RTCA 218 The Grid Bushong Chapter 14 Part
RTCA 218 The Grid Bushong, Chapter 14, Part 2
The Grid Abstract: This section discusses the purpose of a radiographic grid. Emphasis will be placed on the construction of the grid, how the grid works, different types of grids, grids and patient dose and alternatives to grid use. Grid problems with film demonstrations will also be covered. Objectives: At the end of this section, the student will be able to: u discuss the relationship between scatter radiation and image contrast. u define and discuss the function of a grid. u list and discuss the components of a grid. u define and discuss grid frequency and grid ratio. u define and discuss contrast improvement and Bucky factors. u list and discuss the types of grids. u define grid cutoff and discuss the grid cutoff formula. u discuss some alternative methods to grid use.
The Grid Scatter radiation reduces radiographic image _______ and detail, increases film fog and blur u Scatter radiation influenced by k. Vp, radiation field size, patient or part thickness u Scatter radiation can be reduced by beam restricting devices and grids u The grid reduces scatter radiation from the _______ beam u The other beam restricting devices restrict the primary beam – before it interacts with the patient u
The Grids u contains carefully fabricated series of sections of radiopaque material made of ______, alternating with sections of radiolucent material — _______ or plastic u designed to allow the transmission of only the x -rays whose direction come straight from the xray tube u x-rays that strike the interspace material will be transmitted through the grid – those that strike the grid material will be absorbed and prevented from striking the intensifying screen
The Grid
The Grid Construction 1. Grid Ratio u defined as the _____ of the lead strips divided by the ______ between them u Grid Ratio = h / D u Grids with high ratio eliminate more scatter
The Grid The lines of a radiographic grid are 2500 µm high and 10 µm thick, and a distance of 250 µm between them. What is the ratio for this grid? Grid ratio = h / d 2500 µm / 250 µm 10/1 or 10: 1 ratio
The Grid 2. Grid Frequency u defined as the _______ of grid strips or grid lines per inch or cm u as frequency increases the grid strips are thinner resulting in less distinct grid ____ on a radiograph u most grids have a frequency of 60 to 110 lines per inch
The Grid u Grid Frequency = 10, 000 m / cm (T + D) m/line pair u. T = thickness of grid material; D = thickness of interspace material
The Grid The lines of a radiographic grid are 2500µm high and 10 µm thick, and a distance of 250 µm between them. What is the frequency for this grid? 10, 000 m / cm (T + D) m/line pair 10, 000 m / cm (10 + 250) m / line pair 10, 000 / cm 260 / line pair 38. 5 lines/cm or (38. 5 cm)(2. 54 cm/inch) = 96 lines/in.
The Grid 3. Interspace Material u maintain a precise _____ between the lead strips u composed of Aluminum (higher atomic number, easier to manufacture, does not absorb moisture) or plastic fiber (preferred primarily due to its _____ atomic number)
The Grid 4. Grid Material u should be thin and have high x-ray _______ qualities u composed of lead
The Grid Performance 1. Contrast Improvement Factor u ratio of the contrast of a radiograph made ____ the grid to the contrast of a radiograph ______ the grid u most grids have CIF of 1. 5 to 2. 5
The Grid A radiograph produced without a grid yields a contrast of 1. 5. A similar radiograph produced with a grid yields a contrast of 3. 5. What is the contrast improvement factor for the grid that was used? Contrast w/ grid / Contrast w/out grid 3. 5 / 1. 5 = 2. 3
The Grid 2. Bucky Factor u ratio of the amount of the ______ remnant radiation to the amount of the _______ remnant radiation u sometimes referred to as grid factor u as grid ratio increases, Bucky factor increases u as k. Vp increases, Bucky factor increases u See Table 17 -2 on page 252
Bucky Factor @ varying k. Vp Ratio 70 k. Vp None 1 5: 1 2 8: 1 3 12: 1 3. 5 16: 1 4 5: 1 cross 4. 5 8: 1 cross 5 90 k. Vp 1 2. 5 3. 5 4 5 5 6 120 k. Vp 1 3 4 5 6 5. 5 7
The Grid Ratio None 5: 1 8: 1 12: 1 16: 1 Average Bucky Factor 1 2 4 5 6
