Components
Filter
Placed between the x-ray source and the patient (similar to that used in plain film radiography).
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.
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 causing less attenuation of the x-ray beam. 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. A bow-tie filter, as shown in the diagram above, 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
The Collimator is placed between the filter and the patient.
1. Lowers radiation dose to patient
2. Restricts scatter from outside of desired slice
Detector Array
The original single-slice scanners had one row of detectors. Now all scanners are multi-slice and have 8-64 rows of detectors. There are generally 1000-2000 detectors in each row.
Important properties for detectors
- 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)
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)
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 efficiency (~50%)
- High stability
- Consistent sensitivity between detector elements
Ionisation chambers have been superseded by solid-state detectors and are no longer used as they are unsuitable for multislice scanners.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.
Written by radiologists, for radiologists with plenty of easy-to-follow diagrams to explain complicated concepts. An excellent resource for radiology physics revision.
Generations of CT scanner
First generation
Translate-Rotate
- The x-ray beam is picked-up by a single detector.
- The x-ray source and detector then move together (translate)
- The two then rotate together to image a different angle
- This is repeated until a single slice is scanned
- The two then move down the patient to start imaging a different slice
This method took 5 minutes per slice to scan
Second generation
Translate-Rotate
- The x-ray beam is picked-up by a row of up to 30 detectors.
- The x-ray source and detector then move together (translate)
- The two then rotate together to image a different angle
- This is repeated until a single slice is scanned
- The two then move down the patient to start imaging a different slice in the patient
This method took 5-90 seconds per slice
Third generation
Rotate-Rotate
- The x-ray beam hits a row of detectors wide enough to image the whole slice
- The two then rotate together to image a different angle
- This is repeated until a single slice is scanned then the array is moved to a different slice (axial scanning). Alternatively, the detector array is continually moved down the patient as it rotates (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
Fourth generation
Rotate-fixed
- There is a fixed complete ring of detectors
- The x-ray source rotates around to capture a slice
- 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).
- 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 a 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 of the 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
