Week 3-Image Formation PDF

Week 3 Image formation. This week’s lesson we will go over the image formation process. How is the image created, from scanning, to...

Week 3 Image formation. This week’s lesson we will go over the image formation process. How is the image created, from scanning, to planning your coronal and sagittal MPRs. We’re going to start off with the radiation physics which will be a brief review on what goes on at the atomic level what happens to the X-ray beam as it passes through WEEK 3: 1. 2. R A D I AT I O N P H Y S I C S D ATA A C Q U I S I T I O N the image. After all, it is the interactions that occur at that atomic level I M A G E F O R M AT I O N that influence the type of data we acquire, which leads us to section 3. RECONSTRUCTION 4. POST PROCESSING two and three. The formation of the final image involves a two step © Copyright 2019 Pulse Radiology Education LLC process, or separated into two phases. The first is the data acquisition phase, when a scan is taking place to acquire the data about the thing or person we are imaging. The second is the image processing phase which involves reconstruction working in the background, and all the possible post processing options that may be available to us. We have Week 3 section 1, radiation physics. This section is going to be a review of what you may have learned in radiography school. CT is a post primary certification meaning the ARRT requires that you be first an RT credentialed in one of the primary pathways such as radiography or radiation therapy before you pursue CT. This is WEEK 3, SECTION 1: 1. 2. ELECTROMAGNETIC SPECTRUM I N T E R A C T I O N S W I T H M AT T E R because CT builds on the fundamental concepts that one would have R A D I AT I O N P H Y S I C S 3. PHYSICAL PRINCIPLES learned in a structured radiography course. We won’t go too in depth into the details for this section but touch on the broad general ideas © Copyright 2019 Pulse Radiology Education LLC that you may need to know for the ARRT exam. The rabbit hole can go pretty deep when discussing radiation physics. We will leave those details to the physicists and engineers. In this section we will look briefly at the electromagnetic spectrum,  identify the possible interactions that occur when the X-ray beam comes in contact with the patient and lastly we will look at the physical principles of the X-ray beam. 1.1 ELECTROMAGNETIC ENERGY Let’s review real quick on electromagnetic energy and the X-rays or X-radiation is a form of electromagnetic electromagnetic spectrum. This is something that would have been radiation. ๏ Sine waves that travel through space carrying electromagnetic energy. covered in the first few classes of a physics class in radiography Short wavelengths (0.01 to 10 nm) Energy ranges of 100 eV to 100 keV. school. The idea of electromagnetic energy is the basic build block ↓ wavelength (λ) = ↑ frequency = ↑ energy behind how X-radiation works. In CT, we are still working with an X-ray tube, that produces an X-ray beam similar to what might be found in conventional radiography. X-rays, short for X-radiation, was also at one point called Rontgen rays after Wilhelm Rontgen, the person who © Copyright 2019 Pulse Radiology Education LLC discoversd X-rays. X-rays are a form electromagnetic radiation. Electromagnetic radiation is a type of radiation that travels through space carrying electromagnetic energy and they travel in a sinusoidal fashion, or like a sine wave. The electromagnetic spectrum includes radiowaves, microwaves, infrared, visible light, ultraviolet light, X-rays and gamma rays. We can see on the images provided here, that the various electromagnetic radiations all have varying wave lengths. The shorter the wave length, the higher the frequency, the higher the energy of the photons traveling in the electromagnetic wave. In comparison, gamme rays have more potential energy than X-rays. X-rays carry more energy than microwaves, which carry more energy than radio waves. Now in order for X-rays to even be effective at creating images and being able to penetrate specific parts of a patient’s body, the X-ray waves need to carry enough energy to cause ionization to the target atoms of the patient. Therefore, X-rays do typically have short wavelengths, anywhere from 0.01 nanometers to 10 nanometers. The energy ranges can be anywhere from 100 electrovolts to 100 kiloelectrovolts. Wave length is inversely proportional to the frequency and energy level.  1.2 ELECTROMAGNETIC ENERGY Also a quick review on the anatomy of a sine wave. All that X-ray energy, gets focused into a beam targeted at the patient. The photons Wavelength λ Wavelength: The distance over which the wave’s shape repeats, commonly designated by lambda (λ). in that beam all travel in sinusoidal fashion, so traveling in a wave like Amplitude Amplitude: Measure of changes over a single period. the one pictured here. A wave is made up of wavelength, which is the Frequency: Number of occurrences of a repeating event per unit of time, distance over which the wave’s shape repeats itself. Commonly designated by lambda. So this apex to apex would be a measurable expressed as cycles per unit of time. Cycle 1 Cycle 2 Cycle 3 Frequency wavelength. © Copyright 2019 Pulse Radiology Education LLC The amplitude is the measure of changes over a single period of time. Frequency is the number of occurrences of a relating event per unit of time, expressed as cycles per unit of time. In this image, I’ve highlighted three separate cycles.Now image that the time it took for the wavelength to have traveled three cycles took only one second. Then that would mean the wave has a frequency of 3 cycles per second, or 3 hertz. X-ray energy frequencies are in the range of 30 petahertz to 30 exaherts. 