High-Yield:

• Two categories of acute contrast reactions → Physiologic and Allergic-Like (2)

• Physiologic

  • Much more common, dose related and less likely to be severe.

• Allergic-like

  • Much less common (<1% overall) and has a greater chance of being severe.

  • Sensitization due to prior exposure is NOT required for an allergic-like reaction (no IgE ) (1).

    • Serious allergic-like reactions to contrast are mediated by type 1 hypersensitivity reactions. Meaning, they begin within minutes of exposure due to direct stimulation of mast cells/basophils resulting in degranulation (1).

• Distinguishing allergic-like from physiologic reactions is important because patients with physiologic reactions do not require premedication in the future, whereas those with allergic-like reactions may need premedication (1).

• Physiologic and Allergic-like reactions are further characterized as:

  • Mild → Moderate → Severe

• Risk factors for acute adverse reactions to contrast agents (1) :

  • History of severe allergic reactions.

    • This is not limited to contrast and can include medications, food etc.

    • Patient history of previous severe reaction to contrast agents increases the overall risk of subsequent reaction five- to six-fold.

  • History of asthma, bronchospasm or atopy.

  • History of cardiac or renal disease.

  • Patient's >60yo or <5yo.

• Shellfish allergies do not increase a patient's risk of adverse reaction to iodinated or gadolinium based contrast.

  • Iodine is an essential element with no potential to illicit an allergic response.

  • The major allergens in shellfish are tropomyosins, which are unrelated to iodine (1).

• A contrast reaction that occurs despite premedication is a "breakthrough reaction"

  • As long as the patient has been pre-medicated, you can re-inject contrast in someone who has experienced a prior breakthrough reaction.

    • Patients at greatest risk for moderate-severe breakthrough reaction include those with severe allergies to any substance, those with >=4 allergies and those with chronic oral steroid use (1).

• Pregnancy is NOT a contraindication for iodinated contrast use.

  • Iodinated contrast material crosses the placenta and has been demonstrated in fetal tissues; however, to our knowledge, teratogenic effects and hypothyroidism have not been reported. Despite these low theoretical risks, mostly regarding fetal thyroid development, avoiding intravenous contrast agents in pregnant patients is prudent when possible (1).

• MRI contrast (gadolinium based contrast media, GBCM) should be avoided in pregnant patients.

  • GBCM have been classified by the Food and Drug Administration as pregnancy class C drugs (no adequate and well-controlled studies in humans have been performed, although animal reproduction studies have shown an adverse effect on the fetus) and are therefore relatively contraindicated in pregnant patients (2).

  • GBCM crosses the primate placenta and is assumed to cross the placenta in humans. After they enter the fetal bloodstream, these agents are excreted via the urinary tract into the amniotic fluid and are not removed effectively from the fetal environment. To our knowledge, there are currently no reported adverse effects in humans when the clinically recommended doses of GBCM are used in pregnant patients. Authors of one study of 26 women administered GBCA during the first trimester found no subsequent evidence of teratogenesis or mutagenesis (1).

• Per ACR, the greatest risk associated with undergoing steroid premedication is likely a delay in performing the exam, thus potentially delaying the patient's diagnosis.

  • In rare emergency situations where a contrast-enhanced examination must be performed immediately, contrast media may have to be administered without premedication.

    • Exercise caution with this decision.

    • Clearly communicate risks to ordering physician, preferably have them document in EPIC the need for emergent contrast-enhanced exam without premedication and discuss with staff if needed.

• A nursing mother who received iodinated contrast or MRI contrast (gadolinium-based media) can breast feed immediately after their exam.

  • If they're still concerned, they can pump and discard breast milk 12-24hrs afterwards, just know that this is not required per the ACR (2).

• Barium is the preferred enteric contrast agent.

  • However, barium should not be used if esophageal/gastric/bowel perforation is suspected or there is risk for aspiration.

    • Barium in the mediastinum/peritoneum can cause severe mediastinitis/peritonitis.

    • Aspiration can result in severe pulmonary edema, pneumonia and even ARDS.

