Physics Quiz 3 Notes.docx

**Physics Quiz 3 Notes:** **Week 5 Learning Objectives:** 1. State the three possible outcomes when a photon passes through matter. 2. Explain the concept of attenuation and the factors that affect it. 3. State and describe the five possible interactions between photons and matter. 4...

**Physics Quiz 3 Notes:** **Week 5 Learning Objectives:** 1. State the three possible outcomes when a photon passes through matter. 2. Explain the concept of attenuation and the factors that affect it. 3. State and describe the five possible interactions between photons and matter. 4. Describe the differences between coherent and Compton scattering. 5. Determine which of the two interactions (photoelectric absorption or Compton scattering) will likely occur for photons of different energies in specific materials. 6. Differentiate between the probability of photoelectric absorption in different materials for a given photon energy. 7. Describe the similarities and differences between Compton scattering and photoelectric absorption. 8. Use the provided equations on Compton scattering and photoelectric absorption to solve relevant problems. **X-Ray Interaction With Matter:** *When a photon passes through matter it may be:* - transmitted (reaches the detector and has no interaction with matter) - absorbed - scattered **Attenuation:** *Reduction in the number of photons and thus beam intensity. Anything that stops photons from being detected can cause attenuation.* - attenuation is caused by absorption and scattering - attenuation increases as: - thickness of material increases - density of material increases (lead attenuates more than tissue) - atomic number of material increases (higher atomic number means higher density) - energy of photons decreases (higher energies can pass right through without interaction) - attenuation of a material is characterised by its linear attenuation coefficient (**μ)** and mass attenuation coefficient (**μ/ρ**) (dividing linear attenuation coefficient by density of material) **Basic Interaction Mechanisms:** - coherent scattering (*not relevant in diagnostic imaging as it is a low energy interaction -- low energy photons that would undergo coherent scattering are filtered*) - Compton scattering - photoelectric absorption - pair production (does not occur in diagnostic imaging due to energy constraint) - photo disintegration (high energy that sometimes occurs in therapeutic beams) *Note: Whenever a photo interacts it is gone -- a **new photon** is always re-emitted.* **Coherent Scattering:** *A photon incident on an atom is scattered with **no loss of energy** (no change in energy). Also known as **classical** or **elastic** scattering (no energy change).* - occurs for low energy photons (below 10 keV) - the low energy photon excites the whole electron cloud which causes it to vibrate - vibration of the cloud re-emits a photon of the same energy that travels in a different direction - only a few percent of x-rays in the diagnostic range undergo coherent scattering which contributes to film fog (lack of contrast) **Compton Scattering:** *Occurs when an x-ray photon interacts with a loosely held outer-shell electron and ejects it which ionises the atom.* - ejected electron is called a Compton or recoil electron (is provided some kinetic energy) - a newly ejected photon of reduced energy and longer wavelength moves in a different direction - *E = hf and E = hc / lambda* (energy of a photon) - as the energy of a photon decreases its wavelength increases - recoil electrons are absorbed within 1 to 2mm in soft tissue (ionises other atoms) and contributes to patient dose - scatter photons create a radiation hazard for patient and radiographer - main contributor to film fog (decreases image contrast) **Energy of Incident Photon:** - energy before interaction is equal to the energy after interaction - *E(in) = E(scat) + E(b) + E(KE)* - energy of incident photon = energy of scattered photon + binding energy of electron + kinetic energy of recoil electron - incident photon (with all the energy) loses energy when overcoming the binding energy to release the electron - initial energy is divided between the kinetic energy of the recoil electron and scattered photon (photon takes most the energy) as the binding energy of outer shell electrons is small -- meaning that very little energy is required to release the electron - energy of the scattered photon depends on the energy of the incident photon (if an incident photon has high energy then the scattered photon will also have high energy) **Angle of Scatter:** *Dictates the change in energy/wavelength of the photon. Difference between incident and scattered photon. Shifts in energy are angle dependant.