30 Petaherts, is 3 times 10 to the 16th power. That’s a whole lot of cycles per second.  1. 3 X - R AY S Properties of X-rays (and Gamma Rays): ‣ Cannot be detected by human senses (cannot be seen, heard, felt, etc.) ‣ Travels in straight lines at the speed of light. ‣ Not influenced by electrical or magnetic fields. ‣ Angles can change by a small degree during interaction. ‣ Passes through matter until a chance interaction with something. ‣ Degree of penetration depends on energy level and the matter traveling through. ‣ Has enough energy to ionize matter and damage or destroy living cells. © Copyright 2019 Pulse Radiology Education LLC 1.4 ELECTROMAGNETIC ENERGY Now again, depending on the wavelength, which determines the Low energy x-rays interact with whole atoms. energy level of photons in the X-ray beam, it will affect the type of ‣ X-ray beam has long wavelength. interactions that occur. Certain interactions require higher energy Moderate energy x-rays interact with electrons. photons which can be adjusted by adjusting your kVp and mAs ‣ Causes ionization. ‣ Found in diagnostic radiology. settings. Generally low energy X-rays, or X-ray photons that have long High energy x-rays interact with nuclei. wavelengths, less frequency, less energy, these X-rays tend to interact ‣ X-ray photon disappears and creates a nucleon fragment. with the atom as a whole. We will see in a second here, these usually don’t contribute to any image quality and can lead to image fogging. © Copyright 2019 Pulse Radiology Education LLC Moderate energy X-rays are those we find more commonly in diagnostic radiology, so CT, conventional X-rays, mammography, dexa. At these energy levels, the X-rays can cause ionization, which is what is needed in order to generate an image.  High energy X-rays usually found with nuclear studies have so much energy that they can travel straight in and interact with the nucleus of atoms. The X-ray photon disappears, and creates a nucleon fragment. We won’t get too detailed into High energy X-rays as these are more found in nuclear medicine and radiation therapy. 2. 1 I N T E R A C T I O N S W I T H M AT T E R There are five primary forms of interaction with matter when talking Five (5) forms of interaction with matter: about X-rays. Again, this is going to be a quick review of what you 1. Coherent Scattering may have learned in radiography school. All of these principles, can 2. Compton Effect ** 3. Photoelectric Effect ** also be applied to conventional radiography. At the basic level, CT and 4. Pair Production X-ray essentially utilize the same technology but in different ways. 5. Photodisintegration ** Compton effect and photoelectric effect are of particular importance in CT. ‣ This is due to the energy range in which CT units operate. These five forms of interaction are coherent scattering, the compton effect, the photo electric effect, pair production and photo © Copyright 2019 Pulse Radiology Education LLC disintegration. Compton effect and the photoelectric effect are of particular importance when dealing with CT. This has mostly to the do with the energy range in which CT operates. As we saw on the last slide, moderate energy X-rays are the ones we are interested in in diagnostic radiology.  2. 2 C O H E R E N T S C AT T E R I N G Coherent scattering, also known as classical or thompson scatter Known as classical scattering or Thompson scattering. occurs when an x-ray photon interacts with the whole atom causing 1. X-ray interacts with target atom causing it to become excited. the atom as a whole to become excited. The atom releases excess 2. Atom releases excess energy as a scattered x-ray with energy equal to the initial x-ray but energy as a scattered x-ray photon with energy equal to the initial X- ray photon but going in a different direction. Basically in coherent in a different direction. (Same wavelength). Only occurs with X-ray energies below 10 keV. Minimum kVP on most CT scanners is 80 scattering, the x-ray photon just bounces off too another direction. These sort of interactions only occur with X-ray energy levels below 10 Non-ionizing and does not contribute to the image. kiloelectrovots. The minimum kVp setting on most CT scanners is 80 © Copyright 2019 Pulse Radiology Education LLC kVp. This sort of interaction does not frequently occur in CT. There are very small percentages of interactions that result in coherent scattering, but they are non-ionizing and do not contribute to the image at all. They produce slight increase in noise however. 2.3 COMPTON EFFECT The compton effect also known as compton scattering is when a x-ray Known as compton scattering. photon interacts with an outer shell electron knocking it out of orbit. 1. X-ray interacts with outer shell electron, knocking it out of orbit ionizing the atom This is known as ionization. The atom becomes ionized, an excited producing a compton electron. 