Q. Sometimes my doctor orders an M.R.I., sometimes a CAT scan or an ultrasound. If one is best, why not just order it every time?
A. The rationale for selecting an imaging test is complicated, but some general principles can be helpful.
X-ray is the basic imaging test. X-rays are quick and cheap, and while they do involve radiation, the dose is very low. The radiation exposure from a hand or a foot X-ray, for example, is about the same as three hours of background radiation — the environmental radiation we are all continually exposed to. X-ray is usually the best way to assess injuries to the extremities.
A CAT scan, or CT, is a computerized composite of hundreds of X-rays. A CT provides much more detail than an X-ray, but it also entails much more radiation. A CT of the head, for example, can detect a life-threatening intracranial hemorrhage that an X-ray cannot, but the radiation dose is about the same as eight months of background radiation.
M.R.I. provides exquisite detail and does not involve radiation. It is an excellent choice for assessing the soft tissues of joints and for detecting subtle brain abnormalities. But M.R.I. is poor at visualizing air-filled structures, such as the lungs. Also, it is less readily available than CT, is more time-consuming and can feel claustrophobic and provoke anxiety in some patients.
Ultrasound, like M.R.I., does not entail radiation. It excels at imaging pregnancies and fluid-filled organs such as the heart and gallbladder, but it cannot penetrate bone or air.
Specific cases can blur these general distinctions. For example, M.R.I. is the test of choice for chronic headaches, but CT is better for headaches that are sudden and severe, which might suggest an intracranial hemorrhage. Some scans should be enhanced by intravenous contrast, but the chemicals used can cause adverse effects.
A final important point to be considered is that imaging is not always necessary. Unnecessary imaging leads to preventable side effects, excessive costs and overdiagnosis.
The Choosing Wisely Initiative, a consortium of professional societies that includes the American College of Radiology, is fighting the overuse of imaging. Examples of unnecessary imaging highlighted by Choosing Wisely include scans for uncomplicated headaches, routine chest X-rays, CT for uncomplicated sinusitis, and any imaging for nonspecific low back pain.
A physician’s guidance is still essential in making the right imaging decision, particularly when that decision is no imaging at all.
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