Principles of Radiation Physics Part 3: Photons Interactions with Matter (ARRT Registry Review)

When photons, particularly X-ray photons, interact with matter, the outcomes are fundamental to how diagnostic imaging works. These interactions determine the quality of the images produced and the dose absorbed by the patient. In this three-part blog series, we’ll explore how photons behave when encountering human tissue, starting with the three primary photon interactions relevant in the diagnostic energy range: photoelectric effect, Compton scatter, and coherent scatter.

A. Photoelectric Effect: The Engine of Image Contrast

The photoelectric effect occurs when an incoming X-ray photon strikes an atom and ejects one of its inner-shell electrons, typically a K-shell electron. This interaction results in total absorption of the photon’s energy. The ejected electron, now called a photoelectron, carries kinetic energy equal to the energy of the incoming photon minus the binding energy of the electron it displaced.

Once the inner-shell vacancy is created, an outer-shell electron drops into the vacant spot. This downward transition releases energy in the form of a characteristic photon, which is usually absorbed within the tissue and does not exit the body. This is a key reason photoelectric effect contributes to patient dose, but also why it is crucial for image contrast.

Why is contrast so important? High contrast in imaging allows radiologists to differentiate between structures like bone, fat, and muscle. And photoelectric absorption is most likely to occur in elements with high atomic numbers (Z). This is why contrast agents like iodine and barium are used—they absorb X-rays much more than surrounding soft tissues, enhancing the visibility of organs and vessels.

Furthermore, photoelectric effect is inversely proportional to the photon energy and directly related to the cube of the atomic number (Z³) of the tissue. This means lower energy beams increase the likelihood of this interaction, especially in denser tissues like bone.

B. Compton Scatter: A Primary Cause of Radiation Fog

Compton scattering is the most common interaction in the diagnostic X-ray range, especially with outer-shell electrons of low atomic number elements, like those in soft tissue. In this event, a photon strikes a loosely bound outer-shell electron, ejecting it from the atom and imparting it with kinetic energy. This ejected electron is known as a recoil electron.

The incident photon doesn’t disappear entirely—it is deflected, and continues on a new path with reduced energy. The amount of energy it retains depends on the angle at which it scatters: the greater the deflection angle, the less energy it retains.

Compton scatter is problematic in medical imaging for several reasons:

• It leads to image fog, reducing contrast.

• The scattered photons can travel in any direction, which increases the occupational exposure to radiographers.

• Unlike the photoelectric effect, it is only weakly dependent on atomic number and more influenced by tissue density and photon energy.

At higher X-ray energies, Compton interactions dominate, especially in soft tissues, where electron density is more consistent across different structures.

C. Coherent Scatter: Low Energy, Low Impact

Also known as classical, Rayleigh, or Thomson scatter, coherent scatter is the least likely of the three to occur in diagnostic radiography. It takes place at very low X-ray energies, generally below 10 keV. In this process, an incoming photon interacts with the entire atom, causing it to become excited and re-emit a photon of the same energy, but in a different direction.

Key features of coherent scattering:

• No energy is transferred to the atom; therefore, no ionization occurs.

• The scattered photon is unmodified, meaning its energy and wavelength remain unchanged.

• It contributes minimally to image formation and does not significantly affect patient dose.

Despite being virtually harmless, coherent scatter can still contribute slightly to image noise due to the change in photon direction. However, in most clinical settings, its impact is negligible compared to photoelectric and Compton interactions.

Building on our understanding of photoelectric, Compton, and coherent interactions, we now turn to a crucial concept in diagnostic imaging: attenuation. This refers to the reduction in intensity of the X-ray beam as it passes through the body. Attenuation is fundamental because it directly influences image contrast, patient dose, and the diagnostic value of the radiograph.

D. Attenuation by Various Tissues

Attenuation occurs due to both absorption and scattering of X-ray photons. As photons travel through tissue, they are either absorbed (as in the photoelectric effect), scattered (as in Compton or coherent interactions), or transmitted unchanged. The rate and extent of attenuation depend largely on two primary factors:

1. Thickness of the Body Part

One of the most straightforward factors influencing attenuation is tissue thickness. The thicker the body part, the greater the chance that photons will interact with matter before reaching the image receptor. This increases the cumulative attenuation and often requires higher exposure settings to ensure enough photons penetrate the tissue to create a usable image.

For example, imaging a thick abdominal section requires more penetrating power—achieved by increasing kilovoltage peak (kVp)—compared to imaging a wrist. More tissue means more opportunities for interactions like Compton scattering and photoelectric absorption, both of which can affect image clarity and radiation dose.

Thicker tissues not only attenuate more photons but also scatter more radiation, which can degrade image contrast if not controlled with grids or collimation.

