18-02-03 Incidental Hazards: Patient-Related Devices
Implants. It is beyond the scope of this textbook to provide guidelines for every implanted device. For many implants (e.g., pumps, infusion system, and tissue expanders) manufacturers propose detailed MR procedures. Numerous of these implants are "MR unsafe" or "MR conditional" which also means unsafe in daily routine.
There are a number of guides covering all different kinds of implants. A general review and explanation of MR labeling information for implants and devices was presented by Shellock and collaborators [⇒ Shellock 2009]. They also gave an extensive overview of the behavior of implants [⇒ Shellock 2004]. Note that such lists are of only limited use because often it is the interaction between a particular device and a particular MR imaging equipment that can lead to complications.
Foreign bodies. Occult ferromagnetic foreign bodies incorporated in accidents are dangerous, in particular those close to the eyes. The patient's history may help to rule out such foreign bodies. Many patients, however, do not remember such accidents. In case of doubt, x-rays should be taken prior to MR imaging. Ferromagnetic makeup and tattoos cannot only distort MR images, but also can be irritated and makeup can even be pulled into the eye by magnetic forces. Makeup should be removed before the examination, if possible.
IUDs. Most of the commonly used intrauterine contraceptive devices (IUD) do not move under the influence of the magnetic field, do not heat up during sequences usually applied for pelvic imaging, and do not produce major artifacts in vitro or in vivo. Thus, patients with either all plastic or copper IUDs can be safely imaged with magnetic resonance [⇒ Mark].
Joint and limb prostheses. Generally, such prostheses present no risk. However, they can introduce image artifacts. If possible, they should be removed prior to the MR examination.
Skin patches. Pharmaceutical products in transdermal skin patches may cause burns due to the absorption of RF energy. Such patches must be removed prior to MR examinations.
Pacemakers. For a long time the rapidly growing group of patients with cardiac pacemakers (PMs) and implantable cardioverter defibrillators (ICDs) had only limited access to MR imaging. These devices used to be an absolute contraindication. Meanwhile, this restriction has been relaxed to "Potential Contraindication" (Table 18-02). Still, patients with pacemakers and similar devices require specific care.
Contraindications to MR imaging, functional MR, MR spectroscopy, and similar techniques. These contraindications may change from country to country or at different jurisdictions, but adherence to them is strongly recommended. Our earlier distinction between absolute and relative contraindications has been skipped and been replaced by the term "Potential Contraindications".
Potential complications during MR examinations of PM wearers include adverse effects including tissue heating, unpredictable reed switch behavior and alteration of pacemaker programs, asynchronous pacing, or damage to pacemaker circuitry, and electrode and lead displacement.
The US-American Food and Drug Administration underlines that there are weak points in the studies of MR imaging in patients with pacemakers and implantable cardioverter defibrillators [⇒ Faris]. It seems to be imprudent for patients with certain cardiovascualar devices to undergo MR imaging. Judicious scrutiny with careful patient screening and exact and reliable assessment of the implanted device are compulsary before an MR examination of such a patient.
Monitoring of ECG, blood pressure measurement, and pulse oximetry by trained personnel during the MR study is absolutely necessary. Recommended procedures vary; in some places the presence of an anesthesiologist or cardiologist is standard for the preparation and immediate follow-up of the patient.
The current scientific statement from the US-American Heart Association does not comment on SAR limitations [⇒ Levine]. However, the European Society of Cardiology recommends a SAR limit of 2 W/kg [⇒ Roguin]. It is recommended to adjust the highest possible SAR for each sequence to ≤2.0 W/kg, preferably to ≤1.5 W/kg.
Note that the SAR of an identical pulse sequence at 1.5 Tesla quadruples at 3.0 T and is more than twenty times higher at 7.0 T. Therefore, the maximum field strength for studies of such patients is 1.5 T.
Other Implants. Similar considerations hold for pacemakers used for stimulation of the carotid sinus or intracorporal insulin pumps, for instance. Here, no adverse effects have been observed [⇒ Schroeder]. However, interference in electronic cochlear implants and ferromagnetic mechanical stapedial replacements has been reported [⇒ Hepfner]. Prosthetic heart valves are not considered to be dangerous in low fields [⇒ Soulen]. Patients should not undergo MR imaging in high or ultra-high fields if valve dehiscence is clinically suspected.
Hemostatic or other clips in the CNS can move in their position. Dislocation by magnetic attraction or torque presents a risk of hemorrhage. In other parts of the body, we consider this to be a minimal risk, because after the healing phase of six to eight weeks, fibrosis and encasement of the clip help to keep it in a stable position. The label stainless steel is not a guarantee for non-ferromagnetic steel. Implants that involve magnets such as magnetic sphincters, stoma plugs, dental implants, etc., can be demagnetized by the MR equipment. They should be removed prior to the examination.
