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What are the factors affecting X-ray emission?

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Let's examine the factors affecting the X-ray emission.

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First and foremost,

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we want a high-quality X-ray because that will describe the

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penetrability. The quality of the X-rays will

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determine how good it is to penetrate through the material,

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and this quality is determined by the tube voltage.

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I'm talking about the tube voltage set between the anode and cathode, which

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will accelerate the electron at certain speed.

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So higher the speed of the electron hitting the anode

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will result in the X-rays of a greater energy.

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Higher energy X-ray photon have higher quality,

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means they can penetrate much thicker.

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The second factor is the quantity.

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The number of photon, X-ray photons, produced is

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determined by the quantity of X-rays,

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and that's determined by the tube current.

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And it means this tube current is the tube current you set on the

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cathode, where you results in the electron emitting out,

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which are then results in X-ray production.

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Therefore, the quality is defined by the tube voltage.

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The quantity is kind of defined, determined by the tube current.

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Then we have the intensity. Intensity is how

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the X-ray intensity coming out and reaching the patient.

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There are different factors which affect X-ray tube emission.

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First and foremost is the target material.

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What type of target material? The common target material is

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tungsten. We also use molybdenum and rhodium,

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which are used specifically more so in mammography

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X-ray tube. The other factors are the tube voltage,

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the tube current,

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and then the generator X waveform.

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That's a generator of the X-ray generator.

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To a large extent, that is negated now because most of them are constant

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potential generator.

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Then the other factor which affect X-ray emission is the exposure time

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and the beam filtration.

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This is one of the few equations I'm going to show.

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The intensity of the X-ray emission is given by this formula,

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which is a combination of the atomic number of the target,

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that is a Z target, multiplied by the kV square,

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that is applied tube potential,

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and the tube current. And then there is also called as a B,

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attenuation factor, which is the self-absorption of the

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X-rays by the anode itself and any of the

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filtration we're going to insert it, and then multiplied by the K,

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which is a waveform factor.

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Nowadays, K is almost one, so which is more of a factor of the

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Z target, kV square, mA, and the B.

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So here is what is called the relative intensity.

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On the Y-axis is called the relative photon

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fluence,

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and on the X-axis is the energy.

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If we notice, the X-ray energies go anywhere from zero to 100 kV,

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120 kV, not beyond that.

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So you can see here the X-ray spectrum in different levels.

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And here, this is the relative intensity is

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proportional to the square of the kV in a

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diagnostic energy range,

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which means the intensity is proportional to kV square.

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So if we have set the intensity voltage at 80 kV,

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the next one is 160 kV. The ratio is the

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80 kV X-rays have almost a higher intensity, almost 80%

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higher in intensity.

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So you can see here, these are different-- They are all the same tube

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current, but different tube voltage, 60, 80, 100, and

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120. You can see the intensity changing.

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The quantity of the X-ray produced is proportional to Z

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target,

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kV square, and the product of mA

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times time. That is mAs.

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What I want to show here is the relative intensity to show you how

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different factors used in the imaging can

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change the intensities.

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On this particular spectrum, between A and B, the

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target is the same tungsten target,

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and the only difference between these two is the tube current.

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If you look in here, this is a 65.

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This is also 65 tube voltage. That's why you see the end of the photon

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producing the maximum tube voltage is few of them, and it

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ends here.

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The intensity, the number of photon produced increases with the

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tube current here.

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On the other side, the difference between these two, it is done at

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100 kV,

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and now there is 250 mA, and this is 500 mA.

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The spectrum is exactly the same. Now it is moved towards the right side,

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towards the maximum is 100 kV.

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Because the tungsten target has filter, you can see the

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characteristic also showing up here.

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When you talk about X-ray absorption, we also talk about

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attenuation.

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Attenuation is a way to think about how many of the X-rays

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interacting with matter is removed or absorbed.

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It's called as removal of photons from beam of X-ray or

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gamma rays as it passes through matter.

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In this attenuation, the absorption and

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scattering will contribute towards the attenuation.

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Especially in soft tissue,

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the photoelectric absorption dominates at low energy, as we

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discussed earlier.

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For in soft tissue, Compton scattering dominates at higher energy.

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The Rayleigh scatter occurs so low probability it is hardly we discuss it.

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Approximately 10% interaction in mammography and 5%

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in just radiography.

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At very high photon energy, greater than

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1.02 MeV, we see pair production which

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contribute to attenuation, and that's typically discussed in nuclear

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medicine and PET imaging and so forth.

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We define a term called linear attenuation coefficient,

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and also a mass attenuation coefficient.

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What's the difference between the two?

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The linear attenuation coefficient divided by the density of the

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material will give you what is called as a mass attenuation coefficient.

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What is linear attenuation coefficient?

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It is the fraction of the photons removed from

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monoenergetic beam of X-ray or gamma

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rays per unit thickness of material.

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For most of the image radiography, we don't use a

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monoenergetic, we use a Bremsstrahlung radiation, but still,

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some of the concept really still holds.

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So the linear attenuation coefficient is defined as

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mu and the unit is how much X-rays absorbed per

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centimeter of the material it's interacting.

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And if you want to normalize to the density, if you divide this by the

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gram, the density, then the unit will become mass attenuation

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coefficient. Unit is centimeter squared by gram.

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So let's examine here the attenuation through multiple material.

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If there is only one material of certain thickness, and if you're able

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to measure the intensity of the X-rays before it hit the

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material and after it came out of the material, we can

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approximately write a mathematical equation as follows: I

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equals I naught e to the power of minus mu x,

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where I naught is the incident intensity of the incident X-rays,

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and there is what is an attenuation factor.

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That is an exponential factor which depends on the linear

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attenuation coefficient and the thickness of the material.

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If you have a multiple material, you can write similarly the same equation,

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and that's also the principle behind CT, and that's also

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why in CT, we need collection around the object of

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multiple times.

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We also what is called as a 400 kV photon

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transversing through the soft tissue, the attenuation coefficient is

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0.016 per centimeter.

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For every millimeter, about 0.16 is

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absorbed. That means if there are 100

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monoenergetic photon interact with one millimeter thick

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of tissue, approximately 16 will be removed

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from beam due to absorption and scattering.

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That's the reason why it is important to mitigate the scatter.

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Similarly, we can write equation, the same mathematical expression.

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We can calculate the number of photon produced, and then by the

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attenuation coefficient.

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Shown here is a table of linear attenuation

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coefficient of different material of interest in radiography and in

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medical X-ray imaging.

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You can see that the difference in the attenuation is quite

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small between this fatty and ice or water.

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Whereas compact bone,

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water, and air, you can see a dramatic difference.

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That's why you can easily see the difference in the image of these three

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different material.

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We also introduce a concept called half value layer.

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What it means is like it is the amount of material you need to put

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to reduce the intensity of the X-rays by one half.

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Why is this important? We're going to see because object will

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block, the patient will absorb

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certain amount of X-rays as it passes through, and that's defined by

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the half value layer.

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It is an indirect measure of the photon energies of the beam

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when measured in a narrow beam. If we can measure in a narrow beam,

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it'll tell you how much of it is absorbed per centimeter or per millimeter.

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Here is an example of the effective energy with respect to half

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value layer.

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Since X-ray beam are polyenergetic,

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finding half value layer is a way to characterize the

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penetratability of the X-rays. Effective energy is

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estimated by the penetration of power of X-rays.

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Typically, you can see here

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the effective energy is one third to one half of the

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maximum value.