Medradsc 2Y03 Week 1 Lecture 1 Notes PDF

MEDRADSC 2Y03 find more resources at oneclass.com Week 1 Lecture 1 September 4th, 2018 Objectives • Interactions with matter – Bremsstrahlung X-rays and Characteristic radiation • kVp, mAs • Distance changes • Inverse square law • Direct square law • Image fundamentals Course Foundation • Course...

MEDRADSC 2Y03 find more resources at oneclass.com Week 1 Lecture 1 September 4th, 2018 Objectives • Interactions with matter – Bremsstrahlung X-rays and Characteristic radiation • kVp, mAs • Distance changes • Inverse square law • Direct square law • Image fundamentals Course Foundation • Course outline • Course manual/readings • Labs – begin this week on Thursday • Tutorials – start on first week as well Bremsstrahlung • Means – “slow-down radiation” – also known as breaking radiation • Occurs within the diagnostic range, most x-ray are Bremsstrahlung x-rays • Its radiation that results from the breaking of a cathode electron by the nucleus • A cathode electron can lose any amount of its kinetic energy in an • Happens between 0-70 kVp Bremsstrahlung Radiation Production • Produced can be a low or high energy x-ray • A low energy x-ray will have more of an opportunity to be absorbed • A high energy x-ray will have the opportunity to go through find more resources at oneclass.com find more resources at oneclass.com Characteristic Radiation • If the projectile electron interacts with an inner-shell electron of the anode target atom rather than the outer-shell electron, characteristic x-rays (and heat) can be produced • Characteristic x-rays are emitted when an outer-shell electron fills and inner-shell void o As kVp increases the characteristic radiation increases Characteristic x-ray production • Occurs when the electron is knocked out and another one from another shell fills the void • The force is greater than 70 kVp which caused the electron to get pushed out X-rays Emission Spectrum • We set the quantity of radiation differently for every patient • The maximum for bremsstrahlung happens at 1/3 Intensity • Define intensity o Is the magnitude/quantity of photons/radiation per area • Can be measured using the Cobia meter – mR/s, R/s • As technologists we also consider quantity as per the x-ray vendor or equipment • (mR = milliroentgen) Intensity • think about the quantity of radiation or intensity • now think about a flashlight • Flashlight – as the distance from the source to the wall changes does the intensity change? • The farther from the source of the x-ray beam the less the intensity o Less dose • The closer to the patient, the more divergent and the higher the dose o the beam will become more parallel find more resources at oneclass.com find more resources at oneclass.com • Roentgen ® or milliroentgen (mR) X-ray Tube – Study this!! • Cathode is the negative component • The anode is positive • X-ray will come through the window through the patient • The focal spot is where the electrons hit on the anode Relate Technical Factor Selection to Beam Output • mAs – milliampere/second • affects radiation quantity ONLY by controlling number of projectile electrons used for xray production • • kVp – kilovoltage peak affects radiation quantity, average beam energy AND maximum photon energy Change in mAs • involves changing the seconds • a bigger person will have a longer exposure but a small person will have a shorter exposure • 200 mA will give more detail then 400 mA • 200 mA - better image, more time, more dose • 400 mA – okay image, less time, less dose Inverse Square Lae Formula find more resources at oneclass.com find more resources at oneclass.com Definition • the intensity of the beam is inversely proportional to the square of the distance Inverse Square Law Inverse Square Law • describes the decrease in intensity as distance from source is increased • point source - focal spot • photons diverge as they travel away from the source • the inverse square law states that the intensity of radiation at a given distance from the point source is inversely proportional to the square of the distance Inverse Square Law example • an x-rayexposure of 240 mR is recorded at a distance of 20 inches. If the same technical factors are used, what will the exposure be if the distance is increased to 40 inches? • Answer = 60 mR Inverse Square Law • the law should be used when calculating the relationship between the distance and xray intensity (mR) Direct Square Law • as the distance (SID) increases, intensity decreases which causes a decrease in exposure to the image receptor • if this is the case how can we tell this has happened in clinical practice? • Which technical factor is the primary controller of quantity • When changes are made in SID, the technologist can use the direct square law (direct application to radiography) to adjust mAs accordingly so as not to over or under irradiate the patient and maintain image quality Direct Square Law • Ex. A diagnostic image of the abdomen is taken using 25mAs at 80kVp at a 40 inch distance. A second image is requested by the radiologist at 56 inches find more resources at oneclass.com find more resources at oneclass.com • What mAs value should be used to produce an acceptable image if the distance is increased to 56 inches? Review X-ray Emission Spectrum • Factors that affect the size and relative position of the x-ray emission spectrum o Tube current - mAs o Tube voltage - kVp Overview • Image quality is the exactness of representation of the patients anatomy on an image • To produce high-quality images, radiographers apply knowledge of 3 major interrelated categories of radiographic quality … what are they Interrelated Categories 1. IR factors – film screen (analogue or conventional), CR or computed 2. Geometric factors – related to the part or object 3. Subject factors – related to the patient, bone vs soft tissue, contrast vs no contrast What is an image? • 2 – dimensional representation of an object or objects • made of multitude of points, each with location and intensity level • both location and intensity carry information about the object being represented X-ray? Pa, oblique, and lateral X-ray production Recorded Detail • degree of geometric sharpness find more resources at oneclass.com find more resources at oneclass.com • • accuracy of structural lines recorded-line pairs also termed – definition, sharpness, spatial resolution or detail Radiograph Productions Process Step 1: • area beam of uniform intensity is produced by a point source and directed to the object being imaged Step 2: • rays interact with atoms within the object, resulting in non-uniform reduction in beam intensity of the transmitted beam The Remnant Beam • the beam has been MODULATED by the object o ie. Has been transformed by the object to carry information about the object o the information or signal is the magnitude of the radiation exiting the rays path o each path may have caused a different amount of reduction in intensity, so each point location in the remnant beam contains information about one very thin path through the object • from the patient to the image receptor Capturing the Signal • • • • • remnant bean arrives at image receptor rays interact with radiation-sensitive layer(s) within the receptor x-ray energy