CT Scan imaging

CT Scans are like x-rays, but take many more images in a precise and controlled fashion that permits the reconstruction of a 3-dimensional image. CT Scans are often taken in 'slices'. The patient may be given oral constrast and IV contrast to enhance the imaging of soft tissues.

Occluding factors aren't as much of an issue with CT scans as they are with x-rays, because the CT takes multiple fan-shaped images at each point as it rotates around the body, which is enough for a computer to reconstruct both the position and density of the objects in the body that absorb the x-rays.

Conventional CT scans are slow and require the patient to be immobilized during the scan. If the chest is being imaged, the patient may be told to hold his or her breath. Alignment problems between slices can sometimes miss a small mass that lies between slices. A conventional CT scan has a slice thickness of 8 mm to 10 mm.

Higher resolution CT scans, known as HRCT, helical CT and spiral CT, are much quicker. Instead of scanning each slice with the patient bed stationary, the bed is moving continuously as the CT scanner scans around the body (i.e., in a spiral pattern). This is fast enough that the patient doesn't need to hold his/her breath. It is also much higher resolution than conventional CT, so in many cases contrast dye will not be needed. A HRCT scan has a slice thickness of 1 mm to 1.3 mm.

A key limitation of HRCT is the higher resolution can lead to false positives. At higher resolutions more anomalies can be seen, but a much greater number of them turn out to be benign.

Another limitation of CT scanning in general is an inability to view very fine details in soft tissues such as muscles or ligaments. An MRI might be more appropriate in such situations.

Patient motion during a CT scan can cause the images to be "blurry". Also, any metal artifacts in the body can cause 'streaks' in the image. Flat tumors may also be harder to image with a CT scan. Scar tissue may also show up on a CT scan.

CT scanning is slower and more expensive than x-rays and ultrasounds. The radiation exposure from a whole-body CT scan is approximately 100 times that of a chest x-ray.

CT scans are not recommended for pregnant women. The intravenous contrast dye may not be recommended for patients who are allergic to iodine or shellfish.