2Y03 Lecture 9 - 2023 (Student) PDF

Radiographic Imaging and Instrumentation I Unit 1.4: Beam Controlling Devices Lecture 9: Scatter and Grids 1 Housekeeping • Test #1 is Monday, October 2nd @ 9:30am • IAHS room 143 • Covers Units 1.1-1.5 – Lectures 1-10 – Labs 1-4 • Format: – mix of short answer and multiple-choice questions In...

Radiographic Imaging and Instrumentation I Unit 1.4: Beam Controlling Devices Lecture 9: Scatter and Grids 1 Housekeeping • Test #1 is Monday, October 2nd @ 9:30am • IAHS room 143 • Covers Units 1.1-1.5 – Lectures 1-10 – Labs 1-4 • Format: – mix of short answer and multiple-choice questions Inverse Square Law • An image is acquired at an SID of 100cm and an exposure of 425 mR. The technologist decides to repeat the image after repositioning. In order to reposition, they move the tube away from the bucky and forget to move it back. The resulting image has an exposure of 150 mR. • What was the SID for the exposure? How to solve • What we know: I₁ = 425 mR I₂ = 150 mR d₁ = 100 cm d₂ = ? 2 I1 d 2  2 I 2 d1 or  d2  I1    I 2  d1  2 How to solve I₁/ I₂ = d₂ 2 / d₁ 2 d₂ 2 = I₁ x d₁ 2 / I₂ d₂ = √ (I₁ x d₁ 2 / I₂) = √ (425 mR x (100cm) 2 / 150 mR) = 168 cm The SID of the repeat exposure was 168cm Direct Square Law • Eg. A diagnostic image of the abdomen is taken using 25mAs at 80kVp at a 40 inch distance. A second image is requested by the radiologist at 56 inches. • What mAs value should be used to produce an acceptable image if the distance is increased to 56 inches? 2 1 2 2 mAs1 d  mAs 2 d or  d1  mAs1    mAs 2  d2  2 What do we know about scatter? • What is it? • How does it impact image contrast? • Application question: how would scatter impact AEC function? Overview • Grid ratio • Types of grids • Air Gap Technique Grid Construction Parameters • Grid ratio: ratio of height to width of interspaces – Indicates relative dimensions of ray paths – Higher grid ratio has greater ‘stopping power’ for angled scatter photons h D Grid Ratio • Grid Ratio = h/D Where: h = lead strip height D = interspace width Grid Ratio • h = 3.0 mm D = 0.25 mm Grid Ratio = 3.0/0.25 = 12:1 Grid Construction Parameters • Grid frequency: number of radiopaque strips (grid lines) per unit length – Affects visibility of grid on image – stationary • By combining the information from grid ratio and grid frequency = determine the total quantity of lead in the grid Lead Content • Lead content = mass per unit area or grams per cm2 As the Pb content ↑, the ability of the grid to remove scatter and improve contrast (with good design) also increases Grid Surface X-Ray Absorption • As photons reach the grid, a percentage will interact with the lead on the surface • Percentage can be calculated % Grid Surface X-Ray Absorption = width of grid septa/(width of grid septa + interspace width) x 100 Example: Grids and Technical Factors • Image requires a certain amount of energy (photons) to create a good image (remember AEC?) • Grid removes photons (mostly unwanted, some primary beam too) • When using grid must account for photon removal  increase factors Bucky Factor • Factor by which technique must increase using a specific grid vs when non-grid technique used • Also represents increase in patient dose associated with that specific grid • Higher grid ratios = higher bucky factor Example: imaging a knee without a grid uses factors of 70kVp at 5mAs. The same knee is imaged again using a grid with a bucky factor of 2. What factors should be used? 70kVp @ non-grid mAs x bucky factor = 5mAs x 2 = 10mAs Types of Grids • Parallel • Focused Parallel vs Focused Grid Linear Focused Grid Bushong p.237 9th & p.200 10th Crosshatch or Crossed Grid Parallel Grids Grid Cut-off • Undesirable absorption of remnant beam • Due to mismatch of photon path and interspace angle • Can be partial or complete • Worse with improper use Focal Length or Focal Range • Table Bucky – 36 – 46 inches • Chest Radiography – 152 – 183 cm • Focal Range is within which the x-ray tube should be located • Therefore, within the focal range there will be very little or no grid cut off Comparison of Grid Performance Air-Gap Technique • ↓ scatter reaching image receptor • Relies on increased OID Air Gap Technique • Concept: Reduce scatter reaching IR by allowing distance for angled scatter photons to travel so they miss the IR Air-Gap Technique • Advantages – No grid required – No increase in technical factors required • Disadvantages – Geometric unsharpness due to magnification Air-Gap Technique Summary • Grid ratio • Grid frequency • PBL • Air-Gap Technique Readings • Unit 1.4 in course manual & study guide • Bushberg chapter 7, section 7.2 • Bushong chapter 22