SYMPOSIUM REVIEW ARTICLE
Magnetic Resonance Imaging Safety in Cardiothoracic Imaging Daniel Ocazionez, MD, Demetrius L. Dicks, MD, Jennifer L. Favinger, MD, Girish S. Shroff, MD, Sidhdharth Damani, MD, Gregory A. Kicska, MD, PhD, and Gautham P. Reddy, MD, MPH Abstract: Patient safety is a priority for patients undergoing magnetic resonance imaging (MRI). This article reviews MRI safety issues related to devices, pharmacologic stress agents, contrast agents, anesthesia, and external equipment, focusing on cardiothoracic MRI. Key Words: magnetic resonance imaging, safety, pharmacologic stress agents, contrast agents
(J Thorac Imaging 2014;29:262–269)
P
atient safety is a priority for patients undergoing a cardiothoracic magnetic resonance imaging (MRI) examination. It is important for the radiologist to understand the current concepts regarding MRI safety to provide appropriate guidance to referring health care providers on whether a proposed MRI examination is suitable for a particular patient. In this article we will discuss cardiothoracic MRI safety issues related to cardiovascular and noncardiovascular devices, pharmacologic stress agents, contrast agents, anesthesia and sedation, and external equipment such as oxygen tanks. The current Food and Drug Administration (FDA)approved terminology regarding magnetic resonance (MR) safety includes labeling terms and icons (Figs. 1–3).1 These terms include: MR Safe: An item or device that poses no known hazards in any possible MRI environment. They are generally items that are nonconducting, nonmetallic, and nonmagnetic. MR Conditional: An item or device that has been demonstrated to pose no known hazards in a specified MRI environment with specified conditions of use. Specified field conditions include but are not limited to static magnetic field strength, spatial gradient, time rate of change of the magnetic field, radiofrequency (RF) fields, and specific absorption rate (SAR). These items have undergone sufficient testing in the specified environment and have been evaluated for possible conditions such as magnetic-induced displacement and RF heating. MR Unsafe: An item or device that is known to induce hazards in all MRI environments.
From the Department of Radiology, University of Washington, Seattle, WA. Gregory A. Kicska received research funding from GE Healthcare. The remaining authors declare no conflicts of interest. Correspondence to: Gautham P. Reddy, MD, MPH, Department of Radiology, University of Washington, 1959 NE Pacific St, Seattle, WA 98195-7115 (e-mail: [email protected]). Copyright r 2014 by Lippincott Williams & Wilkins
262 | www.thoracicimaging.com
MAJOR CARDIOVASCULAR DEVICES Implantable Cardioverter Defibrillators (ICDs) and Pacemakers The significant increase in the older population during the past decades has led to a substantial increase in the number of patients with comorbidities requiring placement of a pacemaker or an ICD. Many of these patients will need to undergo an MRI examination during their lifetimes. Guidelines from organizations such as the American Heart Association and the FDA previously discouraged MRI examinations in these patients, as the high magnetic fields might cause movement/vibration of the pulse generator or leads, modification of the function (damage to the device components), excessive heating of the leads, or inappropriate sensing, triggering, or activation of the device.2 Recently in the United States and in the European Union, conditional MR-safe pacemaker devices have received approval for 1.5 T MRI systems.2 To conform to MR conditional safety, these devices incorporate design features such as reduction of the ferromagnetic content, adjustment of the reed switches, leads designed to decrease induced heating/currents, and addition of circuitry filters and shielding to lessen undesirable electromagnetic effects.2 Despite the advent of MR-compatible devices, institutional safety committees may insist that a strong clinical
FIGURE 1. MR safe. The device or implant is completely nonmagnetic, nonelectrically conductive, and non-RF reactive, therefore eliminating all the primary potential risks during an MR examination.
