8 Radiation and Medical Imaging

How medical imaging creates images

Radiation is energy carried by particles or waves. Ionizing radiation can remove electrons from atoms, producing charged particles called ions. Medical X-rays and gamma rays are ionizing electromagnetic radiation, while ultrasound and MRI use non-ionizing energy and do not use X-rays.

Medical imaging forms images by detecting differences in radiation transmission or emission. In radiography, an X-ray source sends photons through the body toward a detector. Different tissues attenuate different fractions of the beam, creating a pattern at the detector. Bone typically attenuates more X-rays than soft tissue, while air-filled regions attenuate less; these differences create image contrast. Iodine- or barium-based contrast agents can make selected structures easier to see because they attenuate X-rays strongly.

Several imaging methods use these principles in different ways:

• Computed tomography (CT) measures X-ray transmission from many angles as the source and detectors rotate around the patient. A computer reconstructs cross-sectional images from the measurements.

• Fluoroscopy displays X-ray images continuously or in rapid succession, often to guide procedures.

• Nuclear medicine detects radiation emitted from a radiopharmaceutical inside the body. SPECT detects gamma rays, while PET detects paired photons produced after positrons from the tracer annihilate with electrons. These images can show aspects of tissue function as well as location.

The methods differ in how they collect and display information, but each relies on detecting radiation patterns associated with tissues or tracers.

Attenuation and image contrast

In radiography, image contrast begins with attenuation: as X-rays pass through the body, absorption and scattering reduce or redirect some of the photons that would otherwise reach the detector. A useful simplified model describes a narrow, monoenergetic beam passing through a uniform material:

Here, $I_0$ is the initial intensity, $I$ is the transmitted intensity, $\mu$ is the linear attenuation coefficient, and $x$ is the material’s thickness. Greater thickness or a higher attenuation coefficient results in lower transmitted intensity in this model.

The equation is an approximation rather than a complete description of imaging in the body: real tissues and X-ray energies vary. Even so, it helps explain why tissues create different detector signals. Areas that transmit more photons appear differently from areas that transmit fewer, producing visible contrast. Contrast agents take advantage of strong X-ray attenuation to make selected structures easier to distinguish.

Takeaway: A radiographic image is a map of differences in transmitted X-ray intensity, shaped by tissue composition, thickness, and imaging conditions.

How X-rays interact with tissue

At diagnostic imaging energies, three interactions help explain how X-rays form images and deposit energy in tissue:

• Photoelectric absorption: An X-ray photon transfers its energy to an electron and disappears. This contributes to image contrast and is more likely in higher-atomic-number materials, such as bone or iodine contrast.

• Compton scattering: A photon transfers some energy to an electron and changes direction. If a scattered photon reaches the detector from the wrong direction, it can reduce image contrast. The interaction also deposits energy in tissue.

• Coherent scattering: A photon changes direction without appreciable energy transfer. It has a smaller role in most diagnostic imaging.

The balance among these interactions depends on photon energy and tissue composition. Absorbed energy can ionize molecules and damage cellular structures, including DNA. Cells may repair the damage, misrepair it, or die. Thus, interactions that affect image quality also help determine how radiation affects tissue.

Takeaway: Photoelectric absorption supports contrast, while Compton scattering can add unwanted signal and deposit energy; coherent scattering usually plays a smaller role.

Dose and potential effects

Absorbed dose is the energy deposited per kilogram of material and is measured in gray (Gy). Effective dose, measured in sievert (Sv), is used in radiation protection to compare approximate population-level risk across different kinds of exposure. It is not a precise prediction of an individual patient’s risk.

Medical dose varies with the procedure, body region, patient size, and imaging settings. At the relatively low doses typical of diagnostic imaging, the main radiation-protection concern is a small potential increase in the chance of cancer later in life. Tissue reactions such as skin injury are associated with much higher localized doses. They are uncommon in routine imaging, though they can be a concern during lengthy, complex interventional procedures.

Dose quantities serve different purposes: absorbed dose describes energy deposited in material, while effective dose supports broad comparisons of potential risk. Neither should be treated as a complete description of the individual benefit or risk of a particular examination.

Making imaging safer

Medical imaging uses two central radiation-protection principles:

1. Justification: Perform an ionizing-radiation examination when its expected benefit—such as answering a clinical question or guiding treatment—outweighs its potential harm.

2. Optimization: Use the lowest dose that provides image quality adequate for the clinical task. Measures can include selecting an appropriate test and protocol, limiting the beam to the region of interest, and avoiding unnecessary or repeated images.

Patients should tell their care team if they may be pregnant or have had recent imaging. This information can help the team consider the situation and choose an appropriate approach. When clinically suitable, ultrasound or MRI may avoid ionizing radiation. Necessary imaging should not be avoided solely because it uses radiation: the decision weighs the small potential risk against the expected medical benefit.

For staff, limiting time near the radiation source, increasing distance, and using appropriate shielding reduce occupational exposure.

Takeaway: Choose imaging because it is expected to help, then tailor the examination to obtain adequate information with the least appropriate exposure.