Scattered Radiation and Contrast
Perry Sprawls, Ph.D.

Online Textbook

Table of Contents

 

 

INTRODUCTION AND OVERVIEW

CONTENTS

   When an x-ray beam enters a patient's body, a large portion of the photons engage in Compton interactions and produce scattered radiation. Some of this scattered radiation leaves the body in the same general direction as the primary beam and exposes the image receptor. This scattered radiation reduces image contrast. The degree of contrast loss depends on the scatter content of the radiation emerging from the patient's body. In most radiographic and fluoroscopic procedures, the major portion of the x-ray beam leaving the patient's body is scattered radiation. This, in turn, significantly reduces contrast.

   Contrast reaching an x-ray image receptor was previously described as the difference in exposure to the object area within an image expressed as a percentage of the exposure to the surrounding background. Maximum contrast, i.e., 100%, is obtained when the object area receives no exposure with respect to the background. A previous chapter, X-ray Image Formation and Contrast,  discussed the reduction of  contrast because of x-ray penetration through the objects in the body being imaged. This chapter describes the further reduction of contrast by scattered radiation.

 

CONTRAST REDUCTION

CONTENTS

   The basic concept of contrast reduction by scattered radiation is illustrated below. For simplicity, it is assumed that the object within the body shown here is not penetrated and, if it were not for scattered radiation, would produce 100%  contrast.


Reduction of Image Contrast by Scattered Radiation

   The object is assumed to be embedded in a larger mass of material, such as the human body, that produces the scattered radiation. The exposure to the background area of the receptor, or film, (shown here as the black area) is produced by radiation that penetrates the body adjacent to the object plus the scattered radiation. For a given x-ray machine setting, the background area exposure is proportional to PS, the product of the penetration through the patient (P) and the scatter factor (S). For the same exposure conditions, the exposure to the object area, that is in the shadow of the object,  is proportional to P (S - 1 ). By combining these expressions for relative background and object area exposure, it can be shown that the contrast with scatter (Cs) is inversely related to the value of the scatter factor, as follows:


Cs (%) = 100/S.


   This relationship shows that as the proportion of scattered radiation in the x-ray beam increases, contrast proportionally decreases. For example, if the scatter factor has a value of 4, the contrast between the object and background areas will be reduced to 25%. In other words, the object area exposure is 75% of the exposure reaching the surrounding background. The contrast can also be determined as follows. The ratio of scattered to primary radiation is always S - 1. For a scatter factor value of 4, the scatter-primary ratio is 3. The background area exposure is, therefore, composed of one unit of primary and three units of scattered radiation. The object area receives only the three units of scattered radiation. This yields an object area exposure of 75% of background and a contrast of 25%.

   With respect to image contrast, the scatter factor, S, is also the contrast reduction factor. For example, if the scatter (contrast reduction) factor has a value of 2, the resulting contrast will be 50%. This is a reduction of 100% contrast by a factor of 2. A scatter factor value of 5 reduces contrast by a factor of 5, or down to 20%. The figure below shows the general relationship between contrast and scatter factor. The value of the scatter factor is primarily a function of patient thickness, field size, and x-ray beam spectrum as determined by the KV. In examinations of relatively thick body sections, contrast reduction factors of 5 or 6 are common.



Relationship between Contrast Reduction and Amount of Scatter

   We developed our discussion of contrast reduction using an non-penetrated object (like a piece of metal) that would produce 100% contrast in the absence of scatter. Most objects within the body are penetrated to some extent. Therefore, contrast is reduced by both object penetration and scattered radiation. For example, if an object is 60% penetrated (40% contrast), and the scatter factor, S, has a value of 4, the final contrast will be 10%.
   Since scattered radiation robs an x-ray image of most of its contrast, specific actions must be taken to regain some of the lost contrast. Several methods can be used to reduce the effect of scattered radiation but none is capable of restoring the full image contrast. The use of each scatter reduction method usually involves compromises, as we will see below.

 

COLLIMATION

CONTENTS

   The amount of scattered radiation is generally proportional to the total mass of tissue contained within the primary x-ray beam. This is, in turn, determined by the thickness of the patient and the area or field size being exposed. Increasing the field size increases the total amount of scattered radiation and the value of the scatter contrast-reduction factors. Therefore, one method of reducing scattered radiation and increasing contrast is to reduce the field size with x-ray beam collimators, cones, or other beam-limiting devices, as illustrated below. This method is limited by the necessity to cover a specific anatomical region. However, in most situations, contrast can be improved by reducing the field size to the smallest practical value.