* - the higher the energy of the incident photon the greater the energy of the scattered photon at a given angle (angle affects the change) - change in wavelength between the two photons (incident and scattered) is related to the scatter angle via: - *λ (scat) -- λ (incident) = 2.40 x 10^-12^ (1 -- cosθ)* - scattered photon can be scattered at any angle between 0 and 180 - at 0 degrees there is no energy transferred because the photon proceeds in its original direction (cos 0 = 1) - at 180 degrees the maximum energy is transferred to the recoil electron and the scattered photon has minimum energy (cos 180 = -1) - at any scatter angle the energy given to the electron is less than the energy of the scattered photon **Backscatter Radiation:** - x-rays scattered in the direction of the incident photon are called backscatter radiation - most photons scatter in a forward direction especially as photon energy increases **Probability of Compton Scattering:** - probability of photon undergoing Compton scattering is dependent on electron density and inversely proportional to photon energy - photon energy increases = probability decreases - as electron densities (number of electrons/kg) are similar for all materials then the probability for most materials is similar - Z (Atomic Number) / A (Mass Number Z+N) **Photoelectric Absorption:** *Occurs when an x-ray photon interacts with an inner-shell electron.* - photon gives up all its energy to the electron and disappears and photoelectron is ejected from the atom which ionises the atom - *E(in) = E(b) + E(KE)* - *the kinetic energy of the electron is therefore equal to the difference between the energy of the incident photon and the binding energy of the electron* - photoelectric absorption can only occur if the incident photon has an energy equal to or greater than the binding energy of the electron - binding energies of K-shell electrons for atoms in the body are low e.g., 0.53 keV for oxygen - most of the energy of the incident photon is transformed into kinetic energy of the photoelectron - photoelectrons are absorbed within 1 to 2mm in soft tissue and contribute to patient dose - removal of an inner-shell electron leaves a vacancy which is filled by an electron from a higher-energy shell - the electron loses energy which is emitted in the form of a characteristic photon known as secondary radiation **Probability of Photoelectric Absorption:** - directly proportional to atomic number cubed and inversely proportional to photon energy cubed - as (effective) atomic number increases then probability increases - as photon energy increases probability decreases - as the effective photon number is distinct between soft tissue and bone then there is a bigger difference **Week 6 Learning Objectives:** 1. Define: a. Phosphor Material b. Conversion Efficiency c. Screen Speed d. Spatial Resolution e. Luminescence 2. Describe the process of pair production. 3. Describe the process of photodisintegration. 4. Describe the structure of an intensifying screen and the function of each layer. 5. Describe the two types of luminescence. 6. Describe the desirable characteristics of phosphor materials. 7. Identify and explain the three factors that determine the amount of scattered radiation reaching the film and describe their effects on image quality. 8. Explain the concept of the intensification factor and be able to calculate the intensification factor for a given screen-film combination. **Pair Production:** *Occurs when a high energy photon (1022 keV and above) interacts with the electric field of a nucleus and disappears. Does not occur in diagnostic x-ray range.* - the energy of the photon is used to create an electron and positron (mass = 511 keV) - positron loses energy until it meets a free electron and in the reverse process called pair annihilation they annihilate each other - the mass of both particles is completely converted into energy in the form of two 511 keV photons **Photodisintegration:** *Occurs when a very high energy photon (above 10 MeV) is absorbed by a nucleus and the nucleus emits a proton, neutron, or other nuclear fragment. Does not occur in diagnostic x-ray range.* **Differential Absorption:** *Only Compton scatter and photoelectric absorption are important in diagnostic radiography.* - an x-ray image results from the difference between the x-rays absorbed photoelectrically in the patient and the x-rays transmitted to the film - Compton scattered x-rays contribute no useful information to the image rather only film fog - in the diagnostic range (40 -- 150 keV) Compton scattering predominates - below 40 keV most x-rays interactions with human tissue are photoelectric (most probable outcome is absorption) - as kVp is increased more x-rays penetrate through the film (can be compensated for by halving the mAs value -- 15% rule) **Intensifying Screens:** *Devices which convert x-rays into visible light. Reduces patient dose.