A Computed Tomography (CT) scan of the knee provides detailed, cross-sectional views of the joint. It uses a rotating X-ray beam and computer processing to create multiple “slice” images of the anatomy, which can then be reconstructed into a three-dimensional model. This method offers significantly more detail than a standard X-ray, particularly for visualizing complex structures. CT scans are frequently employed to examine fractures, assess bone structure, detect tumors or infections, and evaluate joint alignment, especially when other imaging, such as MRI, is unsuitable.
Preparing for the Knee CT Scan
Before the appointment, patients should receive specific instructions from their physician, especially concerning food and drink. If the scan requires an intravenous (IV) contrast dye to highlight blood vessels or certain tissues, patients are typically asked to fast for four to six hours beforehand. This preparation helps minimize the chance of adverse reactions to the contrast agent.
Upon arrival, paperwork includes reviewing medical history and signing consent forms. The patient will then be asked to remove all metal objects, including jewelry, belts, eyeglasses, and any clothing with zippers or snaps, as metal interferes with the X-ray beams and degrades image quality. Patients change into a hospital gown to ensure only soft, non-interfering material is near the scanning area.
The Step-by-Step Scanning Process
The CT procedure begins with the patient lying flat on their back on the examination table, usually entering feet first. A technologist will carefully position and stabilize the knee using pillows or straps to ensure the joint remains perfectly still throughout the scan. The table then slides into the donut-shaped machine, known as the gantry, centering the knee within the opening.
If contrast dye is required, a technologist or nurse will administer it through an IV line, usually placed in the arm or hand, shortly before the scan begins. The injection of the iodine-based dye may cause a transient sensation of warmth or flushing, and sometimes a metallic taste in the mouth. This contrast helps the computer differentiate between various soft tissues and blood vessels for a clearer final image.
Once positioned, the technologist moves to an adjacent control room to operate the scanner while maintaining verbal and visual contact with the patient. During the scan, the X-ray tube within the gantry rotates rapidly around the knee, capturing data from multiple angles. The patient will hear clicking and whirring sounds as the machine operates, and the table moves incrementally through the gantry.
The patient must remain absolutely motionless during the brief periods of X-ray exposure to prevent image blurring. The technologist gives instructions over an intercom to ensure stillness. Modern CT scans are very fast, and the actual scanning time for the knee is often less than 20 minutes. The computer then uses the collected data to construct the detailed cross-sectional images, or “slices,” of the knee joint.
After the Scan: Immediate Follow-up and Results
Immediately after the images are acquired, the technologist may perform a quick review to ensure they are clear and complete before removing the patient from the table. If IV contrast was used, the IV line is removed, and the patient may be monitored briefly for adverse reactions. Patients who received contrast are often advised to drink plenty of fluids to help the kidneys flush the dye from the body.
The patient can typically return to normal activities right after leaving the facility. The raw images are then sent to a radiologist, who carefully examines the data and generates a formal, written report. This report details the findings, noting any abnormalities or specific anatomical features.
The radiologist’s report is then electronically sent to the ordering physician who requested the CT scan. For routine scans, this process of interpretation and reporting generally takes between 12 and 48 hours, though emergency cases are prioritized. The ordering physician is responsible for communicating the final results and discussing the diagnostic implications with the patient during a follow-up appointment.
Safety Considerations
CT scans involve exposure to ionizing radiation, which is higher than a conventional X-ray because multiple images are taken. While the dose is low and the risk is very small, a doctor will only order the scan when the diagnostic benefit outweighs this risk. Patients should always inform staff if they are or might be pregnant, as radiation exposure poses a risk to a developing fetus.
The contrast dye, typically iodine-based, presents risks, mainly for individuals with pre-existing kidney issues or known allergies. The kidneys are responsible for removing the dye from the body, so impaired kidney function can be a concern. Patients with a history of allergic reactions to contrast may be pre-treated with medication to mitigate the risk before the procedure.