2. X-ray photon deflected in a different state. it needs to rebalance itself so it ejects the excess energy in the direction with less energy. form of a electron called a compton electron. The original X-ray photon Occurs in diagnostic radiology energy ranges. Negative impact on image quality. gets deflected in a different direction with energy equal to the ‣ Scattered photon provides no useful information and will only degrade quality of image. difference between the incident X-ray photon and the binding energy of the outer shell. © Copyright 2019 Pulse Radiology Education LLC This type of interaction occurs in the energy ranges or kVp ranges that we use in CT, although it has a negative impact on image quality. The scattered photon provides no useful information and will only degrade the quality of the image.  2.4 COMPTON EFFECT This is why. The energy from the incident x-ray photon is divided Energy from incident X-ray photon is divided between scattered X-ray photon and Compton electron. between the scattered x-ray photon and the compton electron. So let’s ‣ Both may have enough energy to undergo more say you have an X-ray beam with a photon carrying 100 ionizing interactions before losing all their energy. kiloelectrovolts. That photon interacts with an outer shell electron of a ๏ Contributes the most to patient dose. ‣ Scattered x-ray is eventually absorbed photoelectrically. material that has a binding energy of 68 kiloelectrovolts. After the Backscatter radiation: X-rays scattered back in the interaction, you would have an ejected electron equal to 68 direction of the incident beam. kiloelectrovolts, then another scattered x-ray photon with an energy level of 32 kiloelectrovolts. © Copyright 2019 Pulse Radiology Education LLC Now the energy levels I am using here are arbitrary. But image if you have many of these interactions occur during an exposure with varying energy levels, you would result in scattered X-rays and compton electrons with varying energy levels. Both of these may have enough energy to undergo more ionizing interactions again before they eventually lose all their energy and are absorbed. This is the ultimate negative effect of this entire process. It contributes the most to the patient’s dose. These scattered x-rays may get absorbed by the skin. They may bounce off in a different direction and get absorbed by something else. They do not contribute to the image quality, and if they do happen to strike a detector, it will only add image fog. The term backscatter radiation refers to when X-rays of the compton effect are scattered back in the direction of the incident beam. In conventional radiography, this is considered the highest contribution to occupational dose.  2.5 COMPTON EFFECT The probability of a compton effect occurring is inversely proportions to the X-ray energy. So as we raise the X-ray energy level by raising The probability of a Compton effect occur is inversely proportional to the X-ray energy. the kVp, the chance of the compton effect decreases and the ↑ x-ray energy (kVp) = ↓ chance of Compton effect probability of a compton effect is independent of the atomic number. The probability of a Compton effect is independent of the atomic number. This can be seen here on the graph. As we increase the X-ray energy level, the relative probability of the compton effect decreases. At a certain energy level, 100 kEv in this case, the probability that the compton interaction will occur in soft tissue and bone will be the © Copyright 2019 Pulse Radiology Education LLC same. At 100 kiloelectrovolts, it won’t matter what the atomic number is meaning it won’t matter what the X-ray is trying to penetrate through. The possibility of a compton effect will be the same regardless. Even at the lower energy levels, the difference between the probability of compton effect occur in soft tissue and bone is 2.6 PHOTOELECTRIC EFFECT The photoelectric effect is also known as photoelectric absorption, or simply absorption. This phenomenon was actually discovered by Known as photoelectric absorption. 1. X-ray interacts with inner shell electron, being completing absorbed and knocking out an inner shell electron, causing the atom to become ionized. Albert Einstein. It’s when an X-ray interacts with an inner shell electron 2. An electron is emitted from the atomhttps://faraday.physics.utoronto.ca/ photoelectron. in the form of a GeneralInterest/Harrison/Flash/ Nuclear/XRayInteract/ XRayInteract.html and getting completely absorbed while knocking out an inner shell ‣ Energy of the photoelectron is dependent upon the difference between energy of incident x-ray and the binding energy of the electron. electron and ionizing the atom. The electron that gets emitted from the Occurs in diagnostic radiology energy ranges. atom is called a photo electron. The energy level of the photoelectron Incident X-ray energy must be >= electron-binding energy is dependent on the difference between the energy of the incident x- ray and the binding energy of the electron. So if you have an atom with © Copyright 2019 Pulse Radiology Education LLC a K-shell binding energy level of 50, and an x-ray photon carries 80 kiloelectrovolts of energy. Upon interaction, you would end up with a photoelectron carrying 30 electrovolts. One thing to note, in order for this interaction to even occur, you need to have an incident x-ray photon with a energy level higher than the binding energy of the inner  shell, otherwise the X-ray photon won’t have enough power ionize the atom. 2.7 PHOTOELECTRIC EFFECT In this picture here, we see an incident photon carrying 100 1. The ejection of a K-shell photoelectron leaves a vacancy that must be filled in the K-shell. kiloelectrovolts interacting with an inner shell electron with a binding energy of 34. After interaction, you get a photoelectron with 66 2. The unnatural ionized state is corrected when an outer- shell electron drops into the vacancy. 