Radiographic Grid. Principles of Imaging Science II (RAD 120) Image-Forming X-Rays. Radiographic Grids

Transcription

1 Principles of Imaging Science II (RAD 120) Radiographic Grids 1 Image-Forming X-Rays Four X-ray paths a. X-rays interact with patient and scatter away from the receptor b. X-rays interact and are absorbed (photoelectric absorption) within patient c. X-rays are transmitted through patient without interaction and strike receptor d. X-rays interact with patient (Compton scatter) and scatter towards C and D are referred to the imageforming x-ray photons 2 Radiographic Grid Used to reduce scatter radiation from reaching the image receptor (IR) through absorption Cleans up scatter radiation Inherent part of bucky, placed between the patient and IR Table or upright bucky usage >60 kvp, 10 cm tissue When primary x-rays interact with the patient, x-rays are scattered from the patient in all directions. 3 1

2 Image Contrast Difference in optical density between adjacent structures High vs Low contrast Skeletal anatomy Abdomen, Chest High Medium Low Radiographs of a cross section of long bone. A, High contrast would result from the use of only transmitted, unattenuated x-rays. B, No contrast would result from the use of only scattered x-rays. C, Moderate contrast results from the use of both transmitted and scattered x-rays. 4 Grid Design Radiolucent interspace material with alternating radiopaque strips Aluminum, plastic or carbon fiber for interspace Lead, tungsten, platinum, gold strips Transmits x-rays traveling in a straight line, oblique x-rays absorbed by strips The only x-rays transmitted through a grid are those that travel in the direction of the interspace. X-rays scattered obliquely through the interspace are absorbed. 5 % Grid Surface X-ray Absorption Formula applied to determine the percentage of x-rays exiting the body that will be absorbed Based upon grid design Lead strip width and interspace width Higher % yields > absorption Surface area of grid 6 2

3 Application 7 Grid Ratio Grid Ratio is the height of the grid strip (h) divided by the thickness of the interspace material (D). T = strip width. Grid Ratio = h/d Affected by changing Height of lead strips Thickness of strips Width of interspace 8 Grid Ratio High ratio grids absorb more scatter yet require higher mas or kvp mas is factor of choice 5:1, 6:1, 8:1, 10:1, 12:1, 16:1 ratio designs High-ratio grids are more effective than low-ratio grids because the angle of deviation is smaller. 9 3

4 Grid Ratio Application 10 Grid Frequency Number of strips or grid lines per inch or cm lines/cm, lines/in lines/cm, lines/in Higher grid frequency requires higher technique Less grid lines appear in image Often used in mammography 80 lines/cm, 200 lines/in Typically higher frequency grids have thinner lead strips 11 Digital Imaging Systems Very high-frequency grids lines/in lines/cm Recommended for use with digital systems Minimizes grid line appearance 12 4

5 Lead Content of Grid Lead content Most important factor in grid s efficiency Measured in mass per unit area g/cm 2 High ratio grids tend to have highest lead content As lead content increases, removal of scatter increases and therefore contrast increases 13 Application 14 Grid Performance Contrast Improvement Factor (k) Comparison of image contrast with a grid to image contrast without a grid k is higher for higher ratio grids K = Radiographic contrast with grid Radiographic contrast without grid Measured at 100 kvp using a step wedge Manufacturer Avg Use of a grid approximately doubles the contrast 15 5

6 Bucky Factor Higher technique required with grid usage mas X bucky factor avg Measurement of technical factor and patient dose increase based upon penetration of primary & scatter radiation through the grid Bucky factor increases with increased grid ratio and increased kvp Grid Performance Approximate Bucky Factor Values Grid Ratio Non- Grid 70 kvp 90 kvp 120 kvp : : : : Avg 16 Approximate Exposure Factor Changes Necessary for Standard Grids Grid Ratio mas Increase kvp Increase Non-Grid 1X 0 5:1 2X :1 3X :1/10:1 4X :1 5X :1 6X Linear/Parallel Vertical lead strips do not coincide with the primary beam Absorption of 1 0 beam (Grid cut-off) occurs with: Short SID Large IR Grid Types Linear Parallel Linear Focused A parallel grid is constructed with parallel grid strips. At a short source-to-image receptor distance (SID), some grid cutoff may occur. 18 6

