CAT Scans (OCR A Level Physics)

• 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 scans take 2D images from multiple positions to create a 3D image

• Advantages of CAT scans:
  • Produces much more detailed images (software can add colour and sharpen images, and parts of the image can be edited out)
  • Can distinguish between tissues with similar attenuation coefficients giving a higher resolution image
  • Soft tissue and bone can be imaged in a single process
  • Produces a 3D image of the body by combining the images at each direction
  • No overlapping images (for example bones obscuring organs)

• Disadvantages of CAT scans:
  • The patient receives a much higher dose than a normal X-ray
  • Possible side effects from the contrast media

X-ray

• The simple X-ray image involves taking a single exposure
• This produces a single 2D image

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
• This involves taking a much greater exposure than the simple X-ray