Continuing Education Activity

Magnetic resonance imaging (MRI) is a diagnostic imaging technique useful for the detailed characterization of the soft tissues without ionizing radiation. MRI imaging is noninvasive, and in some cases, eliminates the need for surgical intervention or invasive procedures when used correctly. This activity outlines the appropriate clinical use of MRI to evaluate for common clinical questions (and is not all-encompassing) and briefly reviews contraindications that are important considerations for the interprofessional team contemplating this imaging modality.

Objectives:

• Describe the appropriate use of MRI imaging broadly organized by anatomy.
• Review the appropriate use of gadolinium-based contrast and risk factors for developing nephrogenic systemic fibrosis.
• Identify contraindications to MRI, specifically related to implanted hardware.

Introduction

Magnetic resonance imaging (MRI) is a diagnostic technique useful for noninvasive visualization of organs and soft tissue structures.[1] The ability to evaluate for structural integrity lends MRI for imaging the neural axis and large joints of the musculoskeletal system where it was used most heavily during its infancy. Since that time, MR's scope and application have broadened significantly and now encompasses abdominopelvic and cardiac imaging. Clinicians frequently order MRI to characterize soft tissue and osseous lesions or masses. In some cases, the varying MRI sequences can determine the composition of these abnormalities. For example, MRI elastography can diagnose and surveil hepatic fibrosis sparing the patient from an invasive and repetitive biopsy. MR angiography, using both contrast-enhanced and non-contrast techniques, can diagnose vascular occlusive disease and stenosis. Faster scan times and gating techniques minimizing cardiac and respiratory motion make MRI a useful non-invasive tool for cardiac evaluations of structure, function, and myocardial perfusion.[2][3]

A major advantage of MRI is the ability to produce high-quality images with superior soft-tissue contrast without using ionizing radiation. The magnet generates images based on the specific and unique magnetic properties of the tissues driven by the spin properties of hydrogen molecules.[4] This makes MRI especially useful to evaluate "high radiation risk" patients like pregnant women and children. MRI is also valuable for patients with chronic conditions requiring routine imaging surveillance, such as multiple sclerosis and inflammatory bowel diseases.[5]

MRI does not exist without hazard – the magnetic field can be dangerous and strict parameters are in place to ensure patient safety. Pre- imaging screening protocols are in place to assess the patient’s risk factors ranging from occupational exposures to surgically implanted devices determined to be incompatible with the magnetic field. Though many of the newer generation implanted devices are MR compatible, it is crucial to consult with both the radiologist and MR technologists. The magnetic field can alter implanted devices and result in loss of function, positioning, and temperature changes. Additionally, while some prosthetic devices- like heart valves, stents, and artificial joints- are MR safe, they may cause signal artifacts that limit the diagnostic quality of the exam.[6] While in no way all-inclusive, this article provides information for clinicians to consider when ordering MR imaging.

Function

Traditionally MRI is an adjunct or complementary imaging exam following initial evaluation with more accessible and cost-effective modalities, like radiographs, ultrasound, or computed tomography (CT). The goal is to characterize a finding using the specific magnetic properties of the tissues and less commonly to guide biopsy.

Neurological: MRI is broadly useful to image the central nervous system. It serves as a beautiful addition to non-contrast CT to evaluate for acute and subacute infarcts, headache and seizure evaluation, as well as following a neoplastic process.

Considering the time-sensitive nature of stroke evaluation, a non-contrast-enhanced MRI of the brain is indicated for patients following a non-contrast CT of the head/brain to evaluate for ischemic stroke. Image sequences based on the diffusion coefficient of water in the brain parenchyma can demonstrate areas of hyperacute ischemia.[7] A small study of 120 patients demonstrated DWI (diffusion-weighted imaging) detection of infarcts within three hours of onset, preceding abnormal signal findings on fluid attenuating inversion images (FLAIR), which may not present for 3 TO 6 hours.[8] There is variable clinical use and availability of perfusion imaging to estimate the extent of infarct and compromised parenchyma and this is beyond the scope of this article.

The gold standard of craniocervical vascular imaging to rule out obstruction, high-grade stenosis, or dissection is CT angiography. MR angiography could be considered in patients with diminished renal function. Time of flight (TOF) MR angiography is a multidimensional (2-D or 3-D) technique mapping the imaged vessels while subtracting the bony structures and brain parenchyma. TOF does not require contrast and instead uses flow-related enhancement properties during image acquisition.[9] Similarly, the dural venous sinuses can be evaluated with MR venography.