7 Parallel Grid With a parallel grid, optical density (OD) decreases toward the edge of the image receptor. The distance to grid cutoff is the source-to-image receptor distance (SID) divided by the grid ratio. 19 Grid Types Linear/Focused Angled lead strips to coincide with primary beam divergence Focal distance set to SID usage to minimize grid cut-off 20 Comparison of Transmitted Photons Parallel & Focused Grids 21 7

8 Grid Focus Convergent Line. Imaginary lines drawn above a linear focused grid from each lead strip meet to form a convergent point. The points form a convergent line along the length of the grid. Convergent Point. The convergent line or point of a focused grid falls within a focal range. Grid Types Crossed (Criss-Cross, Cross-Hatched) 2 parallel grids perpendicular Not common High Grid cut-off if offcentered to CR 23 Grid Types Moving grid mechanism Upright or table bucky activation Eliminate grid lines from image High frequency = less grid lines possible Low frequency = more grid lines possible Reciprocating 2 cm movement transversely Motor drives grid back and forth during exposure Oscillating Circular movement 2-3 cm movement Electromagnet pulls grid to one side Releases it during exposure 24 8

9 Grid Types Grid Cap Permanently mounted grid, IR slides into device Used with a variety of IR sizes Grid Cassette Permanently mounted grid Specific grid sizes Wafer Grid Non-permanent grid mount, must be secured Specific grid size 25 Long vs. Short Dimension Grids Orientation of lead strips for a long- and shortdimension grid. Grid Types Air-Gap (Air Filtration) Common on dedicated Chest X-ray units Part cm from IR ( 4-6 ) Similar to 8:1ratio grid 10 air gap equivalent to 15:1 ratio grid mas increased 10% per cm gap Magnification results unless SID is increased

10 Grid Cutoff A decrease in the number of transmitted photons that reach the image receptor because of some misalignment of the grid Grid Errors Off-level grid Off-center grid Off-focus grid Upside-down focused grid Grid Errors Off-level Parallel & focused Decreased density across image 29 Grid Errors Off-Center (lateral decentering) Focused Grid Decreased density across image Most common error 30 10

11 Grid Errors Off-focus Incorrect SID use Decreased density at edges of image Direct relationship More critical with high ratio grid Upside-down Mobile radiography CR not directed to tube side Marked decreased density at edges of image and points lateral to CR 31 Grid Cutoff Off Level 32 Grid Cutoff Off Center 33 11

12 Grid Cutoff Off Focus 34 Grid Cutoff Upside Down Focused 35 Grid Cutoff Upside-Down Focused Grid Cutoff. Radiograph produced with an upside-down focused grid Off-Center Grid Cutoff. Radiograph demonstrating grid cutoff caused by off-centering. 12

13 Zebra pattern Caused by Grid Errors - Moire Effect Similar grid frequency to laser scanning frequency in CR processing Using a grid cassette in the bucky tray Correct by Selecting a high grid frequency Use a moving grid mechanism Do not use two grids 37 Patient dose Kvp usage Scatter absorption <90 kvp 8:1 satisfactory >90 kvp >8:1 grids used Grid Selection As grid ratio increases, transmission of scatter radiation decreases faster than transmission of primary radiation. Therefore, cleanup of scatter radiation increases. 38 Clinical Consideration in Grid Selection Grid Degree of Scatter Removal Off-Center latitude Off-focus latitude kvp Comments 5:1 + Very Wide Very Wide Up to 80 Low cost; easy to use 6:1 + Very Wide Very Wide Up to 80 Low cost; mobile radiography 8:1/10:1 +/+++ Wide/Wide Wide/Wide Up to 100 General stationary exams 12: Narrow Narrow Over 110 Precise centering; usually fixed mount 16: Narrow Narrow Over 100 Precise centering; usually fixed mount 39 13

14 Summary 40 14