3. This transition emits a characteristic x-ray whose energy is equal to the difference in the binding energies of the shells involved. kiloelectrovolts. This ejection of a K-shell photoelectron leaves a Secondary radiation or characteristic x-rays: ‣ Example: An X-ray photon carrying 100 keV knocks a K- shell electron with a binding energy of 34 keV out of orbit. The resulting photoelectron is 66 keV. An electron vacancy that must be filled in the K-shell. The unnatural ionized state Low energy X-rays can only be corrected when an outer shell electron drops into the from the L-shell drops into the vacancy. The difference between 34 and 5 emits a characteristic X-ray with an Do not contribute to diagnostic value. energy level of 29 keV, below the threshold that can cause anymore interactions and will be absorbed. Act like scatter radiation. vacancy. The transition emits a characteristic X-rays whose energy is equal tot he difference in the binding energies of the shells involved. © Copyright 2019 Pulse Radiology Education LLC just like the ones found during X-ray beam production when the electrons interact with the anode target material. In this scenario, an electron from the L-shell drops into the vacancy. The difference between 34 and 5 emits a characteristic X-ray with an  energy level of 29 kEv, below the threshold that can cause anymore interactions. this characteristic X-ray is low energy and therefore does not contrinbute to diagnostic value and acts essentially like scatter radiation. Characteristic X-rays are undesired. 2.8 PHOTOELECTRIC EFFECT In the compton effect, the atomic number of the material had no effect http://whs.wsd.wednet.edu/faculty/ The effective atomic number of the material that an X-ray photon is interacting on the probability of interaction, especially as the kV increased. busse/mathhomepage/busseclasses/ radiationphysics/lecturenotes/ chapter12/chapter12.html with has an effect on photoelectric absorption. low atomic # (soft tissue) = low K-shell binding energy = ↓ Absorption Photoelectron energy is nearly identical to incident x-ray energy In photoelectric absorptions, the effective atomic number does have high atomic # (bone) = high K-shell binding energy = ↑ Absorption Photoelectron energy is proportionately lower an effect on the probability of interaction. S U B S TA N C E AT O M I C N U M B E R Probability that an incident x-ray will undergo HUMAN TISSUE F AT 6.3 a photoelectric interaction is inversely SOFT TISSUE LUNG 7.4 7.4 proportional to the third power of the X-ray Low atomic number materials such as soft tissue will have a low K- BONE 13.8 C O N T R A S T M AT E R I A L S energy and directly proportional to the third AIR IODINE 7.6 53 power of the atomic number of the absorber. BARIUM 53 shell binding energy level. You would need to use a low energy level or OTHER ↑ x-ray energy (kVp) = ↓ chance of photoelectric effect CONCRETE 17 M O LY B D E N U M 42 TUNGSTEN 74 LEAD 82 low kVp in order for photoelectric effect to occur. Having too high of © Copyright 2019 Pulse Radiology Education LLC an energy level will cause the X-ray beam to just pass through the patient without interaction. In this interaction, the emitted photo electron is nearly identical in energy level to the incident x-ray energy level. Likewise, high atomic number materials like bone have a high k-shell binding energy level. Your chances of absorption are increases. The photoelectron emitted in this scenario is proportional lower because of the higher k-shell energy level. I’ve included a chart here showing different materials and their approximately atomic numbers. Because of the nature of this interaction, the probability that an incident X-ray will undergo a photoelectric interaction is inversely proportional to the third power, orX-ray energy cubed and directly proportional tot he third power or atomic number of the absorber cubed. Increasing the energy level decreases the chance of photoelectric absorption. Too high of an energy level and the photons will just bypass without interaction. Looking at this graph, we see that the chance of photoelectric interaction to occur is higher at lower kV levels. We also see, it’s easier for it to occur at lower energy levels with lower atomic number materials. You need a higher energy level and it is overall harder for photoelectric absorption to occur on higher atomic number materials.  2. 9 PA I R P R O D U C T I O N Pair production requires high energy X-ray photons and can only occur 1. High energy X-ray photons bypass electron shells and when the energy range is above 1.02 mega electrovolts. This occurs interact with the electric field of the nucleus. 2. X-ray photons is completely absorbed and two electrons when the high energy x-ray photons bypass the electron shells, are emitted. 1. Positron (positively charged electron) doesn’t interact witht he electrons but instead interacts with the 2. Negatron (negatively charged electron) electric field of the nucleus. The X-ray photon gets completely ‣ Energy is distributed equally between the two electrons. Only occurs with X-ray energies above 1.02 MeV. absorbed and in it’s place, a pair of electrons are emitted, hence the name pair production. There is a positively charged electron called a Does not occur in CT. Very important in PET imaging. positron, and then an electron, more specifically a negatron. The © Copyright 2019 Pulse Radiology Education LLC amount of energy from the original incident X-ray photon is divided equally between the two electrons. As seen in this picture here, 1.02 mega electrovolts divided by two gives you 0.51 mega electrovolts. 