24. Medical physics
A section of Physics, 9702
Listing 10 of 162 questions
A parallel beam of ultrasound is incident normally on the surface of a layer of fat of thickness 1.1 cm, as shown in . I1 I2 I4 I3 1.1 cm muscle fat ultrasound beam For the ultrasound, I1 is the intensity just after entering the surface of the fat layer, I2 is the intensity incident on the fat-muscle boundary, I3 is the intensity reflected from the fat-muscle boundary, I4 is the intensity received back at the surface of the fat layer. Some data for the fat are given in . specific acoustic impedance Z 1.4 × 106 kg m–2 s–1 density ρ 940 kg m–3 absorption coefficient μ 48 m–1 Calculate the time interval between a short pulse of ultrasound initially entering the layer of fat and then returning back to the surface of the fat layer. time = s Calculate the ratio I2 I1 . ratio = Intensity I4 is 0.33% of intensity I1. Determine the ratio I3 I2 . ratio = The specific acoustic impedance of the muscle is greater than that of the fat. State the effect, if any, on the value of the ratio I3 I2 of an increase in the difference between the specific acoustic impedance of the muscle and that of the fat.
9702_m16_qp_42
THEORY
2016
Paper 4, Variant 2
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Ultrasound is used to produce diagnostic information about internal body structures. Explain how ultrasound waves are detected. An alternating voltage V varies with time t according to V = Vo sin ω t. The voltage is applied to an ultrasound probe. The root-mean-square (r.m.s.) voltage is 66 V. The frequency of the ultrasound generated by the probe is 4.3 MHz. Determine the values of Vo Vo = V ω. ω = rad s–1 Table 9.1 contains information about air and soft tissue. Table 9.1 density / kg m–3 speed of ultrasound / m s–1 specific acoustic impedance / air 1.30 4.3 × 102 soft tissue 1.7 × 106 Determine the unit for the specific acoustic impedance values shown in Table 9.1. Calculate the density of soft tissue. density = kg m–3 Use data from Table 9.1 to explain why ultrasound cannot be used to produce an image inside an air-filled cavity such as the lungs.
9702_m23_qp_42
THEORY
2023
Paper 4, Variant 2
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State what is meant by the specific acoustic impedance of a medium. The specific acoustic impedances Z of some media are given in . Z / kg m–2 s–1 air gel soft tissue bone 4.3 × 102 1.5 × 106 1.6 × 106 7.0 × 106 The density of a sample of bone is 1.7 × 103 kg m–3. Determine the wavelength, in mm, of ultrasound of frequency 9.0 × 105 Hz in the bone. wavelength = mm Ultrasound of intensity I is incident normally on the boundary between two media of specific acoustic impedances Z1 and Z2, as shown in . incident intensity I Z 1 Z 2 reflected intensity IR The intensity of the ultrasound reflected from the boundary is IR. The ratio IR I is given by the expression IR I = (Z1 – Z2)2 (Z1 + Z2)2 . By making reference to the data for air, gel and soft tissue, explain quantitatively why, during medical diagnosis using ultrasound, a gel is usually put on the skin.
9702_s15_qp_42
THEORY
2015
Paper 4, Variant 2
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State what is meant by the specific acoustic impedance of a medium. The intensity reflection coefficient α is given by the expression α = (Z2 − Z1)2 (Z2 + Z1)2 . Explain the meanings of the symbols in this expression. α : Z2 and Z1: A parallel beam of ultrasound has intensity I0 as it enters a muscle. The beam passes through a thickness of 3.4 cm of muscle before being incident on the boundary with a bone, as shown in . muscle bone intensity I0 entering muscle intensity IT entering bone 3.4 cm The intensity of the ultrasound beam as it passes into the bone is IT.
9702_s16_qp_41
THEORY
2016
Paper 4, Variant 1
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State what is meant by the specific acoustic impedance of a medium. The intensity reflection coefficient α is given by the expression α = (Z2 − Z1)2 (Z2 + Z1)2 . Explain the meanings of the symbols in this expression. α : Z2 and Z1: A parallel beam of ultrasound has intensity I0 as it enters a muscle. The beam passes through a thickness of 3.4 cm of muscle before being incident on the boundary with a bone, as shown in . muscle bone intensity I0 entering muscle intensity IT entering bone 3.4 cm The intensity of the ultrasound beam as it passes into the bone is IT.
9702_s16_qp_43
THEORY
2016
Paper 4, Variant 3
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Explain the main principles behind the use of ultrasound to obtain diagnostic information about internal body structures. A parallel beam of ultrasound has intensity I0 as it enters a muscle of thickness 4.6 cm, as illustrated in . 4.6 cm muscle beam of ultrasound I0 IT The intensity of the beam just before it leaves the muscle is IT. The ratio I0 / IT is found to be 2.9. Calculate the linear attenuation coefficient μ of the ultrasound in the layer of muscle. μ = cm–1
9702_s17_qp_42
THEORY
2017
Paper 4, Variant 2
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Questions Discovered
162