M1-2 X-Ray Biomedical Imaging Technology PDF

Module 1-2 : X-Ray Biomedical Imaging Technology 1 Overview of Technology Remember, we described the idea of a planar x-ray in the previous module (M1-1). An x-ray source outside the patient beams x-rays through the patient to a sensor on the other side. The different tissues and bone block (absorb)...

Module 1-2 : X-Ray Biomedical Imaging Technology 1 Overview of Technology Remember, we described the idea of a planar x-ray in the previous module (M1-1). An x-ray source outside the patient beams x-rays through the patient to a sensor on the other side. The different tissues and bone block (absorb) different amounts of the x-rays. Each element in the sensor measures how much of the x-ray beam makes it through a small volume of tissue. The image is a map of how much the x-rays are blocked and therefore a 2D map of tissues and bones. Figure 1: Summary of imaging tools (Image Tburg, 2023). To make the Generalized Biomedical Imaging Framework (GBIF) specific to x-ray imaging, we specified the sensor and processing elements. The remaining elements are very similar to standard photography in that the system captures a picture, which can be enhanced or examined, and then displayed to the user as an image. The unique aspect of x-ray Imaging is that high energy x-rays are beamed at the target and the passage of the waves through the patient is measured. Figure 2 below shows that x-rays have more energy than visible light or ultraviolet light, which goes to their ability to penetrate solid materials (we will talk more about waves next). Version 10-1-2023 Figure 2: X-ray imaging makes unique uses of electromagnetic waves that are passed through or blocked by different materials in the body. 2 Technology Details 2.1 What is an x-ray X-rays are a form of electromagnetic radiation as shown in Figure 3, where the top row of icons in the figure illustrates the ways that different wave properties are used in our daily lives. The main properties of a wave are: • • Wave Property 1: Travel with a constant velocity in vacuum. Rays (straight lines with arrows) are often used to explain the path and reflections of waves. Wave Property 2: Can be modeled as a particle, a photon that is a massless quantum of energy. Version 10-1-2023 Figure 3: X-Rays are high-frequency, high-energy waves that have special usefulness in helping reveal the internal structure of animals and people, and many objects. 2.1.1 Exploring Wave Property 1: Traveling Wave We have already said that x-rays are one type of wave that travels through space (air, bodies, etc). Figure 4 illustrates the two ways that we think about them as waves. On the left we think of the wave as traveling through materials at a certain velocity, which in a vacuum is c = speed of light. The wave is a sinusoidal shape, meaning it gets stronger and weaker along its length. The waves traveling toward a beach, although not the same kind of wave, give a picture of a moving wave. A complimentary way of describing a wave is to use a plot versus time to show how the amplitude (strength) varies with time. Note that both the wavelength and period are measures of how long it takes the wave to repeat. The period and wavelength are shown in multiple locations, it doesn’t matter where on the wave they are measured just so we capture the repeating nature. Figure 4: Waves are described in their spatial behavior by the wavelength (λ) and the velocity (which is c = speed of light in a vacuum). The same wave can be described by a plot of amplitude (strength) versus time where the important measurement is the period. Version 10-1-2023 Three parameters shown Figure 4 describe a wave: • • Period typically represented by the variable 𝑇𝑇 and frequency, typically represented by the variable 𝑓𝑓 Wavelength, typically represented by the variable 𝜆𝜆 These properties are not independent and are related by two simple equations: 𝑓𝑓𝑓𝑓 = 𝑐𝑐 where c is the speed of light = 3x108 m/s In Figure 5, the wave in the left panel is moving with a constant velocity past a fixed point labeled as the “Observer”. As the wave goes past the Observer, it will rise and fall between its maximum and minimum value. If the Observer records the height of the wave at every time, the plot in the right panel would be the result. Be sure to note that the axis for the plot on the left is “Distance” and the plot on the right is “Time”. We are talking about the same wave but from two different perspectives. Figure 5: Left panel shows a moving wave and the right panel shows what the Observer sees as the wave goes past the green peg in the “Distance” plots. To further illustrate the key wave parameters, two waves are illustrated in Figure 6. In the right panel where the time plot is shown, wave2 is shown to be twice the frequency of wave1, meaning it repeats twice as often as wave1. In the left panel where the distance plot is shown, wave2 is now shown to be half the