1Chapter 19 & 20 Image Quality & Techniques There are three geometric factors that affect radiographic quality.MagnificationDistortionFocal Spot BlurWe have explored these factors in the laboratory.
2MagnificationAll objects on the radiograph are larger that their actual size. This is called magnification.The magnification factor is the image size divided by the object size.At 40” (100 cm) factor is 1.1At 72” (180 cm) factor is 1.05
3MagnificationUsually we do not know the size of the object so we must determine the magnification factor another way.Image size S0DMF = =Object Size SID
4Minimizing Magnification Large SID: Use the less divergent beam.Chest X-rays are done at 72” SID to minimize magnification.Lateral C-spine done at 72”Small OID: Get patient as close to the film as possible.Basic principle for positioning.
5DistortionDistortion is the misrepresentation of the true size and shape of the object being radiographed.The amount of distortion depends upon the thickness, position and shape.
6ThicknessThick objects are more distorted than thin objects because of the greater change in Object Image Distance.
7ThicknessThe position of the object relative to the central axis will cause greater distortion with thick and/or irregular shaped objects.
8Object PositionIf the object plane and image plane are parallel the image will not be distorted.If the object plane and image plane are not parallel, distortion will occur.
9Spatial DistortionWhen multiple objects at different OID’s occur, we get spatial distortion due to unequal magnification.Two arrows appear as one.When shifted laterally more distortion occurs
10Object Shape Distortion When the object plane is not parallel to the image plane as when inclined, shape distortion occurs.This will result in foreshortening.
11Focal Spot BlurFocal spot blur is caused by the effective size of the focal spot, which is larger at the cathode side.Focal spot blur is the most important factor in determining spatial resolution.
12Focal Spot BlurFocal spot blur is impacted by the Object to Image Distance.
13Focal Spot Blur Heel Effect There is more to the heel affect than just the attenuation of the beam by the anode.The focal spot blur is smaller at the anode side and larger at the cathode side.
15Did you see a problem ?If the tube is mounted correctly for the AP Full spine, Chest and A-P Thoracic Spine, the patient must stand on his head for the lateral thoracic spine!!!!For erect radiography, the use of the anode heel affect is limited.
16Object Factors that Affect Quality Subject ContrastPatient or part thicknessTissue mass densityAffective atomic numberObject shapekVp
17Radiographic Contrast Radiographic Contrast is how the film looks.It is the combination of receptor contrast and subject contrast.Contrast occurs between structures of different densities.
18ThicknessThe thicker the body part or body section, the greater attenuation of the beam. Contrast is relative to the number of x-rays leaving the body.Remember that x-rays are merely shadows of the anatomy based upon attentation of the beam.
19ThicknessRadiographs of thin patients will have more contrast than those of large patients.Thicker object absorb more rays and will appear lighter than thin objects.
20Tissue Mass DensityDifferent sections of the body have may equal thickness yet different mass density.Two wrist may be the same thickness but the one that is swollen will have greater mass density due to water in the tissues.
21Effective Atomic Number While Compton interactions with tissues are not impacted by the relative atomic number of tissues, the photoelectric effect vary with the cube of the atomic number.When the effective atomic number of adjacent tissues is very different, subject contrast is very high.
22Object shapeThe object shape not only influences the geometry but also through its contribution to subject contrast.A vessel on end has high contrast while other have lower contrast.
23Varying tissue densities Bone absorbs most x-rays leaving a white shadow.Water absorbs less x-rays leaving a light gray shadowFat absorbs fewer x-rays leaving a dark gray shadow.Air absorbs little x-rays and is black on the film.
24Varying densities in the Body Air, oil, water and metal (natural) absorb different degrees of the x-rays and produce contrast.Heavy metals are used as contrast media to enhance contrast in the body in medical radiology. Principle ones are Barium and Iodine.
25kVpWe have little control over the previous factors controlling subject contrast.BUT!!!!!The absolute magnitude of subject contrast is greatly influenced by the kVp of operation.
26kVpkVp also influences film contrast but not to the extend it controls subject contrast.Low kVp = high contrast = short scaleHigh kVp = low contrast = broad scale
27Motion BlurIf any element of the x-ray moves during exposure, we get motion blur.Patient motion is the most common cause of motion blur.Motion blur is more common in erect radiography.
28Ways to Control Motion Blur Use the shortest possible exposure timeRestrict patient motion by instructions or restraining device.Use a large SIDUse a small OID
29Tools to Improve Quality Patient PositioningGet the patient close to the film reduce OID.Center the beam to the area of interest.Get the area of interest parallel to the beam or film.Restrain motion and communicate with the patient.Use short exposure times.
