Imaging basics: X-ray, ultrasound, CT and MRI

"Get a scan" sounds like one decision, but there are several very different machines behind it, each with its own strengths, blind spots and costs. Choosing between them is part of clinical reasoning: the right test is the one that answers the question you are asking, as safely and quickly as possible.

At a glance

X-ray

A plain X-ray (radiograph) sends a beam of X-rays through the body onto a detector. Dense structures such as bone absorb more of the beam and appear white; air lets it through and appears black; soft tissue and fluid fall somewhere in between.

X-rays are fast, cheap and widely available, and the radiation dose is small: a chest X-ray is roughly equivalent to ten days of the natural background radiation everyone receives. Classic uses include:

• Chest X-ray for pneumonia, collapsed lung (pneumothorax), fluid around the lungs (pleural effusion) and signs of heart failure.
• Bone X-rays for fractures and dislocations.
• Abdominal and upright chest X-rays for some bowel obstructions, and for free air under the diaphragm after a perforated bowel.

The main weakness is that an X-ray flattens a three-dimensional body into one picture, so structures overlap and soft tissues are hard to tell apart.

Ultrasound

Ultrasound sends high-frequency sound waves into the body and builds a picture from the echoes. It uses no ionizing radiation, works in real time and can be done at the bedside, which is why emergency clinicians increasingly carry handheld probes (point-of-care ultrasound).

Ultrasound is excellent for:

• Gallbladder and bile ducts, including gallstones and inflammation (cholecystitis).
• Pregnancy, from early dating scans to checking the baby's growth.
• Kidneys and bladder, for example swelling of the kidney when urine flow is blocked.
• Blood vessels, including clots in the leg veins (deep vein thrombosis) and a widened abdominal aorta (aneurysm), with Doppler showing the direction and speed of blood flow.
• The heart: an echocardiogram is an ultrasound of the heart that shows how well it pumps, whether the valves work and whether fluid has collected around it.
• Trauma: a quick scan called FAST looks for free fluid (usually blood) in the abdomen and around the heart.

Sound does not pass well through bone or gas, so ultrasound is poor for the brain in adults, the lungs' interior and bowel full of gas. The quality of the images also depends a great deal on the skill of the person holding the probe.

CT

Computed tomography (CT) rotates an X-ray source around the body and uses a computer to rebuild the measurements into thin cross-sectional slices. Modern scanners can image the whole chest in a few seconds.

CT is the workhorse of emergency imaging:

• Head CT without contrast quickly shows bleeding in the brain, which is essential to rule out before treating a stroke with clot-busting drugs, and injuries after head trauma.
• CT pulmonary angiography uses contrast dye to show blood clots in the lungs' arteries (pulmonary embolism).
• CT of the abdomen and pelvis helps diagnose appendicitis in adults, kidney stones, abscesses, bowel problems and injuries after trauma.
• CT angiography maps arteries elsewhere, for example narrowed leg arteries or a tear in the aorta (aortic dissection).

The trade-offs are radiation and contrast. A CT of the abdomen and pelvis gives a dose roughly equal to three years of natural background radiation, so clinicians avoid unnecessary scans, especially in children and in pregnancy. Iodine-based contrast dye can occasionally cause allergic reactions and needs care in people with poor kidney function.

MRI

Magnetic resonance imaging (MRI) uses a strong magnetic field and radio waves to detect signals from hydrogen atoms, mostly in water and fat. It involves no ionizing radiation and gives the best contrast between different soft tissues.

MRI is the test of choice for:

• The brain, including multiple sclerosis, tumors and very early stroke, which shows up on a sequence called diffusion-weighted imaging.
• The spine and spinal cord, for example suspected cord compression.
• Joints, such as torn ligaments and cartilage in the knee or shoulder.
• The heart: cardiac MRI can show inflammation and scarring of the heart muscle, as in myocarditis.
• Some abdominal organs, such as characterizing liver lesions.

MRI scans take much longer than CT, are noisy and can be hard for people who are claustrophobic or too unwell to lie still. The magnet makes some metal implants unsafe, although many modern devices, including many pacemakers, are designed to be scanned under specific conditions. Gadolinium-based contrast agents are used for some scans.

Other imaging

Nuclear medicine scans use small amounts of radioactive tracers to show how organs work rather than just how they look. Examples include ventilation-perfusion (V/Q) scans for pulmonary embolism, bone scans, and PET-CT, which highlights metabolically active tissue and is widely used to stage cancers. Fluoroscopy is live X-ray, used to guide procedures and to watch swallowing or contrast moving through the bowel.

Choosing the right test

When clinicians choose an imaging test, they weigh:

• The question. Is it about bone, air, fluid, soft tissue or blood flow?
• Speed. A patient who may be bleeding needs the fast answer that CT gives.
• The patient. Pregnancy, kidney function, allergies, implants and the ability to lie still all matter.
• Radiation. Use the lowest dose that answers the question, and none if ultrasound or MRI will do.
• Availability and cost. MRI slots are limited; X-rays are everywhere.

Professional bodies publish guidance to help with these choices, such as the American College of Radiology's Appropriateness Criteria.

Imaging in Stat!

In Syndrome, the Imaging tab lists studies such as X-rays, ultrasound, CT, MRI and echocardiograms. Each comes back normal or abnormal. The name of the study tells you what was examined, so an abnormal echocardiogram points to the heart and an abnormal CT angiogram points to the blood vessels. Pick the scan that would best separate your top diagnoses; the Syndrome guide explains how.