X-Ray grids consist of a series of lead strips placed in front of the X-Ray film, and are used in some diagnostic techniques to improve image quality by stopping scatter radiation from reaching the X-Ray film. The grid was invented by Dr Gustave Bucky in 1913, and is still used in X-Ray machines today. X-Ray grids are produced by X-Ray supplies manufacturers, but are put in place by a technician, and so a choice of using one of several grids or just no grid at all is usually available. There are very few applications where no grid is used, and these include mammography, dental imaging and bone density measurements. Grids are even used when an image intensifier is used instead of X-Ray film.
Lead is well known for its ability to stop X-Rays, and is therefore perfect for use in grids. Thin strips of lead foil are arranged into a grid, with a material that is transparent to x-rays being used to space out the rows of lead. The rows in the grid are aligned so that most of the X-Rays travelling straight from the X-Ray tube's focus spot (the source of the X-Rays) to the X-Ray film can pass between rows. However, X-Rays which collide with a molecule inside the patient's body and are scattered at a random angle will be at the wrong angle to pass between lead strips, and will therefore be blocked. This is useful, as this scatter radiation would otherwise hit the wrong bit of film for the part of the body it had been passing through, thus reducing the quality of the image. A good way to think of it is to imagine a half-open row of blinds - from one angle you can see past them, but from any other angle your view will be blocked. Imagine that light from the sun can pass between them, but sunlight reflected off a nearby car window cannot, and you have a rough model of an X-Ray grid.
The height of the grid is therefore very important - if the blinds aren't very broad, then it's easier to see between them from the wrong angles. The grid ratio is defined as the ratio between the height of the lead strips and the distance between them. For instance, any grid in which the lead strips are eight units high and one unit apart will have a grid ratio of 8:1. The higher the grid ratio, the more scatter radiation is blocked, but then there is also more lead in the grid and therefore more useful X-Rays are blocked, too.
There are several different patterns of grid in use, each having various advantages and disadvantages. The simplest is a linear grid, where the lead strips all run in the same direction, as opposed to a crossed grid, in which two sets of strips run at right angles to one another. Meanwhile, grids can also be divided into parallel and focused varieties. In a parallel grid all of the lead strips are set at exactly the same angle, like a fully opened set of blinds. Imagine yourself as the X-Ray tube's focus spot looking at these blinds - you can only see through the section of blinds in line with where you are standing, with the rest blocking more and more of your view as you look further to the left or right. This is the problem with parallel grids - the useful X-Rays get cut off as well as you move away from the centre of the grid. For this reason, it's generally focused grids which are used in diagnostic radiology. A focused grid has all of its lead strips set at such an angle as to allow the focus spot to be able to 'see' through the entire grid so that only scattered X-Rays are blocked.
For any particular focused grid, the angles of all the lead strips will meet at a certain point, this being where the X-Ray tube's focus spot should be placed. Focused crossed grids only allow for the focus spot being directly above the centre of the grid at a position known as the convergent point. However, with a linear grid, the focus spot can be anywhere on a line directly above the mid-line of the grid, with this line being known as the convergent line. Thinking in terms of blinds, imagine moving up and down while looking through some open vertical blinds - you can see through them perfectly well from anywhere on a vertical line. The focal distance is the perpendicular distance between a grid and its convergent point/line. The focus spot can actually be a little nearer or further from the grid without too much loss of image quality, as indicated by the focusing range. For instance, a grid focused at 40 inches may have a focusing range of 34 to 54 inches. In most countries, the minimal distance between focal spot and patient is given by law, and related to the hardness of the X-Rays being used.
Measuring a Grid's Usefulness
Different grids have differing properties, with some cutting out more scatter radiation but also blocking some useful X-Rays. While the grid ratio provides a useful basic measurement of the grid, the thickness of lead used and the number of lead lines per inch can each vary while the grid ratio remains the same. There are therefore three different measurements of how useful any particular grid is, these being the primary transmission, the Bucky factor and the contrast improvement factor.
The useful X-Rays which pass straight from the X-Ray tube to the film are known as primary X-Rays. Primary transmission is basically the percentage of primary X-Rays which pass through the grid and hit the film, and is thus measured by using the same apparatus with and without the grid in place. The primary transmission can also be estimated from the thickness of the lead strips and the gaps in between, thus giving two formulae:
TP = IP / I'P x 100
TP = primary transmission (%)
IP = intensity with grid
I'P = intensity without grid
TP = D / (D + d) x 100
TP = estimated primary transmission (%)
D = thickness of gaps
d = thickness of lead strips
The second equation supposes that the primary transmission will be higher than realistically possible, as it assumes that the gaps between lead strips will not stop any X-Rays. In actual fact, the gaps in a grid are usually filled with a reasonably, but not completely, X-Ray transparent material, and so the second equation overestimates primary transmission. Grids will generally have a TP between 50% and 75%, with greater primary transmission ratings being better.
The Bucky factor (B) is the ratio of X-Rays arriving at the grid, known as the incident radiation, and those actually being transmitted through the grid. Note that this measurement includes both primary radiation and scatter radiation in each total. This measurement is important as it indicates how much more intense the X-Ray beam has to be to ensure the same number of X-Rays will reach the film once the grid is in place. The formula is quite simple:
B = incident radiation / transmitted radiation
For instance, if for every two X-Rays 'hitting' the grid only one will successfully pass through, then the grid has a Bucky factor of two and the intensity of the X-Ray beam must be doubled to maintain the amount of exposure the film gets.
