X-ray Imaging (AQA A Level Physics)

X-ray photons can be used to treat cancerous tumours in radiotherapy. Some photons are absorbed by healthy tissue before they reach the tumour.

Photons with a range of energies are generated in an X-ray machine.

Table 1 shows the linear attenuation coefficient of brain tissue for photons of energy 100 keV and 500 keV.

Table 1

Deduce whether photons of energy 100 keV or 500 keV are better for treating a brain tumour at a depth of 11 cm.

Metal filters are used in X-ray machines to limit the damage to healthy tissues. Table 2 gives data for possible filter materials.

Table 2

Discuss whether it would be better to use aluminium or copper to filter the X-rays in part (a).

No calculations are required.

State and explain one other method used to limit exposure of healthy cells during X-ray radiotherapy.

High-energy X-rays are used in the treatment of a cancer tumour inside a patient’s body. The patient is given a series of scans before the treatment is started.

Discuss how these scans are used to help provide the best and safest treatment for the patient when using the high-energy X-rays.

Lead is commonly used as shielding when using X-rays due to its small half-value thickness.

Which statement gives the correct meaning of half-value thickness?

The half-value thickness of lead for 500 keV X-rays is 4.2 × 10⁻³ m.

Calculate the mass attenuation coefficient of lead for 500 keV X-rays.

State an appropriate unit for your answer.

density of lead = 1.1 × 10⁴ kg m⁻³

In an X-ray machine, X-rays are emitted from an emission spot on a tungsten target.

Figure 3

Figure 3 shows how a total shadow is produced in the region QR where no X-rays from any part of the emission spot can reach the photographic plate. Partial shadows are formed in regions PQ and RS where X-rays from only part of the emission spot can reach the plate.

Figure 4 shows detail of the formation of edges of the partial shadow PQ. The bottom of the emission spot is 1.0 m vertically above the plate. The horizontal distance across the beam is 1.0 mm at the bottom of the emission spot.

Figure 4

To produce a sharp image of a bone, the partial shadow in region PQ must be no more than 0.10 mm wide.

Calculate the maximum distance between a bone and the plate.

Discuss whether an X-ray image of a chest or an X-ray image of a hand is likely to be sharper when exposed to the same X-ray source.

An X-ray image is to be made of a broken bone.

The image can be formed on

• photographic film

• a flat panel (FTP) detector or

• an intensifying screen using fluoroscopic image intensification.

State and explain which one of these detection methods should be used in this situation.

Go on to discuss why the other two methods are less suitable.

Figure 2 shows an X-ray of a broken bone.

Figure 2

mean diameter of bone = 0.040 m

intensity of incident X-rays = 0.013 W m⁻²

exposure time of X-ray = 0.80 s

linear attenuation coefficient of bone = 58.3 m⁻¹

Calculate an estimate for the X-ray energy that is absorbed by the bone.

State two reasons why the estimate of energy absorption in part (b) may be greater than the actual value.