Abstract
Objective—To determine the functionality of implanted microchips following magnetic resonance imaging (MRI).
Design—Prospective clinical trial.
Animals—53 client-owned patients implanted with a microchip undergoing MRI of various areas of the body for a variety of medical conditions.
Procedures—General anesthesia was induced, and each patient's microchip was scanned with a universal microchip scanner; the chip number was recorded. Patients were transported to the MRI suite, and MRI was completed. Patients were moved out of the magnetic environment, and microchips were scanned again. Patient information and chip number were recorded. Chip numbers before and after MRI were compared.
Results—For all 53 microchips scanned from 53 patients, the same number was read accurately following MRI of a variety of sites.
Conclusions and Clinical Relevance—These data indicated that MRI did not interfere with the functionality of these microchips. This information is valuable for practitioners recommending MRI for their patients and for clients who have invested in implanting a microchip in their pets.
Every year in the United States, millions of pets are lost or abandoned by their owners, causing shelters and rescue agencies to struggle with increasing numbers of unclaimed pets and decreasing resources. In the Knoxville area, 1 animal center took in 15,750 animals in 2010.1 In addition, lack of identification is a common factor leading to the euthanasia of animals in the United States,2 which indicates a need for the use of a reliable and cost-effective identification system, such as implanted microchips. The implantation of a microchip can help a lost pet to be reunited with its owner.3,4
In the past few years, there has been an increase in the number of pets implanted with microchips. Also, advances in medical technology have made MRI more available and relevant to veterinary medicine. Both procedures involve the use of radiowaves with various frequencies. Microchip marketing companies in the United States use carrier frequencies of 125, 128, or sometimes 134.2 kHz.5 In most other countries, microchips operate at a frequency of 134.2 kHz, as instituted by the International Organization for Standardization.6 Magnetic resonance imaging scanners operate on various radiofrequencies depending on the strength of the scanner (in units of Tesla). A 1.0-T MRI unit has a core frequency of 42.57 MHz.
Magnetic resonance imaging units are surrounded by a magnetic field and are shielded to protect from ferromagnetic matter, both for safety and to decrease imaging interference. Microchips currently implanted in pets in the United States have a ferrite core with a radio frequency microtransponder which is bonded to a copper antenna.7,8 There is potential for damage to ferromagnetic components of microchips because of exposure to the magnetic environment within an MRI suite.9 Microchips are activated by radio frequency pulses emitted from a microchip scanner. A unique number is transferred to the scanner through deflected radio frequency pulses and is read and displayed by the microchip scanner. The number of the chip is registered by the owner in a database with the owner's contact information, linking owner and pet.7,8
As the use of MRI as a diagnostic imaging modality increases and the number of patients with implanted microchips increases, it is important to examine whether there is potential for interference between the radio frequency pulses used in each procedure or damage to the ferromagnetic components of the microchip from the magnetic environment. To our knowledge, there are currently no independent clinical studies that indicate whether MRI interferes with the functionality or the life of a microchip. However, some major microchip manufacturersa–c will guarantee the function of an implanted microchip for the life of the animal. The purpose of the study reported here was to determine whether MRI with a 1.0-T magnet affects the functionality of microchips implanted in small animal patients.
Materials and Methods
The experimental protocol was approved by the Institutional Animal Care and Use Committee of the University of Tennessee College of Veterinary Medicine. Fifty-three client-owned patients (dogs and cats) with implanted microchips undergoing MRI at the University of Tennessee Veterinary Medical Center for various medical conditions between June 18, 2008, and March 1, 2009, were included in the study. The patients were anesthetized with a variety of anesthetic protocols commonly used at the study institution. The patients were scanned for their individual microchip numbers with a universal microchip scannerd outside of the magnetic environment before and after MRI. The universal microchip scanner was provided by the manufacturer and was chosen because it has been shown to have the highest sensitivity of popular commercially available microchip scanners.10 The scanning for the microchip implanted in the patient was completed by one of the authors (KAH) using scanning technique guidelines provided by the manufacturer in conjunction with an established standardized scanning protocol.10 Following MRI, the scanning procedure was repeated outside of the magnetic environment and the result was recorded along with patient age, body weight, and site of MRI. The batteries of the scanner were changed whenever a low battery warning was displayed or every 2 weeks. All MRI scans were completed with a 1.0-T magnet.e Magnetic resonance imaging of the brain included routine T1-weighted spin echo, T2-weighted spin echo, fluid-attenuated inversion recovery, and T2*, followed by administration of IV contrast and a postcontrast T1-weighted spin echo with and without fat suppression scans. Magnetic resonance imaging of the spine (cervical, thoracolumbar, or lumbosacral) consisted of routine T1-weighted spin echo, T2-weighted spin echo, and a half-Fourier–acquired single-shot turbo spin echo. Half-Fourier–acquired single-shot turbo spin echo is a myelogram-like sequence highlighting CSF in dorsal and ventral columns above and below the spinal cord. For patients undergoing spinal imaging, contrast medium was injected IV on a case-by-case basis. The image scan times were recorded at the completion of the MRI.
