The best imaging test depends on what you’re looking for

DEAR DR. ROACH: Could you please explain in layman terms the difference between an X-ray, CT scan, MRI and PET scan? — C.D.B. ANSWER: All four of these tests provide images of the body, which your doctor can use to evaluate symptoms or search for a specific anatomical condition. Some conditions, such as depression, have no visible changes on any kind of commonly used imaging, but that doesn’t mean they aren’t real. X-rays are high-energy electromagnetic waves that can travel through the body and are absorbed differently by different structures. X-rays do not travel well through bones, so X-rays are very good at seeing changes in bones due to contrast with soft tissues. They are not as useful for soft tissues, which have little contrast with each other.

A CT scan is a computed tomogram, which uses many different X-rays at the same time, taken while moving around a person. These can be reconstructed by a computer to provide a much better look at both bones and soft tissues in slices through the body. Both X-rays and CT scan use ionizing radiation. This can damage the body in high doses, but the amount in a single X-ray is very small. Often, a contrast agent is given by mouth, in the vein or both to help improve the ability of the scan to tell one structure from another. Modern CT scans are amazingly fast. CT scans are the workhorse of a modern hospital’s radiology department, as they provide excellent detail for many conditions, despite the expense and higher radiation dose. An MRI scan uses magnetic waves, which provide even better resolution of soft tissues, such as the brain, than CT scans. They do not use radiation and are considered safer than X-rays or CT scans. Intravenous contrast also may be given in MRI scans. MRI scans are expensive and slow. Some people get claustrophobic when in the MRI scanner for a long time, sometimes hours. MRI scans are also surprisingly loud, although new technology promises to fix that problem. A PET scan uses positrons -an antimatter particle: a positively charged anti-electron — to identify areas in the body that are metabolically active, especially cancers. PET scans are now mostly combined with CT scans to provide precise imaging of anatomical areas where the increased metabolic activity is. Because the radioactive sugar injected into the body decays by releasing positrons, this imaging technique also exposes the body to additional ionizing radiation. One type of imaging you did not mention is ultrasound, which does not use ionizing radiation but instead uses soundwaves. Ultrasound is very good at telling fluid from solid, although it does not have the fine resolution of CT or MRI.