Grids and best results
Basic principles of grid
The basic purpose of grid use is to enhance the contrast and quality of the medical image by removing the scatter radiation.
1-2. Grid Specifications
The object that scatter radiation arises is thicker than 10cm. The scatter radiation is named “Compton scatter radiation”. Depending on the purpose of radiograph such as chiropractic, veterinary, etc., and the direction of X-ray such as AP(Anterior to Plsterior), PA, Lateral, Oblique, etc., each grid specification should be different. Also, kVp and mA should be adjusted to acquire better X-ray image through a grid.
1-3. Scatter Radiation
Whenever x-rays pass through matter, some of the x-ray photons will interact with the atoms of the matter. When you take an X-ray, many incoming x-ray photons are absorbed by the bone, a high attenuation material, while other tissues absorb less. This produces an image of dark and light on the x-rayfilm. The effect is like holding up paper to a light to see the writing on the other sied. However, absorption is not the end. As part of the absorption, a second photon with less energy will be produced, flying off in a different firection than the photon that was absorbed. These “scattered” photons will create a random grayness on the image, reducing the contrast between body tissues and making it hard to read the image clearly. There are five qays x-rays interact with matter: photo electronic (PE), Compton Scattering(C), Pair Production, Tomson Scattering(R), and Photodisintegration (PD). In low energy x-rays below 100keV, only Photolelectric absorption and Compton scattering are significant.
1-4. Types of grids
Grids are divided into four kinds based on their designs. There are parallel types, focused types, criss-cross types, and tapered types. A single grid may be of more than one type.
1-4.1. Parallel Type
A grid where the absorving strips are parallel to each ohter in their longitudinal axis. Most linear grids are also focused, i.e. their strips are slightly tilted, converging at a line in space (the convergent line). A non-focused linear grid have strips that are parallel when viewed in crosssection; this is called a parallel grid. Many X-ray talesare equipped with linear, focused grids, and the strips in these grids are parallel with the long axis of the table, allowing the X-ray tube to be tilted in this dirction without changing the effectiveness of the grid.
1-4.2. Focused Type
A grid in which the absorbing strips are slightly angled towards the focal spot. The grid can therefore be sued only at a specified focal distance (actually inside a narrow distance interval around this specific distance). Otherwise the gird will absorb the primary radiation and parts of the film are barely exposed. Focused grids may be linear or crossed.
1-4.3. Criss-cross type
A grid consisting of two superimposed parallel grids having ghe same focusing distance. Such grids ard very efficient in removing scattered radiation but must be arranged at exactly the right angle to the beam. The use of such girds is therefore limited.
1-4.4. Tapered Type
A grid that the surface is tapered into the center of grid, functioning similar to a focused grid. All of the strips are paralle to each other, and the tapered surface is toward the focal spot.
1-5. Calculation of line density and ratio
Line density = line number per cm or per inch
Equation: line density = 10 / (interspacer thickness + absorber thickness) lp/cm
Ratio: proportion of interspacer thickness vs grid thickness *
Equation: Ratio = grid thickness/ interspacer thickness
1-6. Checking grid performance
The percentage of radiation that would be transmitted by the grid if all x-rays were coming in perfectly parallel to the absorbers.
Calculation: When interspacer thickness equals to 390um and absorber thickness equals to 450um, ideal primary transmission = interspacer thickness / strip thickness X 100% = 390um / 450um X 100% = 86.7%
1-7. Checking grid performance
This procedure is defined in the IEC 60627 international standard. All grids should comply with the requirement of this standard. When you are discussing the performance of grid, you should use the IEC Numbers measured by this standard.
1-7.1. To determine primary transmission(Tp)
a. 100 kVp, 64mAs(200mA, 0.32sec) is selected for measuring Tp. b. Transmission measurements will be taken with and without a grid in the x-ray beam. c. Calculation for Tp : Tp= Transmission with grid / Transmission without grid.
1-7.2. To determine transmission of total radiation(Tt)
a. 100 kVp, 8mAs(200mA, 40sec) is selected for measuring Tt. b. Transmission measurements will be taken with and without a grid in the x-ray beam. c. Calculation for Tt : Tt = Transmission with grid / Transmission without grid.
1-7.3. To determine transmission of scatter radiation (Ts)
a. 100 kVp, 12.5mAs(200mA, 64sec) is selected for measuring Ts. b. Transmission measurements will be taken with and without a grid in the x-ray beam. c. Calculation for Ts : Ts = Transmission with grid / Transmission without grid.
1-7.4. Bucky Factor
The bucky factor is a measure of how much more radiation exposure is necessary when changing from no grid to using a grid. Calculation of bucky factor(B) B = incident radiation / transmitted radiation = 1/Tt
1-7.5. Contrast improvement factor (K)
The “CIF” (K) is the ratio of the contrast with a grid to the contrast without a grid. “K” can be shown to be the ratio of primary radiation transmission to total radiation transmission. Calculation of CIF : K = contrast with grid / contrast without grid = Tp/ Tt
1-7.6. Selectivity (∑)
The selectivity(∑) is the ratio of the selectivity with a grid to the selectivity without a grid. “∑” can be shown to be the ratio of primary radition transmission to scatter radiation transmission. Calculation of Selectivity : ∑ = selectivity with grid / selectivity without grid = Tp / Ts
1-8. Cut off
One of the important factors in using the x-ray grid is the cutoff, which occurs from the gap of the angle between x-ray beam and the gaps between the x-ray absorbing material (“lamella” or interspacers). This means the grid line causes larger shadows in the image. The cutoff causes non-uniform density on x-ray films and an artificial difference in contrast, so that the doctor may midsiagnose an x-ray due to cut-off. Grids with cut off are normally rejected.
Cut-off can come from four sources besides manufacturing errors. These usually involve the grid not positioned properly:
- The grid is upside down;
- The grid is moved sideways;
- The x-ray tube is moved too far away or too close;
- The grid is at an angle to the tube.
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1-9. Application limits
The angle of the individual lamella is formaed to focus to a particular focal spot in front of the grid. Moving from that focal point will both reduce transmission and create cut-off. Only a certain amount of movement is acceptable for medical practice. These are called “Application Limits.” The application limits are specified in the IEC60627 international standard. Focal distance and application limits should be calculated according to the equation described in the international standard and the grid maker should assure that when the x-ray tube is moved within application limit cutoff doesn’t appear on x-ray film.