AQA · A-Level · Physics

    Course version: 7408 · Version from Sept 2016

    Rotating-anode X-ray tube; methods of controlling the beam intensity, the…

    Full specification wording

    The physics of diagnostic X-rays(A-level only): Rotating-anode X-ray tube; methods of controlling the beam intensity, the photon energy, the image sharpness and contrast, and the patient dose.

    A rotating-anode X-ray tube spins a bevelled metal target so the electron beam strikes a changing area, spreading thermal energy and allowing a higher beam current without melting the anode.

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

    • Point 2 of 2
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    A rotating-anode X-ray tube spins a bevelled metal target so the electron beam strikes a changing area, spreading thermal energy and allowing a higher beam current without melting the anode. Beam intensity is controlled by the tube current, which sets the number of electrons per second and hence the number of X-ray photons. Photon energy is controlled by the tube voltage, since hf_max = eV. Image sharpness improves when the focal spot is small and the object is close to the detector, reducing geometric unsharpness. Contrast depends on differences in attenuation between tissues, which is influenced by photon energy and the use of contrast media. Patient dose rises with tube current, exposure time and tube voltage, so the beam is collimated and exposure is minimised.

    What earns the marks

    • The anode rotates so the electron beam strikes a moving area, preventing local overheating and allowing a higher tube current and continuous operation.
    • Tube current controls beam intensity: a larger current means more electrons per second and therefore more X-ray photons per second.
    • Tube voltage controls photon energy: increasing V increases hf_max = eV and shifts the spectrum to higher energies.
    • Image sharpness is improved by a small focal spot and by reducing the object-to-detector distance, which reduces geometric blurring.
    • Contrast depends on differential attenuation; lower-energy photons generally increase contrast between soft tissues but also increase patient dose.
    • Patient dose is reduced by collimating the beam, minimising exposure time, using appropriate filtration and avoiding unnecessary repeat exposures.

    Common mistakes and how to put them right

    • Saying tube current controls photon energy.

      Put it right: Tube current controls intensity, while tube voltage controls photon energy.

    • Ignoring the effect of focal spot size on sharpness.

      Put it right: A smaller focal spot reduces geometric unsharpness and improves sharpness.

    • Assuming higher photon energy always improves contrast.

      Put it right: Higher energy can reduce contrast because attenuation differences between tissues become smaller.

    Exam tips

    • Link each control to a specific physical quantity: current to intensity, voltage to photon energy, collimation and time to dose.
    • When discussing sharpness, refer to focal spot size and distances rather than vague statements about image quality.
    • For contrast, explain differential attenuation and mention how photon energy or contrast media change it.

    Try the lesson

    The start of the The physics of diagnostic X-rays(A-level only): Rotating-anode X-ray tube; methods of controlling the beam intensity, the photon energy, the image sharpness and contrast, and the patient dose. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Why is the anode in a diagnostic X-ray tube rotated during operation?

    Listen

    A rotating-anode X-ray tube spins a bevelled metal target so the electron beam strikes a changing area, spreading thermal energy and allowing a higher beam current without melting the anode. Beam intensity is controlled by the tube current, which sets the number of electrons per second and hence the number of X-ray photons. Photon energy is controlled by the tube voltage, since hf_max = eV. Image sharpness improves when the focal spot is small and the object is close to the detector, reducing geometric unsharpness. Contrast depends on differences in attenuation between tissues, which is influenced by photon energy and the use of contrast media. Patient dose rises with tube current, exposure time and tube voltage, so the beam is collimated and exposure is minimised. A strong answer earns marks for these points. The anode rotates so the electron beam strikes a moving area, preventing local overheating and allowing a higher tube current and continuous operation. Tube current controls beam intensity: a larger current means more electrons per second and therefore more X-ray photons per second. Tube voltage controls photon energy: increasing V increases hf_max = eV and shifts the spectrum to higher energies. Image sharpness is improved by a small focal spot and by reducing the object-to-detector distance, which reduces geometric blurring. A common slip: Saying tube current controls photon energy. Instead, tube current controls intensity, while tube voltage controls photon energy. Exam tip: Link each control to a specific physical quantity: current to intensity, voltage to photon energy, collimation and time to dose.

    See it

    A top-down flowchart. The rotating anode, where the electron beam strikes a changing area, leads to spreading of thermal energy, which allows a higher tube current without melting the anode. Tube current sets the number of electrons per second, which gives beam intensity in X-ray photons per second. Tube voltage leads to photon energy through hf_max = eV. A small focal spot with the object close to the detector leads to image sharpness with less geometric unsharpness. Differences in attenuation between tissues lead to contrast, influenced by photon energy and contrast media. Collimation and minimised exposure lead to patient dose. Both tube current and tube voltage also lead to patient dose.
    It shows how the rotating anode, tube current, tube voltage, focal spot and object distance, differential attenuation, and collimation and exposure time relate to beam intensity, photon energy, sharpness, contrast and patient dose.

    Key words

    Say what each word means before you turn the card.

    Thermal management in X-ray tubes

    Try answering aloud or in your head before turning the card.

    Control of beam intensity and photon energy

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    Image quality: sharpness and contrast

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    Radiation protection and patient dose

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    See every definition
    Thermal management in X-ray tubes
    The anode rotates so the electron beam strikes a moving area, preventing local overheating and allowing a higher tube current and continuous operation.
    Control of beam intensity and photon energy
    Tube current controls beam intensity, while tube voltage controls photon energy, since increasing V increases hf_max = eV.
    Image quality: sharpness and contrast
    Sharpness is improved by a small focal spot and reduced object-to-detector distance, while contrast depends on differential attenuation.
    Radiation protection and patient dose
    Dose is reduced by collimating the beam, minimising exposure time, using appropriate filtration and avoiding unnecessary repeat exposures.

    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. The rotating anode, where the electron beam strikes a changing area, leads to spreading of thermal energy, which allows a higher tube current without melting the anode. Tube current sets the number of electrons per second, which gives beam intensity in X-ray photons per second. Tube voltage leads to photon energy through hf_max = eV. A small focal spot with the object close to the detector leads to image sharpness with less geometric unsharpness. Differences in attenuation between tissues lead to contrast, influenced by photon energy and contrast media. Collimation and minimised exposure lead to patient dose. Both tube current and tube voltage also lead to patient dose.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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