PA-PSRS received a report of a patient experiencing unanticipated electrical stimulation from an implanted electronic neurostimulator during a computed tomography (CT) scan with a 64-slice CT scanner. When the neurostimulator activated, the patient moved, resulting in an unclear CT image. Another CT scan was performed, and again the neurostimulator fired. However, the patient was able to remain still during the second scan to obtain a clear image. The PA-PSRS report on the patient experiencing unanticipated electrical stimulation is described below:
The patient had an implanted neurostimulator unit that proceeded to give him a shock when he was scanned (CT) in that area. They repeated the scan in that area [because] the patient jumped when it gave him a shock the first time and they did not get a clear image. This time, staff explained to him not to jump so they could get a clear picture. The patient was able to comply even though the scanner [caused him to be shocked] again. Staff member was in the room with the patient when this whole event happened; [the staff member] called the [device manufacturer], and they stated that this has happened before with the 64-slice scanning units, and that the device should have been turned off.
Implantable electronic devices (IEDs) are susceptible to a wide range of external interference. There have been reports of individuals with implanted electronic devices being shocked while passing through retail antitheft systems and airport security systems.1 Less reported is the fact that x-rays produced from CT devices can also interfere with IEDs.
IEDs consist of a sealed package of electronics and of electrodes that are directly connected to the heart, muscle, or nerves.1 Implanted devices generate a series of voltage pulses that are delivered to the patient via the electrodes. Some devices, such as certain pacemakers and implantable cardioverter-defibrillators, sense or measure small voltage changes within the heart (i.e., electrical cardiac signals).2 It is possible that some of those devices can sense the change in voltage induced by ionizing radiation. That change in voltage level may be significant enough to interfere with the normal operation of implanted devices.
A malfunction of the IED during a CT scan leading to a shock to the patient (e.g., from an implantable neurostimulator) could result in the patient experiencing pain or cause the patient to move, thereby compromising the quality of the image. The amount of interference depends on a number of factors, including the amount of tissue between the implanted electronic device and the CT scanner (tissue can attenuate the x-ray beam) and/or the x-ray dose rate.
Evidence of CT Scan Interference
In a study conducted by McCollough et al., the authors observed that 37 of 41 implantable cardiac rhythm management devices (ICRMDs) (i.e., pacemakers, cardioverter defibrillators) were affected by CT irradiation. Seventeen ICRMDs were affected at typical clinical doses and 20 at maximum dose levels. The study was conducted using a 16-slice and a 64-slice CT scanner. Oversensing was the most common anomaly observed for all of the ICRMDs tested. Oversensing is a sensed event other than the intrinsic cardiac activity events of an ICRMD. The effects on the ICRMDs were only observed when the x-ray beam was directly over the devices.2
The study authors observed oversensing effects that would not have a detrimental outcome and effects that could be potentially problematic.2 Specific oversensing effects included inhibition and tracking. Inhibited pacemaker output occurs when depolarization (i.e., heart muscle contraction from the electrical stimulation) is sensed during normal pacing operations, and the sensed event resets the timing cycle of a particular heart chamber. Tracking occurs with devices programmed to the P-synchronous pacing mode, in which a sensed atrial event triggers a ventricular pacing pulse.
The less harmful effects of oversensing include devices programmed to P-synchronous pacing mode and those with atrial antitachycardia features. When in P-synchronous pacing, the arterial sense amplifier of the device may trigger nonphysiologic tracking resulting in inappropriately increased ventricular rates. The end result is considered harmless because the pacing rate is limited by an upper rate limiting feature. In devices with atrial antitachycardia features, oversensing can introduce extra senses that may stimulate an atrial arrhythmia. The resulting atrial arrhythmia may cause a false detection and deliver an unnecessary atrial antitachycardia pacing therapy.2 However, unnecessary atrial antitachycardia pacing was not observed during the McCollough et al. study.
A potentially more serious oversensing event of a device’s ventricular sense amplifier, pacing inhibition, may occur. In this case, pacing inhibition may be clinically significant for pacemaker-dependent patients if the inhibition persists, for example, more than three seconds. This lengthy inhibition may occur during dynamic scanning because during dynamic scanning, a portion of the patient remains stationary within the plane of the x-ray beam.2
Potential Cause and Effects
The cause of CT scanner interference on implantable electronic devices may, in part, be due to newer scanners designed for faster scans. One way to increase faster scan times is to increase the x-ray dose rate. The higher dose rate could increase the likelihood of interference.
The effects of CT scans on implantable electronic devices are typically transient because the interference would occur only during the time of x-ray exposure. In most cases, the implantable device would resume normal operation, but in some cases, the device might need to be reprogrammed by a physician. Irreversible damage may occur if the x-ray dose is very high; high dosage is usually only used during radiotherapy, not diagnostic procedures.1
The effect of a CT scan on patients with IEDs would depend on the type of implantable device. For example, the pacing pattern of an implanted cardiac device (i.e., life-supporting) may change or become interrupted, which may prove harmful to the patient. However, a CT scan’s effect on a non-life-support device (e.g., an implantable neurostimulator) may only result in temporary discomfort for the patient as described in the PA-PSRS report above.
From the PA-PSRS report and the evidence described above, implantable electronic devices can be susceptible to interference from CT scanners. The effects of the interference can be potentially harmful. The degree of harm depends on the patient and the type of implanted device as described in the sections above. In addition to asking the patient to verify an implanted device, facilities have the option of performing a scout view to identify any IED before the actual scan. The CT scout view—a preliminary image prior to performing the major scan—uses a much lower x-ray dose rate and would most likely not interfere with the implanted device.1
In the PA-PSRS report above, the neurostimulator manufacturer reportedly suggested that the implanted neurostimulator device should have been turned off prior to the CT scan. In most cases, non-life-supporting implanted devices such as a stimulator can be turned off during the scan without serious harm to the patient. However, life-supporting implanted devices such as a pacemaker or cardiac defibrillator cannot be turned off. Healthcare facilities typically have protocols in place for procedures that could affect the normal operation of an implanted device. Often, the physician that implanted the device is consulted to determine how best to perform the procedure without undue risk to the patient. Also, implantable device manufacturers are often consulted to determine the best course of action. Physicians who prescribe the CT scan and technicians who perform the CT scan must be aware of the presence of IEDs in order to take appropriate action to prevent harm to the patient during the scan.
- ECRI Institute. CT scans can affect the operation of implanted electronic devices. Health Devices 2007 Apr;36(4):136-8.
- McCollough CH, Zhang J, Primak AN, et al. Effects of CT irradiation on implantable cardiac rhythm management devices. Radiology 2007 Jun;243(3):766-74.