CAT Scans (OCR A Level Physics): Revision Note

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

CAT Scan vs X-ray 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