What is the Difference Between an X-ray, CT, MRI, and Ultrasound?

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X-rays, CTs, MRIs, and ultrasounds are all used to capture images of the internal structure of the body. As a nurse, it is common for patients to ask why they are receiving one type of imaging test over another. In order to educate patients, it is important to understand how these tests differ in terms of cost, time, level of detail, and risk.

X-rays

An X-ray is the quickest, least expensive, and most common type of imaging.

X-rays send electromagnetic waves, or radiation, through the body. Dense areas, such as bones, block the radiation and appear white. X-rays are ideal for capturing problems with the bones, such as fractures, dislocations, or narrowed joint spaces (osteoarthritis). X-rays can also capture images of tissue consolidation, such as pneumonia. However, X-rays are ineffective at capturing images of soft tissues, such as tendons and ligaments.

Even if a provider suspects a soft tissue injury, an X-ray might be ordered first to ensure that a fracture is not also present.

The radiation from a single X-ray is not considered dangerous for adults, but X-rays are generally avoided in pregnant patients due to the risk to the fetus.

CT Scans

A CT scan, or Computed Tomography Scan, also uses radiation, similar to an X-ray. However, it produces much more sophisticated and detailed images with 360-degree views of internal structures.

CT scans take longer than X-rays but are still very quick, often taking several minutes or less. In emergencies, this makes CTs preferable to MRIs because MRIs take more time.

CTs can identify blood clots, organ injuries, and more subtle bone injuries. They are more expensive than X-rays and not always available at more rural hospitals.

Because they use radiation, CT scans can still pose a risk to the fetus of pregnant patients.

MRIs

Magnetic Resonance Imaging, or MRI, does not use radiation. Instead, a powerful magnet sends radiofrequency pulses through the body.

Highly detailed images of the body's soft tissues, brain, nerves, and blood vessels are produced. MRIs are generally used to diagnose musculoskeletal injuries such as herniated discs and torn ligaments or cartilage.

When an MRI is performed, the patient lies on a table that slides into a cylinder-shaped MRI scanner. The machine produces a loud noise, and the total time needed is much longer than an X-ray or CT scan. For example, a knee or brain MRI can take 30-60 minutes. If contrast dye is used for more detailed images, even more time is typically needed.

Because the MRI scanner uses large magnets, a patient cannot have an MRI if there are metal clips or metal implants in their body. They must undergo an extensive screening questionnaire to determine if they are eligible for an MRI. If they are not eligible for an MRI, they will typically have a CT instead.

MRIs are more expensive than X-rays and CT scans and may also not be available at more rural hospitals.

Ultrasounds

Similar to MRIs, ultrasounds do not use radiation. They use high-frequency sound waves to produce images of structures in the body.

Ultrasounds help evaluate real-time flow or movement in the body, such as blood flow through the heart or the movement of a fetus during pregnancy. They are not effective in examining areas with air or bone, such as the lungs or head.

Depending on the part of the body that is being examined, an ultrasound may take 30-60 minutes. A trained technician, or sonographer, uses a small plastic device called a transducer and holds it to the skin near the area that is being examined.

There are no known risks associated with ultrasounds, and price varies depending on body location, but is typically more than an X-ray and less or equal to a CT scan.

Conclusion

As a nurse, you have the opportunity to educate your patient on why a provider may have chosen to order one specific imaging test over another. Ultimately if you have questions, you can consult the ordering provider. They may have already consulted with the radiologist to determine the best imaging test for your patient's condition.

References/Resources

Health: 6 Key Medical Scans and What They Should Cost

Healthline: How Long Does an MRI Take?

Johns Hopkins Medicine: CT Scan Versus MRI Versus X-Ray: What Type of Imaging Do I Need?

Mayo Clinic: Ultrasound

North Central Surgical Center: What's the difference between an X-ray, CT scan and MRI?