J Thorac Imaging
Volume 29, Number 5, September 2014
J Thorac Imaging
Volume 29, Number 5, September 2014
MRI Safety in Cardiothoracic Imaging
FIGURE 2. MR conditional. The device or implant may contain magnetic, electrically conductive, or RF-reactive components that are safe for operation in proximity to the MRI, provided the conditions for safe operation are defined and observed (both for the MR scanner and the device itself).
indication to perform the MRI examination exist, in which case the benefits should clearly outweigh the risks. It is highly recommended that a health care provider with detailed knowledge of pacemaker/ICD programming and advanced cardiac life support skills be present during the entire examination. Continuous electrocardiogram (ECG) monitoring and pulse oximetry are mandatory. An electrophysiology expert should perform a postexamination pacemaker interrogation to ensure appropriate threshold defibrillation settings and to reprogram the device if necessary.3
FIGURE 4. AAA Zenith endovascular graft.
Stents, Filters, and Grafts Coronary artery stents have been proven safe for patients undergoing MRI procedures at a field strength of 3 T or less and a maximum whole-body averaged SAR of 2.0 to 4.0 W/kg for 15 minutes of MRI.4 Noncoronary vascular stents, filters, and vascular grafts have been evaluated with regard to MRI compatibility. A few of these devices showed magnetic field interactions in the immediate postprocedural period, but after 6 weeks they became incorporated safely in tissue.5 Many of these items have been evaluated at 1.5 and 3 T. For example, the Vena Tech IVC filters are nonferromagnetic and therefore acceptable for patients undergoing MR procedures even in the immediate postprocedural setting.6 The AAA Zenith endovascular graft has been deemed MR conditional as per ASTM criteria (Fig. 4; http://www. mrisafety.com/TheList_search.asp?s_list_description=zenith +aaa&s_ANYwords=&s_object_category. Last accessed 08/17/2014). Coil embolization material made from platinum, iridium, and nitinol has been found safe for MRI studies.5,7 Increased force of attraction was reported with stainless steel esophageal stents (Gianturco type stents) but no attraction from the counterpart titanium alloy stents (Ultraflex and Wall stents).8 Therefore it is important to obtain specific information of the type of stent before an MRI examination is performed.5,8
Cardiac Devices FIGURE 3. MR unsafe. Objects that are significantly ferromagnetic and pose a clear and direct threat to persons and equipment within the magnet room. r
2014 Lippincott Williams & Wilkins
Investigational studies have demonstrated that the majority of heart valves and annuloplasty rings may be safely scanned under the following conditions: static magnetic field of 3 T or less, maximum spatial gradient of www.thoracicimaging.com |
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Ocazionez et al
720 gauss/cm or less, and a maximum whole-body averaged SAR of 3.0 W/kg for 15 minutes of scanning (http://ht. edwards.com/scin/edwards/sitecollectionimages/products/heart valves/hvt_mri_ous.pdf?AR11373; Last accessed 08/17/2014). If there is no acute indication for MRI, it is recommended to delay scanning up to 6 weeks after device implantation.5 It is contraindicated for patients with a total artificial heart or left ventricular assist device to undergo an MRI examination. (http://www.fda.gov/ohrms/dockets/ac/04/ briefing/4029b1_final.pdf; http://www.mrisafety.com/TheList_ search.asp?s_list_description=left+ventricular+assist+device &s_ANYwords=&s_object_category. Last accessed 08/17/ 2014).
MAJOR NONCARDIOVASCULAR DEVICES Cochlear Implants Historically, cochlear implants were thought to be MR unsafe, predominantly because of the chances of RFinduced heating and electrode stimulation leading to device malfunction and displacement.9 In recent years, however, several devices have been deemed MR conditional by the ASTM. Crane et al10 have discussed the diagnostic efficacy and safety of cochlear implants in patients older than 8 years of age undergoing MR at 1.5 T.
Pumps Pumps allow the continuous infusion of a medication by means of a catheter. Insulin pumps, which are widely used, can be externally or internally implanted. Almost all of these continuous insulin-delivering devices are considered MR unsafe at magnetic fields exceeding 600 gauss (0.06 T). Other types of infusion pumps may be considered MR safe, conditional, or unsafe as per ASTM criteria (http://www.mrisafety.com/SafetyInfov.asp?SafetyInfoID = 271; Last accessed 08/17/2014). Infusion pumps that are considered MR unsafe should be removed before an MRI is performed; if they cannot be removed, the MRI should not be performed.
