as well. In addition, no evidence of either signal loss or artifact was seen in the acquired MR and CT images. Therefore, we can conclude that the use of this passive RFID tag is safe for a patient undergoing MRI at 0.3 T/12.7 MHz and CT Scanning.

A review and update on the current status of retinal prostheses (bionic eye)

INTRODUCTION/BACKGROUND

The Argus® II is the first retinal prosthesis approved for the treatment of patients blind from retinitis pigmentosa (RP), receiving CE (Conformité Européenne) marking in March 2011 and FDA approval in February 2013. Alpha-IMS followed closely and obtained CE marking in July 2013. Other devices are being developed, some of which are currently in clinical trials.

SOURCES OF DATA

A systematic literature search was conducted on PubMED, Google Scholar and IEEExplore.

AREAS OF AGREEMENT

Retinal prostheses play a part in restoring vision in blind RP patients providing stable, safe and long-term retinal stimulation.

AREAS OF CONTROVERSY

Objective improvement in visual function does not always translate into consistent improvement in the patient's quality of life. Controversy exists over the use of an external image-capturing device versus internally placed photodiode devices.

GROWING POINTS

The alpha-IMS, a photovoltaic-based retinal prosthesis recently obtained its CE marking in July 2013.

AREAS TIMELY FOR DEVELOPING RESEARCH

Improvement in retinal prosthetic vision depends on: (i) improving visual resolution, (ii) improving the visual field, (iii) developing an accurate neural code for image processing and (iv) improving the biocompatibility of the device to ensure longevity.

Recent Innovations in Medical and Surgical Retina

PURPOSE

To highlight recent innovations in the medical and surgical treatment of retinal disease.

DESIGN

Retrospective literature review.

METHODS

The peer-reviewed, published (during 2012) English literature was searched using the Medline database. Articles deemed relevant were selected and highlighted.

RESULTS

A number of important innovations in the treatment of retinal disease have occurred during the past year. We review developments in regenerative medicine (e.g., retinal prosthesis, optogenetics, cell-based therapy), diagnostics (e.g., swept source optical coherence tomography [OCT], intraoperative OCT, multimodal imaging), surgical innovations (e.g., chemical vitreolysis with ocriplasmin, 27-gauge vitrectomy), and pharmacological therapy of retinal vascular disease (e.g., use of ranibizumab and aflibercept in the treatment of macular edema associated with diabetic retinopathy and retinal vein occlusion).

CONCLUSIONS

Many important innovations in regenerative medicine, diagnostic and surgical instrumentation, and pharmacological therapy for retinal vascular disease have occurred during the past year. In view of the number of agents in early phase clinical trials (e.g., treatments for dry age-related macular degeneration) as well as technologies under development (e.g., microrobots for vitreous surgery), this trend will continue.