'A' Level Medical Option Questions - X-Rays


(a) In an X-ray tube, electrons are accelerated from rest through a pd of 72.4 kV before they hit the target anode.

(i) Calculate the kinetic energy of an electron as it reaches the anode. Give your answer to an appropriate number of significant figures.

KE gained by the electron when accelerated by the electric field = QV

1.60 x 10-19 x 72.5 x 103 = 1.16 x 10-14 J 3sf

(2 marks)

(ii) Assuming that the electron gives up all this energy to form an X-ray photon, calculate the wavelength of the photon.

E = hf = hc/λ

λ = (6.63 x 10-34 x 3.00 x 108)/1.16 x 10-14

= 1.71 x 10-11 m

(2 marks)

(b) X-rays are used in a CT scanner. Describe briefly how a CT scanner produces an image.

  • Narrow fan-shaped beam of X-rays
  • X ray generator rotated (in circular path) around patient on a ring
  • Detectors are positioned on the ring opposite the generators in an arc long enough to receive the fan of X-rays.
  • The detectors register the transmitted intensity from a single blast of X-rays
  • The transmitter and detectors are then rotated so that the next section can be scanned. This is repeated while the body slowly moves on a motorised platform. In this way a spiral scan pattern is obtained.
  • Detector output is sent to a computer which (over time) produces a series of cross sectional images that can be analysed
    Any three relevant points.

(3 marks)

(7 marks total)