2 ObjectivesBegin discussing factors that influence image detail or visibility of detailSpatial & Contrast resolutionRadiographic NoiseScatter RadiationWays to reduce scatter & improve image qualityPrimary beam restriction & GridsTechnique adjustments when using grids
3 What are some factors that increase scatter radiation?
4 3 factors contribute to an increase in scatter Increased kVpIncreased x-ray field sizeIncreased patient thickness
5 2 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. (CT measured in cm)
8 Contrast ResolutionDetermined by scatter radiation and other sources of radiographic noiseRadiographic noise (image fog) =A uniform signal produced by scattered x-raysDigital imaging the grainy or uneven appearance of an image caused by an insufficient number of primary x-rays
11 Image-forming x-raysTwo kinds of photons are responsible for the OD and contrast on an image: Photons that pass through without interacting and those that are scattered though Compton.X-rays that exit the patient are remnant and those that interact with the IR are image-forming.
12 However, scatter radiation is a factor that must be managed Ideally, only those x-rays that do not interact with the patient should reach the IR….However, scatter radiation is a factor that must be managedProper collimation has the PRIMARY effect of reducing patient dose by _________ ?Proper collimation also improved image contrast by reducing radiographic noise or fog caused by scatter
17 Patient ThicknessImaging thick parts of the body results in more scatter radiation than thin partsIMAGE TEST TOOL
18 Is patient thickness something the radiographer can control?
19 Patient thickness Normally, No Compression devices improves spatial resolution by reducing patient thickness and bringing the object closer to the IR. Compression also reduces patient dose and contrast resolution
20 Compression Improves spatial resolution Reduces OID Reduces patient doseImproves contrast resolution (reducing fog or noise)
22 Field SizeAs field size increases, intensity of scatter radiation also increases rapidly. Especially during fluoroscopy
23 Compare images: What do you think about radiographic contrast & image noise?
24 Control of Scatter Radiation Technologists routinely use two types of devices to reduce the amount of scatter radiation reaching the IRBeam restrictorsGrids
25 3 Types of beam-restricting devices Aperture DiaphragmCones or CylindersVariable aperture collimator
26 Aperture Diaphragm The simplest of all beam-restricting devices Lead or lead-lined metal diaphragm attached to the x-ray tube headThe opening in the diaphragm is usually designed to cover just less than the IR used
28 Cones & Cylinders Are modifications of the aperture diaphragm Alignmentis one difficultywhen usingconesNow mostlyused with spinesteeth & heads
29 Improved contrast resolution of the frontal sinuses
30 Variable Aperture Collimator The most common beam-restricting device is the light-localizing variable aperture collimatorThe first part of the collimator serves to control off-focus radiation. What is off-focus radiation?
31 Off - focus RadiationX-ray tubes are designed so that the projectile e- interacts with the target. However, some of the e- bounce off the target and land on other areasThis caused x-rays to be produced out side the focal spot
32 Extrafocal Radiation These rays can also be called off-focus radiation Extrafocal radiation is undesirable because it extends the size of the focal spot, increases patient skin dose & reduces image contrast
34 Fixed diaphragm in the tube housing Using a griddoes not reduceextrafocalradiation
35 First-stage entrance shuttering device Has multiple collimator blades protruding from the top of the collimator into the tube housing
36 The second-stage collimator shutters Pb leaves are at least 3 mm thickThey work in pairs and are independently controlled
37 The collimator lamp & mirror Must be adjusted so that the projected light field coincides with the x-ray beamMisalignment of the light field and beam can result in collimator cutoff of anatomic structures
38 Always keep the collimated area smaller than the size of the cassette What is a PBLYou should always see a 4 sided borders.
39 Total Filtration Filtration review… The collimator assembly is usually equivalent to approximately _______ mm Al filtration.Minimum filtration for tubes that can operate about 70 kVp is _______ mm Al or equiv.
41 Contrast & Contrast Resolution Two devices are used to reduce Compton effect beam-restricting devices and radiographic gridsBeam-restricting devices effects what reaches the patient. Grids effect the remnant beam
42 Contrast & Contrast Resolution Contrast = the comparison of areas of light, dark and shades of gray on the imageContrast Resolution = the ability to image adjacent similar tissues
43 Beam-restricting devices Are helpful to improve contrast resolution however the inherent problem is they are placed between the source and the patient. Even under the most favorable conditions, most if the remnant x-rays are scattered.Table pg. 237
44 Effects of Scatter Radiation on Image Contrast Contrast is the degree of difference in OD between areas of an imageIf you could only capture transmitted, unscattered x-rays, the image would be very sharpThe corresponding bone-soft tissue interface, would be very abrupt, and therefore the image contrast would be high
45 Grids Are very effective device for reducing scatter radiation The grid is a series of sections of radiopaque material (grid strips) alternating with sections of radiolucent material (interspace material)The grid is designed to transmit only x-rays that are traveling in a straight line from the source to the IR
46 Grids “clean up” scatter radiation A high qualitygrid can attenuate 80 –90 percent of scatterradiation
47 Grid StripsShould be very thin and have high photon absorption propertiesLead is most commonTungsten, platinum, gold, and uranium have been tried but Pb is still most desirable
48 Interspace MaterialUsed to maintain precise separation between the delicate lead stripsAluminum or Plastic FiberGrid Casing = covered completely by thin aluminum to provide rigidity and to seal out moisture. Yuck!