2. 1 0 P H O T O D I S I N T E G R AT I O N In photodisintegration, the x-ray photon bypasses the electron shells 1. X-ray photon bypasses electron shells and the nuclear field interacting directly with the nucleus. The nucleus and nuclear field and gets absorbed by nucleus. goes into an excited state and emits a nucleon or nuclear fragment. 2. Nucleus goes into excited state and emits a nucleon or nuclear fragment. For interactions like this to occur, it would require X-ray energy ranges Only occurs with X-ray energies above 10 above 10 megaelectrovolts. Because of the required energy range, this MeV. Does not occur in CT. interaction also does not occur in CT. © Copyright 2019 Pulse Radiology Education LLC  2.11 CHANCE OF INTERACTIONS F E AT U R E S O F C O M P T O N S C AT T E R I N G A) WITH OUTER-SHELL ELECTRONS M O S T L I K E LY T O O C C U R B) W I T H L O O S E LY B O U N D E L E C T R O N S A) I N C R E A S E D P E N E T R AT I O N T H R O U G H T I S S U E W I T H O U T INTERACTION A S X - R AY E N E R G Y I N C R E A S E S B) I N C R E A S E D C O M P T O N S C AT T E R I N G R E L AT I V E T O PHOTOELECTRIC EFFECT A S AT O M I C N U M B E R O F A B S O R B E D I N C R E A S E S N O E F F E C T O N C O M P T O N S C AT T E R I N G AS MASS DENSITY OF ABSORBER INCREASES P R O P O R T I O N A L I N C R E A S E I N C O M P T O N S C AT T E R I N G F E AT U R E S O F P H O T O E L E C T R I C E F F E C T A) WITH INNER-SHELL ELECTRONS B) W I T H T I G H T LY B O U N D E L E C T R O N S M O S T L I K E LY T O O C C U R C) W H E N X - R AY E N E R G Y I S J U S T H I G H E R T H A N E L E C T R O N - BINDING ENERGY A) I N C R I N C R E A S E D P E N E T R AT I O N T H R O U G H T I S S U E W I T H O U T INTERACTION A S X - R AY E N E R G Y I N C R E A S E S B) L E S S P H O T O E L E C T R I C E F F E C T R E L AT I V E T O C O M P T O N E F F E C T C) REDUCED ABSOLUTE PHOTOELECTRIC EFFECT A S AT O M I C N U M B E R O F A B S O R B E D I N C R E A S E S I N C R E A S E S P R O P O R T I O N AT E LY W I T H T H E C U B E O F T H E AT O M I C NUMBER (Z³) AS MASS DENSITY OF ABSORBER INCREASES PROPORTIONAL INCREASE IN PHOTOELECTRIC ABSORPTION © Copyright 2019 Pulse Radiology Education LLC 3. 1 X - R AY B E A M In the perfect world, the x-ray tube would generate a mono energetic The quality, or strength of the X-ray beam spectrum is determined by the energy of the photons (kV). x-ray beam. This would be you setting a kVp setting of 100 and having a steady x-ray beam with x-ray photons carrying 100 kiloelectrovolts ‣ Photons need to contain enough energy to ionize atoms. of energy. Unfortunately X-rays produced by cathode x-ray tubes are ‣ High energy X-ray photons penetrates more through matter. The quantity, or number of X-ray photons polyenergetic, that is the photons that make up the x-ray beam vary in found in the X-ray beam is determined tube current (mAs). their energy levels. ‣ Need enough photons for interactions and penetration to occur. Let’s review what happens during X-ray production. The cathode heats © Copyright 2019 Pulse Radiology Education LLC up and boils off electrons via thermionic emission. The amount and strength of the electrons is determined by the tube current and voltage applied to the x-ray tube. This is controlled by kVp and mAs respectively. The flow of electrons strike the anode. Interactions occur at the atomic level creating bremsstrahlung radiation, characteristic  radiation, and heat. Bremstrahlung radiation photons and characteristic radiation photons together make up the X-ray beam. Now remember, the interactions occurring at the anode are random. It’s going to produce electromagnetic radiation at varying levels. Before the X-ray beam even reaches the patient, it needs to go pass some filters and pre-patient collimation further reducing the amount of lower energy photons from the X-ray and increasing the overall average energy level of the X-ray beam. This is known as beam hardening. 3. 2 X - R AY B E A M You may remember from radiography school that the X-ray beam is a Intensity: Product of quality (energy of each X-ray photon) and quantity (number of X-ray Intensity = quality x quantity area x exposure rate polyenergetic beam. The beam carries all the x-ray photons which photons) per unit area per unit time of exposure. each have varying energy levels. We will discuss kVp a bit more in ↑ kVp,↑ mA, ↑ exposure time = ↑ intensity of x-ray beam The X-ray beam is a polyenergetic beam. detail a little later in this lesson but the kVp setting is what determines ‣ Contains a spectrum of photon energies. your kilovoltage peak or the highest energy level a photon can ‣ Highest energy = kilovoltage peak (kVp). possibly carry. This is a poly energetic beam so setting a kVp of 100 ‣ Mean energy = 1/3 - 1/2 of peak. ‣ Also depends on filtration used. doesn’t mean you get all photons with 100 kiloelectrovolts. It will range with the highest being 100 kiloelectrovolts and the average energy © Copyright 2019 Pulse Radiology Education LLC level being about 1/3 to 1/2 of the peak. This chart here is a good representation of that idea. The type of filtration can also have an effect on the overall average energy level known as hardening the beam.  3.3 PHOTON INTERACTION As the X-ray beam passes through an object or a patient, one of three possible scenarios will happen. We’ve already see all the possible Three possible scenarios as the X-ray beam passes through an object: interactions that can occur. Other than the compton and photoelectric 1. Photon penetrates without interaction. 