24. Medical physics
A section of Physics, 9702
Listing 10 of 162 questions
10 Explain the main principles behind the use of ultrasound to obtain diagnostic information about internal body structures. State and explain one advantage of the use of high frequency ultrasound as compared with low frequency ultrasound for medical diagnosis. The absorption coefficient for ultrasound in muscle is 23 m–1. A parallel beam of ultrasound is passed through a muscle of thickness 6.4 cm. Calculate the ratio intensity of transmitted beam intensity of incident beam . ratio = An ultrasound transmitter emits a pulse. Suggest why, when the signal from the pulse is processed, any signal received later at the detector is usually amplified more than that received at an earlier time.
9702_w13_qp_42
THEORY
2013
Paper 4, Variant 2
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Ultrasound may be used to obtain information about internal body structures. Suggest why the ultrasound from the transducer is pulsed. State what is meant by specific acoustic impedance. A parallel beam of ultrasound of intensity I0 is incident normally on the boundary between two media, as shown in . incident beam intensity I0 transmitted beam intensity IT specific acoustic impedance Z1 specific acoustic impedance Z2 The media have specific acoustic impedances Z1 and Z2. The intensity of the ultrasound beam transmitted across the boundary is IT. Explain the significance of the magnitudes of Z1 and of Z2 on the ratio IT / I0.
9702_w16_qp_42
THEORY
2016
Paper 4, Variant 2
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State what is meant by the specific acoustic impedance of a medium. The density of a sample of bone is 1.8 g cm–3 and the speed of ultrasound in the bone is 4.1 × 103 m s–1. Calculate the specific acoustic impedance ZB of the sample of bone. ZB = kg m–2 s–1 A parallel beam of ultrasound passes normally through a layer of fat and of muscle, as illustrated in . beam of ultrasound transmitted beam of ultrasound fat muscle 0.45 cm 2.1 cm (not to scale) The fat has thickness 0.45 cm and the muscle has thickness 2.1 cm. Data for fat and for muscle are given in . specific acoustic impedance Z / 106 kg m–2 s–1 linear attenuation coefficient μ / cm–1 fat muscle 1.3 1.7 0.24 0.23 The intensity reflection coefficient α at a boundary between two media of specific acoustic impedances Z1 and Z2 is given by the expression α = ( ) ( ) Z Z Z Z - + . Calculate the fraction of the intensity of the ultrasound that is transmitted through the boundary between the fat and the muscle. fraction transmitted = State what is meant by attenuation of an ultrasound wave. Data for linear attenuation coefficients are given in . Determine the ratio intensity of ultrasound transmitted through the medium intensity of ultrasound entering the medium for: 1. the layer of fat of thickness 0.45 cm ratio = 2. the layer of muscle of thickness 2.1 cm. ratio = Use your answers in and to determine the fraction of the intensity entering the layer of fat that is transmitted through the layer of muscle. fraction transmitted =
9702_w19_qp_42
THEORY
2019
Paper 4, Variant 2
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10 Briefly explain the principles of CT scanning. A simple section through a body consists of four voxels, as illustrated in . section directions of viewing An X-ray image of the section is obtained by viewing along each of the directions shown in . The detector readings for each direction of viewing are summed to give the pattern of readings shown in . For any one direction, the total of the detector readings is 16. For the pattern of readings of , state the magnitude of the background reading. background reading = On , mark the pattern of pixels for the four-voxel section.
9702_s10_qp_41
THEORY
2010
Paper 4, Variant 1
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Questions Discovered
162