Absorption of X-Rays

Absorption of X-Rays

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X-rays can go through some body parts, but not all of them. Each tissue has a different level of transparency to X-rays. Wilhelm Conrad Röntgen discovered that X-rays get absorbed by different materials at different rates.

Why is X-ray absorption important?

When we use X-rays to create images, we can see different levels of darkness and lightness. These represent how much the X-rays were blocked by the tissues between the X-ray source and detector plate. Each tissue blocks X-rays differently, which creates contrast in the image. This helps us see different structures inside the.

Different types of X-ray absorptions

There are four main mechanisms of X-ray attenuation, and they depend on the energy of the incident photon. The four types consist of two scattering and two absorbing X-ray absorptions.

Simple (Rayleigh) scattering

Diagram showing how simple scattering deflects X-ray photons
Diagram showing how simple scattering deflects X-ray photons

Low-energy photons (1 to 20 keV) are affected by simple scattering. When these photons hit an atom, they don't have enough energy to move an electron. Instead, the electric field of the X-ray photon interacts with the atom, changing the photon's path and causing it to scatter.

Because photons are much lighter than atoms, they scatter without losing momentum. Usually, the photon keeps moving forward in a scattered direction. But sometimes, it can be deflected backwards if it hits an atom head-on. This type of scattering doesn't affect X-ray imaging much because the X-rays used are usually stronger than 20 keV.

Compton scattering

Diagram showing how Compton scattering produces a scattered low-energy photon and free-electron

Compton scattering occurs when an X-ray photon with energy between 30keV and five MeV (megaelectronvolts) collides with an electron of a tissue atom. These photons have sufficient energy to exceed the binding energy of the electron, thus ejecting it from its atomic orbit. In the process, the photon transfers some of its energy to the electron, resulting in a new, lower-energy photon that is scattered by the interaction. This type of scattering produces both a free electron and a scattered, lower-energy X-ray photon.

Check out our explanation on Binding Energy.

Photoelectric effect absorption

Diagram showing how the photoelectric effect allows an atom to absorb an incident photon to eject an electron and emit a new X-ray photon
Diagram showing how the photoelectric effect allows an atom to absorb an incident photon to eject an electron and emit a new X-ray photon

The photoelectric effect occurs when X-ray photons with energies less than 100keV are absorbed by atoms in tissue. When the energy of the photon is equal to the shell binding energy of an atom, the photon and its energy can be absorbed by the atom. The energy is then transferred to an electron, which is ejected from the atom.

As a result of this process, the atom becomes ionized and is in a higher energy state. The atom will return to its ground state by emitting X-rays with a wavelength characteristic of the atom type. These emitted X-rays are at a different energy level than the incident photon and will not travel coherently with X-ray photons from the source.

Pair production absorption

Diagram showing how pair production absorbs a high-energy photon to produce an electron-positron pair and subsequently emit two 511keV photons
Diagram showing how pair production absorbs a high-energy photon to produce an electron-positron pair and subsequently emit two 511keV photons

There are four main mechanisms of X-ray attenuation, each of which depends on the photon energy and the material of the tissue being imaged. Two of these mechanisms, Compton scattering and Rayleigh scattering, scatter photons, while the other two, photoelectric absorption and pair production absorption, absorb photons. Pair production absorption is a mechanism that affects very high-energy photons over 1.022MeV. At these energy levels, a photon can interact with an atom's nucleus, transferring all its energy to produce an electron and a positron. These antiparticles may travel for a short distance before interacting with each other (or other nearby electrons/positrons), where they are annihilated and transformed into a pair of 511keV photons. The pair of newly produced photons travel from the point of annihilation in diametrically opposite directions, ensuring momentum is conserved. The effect of pair production absorption grows as the photon energy increases, meaning it is the dominant mechanism at high energies.

Overview of the different types of X-ray absorptions

Low-energy photons are more easily attenuated than those with higher energy during X-ray scans because the probability of photoelectric absorption (the primary attenuation mechanism at X-ray scan energy levels) is proportional to (Z/E)3, where Z is the atomic number of the atoms in the tissue and E is the X-ray photon energy. This means that the lower-energy photons in the X-ray beam are, on average, absorbed sooner as they pass through the patient, resulting in increasing average photon energy from the front to the back of the patient. Since low-energy photons are more likely to be absorbed, the energy deposition dose is highest at the patient’s skin and decreases as the beam passes through.

Implications of attenuation on the X-ray procedure

Because most energy deposition occurs near the skin, one of the risks of X-rays is skin injuries, such as erythema (reddening of the skin) and epilation (hair loss). This risk is greater for larger patients, as they will require higher doses for the beam to penetrate body parts and produce a helpful image. However, the risk of skin injuries is generally low, as modern X-ray equipment is designed to minimize the dose to the patient while still producing high-quality images.

Attenuation coefficients can estimate what types of tissue different regions represent based on the amount of attenuation of the initial beam intensity. Different tissues have different attenuation coefficients due to differences in their atomic number, density, and thickness. For example, bone has a higher attenuation coefficient than soft tissue, which is why it appears brighter on an X-ray image. By analyzing the attenuation coefficients of different tissues, radiologists can identify abnormalities and diagnose medical conditions.

