Diagnostic-imaging tests offer variety of results

Q. What’s the difference between a CAT scan and an MRI?

The CAT scan, MRI and others are known as diagnostic-imaging tests. Let’s go over the common ones.

X-ray

One of the oldest forms of medical imaging, an X-ray examination uses electromagnetic radiation to make pictures.

An X-ray machine passes a beam through your body and records an image digitally or on film. Body tissues produce different results. Tissues show up in shades of gray. Bones look white. Lungs that contain air appear dark.

Sometimes you take a contrast medium such as barium and iodine to outline an area of your body. This medium may be injected, swallowed or taken as an enema. The contrast medium appears opaque on X-ray film, providing clear images of structures such as your digestive tract or blood vessels.

CT scan

Computed tomography, known as a CT scan or CAT scan, uses X-rays and computers to produce precise images of cross-sections of the body. It is much more revealing than a conventional X-ray.

A CT scan employs a doughnut-shaped machine called a gantry. The patient lies on a table inside the gantry while an X-ray tube rotates around the patient’s body sending radiation through it. Detectors measure the exiting radiation and convert it into electrical signals.

A computer gathers the electrical signals and assigns them a color based upon signal intensity. The computer then assembles the images and displays them on a computer monitor. Some CT scans require a contrast medium.

MRI

Magnetic resonance imaging uses a magnetic field and radio waves, instead of X-rays, to create pictures of cross-sections. In many cases, MRI gives more information than a CT scan or other types of diagnostic imaging. Sometimes contrast agents are used to enhance the images.

Most MRI machines are large cylinders. Inside the machine, the human body produces very faint signals in response to radio waves. These signals are detected by the MRI machine.

A computer then interprets the signals and produces a three-dimensional representation of your body. Any cross-section can be extracted from this representation.

There are MRI machines that are open on all sides. These newer, open MRI scanning systems are useful for the claustrophobic, obese or anyone who feels uncomfortable about lying inside a cylinder.

The MRI often helps with the diagnosis of central nervous system disorders such as multiple sclerosis, because it produces such high-resolution images of the brain and spinal cord.

Nuclear imaging

Nuclear imaging detects radiation from the body after a radiopharmaceutical agent or tracer is either injected or taken orally. The images are recorded on computer and on film.

While other imaging methods assess how the organs look, nuclear imaging shows how organs work. For example, nuclear imaging can analyze blood flow to and from the heart. Nuclear imaging provides information other imaging techniques cannot produce.

Positron emission tomography, also called PET imaging or a PET scan, is a type of nuclear-medicine imaging.

When undergoing a nuclear-imaging exam, the patient lies on a table under a special camera that takes a series of pictures. A computer connected to the camera detects the radiation coming from the body organ being examined and makes a series of images.

Ultrasound

Ultrasound examination, also called diagnostic medical sonography, uses high-frequency sound waves beyond the range of the human ear to produce precise images of structures within your body.

Ultrasound imaging is based on the principles of sonar used by ships to detect underwater objects and by bats to catch flying insects.

During an ultrasound, a sonographer presses a hand-held transducer against your skin. The transducer generates and then receives reflected high-frequency sound waves from your body. However, some ultrasounds are done inside your body. Information about your body is sent from the transducer to a computer. The computer then composes images based on this data.