AQA · A-Level · Physics

    Course version: 7408 · Version from Sept 2016

    Photographic detection with intensifying screen and fluoroscopic image…

    Full specification wording

    Image detection and enhancement (A-level only): Photographic detection with intensifying screen and fluoroscopic image intensification; reasons for using these.

    X-ray film blackens when X-ray photons interact with the emulsion, but direct exposure needs a large dose because film responds weakly to X-rays.

    Checked against the AQA 7408 specification (opens in a new tab) on .

    • Point 4 of 4
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    X-ray film blackens when X-ray photons interact with the emulsion, but direct exposure needs a large dose because film responds weakly to X-rays. An intensifying screen contains a phosphor that absorbs X-ray photons and emits many visible photons, which expose the film far more efficiently. The screen is placed in contact with the film, so one absorbed X-ray photon produces a local burst of light and a darker spot. Fluoroscopic image intensification instead converts the X-ray pattern into visible light, then into electrons, accelerates and focuses them, and produces a small bright output image that a camera or screen can display. Both methods reduce the patient dose needed for a usable image and allow real-time viewing in fluoroscopy.

    What earns the marks

    • Photographic detection uses film whose emulsion is blackened by radiation, but direct X-ray exposure is inefficient and requires a relatively large dose.
    • An intensifying screen contains a phosphor that absorbs X-ray photons and emits visible or ultraviolet photons, which expose the film more effectively.
    • The screen is used in close contact with the film so that light from each X-ray interaction spreads only a small distance, preserving detail while amplifying the signal.
    • Fluoroscopic image intensification converts the X-ray image into a visible-light image, then into an electron image that is accelerated and focused onto a small output phosphor.
    • The output image is much brighter than the original fluoroscopic screen image, so it can be viewed directly or captured by a camera, including in real time.

    Common mistakes and how to put them right

    • Stating that the intensifying screen absorbs visible light and re-emits X-rays.

      Put it right: It absorbs X-rays and emits visible or ultraviolet light.

    • Confusing image intensification with contrast enhancement.

      Put it right: Intensification increases the brightness or signal of the image, whereas contrast media change the attenuation of the subject.

    • Claiming that intensification removes the need for any X-ray dose.

      Put it right: It reduces the dose needed for a usable image but X-rays are still required to form the initial pattern.

    Exam tips

    • Describe the energy conversion sequence in each device in order, naming the input and output radiation at each stage.
    • Explain the benefit in terms of signal per unit dose and, for fluoroscopy, the ability to view the image in real time.
    • Distinguish clearly between intensifying screens used with film and image intensifiers used in fluoroscopy.

    Try the lesson

    The start of the Image detection and enhancement (A-level only): Photographic detection with intensifying screen and fluoroscopic image intensification; reasons for using these. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Why is an intensifying screen placed next to the photographic film in a radiography cassette?

    Listen

    X-ray film blackens when X-ray photons interact with the emulsion, but direct exposure needs a large dose because film responds weakly to X-rays. An intensifying screen contains a phosphor that absorbs X-ray photons and emits many visible photons, which expose the film far more efficiently. The screen is placed in contact with the film, so one absorbed X-ray photon produces a local burst of light and a darker spot. Fluoroscopic image intensification instead converts the X-ray pattern into visible light, then into electrons, accelerates and focuses them, and produces a small bright output image that a camera or screen can display. Both methods reduce the patient dose needed for a usable image and allow real-time viewing in fluoroscopy. A strong answer earns marks for these points. Photographic detection uses film whose emulsion is blackened by radiation, but direct X-ray exposure is inefficient and requires a relatively large dose. An intensifying screen contains a phosphor that absorbs X-ray photons and emits visible or ultraviolet photons, which expose the film more effectively. The screen is used in close contact with the film so that light from each X-ray interaction spreads only a small distance, preserving detail while amplifying the signal. Fluoroscopic image intensification converts the X-ray image into a visible-light image, then into an electron image that is accelerated and focused onto a small output phosphor. A common slip: Stating that the intensifying screen absorbs visible light and re-emits X-rays. Instead, it absorbs X-rays and emits visible or ultraviolet light. Exam tip: Describe the energy conversion sequence in each device in order, naming the input and output radiation at each stage.

    See it

    A top-down flowchart. X-ray photons lead to two branches. In the first branch, the phosphor in an intensifying screen absorbs X-rays, emits many visible or ultraviolet photons, and blackens the film emulsion, with the screen in close contact with the film. In the second branch, fluoroscopic image intensification converts the X-ray image to visible light, then to an electron image, then the electrons are accelerated and focused onto a small output phosphor, producing a small bright output image viewed directly or captured by a camera. Both the film result and the bright output image lead to a reduced patient dose for a usable image, and the bright output image also leads to real-time viewing in fluoroscopy.
    Two detection routes from X-ray photons: an intensifying screen with film, and fluoroscopic image intensification, both leading to a reduced patient dose.

    Key words

    Say what each word means before you turn the card.

    intensifying screen

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    fluoroscopic image intensification

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    dose reduction

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    real-time viewing

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    See every definition
    intensifying screen
    Contains a phosphor that absorbs X-ray photons and emits many visible photons, which expose the film far more efficiently.
    fluoroscopic image intensification
    Converts the X-ray pattern into visible light, then into electrons, accelerates and focuses them, and produces a small bright output image.
    dose reduction
    Both methods increase the signal produced per unit patient dose, so they reduce the dose required for a diagnostic image.
    real-time viewing
    Fluoroscopic image intensification produces a bright output image that a camera or screen can display, allowing real-time viewing in fluoroscopy.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. X-ray photons lead to two branches. In the first branch, the phosphor in an intensifying screen absorbs X-rays, emits many visible or ultraviolet photons, and blackens the film emulsion, with the screen in close contact with the film. In the second branch, fluoroscopic image intensification converts the X-ray image to visible light, then to an electron image, then the electrons are accelerated and focused onto a small output phosphor, producing a small bright output image viewed directly or captured by a camera. Both the film result and the bright output image lead to a reduced patient dose for a usable image, and the bright output image also leads to real-time viewing in fluoroscopy.

    • 1Box 1

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    • 3Box 3

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