Nerve Stimulators Currently, many patients with intractable pain and failure of medical therapy undergo placement of spinal cord stimulator devices, which consist of inserted electrodes connected subcutaneously to an implantable pulse generator. Many of these patients will eventually need to undergo an MRI examination for reasons related or unrelated to their primary condition.11 Of concern are several safety factors, which include: decreased treatment efficacy due to shift of the spinal electrode and impairment of the implantable pulse generator, intolerable stimuli secondary to induced electrical wire pulses, and rapid increase in tissue temperature at the electrode end, which may lead to irreversible spinal cord damage.12 Other types of stimulators include peripheral nerve, vagal nerve, phrenic nerve, deep brain, and bone growth stimulators, each with different MR safety parameters. Most of these devices fall under the unsafe and conditional ASTM categories, with only a few falling under the MR safe category (http://www.mrisafety. com/TheList; Last accessed 08/17/2014).
Bullets, Pellets, and Shrapnel Ammunition may be composed of ferromagnetic or nonferromagnetic material, depending on the country of fabrication and the purpose of the ammunition. Shrapnel is generally made of steel, thereby leading to potential risks
264 | www.thoracicimaging.com
J Thorac Imaging
Volume 29, Number 5, September 2014
for patients undergoing an MRI examination. A potential complication would be damage of a vital organ or structure secondary to excessive heating or movement of the metallic object. The radiologist should perform a risk-benefit assessment before allowing the patient in this circumstance to undergo an MRI examination. Eshed et al13 concluded that MR at 1.5 T may be safe for patients with retained metal fragments if these are not located near a vital structure.
ECG LEADS Conventional ECG leads, which use low impedance conductors or components with ferromagnetic properties, can lead to increased SAR.14 The induction of high voltage through these leads can lead to skin surface heating or skin burns. It is important to use MR safe ECG leads, which are high-impedance leads and do not result in burns or artifacts on MRI.15 Electrical ECG leads should be kept as short as possible to prevent increased RF deposition. To reduce the possibility of burning, either fiberoptic cables or a wireless connection can be utilized to conduct signal transfer between the ECG converted box and the MRI unit.16
PHARMACOLOGIC CARDIAC STRESS AGENTS Cardiac stress tests are used to stratify the risk of patients who either have a history of coronary artery disease or who have risk factors for developing coronary disease. The overall goal of a cardiac stress test is to increase the coronary artery blood flow at “stress” such that a hemodynamically significant coronary stenosis will manifest itself. In most patients, this is done with an exercise stress test. Pharmacologic stress is a well-established technique that can be used in patients who have difficulty with or a contraindication to an exercise stress test. Pharmacologic stress is the preferred method of stress for MRI examinations, because it is usually cumbersome and impractical to set up exercise equipment in the MRI environment. Myocardial perfusion scintigraphy and stress echocardiography are the most commonly performed cardiac stress imaging examinations, but stress cardiac MRI can be performed when the other study is suboptimal (for example, stress echocardiography in an obese patient), to quantify absolute perfusion, or as a component of a comprehensive cardiac MRI examination. The 2 broad categories of pharmacologic stress agents are vasodilators such as adenosine, regadenoson, or dipyridamole and beta agonists such as dobutamine. Vasodilators cause heterogeneity of coronary artery flow, resulting in myocardial ischemia. Adenosine leads to an increase in intracellular cyclic adenosine monophosphate, which causes vasodilation. Dipyridamole inhibits the uptake and metabolism of adenosine, and regadenoson is an agent that selectively inhibits the A2A adenosine receptors, thereby increasing the concentration of interstitial adenosine.17 Adenosine is infused intravenously at a rate of 140 mg/kg/ min, and imaging can begin after 3 minutes. Because it has a relatively short half-life of