Absorption of X-rays formula: attenuation coefficient

The four main attenuation mechanisms outlined above (photoelectric effect, Compton scattering, pair production, and coherent scattering) show that for photons with a given energy, the material (which influences Z) and tissue thickness control the amount of attenuation the X-ray beam undergoes. The intensity of X-rays transmitted through a substance relative to the initial beam intensity is given by the equation below:

I = I0 * e^(-μx)

where I0 is the initial intensity of the photons, x is the tissue thickness (distance travelled), and μ is the linear attenuation coefficient for the photon energy. Larger values of μ indicate greater X-ray attenuation, meaning substances like bone have a larger coefficient than soft tissues. The SI unit of attenuation coefficients is m^-1.

By measuring the intensity of the X-ray beam before and after it passes through a subject, radiologists can calculate the linear attenuation coefficient and use it to estimate the thickness and composition of tissues in the body. This information is essential for the diagnosis and treatment of many medical conditions.

Effect of attenuation on dose

To produce an X-ray image with a good level of detail, the digital detector plate needs to measure a large enough number of photons to stand out against the background noise. The noise comes from photons that have been scattered as they travel through the body, or it can randomly arrive from an alternative source. The ratio of unattenuated photons in the X-ray beam (signal) to background noise is the signal-to-noise ratio (SNR). In X-rays, the SNR is related to the number of photons N in the X-ray dose.

The SNR improves as the number of photons increases, producing an image with more useful detail. We can increase the number of photons in two ways: prolonging the exposure time (mA) or increasing the accelerating voltage in the X-ray tube (as N ∝ KV^3).

Increasing the photon energy level also results in a lower proportion being attenuated by the patient's tissue, which offsets the higher photon energy and results in a lower overall dose being absorbed. However, since the X-ray energy level increases and the attenuation rate decreases, the level of contrast in the image produced is poorer due to attenuation creating the contrast between tissue types. Therefore, balancing the image contrast, noise, and patient dose requires a trade-off between the photon energy/accelerating voltage and exposure time.

Radiologists must carefully consider this trade-off when selecting X-ray parameters to ensure that the resulting image provides sufficient diagnostic information while minimizing the patient's exposure to ionizing radiation. Additionally, modern X-ray equipment often includes advanced noise reduction algorithms and image processing techniques to improve the SNR and enhance the clarity of the image.

Contrast mediums

Some soft tissues have attenuation coefficients too low to create enough contrast in a radiograph image, so we can use contrast mediums to improve the visibility of these structures. Bromine or iodine compounds are the two most commonly used contrast mediums as they are harmless to humans and have large atomic numbers (Z), representing large atoms with many electrons.

The primary attenuation mechanism for X-ray imaging is the photoelectric effect. As this relies on the incoming photon colliding with an electron, larger atoms with a greater number of electrons are more likely to cause photoelectric scattering than smaller ones. Because of this, the photoelectric attenuation coefficient is proportional to the cube of the atomic number (μ∝Z^3), making iodine or bromine far more absorbent than soft tissues, which primarily contain smaller atoms. This allows these compounds to be injected into blood vessels or the digestive tract to capture X-ray images of soft tissue structures.

The use of contrast mediums can significantly enhance the diagnostic value of X-ray imaging, allowing radiologists to identify otherwise invisible structures or abnormalities. However, the use of contrast agents can also pose certain risks, including allergic reactions or adverse effects on kidney function in some patients. Careful consideration and monitoring are necessary to ensure the safe and effective use of contrast mediums in X-ray imaging.

Absorption of X-Rays - Key takeaways

The absorption (or attenuation) of X-rays pass through tissues in a patient's body is what produces contrast in the image and allows us to distinguish tissues. There are four main mechanisms of X-ray attenuation: two, which absorb photons, and two, which scatter photons. The contribution of each of these mechanisms depends on the photon energy E and material (atomic number Z) of the tissue. We can use the attenuation coefficient μ to calculate the expected attenuation for a given material, thickness, and photon energy.

Achieving an image with sufficiently good contrast, low noise, and reasonable patient dose requires balancing the photon energy and exposure time. Radiologists must consider the trade-off between these parameters to ensure that the resulting image provides sufficient diagnostic information while minimizing the patient's exposure to ionizing radiation.

In cases where soft tissues have low attenuation coefficients, contrast mediums can be used to improve image contrast. These compounds typically have large atomic numbers, allowing them to absorb X-rays more effectively and enhance the visibility of certain structures. However, the use of contrast agents can pose certain risks to patients and requires careful consideration and monitoring by medical professionals.

Overall, the selection of X-ray parameters and the use of contrast mediums are critical for producing high-quality images that allow medical professionals to diagnose and treat a wide range of conditions effectively.

Absorption of X-Rays

What does X-ray absorption depend on?

The amount of absorption (or attenuation) of an X-ray beam is affected by the energy of the X-ray photons E and the atomic number(s) Z of the substance the beam is penetrating. The photon energy determines the relative contribution of the four main attenuation mechanisms, while substances with higher atomic numbers (and larger atoms) are more likely to absorb or scatter the beam, resulting in greater attenuation.

What are the kinds of X-ray absorption?

There are four main mechanisms of X-ray beam absorption (or attenuation). There are two scattering mechanisms, simple scattering and Compton scattering, and two absorption mechanisms, the photoelectric effect and pair production.

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