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Scatter radiation is one of the culprit of image quality

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degradation.

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In fact,

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from the time X-rays were developed, X-ray imaging started,

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we have been constantly fighting to develop

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process to eliminate scatter radiation.

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So what is scatter radiation?

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Let's look at the image contrast first.

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Imagine this is an object with two different material, bone and a soft

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tissue. The way we get contrast means the X-rays are

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absorbed differently in these two object.

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Therefore, the signal coming out, if you create an equation like

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this, the signal difference between these two divided

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by the background signal is what we define as a contrast.

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And this contrast is in the absence of scatter.

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There is no scatter, which is purely difference in

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the signal differences between the two.

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Now, the scatter radiation will reduce image contrast,

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and that is influenced by the patient thickness,

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X-ray field area, X-ray energies, and the scatter

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radiation is removed with the Compton scattering, and it is

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removed using grids, which we are going to use it to remove the

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scatter radiation.

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Now, let's say, if there is a scatter,

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the contrast will go

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dramatically with the presence of scatter.

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Before introducing scatter radiation concept, this one, let's look

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at this as a figure.

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In this figure, the X-rays are all primary radiation, which is

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called P, is the primary radiation, passing through the body.

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However, there is one area where the primary radiation

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get deflected, absorbed, and some energy is coming out,

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and this deflected X-rays now reach the same focal

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point.

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Means this is contributed not only by the primary, but also by

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the secondary scatter. And that's what this happen

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is primary... And we use a concept called scatter to

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primary ratio.

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Scatter to primary ratio of three means for every

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primary beam, there is three scatters

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produced, and they are interacting, reaching the same point.

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Imagine now there are 1,000 X-ray photons needed to create

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an image. Now they are all passing through the object, but

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out of this 1,000, almost 300 are now, because they're scattered from different

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area, that's where the fuzziness and the noisiness

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adds, increases. So now how is it happen?

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So the contrast in the absence of any scatter, that is

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the zero scatter to primary ratio,

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let's say is 50%,

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means the contrast between the object, and the background

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is 50%, let's say.

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If there is a scatter,

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for every scatter to primary ratio, if it is one,

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the contrast decreases by 50%,

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almost half.

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If it is scatter primary ratio is two, it further goes down.

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And this is how the contrast changes with respect to the presence

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of scatter.

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An object that has 50% contrast in scatter

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absence has contrast reduced to nearly one-fourth of

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original contrast with the scatter to primary ratio of three.

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In reality, we encounter a scatter to primary ratio of three to four

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easily because of the patient thickness and the field size

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and so forth.

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So now,

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in order to remove the scatter,

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a device was invented, and that's called the anti-scatter

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grid.

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It's basically imagine a mat with a bamboo stick.

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In place between the bamboo stick, you have a lead septum in between,

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and that's how a typical anti-scatter grid looks.

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And the purpose of it is like there is lead septum will

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absorb any scatter coming in different direction.

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Therefore, only primary beam will hit the detector.

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That is idealistic.

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At the same time, when this lead septum will also absorb some

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of the primary beam, therefore, when you have an

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anti-scatter grid introduced, you have to increase the patient

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dose.

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So the grid is placed between the patient and the image receptor,

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and uses geometries, reduce the scatter, and there is

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a concept called as a grid ratio.

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The grid ratio is defined as follows.

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It is ratio, the height of the spectrum, this is the height,

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means the length of the septum, and divided by the width.

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The spacing between the septum is called the grid ratio.

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That can typically go from 10 to one or 12 is to one,

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five is to one, and they're usually about 60 line

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per centimeter. That's why we don't see the grid as an empty space.

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It's all compacted in the carbon fiber.

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Now,

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here are the typical grid ratio.

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For radiography and fluoroscopy, especially for table top

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radiography, the grid ratio is eight is to one,

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which means the height of the septum is eight, while the

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width is one,

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whatever unit you use.

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And if you can notice here, they are aligned in such a way, they are

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all focused towards the X-ray focus spot to

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minimize absorption of the primary beam.

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So radiography is eight is to one,

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and then it goes up to or 10 is to one or 12 is to

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one.

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The larger grid ratio is typically used in chest

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radiography because chest radiographs are taken at a

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longer distance compared to the table top radiograph.

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The tabletop radio X-rays are done at 40 inches,

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whereas chest X-rays are done at 72 inches, therefore,

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you can use 12 is to 1 grid.

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In mammography, we use a smaller grid, five is to one or four

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is to one, because the distance between the source and the

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detector is much lesser than radiography or fluoroscopy.

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Because of the introduction of this grid, we have a new concept

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called Bucky.

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Bucky factor is basically the ratio of the

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entrance exposure with grid to the entrance exposure

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without grid.

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So when there's no grid, the Bucky factor is one.

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But if there is a grid, then the dose with the grid has to be higher,

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therefore the Bucky factor will be higher.

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It can range anywhere from two to four, means when you're using

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grid, the dose required to create an image is four times

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higher than without grid. So there is a trade-off.

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Do you want a grid and increase the dose to remove the scatter,

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or you want to remove the grid, reduce the dose, but you are

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encountered with the noise and scatter radiation?

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There are certain clinical scenario where there is no need for

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grid,

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and that's in pediatric case, where the object is much smaller,

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therefore the scatter is lesser. Therefore, in order to keep the radiation

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dose smaller, we remove the grid in pediatric cases.

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Here, an example shown, the scatter to primary beam

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varies with respect to patient thickness.

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If it is a 30-centimeter patient thickness, abdominal thickness, look at

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here, for a 10-centimeter field of view, the

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scatter radiation is about more than five.

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If it is 20 centimeter or 30 centimeter, which is a normal average

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abdominal size, that scatter primary ratio is about six,

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not three. That's what it determines the Bucky factor.

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There is another way to remove the scatter also,

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by simply moving the patient or the detector

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further away from the object.

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This way, the air gap will remove some of the scatter going

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away. They don't hit the detector,

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and this is called air gap methodology of scatter

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radiation. But in this case, the object get

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magnified, so we are not desired in X-rays, so we are trying

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to keep the grid to remove the scatter radiation.

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But one area where we like to use these opportunities,

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in mammography, when they do a mammogram image and you

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need to magnify the image, the breast is placed at a higher

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distance on a breast plate, magnification plate,

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allowing air to use as a way to reduce

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scatter radiation, there is no grid, and also to minimize the radiation

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dose to the patient. So one area which air gap

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methodology is applied is in mammography.