When Do I Need an MRI Study of My Body? A Discussion Guide for Patients and Caregivers

Richard C. Semelka, MD

Disclosures

September 12, 2005

In This Article

Cost and Weighing Other Imaging Alternatives

Cost is an important consideration when evaluating techniques. Cost is generally about 20% more for MRI ($1200) compared with CT ($1000) and about twice as expensive as ultrasound ($500).

Ultrasound, although not the focus of this particular article, is both safe and relatively inexpensive. Therefore it should be preferentially employed in circumstances in which it provides adequate information. Examples include looking for gallstones and detecting simple processes in the pelvis, such as determining the complexity of an ovarian mass. In addition, follow-up of some disease processes identified on CT or MRI studies may be adequately performed with ultrasound. An example of this is follow-up of kidney cystic lesions.

CT is unmatched in its ability to evaluate the full extent of disease processes in the chest. The presence of air throughout the lungs renders ultrasound ineffective, and the combination of patient motion (from breathing and heart pulsations) and small-volume tissue compared with background air render MRI not so effective at studying many lung diseases. CT is especially superior to MRI in the examination of diffuse noncancer lung disease, such as pneumonias and other air-space disease and interstitial lung disease, such as pulmonary fibrosis. CT is also slightly more consistent in identifying primary lung cancers (the number-one cause of cancer death) and can see smaller nodules of metastatic disease (2 mm vs 4 mm) compared with MRI. An unanswered question is whether, in the new imaging paradigm environment of emphasizing patient safety, MRI is good enough to look at the lungs. In many cases I believe it may be. One recent article that looked at using MRI to study the entire body found that MRI was successful at seeing all lung metastases over 5 mm in size.[6] On newer equipment, that minimal size of consistent detection is 4 mm, which may be adequate.

Imaging of bowel diseases poses challenges for both CT and MRI, and with some conditions CT is superior and with others MRI is superior, from a diagnostic accuracy point of view. Taking safety into consideration, MRI probably should be performed in many of these patients, especially these who undergo multiple imaging studies (eg, patients with Crohn's disease).

Vessel disease is a subject that involves the strengths of MRI (when MRI is used to image vessels it is described as MR angiography, or MRA) and CT angiography (CTA). MRA is excellent for large- and medium-sized vessels, and because of safety considerations should be used to study diseases of these vessels (eg, the abdominal aorta or carotid arteries). Because of the higher spatial resolution of CT (ie, it can acquire thinner sections) small vessels are currently better studied by CT; an excellent example of this is coronary CTA (imaging the arteries in the heart).

Because CT and MRI continue to evolve and improve, it is important to consider the diagnostic accuracy of CT, which is likely to retain advantages over MRI in the foreseeable future. These strengths of CT are based on a few principles: (1) CT is unmatched in its ability to see diseases manifested by the presence of pure calcium or pure air; (2) CT is able to see tubes and catheters (placed inside patients' bodies) well; (3) CT is fast, and (4) there is no danger posed to the patient by medical equipment or other objects brought into the scanning room (this is not true of MRI). On the basis of these points, the relative strengths of CT include: looking for renal stones, looking at lung disease especially primary lung disease, most acute traumas, and intensive care and other very ill patients in whom the placement of tubes and catheters has to be checked.

MRI is unlikely to assume a large role in the imaging of many acute severe traumas due to the superiority of CT in imaging the full range of lung diseases, including injuries that can be critical to assess in the immediate postinjury situation. Also with MRI, great care must be taken that no iron-containing metal objects are brought into the scanning room, as these objects can fly into the bore of the magnet, potentially injuring the patient.

Distinction should be made between single-use studies (eg, a serious motor vehicle accident) and serial-use examinations (eg, follow-up of inflammatory bowel disease such as Crohn's). Single-use studies may not offer unwarranted health risks to the patient; however, careful scrutiny should be made of serial-use situations to consider alternative imaging strategies, such as following patients with Crohn's disease, adrenal mass, or complex kidney cysts with MRI.

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