TwinTree Insert

18-03-01 Incidental Hazards: External Objects

rojectiles. The most imminent danger for both patients and personnel in the mag­­ne­­tic field of an imaging system may result from ferromagnetic objects such as scal­pels, scissors, pens, and even sand bags (not filled with sand but with iron shot) and gas dewars, which can be attracted by the magnet and thus be­have like pro­jec­ti­les.

To prevent such accidents, the installation of a metal detector through which every­body has to pass before entering the MR suite has been recommended, but is rather cum­ber­some. Instead, automatically locking doors with badge access are re­com­mend­ed.

spaceholder redMonitors and respirators. The dependence on physiological monitoring, on me­chan­i­cal respiration, and electric infusion pumps during MR examinations renders dif­fi­cul­ties, and in certain instances does not allow such an exa­mi­na­tion.

However, with the development of appropriate monitoring and life-support equip­ment during the last few years, dependence is no longer a contraindication of MR imag­ing. Details on monitoring can be found, for instance, in an official Ca­na­di­an pub­li­ca­tion [⇒ Medical Advisory Secretariat 2003].

spaceholder redContrast agents. Magnetic resonance contrast agents or other substances which have to be injected or applied in another way may present risks similar to those in any other invasive technique such as x-rays, particularly in patients with kidney diseases. In patients with kid­ney di­sea­ses cer­tain contrast agents are con­­tra­­in­­di­­ca­­ted and were removed from the market in a number of countries (cf. Chapter 13. Ad­ver­se Events).

The clinical experience of administering gadolinium-based or other agents in­­tra­­ve­­nous­­ly to patients has shown that these agents are generally safe and well to­le­ra­ted. Still, all necessary precautions for intensive-care treatment have to be con­si­der­ed when injecting such contrast agents, particularly in patients with a his­­to­­ry of al­ler­gy and drug reactions. When contrast agents are used according to the given guidelines and regulations such side effects are extremely unlikely.

18-03-02 Incidental Hazards: MR Equipment

spaceholder redField Strength and Hazards. Many of the incidental hazards increase with field strength and the existing research on safety cannot straightforwardly be ex­­tra­­po­­la­t­ed to field strengths beyond 1.5 Tesla. Further studies are required to confirm the hazard-free use of MR imaging at ultrahigh fields. Until proven otherwise, patient studies at ultrahigh fields should be considered care­fully.

spaceholder redNoise. The noise created by the switching of the gradients is an additional source of in­con­ve­ni­en­ce and can cause ear damage to the patient and, oc­ca­­si­o­­nal­­ly, personnel [⇒ Radomskij 2002].

Sound pressure levels (SPL) in­crease with field strength. The noise levels at 3 Tesla can approach twice those of those at 1.5 Tesla. Sound pressure levels at the center of a head coil can exceed 130 dB(A). British and US American guidelines stipulate that the maximum permitted daily noise dosage is equivalent to 90 dB(A) for 8 hours; the guide­li­nes for Europe allow only 80 dB(A) for 8 hours [⇒ Foster 2000].

Noise-canceling systems and special earphones are available, and active acous­tic control systems are being developed [⇒ Mansfield 1981]. At fields of 1.5 Tesla ear­plugs and head­phones are strongly recommended; at higher fields they are man­da­to­ry. Even with hearing protection there can be hearing loss after examinations at 3 T [⇒ Jin 2017].

Acoustic noise can cause detrimental effect in both term and preterm neonates; they should be examined at field strengths below 1.5 Tesla, if possible in dedicated equip­ment [⇒ Tkach 2014] (cf. Chapter 3).

MR imaging is used for fetography (Figure 18-07), particularly for imaging the brain. At 3.0 Tesla (or higher magnetic fields), the specific ab­sorp­tion rate (SAR) and the increased noise may, at least potentially, adversely affect the fetus [⇒ Cannie 2016].

Figure 18-07:
MR fetography in the 32nd week of pregnancy.

spaceholder redCooling gases. In superconductive mag­net systems, helium and still occasionally ni­tro­gen are used as cooling gases. In the case of a quench, gases are re­leas­ed to the out­side. Under normal circumstances, the gases should escape through a pipe sys­tem and not reach the magnet-room atmosphere. However, ac­ci­den­tal­ly some gas could be released into the magnet room.

In this case, there are two potential dan­gers. Frostbite can be induced because the gases are extremely cold. Secondly, nitro­gen is to be considered hazardous, in par­­ti­cu­lar under pressure (whereas there is no danger of direct intoxication from helium).

All personnel and patients must evacu­ate the area immediately and return only af­­ter pro­per ventilation of the magnet room.

18-03-03 Incidental Hazards: Patient-Related Devices

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. Devices declared MR Safe for 1.5 Tesla might not necessarily be safe at higher fields.

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. They also gave an extensive overview of the behavior of implants. 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 [⇒ Shellock 2009, ⇒ Shellock 2004].