is transferred to the layer, producing a pattern of changes induced by received energy pattern is latent image because still invisible o the CR image needs to go through a reader to see the image o DR image comes up instantaneously latent image must be extracted and processed Image Contrast • contrast indicated that the signals at two locations are not the same • since signal is relative quantity of radiation transmitted, the information about the object must be relative to reduce radiation transmission, or ability to attenuate radiation o the beam that came out of the tube has interacted with the object that it has either been absorbed or scattered o if it goes through the patient completely it is unattenuated • contrast indicated attenuation differences find more resources at oneclass.com find more resources at oneclass.com Image Contrast • the left image is a lower contrast, the signals are very similar – too much mAs or kVp or both • in the right image, it is a higher contrast image • it has been absorbed in the bone, the photon does not reach the image receptor which causes a brighter image • no interaction with an object which is pure transmission is black – unattenuated What causes attenuation differences? • Structure thickness • Content: o Density (p) ▪ Bone or air o Atomic number • For chest, the kVp is very high but a lower time Principle components of Image Quality • Contrast o The amount of signal represented, highly controlled by the kilavoltage • Spatial resolution o The ability of the imaging system to image objects that are small and close together o Affected by focal spot, mA • Noise o Caused by an insufficient number of photons o Patients size can cause noise after you already set the factors What affects the magnitude of contrast on a radiograph? • Subject contrast o Def: relative magnitude of differences within the remnant beam exiting the object • Image receptors contrast response • Image processing • Noise Summary 1. Besides the image receptor, one must consider both geometric and subject factors regarding the patient and part or object find more resources at oneclass.com find more resources at oneclass.com a. The size, the shape, tissue structure, the mass, 2. Regarding x-ray production, electrons are formed at the cathode 3. The point source on the anode where the e- are stopped and the x-ray is formed is called the focal spot 4. Since signal is relative quantity of radiation transmitted, the information about the object must be relative to reduce radiation transmission, or ability to attenuate radiation find more resources at oneclass.com find more resources at oneclass.com Lecture 2: September 6th, 2018 Objectives • Radiograph production process • Beam divergence • X-ray field • Image geometry • Penumbra • Magnification Subject Contrast • Factors affecting subject contrast: o Non-uniformity of the object ▪ Thickness, density, atomic number of structures within it o The beam energy ▪ The higher the energy of the beam, the narrower the range of differences in attenuation within the object • The higher the subject contrast, the greater the signal difference between two sides of an edge within the object Image Receptor Contrast Response • Receptor receives HUGE range of remnant beam intensity levels (1:10,000) • Range of output signals from receptor will be much narrower due to logarithmic transformation • Response signal is mapped to logarithm of relative exposure received Digital • Detectors – CR – imaging plate • _____ - DR – flat panel detector • Signal to Noise (SNR) ratio in digital is a more relevant description of the contrast potential in the image than its contrast itself o It is higher from the source to object o Lower from the object to the image receptor Image • The hip image was taken with not enough signal, which causes it to be pixelated • The second was taken correctly find more resources at oneclass.com find more resources at oneclass.com Spatial Resolution • 2D image has 3D dimensions = height, width and gray scale • Spatial Resolution – describes the ability of an image system to accurately depict objects in the two spatial dimensions of the image Example of spatial resolution • This is a mammography photo Noise • Types: o Quantum noise – there is not enough signal o Imaging system noise o Irrelevant signal • Noise is not wanted Quantum Noise • Variation in number of photons used by the beam receptor due to statistical fluctuation in number of photons being produced and interacting along the beams path • Initially, there are many photons, as it takes on the personality of the patient, there are less photons o If you start too little it could cause noise Review find more resources at oneclass.com find more resources at oneclass.com 1. Besides the image receptor, one must consider both geometric and subject factors regarding the patient and part or object. What are they? a. Shape, mass, tissue, density 2. Regarding x-ray production, electrons are formed at the cathode 3. The point source on the anode where the electrons are stopped and the x-ray is formed is called the focal spot 4. In digital x-ray, noise or quantum noise occurs when an exposure is taken using too MUCH or too LITTLE signal a. To little Shoulder Image • Too light – the patient was under radiated o Black is more radiation, white is less radiation • There has been differential absorption • The bone (has the highest atomic number) has absorbed most of the radiation Principle components of Image Quality – Review • Attenuation – modifies the beam as it interacts with the object • Contrast – the difference in gray scale between regions that is represented as signal (a number) • Spatial resolution – the ability of an imaging system to see small object that are close together • Noise – the IR receives, you are not looking for it Contrast • The left is rib x-ray and the right is a femur • The left has better spatial resolution What affects the magnitude of contrast on a radiograph • Subject contrast o Relative magnitude of differences within the remnant beam exiting the object • Image receptor contrast response • Image processing • Noise Terminology • Tube focus o It is on the anode, it is the focal spot o Can be controlled by the mA • SID or SDD find more resources at oneclass.com find more resources at oneclass.com • • o o SOD o OID o Source to image receptor distance 40-70 Source to object distance Object to image receptor distance Lab Week 1 1. How many parts to the lab? 2. Are we irradiating a meter or a phantom? 