1. What is the purpose of the intensifying screen?
1. Intensifying screens are used to amplify the incoming x-ray beam by emitting large quantities of light photons when struck by x-rays. The use of intensifying screens will reduce patient radiation dose when compared to non-screen techniques.
Incident x-rays (remnant beam) are absorbed in the phosphor crystals → absorbed x-ray energy is converted to light energy → primarily visible light exposes the silver halide grains within the emulsion layer of film.
2. Define Luminescence.
2. Luminescence is the ability of a material to emit light in response to excitation. Luminescent materials emit light with wavelengths that are characteristic of the particular luminescent material. This results in light emissions of a characteristic colour or spectral emission. There are two types of luminescence.
Fluorescence is the initial light emitted after stimulation and is instantaneous
Phosphorescence is a delay or afterglow of fluorescence and is undesirable in x-ray imaging
3. Is the latent image formed by light or x-ray photons? Explain.
3. The latent image is formed by both light and x-rays. Approximately 99% of the latent image is formed by light photons with less than 1% formed by x-ray photons.
4. List the four qualities that a phosphor must exhibit.
A) High atomic number: increases the probability of interaction of the incident x-ray photon and the phosphor (QDE = Quantum Detection Efficiency)
B) High conversion efficiency: the ability of the phosphor to emit as much light per x-ray photon interaction as possible: this is the speed of the screen: the higher the conversion efficiency the higher the speed of the screen the less radiation dose to the patient
C) Appropriate spectral emission: the wavelength of the light emitted by the phosphor must match the sensitivity of the film to ensure maximum latent image formation
D) Minimal phosphorescence: when the phosphor continues to emit light after the initial stimulation it is called phosphorescence or screen lag/afterglow and this is undesirable as it would degrade the initial latent image. Maximum fluorescence and minimal phosphorescence are desirable.
5. What is quantum mottle?
5. Quantum mottle is caused by an insufficient quantity of photons striking the intensifying screen which results in an image that appears grainy.
• Quantum mottle is caused by an insufficient quantity of photons striking the intensifying screen which results in an image that appears grainy.
• Quantum mottle is controlled by the radiographer i.e. the mAs setting
• Quantum mottle is more commonly observed in fluoroscopy due to the low mA values.
3 | PIR: Part 1: Unit 7, Part 2: Unit 1 and 2 Worksheet
6. What happens when x-ray film and intensifying screens are mismatched?
6. When x-ray film and screens are mismatched this results in a loss in the image process efficiency and may result in an increase in patient dose.
7. List and briefly explain the two important characteristics of film/screen combinations.
7. Relative speed (RS) is the number manufacturers use to rate film/screen combinations; the higher the RS will result in reduced patient dose and decreased resolution.
Resolution (recorded detail or sharpness) are measured in line pairs per millimetre (lp/mm), line spread function (LSF) and modulation transfer function (MTF)
Line pairs per millimetre
measures the minimum size and space between objects
Line spread function
measures the ability of a film/screen system to accurately measure the boundaries of an image
Modulation transfer function
provides the best measurement of the resolving ability of a film/screen combination. It measures the information lost between the subject (patient) and the image receptor
Note: We will study these methods of evaluating resolution further in Part 2: Unit 4 Recorded Detail.
4 | PIR: Part 1: Unit 7, Part 2: Unit 1 and 2 Worksheet
8. Explain how to care and clean cassettes and intensifying screens.
8. Cassettes and screens should be cleaned regularly to remove any artefacts either on the inside or outside. A manufacturer recommended special screen cleaner must be used.
: The film is removed from the cassette, in safelight conditions, and returned to the film bin.
: Begin by cleaning the outside of the cassette (front and back) with the disinfectant solution routinely used in the department.
: Open the cassette and rest it on a work surface. Apply screen cleaner to non-sterile 2X2 sponges or cotton and work in strokes from side-to side.
: Repeat at right angles to the first series of strokes followed by the use of dry sponges to remove any excess liquid.
: Allow the cassette to completely air dry in an
9. What mAs should be used with an 800 RS system when technical factors of 75 kVp and 35 mAs produce an acceptable image with a 200 RS system?
9. 9 mAs
10. What mAs should be used with a 100 RS system when technical factors of 60 kVp and 1.5 mAs produce an acceptable image with an 800 RS system?
10. 12 mAs
5 | PIR: Part 1: Unit 7, Part 2: Unit 1 and 2 Worksheet
Part 2: Unit 1
11. List the similarities between film/screen imaging and digital imaging when creating the beam and creating the image.
11. X-rays are produced in the same manner whether the result is an analog (film/screen) or digital image. Therefore the selection of technical factors, i.e. mAs, kVp, must be considered whenever an x-ray image is being created. Due to x-ray machines having variables from one unit to the next, technologists must familiarize themselves with each x-ray machine when
creating the beam.
Another commonality between analog and digital imaging is the patient. The patient provides varying attenuation due to the tissues and organs within the collimated field as well as the pathology contribution. Other factors within the technologists control when
creating the image are beam restriction and the use of a grid.