Contrast Improvement by Reducing X-Ray Beam Size


 

AIR GAP

CONTENTS

   The quantity of scattered radiation in an x-ray beam reaching a receptor can be reduced by separating the patient's body and receptor surface, as shown below. This separation is known as an air gap. Scattered radiation leaving a patient's body is more divergent than the primary x-ray beam. Therefore, scattered radiation spreads out of the primary beam area. The reduction of scattered radiation in proportion to primary radiation increases with air-gap distance. Several factors must be considered when using this method of scatter reduction. Patient exposure is increased because of the inverse-square effect. The use of an air gap introduces magnification. Therefore, a larger receptor size is required to obtain the same patient area coverage. If the air gap is obtained by increasing the tube-to-receptor distance, the x-ray equipment must be operated at a higher output to obtain adequate receptor exposure.



Contrast Improvement by Using an Air Gap

   Also, increasing the separation distance between the patient and the receptor increases focal spot blurring. It is usually necessary to use relatively small focal spots with an air-gap technique.

One common use of the air gap is in magnification mammography.  Since an air gap is produced by separating the breast from the receptor to produce magnification, it can be used for scatter reduction.  The usual procedure is to remove the grid and rely on the air gap in magnification mammography.

 

GRIDS

CONTENTS

   In most examinations, the most effective and practical method of removing a portion of the scattered radiation is to use a grid. The grid is placed between the patient's body and the receptor, as shown below. It is constructed of alternate strips of an x-ray-absorbing material, such as lead, and a relatively non-absorbing interspace material, such as fiber, carbon, or aluminum. Under normal operating conditions, the grid strips are aligned with the direction of the primary x-ray beam. In most grids, the interspaces are angled so as to align with a specific point in space. These are designated focused grids. The focal point of the grid should coincide with the focal spot of the x-ray tube, which is the source of the primary radiation. In an unfocused grid, the interspaces and strips are parallel and are not aligned with a single point in space. Because the x-ray beam direction is aligned with the grid, much of the primary radiation passes through the interspaces without encountering the lead strips. Scattered radiation, on the other hand, leaves the patient's body in a direction different from that of the primary beam, as shown in the second figure below. Since scattered radiation is not generally lined up with the grid strips, a large portion of it is absorbed by the grid. The ideal grid would absorb all scattered radiation and allow all primary x-rays to penetrate to the receptor. Unfortunately, there is no ideal grid, because all such devices absorb some primary radiation and allow some scattered radiation to pass through.



The General Design of a Grid




Selective Absorption of Scattered Radiation by a Grid

   The penetration characteristics for scattered radiation are largely determined by the dimensions of the lead strips and the interspaces. The significant dimensions are illustrated in the top figure above. The height of the strips, t, is the thickness of the grid and is typically in the range of 2 mm to 5 mm. Another significant dimension is the width of the interspace, d. This dimension varies with grid design, but generally ranges from 0.25 mm to 0.4 mm. With respect to grid performance, the important variable is the ratio of these two dimensions, which is designated the grid ratio. Most grids have ratios ranging from 5:1 to 16:1. The selection of the appropriate grid ratio for a given examination involves the consideration of a number of factors. Although grids with higher ratios eliminate more scattered radiation, they tend to increase patient exposure and x-ray tube loading and require more precise positioning.

 

GRID PENETRATION

CONTENTS

   A knowledge of the total penetration of primary and scattered radiation through a grid is necessary to select appropriate exposure factors for the x-ray machine.

   The total grid penetration is a function of scattered-radiation penetration and penetration of primary radiation. The relationship also involves the proportion of scattered radiation in the beam, S. The figure below shows the general relationship between the two components (primary radiation and scatter) of grid penetration and grid ratio. In general, the penetration of both types of radiation decreases as the grid ratio is increased.



General Relationship between Radiation Penetration and Grid Ratio

   Primary penetration does not change with the amount of scattered radiation, but does change with grid ratio. On the other hand, scattered-radiation penetration is strongly dependent on grid ratio and the amount of scattered radiation in the beam. Because the typical grid removes more scattered than primary radiation, total penetration decreases as the scattered-radiation content of the beam increases. The two major factors, therefore, that determine total grid penetration are the grid ratio and the scatter factor, S.

   It is common to express grid penetration in terms of the Bucky factor, named after Dr. Gustave Bucky, who constructed the first grid in 1913. The Bucky factor is the reciprocal of the total grid penetration, or

Bucky factor = 1 / Grid penetration.

