CT equipment


Components

 

CT machine

 

Filter

Placed between the x-ray source and the patient (similar to that used in plain film radiography).

 

The x-ray beam intensity through a filter

 

1. Removes low energy (soft) x-rays that do not contribute to image formation but do increase patient dose.

2. As the low energy x-rays are removed there is a narrower spectrum of x-ray energies creating a more "monochromatic" beam. Image reconstruction is based upon the assumption of a single energy, monochromatic beam.

 

Bow-tie filter

 

3. In some scanners the filter is shaped to shape the beam e.g. "bow-tie" filter. The lateral edges of a body are thinner than the centre meaning the x-ray beam is less attenuated. A shaped filter compensates for this by attenuating the lateral edges of the beam more than the centre. These filters come in different shapes/sizes depending on the body part imaged. In the diagram above, the filter is designed for imaging the chest or abdomen. If the head was being imaged then a smaller filter would be used, to match the size of the head.

Collimator

 

Collimator of a CT machine

 

The Collimator is placed between the filter and the patient.

1. Defines slice thickness in single slice scanners

2. Lowers radiation dose to patient

3. Restricts scatter from outside of desired slice

Detector Array

A single-slice detector has one row of detectors. Multi-slice detectors have 8-64 rows. There are generally 1000-2000 detectors in each row.

Important properties

  • High detection efficiency for x-rays in CT energy range
  • High dynamic range
  • Narrow gaps between active elements (good geometrical efficiency)
  • Fast response
  • Low cost
  • Small physical size

 

Types of detectors

1. Solid state detector (SSD)

Solid state detector

 

There is a solid scintillator layer that converts the x-rays into visible light photons. The photodiode then converts the photon input into an electrical signal.

Properties:

  • High detection efficiency (~90%)
  • High geometrical efficiency (~80%)
  • Small physical size of detector elements

Most commonly used detector.

 

2. Ionisation chamber detector (no longer used)

Ionisation chamber detector

The detector array is a single vessel filled with gases of a high atomic number (Krypton / Xenon) and subdivided into separate detectors by tungsten septae.

The x-rays ionise the gas and produce a signal at the collection electrodes.

Properties:

  • Lower detection effiiciency (~50%)
  • High stability
  • Consistent sensitivity between detector elements

Superseded by solid-state detectors and no longer used. Unsuitable for multislice scanners.

Gantry

A slip-ring enables continuous rotation of the CT scanner gantry. Brushes on the rotating gantry, through contact with the stationary ring, allows power to be supplied to the gantry and the signal to be passed to the computer. Rotation times are between 0.25 - 3 seconds.

 

Download now on Kindle
Written by radiologists, for radiologists with plenty of easy-to-follow diagrams to explain complicated concepts. An excellent resource for radiology physics revision.

 

FRCR Physics Notes: Beautiful revision notes for the First FRCR Physics exam

Generations of CT scanner

 

1st generation of CT machine

 

First generation

Translate-Rotate

  1. The x-ray beam is picked-up by a single detector.
  2. The x-ray source and detector then move together (translate)
  3. The two then rotate together to image a different angle
  4. This is repeated until a single slice is scanned
  5. The two then move down the patient to start imaging a different slice

This method took 5 minutes per slice to scan

 


2nd generation of CT machine

 

Second generation

Translate-Rotate

  1. The x-ray beam is picked-up by a row of up to 30 detectors.
  2. The x-ray source and detector then move together (translate)
  3. The two then rotate together to image a different angle
  4. This is repeated until a single slice is scanned
  5. The two then move down the patient to start imaging a different slice in the patient to start imaging a different slice

This method took 5-90 seconds per slice

 


3rd generation of CT machine

 

Third generation

Rotate-Rotate

  1. The x-ray beam hits a row of detectors wide enough to image the whole slice
  2. The two then rotate together to image a different angle
  3. This is repeated until a single slice is scanned then moved (axial scanning) or they are continually moved down the patient as they rotate (spiral scanning) see Acquiring an image part 1.

This is the most commonly used method today and takes about 0.3 seconds to image a single slice

 


4th generation of CT machine

 

Fourth generation

Rotate-fixed

  1. There is a fixed complete ring of detectors
  2. The x-ray source rotates around to capture a slice
  3. Both then move down the patient to begin imaging a different slice

This is not commonly used today.

 


Electron Beam Scanner

(Sometimes described as 5th generation CT).

 

5th generation of CT machine

 

  • An electron beam is deflected by an electromagnetic field onto a fixed array of tungsten anode target underneath the patient.
  • The electromagnetic field sweeps the electron beam across the target creating hundreds of x-ray beams firing through the patient to the detector above the patient.
  • Fast scanning of 50-250 milliseconds.
  • Mainly used for certain cardiac imaging.

Σ  Summary

Components of a CT scanner:

Filter:

  • Placed between x-ray source and patient
  • Removes low energy x-rays
  • Produces more monochromatic beam
  • May be bowtie-shaped to even out attenuation once it passes through the body

Collimator:

  • Placed between filter and patient
  • Narrows beam to produce thinner slice 
  • Less scatter from outside slice
  • Lower patient dose

Detector array:

  • Solid state:
    • Most commonly used
    • Solid scintillator layer converts x-rays into light photons
  • Ionisation chamber detector (no longer used):
    • Gas filled single chamber that is ionised by x-rays passing through

Gantry:

  • Slip-ring system allows continuous rotation of the gantry

Generations of CT scanners:

  • 1st: Translate-Rotate with single detector
  • 2nd: Translate-Rotate with row of detectors
  • 3rd: Rotate-Rotate with continuous rotation of a row of detectors. Most commonly used CT type
  • 4th: Rotate-Fixed with complete ring of fixed detectors
  • 5th: Electron beam scanner used in cardiac imaging

Next page: Acquiring an image part 1


  Send us your feedback

Pin it

Get our newsletter

Please note: Your email address will never be shared with any 3rd parties. It will only be used for Radiology Cafe communications. Emails are sent less than once a month on average. Read our Privacy policy for more details.