X-ray imaging

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.

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

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.

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 d 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.

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.

A patient with a suspected broken arm is going to have an X-ray image taken.

Explain the risk to the patient of exposure to X-rays.

Go on to discuss three ways by which the design and use of the X-ray equipment minimises this risk.

The blood vessel called the aorta passes through the abdomen. A second patient with a suspected fault in the wall of the aorta can be given an ultrasound scan or an X-ray of the abdomen.

Suggest, with reasons, which is the better procedure for investigating this suspected fault.

When ultrasound travels across a boundary from blood to the wall of the aorta there is a decrease in acoustic impedance across the boundary. This results in 0.0625% of the intensity of the incident ultrasound being reflected at the boundary.

Calculate the acoustic impedance of the aorta wall tissue.

acoustic impedance of blood = 1.64 × 10⁶ kg m⁻² s⁻¹