A CT scan “with and without contrast” is a two-phase imaging study where your body is scanned twice in the same session: once before any contrast dye is given, and once after. The non-contrast images capture your anatomy in its natural state, while the contrast-enhanced images light up blood vessels, organs, and abnormal tissue. Comparing the two sets of images side by side gives radiologists information that neither scan alone could provide.
Why Both Phases Matter
Contrast material works by blocking X-rays more effectively than your body’s natural tissue does. When an iodine-based dye flows through your bloodstream, it temporarily changes how organs and blood vessels appear on the scan, making them brighter and easier to distinguish from surrounding structures. That’s useful on its own, but certain diagnoses depend on seeing what a structure looks like before and after contrast reaches it.
A kidney mass, for example, might look identical to a simple fluid-filled cyst on a contrast-enhanced scan alone. But if the radiologist can compare pre-contrast and post-contrast images, a mass that “lights up” after contrast (meaning it has its own blood supply) behaves very differently from a cyst that stays dark. That distinction can be the difference between a harmless finding and something that needs a biopsy.
Calcifications, such as calcium deposits in arteries or organs, also pose a problem. Contrast dye appears bright white on CT images, and so do calcifications. Without a non-contrast baseline, the dye can mask or mimic these deposits, hiding important diagnostic clues.
Conditions That Require Both Phases
Ordering a CT with and without contrast doubles your radiation exposure compared to a single-phase scan, so doctors reserve this protocol for situations where comparing the two image sets genuinely changes the diagnosis. According to Johns Hopkins imaging guidelines, the main reasons include:
- Kidney mass vs. cyst: Determining whether a growth on the kidney is solid (potentially cancerous) or a harmless fluid-filled cyst.
- Blood in the urine (hematuria): Evaluating the kidneys, the collecting system that drains urine, and the bladder for cancer.
- Adrenal nodule characterization: Figuring out whether a small growth on the adrenal gland is a benign fat-rich nodule or a metastasis from cancer elsewhere.
- Gastrointestinal bleeding: Identifying the source of active bleeding in the bowel by seeing where contrast dye leaks out compared to the baseline scan.
- Liver mass evaluation: Distinguishing benign liver growths from malignant ones based on how they absorb contrast over time.
- Aortic stent follow-up: Checking for slow leaks (endoleaks) around a previously placed stent in the aorta.
For most routine CT scans, a single phase is sufficient. A scan for kidney stones, for instance, uses no contrast at all because stones show up clearly on their own. A scan looking for an infection or abscess typically needs only the contrast-enhanced phase. The dual protocol is specifically designed for problems where tissue behavior before and after contrast tells the story.
How Contrast Is Given
The type of contrast and how it enters your body depends on what’s being imaged. There are several routes, and some scans combine more than one.
Intravenous (IV) contrast is the most common type for a “with and without” protocol. An iodine-based dye is injected into a vein in your arm, and it travels through the bloodstream to highlight blood vessels and solid organs like the liver, kidneys, and pancreas. This is the contrast most people think of when they hear the term.
Oral contrast is a drink (barium-based or iodine-based) you swallow before the scan, typically used for abdominal and pelvic imaging. It coats the inside of the digestive tract, making the bowel easier to see and helping distinguish loops of intestine from masses or fluid collections. You may be asked to drink it 60 to 90 minutes before your scan to give it time to travel through the gut.
Rectal contrast is occasionally used for specific situations, such as suspected colon injuries, and is administered as an enema shortly before scanning.
Some abdominal scans use both IV and oral contrast together. Evaluating complications from inflammatory bowel disease or diverticulitis, for example, may call for IV contrast to show the organ tissue plus oral or rectal contrast to outline the bowel itself.
What the Scan Feels Like
If your scan includes IV contrast, you’ll have a small IV line placed in your arm. When the dye is injected, most people feel a warm flush that spreads through the body within seconds. It’s common to feel warmth in the face, chest, or pelvis, and some people briefly feel as though they’ve urinated (they haven’t). A metallic taste in the mouth is also typical. These sensations are harmless and usually fade within a minute or two.
For a dual-phase scan, you’ll lie on the CT table and pass through the scanner once before the injection. Then the technologist will inject the contrast through your IV, and you’ll pass through again (sometimes more than once, timed to catch the dye at different stages of circulation). The entire process generally takes longer than a standard single-phase CT, but the actual scanning portions are still quick, often under a minute each.
Risks of IV Contrast
Allergic-like reactions to modern iodine-based contrast are uncommon. A large single-center study of nearly 360,000 contrast injections found that allergic-type reactions occurred in about 0.32% of cases. The vast majority of those were mild, involving hives, itching, or a few sneezes. Severe reactions, which can include difficulty breathing or a dangerous drop in blood pressure, occurred in only 0.005% of injections, roughly 1 in 19,000.
If you’ve had a previous moderate or severe reaction to contrast dye, you aren’t necessarily excluded from getting it again. Your doctor can prescribe a premedication regimen, typically a steroid and an antihistamine taken by mouth starting 12 hours before the scan, to significantly reduce the chance of a repeat reaction.
Kidney function is the other main concern. Your kidneys filter the contrast out of your blood, and in people with already-reduced kidney function, the dye can sometimes cause further strain. For most people with normal kidneys, this isn’t a practical worry. If you take metformin for diabetes and have healthy kidneys, there’s no need to stop it before or after the scan. But if your kidney function is significantly impaired, your doctor may take extra precautions, including pausing metformin for 48 hours after the procedure and checking kidney labs before restarting it.
Preparing for a Dual-Phase CT
Preparation instructions vary by facility, but a few things are consistent. If oral contrast is part of your protocol, you’ll either pick up the drink ahead of time or arrive early enough to finish it before your scan window. Some centers ask you to avoid eating for a few hours beforehand, though this is less universally required than it once was.
You’ll be asked about prior contrast reactions, kidney disease, and diabetes. If your kidney function is borderline, the imaging team may check a recent blood test or order one before proceeding. If you have a known contrast allergy, make sure whoever ordered the scan knows so the premedication protocol can be started the night before.
After the Scan
For most people, there’s nothing special to do after a CT with and without contrast. The IV dye is filtered out by the kidneys within hours. Drinking extra water afterward helps move it along, and most people simply return to normal activities immediately.
If you have reduced kidney function, your care team may recommend more structured hydration, roughly one cup of water per hour for about eight hours after the scan. This helps protect the kidneys during the period when they’re clearing the dye.
Oral barium contrast passes through the digestive tract and is eliminated in your stool over the next day or two. It may cause your bowel movements to look lighter or chalky in color, which is normal and temporary.