Questions And Answers In Magnetic Resonance Imaging Pdf

[Charise Zelnick]

Magneticresonance 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.Access free multiple choice questions on this topic.

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]

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:

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.

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.

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