Magnetic Resonance Imaging Safety in Cardiothoracic Imaging Daniel Ocazionez, MD, Demetrius L. Dicks, MD, Jennifer L. Favinger, MD, Girish S. Shroff, MD, Sidhdharth Damani, MD, Gregory A. Kicska, MD, PhD, and Gautham P. Reddy, MD, MPH Abstract: Patient safety is a priority for patients undergoing magnetic resonance imaging (MRI). This article reviews MRI safety issues related to devices, pharmacologic stress agents, contrast agents, anesthesia, and external equipment, focusing on cardiothoracic MRI. Key Words: magnetic resonance imaging, safety, pharmacologic stress agents, contrast agents
(J Thorac Imaging 2014;29:262–269)
P
atient safety is a priority for patients undergoing a cardiothoracic magnetic resonance imaging (MRI) examination. It is important for the radiologist to understand the current concepts regarding MRI safety to provide appropriate guidance to referring health care providers on whether a proposed MRI examination is suitable for a particular patient. In this article we will discuss cardiothoracic MRI safety issues related to cardiovascular and noncardiovascular devices, pharmacologic stress agents, contrast agents, anesthesia and sedation, and external equipment such as oxygen tanks. The current Food and Drug Administration (FDA)approved terminology regarding magnetic resonance (MR) safety includes labeling terms and icons (Figs. 1–3).1 These terms include: MR Safe: An item or device that poses no known hazards in any possible MRI environment. They are generally items that are nonconducting, nonmetallic, and nonmagnetic. MR Conditional: An item or device that has been demonstrated to pose no known hazards in a specified MRI environment with specified conditions of use. Specified field conditions include but are not limited to static magnetic field strength, spatial gradient, time rate of change of the magnetic field, radiofrequency (RF) fields, and specific absorption rate (SAR). These items have undergone sufficient testing in the specified environment and have been evaluated for possible conditions such as magnetic-induced displacement and RF heating. MR Unsafe: An item or device that is known to induce hazards in all MRI environments.
From the Department of Radiology, University of Washington, Seattle, WA. Gregory A. Kicska received research funding from GE Healthcare. The remaining authors declare no conflicts of interest. Correspondence to: Gautham P. Reddy, MD, MPH, Department of Radiology, University of Washington, 1959 NE Pacific St, Seattle, WA 98195-7115 (e-mail: [email protected]). Copyright r 2014 by Lippincott Williams & Wilkins
262 | www.thoracicimaging.com
MAJOR CARDIOVASCULAR DEVICES Implantable Cardioverter Defibrillators (ICDs) and Pacemakers The significant increase in the older population during the past decades has led to a substantial increase in the number of patients with comorbidities requiring placement of a pacemaker or an ICD. Many of these patients will need to undergo an MRI examination during their lifetimes. Guidelines from organizations such as the American Heart Association and the FDA previously discouraged MRI examinations in these patients, as the high magnetic fields might cause movement/vibration of the pulse generator or leads, modification of the function (damage to the device components), excessive heating of the leads, or inappropriate sensing, triggering, or activation of the device.2 Recently in the United States and in the European Union, conditional MR-safe pacemaker devices have received approval for 1.5 T MRI systems.2 To conform to MR conditional safety, these devices incorporate design features such as reduction of the ferromagnetic content, adjustment of the reed switches, leads designed to decrease induced heating/currents, and addition of circuitry filters and shielding to lessen undesirable electromagnetic effects.2 Despite the advent of MR-compatible devices, institutional safety committees may insist that a strong clinical
FIGURE 1. MR safe. The device or implant is completely nonmagnetic, nonelectrically conductive, and non-RF reactive, therefore eliminating all the primary potential risks during an MR examination.
