AQA A Level Physics Paper 3 Section B: Medical physics June 2017 question paper 7408/3BB

Explain why the unaided defective eye cannot form a clearly focused image of the object placed 24 cm from the eye.

An object is placed 24 cm from the spectacle lens.

Calculate the distance of the image formed from the spectacle lens. Give your answer to a suitable number of significant figures.

What is the name for the position where the image is formed by the spectacle lens? Tick () the correct answer.

Draw a ray diagram to show how this spectacle lens forms an image of the object placed 24 cm from the spectacle lens. On your diagram clearly label the object, image and a principal focus of the lens. Your diagram does not have to be drawn to scale.

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

on a horizontal table and is moved into the PET scanner. The scanner has many detectors positioned in a vertical circular pattern around the patient.

State what is meant by a radioisotope.

The radionuclide used in the PET scan has a physical half-life of 110 minutes. The radionuclide is excreted from the body with a biological half-life of 185 minutes.

Show that the effective half-life of the radionuclide in the body is about 70 minutes.

Discuss what might be a suitable length of time for the patient to relax between injecting the radionuclide and moving the patient into the PET scanner.

The decay of the radionuclide results in the emission of a positron. Two of the detectors, directly opposite to each other, are triggered as they each receive a gamma photon.

Explain the process in which the gamma photons are created.

Figure 1 shows the head of a patient that is 0.2 m across, placed centrally between two of the many detectors in a PET scanner.

To determine the position where the gamma photons are produced between the detectors, the scanner measures the short interval of time t between the triggering of the first detector and the triggering of the second detector.

Discuss, for the detector positions shown in Figure 1, the range of the values of that the scanner must measure to perform a PET scan on the head. Assume that the speed of the gamma photons in the head is 3 × 10⁸ m s⁻¹.

Figure 2 shows an ECG trace for a healthy person.

Complete Figure 2 by adding a suitable unit and scale to the potential axis, and a suitable scale to the time axis.

Figure 3 shows a faulty ECG trace which was obtained for another healthy person.

Discuss three possible reasons why this faulty trace was obtained.

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