Study Guide

Nuclear medicine imaging

PhysicsΒ· 45 min read

1. Radiopharmaceuticals: Ideal properties for imagingβ˜…β˜…β˜†β˜†β˜†β± 15 min

πŸ“˜ Definition

Radiopharmaceutical

A biologically active compound labelled with a radionuclide, designed to accumulate in specific target tissue after injection into the body. Radiation emitted from the radionuclide is detected to produce an image.

Example:

Technetium-99m (Tc-99m) is the most widely used radiopharmaceutical for bone and organ scans.

To be suitable for imaging, a radiopharmaceutical's radionuclide must meet specific criteria: it must emit penetrating radiation that can escape the body, have a half-life matched to the scan duration, and minimise patient radiation dose.

πŸ“ Worked Example

Tc-99m has a half-life of 6 hours. An initial activity of 600 MBq is prepared 12 hours before a patient scan. What is the activity when the scan is performed?

  1. 1

    First calculate the number of half-lives that have passed:

  2. 2
    n=time elapsedt1/2=126=2n = \frac{\text{time elapsed}}{t_{1/2}} = \frac{12}{6} = 2
  3. 3

    Use the radioactive decay relationship for activity:

  4. 4
    A=A0Γ—(12)nA = A_0 \times (\frac{1}{2})^n
  5. 5

    Substitute values to find the final activity:

  6. 6
    A=600Γ—(12)2=600Γ—14=150 MBqA = 600 \times (\frac{1}{2})^2 = 600 \times \frac{1}{4} = 150 \text{ MBq}

2. Gamma Camera Planar Imagingβ˜…β˜…β˜…β˜†β˜†β± 20 min

πŸ“˜ Definition

Gamma Camera

A device that detects gamma radiation emitted by an injected radiotracer to produce a 2D planar image of tracer distribution in the body.

A gamma camera has three core components: a lead collimator, a scintillation crystal, and an array of photomultiplier tubes. The collimator is a critical component that ensures spatial resolution of the image.

πŸ“ Worked Example

Explain why a lead collimator is a required component of a gamma camera.

  1. 1
    1. Gamma photons are emitted in all random directions from radiotracer molecules anywhere in the patient's body.
  2. 2
    1. Without a collimator, photons emitted from any position in the body could hit any point on the detector, resulting in a completely blurred image with no usable spatial information.
  3. 3
    1. The lead collimator has narrow holes that absorb all photons not traveling perpendicular to the detector. Only photons that travel directly from a point in the body to the corresponding point on the detector pass through the collimator.
  4. 4
    1. This preserves spatial resolution, producing a sharp image that maps the distribution of the radiotracer.

Exam tip:

CIE examiners always expect you to state that collimators absorb unwanted gamma photons, not focus them, when asked about their function.

3. Positron Emission Tomography (PET)β˜…β˜…β˜…β˜†β˜†β± 20 min

PET is a 3D functional imaging technique that uses radionuclides that decay by positron (Ξ²+) emission. When a positron is emitted, it travels a short distance through tissue before colliding with an electron, causing annihilation.

e++eβˆ’β†’2Ξ³e^+ + e^- \rightarrow 2\gamma

Annihilation produces two 511 keV gamma photons that travel in exactly opposite directions. A ring of detectors around the patient detects both photons at the same time (coincidence detection), allowing 3D reconstruction of the tracer distribution.

πŸ“ Worked Example

Why does PET produce higher sensitivity 3D images than 2D gamma camera imaging?

  1. 1
    1. Conventional gamma cameras require a lead collimator that absorbs most incoming gamma photons, resulting in low sensitivity (few detected photons per unit activity) and only 2D images.
  2. 2
    1. PET does not need a lead collimator: coincidence detection of the two oppositely traveling annihilation photons allows the line of origin of the event to be determined without blocking photons.
  3. 3
    1. This means far more photons are detected, increasing sensitivity, and the full ring of detectors allows 3D reconstruction of the entire body volume, producing high-resolution 3D images.

4. Common Pitfalls

Wrong move:

Claiming radiopharmaceuticals emit alpha or beta particles for imaging

Why:

Alpha and beta particles are absorbed quickly in body tissue, do not escape to be detected, and deliver unnecessary high radiation dose to the patient

Correct move:

Imaging radiopharmaceuticals emit gamma photons, or positrons that produce gamma photons after annihilation, which escape the body for detection

Wrong move:

Stating that the collimator focuses gamma rays like an optical lens

Why:

Collimators do not refract or focus radiation, they only transmit or absorb

Correct move:

Collimators block off-axis gamma photons via absorption in lead, only allowing perpendicular photons to reach the detector to preserve resolution

Wrong move:

Claiming PET detectors detect positrons directly from tracer decay

Why:

Positrons only travel a few millimetres in tissue before annihilation and never escape the body

Correct move:

PET detectors detect the two 511 keV gamma photons produced by electron-positron annihilation, not the positron itself

Wrong move:

Assuming a longer half-life is always better for a radiotracer

Why:

A half-life much longer than the scan duration increases total patient radiation dose unnecessarily

Correct move:

The ideal half-life is slightly longer than the total scan procedure, to balance imaging time and patient dose

5. Quick Reference Cheatsheet

Feature

Gamma Camera (Planar)

PET

Tracer emission

Gamma photons

Positrons (Ξ²+ decay)

Detection method

Collimated single detector

Coincidence detection, detector ring

Image type

2D planar

3D volumetric

Common tracer

Technetium-99m

F-18 FDG

Common use

Bone scans, organ function

Cancer staging, brain imaging

Approximate resolution

~10 mm

~4 mm

When this came up on past exams

AI-estimated based on syllabus patterns β€” cross-check with official past papers for accuracy. Use only as revision-focus signals.

  • 2022 Β· 2

    Gamma camera collimator function

  • 2023 Β· 4

    Compare PET and gamma imaging

Going deeper

What's Next

Nuclear medicine imaging is a core functional imaging technique that complements structural imaging methods like X-ray and CT, and is a frequently tested topic in CIE A-Level Physics Paper 4. Understanding the principles of tracer selection, radiation detection and image formation here also supports your understanding of broader radioactivity and radiation safety concepts. After mastering this sub-topic, you can review other medical imaging techniques and deepen your knowledge of radiation interactions relevant to the full unit. Follow the links below to explore adjacent topics.