2. Photon interacts with matter and be completely effect, the x-ray photons can also penetrate without interaction. It can absorbed by depositing it’s energy. 3. Photon interacts and scatters to another direction go right through the patient without any interaction and get picked up depositing part of it’s energy. by the detector as a strong signal. This will show up as dark areas on your image. The photon may interact with matter and get completely absorbed by depositing it’s energy as we saw in photoelectric © Copyright 2019 Pulse Radiology Education LLC absorption. And lastly the photon may interact with matter, then deposit some of it’s energy and scatter off into another direction as we saw in the compton effect. 3.4 LINEAR ENERGY TRANSFER We’ve already looked some of the possible interactions that occur Linear Energy Transfer (LET): The rate at which an electron transfers energy to a material. when an X-ray photon interacts with matter. In the energy ranges that ‣ Expressed in terms of amount of energy transferred per unit of distance traveled. we work with in CT, we will either get the compton or photoelectric effect. One is an interaction with a scatter and a resulting electron with ELECTRON ENERGY VS LINEAR ENERGY TRANSFER In a given material, the LET value depends on the kinetic energy (velocity) of the electron. ELECTRON ENERGY (KEV) LET (KEV/MM) 1000 100 0.2 0.3 ‣ LET is inversely related to the electron velocity. As a radiation electron loses energy, it’s velocity decreases, and less energy. The other, we get a total absorption, then a release of a photoelectron. In both of these interactions, there was some form of 10 2.2 the value of the LET increases until all of it’s energy is 1 12.0 dissipated. The LET of the radiation is also related to the effectiveness of a particular radiation in producing biological damage. energy transfer. This is know as the linear energy transfer or LET, which measures the rate at which an electron transfers energy to a material. © Copyright 2019 Pulse Radiology Education LLC It is expressed in terms of amount of energy transferred per unit of distance traveled. In our case, we are working with kiloelectrovolts as the our unit of energy, and millimeters as our unit of distance. We see in this chart here, that in a given material, say soft tissue, the LET value will depend on the kinetic energy or velocity of the electron. How  fast is the electron traveling? The LET value is inversely related to the electron velocity. Notice its inversely related, not inversely proportional. As radiation electron loses enrgy, it’s velocity decreases and the value of LET increases until al of it’s energy is dissipated. The relationship is not proportional. A decrease in energy by two won’t increase the LET by 2. The LET of the radiation is also related to it’s effectiveness in producing biological damage. This is something we will look at when we go over radiation safety principles. 3. 5 AT T E N U AT I O N Attenuation occurs as the X-ray beam passes through the patient to Attenuation: Progressive reduction of intensity of the X-ray beam as it passes through the patient’s tissue. the detectors. It is the progressive reduction of intensity of the X-ray Photoelectric Effect + Compton Effect = Attenuation Detectors measure the attenuation of the X-ray beam by measuring the reduction in intensity of the X-ray beam. beam as it passes through the patient’s tissue. The photoelectric and Intensity: Power per unit cross sectional area. ‣ The intensity decreases exponentially as it passes through an absorbing material. compton effect combined affect attenuation. The detectors measure ‣ Measured intensity at detector based on: the attenuation of the X-ray beam by measuring the reduction in I = I₀ e-µx ๏ Initial intensity of beam entering material (I₀). ๏ The linear attenuation coefficient (µ). intensity of the X-ray beam. Remember the intensity is both the quality ๏ Thickness of the material (x). ๏ e = Euler’s constant and quantity of the x-ray beam. The strength, and amount of photons. Intensity is defined as power per unit cross sectional area. During © Copyright 2019 Pulse Radiology Education LLC attenuation, the intensity of the x-ray beam decreases exponentially as it passes through an absorbing material. The final intensity that gets measured at the detector is based on the  initial intensity of the beam entering the material, the linear attenuation coefficient, and the thickness of the material. You don’t need to know the formula, but in case you were interested, this is the math behind attenuation and how the detectors and computer systems know how much attenuation has occured. E is for euler’s constant which is also a part of this calculation. You don’t need to know what Euler’s constant is other than it’s involved in the calculation of attenuation. 3. 