An imaging exam is any medical test that creates pictures of the inside of your body to help diagnose or monitor a health condition. These exams range from a simple chest X-ray that takes seconds to a full-body MRI that can last an hour or more. Some are routine screening tools, like mammograms for breast cancer detection, while others are ordered to investigate specific symptoms like unexplained pain, swelling, or injury.
Common Types of Imaging Exams
The most widely used imaging exams are X-rays, CT scans, MRIs, ultrasounds, and PET scans. Each works differently and excels at visualizing different parts of the body.
X-rays pass a small beam of radiation through your body to create a flat, two-dimensional image. They’re fast and inexpensive, making them the go-to choice for evaluating broken bones, chest infections, and dental problems. A chest X-ray typically takes just a few minutes from start to finish.
CT scans (computed tomography) use the same basic principle as X-rays but take hundreds of images from different angles, then combine them into detailed cross-sectional “slices.” This makes CT scans far better at showing soft tissue, blood vessels, and internal organs. They’re commonly used to evaluate head injuries, abdominal pain, and suspected cancers.
MRI (magnetic resonance imaging) uses powerful magnets and radio waves instead of radiation. It produces extremely detailed images of soft tissues like the brain, spinal cord, joints, and muscles. Because there’s no radiation involved, MRI is often preferred for conditions that require repeated imaging over time or for patients where radiation is a concern.
Ultrasound uses high-frequency sound waves that bounce off internal structures, creating real-time images on a screen. It’s the standard tool for monitoring pregnancy, evaluating the gallbladder, and examining the heart. Like MRI, it involves no radiation at all.
PET scans (positron emission tomography) work differently from the others. A small amount of radioactive tracer is injected into your bloodstream and collects in areas with high metabolic activity, like cancer cells. PET scans are primarily used in cancer diagnosis and monitoring, and they’re often combined with CT scans for more precise localization.
Radiation-Based vs. Radiation-Free Exams
One of the most important distinctions in imaging is whether the exam uses ionizing radiation. X-rays, CT scans, and PET scans all involve radiation. The doses vary enormously: a single chest X-ray delivers roughly 0.1 millisieverts (mSv) of radiation, while a CT scan of the abdomen delivers around 8 mSv. For context, the average person absorbs about 3 mSv per year just from natural background radiation, so a chest X-ray is equivalent to about 10 days of everyday exposure.
CT scans deliver considerably more. A CT angiogram, one of the higher-dose exams, averages around 16 mSv and can range up to 32 mSv depending on the equipment and the patient’s body size. That said, the diagnostic benefit of a necessary scan almost always outweighs the small added radiation risk.
MRI and ultrasound use no ionizing radiation whatsoever. This makes them particularly valuable for pregnant patients and children. Ultrasound is considered the safest imaging option during pregnancy, while MRI serves as the preferred alternative to CT when deeper tissue detail is needed without exposing the fetus to radiation.
What Contrast Agents Do
Some imaging exams require a contrast agent, a substance that makes certain structures show up more clearly on the images. For CT scans, this is usually an iodine-based liquid that you either drink or receive through an IV. For MRI, the contrast agent is gadolinium-based and always given intravenously.
Contrast agents improve visualization of blood vessels, tumors, infections, and inflammation that might otherwise blend into surrounding tissue. Most people tolerate them without issues, though mild reactions like warmth, a metallic taste, or brief nausea can occur. Serious allergic reactions are rare.
Gadolinium-based contrast for MRI deserves special mention. The FDA has confirmed that trace amounts of gadolinium can remain in the body after injection, with the amount depending on the specific type used. The only known health consequence linked to gadolinium retention is a rare condition affecting patients who already have kidney failure. If you have kidney problems, your doctor will factor that in before ordering a contrast-enhanced MRI.
How to Prepare
Preparation depends entirely on the type of exam and the body part being scanned. For many X-rays and basic ultrasounds, you won’t need to do anything special. For abdominal imaging, you may be asked to fast for six hours beforehand so that food in your stomach doesn’t obscure the images. CT and MRI exams involving contrast may also require fasting or blood work to check kidney function ahead of time.
For any MRI, you’ll need to remove all metal objects: jewelry, watches, hairpins, belts, and clothing with metal zippers or snaps. The MRI machine’s magnet is powerful enough to pull metal objects with dangerous force. You’ll typically change into a hospital gown. If you have metal implants, a pacemaker, or other devices in your body, let the imaging team know well in advance, as some implants are not MRI-compatible.
What Happens During the Exam
Most imaging exams are painless. For an X-ray, you’ll stand or lie in position while a technologist takes images that last only a fraction of a second per exposure. A CT scan involves lying on a table that slides through a large, doughnut-shaped machine. The scan itself often takes less than a minute, though the entire appointment including setup and contrast injection may take 15 to 30 minutes.
MRI is the longest and noisiest of the common exams. You lie inside a narrow tube while the machine produces loud knocking and buzzing sounds. Scans typically last 30 to 60 minutes, and you’ll need to stay very still. Many facilities offer earplugs or headphones with music. If you’re claustrophobic, mention this when scheduling, as mild sedation or an open MRI machine may be options.
Ultrasound is the most interactive. A technologist presses a handheld probe against your skin, using a water-based gel to help transmit sound waves. You can often see the images in real time on the screen beside you.
Who Performs and Reads the Exam
Two different professionals are involved in most imaging exams. A radiologic technologist is the person in the room with you. They position you, operate the equipment, and ensure the images are high quality. A radiologist, a physician who specializes in interpreting medical images, then reviews the results and writes a formal report. That report goes to the doctor who ordered the exam, who discusses the findings with you.
Turnaround time varies. Emergency imaging in a hospital is often read within an hour. Outpatient exams typically have results within 24 to 48 hours, though some facilities take longer. AI tools are increasingly being used to help radiologists flag abnormalities faster, particularly in CT and MRI interpretation, though a human radiologist still makes the final call.
Why Your Doctor Chooses One Exam Over Another
Your doctor selects the imaging modality based on what they’re looking for and where in the body they need to look. Bone fractures are best seen on X-rays. Soft tissue injuries in the knee or shoulder call for MRI. Acute abdominal pain in an emergency room usually warrants a CT scan because of its speed and ability to detect a wide range of problems. Pregnancy monitoring relies almost exclusively on ultrasound.
Cost and accessibility also play a role. X-rays are inexpensive and available in nearly every clinic. MRI machines are expensive to operate and may have longer wait times. For advanced imaging like CT, MRI, and PET scans, many insurance programs require documentation that the exam is medically appropriate before they’ll cover it. Your doctor consults clinical guidelines to match your symptoms with the right test, balancing diagnostic value against factors like radiation exposure and cost.