CAIE A-Level Physics 24 Medical Physics

Practise analysing diagnostic ultrasound, X-ray imaging, CT and PET using their physical principles, calculations, image formation and clinical trade-offs.

Exam points

explain and calculate ultrasound generation, reflection, impedance and attenuation in medical diagnosis
explain X-ray production, image contrast, attenuation and minimum-wavelength calculations
interpret CT image construction and compare its diagnostic trade-offs with single X-ray imaging
explain tracer-based PET from ?? decay through annihilation and opposing gamma detection
calculate annihilation photon energy and interpret detector data as tissue images

Covered syllabus points

24.1 Production and use of ultrasound 24.2 Production and use of X-rays 24.3 PET scanning

[Maximum number: 7]

Piezo-electric transducers are used for the generation of ultrasonic waves.

(a)

State one other use, apart from in ultrasound, of piezo-electric transducers.

[ 1 ]

(b)

Explain the main principles behind the use of ultrasound to obtain diagnostic information about internal body structures.

[ 6 ]

[Maximum number: 6]

(a)

Fig. 8.1 shows a tube in which X-rays are produced at a metal target.

Fig. 8.1

Particles are accelerated from the filament to the target by a constant high voltage applied across the terminals X and Y .

[ 2 ]

(i)

State the name of the particles.

[ 1 ]

(ii)

On Fig. 8.1, use + and - signs to label terminals X and Y to indicate the polarity of the high voltage.

[ 1 ]

(b)

For an accelerating voltage of 32 kV in Fig. 8.1, determine:

[ 1 ]

(i)

the maximum energy, in MeV , of an X -ray photon produced at the target

maximum photon energy =MeV [1]

[ 1 ]

(c)

Explain why X-rays can be used to produce images of internal body structures that have good contrast.

[ 3 ]

[Maximum number: 5]

Fluorine-18 $\left({ }_{9}^{18} \mathrm{~F}\right)$ is a radioactive nuclide that is used as a tracer in positron emission tomography (PET scanning). Fluorine-18 decays to a nuclide of oxygen ( O ) according to

{ }_{9}^{18} \mathrm{~F} \longrightarrow{ }_{P}^{Q} \mathrm{X}+{ }_{8}^{R} \mathrm{O} \text {. }

(a)

State what is meant by a tracer.

[ 1 ]

(b)

Explain how the radioactive decay of fluorine-18 results in the emission from the body of the gamma-ray photons that are detected during a PET scan.

[ 2 ]

(c)

Explain how the detection of the gamma-ray photons is used to produce an image of the tissue being examined.

[ 2 ]

[Maximum number: 8]

(a)

Explain the principles of the generation of ultrasound waves for use in medical diagnosis.

[ 4 ]

(b)

The linear attenuation (absorption) coefficient for a parallel beam of ultrasound waves in air is $1.2 \mathrm{~cm}^{-1}$ .

The parallel beam passes through a layer of air of thickness 3.5 cm .
Calculate the ratio, in dB,
intensity of beam after passing through the layer of air intensity of beam entering the layer of air
ratio = dB

[ 4 ]

$I=I_{0} \mathrm{e}^{-\mu x}$

C1

$\begin{aligned} I / I_{0} =\mathrm{e}^{-1.2 \times 3.5} =0.015 \end{aligned}$

C1

ratio $/ \mathrm{dB}=-10 \lg (1 / 0.015)$ or $10 \lg (0.015)$

C1

$=-18 \mathrm{~dB}$

A1

All question bank results loaded

Covered syllabus points

24. Medical physics question 1

Question (a)

Question (b)

24. Medical physics question 2

Question (a)

Question (i)

Question (ii)

Question (b)

Question (i)

Question (c)

24. Medical physics question 3

Question (a)

Question (b)

Question (c)

24. Medical physics question 4

Question (a)

Question (b)