wavelength of wave1, meaning it repeats twice as often as wave1. Version 10-1-2023 Figure 6: Comparison of two waves. Wave two has twice the frequency and half the wavelength of wave two. 2.1.2 Wave Property 2: Particle Behavior In Figure 7, the wave description of the x-ray is grayed out to highlight that we need to switch to the Wave Property 2: Particle Behavior to understand the amount of energy in an x-ray. In Figure 7, a photon traveling in a straight line is used to show that all of the energy in the wave is concentrated in the photon, that one moving point. When we consider an x-ray interacting with tissue and bone, the photon model of energy is required. An actual beam will comprise many photons. Figure 7: Wave Property 2: Particle Behavior considers the x-ray to be moving like a particle (ignore the wave properties for now). All of the energy of the wave is concentrated in the photon. Energy, typically represented by the variable 𝐸𝐸 is calculated as 𝐸𝐸 = ℎ𝑓𝑓 where ℎ is Planck’s constant = 4.135667696...×10−15 eV/Hz. Revisiting the two waves described in Figure 6, the energy of the waves can be calculated as 𝐸𝐸1 = ℎ𝑓𝑓1 and 𝐸𝐸2 = ℎ𝑓𝑓2 = ℎ(2𝑓𝑓1 )= 2(ℎ𝑓𝑓1 )=2𝐸𝐸1 which means that the higher frequency wave has more energy than the lower frequency wave or the shorter wavelength wave has more energy than the larger wavelength wave. Version 10-1-2023 Two observations we can make from the equations and the chart in Figure 3: • • Increase in frequency means that the wavelength must decrease. The frequency of the different waves increases from left to right in the Figure 3. X-rays have very high frequency, much higher than light. The light blue bar shows that wavelength decreases with frequency. Increase in frequency means that the energy of the wave also increases. The frequency of the different waves increases from left to right in the figure. X-rays have very high energy, much higher than light. The light pink bar shows that energy increases with frequency. Paying a little more attention to the electromagnetic spectrum in Figure 3, we see that x-rays are not just one kind of wave but have a range of wavelengths (and hence frequencies) and energy. We will see that the clinical use of x-rays will call on different wavelengths for specific applications and situations. 2.2 How are x-rays created? The x-rays are created by a multistep process: 1. Create unbound electrons. 2. Accelerate the electrons in an electric field to increase their energy. 3. Convert the high-energy electrons into photons (waves). A device that can produce x-rays using such a process is depicted in Figure 8. Thermal energy is used to eject electrons from a metal, the electrons are pulled across a vacuum where they move faster and gain energy (remember that an electron has mass), and the electrons strike a metal plate where their energy is used to eject photons (~x-rays). Figure 8: Depiction of an x-ray tube (side-view) that shows the three elements of the x-ray generation process. 2.2.1 Step 1: Creating unbound electrons (an electron cloud) An electric current can be used to heat a metal, as illustrated in the two common applications in Figure 9. The pictures show the heating process can cause photons to be emitted, e.g. light in incandescent light bulb. Version 10-1-2023 Figure 9: Common uses of electric current to heat a metal element. Heated metal can also create a cloud of electrons as shown in Figure 10. Electrons that receive enough energy, exceeding the work function of the material, are ejected into a local cloud of electrons around the surface of the metal. Figure 10: Heated metal can eject photons (as light) but also creates a cloud of electrons. X-ray Parameter 1: Heating Current - The current applied to heat the element to create the cloud of electrons must be set (and even varied by the user) to create more or less electrons in the cloud. 2.2.2 Step 2: Accelerating an electron in an electric field An electric field is created by applying a voltage across a gap as shown in Figure 11. The larger the voltage the stronger the electric field. Version 10-1-2023 Figure 11: Left: an electric field is created by applying a voltage across a gap. Right: common voltage sources and size of the voltage. 