When considering MRI to evaluate patients for headaches, seizures, demyelinating disease, and suspected mass, it is generally acceptable to request the use of an intravenous contrast agent. For example, post-contrast-enhancing lesions indicate active disease in a patient undergoing imaging evaluation for multiple sclerosis.[10]

Breast: Breast MR is indicated in very select scenarios due to the use of a specialized breast coil, large field of view, and standardized sequences. It is important to inform the patient that the exam is performed in the prone position with the arms extended overhead. Dynamic contrast-enhanced breast MRI is an important tool to evaluate the extent of breast disease, therapy response, and surveillance of residual or recurrent disease.[11] Under the care and direction of a breast imager, some patients undergo MR guided biopsy of mammographic or sonographic occult masses. Breast cancer screening with MRI is indicated in patients with a high risk of breast cancers, defined as BRCA gene carriers, patients with first degree relatives with breast cancers, and those with >20% lifetime risk of breast cancer, in addition to those with a history of radiation to the chest.[11] Dynamic contrast-enhanced breast MRI is used in conjunction with screening mammography. To assess silicone breast implant integrity and localization of free silicone, a non-contrast MRI of the breasts is performed. Breast imaging departments usually have standard protocols to obtain limited imaging sequences for these purposes.

Chest/Cardiac: MRI with IV contrast agents and customized fields of view are appropriate to evaluate chest wall abnormalities not definitively diagnosable with other modalities. MRI technologists are educated to mark the area of concern with an MRI visible Vitamin E capsule. While contrast-enhanced MR can define the extent of mediastinal soft tissue involvement, a review of the literature shows that the more costly MR was only superior to CT for preoperative planning of posterior mediastinal masses.[12]

Cardiac MRI provides both structural and functional information. It requires the patient to tolerate upwards of 45 minutes of imaging and comply with breath-holding instructions. Non-contrast techniques are emerging, but current cardiac MR protocols typically include intravenous contrast agents; patterns of late gadolinium enhancement are critical to identifying scarred tissue.[13] Tissue characterization using extra-cellular volume fraction and T1 mapping has made cardiac MRIs invaluable in the diagnostic workup of infiltrative cardiomyopathies, oftentimes negating the need for invasive endomyocardial biopsies. Stress cardiac MRIs may also be performed, which provide structural information and serve as an ischemic evaluation and myocardial viability study. 

Abdomen/Pelvis: Multiphase, post-contrast sequences are helpful to characterize abdominopelvic lesions and masses and follow responses to therapy. MR usually follows the more cost-effective and widely available initial imaging like ultrasound or CT, for example:

• Radiologists can confidently characterize a benign versus malignant liver lesion by MR enhancement patterns with liver and biliary specific contrast agents. Studies show that MRI with contrast is superior to ultrasound in diagnosing hepatocellular carcinoma (HCC) in high-risk populations defined as higher rates of detection and decreased false positives.[14]
• A small study of 81 female patients with sonographically indeterminate pelvic masses showed that even limited sequence MR could define uterine vs. extra-uterine origins of a mass, determining and guiding clinical management.[15]
• MRI plays a critical role in diagnosing, staging, treatment, surgical planning, and surveillance of rectal cancer.
• The radiologist may make recommendations for follow up MRI in the appropriate clinical settings. A detailed history is critical when ordering these exams to ensure the acquisition of appropriate sequences. Of note, some institutions have established abdominal MR protocols to evaluate the appendix in pregnant women and small children if ultrasound is not diagnostic.

Musculoskeletal: Magnetic resonance imaging is useful to diagnose internal derangements of the support structures of the joints, occult fractures, bone marrow edema, infiltrative processes of the marrow space, and soft-tissue masses. Radiographs are first-line imaging of the musculoskeletal structures and are an important contribution to MRI interpretation. To highlight this point, consider that independent of radiographic findings, an MRI of the hip changed clinical management in as little as 7% of patients in a retrospective study performed at a single institution.[16] Many published studies agree that MRI is the most useful and affects patient management when ordered by an experienced physician or orthopedic specialist.[16][17]

The aging population of the United States correlates with the increasing popularity and frequency of joint arthroplasties. The 2017 American Joint Replacement Registry reported more than 1 million total joint replacements annually, and that number is predicted to quadruple by 2030.[18] Radiographs are vital in the assessment of the hardware and joint structures. The development of paramagnetic arthroplasty materials, like titanium, make it possible to adequately assess the postoperative patient and minimize metallic susceptibility artifacts by modifying magnet field strength (1.5T being superior to 3T), increasing bandwidth, decreasing slice selection size, increasing matrix size, and use of long echo train lengths with spin-echo sequences.[19]

