CT Overview

A CT (computed tomography) scan is a diagnostic imaging procedure that combines X-rays with computer technology to produce detailed images of the inside of the body — bones, muscles, fat, organs, and blood vessels alike. It goes well beyond what a standard X-ray can show.

How CT differs from a standard X-ray

In a standard X-ray, a single beam of energy passes through the body and a plate on the other side captures the variations left after it travels through skin, bone, muscle, and other tissue. That's useful, but it leaves a lot of internal detail unseen. In a CT scan, the X-ray beam instead rotates in a circle around the body, capturing many different views of the same structure. A computer interprets that data and assembles it into a two-dimensional image on a monitor; modern software can also reconstruct the data into three-dimensional images. CT scans are commonly used to help diagnose tumors, investigate internal bleeding, check for internal injuries, or guide a tissue or fluid biopsy.

Contrast

CT scans may be performed with or without contrast — a substance taken orally or injected through an IV that makes a particular organ or tissue easier to see clearly. Contrast exams sometimes require fasting beforehand. Patients should let their doctor know about any past reaction to contrast media or any kidney problems; a reported seafood allergy on its own is not considered a contraindication for iodinated contrast.

Preparing for a CT scan

• Pregnancy — patients who are or may be pregnant should check with their doctor before scheduling the exam.
• Clothing — a patient gown may be provided; piercings, jewelry, and valuables should be left at home.
• Contrast media — improves a radiologist's ability to spot abnormal structures and understand normal anatomy. Patients with kidney function issues should inform their radiologist in advance, since the scan may need to be done without contrast or via an alternate imaging method. A common approach — a double-contrast study — involves drinking a contrast media before the exam in addition to IV contrast; the more that's tolerated, the clearer the digestive tract appears on the images.
• Allergies — any past allergic reaction to contrast media should be reported when scheduling. IV contrast won't be given after a severe or anaphylactic reaction; milder past reactions are typically managed with pre-medication.
• Eating and drinking — no restrictions for scans without contrast. For scans with contrast, patients are usually asked not to eat for three hours beforehand, though clear liquids and prescribed medications are fine.
• Diabetes — a light meal three hours before the scan is typically recommended, and some oral diabetes medications may need to be paused for 48 hours afterward, per detailed instructions given at the time.

Risks

The radiation dose used in a CT scan is small, though patients can ask their doctor about the specific dose and risk for their situation. Patients who are pregnant or think they may be pregnant should notify their provider. Anyone who is claustrophobic or prone to anxiety should mention it ahead of time — a mild sedative can be prescribed to help.

What happens during the scan

A CT scan itself typically takes only a few minutes and generally follows this process:

• Changing into a gown if needed, with personal belongings secured in a locker and piercings/jewelry removed.
• If contrast is used, an IV line is placed for injected contrast, a liquid preparation is given to swallow for oral contrast, or in some cases contrast is given rectally.
• The patient lies on a table that slides into the scanner's circular opening.
• The technologist operates the scanner from an adjoining room, staying in constant visual and audio contact through a window, speakers, and often a call button.
• As the scanner rotates, X-rays pass through the body in short bursts, producing a clicking sound. The absorbed X-ray data is sent to a computer and reconstructed into images for the radiologist to interpret.
• Staying very still is important, and patients may be asked to hold their breath at points during the scan.
• If contrast is injected, some patients briefly notice a flushing sensation, a metallic taste, a short headache, or nausea. Breathing trouble, sweating, numbness, or heart palpitations should be reported to the technologist right away.
• Once finished, the patient is removed from the scanner and any IV line is taken out.

The scan itself is painless, though lying still for its duration can be uncomfortable, especially after a recent injury or procedure. Technologists aim to complete the scan as quickly and comfortably as possible.

After the scan

If contrast was used, patients may be monitored briefly for reactions like itching, swelling, rash, or difficulty breathing. Pain, redness, or swelling at the IV site after returning home should be reported to a doctor, since it could indicate infection. Generally, no special aftercare is needed — most patients can resume their usual diet and activities unless told otherwise.

Advances in CT technology

Why CT excels at imaging bone

• High contrast with surrounding tissue — bone's calcium content is dense and absorbs X-rays much more strongly than soft tissue, making fractures and bone abnormalities stand out clearly.
• Detailed 3D reconstruction — hundreds of X-ray images taken from different angles are combined into thin cross-sectional slices, which can be reconstructed into a 3D model for examining complex anatomy from any angle.
• Clarity on complex fractures — CT outperforms standard X-ray for fractures of the skull, spine, pelvis, face, and joints (ankle, wrist, knee, shoulder), and can reveal tiny fracture fragments and their exact alignment for surgical planning.
• Speed — most CT scans take only a few minutes, making CT the preferred imaging method for trauma patients with possible multiple injuries.