Wires, other metallic objects, and skin contact. Wire configurations such as pacemaker lead wires, ECG and plethysmographic cables, and surface-coil connections can act as antennae. Gradient and RF fields may induce current into these wires and thus cause fibrillations and burns (Figure 18-07). This presents a risk to the patient and must be eliminated prior to the examination.
In a similar way this holds for all clothing containing metallic threads or components, as well as all metallic objects such as eye glasses, jewelry, hairpins, buttons, watches, bracelets, prostheses, etc. All of these objects must be removed prior to the examination.
The patient’s skin should not be in contact with the inner bore of the magnet. Large-radius wire loops should not be formed by leads or wires that are used in the magnet bore during imaging procedures. If the patient’s arms and legs are not completely covered with clothing, insulating material must be placed between the legs, and between legs and magnet. Leg-to-leg and leg-to-arm skin contact must be prevented in order to avoid the risk of burning due to the generation of high current loops if the legs or arms are allowed to touch.
18-02-04 Incidental Hazards: Other Considerations
Claustrophobia. This is a very real psychological danger for some patients. Claustrophobia and other psychological stress situations have been reported to be severe enough to interrupt the examination in about 1-4% of cases.
A high incidence of claustrophobia requires additional psychological training of doctors and staff. Explanation of the imaging procedure and the equipment prior to the examination helps to reduce claustrophobia significantly. Friendly, compassionate, and caring bedside manners help avoid claustrophobia. Small, open, wide-bore and low-noise MR imagers are advantageous because the percentage of claustrophobic incidents drops significantly.
The contact, dialogue and understanding between patient and radiographer are among the most important ingredients of a successful MRI examination. Communication leads to the choice of the right strategy for the individual patient, as the radiographer can act and adjust the examination on the response of the patient. Most claustrophobic patients are able to complete their examination when some effort is made to support them corresponding to their individual needs [⇒ Landrø Svarliaunet 2014].
Communication with the patient should include:
general information in advance, including the room and the location of the
operators' console, the MR machine and the positioning of the patient;
explanation of the strategy and handling of the examination;
Which strategy to be used depends on the patient. When given some advice about how to handle stress inside the magnet, most patients achieve the needed feeling of self-control.
Particular considerations that proved helpful to patients according to the Norwegian study are summarized in Table 18-03.
Avoiding or relieving claustrophobia.
These actions are simple, but very practical and helpful. They indicate a positive attitude towards the patients' problems and aid the patients in building up the confidence in themselves to manage the situation. The actions are, however, time consuming and require well trained radiographers with an understanding of psychology. They do not fit the trend to industrialized assembly line patient examinations.
The possibility of the patient falling from the examination couch and hypotonic syndrome (due to heat, motionless horizontal lying for a certain time, and psychological agitation) are additional hazards.
Sedation. MR has become an important tool in pediatric imaging. Since some infants and children are unable to cooperate with the examiners, there is an increased demand for sedation. Some infants sleep soundly through an MR examination, particularly if they have eaten; however, many infants and children up to eight years require sedation, even if they are accompanied by their parents into the scanner room. In most instances, teenagers can be treated like adults.
Pregnancy. There is no evidence that MR can harm the fetus or embryo. MR imaging is used for fetography (Figure 18-08), particularly for imaging the brain.
An epidemiological study by Kanal, et al. concluded that data collected from MR imaging technologists were negative with respect to any statistically significant elevations in the rates of spontaneous abortion, infertility, and premature delivery [⇒ Kanal 1993]. As a safety precaution, MR scanning should be avoided in the first three months of pregnancy. MR imaging is indicated for use in pregnant women if other non-ionizing forms of diagnostic imaging are inadequate, or if the examination provides important information which would otherwise require exposure to ionizing radiation such as x-ray or CT. At 3.0 Tesla, the specific absorption rate (SAR) and the increased noise may, at least potentially, adversely affect the fetus. With imaging times shorter than 30 minutes, normal SAR regimens did not lead to temperature increases above 1° C in pregnant animals. However, longer imaging times can lead to an increase of 2.5° C [⇒ Cannie]. Therefore some major hospitals refer pregnant women to 1.5-Tesla machines.
Similar considerations hold for pregnant staff of a magnetic resonance department. Mainly for psychological reasons, it might be a wise precaution that pregnant staff members do not remain in the scan room during actual scanning; however, they are allowed to prepare and position the patient, administer contrast agents, and scan.