10.5.4 X-ray Detection

• X-rays can be detected and images can be produced from three main detection methods:
  • X-ray flat panel (FTP) detectors
  • Photographic film
  • Fluoroscopic image intensification

Flat-Panel Detectors

• X-ray flat panel (FTP) detectors are the most common type of detection method used in medical facilities today
• They are made up of three layers, or 'panels':
  • The scintillator layer
  • The photodiode pixel layer
  • The electronic scanner layer

X-ray Flat Panel Detector Structure

The process of forming a digital image using an X-ray flat panel detector

• Once the X-rays arrive at the FTP detector behind the patient:
  • The electrons in the scintillator layer absorb the high-energy X-rays and emit visible photons
  • The emitted visible photons are then absorbed by photodiode pixels and trigger the release of electrons
  • The release of electrons generates a p.d. (electrical signal) which is processed and transmitted as a digital image to be stored on a computer

• FTP detectors can produce high-quality images of most solid structures in the body, such as bones and joints
  • These types of detectors are also used in most commercial uses of X-rays, such as airport security 

Photographic Film Detection

• Before digital methods, the original X-ray detectors used photographic film
  • In medicine today, however, photographic detection is rarely used

• An intensifying screen or 'cassette' is a device containing two fluorescent screens placed on either side of a double-sided X-ray film

X-ray Intensifying Cassette

In an intensifying cassette, photographic film is sandwiched between two sheets of fluorescent material 

• The X-rays expose the photographic film but the fluorescent screens emit light that exposes the film faster
• Each X-ray absorbed by the fluorescent material causes several visible light photons to be emitted
  • These visible photons contribute to the darkening of the film, allowing the image to form about 20 times faster than using X-rays alone

Structure of an Intensifying Screen

The fluorescent screens on both sides of the film significantly shorten the exposure time required to produce the X-ray image

• Using an intensifying screen allows the overall exposure time of X-rays to be shortened
• This is beneficial to the patient because
  • Reducing the exposure time reduces the ionising dose of radiation received by a patient
  • The patient does not have to be stationary for so long

The Image Intensifier

• A fluoroscopic image intensifier is a device which consists of
  • An evacuated glass tube
  • A photocathode
  • Multiple anodes
  • Two fluorescent screens, one at each end of the evacuated tube

• The operation of an image intensifier is as follows:
  • An image forms on the first fluorescent screen as incident X-rays are absorbed and re-emitted as visible photons
  • Visible photons cause electrons to be emitted from the photocathode
  • The emitted electrons are accelerated through a large p.d. (about 25 kV) towards the anodes which focus them on an output window
  • The intensified image is formed on the fluorescent viewing screen at the end of the evacuated tube 

• Often a camera is attached to the output window to allow the images to be viewed on a TV screen

Structure of a Fluoroscopic Image Intensifier

An image intensifier converts X-rays to photons using fluorescent screens and increases the brightness through the acceleration of electrons to show processes in real-time

• The final image on the fluorescent viewing screen is about 5000 times brighter compared to the initial image on the first fluorescent screen because the electrons are:
  • Focused onto a smaller area for a given power output, hence intensity increases
  • Given a large amount of energy due to the acceleration by the anodes which means several photons are produced for every electron arriving at the fluorescent viewing screen

• This method of X-ray detection is used for imaging movement
  • This means real-time images can be observed and recorded
  • For example, dynamic processes such as swallowing or blood flow in and around organs

• This method involves a higher radiation dose to the patient than in X-ray imaging involving a single exposure
  • This is because a continuous beam of X-rays is required for the duration of the procedure
  • However, if the image intensifier is used with a TV camera, the radiation dose is minimised compared to taking several images of the same region

• Previously, X-ray images were predominantly produced using photographic film
  • Now, digital methods, such as flat panel detection (FTP), are preferred

• The key advantages of FTP detectors compared with photographic detection are:

1. Flat-panel detectors are faster than film

• • This means X-ray images can be produced in real time, which allows for quicker diagnoses
  • Whereas photographic film requires time to be processed and developed

2. Flat-panel detectors are more sensitive than film

• • This means a lower dose of radiation can be administered to the patient to produce an image of the same quality compared to one produced by film

3. Flat-panel detectors produce digital images

• • Digital images can be processed quickly, as well as stored and transferred with ease