Module 4-5 Physics PDF

Module 4: Physics Lesson 1: X-ray Interactions Five types: Coherent Scatter​ ○ (aka: unmodified, elastic, classical scatter​ Photoelectric Effect​ ○ (aka: absorption, attenuation)​ Compton Scatter​ ○ (aka: modified, inelastic, incoherent scatter)​ Pair Production​...

Module 4: Physics Lesson 1: X-ray Interactions Five types: Coherent Scatter​ ○ (aka: unmodified, elastic, classical scatter​ Photoelectric Effect​ ○ (aka: absorption, attenuation)​ Compton Scatter​ ○ (aka: modified, inelastic, incoherent scatter)​ Pair Production​ Photodisintegration Coherent Scatter​ Photon interacts with bound electron (s)​ Single electron interaction: Thompson ​ Multiple electron interaction: Rayleigh ​ Photon changes direction, but no change in E, ƛ,ѵ(frequency)​ Energies below 10 kEv​ Only interaction with no ionization of atoms​ Photoelectric Effect (PE)​ Photon interacts with bound electron​ KE of incident photon completely absorbed by electron (true absorption)​ KE of ejected photoelectron is = to KE of the incident photon​ For PE interaction to occur the incident photon must have an energy > the BE of the orbital electron.​ Products of a photoelectric interaction ○ Photoelectron ○ Characteristic Photon ○ Remaining positive ion Probability of Photoelectric effect is Inversely proportional to the third power of the photon energy. (1/E)3​ Probability of Photoelectric effect is directly proportional to the third power of the Z# of the tissue. ​ K edge absorption​ ○ most likely when → photon E = B.E. of electron​ Significance in Radiology ​ ○ contrast enhancement​ ○ greater patient radiation dose​ Compton Scatter​ photon interacts with free (outer shell) electron​ energy of photon partially absorbed by recoil electron​ scattered photon has different E, ƛ, ѵ​ energy loss by photon depends on angle of scatter​ predominate interaction in DI. WATER VS BONE Pair Production​ occurs when photon is above 1.02 MeV​ photon interacts with nucleus​ energy of photon is completely absorbed by nucleus​ results in emission of positron and electron​ Not a factor in DI but used in positron emission tomography (PET), where it aids in creating images for nuclear medicine Photodisintergration​ photon interacts with nucleus​ photon energy is completely absorbed​ several particles are emitted (p, n, ᵦ+, ᵦ-, α)​ most probable with v. high energy photons (5 -17 MeV)​ ​ Lesson 2: Attenuation & Differential Absorption Differential Absorption​ Occurs due to:​ ○ X-rays absorbed photoelectrically ​→ Radiopaque structures​ ○ Compton scattering​ X-rays transmitted through the patient. → Radiolucent structures Factors Affecting Differential Absorption​ Quality of Radiation: kVp​ Mass Density (ρ) ​ Atomic Number (Z) Differential Absorption​ Careful selection of technical factors is important ​ Image contrast is dependent on kVp​ ○ as kVp increases = Differential Absorption decreases Image Contrast​High kVp ​ VS Image Contrast​High kVp Mass Density​ Mass density = quantity of matter per unit volume (kg/m3 or g/cm3)​ Tells how tightly atoms are packed together ​When density is doubled, probability of PE and Compton scatter increases ​ ○ There are double the # of electrons available for interaction​ Atomic number (Z#)​ PE increases with an increase in Z#​ To image small differences in soft tissue:​ ○ low kVp to get maximum differential absorption​ ○ High contrast , short scale​ High kVp is used for: ​ ○ low contrast exams or chest radiography, long scale​ Important for mammography (very low kVp) Attenuation​ Total reduction in the number of x-ray photons in an x-ray beam as it travels through matter.​ Either by absorption or scattering of the incident photons 1% of photons incident on a patient reach the image receptor​ ᐸ than ½ of those form the image approx 0.5%​ Linear attenuation coefficient​ Measurement of attenuation per cm of absorber​ Symbolized by μ (mu)​ μ water > μ ice > μ water vapor​ Will decrease with increased energy except at K edge​ ○ T ​ he linear attenuation is the most useful measurement, because it tells us how much radiation we can expect to be attenuated as it travels through a certain thickness of tissue. This is how we derive the Houndsfield #’s (CT #’s) that make up the gray scale values for digital imaging. ​ Mass attenuation coefficient Determines the attenuation of materials independent of their physical state (recall water example)​ All states of water have the same mass attenuation coefficient​ 𝑙𝑖𝑛𝑒𝑎𝑟 