1.5V is small and 1kV is large. As illustrated in Figure 12, the voltage represents the potential to do work on a charge, much the way a hill has the potential to do work on a rock. Figure 12: The electric field is analogous to a rock on a hill. It takes energy to move the rock up the hill. The hill can supply energy to move the rock. The magnitude of the applied voltage can increase or decrease the electric field. Larger voltages will produce a larger electric field which will accelerate the electron faster. The electron in the higher electric field case will be going faster and have more energy at the end of the travel. As shown in Figure 13, the analogy of the rock rolling down the hill holds. A steeper hill will produce a faster moving rock with more energy. Version 10-1-2023 Figure 13: Continuing the hill analogy, a higher hill takes more energy to push the rock up and can provide more energy to move the rock. Now put the two processes together as shown in Figure 14. Thermal energy creates a cloud of electrons. The electric field accelerates these electrons toward the positive plate. Two variables control this process: the temperature of the heating the plate and the voltage. More electrons can be available in the electron cloud by increasing the thermal activity; that is, increase current to increase thermal activity. The strength of the electric field can be increased by increasing voltage, and hence the amount of energy of each electron moving in the field is increased. As we leave this section, lets review what we have achieved: we have a beam of high energy electrons racing toward a metal plate. Version 10-1-2023 Figure 14: Thermal energy creates a cloud of electrons and the electric field accelerates these electrons toward the positive plate. 2.2.3 Step 3: Creation of x-rays When the electrons hit the metal plate, the energy of the electrons is affected in one of two ways: collision with a valence electron or slowing by forces in the atoms. In either case, the energy of the incoming electron is transformed into a photo emission. Figure 15 below illustrates the two types of interacts and resulting emissions. The electrons that hit valence electrons produce characteristic radiation, which is related to the energy levels of the electrons in the metal atoms. The electrons that are slowed by the atoms emit braking radiation. Version 10-1-2023 Figure 15: The electrons that hit valence electrons produce characteristic radiation, which is related to the energy levels of the electrons in the metal atoms. The electrons that are slowed by the atoms emit braking radiation. For the characteristic radiation, the incoming electron ejects an electron in the atom. An electron from a lower energy shell will move to replace the missing electron. The difference in binding energies between the original electron position and the new position is given off as energy in a photon (an x-ray wave). The frequency of the emitted wave, is set by the difference in binding energies as 𝐸𝐸 = ℎ𝑓𝑓 where ℎ is Planck’s constant -> 𝑓𝑓 = ∆𝐸𝐸 , ℎ where ∆E is the notation to represent this change in energy. Since the difference in binding energies are always the same for any transition between shells and there are a limited number of electron shells, there are a limited number of frequencies that can be emitted by a specific metal. Each metal will have a Version 10-1-2023 signature of wavelengths (characteristic radiation) that can be emitted in this manner. The first plot on the right of Figure 16 shows that characteristic radiation occurs at just a few frequencies. The braking radiation is a lot less defined process and can represent a range of energy changes. Again, since frequency is proportional to energy, there are a range of frequencies emitted by braking radiation. The second plot in Figure 16 illustrates that there are too many frequencies to count and so we see a line across a range of frequencies. The first two plots in Figure 16 illustrate that the quantity of the characteristic radiation is higher. The total radiation is the sum of the two types as represented in the bottom plot of Figure 16. Figure 16: Total x-ray emissions from a metal is the characteristic radiation plus the braking radiation. Only about 1% of incident electrons are converted to x-rays, the rest is converted to heat in the metal. X-ray Parameter 2: Voltage - The voltage applied to create the electric field must be set (and even varied by the user) to create x-rays with a specific energy. 2.2.4 How are x-rays directed at patient? The final imaging process details rely on filters, shielding, and geometry to direct the x-rays at the patient. A photo of an x-ray tube shown in Figure 17 illustrates the x-rays radiate from the tube in a cone-shaped beam. Version 10-1-2023 Figure 17: Photo of x-ray tube (iStock downloaded 2/13/2023) annotated to show electrons strike the angle plate to create a cone-shaped beam of x-rays. The x-ray imaging system is set up to direct the create a cone of x-rays at an area of the patient as shown in Figure 18. Shaping and filtering are done to limit the frequencies and direction of the waves. Version 10-1-2023 Figure 18: X-Rays generated by x-ray source are directed toward a patient. 2.2.5 What happens when x-rays hit a material? The basic principle is that different tissue will absorb and scatter X-rays at different rates, for example bone will absorb X-rays more effectively than soft tissue. The x-rays enter a tissue and are attenuated, their energy is reduced. There are two main mechanisms: photoelectric and Compton scattering. 