Each imaging test uses a different kind of energy to see inside the body. Choosing the right one depends on the clinical question, the urgency and the patient.
X-ray
X-rays pass through soft tissue and are blocked by dense structures, so bone looks white, air looks black and soft tissue and fluid look gray. It is fast and cheap, with a small dose of radiation.
CT
CT takes many X-ray images in a circle and builds thin slices of the body. It is quick and detailed, ideal for trauma, stroke and bleeding, but the radiation dose is higher and contrast dye can harm weak kidneys or cause allergy.
MRI
MRI uses a strong magnet and radio waves, with no radiation. It shows soft tissue best, such as the brain, spinal cord, ligaments and joints, but it is slow and every metal implant or device must be checked first.
Ultrasound
Ultrasound sends sound waves and reads the echoes in real time, with no radiation. Fluid looks black, which makes it ideal for the heart, gallbladder, pregnancy, vessels and bedside FAST exams, while bone and air block the waves.
| Test | Radiation | Best for |
|---|---|---|
| X-ray | Low | Fractures, chest, bowel gas |
| CT | Higher | Trauma, stroke, bleeding, abdominal pain |
| MRI | None | Brain, spinal cord, joints, soft tissue |
| Ultrasound | None | Pregnancy, gallbladder, heart, free fluid in the abdomen |
At the bedside
Before a scan, ask about pregnancy, kidney function and contrast allergy, and screen every patient for metal before MRI. Then compare what the image shows with what you found on exam.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).