6 AT T E N U AT I O N In the previous formula, one of the determining factors of the The number of photons measured by the detectors is reduced exponentially as the calculated attenuation is x, thickness of material. thickness of the tissue it needs to pass increases. X-ray beams are polyenergetic. As the x-ray beam passes through the patient, During attenuation the number of photons measured by the detectors the number of photons is reduced and the average energy is increased. — Beam is reduced exponentially as the thickness of the tissue that the x-ray hardening. Low energy photons are more easily Example: 100 photons with varying energy levels interact with material that is 1 unit thick. Some of the photons interact with the material and some pass through. The interactions photon needs to penetrate increases. In this example, we start off with 100 photons all carrying varying energy levels. These photons interact that occur are either photoelectric or Compton. This is attenuated. attenuation, the removal of some of the photons from the beam. After interaction, there are 90 photons left. with a material that is 1 unit thick, or 1 centimeter. Some of the © Copyright 2019 Pulse Radiology Education LLC photons interact with the material and some pass through it. The interactions that do occur are either photoelectric or compton in nature. This is what attenuation is, the removal of some of the photons from the beam. After interaction, there are 90 photons left. Point to take away here, the thickness of the tissue that the X-ray beam needs  to penetrate, the less photons that get picked up by the detectors. Also remember that the x-ray beam is poly energetic meaning it carrying many different energy levels. as the x-ray beam passes through the patient, the number of photons will be reduced. When this happens, the average energy is increased. This is the beam hardening effect at play. This is due to the fact that the lower energy photons are more easily attenuated. 3. 7 L I N E A R AT T E N U AT I O N C O E F F I C I E N T The linear attenuation coefficient also found in the previous formula describes the fraction of the X-ray bee that gets absorbed or scattered Linear Attenuation coefficient (µ): Describes the fraction of the X-ray beam that is absorbed or scattered per unit thickness of matter. ‣ Indicates the rate at which photons interact as they move through material. per unit thickness of matter. It gives a quantitative value to indicate the ‣ Inversely related to the average distances photons travel before interacting. rate at which photons interact as they move through material. How ‣ The attenuation coefficient value (rate of interaction) is determined by the energy of the X-ray photons and the atomic often do interactions occur when passing through material. It’s inversely related to the average distances photons travel before number and density of the material. Example: Bone is a better absorber of X-rays than soft tissue. bone has a higher µ. interacting. The farther a photon needs to travel before interacting, the less chance an interactions to occur. © Copyright 2019 Pulse Radiology Education LLC The attenuation coefficient value or rate of interaction, how often an interaction occurs is based on the energy of the X-ray photons, set by your kVp and the atomic number and density of the material. Example, bone is a better absorber of x-rays than soft issue, bone has a higher  linear attenuation coefficient. It’s harder for an X-ray photon to pass through bone. The chance of interaction is pretty high as long as the energy level of the photons are within range to cause an interaction. We can see here that there is a certain energy range where photoelectric interactions occur. If you increase the kVp higher however, the attenuation coefficients begin to decrease since the photons have enough energy to transmit through a material. Increasing kVp decreases the chances of interaction, the rate of interaction and the linear attenuation coefficients. A balance must be found between transmitted and absorpted X-rays as we saw with he idea of differential absorption. 3. 8 L I N E A R AT T E N U AT I O N C O E F F I C I E N T During the reconstruction process, the array processor calculates the During reconstruction, the array processor calculates the total distribution of attenuation coefficients from total distribution of attenuation coefficients from a set of projections a set of projections specified by an angle. ‣ The CT image is based on the attenuation profiles http://www.sprawls.org/resources/ CTIMG/ctimg25.jpg specified by an angle. Translation, the CT image that we see on the that has occurred over a region based on a set of projections from multiple angles and the measured monitor, is based on the amount of attenuation that has occurred over linear attenuation coefficients. the scanned area or region. The measured attenuation information, ๏ Fan beam = multiple angles. ‣ The linear attenuation coefficients can be further known as attenuation profiles are based on all the projections that we converted into Hounsfield Units (HU). need to take when the tube and detector frame assembly rotate around the patient and the measured linear attenuation coefficients. In © Copyright 2019 Pulse Radiology Education LLC CT, we work with either fan or cone beams, so as the frame assembly spins around the patient, we acquire attenuation profiles from multiple angles. With the value of the linear attenuation coefficients, those can be  further converted into Hounsfield Units. We hear the name Hounsfield again. 