2. Type of Tissue (Atomic Number)

The composition of the tissue, particularly its atomic number (Z), significantly influences how X-rays are attenuated. Different tissues—bone, muscle, fat, air—have varying densities and atomic structures, which means they interact with photons differently.

• High Z materials, such as bone (calcium-rich) or contrast agents like iodine and barium, increase attenuation primarily via the photoelectric effect. Since photoelectric interactions are proportional to Z³, even small increases in atomic number dramatically raise the probability of photon absorption.

• Low Z materials, like fat and soft tissue, tend to allow more transmission and are more likely to scatter photons via Compton interactions.

This explains why bone appears white on a radiograph (more attenuation due to absorption), while air-filled lungs appear black (minimal attenuation). The presence of high-Z materials not only increases patient dose but also improves image contrast, making the visualization of specific structures clearer.

Contrast agents are used in many diagnostic studies precisely for this reason—they sharply increase photoelectric absorption in areas of interest, such as blood vessels or the gastrointestinal tract, creating a bright outline on the radiograph.

The Role of Beam Energy in Tissue Attenuation

While tissue thickness and composition play dominant roles, beam energy also critically influences attenuation:

• At low kVp (e.g., 50–70 kVp), photoelectric absorption is more likely, especially in tissues with higher atomic numbers. This yields high contrast images but increases patient dose.

• At high kVp (e.g., 100–130 kVp), Compton scatter dominates. These settings produce lower contrast but lower patient dose, as more photons pass through the body without interaction.

This trade-off is central to image quality decisions. For example, mammography uses low-energy X-rays to enhance contrast through photoelectric interactions in soft tissue, whereas chest radiography uses high-energy beams to reduce dose and image lung structures with lower contrast needs.

Having explored the types of photon interactions and the role of tissue characteristics in attenuation, we now bring the concepts together to understand how they shape real-world imaging and radiation safety practices.

Clinical Implications of Photon Interactions

Understanding how photons interact with matter is not just an academic exercise—it’s essential for every imaging professional who must strike a balance between image quality and radiation dose. Let's examine how these principles apply to actual clinical imaging:

Balancing Contrast and Dose

The photoelectric effect is crucial for producing high-contrast images, but it comes with a cost: increased patient dose. This is why it's typically favored when high detail is needed, such as in imaging bones or when using contrast agents. For example, iodine in vascular imaging or barium in gastrointestinal exams significantly boosts photoelectric absorption, enhancing the visibility of structures that might otherwise blend into the background of soft tissue.

On the other hand, Compton scatter dominates in most soft tissue examinations at higher beam energies. While it reduces contrast, it also allows for lower patient doses because fewer photons are absorbed and more pass through the body. This is preferred in high-throughput imaging like chest radiography, where minimizing dose is critical due to population scale and repeated exposure.

Image Quality Control: Reducing Scatter

Compton scatter’s major downside is the fogging effect it introduces, lowering contrast by adding unwanted exposure to the image receptor. Radiographers use several techniques to minimize this:

• Collimation: Restricts the X-ray beam to the area of interest, reducing the amount of tissue irradiated and thus the scatter produced.

• Grids: Absorb scattered photons before they reach the image receptor.

• Air gaps: Increasing the distance between the patient and the detector allows some scatter to diverge away from the detector path.

These strategies are vital in controlling scatter and ensuring that diagnostic images retain clarity and detail.

The Role of Beam Energy Selection

Selecting the appropriate kilovoltage peak (kVp) is one of the most powerful tools in managing both image contrast and patient dose. Here’s how different energies influence photon interactions:

• Low kVp (40–70): Increases the probability of photoelectric absorption. Useful for extremities or contrast-enhanced studies.

• High kVp (100+): Encourages Compton scatter. Best for chest or abdominal studies where contrast demands are lower and dose considerations are critical.

Radiographers adjust mAs (milliampere-seconds) in tandem with kVp to maintain exposure while keeping doses within safe limits. Understanding the interaction behavior at different energy levels allows for smart technique customization depending on the patient and the exam type.

Summary: Why Photon Interactions Matter

To recap:

• Photoelectric interactions provide image contrast but increase patient dose.

• Compton interactions dominate soft tissue imaging and contribute to scatter and occupational exposure.

• Coherent interactions are rare and clinically insignificant but do exist at very low energies.

• Tissue thickness and atomic number heavily influence attenuation, and both must be considered when selecting imaging parameters.

• Optimizing kVp and mAs, using grids, and proper collimation are key strategies for balancing diagnostic quality with radiation safety.

Radiologic imaging is a precise science, and every exposure carries implications for both the quality of the image and the safety of the patient and radiographer. Mastery of photon interactions and attenuation principles is essential to achieve excellence in diagnostic radiology.