3. What is the goal of the lab? Radiograph Production Process • The x-ray tube emits x-rays directed toward the patient. After the x-rays interact with the patients anatomy, the image detector records the altered x-ray distribution Anode Heel Effect • Definition – the area on the anode that is bombarded by the electrons from the cathode • Focal Spot – one difference between large and small focal spots is the capacity to produce x-rays • Large focal spot = more x-rays, higher anode heat capacity, less detail o 400mA will be less detail o 100mA is more detail Key Terms Focal Spot • very small zone of x-ray production on anode in x-ray tube • also called “focus” • sufficiently small to behave as an approximately point source of radiation Think/Pair/Share 1. Describe remnant beam and which view would result in an image with high contrast or less grey scale? PA hand, PA chest, AP abdomen a. PA hand 2. Rearrange the wrist from highest atomic number to the lowest atomic number (air vs. bone) a. FEMUR, ARTERY, LUNG 3. Magnitude of contrast on an image is affected by a. Density, image receptor, noise find more resources at oneclass.com find more resources at oneclass.com The Beam • There will always be more x-rays on the cathode side of the tube The Anode • Smaller mA, more concentrated heat area on the anode but better detail • Need this for a wrist but not for an abdomen • • • • • The source starts at the anode focal spot Divergent beam is the x-ray beam In the middle of the light field is the cross hairs o This is 100 As that beam diverges towards the anode side it decreases The anode heel affect is not well imaged when we image a finger because it is very small collimation Image Geometry • Projection imaging relies on geometric principles to produce predictable results • We have to try to make sure that the central ray (100% the x) is perpendicular to the image receptor and the object • We also want to get the object close to the image receptor which will be in the same plane find more resources at oneclass.com find more resources at oneclass.com Image Geometry Controls: • Distortion of the image relative to the actual object: o Object thickness o The shape of the object • Geometric unsharpness of edges on image o Object that are thicker create more area that is unsharp (conumbra) More Key Terms • Central ray o Ray produced in center of area beam emitted from collimator o Location is indicated by cross-hairs of collimator light field o It is the x in the collimator – most uniform area • Parallel ray o More parallel at 72 • Oblique ray o For triangular at 40 Beams Divergent Beam • Emitted from point source (anode focal spot) • Consists of central ray and peripheral rays that are at increasingly oblique angles from the central ray Parallel Beam • All rays in parallel beam meet at infinity • We get approximately parallel rays when angle of divergence is so small as to be negligible: o Using rays close to central ray o Using rays at very long distance from point source find more resources at oneclass.com find more resources at oneclass.com Review • bone has the highest atomic / in the body • contrast is the difference in the image gray scale between closely adjacent regions on the image • Femur and Anode Heel – Which end where? o Want to capture from hip to knee o Hip is thicker, knee is thinner o Hip should be placed under the cathode because it would be 120% there and the knee would receive less • Quantity of remnant beam is higher than primary beam. Y or N o yes • If distance is increased (tube to patient) the beam becomes more parallel. Y or N o yes find more resources at oneclass.com find more resources at oneclass.com Week 2 Lecture 1 September 11th, 2018 Objectives • X-ray equipment • X-ray lab • X-ray tube components • Distance and geometry X-ray Equipment: Introduction to external components Ceiling Support System • Most frequently used • 2 perpendicular mounted ceiling rails • allows movement in many different angles • in 4,6, and 3 Equipment – Shoot Through Hip – trauma • not a good photo o you cannot see the end of the pin X-ray Equipment: Introduction to external components Telescoping Column • allows variable distances • called source-image-distance in x-ray o most common = 40 inches (100 cm) and 72 inches (180 cm) • detent lock X-ray Equipment: X-ray Console – Control Panel Operating Console find more resources at oneclass.com find more resources at oneclass.com • • • most familiar allows to control the x-ray tube current and voltage so that useful x-ray beam of proper quantity and quality provides controls for kVp, mA or mAs, AEC, density, time, and focal spot size o AEC is automatic exposure control o Time affects the amount of photons X-ray Equipment Automatic Exposure Control • three chambers o once the chamber receives a certain number of photons it will stop exposing more because it thinks it has enough radiation • the technologist must position the patient directly over the chamber X-ray Equipment: Introduction to external components x-ray tube housing covers the glass envelope • contains the x-ray tube • bathed in oil • insulates, protects, and supports the x-ray tube insert from the environment • the x-ray tube cools itself by the anode rotating, the bathed in oil, the glass is heat resistant glass and there are also fans • taking to many exposures will cause the tube to heat up X-ray Equipment: Introduction to external components X-ray Tube Housing • lead shielding • federal/provincial regulation • microswitch B114 Room Orientation: B114 C = Room 3 – PHILIPS • 5 AEC chambers find more resources at oneclass.com find more resources at oneclass.com B114 Room: Room 4 – SIEMENS • • pitcher batter catcher 3 AEC chambers B114: Room 5 – IDC unit • similar to what we will see in clinics • get familiar with the ALL button find more resources at oneclass.com find more resources at oneclass.com Room 6: GE – General Electric • ceiling tube mount (CTM) find more resources at oneclass.com find more resources at oneclass.com B114 Equipment Review 1. Which x-ray room(s) in b114 has only one detector? • 6 and 5 2. Which x-ray room(s) in B114 uses a ceiling tube mount? • 3,4, and 6 3. Which x-ray rom(s) in B114 are best for a shoot through hip projection? • 4, 6 and 3 (ceiling tube mount) 4. Ho many of the x-ray suites in B114 have the option of using Automatic Exposure Control? • All of them 5. Which x-ray suite possess a challenge for trauma cases such as shoot through hip? Where would you find this unit in clinic practice? • 5, in a small community clinic 6. What does SID stand for? • Source image distance 7. List two common distances used in radiography • 40 (100cm) and 72 (180cm) 8. Why isn’t one longer distance used for everything? • Farther away is less dose • Smaller parts, getting closer is more advantageous 9. Which x-ray vendor in the B114 x-ray lab uses a atypical SID • 4 (seimens) 10. As the x-ray tube gets closer to the patient, does the skin dose increase of decrease? • Increases find more resources at oneclass.com find more resources at oneclass.com Week 2 Lecture 2: Radiographic Image and Instrumentation September 13th, 2018 Objectives • Image geometry • Distortion • Factors affecting distortion • Types of distortion • Significance of distortion Image Geometry • Projection imaging relies on geometric principles to produce predictable results • The central ray is perpendicular to the object • The object and the image receptor are parallel • It comes out as a divergent property Image Geometry Controls: • Distortion of the image relative to the actual object: o Object thickness, object shape object size o Make sure that the SID is exact to the one that is prescribes (either 40 or 72) o We can switch the mA (focal spot) • Geometric unsharpness of edges on image AP Coccyx vs AP Sacrum • Food for thought • They would lay on their back to bring the objects closer to the image receptor o This will decrease the OID • AP coccyx, we will angle the tube at 10 degrees towards the feet (caudal) More Key Terms