(b) gamma camera; components – collimator, scintillator, photomultiplier…

The idea

A gamma camera images a patient after a gamma-emitting tracer is administered. The collimator is a lead block with parallel holes that admits only near-parallel gamma rays, so each point on the detector maps to a known direction. Gamma photons strike a large scintillator crystal, commonly sodium iodide doped with thallium, producing many visible-light photons. These reach an array of photomultiplier tubes; each tube's photocathode releases electrons by the photoelectric effect, and dynodes multiply the signal by secondary emission. The computer compares pulse heights across tubes to calculate the interaction position, building a two-dimensional image of tracer distribution. The display shows counts as brightness or colour, so regions of high uptake appear bright. The collimator limits sensitivity but preserves spatial resolution.

What earns the marks

• The collimator is a lead grid with parallel channels that absorbs oblique gamma rays so only near-parallel rays reach the scintillator, defining the line of sight.
• The scintillator, often NaI(Tl), absorbs a gamma photon and emits a burst of visible-light photons whose number is proportional to the deposited energy.
• Photomultiplier tubes convert light into an electrical pulse: the photocathode emits electrons, and dynodes produce secondary emission, multiplying the signal.
• The computer uses the relative pulse sizes from several photomultiplier tubes to locate each scintillation event and accumulates counts to form the image.
• The display presents the count distribution as brightness or colour, allowing regions of high and low tracer uptake to be distinguished.

Common mistakes and how to put them right

• Thinking the collimator focuses gamma rays like a lens.

  Put it right: It does not refract or focus, it absorbs oblique rays so only near-parallel rays pass.

• Confusing the scintillator with the photomultiplier tube.

  Put it right: The scintillator converts gamma photons to visible light, while the photomultiplier tube converts light to an electrical pulse.

• Believing the gamma camera detects positrons or emits radiation.

  Put it right: It passively detects gamma photons emitted from within the patient.

• Assuming the computer produces a three-dimensional image.

  Put it right: A standard gamma camera produces a two-dimensional projection image.

Exam tips

• Link each component to its function in one sentence: collimator selects direction, scintillator converts gamma to light, photomultiplier tubes amplify and convert to electrical signal, computer locates events, display shows the image.
• Use the correct order of energy conversion: gamma photon to visible photons to electrons to electrical pulse.
• When explaining image formation, state that the computer uses the relative signals from several photomultiplier tubes to calculate the position of each event.