Absorption of X-Rays and Contrast Media

A set of 50 flashcards covering X-ray absorption processes, attenuation mechanisms like the photoelectric effect and Compton scattering, and the application of contrast media based on lecture notes.

How do heavy elements such as calcium interact with X-rays compared to lighter elements?

Heavy elements are very good absorbers of X-rays, while lighter elements are generally poor absorbers.

How are X-ray ‘shadow pictures’ produced?

By placing a body between an X-ray source and photographic film, as X-rays can blacken photographic film.

In an X-ray shadow image, what do the light areas represent?

Areas where there has been little or no X-ray exposure on the film because the X-rays were absorbed by bone material.

What causes dark areas on an X-ray film?

The film has been exposed to X-rays and blackened due to the low absorption of X-rays by soft tissues.

Why are soft tissues like fat, muscle, and tumors difficult to distinguish in standard X-ray pictures?

They all have similar absorption characteristics for X-rays.

What is the definition of the attenuation of an X-ray beam?

The reduction in its intensity due to the absorption and/or scattering of some of the X-ray photons out of the beam.

Which three processes can cause X-rays to be attenuated in biological tissue?

Photoelectric Effect, Compton Scattering, and Pair Production.

What occurs during the photoelectric effect when an X-ray photon is absorbed?

The entire quantum energy is absorbed and an electron, known as a photoelectron, is ejected from the material.

The photoelectric effect primarily involves photons in what energy range?

Primarily low energy photons $< 0.5\,MeV$.

What is a potential biological hazard of the photoelectric effect?

The ejected photoelectron can have energy up to $~50,000\,eV$, causing it to ionize many other atoms within the tissue.

What is the specific ionization energy for hydrogen mentioned in the notes?

$13.6\,eV$.

How is the work function ($\Phi$) defined in the context of X-ray absorption?

The minimum amount of energy required to free an electron from an atom.

When an electron from a higher level falls into a vacant orbit created by the photoelectric effect, what is emitted?

A photon (unlikely to be an X-ray photon).

What happens during Compton scattering?

An incident X-ray photon interacts with an atomic electron, causing it to recoil out of orbit, while the photon loses energy and scatters into a different path.

Compton scattering primarily involves photons in what energy range?

Primarily medium energy photons $> 0.5\,MeV$ and $< 5\,MeV$.

In Compton scattering, how does the frequency of the scattered photon ($f'$) compare to the incident frequency ($f$)?

f' < f

Which attenuation process is more common than the photoelectric effect for clinical X-rays?

Compton Scattering.

What is one consequence of the ejected electron in Compton scattering?

It may have sufficiently high energy to ionize surrounding tissue, leading to secondary damage.

How does Compton scattering affect medical image quality?

The scattered X-ray photons can lead to poor contrast.

What is the process of pair production?

A high energy X-ray or gamma ray photon enters an intense electric field of a nucleus and is converted into an electron and a positron.

Pair production primarily involves photons in what energy range?

Primarily high energy photons $> 1.02\,MeV$.

Why is pair production not common within clinically useful ranges of X-rays?

It requires very high energy photons.

What is pair annihilation?

The process where a positron interacts with another electron, converting their mass-energy into two photons.

What is the energy of each of the two photons produced during pair annihilation?

$$511\,keV$$

In what direction do the two photons from pair annihilation travel?

In opposite but co-aligned directions.

Why do heavy elements like barium, lead, and calcium absorb X-rays efficiently?

They have energy levels in the inner shells with separations comparable to X-ray photon energies.

Why are bulk materials made of light atoms, such as soft tissue, almost transparent to X-rays?

The energy level separations are smaller, so atoms cannot absorb X-ray photons directly.

What is the purpose of using contrast media in X-ray studies?

To modify the absorption characteristics of organs that are not normally easily visible.

What physical characteristic do clinically useful contrast media generally possess?

A high Z-number.

Where is iodine typically introduced as a contrast medium, and what does it allow clinicians to observe?

It is introduced into the bloodstream to observe arteries and veins via Digital Subtraction Angiograpy.

Which contrast medium is used effectively to study the gastrointestinal (GI) tract?

Barium.

What specific abnormalities can a barium enema help detect in the colon?

Tumors, strictures, polyps, or other structural abnormalities.

As absorber thickness ($t$) increases, what happens to the measured intensity ($I$) of transmitted X-rays?

The intensity decreases exponentially.

What is defined as behavior where a parameter decreases by a constant fraction for each fixed increment of a second parameter?

Exponential Decay.

What is the rate of decrease for a quantity undergoing exponential decay?

It decreases at a rate proportional to its current value.

What are four examples of exponential changes mentioned in the text?

Nuclear decay, radiation absorption in tissue, disease infectivity, and population growth.

Which law describes the standard exponential decay behavior of X-ray attenuation?

The Lambert-Beer Law.

What does the symbol $\mu$ represent in the Lambert-Beer Law?

The Linear Attenuation Coefficient.

What are the units for the Linear Attenuation Coefficient ($\mu$)?

$m^{-1}$ or $cm^{-1}$

How does an increase in the Linear Attenuation Coefficient ($\mu$) affect the attenuation graph?

It results in more absorption per unit thickness, making the graph steeper.

The Linear Attenuation Coefficient ($\mu$) depends on which two factors?

The material of the absorber and the energy of the X-ray photons.

How is the Half-Value Layer (HVL) defined?

The thickness of material required to reduce the transmitted intensity by a factor of $2$.

What is the Mass Attenuation Coefficient ($\mu_m$)?

The attenuation coefficient normalized by the density ($\rho$) of the material, representing attenuation per unit mass.

The Mass Attenuation Coefficient ($\mu_m$) is measured in what units?

$$cm^2/g$$

In the rewritten Lambert-Beer Law $I = I_0 e^{-\mu_m(\rho t)}$, what is the term $\rho t$ called?

Area density.

What is a K-edge?

A sharp discontinuity in the absorption graph where the amount of absorption increases sharply at certain energies.

What are the specific K-edge energies for Iodine and Lead?

Iodine has a K-edge at $33\,keV$ and Lead has a K-edge at $88\,keV$.