What happens to photons that scatter as they pass through the body?

X-ray scatter due to Compton Scatter generates background haze in x-ray images and there is a desire to reduce the scatter impact on image quality. X-ray scatter depends on several physical factors including: body habitus (increases with increased habitus), kVp, collimation (increases with increased kVp), air gap (decreases with increased air gap), and anti-scatter grids (decreases with increased grid ratio). When a grid is used to reduce x-ray scatter it increases the contrast by a contrast improvement factor (k), but additional radiation dose is needed to compensate for the photons blocked by the grid (Bucky Factor).

We have the full blog post below with a description of X-ray scatter with the video links embedded directly in the descriptions. However, if you would like to skip straight to the YouTube videos that is also and option here.

As an x-ray technologist or radiographer one of the important technical considerations is the contribution of x-ray scatter in the images. This image shows a standard reference condition, where the green lines indicate primary photons that are transmitted through the patient, deposit their energy locally (via the photo-electric effect) and the red lines indicate photons that undergo Compton Scattering within the body.

The contrast in X-ray images is generated by the difference between the absorbed photons and the transmitted photons that are measured on the detector.

Primary photons that hit the detector and are measured are the green ones but some photons scatter and change their directions (red ones). This can even happen multiple times while the photons travel through the body.

Since both the primary and scattered photons are measured in the detector the scatter signal degrades the image. Specifically, in x-ray imaging Compton scattering leads to an additional background haze in the image.

In this post we will go through the dependencies of x-ray scatter on the technical parameters in x-ray imaging. 

Collimation

Depending on the required imaging area for a given clinical task we can change the coverage of x-rays irradiating the body by opening or closing collimator blades. This is termed the x-ray field and is typically also visualized with a light field covering the same area on the patient.

As shown on the picture, a narrow collimation (ie. a small x-ray field) results in smaller yellow area and a smaller volume of the patient where scatter can be generated.

If we increase the size of the x-ray field, (i.e. make the yellow irradiated regions larger) then more of the body is irradiated at once.

Since more of the tissue in the body is irradiated, and there is some probability of Compton scatter for all of this tissue, there will be more x-ray scatter measured on the detector.

In general, when we just need to see a certain part of the anatomy, it is always beneficial to collimate down, so that we are at the minimum region needed to see all the anatomy of interest for that clinical test.

Rad Take-home Point: narrower collimation to a smaller region irradiated will reduce scatter, and when we have to go to wider collimation, this will lead to an increase in X-ray scatter.

Thickness Dependence

X-ray scatter depends on the thickness of anatomy we are imaging using x-ray. As we already know, scatter leads to overall haze in the background of the image.

In the case of a thicker body, when patient body habitus is larger, the photons have to travel more distance in the body and scatter events will take place more frequently.

Thicker tissues mean more scatter events (see more red in-scattering photons in this Figure compared with the reference), and more background haze. This leads to very simple rule: thicker the anatomy, the higher the scatter, the thinner the anatomy, the lower the scatter. This is the general reason why we want to use compression in areas where it is possible to compress, such as when doing x-ray images of the breast for mammography.

Rad Take-home Point:  Remember that thinner habitus means less scatter, this is one reason to use compression whenever possible in mammography. 

KVP Dependence

In clinical x-ray imaging there are a number of trade-offs to consider in the selection of the technical parameters. One factor to consider is to ensure that sufficient x-ray photons will pass through the patient and be incident on the detector.

Thus for large patients it can be necessary to use higher kVp in order to have sufficient penetration through the patient. However, one downside to consider when making this choice is the impact of x-ray scatter.

Remember, we have two dominant types of interactions – photoelectric and Compton.

Photoelectric interaction is the most desirable because the photons are traveling in straight lines and they either stop in the body or continue to the detector. While Compton scatter leads to a background haze in the detector.

From this Figure we can see that as we increase the kVp the likelihood for Compton interactions goes up significantly. Therefore, it is desirable from a scatter perspective to use lower kVp.

Rad Take-home Point: use lower energy when possible to avoid degradation of quality of the resulting image due to the effect of Compton interaction. 

Air Gap

Scatter measured on the detector depends on the distance between body and detector. This distance is called Air Gap.

The question is how does an Air Gap affect the quality of an x-ray image in terms of the x-ray scatter?

In reference case we have primary events, green photons going through the body, and secondary events shown as red that lead to scatter.