   Grid penetration and Bucky factor values are shown below for various grid ratios and scatter factors.


Grid Penetration and Bucky Factor Values

   It should be recalled that, in most cases, there is a correlation between S and KV. Since high KV values are generally used for thick body sections, and both factors increase S, grid penetration appears to decrease as KV increases. This is because scatter radiation content at the higher KV values is generally greater than for primary radiation.
   The amount of radiation delivered to a patient's body must be increased to compensate for the radiation absorbed by the grid. Patient exposure is directly proportional to the Bucky factor. For example, if a grid with a Bucky factor of 3 is replaced by one with a Bucky factor of 6, the exposure to the patient must be doubled to compensate for the additional grid absorption.

 

   Scatter Penetration

CONTENTS

   The relationship between the quantity of scattered radiation that passes through the grid and the grid ratio can be visualized by referring to the illustration shown here.. Consider the exposure that reaches a point on the receptor located at the bottom of an interspace. Since no radiation penetrates the lead strips, radiation can reach the point on the receptor only from the directions indicated. The amount of radiation reaching this point is generally proportional to the volume of the patient's body in direct "view" from this point. As grid ratio is increased, this volume becomes smaller, and the amount of radiation reaching this point is reduced. In effect, with a high-ratio grid, each point on the receptor surface is exposed to a smaller portion of the patient's body, which is the source of scattered radiation.
Using basic geometrical relationships, the theoretical penetration of scattered radiation through grids of various ratios can be determined. This is shown graphically in the figure above titled, "General Relationship between Radiation Penetration and Grid Ratio."
In actual usage, the relationship can differ from the one shown, especially for certain grid ratio-KV combinations.



 

 

   Primary Penetration

CONTENTS

   Because of the presence of the lead strips, grids attenuate part of the primary radiation. The penetration of primary radiation through the grid is generally in the range of 0.6 to 0.7. This value depends on grid design and is generally inversely related to grid ratio.

 

   Contrast Improvement

CONTENTS

   It has been shown that as the grid ratio is increased, a greater proportion of the scattered radiation is removed from the beam. By using the scattered-radiation penetration shown in the figure above titled, "General Relationship between Radiation Penetration and Grid Ratio," and an average primary penetration of 0.65, it is possible to calculate the expected contrast for various combinations of grid ratio and scatter factors, S. Some values are shown in the figure below.



Relationship of Image Contrast to Scatter and Grid Ratio

   For a grid ratio of 0, that is, no grid, the contrast percentage is equal to 100 divided by S. As grid ratio is increased and scatter penetration decreases, contrast improves. For relatively small amounts of scattered radiation, that is, S = 2, a grid ratio of 8:1 restores the contrast to 90%. The additional improvement in contrast with higher grid ratios is relatively small. It should be noticed, however, that even with high grid ratios, all contrast is not restored. When the proportion of scattered radiation in the beam is higher, for example, when S has a value of 6, the situation is significantly different. At each grid ratio value, the contrast is much less than for lower scatter factor values. Even with a high-ratio grid , such as 16:1, the contrast is restored to only about 76%. This graph illustrates that contrast is not only a function of grid ratio, but is also determined by the quantity of scattered radiation in the beam, the value of S.

   It might appear that the data in the figure above indicate that grids do not remove as much scattered radiation when the amount of scattered radiation in the beam is relatively large, such as for a value of S of 5 or 6. The relatively lower contrast obtained with large amounts of scattered radiation is because of the very low contrast values present without the grid. Actually, grids improve contrast by larger factors when the proportion of scattered radiation in the beam is higher. This can be illustrated by observing values of the contrast improvement factor, K, as shown in the figure below. The contrast improvement factor is the ratio of the contrast when a specific grid is used compared with the contrast without the grid. It is a function of the grid penetration characteristics and the amount of scattered radiation, S.



Relationship of Contrast Improvement Factor to Scatter Factor and Grid Ratio

   The value of the contrast improvement factor, K, generally increases both with grid ratio and with the quantity of scattered radiation in the beam, S. Although it is true that grids improve contrast by larger factors under conditions of high levels of scattered radiation, one significant fact should not be overlooked: the total restoration of contrast for a given grid is always less for the higher values of scattered radiation. This becomes apparent by comparing the value of the contrast improvement factor to the value of the contrast reduction factor, which is equal to the value of S. This expresses the ability of a grid under various scatter conditions to recover lost contrast. For example, in the figure above it is shown that when S is equal to 5 (contrast reduced to one fifth) a 16:1-ratio grid produces a contrast improvement factor of 4. The contrast recovery, K/S, is four fifths, or 80%. However, at a lower level of scattered radiation, such as S = 3, the same grid produces a contrast improvement factor of 2.7, which represents a contrast recovery of 2.7/3, or 90%.