  • In these situations use water-soluble iodinated contrast (e.g. Omnipaque) instead.

    • In cases of suspected esophageal/gastric/bowel perforation, you can consider using barium if you are confident there is no perforation after imaging with water-soluble contrast. Exercise caution with this decision, in many cases the diagnosis can be made with water-soluble contrast alone.

• Metformin can cause lactic acidosis in the setting of renal failure.

  • Use of contrast media is not an independent risk factor for patients taking metformin (1).

  • However, avoid iodinated contrast use in patients taking metformin who are at increased risk of contrast induced nephropathy (CIN). RF's for CIN include AKI and eGFR <30.

  • Discontinuation of metformin is not necessary after administration of GBCM in the recommended dose range (0.1–0.3 mmol/kg of body weight) (1).

1. Safe Use of Contrast Media: What the Radiologist Needs to Know, Katrina R. Beckett, Andrew K. Moriarity, and Jessica M. Langer, RadioGraphics 2015 35:6, 1738-1750

2. ACR Committee on Drugs and Contrast Media. ACR manual on contrast media: 2021 version. American College of Radiology

How can MRI hurt the patient?

This section offers a more in-depth discussion of MRI safety issues. For a quick/convenient list of commonly encountered MRI safety issues and how to handle them, click the MRIquestions.com and the UCSF links at the top of the page.

• On a basic level, the MR unit consists of three main systems. Each with their own potential safety risks.

  • Main magnet

  • Gradient coils

  • Radiofrequency (RF) send-receive coils

• Main magnet

  • What does it do?: Generates the main magnetic field.

  • Potential Danger:

    • Projectile injury:

      • The magnet is strong........very strong. Strong enough to turn an oxygen tank into a torpedo. Do not bring ferromagnetic objects into the magnet room (Zone IV).

      • All equipment brought into the MR imaging unit room should be evaluated for MR imaging compatibility and labeled with proper terminology. Unlabeled or unverified equipment should be assumed to be unsafe.

• Gradient Coils

  • What do they do?: Apply gradients to the main magnetic field to spatially encode MR imaging signals.

  • Potential Danger:

    • Translational force and torque :

      • This refers to the forces that make small metal implants/objects inside the patient move/rotate (i.e. aneurysm clips, bullet fragments, screws, etc.)

      • How this happens is somewhat counterintuitive. The gradient of the magnetic field is a measure of how rapidly the field strength increases as the distance to the magnet decreases. Although the strongest magnetic field is at the isocenter of the magnet, the strongest forces are present where the gradient is largest. The largest gradients occur well away from the isocenter and in some cases may be near the ends of the magnet bore (3).

        • Most devices are passive—that is, they do not contain any electronic components. Examples include surgical sutures, vascular and biliary stents, clips, plates, and screws. Many of these devices are composed of nonferromagnetic materials that do not pose a risk of force-related injury. Those that do contain some ferromagnetic materials may be deemed MR safe if the amount of material is too small to cause any substantial force or if the device is anchored securely (eg, most dental implants and orthopedic screws) (3).

        • However, other devices require more caution. Aneurysm clips, for example, are attached to soft-tissue structures only, and there has been one documented case of a fatality attributed to the rotation of such a clip while the patient was adjacent to the magnet; in that case, the ferromagnetic content was underestimated because the clip was incorrectly identified (3).

        • Several types of implants may require a waiting period before an MR imaging examination can be performed. Many cardiac and vascular stents, for example, do not become securely embedded into the vessels until 6 weeks after implantation. These stents are considered MR safe afterward, although imaging can be performed earlier on a case-by-case basis if there is a clinical necessity. Some gastrointestinal endoclips, typically used for hemostasis, can translate or rotate within a magnetic field, but the majority are sloughed off and passed at 2 weeks. Delaying a nonemergent MR imaging examination in this case would bypass any potential safety issues and could eliminate imaging artifacts.

