5kVp Kilovolts controls how fast the electrons are sent across the tube Controls, quality, penetrability & contrastIncreasing kVp also increases scattered photons reducing image qualityDoes kVp influence OD?
8mADetermines the number of photons, radiation quantity, OD & patient doseChanging mA does not change the kinetic energy of e-Available mA stations are usually 50, 100, 200, 300, 400 & 600
9Exposure Time mA X s = mAs mAs controls OD Should be kept as short as possible, for most examinations. To minimize the risk of patient motionmA X s = mAsmAs controls ODmAs determines the number of photons in the primary beam
10Distance Affects exposure of the IR because of the Inverse Square Law SID largely determines the intensity of photons at the IRDistance has no effect on radiation quality
15Tube voltage (kVp) Determines the maximum energy in the beam spectrum and affects the quality of the output spectrumEfficiency of x-ray production is directly related to tube voltage
16Influencing factors: kVp 15% rule:15% kVp = doubling of exposure to the film 15% kVp = halving of exposure to the film15% rule will always change the contrast of the image because kV is the primary method of changing image contrast.Remember :15% change ( ) KVP has the same effect as doubling or ½ the MAS on density
17kVp ChangesThe kVp setting must be changed by at least 4% to produce visual changes an image
19Radiographic Technique Technique charts are based on the “average patient”The thicker the part the more x-radiation is required to penetrate. Calipers should be usedKeep in mind not only the measurement but the type of tissue you need to penetrate (fat vs muscle)
20Technique In general, Soft tissue = low kVp and high mAs Extremity (soft tissue & bone) = low kVpChest (high subject contrast) = high kVpAbdomen (low subject contrast) = middle kVp
21Pathology Can appear with increased radiolucency or radiopacity Some pathology is destructive causing tissue to be radiolucentOthers can be additive causing tissue to be radiopaque
22Technique selection – Fixed kVp For each anatomic part there is an optimum kVpmAs is varied based on part thickness or pathological condition
25Image QualityIs the exactness of the representation of the patient’s anatomy3 major factors affecting image quality that is under the control of the technologist: Image Receptor selection/use, Geometric factors & Subject factors.
26Judging Image QualityThe most important characteristic of radiographic quality are: Spatial Resolution, Contrast Resolution, Noise & Artifacts
34ResolutionIs the ability to image two separate objects and visually distinguish one from the other.Spatial resolution is the ability to image small objects that have high subject contrast. Ex: bone-soft tissue interface, breast calcifications, calcified lung noduleConventional radiography has excellent spatial resolution
35RESOLUTION TESTTOOLSLINE PAIRS/ MMDepicts how well you can see the differences in structuresMore lines=more detail
36Measuring Resolution for an x-ray imaging system
39SMPTE Test PatternIn 1985 the Society of Motion Picture and Television Engineers (SMPTE) published a recommended practice (RP-122).Specifications for Medical Diagnostic Imaging Test Patterns for Television Monitors and Hard-copy Cameras.
41Focal Spot Size Smaller x-ray beam width will produce a sharper image. Fine detail = small focal spot (i.e. small bones)General radiography uses large focal spotBeam from penlight size flashlight vs. flood light beam
44Modulation Transfer Function The ability of a system to record available spatial frequencies.The sum of the components in a recording system cannot be greater than the system as a whole.When any component’s function is compromised because of some type of interference, the overall quality of the system is affected.
45Contrast ResolutionIs the ability to distinguish anatomic structures of similar subject contrast. Ex: liver-spleen, gray matter-white matterMagnetic Resonance Imaging has the highest contrast resolutionComputed Tomography is excellent as well
57Radiographic Noise Four components: Film graininess, structure mottle, quantum mottle & scatter radiation
58Radiographic NoiseFilm graininess – distribution & size of the silver halide grains in the emulsionStructure mottle – speed of the intensifying screen. Phosphor size & DQE/CENot under the control of the technologist
59Image Noise Speckled background on the image Caused when fast screens and high kVp techniques are used. Noise reduces image contrastThe percentage of x-rays absorbed by the screen is the detective quantum efficiency (DQE)The amount of light emitted for each x-ray absorbed is the conversion efficiency (CE)
60Quantum MottleAn image produced with just a few x-rays will have more quantum mottle.The use of very fast intensifying screens or not enough mAs or kVp will increase quantum mottle
71The latitude of an image receptor is the exposure range over which it responds with diagnostically useful OD.Depending on the manufacturing characteristics radiographic film will respond differently to radiation exposure
74Unexposed film Appears like a frosted glass window ODs of unexposed film are due to base density and fog densityBase density – tint added to the base to reduce eye strain and crossover. Has a densitometer value of approximately 0.1
75CROSSOVERReducing crossover by adding a dye to the base
76Unexposed filmFog Density – inadvertent exposure of film during storage, chemical contamination, improper processing, radiation exposure, etc.Fog density contributes to reduction of radiographic contrastFog density should not exceed 0.1Base + fog OD = 0.1 to 0.3
782 principal characteristics of any image are Spatial & Contrast Resolution Spatial resolutionResolution is the ability to image two separate objects and visually distinguish one from the otherSpatial resolution is the ability to image small objects that have high subject contrast (eg. bone-soft tissue interface, calcified lung nodules)Determined by focal-spot size and other factors that contribute to blurDiagnostic x-ray has excellent spatial resolution. It is measured in line pairs per mm.
