4.3 Photon Interactions with Matter & Tissue Attenuation

Why This Topic Is Tested

The final leaf item under Radiation Physics is "photon interactions with matter," naming three specific interactions — photoelectric, Compton, and coherent (classical) — plus "attenuation by various tissues." This is arguably the highest-yield physics topic on the CT exam because it connects directly to image production concepts you will study later: the Hounsfield Unit (HU) scale, beam hardening artifacts, photon starvation streaks, and even why scatter radiation is the primary occupational hazard for CT staff. The exam expects you to know not just the definition of each interaction, but which one dominates under which conditions, and why that matters clinically. Note: ARRT's outline names only these three interactions plus attenuation — pair production and photodisintegration occur only at megavoltage energies far above diagnostic CT and are not part of this content outline, a useful fact for eliminating distractor options.

Photoelectric Effect: Total Absorption

In the photoelectric effect, an incoming photon transfers all of its energy to a tightly bound inner-shell electron (typically K-shell), completely ejecting that electron (now called a photoelectron) and ionizing the atom. The photon itself ceases to exist — it is fully absorbed, producing no scatter. Photoelectric interaction probability depends heavily on two factors: it increases sharply with the atomic number (Z) of the absorbing tissue (roughly proportional to Z-cubed) and decreases sharply as photon energy increases (roughly proportional to 1/E-cubed). This double dependence explains several clinically important effects:

• Bone (higher effective Z than soft tissue) and metal implants (very high Z) absorb far more photons by this mechanism than soft tissue does, producing strong CT contrast — but also photon starvation when so few photons make it through that the detector signal becomes noisy or absent, a common cause of streak artifacts near hip prostheses or dental hardware.
• Iodinated contrast media (iodine, Z = 53) and barium (Z = 56) attenuate strongly via the photoelectric effect, especially at lower kVp, which is exactly why low-kVp CT angiography protocols and dual-energy CT techniques are used to maximize iodine conspicuity.
• Because the photoelectric effect fully absorbs the photon locally, it also deposits its entire energy in that tissue — contributing directly to local patient dose.

Compton Scatter: Partial Energy Transfer

Compton scatter occurs when an incoming photon interacts with a loosely bound outer-shell (valence) electron. Unlike the photoelectric effect, the photon is not absorbed — it transfers only part of its energy to eject the electron, then continues traveling in a new direction (scattered) with reduced energy (longer wavelength). Compton probability is roughly independent of atomic number but depends on the electron density (physical/mass density) of the tissue, and it is the dominant interaction at typical diagnostic CT energies (roughly 60-140 keV effective beam energy) in soft tissue. Compton scatter matters for two major reasons tested on the exam:

1. Image degradation — scattered photons that still reach the detector array carry no useful spatial information and add noise/reduce contrast.
2. Occupational radiation exposure — Compton-scattered photons leave the patient in many directions, including back toward the technologist, making scatter the primary source of staff dose in CT and the reason for the distance, shielding, and personnel-protection strategies covered in Chapter 5.

Coherent (Classical/Rayleigh) Scattering

Coherent scattering (also called classical or Rayleigh scattering) happens when a low-energy photon interacts with an atom as a whole rather than a single electron. The atom absorbs and instantly re-emits a photon of the same energy but in a different direction — no ionization occurs and no energy is lost. At CT diagnostic energies, coherent scattering is a minor contributor (typically only a few percent of interactions), but it is frequently used as a distractor option because it is the only listed interaction that involves no energy loss and no ionization.

Comparing the Three Interactions

Attenuation by Various Tissues

Attenuation is the overall reduction in beam intensity as it passes through tissue, combining the contributions of all interactions above. It follows an exponential relationship described by the linear attenuation coefficient (mu):

I = I0 x e^(-mu x thickness)

where I0 is the initial intensity and I is the intensity after passing through a given thickness of tissue. The half-value layer (HVL) relates to mu by HVL = 0.693 / mu — the thickness needed to cut intensity in half. Different tissues have very different attenuation coefficients, and this hierarchy — from least to most attenuating — is: air < fat < water/soft tissue < iodinated contrast < bone < metal. This exact hierarchy is the physical foundation of the Hounsfield Unit (HU) scale you will study in depth under Image Production: water is defined as 0 HU, air as approximately -1000 HU, and dense cortical bone typically ranges from +1000 to +3000 HU, with metal often exceeding that range and causing scale truncation and artifacts.

Exam Scenario

A patient with bilateral metal hip prostheses undergoes a pelvis CT. The extremely high atomic number of the metal causes overwhelming photoelectric absorption, and so few photons survive to reach the detector along those paths that the system experiences photon starvation, producing dark and bright streak artifacts radiating from the metal (a beam-hardening artifact you will study fully in Chapter 9). Recognizing that this artifact traces back to the photoelectric effect's strong Z-dependence — not Compton scatter, which is roughly Z-independent — is a common way the exam links physics knowledge to artifact recognition.