Filters, Collimators and Grids Christensen’s physics of diagnostic radiology Dr. G. Dileep Bhoopal

Filters Filtration is the process of shaping the X-ray beam to increase the ratio of photons useful for imaging to those photons that increase patient dose or decrease image contrast

Diagnostic X-ray beam is polychromatic, comprising of whole spectrum of energies. The mean energy will vary from 1/3rd to one half of the their peak energy. The first few centimeters of the tissues receive much more radiation than the rest of the body tissues of the patient

Filters Filters are sheets of metal, attached at the opening of tube housing, which will absorb the low energy photons from the X-ray beam before it reaches the patient

Types of filtration The X-ray beam is filtered by absorbers at three different levels Inherent Filtration Added filtration Patient

Inherent filtration Glass envelope The insulating oil surrounding the tube Window in the tube housing

Added filtration Aluminum At.No. 13 Copper At. No. 29 Compound Filter copper + Aluminum

Advantage of compound filtration Copper is used to cut down the thickness of filter Copper will absorb high energy photons and Aluminum will absorb the characteristic radiation from copper (8 keV)

Measurement of filtration Filtration is measured in Aluminum equivalents which is defined as the thickness of the Aluminum that would produce the same degree of attenuation as the thickness of material in question

Effects of filtration Patient exposure and Exposure factors

60 –kVp beam Aluminum Exposure dose Decrease in filtration (mm) to skin (mR) exposure dose (%) None 2380 0.5 1850 22 1.0 1270 47 3.0 465 80

Inherent filtration at the tube housing is 0.5 to 1.0 Al.Eq. Below 50 kVp 0.5 mm Aluminum 50-70 kVp 1.5 mm Aluminum Above 70 kVp 2.5 mm Aluminum

Effect on exposure factors There will be reduction in the intensity of X-ray beam as the filters absorb some photons at all energy levels. To Compensate the loss of high energy photons, increase in the exposure factors (mAs) is required

Types of filters Single Filter Aluminum Compund Filter Aluminum +Copper Wedge Filter Used in the angiography Molybdenum Filters used in Mammography. K. Edge filter

Molybdenum filters Used in molybdenum target X-ray tubes used for mammography 17.5 kev K alpha and 19.6 kev K beta characterstic radiation of Mo When operated at 30-40 kVp, Mo will produce bremsstrahling with energies higher than 20 kev Mo filter attenuates these high energy rays

K- Edge filters These filters make use of K absorption edge of elements with atomic No. greater than 60. The purpose of heavy metal filters or K edge filters is to produce an X-ray beam that has a high number of photons in the specific energy range Enhance contrast for Iodine and barium, reduce patient dose, and increase tube loading

X-ray beam restrictors

X-ray beam restrictors An X-ray beam restrictor is a device that is attached to the opening in the X-ray tube housing to regulate the size and shape of the X-ray beam

Types of restrictors Aperture diaphragms Cones & cylinders Collimators

Aperture diaphragm It consists of a sheet of lead with a hole in the center. The size and shape of the hole determine the size and shape of the X-ray beam.

Cones Cones are usually flare shaped Ideal geometric configuration for an X-ray beam restrictor. The flare of the cone is greater than the flare of the x ray beam.

Cylinders Beam restriction with cylinder takes place at the far end of the barrel, so there is less penumbra.

P Aperture diaphragm cone cylinder

Disadvantages Penumbra: Partially exposed periphery of the X-ray field is called penumbra Another major disadvantage with cones and cylinders is severe limitations they place on the number of available field sizes

Collimators Collimator is the best all round X-ray beam restrictor These are two types: Manual collimator Automatic collimator or PBL (Positive beam limitation device or automatic light localised variable aperture collimator)

Advantages It provides infinite variety of rectangular or square X-ray fields The light beam shows the exact center and configuration of X-ray field

Construction of collimator Collimator shutters X ray filter mirror X ray &light beam bulb Lower shutter

Testing of X- ray beam and light beam alignment Periodic check of alignment of X-ray beam and light beam is essential, because the mirror gets out of adjustment due to frequent daily use.

Automatic collimators When the cassette is loaded in the Buckey tray the sensors in the tray identify the size and alignment of the cassette and relay the information to collimator motors, which positions shutters to the exact size of the film used.

Functions of X ray beam restrictors Protects the patient from unnecessary radiation It decreases the scatter radiation

The number of scattered radiation depends upon field size Small fields generate little scatter, as the field increases scatter increases Collimators are only successful in decreasing the scatter radiation with small fields, so we should reduce the size of X-ray beams as much as possible

GRIDS

The radiographic grid consists of a series of lead foil strips separated by X ray transparent spacers. It was invented by DR.GUSTAVE BUCKY in 1913. Grid is still the most effective way of removing the scatter radiation from large radiographic fields

Primary radiation is oriented in the same axis as the lead strips and passes between them . Scatter radiation arises from many points within the patient and most of it is absorbed by the lead strips

The interspaces of the grids are filled either with aluminium or some organic compound. The main purpose of the interspace material is to support the thin lead foil strips.