Central Ray • Ray produced in center of area beam emitted from collimator find more resources at oneclass.com find more resources at oneclass.com • Location is indicated by cross-hairs of colliator light field Parallel ray Oblique ray Similar Triangles of Projection Image • Sides and altitudes (heights) of similar triangles are proportional • a/A = b/B = c/C = h/H Penumbra • umbra o the area that is the most sharp • penumbra o the area of unsharpness • this can be influenced by the focal spot size that is reflected o this depends on the mA o focal spot = 800 mA = more penumbra • width of penumbra can be calculated • P = F.S.S. x OID/SOD o Focal spot size x object to the image receptor distance/source object distance Calculation of Penumbra Calculate the penumbra for an image taken with a 1.0 mm focal spot, at 40 inches and an OID of 3 inches. P = 1.0 x 3/37 P = 0.081 mm Spatial Resolution Review • A 2D image really has 3 dimensions: height, width, and gray scale. The height and width dimensions are spatial, and have units such as millimeters. Spatial resolution is a property that describes the ability of an imaging system to accurately depict objects in the two spatial dimensions of the image • Sharpness of the image detail is best measured by spatial resolution • Small objects that are very close together o Want to make sure we have grey scale and sharpness Unsharpness – Some other causes • Factors that generally control the sharpness of image detail are the geometric factors – focal spot size, SID, and object to image receptor distance (OID) • Patient motion blurr – voluntary of involuntary o Could be challenging depending on the patient o Involuntary – peristalsis, the heart, • Thickness of the patient of part being imaged find more resources at oneclass.com find more resources at oneclass.com o If we want to change the thickness of the patient → lay them down Magnification • Magnification factor = SID/SOD • SID = 100cm and SOD = 75 cm then, • M.F = ?? = 1.333 → it is about 30% magnified • M.F permits calculation of the actual object size that is projected as an image Magnification • With magnification, geometric blurring of the object occurs in the image • Similar triangles allow the calculation of the edge gradient blurring, f, in terms of magnification M, and focal spot, F: • f = OID F SOD Magnification • the magnification formula assumes that the focal spot is a point source. Because it is not, when object size approaches the effective focal spot size or smaller, special problems develop form penumbral overlap • therefore, objects smaller than the effective focal spot cannot be demonstrated Lab Week 2 How many parts to the lab? 2 How many exposures in Part A? 3 What is being irradiated in Part A? hand or elbow How many exposures in Part B? 3 What is being irradiated in Part B? How much time between the exposures? 30 seconds Are we using the technique chart for the rooms? No. we will change the factors ourselves Distortion • what is distortion? • The size or shape is misrepresented • Can be due to the part, the set up • Objective of radiography Factors Affecting Size distortion • Controlled by distances • SID and OID • Decrease magnification increase resolution of recorded detail Geometric Magnification find more resources at oneclass.com find more resources at oneclass.com Magnification • Magnification size distortion is controlled by • Positioning of the body part • Utilizing SID concept Source-to-Image Receptor Distance (SID) • The greater the SID, the smaller the magnification • Common SID’s = 40 inches and 72 inches o This is when the object is where it normally is on the walking and talking patient o Not always the case • As the SID, the beam becomes less divergent, hence minimizes magnification Full Spine AP Object-to-Image Receptor Distance • Critical distance • Dosimetry • Positioning techniques Factors Affecting Shape Distortion • Shape distortion? • Elongated or foreshortened o We use elongation when we take images of the scaphoid • Structures lie at different levels or angles find more resources at oneclass.com find more resources at oneclass.com • Divergence of the beam X-ray Image • Not a good image • The left side of the ribs are elongated and the right are forshortened Do similar triangles still exist when the beam is angled? • Yes, they always exist Object Position • Depending on where the object is relative to the central ray • The object could look elongated Limitation of Similar Triangles • Only exist when plant of object is parallel to plane of image receptor • When object and image receptor are not parallel, M is not the same across the image find more resources at oneclass.com find more resources at oneclass.com Shape Distortion • If it is on an angle is could look foreshortened or elongated Elongation • Increased size of the image relative to the object unevenly in one of two dimensions • Occurs when the edge at larger OID is projected wheh a more oblique ray than the ray used to project the edge that has the smaller OID • Degree of elongation depends on magnitude of obliquity of rays and on magnitude of difference in OID between sides of object Foreshortening • Reduced size of image relative to object due to partial superimposition of the object on itself • Occurs when the edge at the larger OID is projected with a less oblique ray than the ray used to project the edge that has the smaller OID Minimization of Shape Distortion • Orient object so that its main plane is parallel to image receptor (minimize differences in OID) • Orient object so that rays used to image it have minimal obliquity Review Name 3 conditions which contribute to image distortion • Distance (SID/OID) • Thickness of the object find more resources at oneclass.com find more resources at oneclass.com • Size of the object • Shape of the object • Focal spot size Penumbra in the image is the area of unsharpness? T or F • true Magnification Factor = SID/SOD. How do you calculate SOD? • SID-OID The x-ray beam can be delivered perpendicular to the part. It can also be delivered on an angle (tube angle). What are the directions of angulation called?? • Cephalad (angled up) • Caudad (angled down) • Tube cannot be angled side to side • Always start pitcher batter catcher find more resources at oneclass.com find more resources at oneclass.com Week 3 Lecture 1: Location Distortion September 18th, 2018 Review – Small Group Chat What is distortion? • Image does not appear how it should in its proper image position • Misrepresentation of the objects size and shape • Can we used in our advantage: scaphoid to open up the waist Describe the alignment of the: Central ray • Perpendicular to the IR Object of anatomical part • Parallel to the IR and close • Perpendicular to the central ray Image receptor • Perpendicular to the central ray • Need to be in day taunt to make sure this occurs Location (Position) Distortion Definition • Misrepresentation of an objects location on the image relative to other objects, due to objects being at different distances from image receptor and imaged with oblique rays How it occurs: • Object at larger OID is projected further from central ray than object with smaller OID, so that larger OID appears to be more peripheral than it actually is Bilateral AP