30Tools to Improve Quality Image receptorsUse the correct film & screen combination for the examination. Extremity for wrist. Regular for spines.Intensifying screens reduce patient exposure at least 20 times.As the speed of the image receptor increases, radiographic noise and contrast resolution decreases.
31Tools to Improve Quality Image receptorsLow contrast imaging procedures have wider latitude and a larger margin of error in producing acceptable radiographs.Use the highest speed system that will provide adequate contrast and density over the entire spectrum of examinations.
32Tools to Improve Quality Technique selectionWe must select the optimum technical factors.We must get the exposure time as low as possible so high frequency machine are important.Contrast controlled by the kVp usedDensity controlled by the mAs used.
33Tools to Improve Quality kVp has a greater influence than mAs.Technique selection is a balancing act. Balancing contrast, density and exposure.
34Chapter 20 Radiographic Technique Several factors influence the selection of technical factors. The primary factors that impact exposure and image quality are:SIDmAskVp
35Patient FactorsThe anatomic thickness and body composition greatly impact the technical factors.The technique chart is based upon the Sthenic Body Type.
36Patient Factors Sthenic is strong & active Hyposthenic is thin but healthyHyperstenic is obeseAstenic is small, frail sometime emaciated, and often elderly
37Patient Thickness The thickness of the patient should not be guessed. It should be measured with calipers.Patient thickness is measured in cm.
38Body compositionThe type of tissue in the area of exposure will impact the technical factors.The tissue types in the chest are different from the abdomen.Disease processes will also impact the exposure factors. Obtaining a good clinical history is important. History must be communicated to the radiographer.
40Image Quality Factors Image quality factors include ODContrastImage DetailImage DistortionOD is the optical density or radiographic density. OD is controlled by the mAs and SID.
41Optical Density Numerically low OD is a low number like 0.25. Dark is a high number like 2.20 to 4.0Light is underexposedDark is over exposedIf density is the only factor that needs to be changed, change the mAs.
42Optical DensityA 30% change in mAs is needed to make a perceptible change in optical density.Usually when a change in optical density is needed, the mAs is either doubled or halved.kVp must be changed by 4% to produce the same change in optical density.Changing kVp will also impact penetration and contrast.
4330 - 50 Rule If the film is under exposed, double the mAs. If the film is over exposed, cut the mAs in half.If the film is slightly underexposed, increase the mAs 30%.If the film is slightly overexposed, reduce the mAs 30%.
4430% Density ChangeThe lower image was the first image taken. It was dark but normally would be acceptable.The top image was the mAs reduced 30%. The air fluid levels in the sinus is easier to see.
4515% RuleThe OD can be changed with kVp but it will also impact exposure and contrast also.Increase of 15% in kVp is equal to cutting the mAs in half.Decrease of 15% in kVp is equal to doubling the mAs.If the film is underexposed, increase kVp 15%.If the film is overexposed, decrease kVp 15%.
46Contrast The function of contrast is to make the anatomy more visible. Contrast is the difference in density of adjacent structures.The relative penetrability of the x-ray through different tissues determines the image contrast.
47ContrastContrast can be measured as the Gray Scale of Contrast. It is the range of optical density from white to black on the image.Contrast is controlled by kVp.
48Adjusting Contrast with 15% Rule An 15% increase in kVp and a reduction of mAs by 50% will produce the same OD but lower contrast.Used to reduce exposure or reduce exposure time/An 15% decrease in kVp and doubling the mAs will produce the same OD but higher contrast.
49Image DetailThe sharpness of image detail refers to the ability to see structural lines or borders of tissue in the image.The visibility of image detail is best measured by the contrast resolution.The geometric factors of focal spot selection, SID and OID will impact sharpness.
50Image DetailVisibility of image detail is impacted by factors such as image fog.Scatter radiation reduces the ability to visualize lines of detail.Light fog or processing can impact the visibility of structures.Collimation, screen combination and the use of a grid are other factors that impact image detail.
51DistortionThe position of the x-ray tube greatly impacts distortion of the image. The image may be elongated or foreshortened.The proper Positioning of the tube, anatomic part and image receptor greatly impacts distortion.
52Types of Technique Charts There are four primary means to establish techniques.Variable kVp Fixed mAsFixed kVp with varying mAs.High kVp with varying mAsAutomatic Exposure Charts when AEC is used.
53Variable kVp ChartsThe mAs is fixed and the kVp is varied based upon patient thickness.Usually by a formula such as 2 x thickness +30 = kVp for single phase24 cm patient= 24*2+30=78kVpFor high frequency use 23 and for three phase use 25.Small patient used low kVp= high contrastLarge patient used high kVp= low contrast
54Variable kVp Charts This type of chart should be avoided. Contrast was very inconsistent.Very little latitude on smaller patients.Higher radiation exposureThis type of chart should be avoided.