For clinical MR examinations MR Conditional devices must be consider­ed unsafe if the contrary has not been proven. Some equipment manufacturers of­fer an auto­ma­tic adjustment of imaging procedures for MR Conditional devices.

The rate of increase of the magnetic field within the fringe field is described as the spatial field gradient (SFG or SG) and expressed in Tesla/meter or Gauss/ centi­meter. A description is given by the International Electrotechnical Commission or, for instance, in a review by Steckner [⇒ IEC 2016, ⇒ Steckner 2012].

spaceholder redForeign bodies. Hemostatic or other clips in the CNS can move in their position (Figure 18-08). 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.

Figure 18-08:
X-ray of a head showing clips after a brain operation: safe or unsafe for MR imaging?

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 can be irritated and makeup can even be pulled into the eye by mag­ne­tic for­ces. Make­up should be re­moved before the examination, if possible. 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.

spaceholder redIUDs. 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 at medium, high and ultrahigh fields (up to 9.4 Tesla). Thus, patients with either all plastic or copper IUDs can be safely imaged with magnetic resonance [⇒ Mark 1987, ⇒ Gaa 2017].

spaceholder redJoint 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.

spaceholder redPacemakers. 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. Still, patients with pacemakers and similar devices require specific care.

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 2006]. 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 2007]. However, the Euro­pean So­cie­ty of Car­dio­logy recommends a SAR limit of 2 W/kg [⇒ Roguin 2008]. 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.

spaceholder redOther Implants. Similar considerations hold for pacemakers used for sti­mu­la­tion of the carotid sinus or intracorporal insulin pumps. Here, no ad­ver­se effects have been observed [⇒ Schröder 1987]. How­ever, interference in elec­tro­nic cochlear im­plants and ferromagnetic mechanical stapedial replacements has been reported [⇒ Hepfner 1985]. Prosthetic heart valves are not considered to be dangerous in low fields [⇒ Soulen 1985]. However, patients should not undergo MR imaging in high or ul­tra­high fields if valve dehiscence is clinically suspected.

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.

The label stainless steel is not a guarantee for non-ferromagnetic steel.

spaceholder redWires, other metallic objects, and skin contact. Wire configurations such as pace­maker lead wires, ECG and plethysmographic cables, and surface-coil con­nec­tions can act as antennas. Gradient and RF fields may induce current into the­se wires and thus cause fibrillations and burns (Figure 18-09). This presents a risk to the patient and must be eliminated prior to the examination.

Figure 18-09:
Burn on the back of a patient who un­der­went MR imaging while lying on an ECG lead.

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. 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.

18-03-04 Incidental Hazards: Other Considerations

spaceholder redClaustrophobia. 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. The contact, dialog and understanding between patient and radiographer are among the most important ingredients of a successful MRI examination.

According to a Nor­we­gian study 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]. The following 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 patient should be given:

spaceholder darkbluegeneral information in advance, includ­ing the room, the lo­ca­tion of the op­era­tors' con­sole, the MR machine and the positioning of the patient;

spaceholder darkbluean explanation of the strategy and handling of the examination;

spaceholder darkbluea debriefing.

Concerning the examination itself:

spaceholder darkblueA mirror to the head coil helped a quarter of the patients to feel more com­fort­able;

spaceholder darkbluewhenever possible patients should enter the machine feet first;

spaceholder darkbluean accompanying person, laying a hand on the patient's feet, makes it easier for the patient to cope with the situation;

spaceholder darkbluepatients in pain should be comforted with, e.g., an extra cushion or straps, help­ing them to find a comfortable position that they can keep during the exa­mi­na­tion;

spaceholder darkblueto shorten the examination time for patients in pain or being extremely un­com­fort­able inside the magnet, the application of pre-planned "short pulse se­quen­ces" for the examinations have proven helpful;

spaceholder darkblueusing a coil which might not be the optimal or common for the kind of study but suits the patient;

spaceholder darkblueif possible, allowing certain patients to come out of the magnet between se­quen­ces;

spaceholder darkblue(a lot of) "chatting" between the scans.

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. 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.

Small, open, wide-bore and low-noise MR machines are advantageous because the per­­cen­­ta­ge of claustrophobic incidents drops significantly.

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.

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Claustrophobia, MRI and the human factor.

inkpot A little more about this topic …

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spaceholder redPregnancy. There is no evidence that MR can harm the fetus or embryo. An epi­de­mio­lo­gi­cal study by Kanal et col­la­bo­ra­tors 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.

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. The­re­fore some major hospitals refer preg­nant women to 1.5-Tesla (or lower field strength) machines.

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.

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 to scan.

spaceholder redSedation. 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.

spaceholder redDetails on sedation and procedures can be found in the literature [⇒ American Academy of Pediatrics 1992, ⇒ Kanal 1992].