Standing Knee • Back towards the detector and feet are facing front • CR is pointed in the middle of the two needs (not over a part of the body) • Largest OID – the patella • That diverging ray will make the patella look like it is off the knee (distortion) Location Distortion • We can shift things around by distortion and superimposition find more resources at oneclass.com find more resources at oneclass.com Minimization of Location Distortion • Orient objects so that rays used to image them have minimal obliquity relative to image receptor • Line things up so that they are straight Radiographic Technique to Intentionally Distort Why do it? • Alter normal appearances to improve visibility of structures How? • Remove superimposition of two objects by applying location distortion • “map” object onto larger area of image receptor by applying magnification • distortion happens more with irregular anatomy • distortion is minimized if the object is flat Summary What is distortion Side distortion Factors affecting magnification Shape distortion • elongation – making them longer • foreshortening – making them shorter Over view • mAs/kVp • differential attenuation • distance • photon fluence o number of photons or electrons passing through a unit or cross sectional area Differential Attenuation • of the 5 ways x-ray can interact with the patient, only 2 are important in radiography • what are they? find more resources at oneclass.com find more resources at oneclass.com o Photoelectric affect - absorption o Compton – scattering → leads to brems o If Compton reaches the image receptor (scatters) it degrades the image → less sharp and more grey Interaction with matter x-rays interact at various structure levels in matter through 5 mechanisms 1. pair production a. 1.02 milaelectron volts 2. photoelectric effect 3. coherent scattering 4. Compton scattering 5. Photodisintegration Compton Scattering • Occurs throughout the diagnostic range • Interaction with an outer-shell electron that not only scatter the xray but reduced its energy and ionizes the atom as well • A scattered x-ray is produced and a Compton electron • High energy → wavelength is short o More likely to go through • Low energy → long wavelength o More likely to be absorbed o More likely to be white like bone Compton Scattering • The ejected electron is called the Compton electron • Energy is divided between the Compton e and the scattered e • As x-ray energy increases the probability of Compton decreases Photoelectric Effect • Occurs in the diagnostic range, by ionizing interactions with inner-shell electrons • The x-ray is not scattered but totally absorbed, producing a photoelectron • A photoelectric interaction cannot occur unless the incident x-ray has energy equal to or greater than the electron binding energy • Occurs in bone and soft tissue → NOT air Differential Absorption • As kVp decreases, differential absorption increases o Less likely to transmit find more resources at oneclass.com find more resources at oneclass.com • • • Lower kVp with extremity Higher kVp with the chest High quality radiographs require the proper selection of kVp, so that the effective x-ray energy results in maximum differential absorption Use of Contrast • The degree of difference between light and dark area on a radiographic image • Barium and iodine compounds can be used • Long scale low contrast → many shades of grey o chest • If the scale is shorter from high to low → called short scale high contrast o finger Contrast Study Imaging Goals • Optimize visibility of anatomical structures • Minimize detriment to: o Patient ▪ Lowest patient dose o Equipment o Operator ▪ Keeping operator safe → no dose to us o Budget ▪ Do not ruin the equipment Visibility of anatomical structure is affected by: • Fineness of recorded detail o Geometric unsharpness = magnification, distortion, focal spot blur o Motion blur o Spatial resultion of image receptor – tied to focal spot blur, cannot resolve objects smaller than focal spot • Control of structure shape, size and position • Focal spot blur – SOD/OID – pay attention to this o Effective vs actual focal spot → which is larger? find more resources at oneclass.com find more resources at oneclass.com Visibility of anatomical structure is affected by: Demonstration of differences in beam transmission 1. Subject contrast 2. Image receptors contrast resolution 3. Processing of image information 4. Display conditions 5. Noise 6. Signal quantity reaching IR 7. Signal quantity captured by IR Problem: conflicting effects when selecting factors • No ideal combination of factors and technique exists to meet all goals equally well • For every part of the body there is give and take, trade offs regarding technical factor selection • Low kVp for smaller body parts X-ray emission • X-rays produced = number of electrons crossing the tube in a given amount of time • 100mA x 0.1s = 10mAs o longer exposure • 200mA x0.05s = 10mAs o less detail • mAs is a product of mA and the exposure time • we can change mA and t and keep the same mAs Group work • assuming a constant exposure time of 0.1 second, mA is changing from q00 mA to 200 mA • explain in terms of quantity and patient dose • 100mA x 0.1s = 10mAs • 200mA x 0.1s = 20mAs → more signal • if you take an image on small but it was actually medium, we would double the mA mAs • • • • • • more is better for increasing number of photons reaching receptor so more signal less is better for reducing head production at anode and quantity of radiation absorbed by patient x-ray quantity is directly proportional to mAs mAs controls quantity = dose!! As mAs increases so does image receptor exposure A change in mA or mAs (200 to 400mA) results in a change in amplitude of the x-ray emission spectrum at all energies find more resources at oneclass.com find more resources at oneclass.com • Dose changes in the area of the amplitude, 1/3 is wehre you see the max Focal spot size • Smaller is better for minimizing geometric unsharpness (mA is the focal spot) • Larger is better for reducing concentration of head loading on anode target Prime Factors • Three factors that affect x-ray emission o mAs o kVp o distance • direct control of the radiographer SID • • • kVp • • larger is better for minimizing geometric unsharpness and magnification distortion smaller is better for reducing quantity of radiation to be produced (so less head produced at anode) if SID goes to 72 feet it minimizes distortion lower is better for increasing subject contrast (greater differential attenuation), decrease kVp, increase subject contrast higher is better for increasing % (amount) of beam transmitted, so more signal to IR and less absorption by patient Kvp Controls … • contrast • as kVp goes down, penetration goes down o absorption would go up → more white on the image Scatter (kVp) • three factors contribute to increased scatter radiation o kVp o field size o patient thickness • increase kVp causes and increase in speed and energy of electron applied across the tube • increase the speed and decreased he wavelength → more energy → more transmission • if speed is lowered → more scatter Test #1 • 50 min of class time find more resources at oneclass.com find more resources at oneclass.com • • • • 100% multiple choice 