The Grid A radiograph produced without a grid requires exposure factors of 70 k. Vp @ 5 m. As. If a 8: 1 grid is added, what new m. As should be used to produce a similar radiograph? 8: 1 Bucky factor = 4 5 m. As x 4 = 20 m. As
The Grid A radiograph produced with a 5: 1 grid requires exposure factors of 80 k. Vp @ 20 m. As. If a similar radiograph is performed using a 12: 1 grid, what new m. As should be used? 5: 1 Bucky factor = 2; 12: 1 Bucky factor = 5 Step 1: 20 m. As ÷ 2 = 10 m. As (non-grid technique) Step 2: 10 m. As x 5 = 50 m. As
The Grid 3. Selectivity u ratio of transmitted _______ radiation to transmitted _______ radiation
The Grid u Grids with high _____ have high CIF u Grids with high frequencies have thin interspace material u _____ grids (contain more lead) have high selectivity and CIF
The Grid Types of Grids 1. Linear Parallel Grids u simplest type – _____ lead strips are parallel
The Grid u Grid cutoff 1. _______ clean up of primary xrays 2. can occur if grid is not used ______– improper CR, angulation, SID, upside down grid 3. Grid Cutoff Formula – Distance to cutoff = SID / Grid Ratio
The Grid 2. Crossed Grid (Cross. Hatched) u two linear grids placed _________ on top of one another u very little room for error u cannot ______ xray tube at all, CR must be precise
The Grid 3. Focused Grid u parallel grid lines vertical at the center of the grid but are _____ at the edges u allow the ______ rays of the remnant beam penetrate the grid u have a focal range – requires its use within a specified SID
The Grid 4. Moving Grids u ______ grid lines on radiograph u Single-stroke grid: most simple u Reciprocating grid u Oscillating grid
The Grid Problems u Off-level error 1. improper _______ of the x-ray tube with the grid 2. grid cutoff occurs across the _____ radiograph
The Grid u Off-center error 1. improper ________ of the CR with the grid 2. lateral shift in the CR will result in grid cutoff across the ______ radiograph
The Grid u Off-focus error 1. using an ______ outside the specified range 2. high grid ratios have a ______ focal range latitude 3. grid cutoff will occur at the edges of the radiograph
The Grid u Upside-down grid error 1. severe grid cutoff on ____ sides of the central axis of the grid 2. ____ side of the grid is indicated on the grid
Grid Cutoff – Grid Lines on the Radiograph
Properly Used Grid – CR centered; directed ┴ to grid; proper focal range; right side up
Off Center Error With Grid Lines Against Grid Lines
Off Level Error CR angled against GL CR angled with GL
Off Level Error GL angled against CR GL angled with CR
Off Focus Error SID too Long SID too Short
Upside Down Grid
The Grid Selection u 8: 1 grids – below ____ k. Vp u above 8: 1 should be used with k. Vp levels above 90
The Grids and Patient Dose u Patient Dose _______ with increased grid ratio u High grid ratio used for high k. Vp procedures u Patient dose at high k. Vp when x-rays are transmitted through tissue is _____ than that at low k. Vp when x-rays are absorbed in tissue
Type of Grid Degree of Scatter Removal Off Center Off Focus Recommended Technique Remarks 5: 1, linear + Very wide Up to 80 k. Vp It is the least expensive. It is the easiest to use. 6: 1, linear + Very wide Up to 80 k. Vp It is the least expensive. It is ideally suited for bedside radiography. 8: 1, linear + Wide Up to 100 k. Vp It is used for general stationary grids. 10: 1, linear +++ Wide Up to 100 k. Vp Reasonable care is required for proper alignment. 5: 1, crisscross +++ Narrow Very wide Up to 100 k. Vp Tube tilt is limited to 5 degrees. 12: 1, linear ++++ Narrow Over 110 k. Vp Extra care is required for proper alignment. It usually is used in fixed mount. 6: 1, crisscross ++++ Narrow Very wide Up to 110 k. Vp It is not suited for tilted-tube techniques. 16: 1, linear +++++ Narrow Over 100 k. Vp Extra care is required for proper alignment. It usually is used in fixed mount. 8: 1, crisscross +++++ Narrow Wide Up to 120 k. Vp It is not suited for tilted-tube techniques.
The Grid Alternative to Grid Use u Air gap technique u Long _____ and ______ u Long OID allows for scatter radiation to ______ away from image receptor u Long SID reduces magnification due to increased OID
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