So, if we move detector back further away, ie. we increase distance, Air Gap. Then scatter events still take place but some of photons will out-scatter and won’t be measured on the detector (see example red photons in the Figure. This means we will have reduced scatter.

Rad Take-home Point: Larger Air gap leads to less scatter, while smaller Air Gap will lead to more scatter. 

Anti-scatter Grids

All of the sections above deal with the impact of properties of the patient of standard technical parameters of the x-ray system on x-ray scatter. In this section we will discuss a mechanism designed explicitly to reduce scatter in x-ray images.

As mentioned above larger patients generate more scatter so typically anti-scatter grids are not required for the smallest anatomy such as extremities, but anti-scatter grids are used for brain, spine, abdomen, breast and contrast studies.

The first x-ray anti-scatter grid was proposed in 1913 and significant improvements have been made over time.

Main idea of introducing an anti-scatter grid is to reduce the fraction of scattered x-rays that reach the detector.

For anti-scatter grids an x-ray attenuating material, such as lead, is commonly used. So, primary photons will pass through grid plates while scatter x-rays will more likely be stopped by the plates of the grid.

The grid ratio is the ratio of the height of the grid plate, often called grid septa, to the grid width.

The grid ratio is the ratio of the height of the grid plate, often called grid septa, to the separation width (grid Ratio = h/D). If the width of the grid plates is left constant then higher grid plates will block more scatter. Common grid ratios used clinically are 4:1, 6:1, 10:1 or 12:1. There is also a desire to have the grid septa width (t) to be as thin as reasonably possible so that fewer primary photons are blocked before reaching the detector. Another parameter of anti-scatter grids is the grid frequency which is how often the grid repeats and it is defined as 1/(D+t).

In x-ray imaging the x-rays are emitted from the anode of the x-ray tube. The x-rays all come out from there in what is called a divergent geometry.

The primary x-rays all travel in straight lines from the x-ray tube to the detector. Therefore, in order to block fewer primary photons it is best to have a focused grid where the septa are directed toward the x-ray source.

From this focused grid figure it is clear that there should be more focusing of the grid as the source to detector distance is smaller and less focusing as the source to detector distance is larger.

Therefore, in clinical practice several x-ray grids are used, each of which has a range of distances that it operates best.

Also, moving grids are used to reduce image artifacts as the effect of the grid will be averaged out during the acquisition.

The main purpose for using the scatter grid is to increase the image contrast by reducing the background fog due to scatter.

The contrast enhancement factor (k) measures how impactful the grid is at improving the contrast. It is the ratio of image contrast with the grid present to image contrast without the grid.

The contrast enhancement factor will generally be higher for higher grid ratio grids, and the average k for antiscatter grids is ~1.5-2.5.

However, these grids do block more of the photons from hitting the detector and therefore the x-ray intensity needs to be increased.

The Bucky Factor is named after Gustav Bucky who invented anti-scatter grids.

Since anti-scatter grids block more of the x-rays in order to keep the x-ray intensity the same on the detector the incident x-ray intensity must be increased.

The Bucky Factor is the ratio of the increase in patient dose with the grid to the patient dose without the grid. If the Bucky Factor is 2, then twice the mA is required to keep the same detector exposure.

Therefore, when changing grids patient dose and tube loading concerns must be address and the Bucky factor helps us keep track of this effect.

Rad Take-home Point: anti-scatter grids can be effective at reducing the contribution of x-ray scatter in and a higher grid ratio makes the grid more efficient at reducing x-ray scatter.

Summary

Collimation is the area of the x-rays exposed at one time. If it increases that means we have a larger volume of the patient that has been covered with x-rays, and therefore there’s going to be a higher chance of scattered x-rays ending up measured on the detector.

Thickness of the patient: We usually don’t have a lot of control over the patient’s thickness. But we can compress breast to achieve smaller thickness of this part of body, so x-rays will be passing through the tissue.

KVP Dependence: As we move to the higher energies, we’re going to have more x-ray scatter from Compton scattering events. So when possible we want to reduce the KVP keeping in mind that we do need a higher KVP to penetrate thicker individuals.

Air Gap:  If we increase the Air Gap, the distance between the patient and the detector, it will reduce x-rays scatter measurements on the detector.

Ant-scatter grids: They are designed specifically to reduce scatter. An increase in the grid ratio will make the grid more effective at reducing scatter, but will also require higher x-ray tube technique.