J Thorac Imaging
Volume 29, Number 5, September 2014
J Thorac Imaging
Volume 29, Number 5, September 2014
MRI Safety in Cardiothoracic Imaging
FIGURE 2. MR conditional. The device or implant may contain magnetic, electrically conductive, or RF-reactive components that are safe for operation in proximity to the MRI, provided the conditions for safe operation are defined and observed (both for the MR scanner and the device itself).
indication to perform the MRI examination exist, in which case the benefits should clearly outweigh the risks. It is highly recommended that a health care provider with detailed knowledge of pacemaker/ICD programming and advanced cardiac life support skills be present during the entire examination. Continuous electrocardiogram (ECG) monitoring and pulse oximetry are mandatory. An electrophysiology expert should perform a postexamination pacemaker interrogation to ensure appropriate threshold defibrillation settings and to reprogram the device if necessary.3
FIGURE 4. AAA Zenith endovascular graft.
Stents, Filters, and Grafts Coronary artery stents have been proven safe for patients undergoing MRI procedures at a field strength of 3 T or less and a maximum whole-body averaged SAR of 2.0 to 4.0 W/kg for 15 minutes of MRI.4 Noncoronary vascular stents, filters, and vascular grafts have been evaluated with regard to MRI compatibility. A few of these devices showed magnetic field interactions in the immediate postprocedural period, but after 6 weeks they became incorporated safely in tissue.5 Many of these items have been evaluated at 1.5 and 3 T. For example, the Vena Tech IVC filters are nonferromagnetic and therefore acceptable for patients undergoing MR procedures even in the immediate postprocedural setting.6 The AAA Zenith endovascular graft has been deemed MR conditional as per ASTM criteria (Fig. 4; http://www. mrisafety.com/TheList_search.asp?s_list_description=zenith +aaa&s_ANYwords=&s_object_category. Last accessed 08/17/2014). Coil embolization material made from platinum, iridium, and nitinol has been found safe for MRI studies.5,7 Increased force of attraction was reported with stainless steel esophageal stents (Gianturco type stents) but no attraction from the counterpart titanium alloy stents (Ultraflex and Wall stents).8 Therefore it is important to obtain specific information of the type of stent before an MRI examination is performed.5,8
Cardiac Devices FIGURE 3. MR unsafe. Objects that are significantly ferromagnetic and pose a clear and direct threat to persons and equipment within the magnet room. r
2014 Lippincott Williams & Wilkins
Investigational studies have demonstrated that the majority of heart valves and annuloplasty rings may be safely scanned under the following conditions: static magnetic field of 3 T or less, maximum spatial gradient of www.thoracicimaging.com |
263
Ocazionez et al
720 gauss/cm or less, and a maximum whole-body averaged SAR of 3.0 W/kg for 15 minutes of scanning (http://ht. edwards.com/scin/edwards/sitecollectionimages/products/heart valves/hvt_mri_ous.pdf?AR11373; Last accessed 08/17/2014). If there is no acute indication for MRI, it is recommended to delay scanning up to 6 weeks after device implantation.5 It is contraindicated for patients with a total artificial heart or left ventricular assist device to undergo an MRI examination. (http://www.fda.gov/ohrms/dockets/ac/04/ briefing/4029b1_final.pdf; http://www.mrisafety.com/TheList_ search.asp?s_list_description=left+ventricular+assist+device &s_ANYwords=&s_object_category. Last accessed 08/17/ 2014).
MAJOR NONCARDIOVASCULAR DEVICES Cochlear Implants Historically, cochlear implants were thought to be MR unsafe, predominantly because of the chances of RFinduced heating and electrode stimulation leading to device malfunction and displacement.9 In recent years, however, several devices have been deemed MR conditional by the ASTM. Crane et al10 have discussed the diagnostic efficacy and safety of cochlear implants in patients older than 8 years of age undergoing MR at 1.5 T.
Pumps Pumps allow the continuous infusion of a medication by means of a catheter. Insulin pumps, which are widely used, can be externally or internally implanted. Almost all of these continuous insulin-delivering devices are considered MR unsafe at magnetic fields exceeding 600 gauss (0.06 T). Other types of infusion pumps may be considered MR safe, conditional, or unsafe as per ASTM criteria (http://www.mrisafety.com/SafetyInfov.asp?SafetyInfoID = 271; Last accessed 08/17/2014). Infusion pumps that are considered MR unsafe should be removed before an MRI is performed; if they cannot be removed, the MRI should not be performed.
