Computed tomography scanning
- A simple X-ray image can provide useful, but limited, information about internal structures in a 2D image
- When a more comprehensive image is needed, a computerised axial tomography (CAT or CT) scan is used
- The main features of the operation of a CT scan are as follows:
- An X-ray tube rotates around the stationary patient
- A CT scanner takes X-ray images of the same slice, at many different angles
- This process is repeated, then images of successive slices are combined together
- A computer pieces the images together to build a 3D image
- This 3D image can be rotated and viewed from different angles
CAT scan diagram
CAT scans take 2D images from multiple positions to create a 3D image
Advantages & Disadvantages of CAT Scans
- Advantages of CAT scanners:
- They produce images of much higher resolution (compared to a single X-ray image)
- They can distinguish between tissues with similar attenuation coefficients
- They produce a 3D image of the body by combining the images taken in each direction
- Disadvantages of CAT scanners:
- The patient receives a much higher radiation dose than a normal X-ray
- There are possible side effects from the contrast media
Worked example
An X-ray image is taken of the skull of a patient. Another patient has a CT scan of his head.
By reference to the formation of the image in each case, suggest why the exposure to radiation differs between the two imaging techniques.
Answer:
Step 1: Describe how an image is formed from a simple X-ray scan
- The simple X-ray image involves taking a single exposure
- This produces a single 2D image
Step 2: Describe how an image is formed from a CT scan
- The CT scan requires taking several exposures of a slice from many different angles
- This is then repeated for different slices before being combined together to build a 3D image
Step 3: Suggest why the exposure to radiation differs
- The resolution provided by a CT scan means the exposure to the patient is much greater than if they had a simple X-ray scan