Contrast Improvement Factor
The contrast of an X-Ray is measured by looking at the difference in photographic density of sections of a film that have either been fully exposed or blocked by a prescribed 'phantom', usually a volume of water placed between the X-Ray tube and the film. The contrast improvement factor (K) is a measurement of the change in contrast gained by adding a grid in front of the film, and is found using the following formula:
K = contrast with grid / contrast without grid
The contrast improvement factor varies depending on the kilovoltage used in the x-ray tube, as this affects the amount of scatter radiation produced as the X-Rays pass through the patient. For this reason, K is usually measured at a peak kilovoltage of 100, with a 20cm thick water phantom between the X-Ray tube and film. The contrast improvement factor is a very useful measurement as it determines how good the grid is at doing its job, although an increase in K may be at the price of an increased Bucky factor, thus requiring a greater patient exposure to give the same level of film exposure.
Another useful measurement is that of a grid's lead content, which is found by measuring the number of grams per square centimetre of grid. Provided that the grid is well-designed, a greater lead content will mean an improved contrast, whereas two grids with the same lead content will generally fall in the same ball park. For instance, if a grid with an 8:1 grid ratio is produced with the same amount of lead as a 5:1 grid, it will have roughly the same contrast improvement factor as the 5:1 grid, as the lead strips will each have to be thinner or shorter. Higher grid ratios generally bring improved contrast due to their increased lead content, which also leads to an increased Bucky factor.
It is not impossible for a technician to make mistakes when setting up an X-Ray grid, leading to the grid inadvertently stopping some of the X-Ray beam from reaching the film. There are several ways this can happen, with mistakes including putting the grid upside-down and misaligning the X-Ray source in various ways.
If a focused grid is placed upside-down1 then most of the lead strips will end up facing in entirely the wrong direction, with only the straight strips in the middle allowing X-Rays to pass through 'the wrong way'. This leads to the X-Ray film being exposed only in the centre, a telltale sign of an upside-down grid.
Lateral decentring occurs when the X-Ray focus spot is moved away from the convergent point/line while remaining at the same distance from the grid. This leads to the same effect as seen when moving sideways while looking through an open set of blinds, with the view through the blinds quickly disappearing as the viewer deviates from the centre line. However, the loss of X-Rays is spread out across the entire film, leading to an underexposed film with no telltale signs of lateral decentring. The formula for the primary radiation lost through lateral decentring is:
L = rb / f0 x 100
L = primary radiation lost (%)
r = grid ratio
f0 = grid focal distance
b = amount of lateral decentring (cm)
Lateral decentring is therefore more disruptive when using grids with high grid ratios and small focal distances. When exact centring of the X-Ray source is not possible, it is best to use low ratio grids with long focal distances. Tilting the X-Ray grid and film with respect to the X-Ray source causes a similar effect, and so the same advice applies in situations where off-level grids are a problem.
Focus-Grid Distance Decentring
Focus-grid distance decentring occurs when the x-ray source is too far from or too near to the grid, leading to a difference between the angle of the lead strips in the grid and the angle of the X-Rays in the beam. Far focus-grid distance decentring is more forgiving than near focus-grid distance decentring, though the disruption caused by either can be determined using the following formula2:
L = rc |1/f0 - 1/f1| x 100
L = primary radiation lost at point C (%)
r = grid ratio
f0 = grid focal distance
f1 = actual focus-grid distance
c = distance between point C on grid and centre of grid
As you can see, the amount of radiation lost depends on the distance from the centre of the grid, as well as the magnitude of the error, the grid's focal distance and the grid's ratio. Grids with a higher grid ratio are more prone to radiation loss through focus-grid distance decentring, which is why grids with higher ratios have narrower focal ranges. Parallel grids automatically cause focus-grid decentring, as their focus point lies at infinity.
Combinations of lateral and focus-grid distance decentring are also possible, and lead to a telltale decrease in film exposure on one side compared to the other. This is due to the fact that moving the X-Ray source away from the convergent spot/line both horizontally and vertically means that the beam will still match the angle of the lead strips on one side of the grid while being blocked by those on the other side.
Invented in 1920 by Dr Hollis E Potter, moving grids are used to prevent the grid from leaving a visible shadow on the X-Ray film. Each time an X-Ray exposure is made, the grid moves quickly to a position between 1 and 3cm from its starting position, thus avoiding blocking the same bit of film throughout the exposure. For instance, Bucky systems use a moving grid suspended by levers which starts to swing just before the exposure is taken, with the image being produced as the grid reaches its maximum velocity halfway through the swing. However, there are several issues caused by using a moving grid:
The grid must move very quickly in order to blur the image of its shadow, and this can cause the X-Ray table - and therefore the patient - to vibrate, leading to reduced image quality.
The grid must not move in synchrony with the pulses3 of the X-Ray tube. If a grid line moves into the same position the previous line was occupying, in the same time it takes the X-Ray machine to go from one pulse to the next, then each grid line will be superimposed on the last, creating a shadow.
The slowness of moving grids leads to a need for extended exposure lengths, leading to an increased patient exposure. Moving grids also require a greater exposure length than still grids because still grids 'concentrate' the X-Rays in a way that moving grids do not.
Which Grid Should I Use?
X-Ray grids are always a form of compromise, in that while they increase contrast, the increase in Bucky factor leads to a need for increased patient exposure to X-Rays. However, the drop in scatter radiation between a 16:1 grid and a 12:1 grid is nowhere near as great as that between an 8:1 grid and a 4:1 grid. For a peak kilovoltage below 90, an 8:1 grid will usually do, whereas above 90 kVp a 12:1 grid is better. However, for cases of unavoidable decentring, a low grid ratio and long focal distance should be used.