# CAT Scans(OCR A Level Physics)

Author

Katie M

Expertise

Physics

## 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

#### Exam Tip

Don’t confuse CAT scans with MRI scans.  The machines both look like large doughnuts but MRI uses magnetic fields not X-rays!

## CAT Scan vs X-ray Image

• 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)

• The patient receives a much higher dose than a normal X-ray
• 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.

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

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