55Fixed kVp TechniquekVp is fixed and mAs varies by patient thickness. Usually 30% per two cm.Uses Optimum kVp for the body partContrast is constant.Wider latitudeLower exposure
56Fixed kVp Technique Variations High kVp technique uses over 100 kVpNo longer used for bone.Long ago used for spine but images are too gray. Low contrastMostly used for chest and barium contrast studies.
57Fixed kVp Technique Variations Automatic Exposure Technique ChartsUses optimum kVp and high backup mAs.Ion chamber or photo cell determines when correct density is achieved on film and terminates exposure.Proper positioning is critical to get the area of interest over the ion chamber.
58Using the Technique Chart The chart is not the Bible but is a guide.Works about 85% of the time so it is a great starting point.Lists factors used for each view based upon measurement of the patient.Can include as much as you want to include.
59Using the Technique Chart Recommended factors for chart:optimum kVp for viewmAs based upon cm measurementfilters usedSID & tube angle usedBucky or non-BuckyCassette & film type
60Using the Technique Chart Charts should be:accessibleeasy to readnot hand writtenbased upon the type of machine and machine controls.mAs or mA and time
61Technique Variables Variable machine & electrical output Incoming power and ability of machine to compensate for variations in incoming power.Type of High-voltage PowerSingle phase to High Frequency reduce mAs 50%High Frequency to Single phase double mAsGrid ratioNon-Bucky Holder
62Technique Variables Variable machine & electrical output Relative Speed Value of cassettes & film combination.400 speed to 200 speed = double mAs200 speed to 400 speed = reduce mAs 50%
63mAs & kVp RelationshipThere are some basic rules for mAs and kVp that are used to adjust the technical factors.Remember x-rays are like toast.Dark is too denseLight has inadequate densityThis tells you which was to go.
64mAs RulesSince mAs controls density, it is usually used to adjust density.30% increase needed to make a noticeable change in density.50% mAs reduction will reduce density 50%Doubling mAs will doubles density.
65mAs Rules If image is too dark reduce mAs 50%. If image is too light double mAs.Doubling mAs can be done by doubling mA or time.Doubling time increases chance for motion blur.
66kVp Rule kVp will also change density. A light film from low kVp is called under exposed or under penetrated.Very white image because no x-rays reached the film.Too dark is over exposed, some say over penetrated. They are different.
67kVp RuleOver penetrated will result only if the kVp used is too high for the view. It will be dark and very flat (lacking contrast)Density is very sensitive to changes in kVp.A 2 kVp (HF) to 4 kVp change is noticeable. About 4%.The 15% rule works with density adjustment.
68kVp Rule 15% increase in kVp will double density. 15% decrease in kVp will reduce density 50%.15% increase in kVp = doubling mAs15% decrease in kVp = half the mAs10 kVp = 15% change in the 60 to 90 kVp range.
69Optimum kVpOptimum kVp will provide the best contrast with the least amount of radiation.If using the optimum kVp you should not need to adjust kVp.kVp can be changed based upon body habitus and disease.
70Optimum kVp Small Extremity Large Extremity Cervical Spine AP or Lat APOMThoracic APThoracic LatLumbar APLumbar ObliqueLumbar LateralPelvisAbdomenRibsChest55-6565-7070-7475-787580749070110+
71Dark film Dark & No contrast = reduce kVp no change to mAs Black no structures seen reduce both.Dark = look at the optimum kVp range.If reducing kVp goes beyond optimum kVpreduce mAs 50%
72Light FilmIf film is so light that no structures are seen then it is under penetrated so increase kVp.kVp controls penetration.If structures seen but lacks density increase double mAs.
73Patient Factors Very muscular or large boned increase mAs by 50% Very muscular and large boned increase mAs 50% and kVp 4 to 6 kVp.Obese increase mAs 50%Edema increase mAs 30%Frail decrease kVp 5 to 15%
74Patient FactorsOsteoporotic patient : over 60 years old decrease mAs 30% to 50%6 to 12 years old reduce mAs 30 to 50%Infants to 6 years old decrease mAs 75%
75Item that Affect Detail Spatial Resolution controlled by focal spot size and image receptor.Detail influenced by:SIDOIDMotion BlurDensity & Contrast of Image
76Items that Affect OD Optical density is controlled by mAs OD influenced by:kVpSIDThicknessDensityCollimation
77Items that Affect OD OD influenced by: Grid Ratio Development time and temperatureImage receptor speed
78Items that Affect Contrast Contrast controlled by kVpContrast influenced by:mAsDevelopment Time & TemperatureCollimationGrid ratioImage receptor
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