20% of grade based on lecture notes, class discussions, tutorials, labs, and readings 40 questions find more resources at oneclass.com find more resources at oneclass.com Week Three: Lecture 2 September 20th, 2018 Objectives • Summary of basics surrounding x-ray production, tube and factors • Expand details surrounding component parts and connections to technical factors • AEC Recap X-ray tube components Envelope • The cathode has a negative charge. The anode has a positive charge • How do the electrons know to travel from negative to positive_. What signals the move? o There is a potential difference and the kVp is applied • Describe the medium the electrons travel through o Vacuum • What is the x-ray tube made of? o Glass – heat resistant glass Name the part of the x-ray tube • Hints o Part of cathode (-) o Surrounds filament(s) o Shapes electron beam o Called the cathode focusing cup • What is it made of? o Tungsten o It will emit electrons as is it heated o Causes the electron stream which is applied by the kV to be focused • Why? o Negative charge (potential difference) applied to shape electron cloud X-ray components: Target • The anode o Positive part of the diode tube o Area to be struck by the electron beam • What is it made of? o tungsten and rhenium most modalities o mammography uses molybdenum or rhodium ▪ they want kVp at a lower level • why? o Tungsten has high melting point to withstand heat, and high Z for more efficient x-ray production o rhenium adds strength and prevents surface damage find more resources at oneclass.com find more resources at oneclass.com o lover Z of molybdenum/rhodium beneficial for mammography X-ray Tube Components: Anode Stem • what does it do? o Connects anode disc to rotor assembly in rotating design o The rotor helps it rotate – this is to help dissipate some of the heat • What is it made of? o Molybdenum o This does not conduct – • Why? o we do not want the heat to travel X-ray Tube Components: Rotor • What does it do? o Part of induction motor inside the tube o Rotates the anode disc • What is it made of? o Copper and iron o Copper – very good electrical conductor o Iron – magnetic properties → it will stay close to copper X-ray Tube Components: Stator • What does it do? o Part of induction motor outside of tube around the rotor o Stationary o Initiates spinning of rotor assembly inside tube • What is it made of? o Electromagnets • Why? o To power the induction motor Induction Motor Inside the Tube • Rotor assembly is in the moving magnetic field created by the stator • Moving magnetic field induces an electrical current in the copper of the rotor • Current in the rotor creates an opposing magnetic field to stator magnetic field → rotor spins! Focal Spot • Area of target bombarded by electrons • Conflicting needs o Optimal image production → smallest area possible find more resources at oneclass.com find more resources at oneclass.com • o Optimal heat loading → largest area possible o Small focal spot, we do not want a large mA Compromise → line focus principle! o In regards to the focal spot is on an angle → has to be on an angle to hit the object (7-20 degrees) o The larger the focal spot, the more unsharpness (penumbra) Focal Spot – Actual vs Effective • Application of line focus principle o Angling the target creates the illusion of a smaller focal spot when viewed from beneath the x-ray tube o The steeper the angle, the smaller the spot appears • Actual focal spot o Target surface area that is struck by electrons • Effective focal spot o Side that the focal spot appears to be when viewed from directly beneath the tube Main Technical Factor Selections • kV or kVp setting: o main factor for controlling average energy of the beam o controls maximum energy of a photon in the beam • mA setting: o controls the quantity of current flow from cathode to anode per unit time • mAs setting: o controls the total quantity of charge moving from cathode to anode while the tube is energized o main factor for controlling total quantity of radiation produced during an exposure Beam Energy • x-ray tubes produce photons of varying energies • called a polyenergetic/heterogenous beam • can be demonstrated by graphing emission spectrum • controlled by kVp Change in mAs find more resources at oneclass.com find more resources at oneclass.com Voltage Waveform During Exposure • newer technology systems use steady (or almost steady) voltage (kV) • current across tube is (almost) steady • radiation output is (almost) steady • average photon energy of beam is higher than average photon energy for pulsating voltage system Voltage Waveforms • depending on the equipment • when it is consistent the exposure is better, there is less ripple o emission is consistent o the exposure should come out properly • the area underneath is greater because the x-ray spectrum is higher and more consistent Control of Tube Current • mA selector actually controls quantity of current fed through filament of cathode o filament current controls heat production (and temperature rise) that causes thermionic emission o filament temperature controls number of electrons in the electron cloud o controls number of electrons per unit time that move form cathode to anode find more resources at oneclass.com find more resources at oneclass.com The relationship between filament current and tube current • as you change your current, it will change the filament Automatic Exposure Control (AEC) • also called phototimer • used instead of manual settings o automatic exposure • compensation for patient thickness/density • x-rays → patient → AEC (located inside the bucky or detector) • AEC can be activated from the control panel AEC • • • Function: terminate the exposure as soon as a predetermined quantity of remnant beam has reached the image receptor Goal: obtain consistent quantity of remnant beam radiation to the image receptor to get standard quality of image AEC devices provide diagnostic quality exposures only for stuctures positioned directly above the ionization chamber AEC Operation • Sensors are normally located just in front of image receptor (BUCKY) o Between the patient and the image receptor o The remnant beam is also here o The grid is also in this area • Each sensor releases electrons in proportion to quantity of radiation received AEC Operation • Electrons are fed into a trigger device that will activate once it has received a predetermined quantity of electrons AEC calibration and settings find more resources at oneclass.com find more resources at oneclass.com • • • Installer must take into account the quantity of remnant beam that the IR required to produce an acceptable image The more radiation needed, the higher the number of electrons required to reach the trigger device before termination occurs Settings on control panel allow adjustment of the amount of radiation necessary to send the exposure termination signal – Density controls o Regulate how dark the image can get additional to what is already set o Increase density → darker image Shoot Through Lateral Cervical Spine • Would increase the density just incase for the image to show up Back-Up Timer • Connected to AEC • Safety feature • A normal mAs plus half (patient could get 50% more) if it doesn’t stop early, we