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RAD 354 Chapt . 11 Control of Scatter

RAD 354 Chapt . 11 Control of Scatter. Break down into: Those that reduce pt. dose and those that are geometrical in nature and those NOT! 3 factors affecting scatter (primary) Increased kVp Increased field size Increased pt. thickness. Spatial Resolution & Contrast Resolution.

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RAD 354 Chapt . 11 Control of Scatter

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  1. RAD 354 Chapt. 11 Control of Scatter • Break down into: Those that reduce pt. dose and those that are geometrical in nature and those NOT! • 3 factors affecting scatter (primary) • Increased kVp • Increased field size • Increased pt. thickness

  2. Spatial Resolution & Contrast Resolution • Spatial resolution may be thought of as GEPMETRIC in nature (F.S. size, emission spectrum, etc. dealing with geometric image formation • Contrast resolution – driven by scatter and other sources of “noise”

  3. Scatter • Increased field sizes = more scatter – collimation is the most readily available and easiest thing to lower the amount of scatter • Patient thickness also increases scatter- compression may be used to help avaid this (IVP’s and mammo are examples)

  4. Beam restricting devices limit the radiation to the patient • Aperaturediaphram (size and resultant filed size are a direct proportion – draw the damn picture and figure the problems) • Cones and cylinders – great for absorbing scatter, but are circular shaped = great for improving contrast and removing scatter BUT require MUCH MORE mAs as a result!

  5. Variable aperaturediaphram • Mandated in 1974 by the US Dept. of Food and Drug Admin. (mandate later removed) • Positive Beam Limitation Devices (PBL’s) • Automatically collimate to the size of the cassette/receptor in the bucky and CANNOT be a bigger size than the cassette/receptor

  6. Filtration/Grid • Filtration also will decrease the low energy rays and limit patient dose and some scatter • Grids remove “angled scatter” so only FORWARD scatter is of any benefit to the radiographic image – all other scatter degrades the image!

  7. Scatter = LOWER contrast • Using a grid (alternating strips of fine/thin leaded strips with alternating radiolucent interspace material) can effectively reduce the amount of ANGLED scatter from reaching the film/recepter

  8. Grid Terms • Grid Ratio = height of the lead lines divided by the interspace WIDTH • Grid frequency/lines per inch = the more lines per inch, the more clean up • Grid clean up = scatter w/o a grid vs scatter reaching film with a grid AKA “Contrast Improvement Factor” • Grid function = improve image contrast

  9. Bucky Factor • Refers to the AMOUNT of radiation to the patient with a grid vs W/O a grid • Higher the grid ratio, the higher the “bucky factor” • The higher the kVp, the higher the “bucky factor” Grid weight refers to how heavy it is – DUH! The more lead, the heavier it is!

  10. Grid Types • Parallel • Crossed (cross-hatch) • Focused • Focused - crossed

  11. Grid Problems • Grid cut-off = short SID’s result in the vertical, parallel strips absorbing the “diverging” beam at the outer margins of the grid/film/rescepter MOST pronounced at short SID’s • Most grid problems are “positioning related” • Uneven grid/off level grid • Off centered (lateral decentering) • Off focus grid • Upside down focused grid

  12. Focused Grid Misalignment • Off level = grid cutoff across image; underexposed image (light) • Off center = ditto • Off focus = CR centered to one side or the other of a focused grid • Upside down grid = severe grid cut-off )no density) at BOTH sides of the image

  13. Grid Ratio Selection • 8:1 grid is the most widely used • 5:1 grid most widely used PORTABLE grid • Grid ratio is kVp driven • Higher kVp’s warrant higher grid ratios • Higher grid ratios = higher patient dose (more radiation needed to produce an image) • As kVp increases past MAXIMUM OPTIMUM kVp, patient dose INCREASES

  14. mAs- Grid considerationsAs grid ratio increases, so must mAs! • 5:1 grid = 2 X mAs • 8:1 grid = 4 X mAs • 12:1 grid = 5 X mAs • 16:1 grid = 6 X mAs

  15. Air gap technique • By allowing the scatter radiation to “diffuse” in the atmosphere after the patient but BEFORE the film/receptor as the scatter diffuses and does NOT reach the film/receptor • C-spine lateral is a good example of this

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