𝑎𝑡𝑡. Determined by: ​ 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 Expressed as grams per square centimeter​ Three Factors Affecting Scatter​ kVp​ ○ Increase kVp → Increase scatter. Increase in Compton Scatter interactions​ Part thickness​ ○ Increase part thickness → Increase scatter​ Field size​ ○ Increase field size (decrease collimation), → Increase scatter​ Attenuation: 𝑇𝑜𝑡𝑎𝑙 # 𝑜𝑓 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 𝑖𝑛 𝑏𝑒𝑎𝑚 = (𝑡𝑜𝑡𝑎𝑙 # 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 + 𝑡𝑜𝑡𝑎𝑙 # 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 𝑠𝑐𝑎𝑡𝑡𝑒𝑟𝑒𝑑) + 𝑡𝑜𝑡𝑎𝑙 # 𝑜𝑓 𝑝ℎ𝑜𝑡𝑜𝑛𝑠 𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑡𝑡𝑒𝑑 ○ (total # of photons ATTENUATED) Lesson 3: Filters X-RAY Beam Review​ Polychromatic X-ray emission​ High energy X-rays: penetrate tissue further​ Lower energy X–rays: are absorbed by the tissue​ Penetrability of the beam “QUALITY” Filtration Primary purpose to remove lower energy photons ○ INCREASE QUALITY – PENETRABILITY ​ Remove photons that contribute only to patient dose​ Some attenuation is required to produce the image (contrast) Half Value Layer (HVL)​ The amount of filtration that reduces the intensity of the beam by ½ is the beam’s HVL.​ Diagnostic X–ray beam HVL = 3-5 mm of Al​ HVL is determined by measuring the effects of filtration on beam intensity​ Rad’n measurements are made as Al absorbers are added​ Inherent Filtration Glass envelope​ Metal tube housing​ Mirror of the light-localizing collimator​ Added Filtration​ Sheet of Al between the tube housing and collimator ​ Can be adjusted by technologist​ Usually totals 2 – 3 mm of Al​ Compensating filtration​ Types of Filtration​ Aluminum (Z = 13)​ Copper (Z = 29)​ Tin (Z = 50)​ Gadolinium (Z = 64)​ Plastics​ Compensating Filters​ Body parts vary in thickness​ Produces a uniform OD​ Image A: ○ Wedge filter, AP T spine. thick portion partially attenuates beam over upper T spine. Nonfilter area receives full exposure penetrating the thick portion of the spine. ​ ○ An even image density results. ​ Image B ○ Trough filter, AP chest. Two side wedges partially attenuate beam over the lungs. While the mediastinum receives the full exposure. ​ A → better-quality image of the chest and mediastinal structures results.​ ​ → Trough filter. A B → “Bow-tie” filter for use in computed tomography. ​ C → Wedge filter. D → Conic filters for use in digital fluoros​ A → Wedge: collimator-mounted ClearPb filter, ​ ○ AP projection of the hips, knees, and ankles (1 image)​ B → Trough: collimator mounted, AL filter, double-wedge​ ○ AP projections of the spine. ​ C → Boomerang contact filter: ○ AP shoulder ​ D → Boomerang, collimator-mounted: ○ AP and PA oblique (scapular Y) shoulder. ​ E → collimator-mounted: ○ laterals of the cervicothoracic region (swimmer's technique) and axiolateral projections ​ Filters Boomerang collimator-mounted: positioned on the collimator for an AP shoulder.​→ Boomerang contact filter: positioned for an​AP shoulder. → Compound Filters → K edge filters​ 2 materials used in construction of filter​ Usually copper and aluminum​ Layers of copper and aluminum are arranged so that the highest # Z faces the ​X–ray tube ​ Decreases the thickness of filter required​ Most absorption occurs in Cu then in Al​ Beneficial for high contrast exams requiring high kVp​ Filtration​ ○ A: Axiolateral projection of the hip without compensating filter. ​ ○ B: Same projection with swimmer's filter.​ ○ A: Axiolateral projection of the hip without compensating filter. ​ ○ B: Same projection with swimmer's filter. ○ A: T spine without compensating filter. ​ ○ B: Same projection with wedge filter. Note more even density of the spine, and all vertebrae are shown.​ ○ A: T spine without compensating filter. ​ ○ B: Same projection with wedge filter. Note more even density of the spine, and all vertebrae are shown.​  ○ A: AP chest without compensating filter. ​ ○ B: Same projection with trough filter. ​ ○ Note lower lung fields and mediastinum are better demonstrated ○ A: Lateral cervicothoracic region (swimmer's technique) without compensating filter. ​ ○ B: Same projection with swimmer's filter. ​ ○ Note how C7 and T1 area is penetrated and shown.​ ○ A: Lateral cervicothoracic region (swimmer's technique) without compensating filter. ​ ○ B: Same projection with swimmer's filter. ​ ○ Note how C7 and T1 area is penetrated and shown.​ ○ A: AP shoulder without compensating filter. ​ ○ B: Same projection using the Boomerang contact filter.