do this incase the back up timer does not terminate Lab • Study distance changes Cobia • Measures the intensity of the x-ray beam Summary 1. The actual focal spot represents the target surface area that is struck by electrons 2. How many voltage waveforms are there? a. 5 3. The difference between 3-phase-6 pulse and 3-phase-12 pulse is reduced ripple with 12pulse 4. The lower the ripple the more efficient the generator at producing the set (mA, time, mAsor kVp) a. kVp find more resources at oneclass.com find more resources at oneclass.com 5. Think of a PA chest image. Manually, the exposure is set at 120kVp 200 mA at 1/40 sec (0.025 sec). Now we are going to switch and perform the same PA chest image using AEC. What time would we set? (.05, .017, .04) a. One and a half of 0.025 is 0.04 find more resources at oneclass.com find more resources at oneclass.com Week 4 Lecture 2 September 27th, 2018 Overview • Image receptors • KERMA • Primary Beam • Digital • AEC • Scatter • Grids Beam Quality • Detecting photons • Intrinsic efficiency of a detector or CR cassette is its ability to detect photons • Intrinsic efficiency is determined by the atomic number, photon energy (kVp), and detector thickness • CR receives DR images differently o They have different abilities to quantum detect photons Dose • Absorbed dose – measured in Gray • KERMA – is the energy absorbed per unit mass from the initial kinetic energy released in matter of all the electrons liberated by x-rays • KERMA – acronym – kinetic energy released in matter • The energy imparted directly to the electrons, per unit mass, is the kerma • “is the tissue absorbing it” Mass and Attenuation Coefficients • Mass energy transfer coefficient. The mass energy transfer coefficient and the fraction of energy transferred to the charged particles as kinetic energy, by the interacting incident photons • The mass attenuation coefficient or mass narrow beam attenuation coefficient of the volume of a material characterizes how easily it can be penetrated by a beam of light, sound, particles, or other energy or matter • The detection of the IR change depending on where we are – department, OR, portable, etc. • The DR receptor is more sensitive – it is more efficient Image Receptors Digital radiography has: find more resources at oneclass.com find more resources at oneclass.com • Excellent ability to capture a wide range of remnant beam signal magnitudes excellent ability to customize processing and display Image Receptors • Detective quantum efficiency – is a descriptor used in medical imaging • Absorption coefficient • DQE – is a measure of x-ray absorption efficiency • If the photons that reach the detector are low energy they would be more absorbed Image Receptors Film-screen has: • Best spatial resolution • This is not taught anymore Digital Radiography has: • Adequate spatial resolution for most body parts (more expensive system tend to be better) • It has a very long contrast resolution Beam Content – terminology Heterogeneous beam • Beam contains a range of photon energies Beam Intensity • Quantity of energy flowing through an area per unit time • Depends on both number of photons and energy of photons Energy fluence • Total quantity of energy flowing through an area • This is kVp Photon fluence • Total number of photons flowing through an area • This is mAs Changing one Variable – think about what happens to the: Primary/Incident beams • Photon fluence • Energy fluence Remnant beams • Photon fluecne • Energy fluence find more resources at oneclass.com find more resources at oneclass.com • • SNR Contrast If we raise mAs • Patient size will determine if we need to change the mAs • Small, medium, large, If we raise mAs Primary/Incident beams • Photons fluence increases • Energy fluence no change – because we did not change the kVp Remnant beams • Photon fluence increases • Energy fluence no change • SNR increase • Contrast no change – it is influences by kVp If we raise kVp • We want more penetration • Determined by the thickness of the body part If we raise kVp Primary/Incident beams • Photons fluence increase • Energy fluence increase Remnant beams • Photon fluecne increase • Energy fluence increase • SNR increase • Contrast decrease – more shades of grey – long scale low contrast – more similarity If we raise SID (change nothing else) – similar to decreasing the kVp Primary/Incident beams • Photons fluence decrease • Energy fluence decrease Remnant beams • Photon fluence decrease • Energy fluence decrease • SNR decrease • Contrast – becomes longer scale, lower contrast find more resources at oneclass.com find more resources at oneclass.com What about focal spot size? Primary beam is • Remnant beam is How to balance goals Consider the part being imaged: • Thickness • Inherent differences in ability to attenuate radiation • Size of detail required to be visible • Sensitivity of part to radiation Consider the equipment available • X-ray generator (room 5 vs 4) • Image receptor system Kvp • • • News Beam quality and quantity vary with changes in kVp kVp 15% rule 15% rules (kVp) • an increase of 15% in kVp, is equivalent to doubling the mAs • reducing the kVp by 15%, is equivalent to halving the mAs • kVp is very powerful Using standard output values • use known relationships to create formula for calculating exposure • exp is proportional to o mAs (dose, quantity) o kV2 (quality) o 1/d2 (inverse square law) Exposure is proportional to… • KV2 • Exposure is nearly proportional to the energy fluence of the x-ray beam. Exposure is approximately proportional to the square of the kVp in the diagnostic range (30 to 150 kVp) X-ray quantity and KVP • Lateral chest calls for 110 kVp, 10 mAs, the resulting intensity is 0.32 mGy (32 mR). If kVp is increased to 115 but mAs remains unchanged, what will be the intensity? find more resources at oneclass.com find more resources at oneclass.com • • the answer is (110)2/(115)2 Review – Inverse Square Law • must know 3 of 4 parameters, tow distances and two intensities • ex. For a given technique, x-ray intensity at 1m is 450mR. What is the intensity at 3m?? Review – Direct Square Law Review – Automatic Exposure Control • is a device … that measures the quantity of radiation that reaches the IR AEC – Phototiming • when AEC systems are used, the selection of exposure time factors, and thereby mAs selection, are eliminated • the radiographer must still exercise professional judgment to determine all other factors, mA, kV, and distance • APR – anatomically programmed radiography Use of AEC in controlling the Remnant Beam • AEC is calibrated to terminate the exposure once a pre-determined energy fluence has reached the detectors • Pre-determined quantity of energy fluence is the amount needed to achieve a specific quantity of signal on the receptor Clinical Use of AEC User sets up equipment • Beam must be centered to the IR location being used (CR cassettes, bucky is used) • Number, location and sensitivity of sensors are selected on control panel • kV is selected to control beam penetrability • focal spot and SID are selected to control image geometry Correct Detector (or sensor or cell) selection find more resources at oneclass.com find more resources at oneclass.com • • number of cells is