ELEC5622 Chap.6 X-ray Physics and Computed Tomography
X-ray Physics and Computed Tomography
This chapter covers X-ray physics and computed tomography. Tube voltage sets the most energetic photon, and photons are transmitted, absorbed by the photoelectric effect, or scattered. Differential attenuation forms the image. CT reconstructs thin slices from projections at many angles, and seven scanner generations trade motion, detectors and speed, with resolution and dose as the lasting compromise.
Photon energy limits are computed from the tube voltage. Expect questions that join the physics to the picture, such as why bone looks light or why a scanner design changed. A short worked example links tube voltage to the shortest wavelength and the most energetic photon in the beam.
What this chapter covers
- 01
Photon energy, wavelength and the 12.398 relation
- 02
Transmission, absorption and scattering
- 03
Photoelectric effect versus Compton scatter
- 04
Attenuation and image contrast
- 05
Tomography versus projection radiography
- 06
Seven generations of CT scanner
- 07
Spatial resolution, contrast resolution and dose
Worked example · free
Photon energy limits at 120 kV
- 1λmin = 12.398 / 120 = 0.103 Å.
- 1E = 12.398 / 0.2 = 62.0 keV.
- 162.0 keV is below the 120 keV ceiling, and 0.2 Å is longer than λmin, so the photon is possible.
Key terms
- Compton scatter
- Compton scatter is an X-ray interaction in which a photon is deflected with energy loss, adding fog rather than information.
- Photoelectric effect
- The photoelectric effect is the full absorption of a photon in ejecting an inner-shell electron, favoured in high atomic number tissue.
- Attenuation
- Attenuation is the total loss of photons from an X-ray beam by absorption and scattering as it crosses tissue.
- Spiral CT
- Spiral CT acquires data while the tube rotates and the table moves, giving one continuous volume.
- Tomographic image
- A tomographic image is a reconstructed picture of a single slab through the body rather than a projection.
X-ray Physics and Computed Tomography FAQ
Why does bone look light on an X-ray image?
Bone has high atomic number and density, so it attenuates strongly, mainly through photoelectric absorption at diagnostic energies. Fewer photons reach the detector behind it, which produces the light areas, while soft tissue and air transmit more and appear darker.
What problem did fourth-generation CT solve?
Third-generation scanners rotate the tube and detectors together and need very stable detectors to avoid ring artefacts. The fourth generation used a stationary detector ring with only the tube rotating, removing ring artefacts but using under a quarter of the detectors at any time.
Does higher CT resolution cost anything?
Yes. The lecture notes that higher-resolution imaging and more complex scan techniques raise radiation dose, while improved radiation efficiency lowers it, so resolution and dose have to be balanced for each examination.
What is the difference between projection radiography and CT?
Projection radiography records every structure along each ray on one image, so overlapping anatomy is superimposed. CT reconstructs a thin cross-sectional slice from projections taken at many angles, which removes superimposition.
Why does Compton scatter not help the image?
Its probability is roughly the same for bone and soft tissue atoms and depends mainly on photon energy, so it does not separate tissues. The deflected photons land in the wrong place and add fog instead of diagnostic contrast.
What is the difference between spatial and contrast resolution?
Spatial resolution is the ability to tell apart two small objects that sit close together. Contrast resolution is the ability to tell apart neighbouring areas whose attenuation differs only slightly. CT is strong on contrast, and its spatial resolution depends on slice thickness, detector width, focal spot and matrix size.
Exam move
Memorise E = 12.398 / λ and its λmin form and practise both directions with round voltages. Build a two-row comparison of photoelectric and Compton interactions covering energy dependence, atomic number and effect on the image. Then learn the generations as a sequence of problems and fixes rather than a list.
Finish by writing one exam-style paragraph that moves from tube voltage to interaction to image appearance to dose, since that chain links the whole chapter. Keep a list of the image-quality factors from the lecture and say, for each, whether improving it usually raises or lowers patient dose.
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