Explore the basic tenets of medical Physics (nervous system, electro-cardiography, diagnostic imaging and radiotherapy)

Radiation used in medicine is classified into two categories based on its ability to ionize atoms.

Ionizing Radiation

Radiation with enough energy to remove electrons from atoms, causing ionization. Examples include X-rays, gamma rays, alpha particles, and beta particles. Used in X-ray imaging, CT scans, and radiation therapy.

Non-Ionizing Radiation

Radiation without sufficient energy to ionize atoms. Examples include visible light, radio waves, and ultrasound. Used in MRI and ultrasound imaging.

X-ray Imaging

X-rays are produced when high-energy electrons strike a metal target in an X-ray tube. Different tissues absorb X-rays differently—bones absorb more and appear white, while soft tissues appear gray and air-filled areas appear dark. X-rays are commonly used to detect fractures, tumors, and infections.

Computed Tomography (CT)

CT uses X-rays with computers to create cross-sectional images of the body. Unlike conventional X-rays producing 2D images, CT provides detailed 3D views of internal organs. The scanner rotates around the patient, taking multiple X-ray images from different angles, which a computer processes to generate slice images.

Magnetic Resonance Imaging (MRI)

MRI uses strong magnetic fields and radio waves to create detailed images without ionizing radiation. The technique aligns hydrogen atoms in the body with a strong magnetic field; when radio waves are applied, these atoms emit signals detected by the MRI machine. MRI is particularly useful for examining soft tissues like the brain, spinal cord, muscles, and joints.

Ultrasound Imaging

Ultrasound uses high-frequency sound waves to generate real-time images. A transducer emits sound waves that reflect off tissue boundaries, creating echoes. The time delay and intensity of echoes determine the depth and nature of tissues. Ultrasound is safe, non-invasive, and does not use ionizing radiation, making it ideal for monitoring fetal development during pregnancy.

Nuclear Medicine Imaging

Nuclear medicine involves introducing radioactive substances (radiopharmaceuticals) into the body. Techniques include:

• PET (Positron Emission Tomography): Detects positrons emitted by radioactive isotopes to reveal metabolic activity
• SPECT (Single Photon Emission CT): Uses gamma rays to create 3D images of organ function

Radiation therapy uses high-energy radiation to kill cancer cells or shrink tumors by damaging their DNA.

External Beam Radiation Therapy (EBRT)

High-energy X-rays or electrons are directed at the tumor from outside the body using machines like linear accelerators (LINACs). By directing beams from multiple angles, doctors deliver high doses to cancer while protecting nearby healthy tissues.

Brachytherapy

Radioactive sources are placed directly inside or near the tumor. The radiation travels only a short distance, delivering strong doses to the tumor while minimizing damage to surrounding tissues. Used for cancers of the cervix, prostate, and breast.

Therapeutic Nuclear Medicine

Radioactive materials are introduced into the body, targeting specific organs or tumors. For example, iodine-131 treats thyroid cancer because the thyroid naturally absorbs iodine.

Ultrasound intensity decreases as it travels through tissue according to:

$I = I_0 e^{-\mu x}$

Where:

• $I_0$ = initial intensity
• $I$ = intensity at distance $x$
• $\mu$ = attenuation coefficient
• $x$ = distance traveled

Problem: An ultrasound wave with initial intensity $I_0 = 100 \text{ mW/cm}^2$ travels 5 cm in muscle tissue with attenuation coefficient $\mu = 0.5 \text{ cm}^{-1}$. Calculate the emerging intensity.

Solution:

$I = 100 \times e^{-0.5 \times 5} = 100 \times e^{-2.5}$

$I = 100 \times 0.0821 = 8.21 \text{ mW/cm}^2$

Only about 8.2% of the original intensity remains after traveling 5 cm through muscle tissue.

Medical physicists are essential members of healthcare teams who:

1. Ensure quality and safety in radiation therapy through treatment planning and equipment calibration
2. Optimize medical imaging protocols to balance image quality with patient safety
3. Perform dosimetry calculations to determine precise radiation doses
4. Implement radiation protection protocols following the ALARA principle (As Low As Reasonably Achievable)
5. Train healthcare staff on safe radiation practices

Three fundamental principles guide radiation protection:

1. Justification: Any radiation use must produce more benefit than harm
2. Optimization: Keep radiation doses as low as reasonably achievable (ALARA)
3. Dose Limitation: Set limits on radiation exposure for workers and the public

Protection methods include limiting exposure time, maximizing distance from radiation sources, and using shielding materials like lead or concrete.

In Tanzania, medical physics principles are applied at hospitals like Bugando Medical Centre in Mwanza and Muhimbili National Hospital in Dar es Salaam, where CT scanners and X-ray machines aid in diagnosing conditions such as tuberculosis and injuries from road accidents. Understanding these principles helps future healthcare professionals operate equipment safely and effectively, contributing to improved patient care across the country's healthcare system.