​  ○ A: AP shoulder without compensating filter. ​ ○ B: Same projection using the Boomerang contact filter.​  Module 5: Physics Lesson 1: Image Creation​ Technical Factor Selection​ kVp​ ○ Primary controller of radiographic contrast (not in DR)​ ○ Increases in kVp= increases in quality and quantity​ ○ Use of grids can compensate for reduced rad contrast​ mA​ ○ Increases in mA increases # of incident photons​ ○ Influences optical density or brightness​ S, time​ ○ Component of mAs​ ○ Increase mAs increase patient dose​ ○ Decreasing S reduces motion​ ○ Increasing S can be used for blurring of structures Radiographic Contrast​ Degree of difference between light and dark areas on a radiographic image​ Contrast Scale​ ○ Number of shades of grey​ ○ Short Scale – fewer shades​ More black and white​ ○ Long Scale – many shades IR Exposure/Brightness​ An optimal IR exposure permit maximum visualization of contrast ​ Excessive or inadequate IR exposure decreases contrast​ As kVp increases, the percentage of Compton interactions versus photoelectric absorption increases. ○ This results in more scatter radiation reaching the image receptor, decreasing contrast​ kVp Changes​ kV Effect on Image Appearance​: Inadequate exposure​→ Excessive exposure​→ ​ (Under exposed) (Over exposed) Intensity​: Over exposed → Under exposed​→ mAs – Proportional relationship Intensity ​ Effect of mAs change​ Technical Factor Selection – effect of kVp​ Technical Factor Selection – effect of kVp​ 2 different Z# contrast media​ Enhances contrast​ Destructive condition: Contrast scales?​ Breathing technique​ Lesson 2: The Patient​ Human body as an attenuator The patient is the radiographer’s greatest variable, therefore the technologist must learn to select and adjust technical factors accordingly The Patient:​Attenuation​ Reduction in x-ray photons remaining in beam after passing through a given thickness of material​ Increased part thickness results in increased attenuation​ Higher atomic number attenuate a greater percentage of the beam​ Higher density (kg/m3) attenuate a greater percentage of the beam​ Substance Effective Atomic Number Density (kg/m3 ) Air 7.78 1.29 Fat 6.46 916 Water 7.51 1000 Muscle 7.64 1040 Bone 12.31 1650 Patient's Relationship to Image Quality Subject density: ○ IR exposure will be altered by changes in the amount and/or type of tissue being irradiated Subject contrast: ○ Degree of differential absorption resulting from the differing absorption characteristics of tissues in the body Pathology and Radiation Absorption​ A. Pathology can alter the thickness and composition of patient’s tissues​ B. Challenge for the technologist: awareness of the changes specific pathology conditions have on tissues​ C. Small localized pathology does not require a change in technical factors MRT’s responsibilities​ Read & interpret request for each radiological examination ​ ○ Take an accurate patient history​ Close observation of the patient​ ○ Draw on theoretical knowledge & clinical experience to decide on optimal technical factors​ Keep patient dose as low as possible without compromising image quality​ ○ ALARA Pathology and Radiation Absorption​ Additive conditions​ ○ Increase attenuation​ ○ Require an increase in technical factors to properly expose the image receptor​ General compensation is to increase kVp​ Destructive conditions​ ○ Decrease attenuation​ ○ Require a decrease in technical factors to properly expose the image receptor​ General compensation is to decrease mAs​ Increased Attenuation (Additive) Conditions Conditions affecting multiple sites​ ○ Abscess, edema, tumour ​ Conditions of the chest​ ○ Cardiomegaly, congestive heart failure, pleural effusions, pneumonia, pulmonary edema, tuberculosis​ Conditions of the abdomen​ ○ Aortic aneurysm, ascites​ Conditions of the extremities and skull​ ○ Acromegaly, chronic osteomyelitis, osteoblastic metastases, Paget's disease (osteitis deformans), sclerosis, ​ Decreased Attenuation (Destructive) Conditions​ Conditions affecting multiple sites​ ○ Anorexia nervosa, atrophy, emaciation ​ Condition of the chest​ ○ Emphysema​ Condition of the abdomen​ ○ Bowel obstruction ​ Conditions of the extremities and skull​ ○ Active osteomyelitis, aseptic necrosis, carcinoma, multiple myeloma, osteolytic metastases, osteomalacia, osteoporosis Patient size​ Bariatric Patient ​ Adjustments in technical factors required​ Multiple images required to image area of concern​