Medical Physics

Medical imaging, radiotherapy, clinical dosimetry, ultrasound, MRI, and radiological safety.

General idea of the topic

Medical physics applies nuclear physics, ionizing and non-ionizing radiation, acoustics, and electromagnetism to modern healthcare: high-resolution anatomical/functional diagnostic imaging and precision radiation oncology. It combines fundamental microscopic radiation-matter interactions with advanced signal processing and tomographic image reconstruction algorithms.

This node covers photon and charged-particle interactions with biological matter (photoelectric, Compton, pair production, Bethe-Bloch stopping power), radiation dosimetry, X-ray Computed Tomography (CT), Magnetic Resonance Imaging (MRI), Positron Emission Tomography (PET), and external beam radiotherapy.

What you will truly understand

You will understand the physics of the Bragg peak exhibited by proton and heavy-ion beams compared to exponential X-ray attenuation: heavy charged particles deposit the overwhelming majority of their ionization energy at a precise millimeter tumor depth, sparing distal healthy tissue.

You will master the Radon transform and the Filtered Back-Projection (FBP) algorithm used to reconstruct cross-sectional tomographic density slices from continuous angular projection data.

Recommended learning route

1. Radiation-matter interactions: X-ray attenuation coefficients, Compton scattering cross-sections, and the relativistic Bethe-Bloch stopping power.
2. Dosimetry and radiation protection: absorbed dose ($D$ in Grays), equivalent dose ($H$ in Sieverts), radiation weighting factors $W_R$, and ionization chamber calibration.
3. Diagnostic imaging physics: X-ray tube physics, CT Radon transform reconstruction, NMR relaxation dynamics ($T_1, T_2$ weighted MRI), and coincidence PET positron-electron annihilation.
4. Radiation oncology: medical linear accelerators (LINACs), multileaf collimators (MLCs), Intensity-Modulated Radiation Therapy (IMRT), and proton beam Bragg peak delivery.

Physical criterion you must acquire

Differentiating between physical absorbed dose (Joules of ionizing energy per kilogram of mass) and biologically effective equivalent dose that weights DNA double-strand break severity across radiation modalities (photons vs neutrons vs alpha particles).

Connection with real problems

• Clinical radiotherapy treatment planning optimization in hospital oncology departments.
• Commissioning, calibration, and quality assurance of medical CT, MRI, and LINAC systems.
• Theranostic radiopharmaceutical synthesis in targeted molecular nuclear medicine.

Final goal of the topic

Master the physical principles of ionizing radiation, tomographic image reconstruction, and clinical dosimetry in diagnostic and therapeutic medicine.