Attenuation of X-rays

Figure 5 shows a rotating-anode X-ray tube used to produce an image of a bone.

X-rays originate from a region on the rotating anode known as the focal spot.

Explain:

• one advantage of using a small focal spot

• why the focal spot should not be too small.

State and explain three methods used to limit exposure of the patient’s healthy cells to X-rays during a diagnostic procedure.

Figure 6 shows a scale diagram of a cross-section through a human bone called a femur.

A dashed line has been drawn from point A to point B through the centre of the femur. Along AB the femur has a circular cross-section with an external diameter of 27 mm. The femur has a hollow centre.

A beam of X-ray photons is incident on the femur at A in the direction AB. For these photons, the half-value thickness of bone is 15 mm.

Determine, using the scale diagram in Figure 6, the percentage of the X-ray photons incident on A that emerge at B.

Figure 6 shows an X-ray image of a chest on photographic film. Figure 7 shows a diagram of a horizontal cross-section of the chest along the dashed line in Figure 6.

Figure 6 Figure 7

Discuss the half-value thicknesses of the labelled body parts relative to each other.

In your answer you should:

• explain how the intensity of the X-rays affects the brightness of the image

• explain how the thickness of the labelled parts affects the intensity of the X-rays

• compare the relative half-value thicknesses of the labelled body parts.

The stomach is not clearly visible in Figure 6.

Explain the method used to improve the image of the stomach on X-ray photographic film.

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.

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 Question 02.2 may be greater than the actual value.

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.

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.

Discuss whether it would be better to use aluminium or copper to filter the X-rays in Question 03.1. No calculations are required.

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

Figure 3 shows the X-ray spectrum produced in a medical X-ray machine at a particular anode potential difference (pd).

In an X-ray tube, electrons collide with a tungsten target.

Explain how the continuous spectrum and the characteristic spectra are produced by these electron collisions.

Continuous spectrum

Characteristic spectra

The dashed line on Figure 4 shows the X-ray spectrum for the initial anode pd.

Sketch on Figure 4 the X-ray spectrum produced when the anode pd is increased.

In the medical X-ray machine, the X-rays produced with the initial anode pd are now passed through an aluminium filter. The dashed line on Figure 5 shows the X-ray spectrum for the initial anode pd.

Sketch on Figure 5 the X-ray spectrum of the X-rays that emerge from the filter.

In the past, doctors could only use a simple X-ray image to assess head injuries. A CT scan is now a preferred technique.

Discuss why the CT scan has replaced the simple X-ray image to assess head injuries, but a simple X-ray procedure is suitable for assessing other injuries.

In your answer, you should:

• describe the basic principles of a CT scanner

• discuss the advantage of the CT scan over a simple X-ray image for head injuries

• explain why a simple X-ray procedure is more suitable for assessing other injuries.

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? Tick (✓) the correct answer.

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.

mass attenuation coefficient =