Medical Physics Applications
Discover how physics principles power modern medical imaging and treatment technologies, from X-rays and MRI to PET scans and radiation therapy.
Pax says: "Physics saves lives every day! From seeing inside the body without surgery to targeting cancer cells with pinpoint accuracy -- let's explore the physics behind modern medicine."
X-rays and CT Scans
X-rays are high-energy electromagnetic waves that can pass through soft tissue but are absorbed by dense materials like bone and metal. This differential absorption creates contrast in images, allowing doctors to see inside the body. CT (Computed Tomography) scans take X-ray images from many angles and use computer processing to create detailed cross-sectional images.
X-ray Tube Operation
Cathode (heated filament)
Emits electrons by thermionic emission
Anode (tungsten target)
Electrons decelerate rapidly, emitting X-rays
X-ray beam
~99% energy becomes heat, ~1% becomes X-rays
Standard X-ray
Produces a 2D shadow image. Dense structures (bone) appear white; soft tissue appears in shades of grey; air appears black.
Uses: Fractures, dental imaging, chest X-rays
CT Scan
Rotates the X-ray source around the patient, creating detailed 3D cross-sections. Much higher radiation dose than standard X-ray.
Uses: Internal injuries, tumour detection, brain imaging
MRI and Ultrasound
Unlike X-rays, MRI (Magnetic Resonance Imaging) and ultrasound do not use ionising radiation, making them safer for repeated use and for imaging vulnerable patients such as pregnant women.
Comparison of Imaging Techniques
| Feature | MRI | Ultrasound |
|---|---|---|
| Principle | Nuclear magnetic resonance of hydrogen atoms | Reflection of high-frequency sound waves |
| Radiation | None (uses magnetic fields and radio waves) | None (uses sound waves) |
| Best for | Soft tissue detail (brain, joints, muscles) | Real-time imaging (foetus, heart, blood flow) |
| Limitations | Expensive, slow, not for patients with metal implants | Lower resolution, cannot penetrate bone or air |
How MRI Works: A powerful magnetic field aligns hydrogen nuclei (protons) in the body. Radio frequency pulses disturb this alignment. As protons relax back, they emit radio signals that are detected and processed into detailed images. Different tissues relax at different rates, creating contrast.
PET Scans and Radiation Therapy
PET (Positron Emission Tomography) scans use radioactive tracers to image metabolic activity in the body. Radiation therapy uses targeted ionising radiation to destroy cancer cells. Both technologies demonstrate how nuclear physics directly impacts patient care.
How PET Scanning Works
1. Radiotracer Injection
Patient receives a glucose-based radiotracer (e.g., FDG with fluorine-18)
2. Positron Emission
Radioactive fluorine-18 emits positrons (β+)
3. Annihilation
Positron meets electron, producing two gamma rays at 180°
4. Detection and Imaging
Ring of detectors locates the annihilation site to build 3D image
Radiation Therapy Principles
Radiation therapy works by damaging the DNA of cancer cells, preventing them from dividing. Techniques include:
- External beam: High-energy X-rays or gamma rays focused on the tumour from outside the body
- Brachytherapy: Radioactive source placed inside or next to the tumour
- Proton therapy: Uses protons instead of X-rays, with more precise dose delivery (Bragg peak)
Key Vocabulary
Ionising Radiation
Radiation with enough energy to remove electrons from atoms, including X-rays and gamma rays. Can damage DNA and living tissue.
Magnetic Resonance
The phenomenon where atomic nuclei in a magnetic field absorb and re-emit radio frequency energy, forming the basis of MRI technology.
Positron
The antimatter counterpart of the electron, with the same mass but positive charge. Emitted in beta-plus decay and used in PET scanning.
Piezoelectric Effect
The ability of certain crystals to produce sound waves when an electric field is applied (and vice versa), used in ultrasound transducers.
Worked Examples
An X-ray tube operates at 80 kV. Calculate the maximum energy (in eV and joules) of X-ray photons produced.
Step 1: Maximum photon energy equals the kinetic energy gained by the electron: E = eV
Step 2: E = 80,000 eV = 80 keV
Step 3: In joules: E = 80,000 × 1.6 × 10-19 = 1.28 × 10-14 J
Answer: The maximum X-ray photon energy is 80 keV or 1.28 × 10-14 J. Most photons will have lower energy than this maximum.
An ultrasound pulse is sent into the body and reflects off an organ 0.08 s later. If the speed of sound in tissue is 1540 m s-1, how deep is the organ?
Step 1: The pulse travels to the organ and back, so total distance = 2d.
Step 2: 2d = v × t = 1540 × 0.08 = 123.2 m
Step 3: d = 123.2 / 2 = 61.6 m... This is unreasonably large! Check: t should be 0.08 ms = 0.00008 s. Then d = 1540 × 0.00008 / 2 = 0.0616 m = 6.16 cm. This is a realistic depth for an organ.
In a PET scan, a positron annihilates with an electron. Calculate the total energy released. (Mass of electron/positron = 9.11 × 10-31 kg, c = 3 × 108 m s-1)
Step 1: Total mass annihilated = 2 × 9.11 × 10-31 = 1.822 × 10-30 kg
Step 2: E = mc2 = 1.822 × 10-30 × (3 × 108)2 = 1.64 × 10-13 J
Answer: Total energy released is 1.64 × 10-13 J (about 1.022 MeV), split equally between two gamma ray photons of 0.511 MeV each, emitted in opposite directions.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
X-rays produce images of the body because they are:
Question 2
Which imaging technique uses NO ionising radiation?
Question 3
In a PET scan, the two gamma rays produced by positron-electron annihilation are emitted:
Question 4
Ultrasound imaging works by detecting:
Question 5
The advantage of proton therapy over conventional X-ray radiation therapy is that protons:
Key Concepts Summary
- ●X-rays create images through differential absorption by different tissues; CT scans use rotating X-rays to produce 3D cross-sections.
- ●MRI uses magnetic fields and radio waves (no ionising radiation) to image soft tissue with exceptional detail.
- ●Ultrasound uses reflected sound waves for real-time imaging, is safe and portable, but has limited resolution.
- ●PET scans detect gamma rays from positron-electron annihilation to map metabolic activity, particularly useful for cancer detection.
- ●Radiation therapy uses targeted ionising radiation to damage cancer cell DNA, with proton therapy offering superior dose precision via the Bragg peak.