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Let's look at the impact of X-ray beam hardening.

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What is beam hardening?

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When X-rays are produced from an X-ray tube, the X-rays are

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all different energies. There are some low-energy X-rays, some very high-energy

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X-rays. The maximum energy produced is defined by the tube

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voltage we set. But a lot of the low energy is useless for

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image creation. It basically increases the radiation exposure

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to patient.

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So we need to remove some of them,

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and that's done, we are process called as

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hardening the X-ray beam, called as beam hardening.

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And it is done as follows.

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So, the soft X-rays get absorbed on the skin

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surface and do not contribute to formation.

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Therefore, you want to harden the beam to remove the

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soft X-rays and harden the beam so that it has

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sufficient energy to penetrate patient.

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So,

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here is the photon energy and HVL increases.

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So here is the first interaction, all energy range shown by

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different frequencies and wavelength here.

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Lot of the low wavelength is absorbed.

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As you go more and more thickness is placed in between, you

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get more of an average higher energy.

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And that can be shown in this particular spectrum.

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So here is again, this is a typical X-ray spectrum,

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and if I look at here, I can say the tube voltage is 100

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kVp because the maximum energy produced is

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100 kV, we are not beyond that. In this

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particular target or material, there was a characteristic around this

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area, around 60 to 72 here. It may be due to the X-ray.

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So now you calculate what is called as an average energy.

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This is called the peak energy, and if you take all the X-rays

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of different photon,

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number of X-rays in different energy, you average it, we get what is called as

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average energy.

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The average energy is usually between one third to

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two third of the maximum energy. So if it is 100

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kVp, I can confidently say the average energy of

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the X-ray coming out is around 33 to

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66 keV.

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What I'm trying to show here is like the spectrum already has blocked off

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a lot of this low energy, and that's done by the window of the

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X-ray tube or the glass casing, which absorb a lot of the low energy.

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Now, for between these two, it's done at 100 kV,

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250 mA, everything same, except now you add

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one millimeter of aluminum at the point of X-rays coming out of

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the X-ray tube.

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Immediately what happen is like the average energy

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will increase.

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Means the energy coming out is now bit more stronger,

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more penetrable, and it can penetrate more larger body,

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and that's where the shift happened to the right side.

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But notice also there is a intensity goes down because

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some of the photons are lost by absorption. That's why we get this type.

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Now, shown here is no filtration

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with one millimeter filtration, and now with a two and a half

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millimeter of aluminum added, it can go further down,

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and this is with the copper is even more further.

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Intensity goes down, but it's the shift towards the right side.

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Then one can say you can keep on adding more and more material.

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If we add more and more filter material, if the average energy

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is very high, that will begin to impact the image

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contrast.

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Bremsstrahlung process produces wide spectrum of energy, which is that's why

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we call it polyenergetic X-ray beam.

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The beam hardening will shift off X-ray spectrum to

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higher effective energies due to removal of lower

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energy X-ray photon.

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In one slide, basically, the average energy

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keeps decreasing or increasing to the higher level, but the

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intensity will decrease. But this is the

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impact of the beam hardening

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as shown here.

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How does it matter?

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That will directly impact on the image contrast.

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On the upper side, here shown is an average size patient.

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The thickness is about 23 centimeter.

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Anywhere from 20 to 30 centimeter is considered as average.

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The distance between the source and the receptor is 100 centimeter.

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Now, in this scenario, for a 23 centimeter,

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if you set the tube voltage at 50 kV, this

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exposure is 1,656 mR.

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That is the amount of radiation you want to produce or amount of X-rays

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needed to create an image.

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Lower kV means higher dose to the patient.

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It can produce higher contrast, but it will also produce higher patient

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dose.

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Let's say if you go down the list, at 140 kV, the

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entrance X-rays is only 153 mR,

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but that will lose the contrast, showing the contrast differentiating with the

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tissue.

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Therefore, for diagnostic radiology, there's a very limited

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range of kV we can use and still

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producing a good image quality.

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So here is the diagram show the dose

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or contrast percentage, which is the kVp.

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If you look at this, this is the entrance dose,

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and this is the contrast agent, and this is where the dose to the

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patient goes when you're imaging a one-millimeter bone chip.

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Back again to demonstrate, we are working in the range of

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10 to 100 kV for diagnostic imaging.

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The common point is Compton scattering and the photoelectric

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absorption.

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The HVL is defined as follows,

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is given as in a narrow beam geometry, if X-ray is passing

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through, you keep putting the material until the intensity

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goes by 50%, and that material is called half value

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layer.

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Why is that important? I'm trying to term in terms of the patient

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thickness.

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Let's say for a patient thickness and a soft tissue of

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37 millimeter,

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just about four centimeter,

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for a 70 kV X-rays,

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it only requires 3.37 millimeter to cut the intensity

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by half.

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For a 140 kV gammas, it requires about 44 millimeter.

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What I'm trying to show you is

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in a tissue,

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X-rays are absorbed.

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Half of the X-rays incident on the tissues are

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absorbed every four centimeter in distance.

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So approximately four centimeter means if a patient abdominal

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thickness is 20 centimeter,

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after every 40 millimeter, four centimeter, half of the

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X-rays are remaining. So by the time X-rays comes out, it's

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approximately 15% of the incident radiation,

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which is used for image creation. So that's where the challenge is.

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One other point which I want to bring here is

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for lead. You can see here the lead,

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every 0.2 millimeter will absorb by half of the radiation.

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That's one of the reason why we use lead apron, because any

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scattering come will be completely absorbed by the lead.

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How does the attenuation makes the image contrast?

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The image contrast is due to variation in attenuation.

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In this case, the thickness is same

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in both the situation,

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but this is a wax and this is aluminum.

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The lower attenuation material, wax, has lower contrast,

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whereas this has a higher contrast because lot of it is absorbed

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compared to the background, therefore, this is much more differentiated.