   The relationship between the improvement in contrast and grid ratio strongly depends on the proportion of scattered radiation in the beam emerging from the patient's body. This, in turn, is a function of patient thickness, field size, and KV. Under conditions that produce high scatter radiation values, a given grid improves contrast by a greater factor, but cannot recover as much contrast as is possible at lower scattered radiation levels.

 

   Artifacts

CONTENTS

   Since the grid is physically located between the patient and the receptor, there is always a possibility that it will interfere with the formation of the image. This interference can be in the form of an image of the grid strips (lines) on the film, or the abnormal attenuation of radiation in certain portions of the field.

 

   Grid Lines

CONTENTS

   To some extent, the appearance of grid lines in the image depends on the thickness of the strips and the interspaces. This is usually specified in terms of the number of strips, or lines, per unit distance. The spacing of lines in grids normally encountered ranges from approximately 24 lines to 44 lines per centimeter (60 to 110 lines per inch). The grid lines are generally less distracting for the higher spacing densities.

   A method frequently used to eliminate grid lines in the image is to blur them by moving the grid during the exposure. The mechanism for accomplishing this was first introduced by Dr. Hollis Potter, and a moving grid system is often referred to as a Potter-Bucky diaphragm. In a Potter-Bucky system the grid moves at right angles to the grid lines. The speed at which the grid moves determines the shortest exposure time that will not produce grid lines.

   Grid motion during exposure also helps eliminate image patterns created by the irregular spacing of grid strips. This type of interference is generally less when grids with aluminum interspaces are used.

 

   Grid Cutoff

CONTENTS

   The basic function of a grid is to absorb radiation that is moving along a path that is not aligned with the grid interspaces. It is desirable that the primary radiation from the x-ray tube focal spot pass through the grid with a minimum of absorption. Maximum grid penetration by primary radiation can occur only if the x-ray tube focal spot is located at the grid focal point. If these two points are not properly aligned, as shown in the figure below, the direction of the primary radiation might be such that the radiation does not adequately penetrate certain sections of the grid.



Two Forms of Grid Misalignment That Can Produce Cut-off Artifacts


   Misalignment of the x-ray tube focal spot with respect to the focal point of the grid can be either lateral or vertical, or a combination of both. Lateral misalignment causes the x-ray beam to be misaligned with all interspaces, and grid penetration is decreased over the entire beam area. The amount of penetration reduction is related to the amount of misalignment and the grid ratio. Alignment becomes more critical for higher ratio grids. That is, the loss of grid penetration because of a specific misalignment is much greater for a high-ratio grid.

   Vertical misalignment does not alter penetration in the center of the grid, but decreases penetration near the edges. The loss of penetration is related to the degree of misalignment and the grid ratio. The reduction in penetration for a given degree of misalignment increases with grid ratio. Focused grids are labeled with either a focal distance or a focal range, which should be carefully observed to prevent this type of grid cutoff. Cutoff toward the edges of the image area will also occur if a focused grid is turned upside down because the primary radiation will be unable to penetrate except near the center. This produces an artifact similar to vertical misalignment but usually much more pronounced.

 

GRID SELECTION for CLINICAL APPLICATIONS

CONTENTS

   A number of factors must be considered when selecting a grid for a specific application. In most cases, a grid is selected that provides a reasonable compromise between contrast improvement and patient exposure, machine loading, and positioning.

   The advantages of a 5:1-ratio grid are that it is easy to use and does not require critical positioning. Its use must be restricted, however, to situations in which the amount of scattered radiation is relatively small (thin body section, low KV
) or in which maximum image contrast is not necessary. On the other hand, a 16:1-ratio grid produces high-contrast recovery but significantly increases patient exposure. With a high-ratio grid of this type, there is very little latitude in positioning. Many applications are best served by grid ratio values between these two extremes. Such grids generally represent compromises between image quality and the other factors discussed.

   Some grids have strips running at right angles to each other, generally designated crossed grids. This design generally increases contrast improvement but cannot be used in examinations in which the x-ray tube is tilted.

   In stationary grid applications in which lines in the image are undesirable, grids with a high spacing density (lines per centimeter) can be used. An increase in the spacing density generally requires a higher ratio grid to produce the same contrast improvement.