79Other factors affecting the finished radiograph The concentration of processing chemicalsThe degree of chemistry agitation during developmentDevelopment timeDevelopment temperature
85Spatial Resolution determined by: Pixel size.CR- sampling frequencyDR – DEL size There are relationships betweenPixel sizeReceptor sizeMatrix size pixel size = larger matrix receptor size = larger matrixSpatial resolution is not related the amount of exposure
89Signal Sampling Frequency Good sampling under sampling
90Nyquist Frequency The Nyquist Frequency will be ½ of the sampling frequency.A plate that is scanned using a sampling frequency of 10 pixels per millimeter would not be able to demonstrate more than 5 line pairs per millimeter based upon the Nyquist Frequency.The Nyquist Frequency allows thedetermination of the spatial resolution fora given sampling frequency.
97Geometric FactorsProducing high quality radiographs. Technologists must maximize geometric conditionsThree principal geometric conditions affect radiographic quality: Magnification, Distortion & Focal-spot blur.Review table 16-4, pg. 295 for summary
98Object UnsharpnessMain problem is trying to image a 3-D object on a 2-D film.Human body is not straight edges and sharp angles.We must compensate for object unsharpness with factors we can control: focal spot size, SID & OID
99MagnificationAll image on the radiograph are larger than the object they represent.For most exams minimizing magnification is desired. There are a few exams where some magnification can be helpfulTUBE CLOSE TO THE PART (SID),PART FAR FROM THE CASSETTE (OID)
100Minimizing Magnification Large SID: use as large a source-to-image receptor distance as possibleSmall OID: place the object as close to the Image receptor as possibleIn terms of recorded detail and magnification, the best image is produced with a small OID and a large SID.
101Size Distortion & SID Major influences: SID & OID As SID , magnification Standardized SID’s allow radiologist to assume certain amt. of magnification factors are presentMust note deviations from standard SID
102The position of the tube (SID) to IR Will influence how the structures appear on the image The farther away – the less magnified ↑SID ↓ MAGNIFICATION
103SIDShine a flashlight on a 3-D object, shadow borders will appear “fuzzy”-On a radiograph called PenumbraPenumbra (fuzziness) obscures true border – umbraFarther the flashlight from object = sharper borders. Same with radiography.
106OID Object to Image Distance The closer the object to the film, the sharper the detail.OID , penumbra , sharpness OID , penumbra , sharpness Structures located deep in the body, radiographer must know how to position to get the object closest to the film.
107Size Distortion & OIDIf source is kept constant, OID will affect magnificationAs OID , magnification The farther the object is from the film, the more magnification
108The position of the structure in the body will influence how magnified it will be seen on the image The farther away – the more magnified
117Finding SODSID – OID = SODSID = 100 cmOID = 7 cmWhat is the SOD?
118Finding magnification of the heart on a lateral CXR SID – OID = SODSID = 72 inchesOID = 8 inches (estimated)What is the SOD?What is the Mag Factor?
119Distortion Misrepresentation of the true size or shape of an object -MAGNIFICATION (size distortion)-TRUE DISTORTION (shape distortion)Shape distortion: unequal magnification of different portions of the same object
129Position DistortionSpatial distortion = anatomy positioned at various OIDs but superimposed, only one can be seen
130Position Distortion – Irregular Anatomy Anatomy or objects can cause considerable distortion when imaged off the central axis
131Focal-Spot Blur Dependent on the size of the effective focal spot Smaller the effective focal spot = less blur and better spatial resolutionFocal-spot blur is the most important factor in determining spatial resolution
132Focal Spot Size Smaller x-ray beam width will produce a sharper image. Fine detail = small focal spot (i.e. small bones)General radiography uses large focal spotBeam from penlight size flashlight vs. flood light beam