Results
Of the 53 patients, there were 2 cats and 51 dogs. Twenty-one patients underwent MRI of the brain, and 23 patients underwent MRI of the spine. Of those 23, 6 dogs had imaging of the cervical spine, 10 dogs had imaging of the thoracolumbar spine, and 7 dogs had imaging of the lumbosacral spine. Seven of the dogs undergoing spinal MRI were administered contrast medium IV, and additional T1-weighted spin echo with fat suppression scans were completed. The mean image scan time was longer in patients requiring contrast administration and postcontrast imaging. The additional image scan time ranged from 6 to 12 minutes, depending on the size of the animal and the area to be imaged. The mean scan time for patients undergoing MRI of the brain was 42 minutes (median, 42 minutes; range, 39 to 46 minutes) including postcontrast imaging. Five patients had 2 sites imaged, with a mean image scan time of 84.6 minutes (median, 83 minutes; range, 75 to 99 minutes). One patient had 3 areas imaged, with an image scan time of 155 minutes. Three patients had other areas scanned, including bilateral shoulder joints, bilateral brachial plexus, and right stifle. The mean image scanning time for these 3 patients was approximately 43 minutes (median, 41 minutes; range, 28 to 60 minutes). The mean scan time for all patients was 33 minutes (median, 45 minutes; range, 21 to 155 minutes).
In each of the 53 clinical cases, the patient's microchip number before MRI matched the recorded number after MRI. These data indicate that the site of imaging, mean MRI scan time, brand or frequency of microchip, patient age, or patient weight did not affect the function of the microchip. In addition, we did not identify any malfunctioning microchips in the patients that received repeated MRI. This indicated that MRI did not interfere with the functionality of the implanted microchips.
Discussion
The results of the present study indicated that MRI with a 1.0-T magnet in small animal patients did not interfere with the functionality of the microchips tested. Another study10 of microchip function after implantation in dogs and cats not undergoing MRI indicated that microchip failure rate could be 0.4%. The information reported here is important to eliminate MRI as a potential cause for microchip failure. The results are useful for veterinarians recommending MRI to their clients and also for the thousands of owners whose pets have an implanted microchip which have already undergone MRI.
Although providing valuable information, the present study had several limitations. We did not collect information on the age of the microchip (ie, the length of time each pet had been implanted with the microchip). Also, we examined patients during a short portion of the life of the pet and microchip. Some of the patients underwent > 1 MRI; however, all imaging procedures were completed within a 3-month period and no follow-up testing was completed. Consequently, the longevity of the microchips was not examined. An additional limitation is the strength of the magnet. This study was conducted with the 1.0-T MRI unit at our institution. Currently, clinical veterinary MRI units range in strength from 0.2 to 3.0-T. The effect of higher-strength MRI units on microchip functionality was not investigated in the present study. Finally, the sample size of 53 patients was very small. Similar studies10,11 of implanted microchips set in shelter environments have collected data for thousands of animals. It may be useful to examine a larger sample size in the future, but this will always be limited by clinical case load for this type of study. A recent study12 reported the occurrence of imaging artifact caused by microchip implantation, microchip migration, and damage to the tissues surrounding an implanted microchip following MRI. The authors found the incidence of susceptibility artifact (ie, focal signal void and image distortion) to be 100%, with no evidence of microchip migration. The damage to surrounding tissues was negligible.12 Future studies with stronger MRI scanners and larger numbers of cases are suggested. However, the results of the present study indicate that MRI with a 1.0-T magnet is not a potential cause for microchip failure, which is valuable information for practitioners recommending MRI for their patients and for clients who have invested in implanting a microchip in their pets.
ABBREVIATION
| MRI | Magnetic resonance imaging |
American Veterinary Identification Devices (AVID), Norco, Calif.
Home Again, East Syracuse, NY.
24PetWatch, Oakville, ON, Canada.
Universal WorldScan Reader, Digital Angel Inc, Saint Paul, Minn.
Harmony 1.0-T magnet, Siemens, Erlangen, Germany.
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