GRID RATIO Is defined as the ratio between the height of the lead strips and the distance between them.

The lead strips are approximately 0.05 mm thick ( lead foil). The interspaces are much thicker. Grid ratios are usually expressed as two numbers, such as 20:1 Ratios usually range from 4:1 to 16:1 the Higher the ratio, the better the grid functions.

Grid pattern Is the orientation of the lead strips in their longitudinal axis. The two basic grid patterns are : Linear and Crossed.

Linear grid They allow us to angle the x-ray tube along the length of the grid .

Crossed grids A crossed grid is made up of two superimposed linear grids that have the same focussing distance The grid ratio of crossed grids is equal to the sum of the ratios of the two linear grids. A crossed grid made up of two 5:1 linear grids has a ratio of 10:1. Crossed grids cannot be used with oblique techniques requiring angulation of the X-ray tube

Focussed grid Is a grid made up of lead strips that are angled slightly so that they focus in space. A focussed grid may be either linear or crossed. Linear focused grids converge at a line in space called the convergent line. Crossed grids converge at a point in space called the convergent point. The focal distance is the perpendicular distance between the grid and the convergent line or point.

Focussing range Indicates the distance within which the grid can be used without significant loss of primary radiation It is fairly wide for a low-ratio grid and narrow for a high ratio grid. A 5:1 grid focused at 40 inches has a focusing range of approximately 28 to 72 inches. While a 16:1 grid focused at 40inches has a range of only 38 to 42inches.

Parallel grid A parallel grid is one in which the lead strips are parallel They are focused at infinity. can only be used with either very small Xray fields or long-target grid distances. They are frequently used in fluoroscopic spot film devices.

Lines per inch Is the number of lead strips per inch of the grid. Lines per inch = 25.4/D+d D= thickness of the interspaces d=thickness of the lead strips(both in millimeters)

Grid cassette Usually used for portable radiography , with a grid built in to the front of the cassette. Are focussed and Have a grid ratio of 4:1 to 8:1

Evaluation of grid performance The three methods of evaluating grid performance: 1.Primary transmission(Tp) 2.Bucky factor (B) 3.Contrast improvement factor(K)

Primary transmission Is the percentage of primary radiation transmitted through the grid. Ideally , a grid should transmit 100% of the primary radiation. The first measurement is made with the grid in place The second measurement is made after removal of the grid

Tp = _____________________ X100 intensity without grid I’p A ratio of the intensity with the grid to the intensity without the grid gives the fractional transmission, which is multiplied by 100 to give the percentage of transmission. intensity with grid Ip Tp = _____________________ X100 intensity without grid I’p There is a significant loss of primary radiation with grids, more with cross grids

Measured primary tranmission The measured primary transmission is always less than the calculated primary transmission: absorption by the interspace material manufacturing imperfections

Bucky factor Is the ratio of the incident radiation falling on the grid to the transmitted radiation passing through the grid. It indicates how much we must increase exposure factors when we change from a non grid to a grid technique. The Bucky factor indicates the absorption of both primary and secondary radiation. It is determined with a large X-ray field and a thick phantom

incident radiation B = ____________________ The Bucky factor( B) is a measure of the total quantity of radiation absorbed from an X-ray beam by a grid incident radiation B = ____________________ transmitted radiation The transmitted radiation is measured with the grid in place ,and The incident radiation is measured after the grid has been removed

Grid ratio 70 kVp 120 kVp No grid 1 1 5:1 3 3 8:1 3.5 4 12:1 4 5 16:1 4.5 6 High ratio grids absorb more scatter radiation and have larger Bucky factors than low-ratio grids Higher energy beams generate more scatter radiation and place a greater demand on a grid’s performance than lower energy radiation.

The higher the Bucky factor,the greater the exposure factors and radiation dosage to the patient. If the Bucky factor for a particular grid-energy combination is 5, then exposure factors and patient exposure both increase 5 times

Contrast improvement factor (K) The contrast improvement factor(K) is the ratio of the contrast with a grid to the contrast without a grid. contrast with a grid K = _________________ contrast without a grid Is the ultimate test of grid performance. The contrast improvement factor is dependent on kVp,field size and phantom thickness. These three factors determine the amount of scatter radiation

The larger the amount of scatter radiation,the poorer the contrast, and the lower the contrast improvement factor. It is more closely related to the lead content of the grid than any other factor. Generally, the higher the grid ratio,the higher the contrast improvement factor.