* - x-ray film is usually sandwiched between two screens which are permanently placed within a radiographic cassette **Structure:** - base - made of polyester - has strength + flexibility + chemically inert - 1mm thickness - reflective layer - made of highly reflective layer (magnesium oxide or titanium dioxide) - ensures that any light produced in the phosphor that moves towards the base is reflected back towards the film - reduces the amount of radiation needed for an examination - increases blurring (reduces image detail) - fluorescent or phosphor layer (active layer) - converts x-ray photons into visible light photons - consists of fluorescent particles suspended in a binding substance - compounds of the rare earth materials (gadolinium + lanthanum + yttrium) are now widely used as phosphors - layer is about 150 -- 300 micrometres thickness - protective coating - thick layer of cellulose acetate - protects phosphor layer from abrasions and damage - transparent to allow the light produced in the phosphor to reach the film (does not absorb light) **Luminescence:** *The ability of a substance to emit light in response to excitation (usually by increasing the energy of outer-shell electrons).* - x-ray radiation can produce luminescence - two types of luminescence (rare earths): - fluorescence or instantaneous light emission (within 10^-8^s) -- immediate - phosphorescence or delayed light emission -- delayed - radiographic intensifying screens fluoresce only while being struck by x-rays (instantly fluorescing the layer) **Phosphors:** *The rare phosphors exhibit all the below characteristics.* - high atomic number = high probability of x-ray absorption - high conversion efficiency = many light photons are produced per x-ray photon (typical conversion efficiency is 1000 for a 50 keV photon) - appropriate spectral emission = refers to the wavelengths of light emitted by the phosphor which must match the spectral sensitivity of the film - minimal phosphorescence or afterglow (must only expose the film when the x-ray is on) **Screen Characteristics:** *Three primary screen characteristics:* - screen speed - image noise - spatial resolution **Screen Speed:** - relative number that describes how efficiently x-rays are converted into usable light - speeds range from 100 (slow and detailed) to 1200 (very fast) - a measure of screen speed is the intensification factor (IF) - the rate of exposure required to produce an optical density (darkness of film) without a screen to the exposure required to produce the same optical density with the screen - *IF = exposure required without screen / exposure required with screen* - the intensification factor thus measures the magnitude of dose reduction to the patient **Image Noise:** - appears on radiographs as a speckled background - occurs more often when fast screens and high kVp are used - reduces image contrast **Spatial Resolution (Image Detail):** - refers to how small an object can be imaged - measured by the number of line pairs per millimetre (lp/mm) that are imaged - screens somewhat reduce spatial resolution **Beam Restricting Devices:** - two types of x-rays reach the film: - those that exit the patient without interacting (primary x-rays) - those that are scattered via the Compton interaction (contribute to film fog and reduce image contrast) - three factors influence the relative intensity of scattered radiation reaching the film - kVp - x-ray field size - tissue thickness **kVp:** - as kVp and photon energy increase the probability of a photon undergoing photoelectric absorption decreases much more rapidly than the probability of it undergoing Compton scatter - more likely to have scattered photons reaching the film - the **relative** number of scattered photons reaching the film increases as kVp increases and thus lowers contrast - decreasing kVp means fewer and less penetrating x-rays and greater patient dose - a compromise kVp value is used so that adequate contrast is achieved at the lowest possible dose **X-Ray Field Size:** - scatter radiation increases as the field size increases because there is a greater number of atoms the photons can interact with via the Compton effect **Tissue Thickness:** - scatter radiation increases as the thickness of the body part being imaged increases because there is a greater number of atoms the photons can interact with via the Compton effect