49 Grid Ratio3 important dimensions on a grid: The thickness of the grid strips, the width of the interspace material, and the height of the gridThe grid ratio is the HEIGHT of the grid divided by the INTERSPACE WIDTH:Grid ratio = hD
50 h = height of the grid, T = thickness of the grid strip, D = width of the interspace material
51 Grid RatioHigh-ratio grids are more effective in cleaning up scatter radiation than low-ratio gridsThe angle of deviation is smaller for high-ratio grids. (the photon must be traveling in a straighter line to make it through the grid)However, the higher the ratio the more radiation exposure necessary to get a sufficient number of x-rays through the grid to the IR
52 The higher the ratio the straighter the photon must travel to reach the IR Grid ratios rangefrom 5:1 to 16:1Most common8:1 to 10:1A 5:1 grid willclean up 85%16:1 clean up 97%
53 Grid FrequencyThe number of grid strips or grid lines per inch or centimeterThe higher the frequency the more strips and less interspace material and the higher the grid ratioAs grid frequency increases, patient does is increase because more scatter will be absorbed
54 Grid FrequencySome grids reduce the thickness of the strips to reduce the exposure to the patient, this over all reduces the grid clean upGrids have frequencies in the range of 25 to 45 lines per centimeter (60 to 110 lines per inch)
55 Higher frequency with the same interspace distance reduces the grid effectiveness
56 Grid PerformanceThe principal function of a grid is to improve image contrastContrast Improvement Factor (k) = the ratio of the contrast of a radiograph made with a grid to the contrast of the radiograph made without a grid. A contrast improvement factor of 1 indicates no improvementsThe higher the grid ratio & frequency the higher the k
57 Bucky Factor Using grids require more patient dose. Why is this? When a grid is used technique must be increased to maintain ODThe amount of increase is given by the Bucky factor (B) or grid factor
58 Bucky Factor or grid factor The higher the grid ratio or frequency the higher the bucky factorThe Bucky factor increases with increasing kVpPg 235: We will use the average values for calculations.
59 Selectivity or ability to “clean up” the heavier the grid the more Pb it contains
60 Grid Types Parallel Grid – simplest type of grid All the lead strips are parallelOnly clean up scatter in one direction (along the axis of the grid)Easy to make, however can cause grid cutoff with short SID’s.
61 Grid cutoff Distance to cutoff SID Grid ratio With decreasing SID more potentialfor grid cutoffIR size will alsoInfluence grid cutoff
62 Grid Cutoff – Parallel grid THE HIGHER-RATIO THE MORE CUTOFF POTENTIAL
63 Crossed GridHave lead strips running along the long and short axes of the gridMade by placing two parallel grid on top of each other
64 Crossed Grid Have twice the grid ratio as linear grids However, CR vs grid placement is critical. The CR must align with the center of the grid and the grid and CR must be exactly parallel or grid cutoff will occur
65 Focused Grid Designed to minimize grid cutoff Lead strips are aligned with the divergence of the x-ray beamEach focused grid must be identified with the appropriate SIDWrong SID = Grid cutoff
66 Focused grid have a little SID latitude (eg Focused grid have a little SID latitude (eg. 100cm grid could be used at 90cm – 110cm)
67 Moving GridsAll stationary grids will give you grid lines on your radiograph. Thinner Pb strips will give you less noticeable lines. However, thinner strips have less Pb content not “cleaning up” as wellGrid Lines are made when primary x-rays are absorbed in the grid strips.
68 Focused grids are usually used as moving grids The grid is placed in a holding mechanism that begins moving just before the x-ray exposure and continues moving after the exposure ends2 types of movement Reciprocating & Oscillating
69 Grid MotionReciprocating = moves several times about 2cm back and forth during the exposureOscillating = moves several times about 2 – 3 cm in a circular patternMost grids are moving. Except for portable imaging
70 Grid Problems Increased OID, especially with moving grids The biggest problem with grids is misalignmentGRID PROBLEMSRESULT IN:UNDEREXPOSED IMAGEOR UNDEREXPOSEDEDGES OF IMAGE