22.8.1 Medical Tracer

What is a Medical Tracer?

A medical tracer (or radiotracer) is a radioactive isotope (radioisotope) that is introduced into the body — either by injection, ingestion, or inhalation — to monitor the function of specific organs, track blood flow, or locate blockages and abnormalities. The emitted radiation is detected by instruments placed outside the body, allowing doctors to obtain diagnostic information without surgery.

---

Properties Required of a Good Medical Tracer

For a radioisotope to be suitable as a medical tracer, it must satisfy the following conditions:

1. Gamma emitter — The tracer must emit gamma ($\gamma$) rays, which are highly penetrating and can pass through body tissues to reach external detectors. Alpha ($\alpha$) and beta ($\beta$) particles are absorbed by surrounding tissue, causing unnecessary radiation damage and cannot be detected externally.

2. Short half-life — The half-life must be long enough for the diagnostic procedure to be completed, but short enough that the tracer decays quickly after use, minimising the long-term radiation dose to the patient. Typical half-lives range from a few hours to a few days.

3. Chemically compatible — The tracer must be attachable to a biological molecule that is naturally taken up by the target organ, so it concentrates where imaging is needed.

4. Low toxicity — The chemical form of the tracer must be non-toxic to the patient.

---

Common Medical Tracers

Technetium-99m (${}^{99m}\text{Tc}$)

Technetium-99m is the most widely used radioisotope in nuclear medicine. The 'm' stands for metastable, meaning the nucleus exists in an excited energy state and releases a gamma photon to reach a lower energy state.

Its 6-hour half-life is ideal: long enough for imaging procedures, short enough to minimise patient dose. It can be chemically bound to many different biological molecules to target specific organs.

Iodine-131 (${}^{131}\text{I}$)

The thyroid gland naturally absorbs iodine from the bloodstream. When ${}^{131}\text{I}$ is ingested, it concentrates in the thyroid. By measuring the rate and distribution of iodine uptake, doctors can diagnose:

• Hyperthyroidism (overactive thyroid — absorbs iodine too rapidly)
• Hypothyroidism (underactive thyroid — absorbs iodine too slowly)
• Thyroid cancer

Sodium-24 (${}^{24}\text{Na}$)

Sodium-24 is injected into the bloodstream to act as a tracer for blood flow. It can locate obstructions, blockages, or leaks in the circulatory system by detecting where the tracer fails to flow normally.

---

Detection of Gamma Rays Outside the Body (SLO P-12-F-42)

Once the tracer is inside the body and emitting gamma rays, the radiation must be detected externally. This is achieved using a gamma camera (also called a scintillation camera):

How a Gamma Camera Works

1. The patient is injected with the gamma-emitting tracer, which concentrates in the target organ.
2. Gamma rays emitted from inside the body pass through the tissues (due to their high penetrating power).
3. The gamma rays strike a scintillator crystal (typically sodium iodide, NaI) in the gamma camera.
4. Each gamma photon causes the crystal to emit a tiny flash of visible light (scintillation).
5. Photomultiplier tubes (PMTs) behind the crystal detect these light flashes and convert them into electrical signals.
6. A computer processes the signals to produce a 2D image (scintigram) showing the distribution of the tracer within the organ.

Areas of high tracer concentration appear as 'hot spots' (indicating high metabolic activity or blood flow), while areas of low concentration appear as 'cold spots' (indicating blockage or reduced function).

Why Gamma Rays Are Ideal for External Detection

• High penetrating power: Gamma rays pass through several centimetres of tissue without significant absorption.
• Externally detectable: Unlike alpha or beta particles, gamma rays reach the detector outside the body.
• Minimal tissue damage: Low-energy gamma rays (as used in tracers) cause less ionisation damage than alpha or beta radiation.

---

Tracer vs. Radiotherapy