determined by o size of image field location of cells is determined by o shape, size + orientation of structures Examples of AEC Cell selection: knee • PA projection – includes mostly bone and some soft tissue structures at joint space and sides o Select ?? o Bone will be light gray, joint space and soft tissue will be dark find more resources at oneclass.com find more resources at oneclass.com Week 5 Lecture 1 October 2nd, 2018 Overview • Contrast • Scatter • Beam restricting devices o A collimator • Grids What do you know about scatter? • How does scatter influence an image receptor and AEC device? • AEC lives between the patient and IR • If unattenuated, it will reach the AEC sensor and IR • Because of attenuation, some photons do not reach it to the AEC/IR • Higher Kvp will increase the amount of photons that will reach the IR Effect of Scatter: Key point • Scatter adds to the remnant beam photons that don’t represent transmissions, so subject contrast is reduced • Scatter will cause the image to have more noise (more gray) • More scatter reaching the IR will increase the EI number (dose) More Scatter Talk Describe attenuation • Decrease of intensity of the beam as it goes through the object → absorption or scatter What is KVP’s relationship to scatter • Increase the kVP will increase the amount of scatter to reach the IR • Less photoelectric absorption because the energy is higher Collimators and grids are used to reduce contrast • Help us deal with scatter • Thicker body part = more scatter • More scatter will cause AEC to terminate sooner → this could create an under exposed image Scatter What is collimation’s roll when it comes to scatter find more resources at oneclass.com find more resources at oneclass.com • • FOV (field of view) is collimation FOV is expected → a certain size that will lower the chance of scattered x-rays Object thickness – how does that affect scatter • There is more room to bounce around o The photon will hit something and change its direction • Lay the patient down to make them smaller Methods of reducing number or scattered photons produced • Reduce field size by collimation • Reduce thickness of ray paths by compressing the object (spread object laterally) • Use a higher beam energy (change kV and/or filtration) PBL • • • Positive beam limitation o Automatic collimation → machine will automatically change the field of view o Room 6 Manual Aggressive FOV – reduces scatter Problem with using beam energy to control scatter production • The higher a photons energy, the less likely it is to scatter o % of photons of the beam having a scatter interaction decreases when beam energy increases o higher energy (short wavelength) → higher transmission • the higher a photons energy, the smaller the change in direction the photon has when it scatters o of the photons that scatter, a greater % will now head toward the IR instead of missing it o photons will interact with an AEC chamber or IR →they will add more gray to the image Thank Goodness for GRIDS • a device consisting of a series of lead strips closely spaced on their edges and separated by spacers of low density material such as aluminum • used to reduce the amount of scatter radiation reaching the image receptor • with a grid in the dose will go up because you need to get through the grid with the lead strips find more resources at oneclass.com find more resources at oneclass.com Contrast • areas of light, dark, and shades of gray on the x-ray image • variation → shades of gray • long scale and short scale contrast • contrast is influenced by the factors that we set Scatter • definition • scatter radiation • adds noise, decrease contrast resolution, and decrease image contrast Grid Function • contains a series of radiopaque strips alternating with radiolucent strips • radiopaque – a dense material and high atomic number = lead (Pb) • radiolucent = aluminum or fibre Scatter Radiation • lower kVp for wrist – more black and white • higher for chest • abdomen is most gray Scatter • most x-rays have energy that matches the k-shell binding energy • the scattered x-ray beam (remnant) has lower energy than the primary x-ray beam • thinking of the DGE (detection quantum efficiency) – scattered x-rays increase absorption of image-forming x-rays by the IR Radiation in Radiography Expressed in rad or gray • absorbed dose • is the energy transferred from ionizing radiation per unit mass of irradiated material. (biological effects) Expressed in gray (Gy) • kerma • is the energy absorbed per unit mass from the initial kinetic energy released in matter of all the electrons liberated by x-rays of gamma rays (similar to intensity or roentgen units) find more resources at oneclass.com find more resources at oneclass.com Patient Dose Descriptions • CT is more dose because the radiation is on the entire circular motion that it makes Collimation – 3 types of beam restricting devices Question Name two tools within the technologists reach to enhance contrast by reducing scatter • Collimation → reducing the field size • Grid • Change the object thickness How does collimation and object thickness affect scatter • Thick patient more scatter • 10cm rule • increase FOV = more scatter find more resources at oneclass.com find more resources at oneclass.com What is PBL and how does it aid the technologist? • It gives us a good starting point for collimation • Always go smaller Compare different IRs (film, CR, DR) • Film and CR both have cassettes • CR takes more time than DR What is AEC, is it foolproof? • Automatic exposure control • Sense it properly and position correctly • make sure collimation is correct • set 1 and a half more than the normal time What is the direct square law? Name one practical application in radiography • in recovery or in the OR • not in the department Name 2 terms that are located between the patient and the IR • the grid • the AEC chamber find more resources at oneclass.com find more resources at oneclass.com Objectives • Grids • Filters Grid Construction Parameters • Grid ratio: ratio of height to width of interspaces o Indicates relative dimensions of ray paths o Higher grid ratio has greater “stopping power” for angled scatter photons o Interspace can be aluminum or carbon fibers → they can be penetrated (x-ray can go through them) Grid Ratio • Grid ratio = h/D • Where: h = lead strip height, D = interspace width • Higher the grid ratio, more scatter is absorbed → less scatter in general • 12:1 requires more dose than 5:1 Grid Ratio • Calculate the grid ratio • h = 3.0 mm • D = 0.25 mm • Answer = 12 Radiographic Grids • High grid ratios increase patient dose • Do grids move? o They move → it shakes to blur the lead strips, we do NOT want grid lines • Grid frequency: number of radiopaque strips (grid strips) per centimeter (25-80 lines/cm). Grids with high grid frequency show less distinct on an x-ray compared to grids with a low grid frequency • By combining the information from grid radio and grid frequency – determine the total quantity of lead in the grid Lead Content • Lead content = mass per unit area or grams per cm2 • As lead content increases, grid efficiency • Increase lead content → increase grid ratio 12:1 → increase grid frequency (number of grid lines goes up) Grid Radio find mo