Radiation dose, safety, and patient imaging with X-ray and CT (part 2) Flashcards
What is ionising radiation?
Ionising radiation are electromagnetic waves that have sufficient energy to ionise atoms by detaching electrons from them.
What is particle radiation (or particulate radiation)?
- Particle radiation is made up of any subatomic particles such as protons, neutrons and high-speed electrons, that are capable of causing ionization.
- Alpha and beta particles are two of the more common types of particle radiation. But electrons and protons are also types of particle radiation.
- Such particles originate from the nuclei of radioactive atoms through radioactive decay.
Explain in short how projectional radiography works.
- Projectional radiography produces 2D images by X-ray radiation.
- For the generation of these X-rays, an X-ray tube is used. The tube converts electrical input power into X-rays. The tube uses heating, which frees the electrons from the filament within the tube. Electrons are then accelerated towards the anode, which collects the electrodes. This collection of electrodes produces an electrical current known as the beam.
- The efficiency is very slow, < 1% X-rays are produced and >99% of heat is produced.
What is Bremsstrahlung radiation?
Electromagnetic radiation produced by the deceleration of a charged particle when being deflected by another charged particle. The moving particle loses kinetic energy, which is converted into radiation (i.e. photons).
When the X-ray tube generates X-rays, it is passed on to the beam that reflects on the patient. Subsequently, an X-ray photo is made. With the use of these X-rays, how is it possible that an X-ray photo is produced?
- Soft tissue such as skin and organs cannot absorbe the high energy X-rays, thus the beam passes through these organs. On the X-ray photo, these organs are depicted as black.
- Dense tissue like bones are able to absorb the energy X-ray. On the X-ray photo, these organs are depicted as white.
What determines the contrast on an X-ray photo?
Contrast: difference black and white tissue depicted on X-ray photo.
- Type of tissue (thickness, density, effective atom number)
- X-ray (energy, filtration)
- Scattering within tissue
How can X-rays that are used to generate a X-ray photo be detected?
With a digital detector behind the patient.
What types of digital detectors are there?
- Charged Coupled Device (CCD) camera
- Computed radiography (CR)
- Direct/Digital radiography (DR)
How does the charged-coupled device camera work as a digital detector for X-rays?
Indirect conversion of x-ray photons into an electric charge (indirect because the x-ray photons are first converted into light via a scintillating screen).
How does computed radiography (CR) with the use of phosphor plates work as a digital detector for X-rays?
CR uses phosphor plates that are exposed to x-rays. Here, an elecronic charge is stored at the location of the absorbed x-ray. The charges are stored in ‘traps’ and are proportional to the amount of incident x-rays. This image is read by a CR reader by a red precision laser beam that discharges the traps causing stimulation emission of blue light. The light is collected by light guides that direct the light into photo-multiplier tubes. The signal is amplified, digitized and processed for display.
How does digital radiography (DR) work as a digital detector of X-rays?
DR uses X-ray-sensitive plates to directly capture X-ray data. Instead of X-ray film, DR uses a digital image capture device.
Flat panel detectors (FPDs) are the most common kind of direct digital detectors for X-ray imaging. FPDs are classified as:
- indirect FPDs
- direct FPDs
Explain how an indirect FPD works in regard to the detection and conversion of X-rays.
Indirect FPDs uses amorphous silicon (a-Si) in combination with a scintillator (outer layer of detector) made from caesium iodide (CsI) or gadolinium oxysulfide (Gd2O2S). This combination of a-Si and a scintillator converts X-rays to light. The light is channeled through the a-Si photodiode layer where it is converted to a digital output signal. This signal is then read out by thin film transistors (TFTs) or fiber-coupled CCDs.
Note: because of this conversion, the a-Si detector is considered an indirect imaging device.
Flat panel detectors (FPDs) are the most common kind of direct digital detectors for X-ray imaging. FPDs are classified as:
- indirect FPDs
- direct FPDs
Explain how a direct FPD works in regard to the detection and conversion of X-rays.
Direct FPD uses amorphous selenium (a-Se). Here, X-ray photons are converted directly into charge. The outer layer of the direct FPD is typically a high-voltage bias electrode. X-ray photons create electron-hole pairs in a-Se and the transit of these electrons and holes depends on the potential of the bias voltage charge. As the holes are replaced with electrons, the resultant charge pattern in the selenium layer is read out by a TFT array.
What happens to the contrast and dose when there is an increase in:
1. X-ray tube voltage (kV)
2. milliampere-seconds (mAs) and tube current (mA) (or so called number of pulses)
3. filter
4. size field
5. posture
6. grating
- X-ray tube voltage (kV) → contrast and dose decrease (due to increased scatter and less exposure needed)
- milliampere-seconds (mAs) and tube current (mA) (or so called number of pulses) → dose increases and no change to contrast
- filter → contrast and dose decrease
- size field → decrease in contrast and increase in dose
- posture → decrease in contrast and increase in dose
- grating → contrast and dose increase
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