Traditionally gadolinium-based contrast is administered when evaluating for masses or bone lesions. The clinical benefits of MRI outweigh the cost when characterizing bone lesions or masses, supporting the role imaging plays in diagnosing neoplasms.[16] Imaging evaluation of the musculoskeletal system is not limited to plain radiographs and MRI. It is important to remember nuclear medicine bone scans, positron emission tomography, ultrasound, and computed tomography, all of which contribute to achieving an accurate diagnosis.

Issues of Concern

Issues of concern pertaining to MRI revolve around the use of nonionizing radiation when imaging vulnerable patient populations, the use of contrast agents, and logistical concerns for the patients, some of which are addressed earlier in this article. When ordering an MRI, it is important to scrutinize the clinical question, patient’s clinical status, psychological factors, length of time required for the exam, and weigh the risks and benefits.

• There is abundant information published about gadolinium, paramagnetic contrast agents. A glomerular filtration rate of less than 30 is a contraindication to receiving gadolinium contrast. These patients are at increased risk of developing a systemic phenomenon called nephrogenic systemic fibrosis due to impaired clearance of contrast.
• The effect of MRI contrast on pregnancy is largely unknown and therefore avoided. Since gadolinium contrast is excreted quickly, nursing mothers can resume breastfeeding 36 to 48 hours after having an MRI. No deleterious effects have been documented from MRI during pregnancy, and for this reason, a medically-indicated MRI can be performed at any trimester. The risk of harming the pregnancy versus the benefit of the scan should be considered, and the MRI should wait until after pregnancy when appropriate.[20]
• A common patient concern regarding the utilization of MRI imaging is claustrophobia and anxiety during the MRI exam. Anxiety attacks have been found to occur in as many as 2% of MRIs, with as many as 1% resulting in premature cessation of the study.[4]
• Proximity to the magnetic field is a source of risk for the patient, and stepwise measures are in place to ensure that ferromagnetic materials do breach specific zones surrounding the magnet. All patients are screened before MR imaging is approved, but special attention should be paid to those with known exposure to shrapnel, bullets, or other metal fragments. Implanted medical devices are susceptible to the magnetic field with the potential for altered function, positioning of the hardware, and possible temperature changes. Many implanted devices, such as prosthetic heart valves, stents, and artificial joints, are safe for MRI but cause imaging artifacts.[6] Therefore, it is important to communicate and consult with the radiologist and MR technologist who can confirm or deny device compatibility with MRI.

Clinical Significance

MRI in the clinical setting has improved patient care by offering a non-ionizing radiation alternative method of imaging. The value in not relying solely on standard X-ray based imaging techniques is best illustrated in cases requiring regular monitoring of disease progression, most notably multiple sclerosis, and inflammatory bowel disease. MRI also offers a lower-risk diagnostic evaluation for those who are at a high risk of long term radiation-related complications, such as children and pregnant women.

The alternative non-contrast techniques used in MRI are important for time-sensitive neurological imaging, for example, MR arteriography assists in the assessment of the craniocervical vasculature in suspected stroke patients with poor renal function or severe iodinated contrast allergies. In summary, the judicious use of MRI by ordering clinicians is critical for positive patient outcomes, timely diagnosis, and healthcare costs.

Enhancing Healthcare Team Outcomes

As healthcare costs continue to soar, it is the responsibility of healthcare providers to order advanced imaging judiciously. Not all facilities have access to MRI, and, as mentioned, there are strong preliminary imaging modalities widely available that can guide care and treatment. The clinician can inquire if the clinical question was addressed on recent existing imaging. Conversation with your radiology colleagues can also answer if the exam being ordered is appropriate.

While MRI is our most technologically advanced modality, it is not appropriate in all situations. Medicare spends $10 billion annually on imaging, a good portion of which is repeat or unnecessary imaging.[21] Fragmented exchange of health information between healthcare systems contributes to repeat imaging because of the lack of access to prior exams. A study of 196,314 patients showed 7.7% of imaging was repeated within 90 days when prior imaging was inaccessible.[22] Thus, all interprofessional team members can contribute to improved allocation of resources by integrating the radiology department into the multidisciplinary care team.