Nuclear and atomic physics aren’t just abstract theory — they’re the foundation of the most important tools in modern medicine. Every time a doctor orders an X-ray, CT scan, PET scan, or MRI, they’re applying the principles from this chapter: photon energy, electron transitions, radioactive decay, electromagnetic induction.
The MCAT loves these technologies as passage topics because they neatly tie the physics of the last 11 sections to real clinical practice. This section walks through each modality (X-ray, CT, PET, MRI), explains the physics behind radiation shielding and the inverse square law, and covers basic dosimetry.
X-Rays
X-rays are high-energy photons with wavelengths between about 0.01 and 10 nm, placing them between ultraviolet light and γ rays on the electromagnetic spectrum. They penetrate soft tissue but are absorbed by dense materials like bone and metal, which is why X-ray images show bones as bright white against darker soft tissue.
PET Scans
Positron Emission Tomography (PET) directly uses the nuclear physics you learned in Section 9.6. A patient is injected with a tracer molecule labeled with a positron-emitting isotope, most commonly fluorine-18 attached to a glucose analog (FDG).
Here is the physics chain:
Fluorine-18 undergoes β-plus decay, emitting a positron.
The positron travels a very short distance before encountering an electron.
The positron and electron annihilate, converting their combined mass into two γ rays that fly off in exactly opposite directions (180 degrees apart).
A ring of detectors around the patient records these coincident γ rays. By tracing the lines between pairs of detectors, the computer reconstructs where each annihilation occurred.
MRI
Magnetic Resonance Imaging uses no ionizing radiation at all. Instead, it exploits the magnetic properties of hydrogen nuclei (protons), which are abundant in water and fat throughout the body.
Radiation Shielding
Different types of radiation require different shielding because of their vastly different penetrating abilities:
Radiation
Stopped by
Why
Alpha (helium nuclei)
Sheet of paper, skin
Large, heavy, doubly-charged - interacts strongly with matter, loses energy quickly
Beta (electrons/positrons)
Thin aluminum sheet (~mm)
Lighter and singly-charged - penetrates farther than α but still interacts frequently
Gamma (photons)
Thick lead or concrete
No charge, no mass - interacts weakly with matter, can travel through many centimeters of material
Inverse Square Law for Radiation
The intensity of radiation from a point source decreases with the square of the distance:
This has big practical implications: simply stepping back from a radioactive source sharply reduces your exposure. Moving from 1 meter to 3 meters away reduces intensity by a factor of 9.
Dosimetry Concepts
In a PET scan, fluorine-18 emits a positron. What happens to the positron, and what is detected by the scanner?
Click to reveal answer
The positron annihilates with a nearby electron, producing two 511 keV γ rays that travel in opposite directions. The PET scanner's ring of detectors identifies coincident γ ray pairs and uses their trajectories to reconstruct where in the body the annihilation occurred.
A technician stands 2 meters from a γ source and receives a dose rate of 100 mSv/hr. What is the dose rate at 6 meters?
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About 11.1 mSv/hr. By the inverse square law: I2=I1(r1/r2)2=100×(2/6)2=100×(1/3)2=100/9=11.1 mSv/hr. Tripling the distance reduced the intensity by a factor of 9.
