X-Ray vs. CT vs. MRI: What's the Difference?
When a child comes in to the clinic after a fall with pain in an extremity, as a nurse practitioner you know what to do. You order an X-ray of the affected area, right? Sometimes, however, selecting the appropriate diagnostic test isn't so simple. Ultimately, test selection depends on the condition(s) you are looking for or trying to rule out as determined by the patient's history and physical exam. If you're not quite sure what to order and when, here are some general differences between common diagnostic imaging tools which may help you make a decision.
What is it?
An X-ray is nothing more than a wave of energy or radiation. In the clinical setting, X-ray machines send such beams of energy towards a film. These are sent toward the patient's body and either pass through, are absorbed, or scatter creating a resulting picture, known as a 'map of attenuation', on a film. The resulting picture allows us to view certain aspects of body composition. Bone appears light while air appears black. Soft tissues and fat show as shades of grey in between.
When do you use it?
X-rays are helpful in the case of, or suspected, skeletal injury or abnormalities. They may also be used to look for signs of infection such as pneumonia or osteomyelitis. X-rays are useful for spotting some abnormalities of the abdomen such as a bowel obstruction or kidney stones. They are often used as an inexpensive, quick screening tool before proceeding with more expensive or invasive testing.
X-rays are quick, painless, and relatively inexpensive to obtain. They are readily available making them an easy go-to for nurse practitioners in the clinical setting. While X-rays do emit some radiation, the amount is 100 or more times less than the dose emitted by CT scans.
Since X-rays are just 2-dimensional, they give limited diagnostic information. In some cases, this is sufficient to proceed with diagnosis and treatment. In others, it is not. Nurse practitioners must take caution when interpreting X-ray studies to be sure further investigation into the patient's symptoms is not warranted. Given that X-rays emit radiation, this also poses a risk to patients, although minimal compared to other diagnostic testing options.
What is it?
Computerized Tomography, or CT, uses multiple X-ray beams to generate a 360 degree view of the body. Images are taken around an axis of rotation and compiled allowing the scanner to generate 3-dimensional images of the body. The provider may view these images in 2-D cross-sections on a computer screen. The picture generated by a CT scan is far more detailed than that of an X-ray as the machine detects hundreds of levels of density and therefore allows for the visualization of structures such as the brain and other internal organs as well as soft tissues and blood vessels. Dye administered orally or through an IV can enhance the visualization of some structures and may be indicated depending on the patient's presentation.
When do you use it?
CT scans have an infinite number of indications. They may be used to visualize skeletal structures when X-ray images are insufficient or not indicated. CT scans are most helpful for making diagnoses related to the brain and internal organs in the chest, abdomen and pelvis. CT angiograms may show problems with blood vessels in the head, neck, trunk or extremities. CT scans are typically the test of choice to detect acute bleeding as well as bony abnormalities for which X-ray is not indicated. In general, these images can be helpful in evaluating patients with trauma, masses or infections, or pain.
While not as convenient as X-rays and significantly more costly, CT scans are still widely available. They require just 5 to 20 minutes to complete and produce a 360 view eliciting significantly more detail about the body's structures than plan film X-rays. The ability to image bone, soft tissues, and blood vessels gives the provider significantly more information to aid in making a diagnosis. The speed of CT scans is helpful in patients who may have difficulty lying still for an MRI such as pediatric patients or those with claustrophobia. Unlike MRI, CT does not use magnetic radiation and may be helpful in giving a detailed picture in patients where MRI is contraindicated.
CT technology should be used with extreme caution in pediatric patients. While the test is sometimes clinically indicated, CT scans produce a significant amount of radiation posing a cancer risk to patients, especially children. Pregnant women should also avoid CT scans.
What is it?
Like CT scans, Magnetic Resonance Imaging (MRI) produces cross-sectional, detailed views of the body. Unlike CT scans, MRI uses magnetic fields and radio waves rather than radiation to produce an image. These images are highly detailed making MRI the most sensitive diagnostic tool.
When do you use it?
MRI is the diagnostic test of choice for detecting small or subtle abnormalities such as lesions, masses, or tumors. It may also be helpful to detect joint abnormalities such as tendon, muscle, and ligament injuries or abnormalities. Finally, MRI is often helpful in detecting more subtle brain abnormalities in patients with problems such as dementia or with suspected cerebral infarct.
MRI does not expose patients to radiation and therefore is lower-risk than CT imaging. It also produces highly detailed images and therefore shows more subtle abnormalities than can be detected with CT scans.
While MRI is a highly sensitive diagnostic tool, it is also the most difficult of these imaging tests to perform. An MRI may take 30 to 90 minutes to complete. The test is time consuming and may not be practical in some patients with acute illness or injury. Patients with any kind of metal in or on their body may not have an MRI given that the test uses a magnetic field. Many MRI machines also require that patients lie in an enclosed space, which may not be well tolerated. Furthermore, the machine is noisy which may also make the test difficult to complete for some patients. Finally, MRI is an expensive diagnostic test and not as widely available as other options.
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