Page 538 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
Summary
■■ X-rays are short-wavelength, high-frequency electromagnetic radiation, produced when electrons are decelerated.
■■ The intensity of an X-ray beam is the power transmitted per unit cross-sectional area.
■■ The intensity of a collimated X-ray beam decreases exponentially according to the equation I = I0 e−μx, where μ is the attenuation coefficient of the medium. μ has units of m−1 (or cm−1 or mm−1).
■■ X-ray images can be improved using image intensifiers and contrast media (such as barium or iodine).
■■ Ultrasound is a longitudinal wave with a frequency greater than 20 kHz.
■■ Ultrasound transducers use the piezo-electric effect to generate and detect ultrasound waves.
■■ The acoustic impedance Z of a material depends on its density ρ and the speed c of sound:
Z=ρc
■■ The fraction of the intensity of an ultrasound wave reflected at a boundary is given by:
Ir = (Z2−Z1)2 or Ir = Z2−Z1 2 I0 (Z2 + Z1)2 I0 Z2 + Z1
■■ To transfer a high proportion of the intensity of
an ultrasound pulse into the patient’s body, an impedance-matching gel must be used with acoustic impedance almost the same as that of the skin.
■■ In MRI scanning, spinning, precessing protons are forced to resonate using radio frequency pulses. RF radiation from relaxing protons is used to obtain diagnostic information about internal organs, particularly soft tissues.
■■ The main components of an MRI scanner are: superconducting magnet, RF transmitter coil,
RF receiver coil, set of gradient coils and computer.
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End-of-chapter questions
1 a Explain what is meant by ionising radiation and explain why it can be harmful to humans. [2] b Which of the following scans use ionising radiation? [2]
X-ray shadow imaging ultrasound A-scan ultrasound B-scan
magnetic resonance imaging CT scan
2 Calculate the minimum wavelength (in air) of X-rays produced when the accelerating potential across the
source is 20 kV. [2]
3 Explain why a gel is used between the skin and the transducer when an ultrasound scan of a fetus is taken. [2]
4 Forultrasoundoffrequency3.5MHz,theacousticimpedanceofmuscleis1.78×106kgm−2s−1, andthatof
soft tissue is 1.63 × 106 kg m−2 s−1.
Calculate the percentage of the incident ultrasound reflected at a muscle–soft tissue boundary. [3]