        • Ballistic implants, such as shrapnel and bullets, warrant special consideration because their ferromagnetic composition may not be known and their anatomic position is variable. Although most fragments do not pose any translational or rotational hazard, proximity to nearby vital structures may preclude imaging (3).

• • • Acoustic Injury:

      • Rapidly changing currents are applied to gradient coils during imaging. These currents cause microscopic movements within the coils resulting in the characteristic knocking/buzzing noises heard during MR imaging.

      • The FDA sets a maximum of 140 dB for an MR imaging system and a maximum of 99 dB for a patient with hearing protection (3). .

        • Pulse sequences that are gradient intensive, such as echo-planar imaging (used to obtain diffusion weighted sequences), are the loudest, but even these usually fall under the required maximum .

• • • Burns:

      • Electromagnetic induction, where generated current from changing magnetic fields produces an excessive amount of heat, analogous to an excessive local SAR. Gradient or radiofrequency coils provide the source of the fluctuating magnetic fields, but the current can be produced within any conducting material, either internal or external to the body (3).

      • How to avoid (click here)

• Peripheral neurostimulation:

  • Induced electrical currents can produce painful neurostimulation in patients. This stimulation is most often felt in the arms and legs, where the gradient magnetic field is changing most rapidly.

  • The risk of peripheral neurostimulation is dictated by the rate of change of the magnetic field over time, termed dB/dt and expressed in teslas per second. The FDA requires only that dB/dt be set to levels that do not result in peripheral neurostimulation, without a specific number. Sensitivity to peripheral neurostimulation varies widely among individuals, and it is possible that an imaging examination that is well tolerated by one patient will be uncomfortable for another. MR imaging studies that pose the greatest risk of peripheral neurostimulation are those that involve high-bandwidth readouts and/or rapid gradient switching, such as echo-planar imaging. Reducing the read bandwidth or increasing the repetition time can reduce dB/dt.

• RF send-receive coils

  • What do they do?: Send RF coils send pulses to excite nuclear magnetization in the patient and receive coils acquire MR signal from the patient used for image formation.

  • Potential Danger:

    • Burns:

      • Skin contact against radiofrequency transmit and receive coils and cables (3) can result in direct burns.

        • To minimize this risk, modern coils and cables are typically insulated and sealed within a thicker plastic protective sleeve to provide a minimum safe distance.

        • Even when coils and cables are appropriately insulated, if they are pressed tightly against bare skin, a direct burn can potentially occur as a result of arcing through the insulation (3).

        • It is critical that all coils are accounted for and properly connected before any imaging examination can begin (3).

      • Electromagnetic induction, where generated current from changing magnetic fields from either as described in the above section.

      • The “antenna effect”

        • An uncoiled wire resonates with the electric field of the radiofrequency coil, similar to a radio tuned to a station, generating large electric fields in the vicinity of the lead tip (3).

        • MR-conditional intracardiac pacemakers contain electronic filters that nearly eliminate the possibility of the antenna effect. However, abandoned intracardiac pacer wire leads lack such protection and are currently considered a contraindication to MR imaging (3).

          • Retained wires that are short and have no potential to form loops may be safe for MR imaging. A retained lead from prior temporary epicardial pacing is one such example; no additional screening is required for these patients (3).

        • How to avoid (click here)

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MR Imaging of Unconscious or Incapacitated Patients

• MR imaging may be indicated in patients who are unable to provide answers to the screening profile (click here for an example screening form [3]).

• The screening form may be completed by the patient’s health care proxy and be confirmed with one of the patient’s health care providers (physician, physician assistant, or nurse practitioner), who can also review the medical records.

• If the medical history is incomplete, recent CT or radiography studies can be reviewed, or screening radiographs can be obtained, starting with the skull, chest, and abdomen. The MR imaging personnel performing the examination should also perform a physical examination of the patient to look for surgical scars that might warrant radiography prior to MR imaging.

• MRI should be avoided in unconscious/incapacitated patients who have MR unsafe/MR conditional devices or if there is indeterminate metal debris/shrapnel in locations that could result in significant patient harm if they were to move/heat during the course of the exam.