2.2.5.1 Photoelectric Attenuation This is very similar to the creation of x-rays in the metal of the x-ray source. The incoming electron ejects an electron in an atom of a tissue. An electron from a lower energy shell will move to replace the missing electron. The difference in binding energies between the original electron position and the new position is given off as energy in a photon (an x-ray wave). The frequency of the emitted wave, is set by the difference in binding energies as E=hf where h is Planck’s constant -> f=∆E/h, where ∆E is the change in energy. Now, the transitions that can occur are different than the metal in the source. The possible transitions are defined by the atoms making the tissue, typically carbon, oxygen, Version 10-1-2023 and calcium. Each element has a signature of wavelengths (characteristic radiation) that can be emitted in this manner. The first plot on the right of the figure shows that characteristic radiation occurs at just a few frequencies. 2.2.5.2 Compton Scattering Compton scattering changes the angle and energy of the incoming x-ray without completely absorbing it. The figure shows that the interaction of the incoming x-ray causes the x-ray to lose energy and direction when it ejects a loosely held electron for its shell. 2.2.6 Attenuation Both photoelectric effect and Compton scattering act to attenuate the incident x-rays. The number of xrays that get through a material depends on the type of material and the thickness of the material. The number of rays that make it through are a function of the x-ray energy, the material type, and the thickness (x) of a material according to Figure 19 illustrates this point and emphasizes that there are fewer x-rays exiting the material than enter. It is amazing what we know at this point. We have a beam of x-rays with characteristics we set, namely changing the x-ray source to change the energy and frequency and number of x-rays in the beam, that are shown onto a tissue. We have good models of how different materials and thickness of materials reduce and transform the incident x-rays. Now, if we can measure the x-rays coming out of the patient, we will be able to “see”, via an image, the types and thicknesses of the tissues. Version 10-1-2023 Figure 19: Fewer rays exit the material than enter. 2.2.7 How are x-rays detected? The final technology is to measure the x-rays exiting the patient to create an image. As shown in Figure 20, an array of sensors is used to detect the number of x-rays in each sensor area. Version 10-1-2023 Figure 20: Array of sensors to detect x-rays. 3 X-ray technology Figure 21 illustrates the two parameters that affect the beam quality: • • Voltage determines the energy of the waves to penetrate (kV) (25kV-140kV) Current is proportional to the number of photons emitted per time (A). The table on the right illustrates the adjustment scenarios. In the first column, increasing the current value directly increases the number of x-rays produced, with no change in voltage there is no change photon energy. There are just more photons of the same energy. In the first row, increasing the voltage increases the photon energy, without an increase in current, no more photons are produced but each one now has more energy. The bottom right corner indicates that we can increase current and voltage to produce more photons of higher energy. The amount of time the x-ray beam is on is the Exposure. The current determines the number of photons per time and then we must multiple by the time the beam is on (ms) to get the exposure. Version 10-1-2023 Generally the time is fixed and the current is adjusted to set the exposure; however, the exposure time can also be adjusted with the current held constant. Figure 21: Two parameters that directly affect beam quality. Figure 22 shows the impact on the resulting image of adjusting voltage and current. Figure 22: Human chest radiograph (left) is underpenetrated, requiring increase in x-ray tube voltage. After voltage (kV) has been increased (middle), radiograph is too dark. After x-ray tube output (milliampere-second [mAs] value) has been reduced (right), radiograph has correct image density. (from Radiographic Techniques, Contrast, and Noise in X-Ray Imaging | AJR (ajronline.org)) 4 Summary We can now put it all together as described in Figure 23. For each sensor, compare the incoming x-ray waves to the outgoing x-ray waves to measure the change that a small beam of x-rays sees traveling through the subject experiences. The Figure 23 illustrates that we can create an image from these Version 10-1-2023 difference measurements. By convention, we typically create images where the dark areas mean less attenuation (more exposure at the sensor) and the bright areas mean more attenuation (less exposure at the sensor). The image can be modified to convey even more information, for example a specific bone could be colored. Figure 23: Final view of x-ray measurements of a subject. 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