Medical physics — WJEC A-Level Physics Revision
This topic explores the physical principles and technological applications of medical imaging and radiation therapy. It covers the production and attenuati
Topic Synopsis
This topic explores the physical principles and technological applications of medical imaging and radiation therapy. It covers the production and attenuation of X-rays, the use of ultrasound for diagnostic imaging, the principles of magnetic resonance imaging (MRI), and the use of radionuclides in tracers and PET scanning.
Key Concepts & Core Principles
- Attenuation of X-rays: The exponential reduction in intensity as X-rays pass through matter, described by I = I₀ e^(-μx), where μ is the linear attenuation coefficient. Different tissues attenuate differently, producing contrast in radiographs.
- CT scanning: Uses multiple X-ray projections from different angles to reconstruct cross-sectional images via computed tomography. The Hounsfield scale quantifies tissue density, with water at 0 HU, bone at +1000 HU, and air at -1000 HU.
- Ultrasound imaging: Uses high-frequency sound waves (1–20 MHz) and the piezoelectric effect. Reflection at tissue boundaries (acoustic impedance mismatch) creates echoes; the time-of-flight gives depth. Doppler ultrasound measures blood flow velocity via frequency shift.
- Radionuclide imaging (e.g., PET, gamma camera): Involves injecting a radioactive tracer (e.g., technetium-99m) that emits gamma rays. The gamma camera detects these rays to form functional images. Key concepts include half-life, specific activity, and the use of collimators.
- Radiotherapy: Uses ionising radiation (e.g., gamma rays from Co-60, or X-rays from linear accelerators) to destroy cancer cells. The Bragg peak for protons and the concept of dose (Gray, Sievert) are critical. Treatment planning ensures maximum dose to tumour while sparing healthy tissue.
Exam Tips & Revision Strategies
- Ensure all units are converted to SI before substituting into the attenuation equation
- Be prepared to compare the advantages and disadvantages of different imaging modalities (X-ray, ultrasound, MRI)
- Clearly distinguish between the physical principles of X-ray production (therapy vs diagnosis)
- Practice calculations involving radiation weighting factors and tissue weighting factors
Common Misconceptions & Mistakes to Avoid
- Confusing absorbed dose (Gy) with equivalent/effective dose (Sv)
- Misapplying the attenuation equation by failing to use consistent units for x and μ
- Neglecting the necessity of a coupling medium for ultrasound due to acoustic impedance mismatch
- Incorrectly identifying the components of a gamma camera or their specific functions
Examiner Marking Points
- Attenuation of X-rays using I = I0 exp(-μx)
- Production of X-ray spectra and control of beam intensity/energy
- Acoustic impedance Z = cρ and its role in ultrasound reflection/transmission
- Doppler equation Δf = 2v cosθ for blood flow measurement
- Larmor frequency f = 42.6 × 10⁶ B for MRI
- Units of absorbed dose (Gray, Gy) and equivalent/effective dose (Sievert, Sv)
- Calculation of equivalent dose H = DW R and effective dose E = HWT
- Principles of gamma camera components: collimator, scintillation counter, photomultiplier/CCD