understand that a tracer is a substance containing radioactive nuclei that can be introduced into the body and is then absorbed by the tissue being studied

1. Introduction to X‑rays

X‑rays are high‑energy electromagnetic radiation produced when fast electrons are decelerated or when inner‑shell electrons in atoms are displaced. Their short wavelength (\$\lambda \approx 0.01\text{–}10\ \text{nm}\$) gives them the ability to penetrate matter, making them valuable in both diagnostic and therapeutic medicine.

1.1 Key properties

• Frequency: \$f \approx 3\times10^{16}\text{–}3\times10^{19}\ \text{Hz}\$
• Energy of a photon: \$E = h\nu\$, where \$h = 6.626\times10^{-34}\ \text{J·s}\$
• Wavelength–energy relation: \$\lambda = \frac{hc}{E}\$

2. Production of X‑rays

2.1 Bremsstrahlung (braking radiation)

When high‑speed electrons are abruptly decelerated upon striking a metal target, their kinetic energy is converted into a continuous spectrum of X‑ray photons.

• Electron kinetic energy: \$E_k = eV\$, where \$V\$ is the accelerating voltage.
• Maximum photon energy equals \$Ek\$; thus the minimum wavelength is \$\lambda{\min} = \frac{hc}{eV}\$

2.2 Characteristic X‑rays

These arise when an incident electron ejects an inner‑shell electron from the target atom. An outer‑shell electron then falls into the vacancy, emitting a photon with energy equal to the difference between the two energy levels.

2.3 X‑ray tube components

1. Electron source (cathode) – heated filament emits electrons.
2. Accelerating voltage – creates a potential difference (typically 30–150 kV).
3. Target (anode) – usually tungsten for its high melting point.
4. Window – thin beryllium allows X‑rays to exit.

3. Uses of X‑rays

3.1 Diagnostic imaging

Radiography, computed tomography (CT) and fluoroscopy rely on differential absorption of X‑rays by tissues.

• Bone absorbs more X‑rays → appears white.
• Soft tissue absorbs less → appears gray.

3.2 Therapeutic applications

High‑energy X‑rays are used in radiotherapy to destroy cancerous cells. The dose is carefully controlled to maximise tumour damage while sparing healthy tissue.

3.3 Industrial and scientific uses

• Non‑destructive testing of welds and castings.
• Crystallography – determination of atomic structures.

4. Radiation safety

Because X‑rays are ionising, exposure must be minimised.

1. Time – reduce the duration of exposure.
2. Distance – intensity follows the inverse‑square law: \$I \propto \frac{1}{r^{2}}\$
3. Shielding – use lead aprons, walls, and collimators.

5. Radioactive tracers – linking to X‑ray techniques

5.1 Definition of a tracer

A tracer is a substance that contains radioactive nuclei and can be introduced into the body. It is absorbed by the tissue or organ under investigation, allowing its distribution to be followed.

5.2 Types of tracers used in medical imaging

5.3 How tracers complement X‑ray techniques

• Hybrid imaging (e.g., PET/CT) combines functional information from a radioactive tracer with anatomical detail from X‑ray CT.
• Tracer uptake highlights metabolic activity, while X‑ray images locate the region precisely.

5.4 Safety considerations for tracers

1. Use the minimum activity necessary for a clear image.
2. Allow sufficient time for decay before patient discharge when possible.
3. Follow strict contamination control protocols.

6. Summary

Production of X‑rays involves bremsstrahlung and characteristic processes within an X‑ray tube. Their penetrating ability makes them indispensable for medical diagnosis, therapy, and many industrial applications. Understanding radiation safety is essential. Radioactive tracers, containing short‑lived nuclei, can be introduced into the body to study physiological processes; when combined with X‑ray based imaging (CT), they provide powerful hybrid diagnostic tools.