Principles of Imaging Science II (RAD 120) Radiographic Grids
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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
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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.
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Image Contrast • Difference in optical density between adjacent structures • High vs Low contrast
High
– Skeletal anatomy – Abdomen, Chest 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.
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% 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
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Application
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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.
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Grid Ratio Application
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Grid Frequency • Number of strips or grid lines per inch or cm – 25 – 45 lines/cm, 60 – 110 lines/in – 25 – 80 lines/cm, 60 – 200 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
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Digital Imaging Systems • Very high-frequency grids – 103-200 lines/in – 41-80 lines/cm
• Recommended for use with digital systems – Minimizes grid line appearance
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Lead Content of Grid • Lead content – Most important factor in grid’s efficiency – Measured in mass per unit area • g/cm2 – High ratio grids tend to have highest lead content – As lead content increases, removal of scatter increases and therefore contrast increases
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Application
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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 1.5 – 2.5 – Use of a grid approximately doubles the contrast
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Grid Performance • 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
Approximate Bucky Factor Values Grid Ratio
70 kVp
90 kVp
120 kVp
Avg
NonGrid
1
1
1
1
5:1
2
2.5
3
2
8:1
3
3.5
4
4
12:1
3.5
4
5
5
16:1
4
5
6
6
16
Approximate Exposure Factor Changes Necessary for Standard Grids Grid Ratio
mAs Increase
kVp Increase
Non-Grid
1X
0
5:1
2X
8 – 10
6:1
3X
11 – 12
8:1/10:1
4X
13 – 15
12:1
5X
20 – 25
16:1
6X
30 – 40
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Grid Types Linear Parallel Linear Focused
• Linear/Parallel – Vertical lead strips do not coincide with the primary beam – Absorption of 10 beam (Grid cut-off) occurs with: • Short SID • Large IR A parallel grid is constructed with parallel grid strips. At a short source-to-image receptor distance (SID), some grid cutoff may occur.
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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.
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Grid Types • Linear/Focused – Angled lead strips to coincide with primary beam divergence – Focal distance set to SID usage to minimize grid cut-off
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Comparison of Transmitted Photons Parallel & Focused Grids
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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
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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
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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 is @10-15 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
6”
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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
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Grid Errors • Off-Center (lateral decentering) – Focused Grid – Decreased density across image – Most common error
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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
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Grid Cutoff – Off Center
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Grid Cutoff – Off Focus
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Grid Cutoff – Upside Down Focused
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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.
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Grid Errors - Moire Effect • Zebra pattern • Caused by – 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
Grid Selection
– <90 kVp 8:1 satisfactory – >90 kvp >8:1 grids used
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
12:1
++++
Narrow
Narrow
Over 110
Precise centering; usually fixed mount
16:1
+++++
Narrow
Narrow
Over 100
Precise centering; usually fixed mount
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Summary
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