  • In conscious patients, indeterminate metal debris in low-risk locations is generally of little concern because the patient is able to communicate any discomfort immediately to the MR technologist by squeezing a bulb/alarm device.

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MR Imaging of Pregnant Patients

• There is currently no definitive evidence of harmful effects from performing routine (nonenhanced) MR imaging examinations in pregnant patients (3).

• The ACR considers use of MR imaging to be relatively risk free during pregnancy, and no special consideration is recommended for the first, versus any other, trimester in pregnancy (3).

  • The possible risks, although not conclusively documented to be present, include but are not limited to the following: possible bioeffects of the static magnetic field of the MR imaging system, risks associated with exposure to the gradient magnetic fields, potential adverse effects of the radiofrequency energy, possible adverse effects related to the combination of these three magnetic fields, and possible effects of acoustic noise in the MR imaging environment on the fetus.

  • However, these have only been observed in animal studies and have not be observed in humans (3).

• No adverse outcomes to fetuses have been reported after a review of studies in pregnant patients who received gadolinium based contrast agents (GBCAs), although the sample sizes of these studies were small (3).

  • Although no adverse effects to the fetus or neonate have been established, intravenously administered GBCAs are known to enter fetal circulation and to persist within the amniotic fluid. The FDA has classified GBCAs as pregnancy category C drugs, meaning that their safety in humans has not been proven but that they may be used in cases where the potential benefits outweigh the risks (3).

3. A Practical Guide to MR Imaging Safety: What Radiologists Need to Know, Leo L. Tsai, Aaron K. Grant, Koenraad J. Mortele, Justin W. Kung, and Martin P. Smith, RadioGraphics 2015 35:6, 1722-1737

Determining device and implant compatibility

• • This is a complicated topic and no one source is perfect. Practically speaking, often the best you can do is look up the exact device in question on mrisafety.com.

  • Any metallic or electronic medical device has the potential to cause harm within an MR imaging environment (3).

  • Current guidelines (from ASTM and FDA) categorize devices as MR safe, MR conditional and MR unsafe (3).

    • MR safe: nonhazardous in all MR imaging environments.

    • MR conditional: Compatible only in specific operating conditions with the following information required:

      • Main magnetic field strength

      • Maximum magnetic field strength

      • Maximum magnetic field gradient

      • Maximum specific absorption rate (SAR)

      • Description of testing conditions used to arrive at this designation.

    • MR unsafe: Contraindicated in any MR imaging environment.

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MR safe

These devices are considered MR compatible but still cause image artifact often limiting the exam:

1. All orthopedic implants

2. Dental braces

3. Spinal rods

4. Sternal wires and most all post surgical sutures

5. External fixators (these, however, have heated in select patients and the exam had to be aborted)

MR conditional

These are generally considered MR compatible as long as certain steps are followed (not an exhaustive list):

1. It is recommended that all coils, stents, and filters be in place for 60 days before an MRI scan.

2. Pain stimulators (these devices must be turned off before MR and reset after)

3. Baclofen pumps (must be reset by patient’s physician)

4. Most mechanical implants are MR conditional and should come with an MR compatibility card.

5. Replacement heart valves.

MR UNSAFE

Below are the most commonly encountered MRI unsafe items at UMMC.

1. Aneurysm clips (unless put in here by UMMC Neurosurgeons and a form of device identification is provided)

2. Cardiac pacemakers*

• These should be considered unsafe unless you know the exact make/model and cardiology staff has evaluated the device.

3. Cochlear implants

4. Codman devices

5. Any wire guided devices or tubes that still have wire in place.

6. Any “unknown” metal (ex: shrapnel, metal fragments) particularly if patient is non-responsive/sedated.

7. Vagus nerve stimulators

8. Extremity prosthetics that cannot be removed.

3. A Practical Guide to MR Imaging Safety: What Radiologists Need to Know, Leo L. Tsai, Aaron K. Grant, Koenraad J. Mortele, Justin W. Kung, and Martin P. Smith, RadioGraphics 2015 35:6, 1722-1737