3.9 DIFFERENTIAL ABSORPTION The result of the compton scatter, photo electric effect and the Differential Absorption: The result of compton scattering, photoelectric effect, and X- rays transmitted through the patient. transmitted x-rays is differential absorption. Compton scatter: ‣ Provide no useful information. ‣ Can cause artifacts. X-rays that penetration: ‣ No interactions. ‣ Appears radiolucent - permeable to radiation. Compton scattering don’t provide any useful information, and can Photoelectric interactions: ‣ Absorbed by patient ‣ Represent anatomical structures contribute to artifacts. This will affect the final image. ‣ Appears radiopaque - capacity of matter to obstruct transmission of radiation. The difference between the radiopaque and radiolucent areas of the body give contrast to the image, based on the differential absorption. ↑ Differential Absorption = ↓ kVp (less transmitted X-rays) The x-rays that penetrate will not have undergone any interactions. They go straight to the detectors. This is how radiolucent or dark areas © Copyright 2019 Pulse Radiology Education LLC of the image are formed. It represents areas of the body that are permeable to radiation or easily penetrated by radiation. Photoelectric interactions are what get absorbed by the patient. They appear radiopaque or bright. This is based on how well a particular organ can stop an X-ray from passing through. Bone is dense, and therefore can  stop X-rays better than say fat. Differential absorption is the difference between the radiopaque and radiolucent areas of the body which is what give contrast to the image so structures can be differentiated. As you decrease the kVp, the chance of differential absorption increases. 3.10 EFFECTS ON DIFFERENTIAL ABSORPTION Differential absorption is dependent on atomic number. The greater the Differential absorption is dependent on atomic number. ‣ Greater the atomic number, higher chance of interaction. atomic number the higher chance of interaction because this means Differential absorption is also dependent on mass density. there is a greater chance that an X-ray photon won’t be able to ‣ Thicker body part = greater chance of interaction. S U B S TA N C E AT O M I C N U M B E R HUMAN TISSUE S U B S TA N C E MASS DENSITY (KG/M³) HUMAN TISSUE transmit through. Differential absorption is also dependent on mass density because the greater the mass density, the higher the chance of F AT 6.3 LUNG 320 F AT 910 SOFT TISSUE 7.4 SOFT TISSUE, 1000 LUNG 7.4 MBUOSNCEL E 1850 BONE 13.8 C O N T R A S T M AT E R I A L S C O N T R A S T M AT E R I A L S AIR 1.3 interaction. Comparing bone to fat here. Bone has higher mass density AIR 7.6 BARIUM 3500 Contrast materials may be IODINE 53 IODINE 4930 BARIUM 53 OTHER administered to enhance visualization CALCIUM 1550 OTHER CONCRETE 2350 when compared to fat. Bone will appear bright. Fat will appear dark. CONCRETE 17 of low contrast structures. M O LY B D E N U M 10,200 M O LY B D E N U M 42 LEAD 11,350 TUNGSTEN 74 RHENIUM 12,500 LEAD 82 TUNGSTEN 19,300 Air with it’s extremely low mass density shows up black on an image. © Copyright 2019 Pulse Radiology Education LLC Contrast materials such as barium and iodine may be administered to enhance visualization of low contrast structures, like the gastrointestinal tract. Looking at this chart here, we see that as we increase the kVp, the  chance of interaction for either photoelectric or compton effect decreases. If you think about it, if you increase kVp and increase the overall energy of the X-ray beam, those photons have a higher chance of just going through the patient instead of interacting with the patient. It will air quotes burn through the image. 3.11 EFFECTS ON DIFFERENTIAL ABSORPTION In order to produce high quality images, proper selection of parameters is required so that there is maximum differential To produce high-quality images, proper selections of parameters is required so that there is maximum differential absorption. absorption. ‣ Decreasing the kVp to increase absorption can increase contrast resolution but may cause an increase in patient dose. ‣ A compromise is necessary. ๏ As Low As Reasonably Achievable (ALARA) to generate a diagnostic image. CHARACTERISTICS OF DIFFERENTIAL ABSORPTION A S X - R AY E N E R G Y INCREASES A) B) C) FEWER COMPTON INTERACTIONS. MAN FEWER PHOTOELECTRIC INTERACTIONS. MORE TRANSMISSION THROUGH TISSUE. Decreasing the kVp to increase absorption can increase the contrast resolution but it may also inadvertently cause an increase in patient A) NO CHANGE IN COMPTON INTERACTIONS. A S T I S S U E AT O M I C B) MANY MORE PHOTOELECTRIC INTERACTIONS. NUMBER INCREASES C) L E S S X - R AY T R A N S M I S S I O N. A) PROPORTIONAL INCREASE IN COMPTON INTERACTIONS. AS TISSUE MASS dose since the patient is absorbing more of those X-rays and ionizing B) PROPORTIONAL INCREASE IN PHOTOELECTRIC INTERACTIONS. DENSITY INCREASES C) P R O P O R T I O N A L R E D U C T I O N I N X - R AY T R A N S M I S S I O N. more of the patient’s atoms. A compromise or balance is necessary. © Copyright 2019 Pulse Radiology Education LLC CT technologists have the ethical responsibility of keeping radiation dose to as low as reasonable achievable to generate a diagnostic image.  WEEK 3, SECTION 2: 1. 2. ACQUISITION MODES S C A N N I N G A N D R E C O N S T R U C T I O N PA R A M E T E R S D ATA A C Q U I S I T I O N © Copyright 2019 Pulse Radiology Education LLC 1.1 SCANNING METHODS There are three primary scanning methods also known as the There are three (3) primary scanning methods (acquisition geometry): acquisition geometry or how the image is acquired. These are the ‣ Localizer scan localizer scans, the conventional scan, also called the axial scan or ‣ Conventional/Axial/Serial scan (step and shoot) G E U S E R I N T E R FA C E serial scan and is sometimes referred to as the step and shoot ‣ Spiral/Helical/Volumetric scan method, and last is the spiral scan also known as the helical or ๏ Only found