Nerve Stimulators Currently, many patients with intractable pain and failure of medical therapy undergo placement of spinal cord stimulator devices, which consist of inserted electrodes connected subcutaneously to an implantable pulse generator. Many of these patients will eventually need to undergo an MRI examination for reasons related or unrelated to their primary condition.11 Of concern are several safety factors, which include: decreased treatment efficacy due to shift of the spinal electrode and impairment of the implantable pulse generator, intolerable stimuli secondary to induced electrical wire pulses, and rapid increase in tissue temperature at the electrode end, which may lead to irreversible spinal cord damage.12 Other types of stimulators include peripheral nerve, vagal nerve, phrenic nerve, deep brain, and bone growth stimulators, each with different MR safety parameters. Most of these devices fall under the unsafe and conditional ASTM categories, with only a few falling under the MR safe category (http://www.mrisafety. com/TheList; Last accessed 08/17/2014).
Bullets, Pellets, and Shrapnel Ammunition may be composed of ferromagnetic or nonferromagnetic material, depending on the country of fabrication and the purpose of the ammunition. Shrapnel is generally made of steel, thereby leading to potential risks
264 | www.thoracicimaging.com
J Thorac Imaging
Volume 29, Number 5, September 2014
for patients undergoing an MRI examination. A potential complication would be damage of a vital organ or structure secondary to excessive heating or movement of the metallic object. The radiologist should perform a risk-benefit assessment before allowing the patient in this circumstance to undergo an MRI examination. Eshed et al13 concluded that MR at 1.5 T may be safe for patients with retained metal fragments if these are not located near a vital structure.
ECG LEADS Conventional ECG leads, which use low impedance conductors or components with ferromagnetic properties, can lead to increased SAR.14 The induction of high voltage through these leads can lead to skin surface heating or skin burns. It is important to use MR safe ECG leads, which are high-impedance leads and do not result in burns or artifacts on MRI.15 Electrical ECG leads should be kept as short as possible to prevent increased RF deposition. To reduce the possibility of burning, either fiberoptic cables or a wireless connection can be utilized to conduct signal transfer between the ECG converted box and the MRI unit.16
PHARMACOLOGIC CARDIAC STRESS AGENTS Cardiac stress tests are used to stratify the risk of patients who either have a history of coronary artery disease or who have risk factors for developing coronary disease. The overall goal of a cardiac stress test is to increase the coronary artery blood flow at “stress” such that a hemodynamically significant coronary stenosis will manifest itself. In most patients, this is done with an exercise stress test. Pharmacologic stress is a well-established technique that can be used in patients who have difficulty with or a contraindication to an exercise stress test. Pharmacologic stress is the preferred method of stress for MRI examinations, because it is usually cumbersome and impractical to set up exercise equipment in the MRI environment. Myocardial perfusion scintigraphy and stress echocardiography are the most commonly performed cardiac stress imaging examinations, but stress cardiac MRI can be performed when the other study is suboptimal (for example, stress echocardiography in an obese patient), to quantify absolute perfusion, or as a component of a comprehensive cardiac MRI examination. The 2 broad categories of pharmacologic stress agents are vasodilators such as adenosine, regadenoson, or dipyridamole and beta agonists such as dobutamine. Vasodilators cause heterogeneity of coronary artery flow, resulting in myocardial ischemia. Adenosine leads to an increase in intracellular cyclic adenosine monophosphate, which causes vasodilation. Dipyridamole inhibits the uptake and metabolism of adenosine, and regadenoson is an agent that selectively inhibits the A2A adenosine receptors, thereby increasing the concentration of interstitial adenosine.17 Adenosine is infused intravenously at a rate of 140 mg/kg/ min, and imaging can begin after 3 minutes. Because it has a relatively short half-life of
