22.5 Medical Applications of Radioactivity: Diagnostic Imaging and Radiation

Nuclear Imaging Techniques

Nuclear imaging lets doctors see how organs and tissues function, not just what they look like structurally (that's what CT or MRI does best). By tracking radioactive tracers through the body, these scans reveal metabolic activity, blood flow, and chemical processes in real time.

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PET Scans

PET (Positron Emission Tomography) works through a specific chain of physics:

1. A radioactive tracer that emits positrons is injected into the patient (commonly $^{18}F$-fluorodeoxyglucose, a glucose analog).
2. Each emitted positron travels a short distance before colliding with a nearby electron.
3. This matter-antimatter collision (annihilation) produces two gamma ray photons that fly off in opposite directions at 180° from each other.
4. A ring of detectors surrounding the patient picks up these paired gamma rays simultaneously.
5. A computer reconstructs a 3D image showing where the tracer concentrated.

Because cancer cells consume glucose at a much higher rate than normal cells, they light up on PET scans. This makes PET especially useful for detecting tumors, staging cancer, and monitoring whether treatment is working.

SPECT Scans

SPECT (Single Photon Emission Computed Tomography) is similar in concept but differs in mechanism:

• The tracer emits single gamma rays directly (no positron annihilation step).
• A rotating gamma camera orbits the patient, capturing gamma rays from multiple angles.
• A 3D image is reconstructed from these projections.

SPECT is widely used for imaging blood flow to the heart (myocardial perfusion imaging) and studying brain receptor activity in neurological disorders.

Both PET and SPECT provide functional information that helps diagnose cancer, heart disease, and neurological conditions, and both allow for early detection and treatment response monitoring.

Radiation Effects on Cells and Cancer Treatment

How Radiation Damages Cells

Ionizing radiation damages cells through two pathways:

• Direct damage: Radiation strikes DNA molecules directly, causing strand breaks or chemical modifications to the bases.
• Indirect damage: Radiation interacts with water molecules in the cell, producing highly reactive free radicals (like $OH \cdot$). These free radicals then attack and damage nearby DNA.

The severity of DNA damage matters:

• Single-strand breaks are often repairable by the cell's own repair enzymes.
• Double-strand breaks are much harder to repair and can lead to chromosomal aberrations, where chunks of chromosomes get rearranged or lost.
• When DNA damage accumulates faster than the cell can fix it, mutations build up. Over time, this accumulation can contribute to cancer development.

Radiation Therapy

Radiation therapy deliberately exploits this cell-killing ability to treat cancer. It works because rapidly dividing cancer cells are more susceptible to radiation damage than most normal cells. Normal cells generally have better DNA repair mechanisms and slower division rates, giving them a survival advantage.

Key principles of radiation therapy:

• High doses of targeted radiation kill cancer cells and shrink tumors.
• Fractionation splits the total dose into many smaller treatments over several weeks. Between sessions, normal cells recover more effectively than cancer cells, which minimizes side effects while still destroying the tumor.

Delivery Methods

• External beam radiation therapy (EBRT): A machine directs a focused beam of radiation at the tumor from outside the body. The beam can be shaped and aimed to match the tumor's geometry.
• Brachytherapy: Radioactive sources are placed directly inside or immediately next to the tumor. This delivers a very high local dose while sparing surrounding tissue, and is commonly used for cervical, prostate, and certain head/neck cancers.

Radiation Dose Units and Biological Effects

Dose Units

Three related but distinct quantities describe radiation dose:

Absorbed dose measures the raw energy deposited per unit mass of tissue.

$D = \frac{E}{m}$

• Unit: gray ($1 \text{ Gy} = 1 \text{ J/kg}$)
• This tells you the physical energy delivered but says nothing about biological harm, because different types of radiation cause different amounts of damage per unit of energy.

Equivalent dose accounts for how biologically damaging a particular radiation type is.

$H = D \times W_R$

• Unit: sievert ($\text{Sv}$)
• $W_R$ is the radiation weighting factor. For X-rays and gamma rays, $W_R = 1$. For alpha particles, $W_R = 20$, meaning alpha radiation is 20 times more biologically damaging per gray than gamma radiation.

Effective dose goes one step further by accounting for how sensitive different organs are to radiation.

$E = \sum (H_T \times W_T)$

• Also measured in sieverts ($\text{Sv}$)
• $W_T$ is the tissue weighting factor for each organ. Bone marrow and gonads have higher $W_T$ values than skin, for example, because they're more radiation-sensitive.
• Effective dose represents the overall health risk from a non-uniform exposure.

Biological Effects

Radiation's biological effects fall into two categories:

1. Deterministic effects occur only above a threshold dose and get more severe as the dose increases. Examples: skin reddening (erythema), cataracts, and acute radiation syndrome. These are predictable once the threshold is crossed.
2. Stochastic effects have no threshold; they can occur at any dose, but the probability increases with dose. The severity doesn't change, only the likelihood. Examples: cancer induction and heritable genetic mutations.

Radiation Protection

The guiding philosophy is the ALARA principle: keep exposure As Low As Reasonably Achievable.

• Regulatory bodies set dose limits for both occupational workers and the general public, designed to prevent deterministic effects entirely and minimize stochastic risk.
• Shielding (lead aprons, concrete barriers, leaded glass) reduces exposure in medical and occupational settings.
• In practice, protection relies on three factors: minimizing time near the source, maximizing distance from it, and using appropriate shielding material.

Radionuclides in Medical Applications

Radionuclides are unstable atomic nuclei that undergo radioactive decay, emitting radiation in the process. Their usefulness in medicine depends on matching the right isotope to the right application.

Half-life ($t_{1/2}$) is the time required for half of a radioactive sample to decay. This property is critical for choosing medical isotopes:

• Too short a half-life and the tracer decays before useful images can be captured.
• Too long a half-life and the patient receives unnecessary radiation exposure after the procedure is over.
• For example, $^{99m}Tc$ (technetium-99m) has a half-life of about 6 hours, which is long enough for most imaging procedures but short enough to limit patient dose. It's the most widely used radionuclide in diagnostic imaging.

Radiation detectors (gamma cameras, PET detector rings, scintillation counters) measure the radiation emitted by these tracers, converting the signals into the images physicians use for diagnosis.