X-radiography

Question 1

Describe how X-radiography works

Answer 1

• X-rays produced in an X-ray tube
• Beam passes through body, some X-rays interact via CS and the PE
• This results in an attenuation of the beam
• Different tissues have different attenuation coefficients, which depend on: gamma energy, density of tissue, atomic number Z of tissue
• Each point on the projected image contains information in the form of a line integral of the attenuation coefficient through the object. The full image reveals transverse variations of this integral in two dimensions only. It contains no depth information along the path of the radiation.
• An anti-scatter grid in front of the detector filters scattered photons. Note a scattered “false parallel” photon leading to the wrong line of sight is possible.
• The radiographic image is formed by the interaction of X-ray photons with a photon detector (scintillator or semiconductor)
• Only primary photons are useful. Making use of information from secondary photons is an area of research.

Question 2

Give the five principal densities and their order of density

Answer 2

• Appearance from black to bright white: air/gas, fat, soft tissue, bone, contrast materials

Question 3

Why is X-radiography good for skeletal details but not for tumours?

Answer 3

• Because no clear difference in Z between tumourous / non-tumourous tissue
• But there is clear difference between bone and non-bone

Question 4

How does a semiconductor detector work?

Answer 4

Direct detectors (semiconductor detectors) use a detector screen containing silicon or germanium doped with lithium.

Ionizing radiation produces free electrons and holes. A number of electrons are transferred from the valence band to the conduction band, and an equal number of holes are created in the valence band. The number of electron-hole pairs is proportional to the energy of the radiation to the semiconductor.

Under the influence of an electric field, electrons and holes travel to the electrodes, where they result in a pulse that can be measured in an outer circuit

Question 5

How does a scintillation detector work?

Answer 5

X-rays –> visible photons –> electrons –> signal

Question 6

How is contrast measured for the analysis of image quality?

Answer 6

difference between absorbed energy inside and outside target image, divided by that inside target image

Question 7

How is unsharpness measured for the analysis of image quality?

Answer 7

This is a measure of blurring of the image

One way to express is to use FWHM of point or line spread function. Lower FWHM corresponds to sharper image.

Question 8

What affects the noise of an X-radiography image?

Answer 8

• Statistical fluctuations in number of X-ray photons detected (quantum noise)
• Fluctuations due to image receptor and display system