5 Subject contrast Causes of subject contrast The difference in the x-ray intensities transmitted through the subjectIt is the shortened form of the radiation contrast of the subjectCauses of subject contrastDifferential attenuationScattered radiation
6 Differential attenuation Differential attenuation is the result of the attenuation caused by Photoelectric absorption and Compton scattering.Depends onThickness of the anatomical structureEffective atomic number of the body tissuesPhysical density of the body tissuesPresence of radiological contrast mediumX-ray tube kilovoltage employedX-ray beam filtration
7 Effective atomic number & Subject contrast For a given Photon energy the photo electric absorption is higher when the atomic number is high ( bone absorbs more radiation than soft tissue)E.g. if the three tissues A,B,C have effective atomic numbers as Z1 > Z2 > Z3Incident intensityAZ1BZ2CZ3Subject contrast A-BTransmitted intensitySubject contrast A-CSubject contrast B-C
8 X-ray tube kilovoltage & subject contrast Photo electric absorption predominates at low kilovoltages, therefore at low kilovoltages the subject contrast is high, and when the kilovoltage is increased the subject contrast tend to be reduced.At high kilovoltages approaching 150kV the contrast is mainly caused by the compton effect which mainly depends on the density difference of the anatomical structures.
9 kV & subject contrast Low kV E B B1 Air E B1 E B2 Higher differences Supporting tissue (m)BB1T3B2T1T2AirE B1E B2Higher differencesET1EMEMET2ET3EA
10 kV & subject contrast High kV E B B1 Air Lower differences E B1 E B2 Supporting tissue (m)BB2B1T3T1T2AirLower differencesE B1E B2EMEMET1ET2ET3EA
11 X-ray beam filtration & Subject contrast Filtration reduces the low energy components of the x-ray beam. Hence increasing the filtration has the effect of increasing the effective photon energy of the beam. This influences the photoelectric absorption in a similar way as increasing the tube kilovoltage.Therefore increasing the filtration will decrease the subject contrast
13 Scattered radiation & subject contrast When the primary beam from x-ray tube interacts with matter scattered radiation is produced.Scattered radiation travels in different paths from the primary beam and will reduce the subject contrast of the invisible x-ray image.Not only the subject contrast but it will reduce the signal to noise ratio also.
14 Scatter reduces the subject contrast Supporting tissue (m)BB2B1T3T1T2AirScatter Lowers the differencesE B1E B2EMEMET1ET2ET3EA
15 How to minimize the effect of scatter on subject contrast? Reduce the amount of scatter produced at the object (patient) by:Collimating the primary beamReducing the proportion of forward scatter using low kVReducing the tissue thicknessAvoiding other sources of scatter, such as bucky trayProtecting the image receptor byUse of secondary radiation gridEmploying an air gap
16 Use of gridLead stripsImage receptorRadiolucent inter-space
17 Employing Air gap Air gap Image plane 2Image plane 1ObjectScatterAir gapPercentage of oblique ray reaching the image receptor plane is reduced at image plane 2
18 Sharpness of Invisible x-ray image The sharpness is determined first by the geometry of image formationThe size of the source of radiation is of primary concernedInfinite size (Point source)Finite size ( larger than a point)When the size of the x-ray source (Focus) is large the sharpness of the image is less
20 Intensity distribution at previous situations Intensity of x-rays at image planeIntensity of x-rays at image planeUUDistance across image planeDistance across image plane
21 Geometric unsharpness The formation of unsharpness due to a penumbra is a direct consequence of the finite size of the x-ray source.This form of unsharpness is known as Geometric unsharpness (UG)It can be shown thatfocal spot size x object-image distanceGeometric =Unsharpness focus-object distance
22 Evaluation of Geometric unsharpness SourceABTriangles OAB & OCD are similar.AB/CD = OB/OCRe-arrangingCD = AB x OC/OBUG = focal size x OFD/FOBObjectOImage planeCD
23 Factors governing geometric unsharpness Focal spot sizeSmall focus gives minimum geometric unsharpnessObject image (film) distanceShorter OFD gives less geometric unsharpnessFocus to object ( Focal film) distanceLonger the FFD lesser the geometric unsharpnessIncrease the FFD when OFD cannot be reduced, to minimize the geometric unsharpnessEdge penetration
24 Focal spot size & Geometric unsharpness Unsharpness increases, when apparent focal area increasesApparent (effective) focal area = Actual focal area x Sine of target angleTherefore Unsharpness increases when target angle increases for a given actual focal spot sizeGeometric Unsharpness can be reduced by using small focus but that reduces the maximum tube loading capacity
25 Unsharpness due to Edge penetration This is due to the shape of the objectThe edges of the object absorb less amount of radiation and the absorption increases towards the centreThis creates a intensity gradient producing inherent unsharpnessIntensity of x-rays at image planeDistance across image plane
26 Movement unsharpnessVoluntary & involuntary movement of the organs or body parts or the patient as a whole will cause changes in the pattern of x-ray intensities forming the invisible x-ray imageThis changes are referred to as movement unshrpness : UMIf they occur during image recording they will produce unsharpness in the final image
27 Noise in the invisible x-ray image The kinds of noise present in the invisible x-ray image areFog due to scatter radiationQuantum noise – presence of less number of photons in the invisible x-ray image, making the identification of gaps between individual photons and finally making the recorded image looks grainy.Quantum noise can be avoided by using adequate exposure factors producing high enough x-ray intensity
28 Resolution of invisible x-ray image The resolution depends oncontrast,sharpness andnoise.We must try to obtain maximum resolution at this stage because the resolution becomes less and less in the next stages of image production
29 ConclusionIt is important to know the details of production and characteristics of the invisible x-ray image because;If the invisible x-ray image is of poor quality, it is extremely difficult to produce an adequate standard of final visible image.It is during the production of the invisible x-ray image that the radiographer has the greatest scope for control of image quality, particularly in conventional radiography.