Lead content The lead content of a grid is expressed in gm/cm2 Imagine cutting a grid up in to 1 cm squares and then weighing one square. Its weight in grams is the lead content of the grid(ignoring the interspace material) The amount of lead in a grid is a good indicator of its ability to improve contrast. There is a definite relationship between the grid ratio,lead content and the number of lines per inch.

when grids are constructed with many lines per inch, both the thickness and height of the lead strips are decreased. These grids are thinner, and improve contrast less than grids of comparable ratios with fewer lines per inch.

Grid cut off Grid cut off is the loss of primary radiation that occurs when the images of the lead strips are projected wider than they would be with ordinary magnification It is the result of a poor geometric relation ship between the primary beam and the lead foil strips . The resultant radiograph will be light in the area in which the cutoff occurs. With linear grids there may be uniform lightening of the whole film, one edge of the film ,or both edges of the film,depending on how the cutoff is produced.

There are 4 situations that produce grid cutt off The amount of cutt off is always greatest with high ratio grids and short grid focus distance There are 4 situations that produce grid cutt off Focussed grids used upside down Lateral decentering Focussed grid distance decentering Combined lateral and focus grid distance decentering

Upside down focussed grids When a focused grid is used upside down, there is severe peripheral cutoff with a dark band of exposure in the center of the film and no exposure at the film’s periphery

Lateral decentering Results from the X-ray tube being positioned lateral to the convergent line but at the correct focal distance

All the lead strips cutoff the same amount of primary radiation,so there is a uniform loss of radiation over the entire surface of the grid ,producing a uniformly light radiograph This is probably the most common kind of grid cutoff, but it cannot be recognised by inspection of the film. All we see is a light film (that is usually attributed to incorrect exposure factors.) The films become progressively lighter as the amount of lateral decentering increases.

Three factors affect the magnitude of cutoff : Grid ratio Focal distance and Amount of decentering The equation for calculating the loss of primary radiation with lateral decentering is L=rb/fo x 100 L= loss of primary radiation(%) r=grid ratio b=lateral decentering distance (inches) fo =focal distance of grid (inches)

When exact centering is not possible , as in portable radiography, low ratio grids and long focal distances should be used whenever possible

Off level grids When a linear grid is tilted , as it frequently is in portable radiography, there is a uniform loss of primary radiation across the entire surface of the grid

Focus-grid distance decentering In focus -grid distance decentering , the target of the X-ray tube is correctly centered to the grid , but it is positioned above or below the convergent line. If the target is above the convergent line ,it is called FAR focus-grid distance decentering If the target is below the convergent line , it is called NEAR focus-grid distance decentering. The cut off is greater with NEAR than with FAR focus-grid distance decentering. The cutoff becomes progressively greater with increasing distance from the film center.

The central portion of the film is not affected, but the periphery is light.

Parallel grids are focused at infinity A film taken with a parallel grid has a dark center and light edges because of near focus-grid distance decentering

Combined lateral & focus-grid distance decentering The most commonly recognised kind of grid cutoff is from combined lateral and focus grid distance decentering It causes an uneven exposure,resulting in a film that is light on one side and dark on the other side. The projected images of the lead strips directly below the tube target are broader than those on the opposite side, and the film is light on the near side. Cutoff is greatest on the side directly under the Xray tube.

The projected images of the lead strips are broader on the side opposite the tube target than on the same side, and the film is light on the far side. Cutoff is least on the side under the Xray tube.

Moving grids Grids are moved to blur out the shadows cast by the lead strips. Most grids are reciprocating,which means they continuously move 1 to 3 cms back and forth through out the exposure. They start moving when the Xray tube anode begins to rotate. They eliminate grid lines from the film

Moving grids precautions The grid must move fast enough to blur its lead strips The transverse motion of the grid should be synchronous with the pulses of the Xray generator

Disadvantages They are costly Subject to failure May vibrate the Xray table Put a limit on the minimum exposure time because they move slowly increase the patient’s radiation dose

Grid selection Usually 8:1 grid will give adequate results below 90kVp. Above 90kVp,12:1 grids are preferred. There is little decrease in transmitted scatter beyond an 8:1 ratio grid, And almost no change between 12:1 and 16:1 For this reason12:1 grids are preferable to 16:1 grids for routine radiography

Air gap technique Scattered radiation arising in a patient from comption reactions is dispersed in all directions. With an air gap the concentration of scatterd radiation decreases because scatters photons fail to reach the film

Used in 2 clinical situations Magnification radiology Chest radiology With magnification techniques the object -film distance is optimised for the screen focal spot combination and the air gap technique reduces the scatter radiation In chest radiography the focal film distance is usually lengthened from 6-10 ft to restore sharpness

Exposure factors with air gaps X-ray tube exposure must be increased for the air gap technique because of larger focal film distance Patients exposures are usually less with air gap technique The air gap loses less primary radiation, so the patient’s exposure is less

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