For an object or anatomical structure to be visible in an x-ray image, it must have physical contrast in relationship to the tissue or other material in which it is embedded. This contrast can be a difference in physical density or chemical composition (atomic number).
When an object is physically different it absorbs either more or less x-radiation than an equal thickness of surrounding tissue and casts a shadow in the x-ray beam. If the object absorbs less radiation than the surrounding tissue (ie, gas surrounded by tissue), it will cast a negative shadow that appears as a dark area in a radiograph. The third factor that affects object contrast is its thickness in the direction of the x-ray beam. Object contrast is proportional to the product of object density and thickness. This quantity represents the mass of object material per unit area (cm2) of the image: For example, a thick (large diameter) vessel filled with diluted iodine contrast medium and a thin (small diameter) vessel filled with undiluted medium will produce the same amount of contrast if the products of the diameters and iodine concentrations (densities) are the same.
The chemical composition of an object contributes to its contrast only if its effective atomic number (Z) is different from that of the surrounding tissue. Relatively little contrast is produced by the different chemical compositions found in soft tissues and body fluids because the effective atomic number values are close together. The contrast produced by a difference in chemical composition (atomic number) is quite sensitive to photon energy and the spectrum of the x-ray beam (KV).
Most materials that produce high contrast with respect to soft tissue differ from the soft tissue in both physical density and atomic number. The physical characteristics of most materials encountered in x-ray imaging are compared in the following table.
Physical Characteristics of Contrast-Producing Materials
The contrast in the invisible image emerging from the patient's body is an image of the different attenuations through the body and is sometimes referred to as subject contrast, especially in older publications. The contrast in the attenuation image is represented by the difference in exposure among various points within the image area.
For an individual object, the significant contrast value is the difference in exposure between the object area and its surrounding background. This exposure difference is generally expressed as a percentage value relative to the background exposure level. Contrast will be present if the exposure in the object area is either more or less than in the surrounding background.
X-ray contrast is produced because x-ray penetration through an object differs from the penetration through the adjacent background tissue. For objects that attenuate more of the radiation than the adjacent tissue, contrast is inversely related to object penetration. Maximum (100%) contrast is produced when no radiation penetrates the object. Metal objects (lead bullets, rods, etc.) are good examples. Contrast is reduced as x-ray penetration through the object increases. When object penetration approaches the penetration through an equal thickness of surrounding tissue, contrast disappears.
The amount of x-ray contrast produced is determined by the physical contrast characteristics (atomic number, density, and thickness) of the object and the penetrating characteristics (photon energy spectrum) of the x-ray beam.
NOTE: The contrast in the x-ray beam coming from a patient's body is greatly diminished by the scattered radiation produced within the body. That is considered in the next chapter.
The x-ray images are captured by the receptor and recorded in some form. This can be on film, or on stimulable phosphor plates or some digital media for digital radiography. A desirable characteristic of radiographic recording methods is that all of the contrast in the x-ray image is fully recorded.
Both the stimulable phosphor plates and direct digital recording methods generally capture and record all of the contrast in the x-ray image because of their wide dynamic range for exposure.
One of the limitations of film for recording x-ray images is its relatively narrow exposure latitude or dynamic range. Image contrast is reduced or completely lost when the exposures are not well within the film latitude. This is the contrast transfer characteristics of the film, which are discussed in the chapter "Film Contrast Characteristics."
Both images recorded on film and digital media undergo some form of processing before they are displayed as visible images.
Film is chemically processed to convert the invisible recorded image into a visible image. If the chemical processing is deficient (because of inadequate processor quality control, incorrect chemistry, etc) some of the recorded image contrast ( and visibility of objects) might not be transferred into visible image contrast.
One of the great advantages of digital radiography is the ability to perform digital processing to enhance image contrast for specific clinical applications.
The next type of contrast is the contrast that appears in the visible image. The contrast in a radiograph recorded on film is in the form of differences in optical density values between various points within the image, such as between an object area and the surrounding background.
The contrast in an image on an electronic display or monitor (digital radiographs, fluoroscopic images, etc) is in the form of different brightness or brightness ratios between various points within the image area.
Displays for digital images might have there own contrast characteristics that will alter or limit the contrast in displayed images. The contrast characteristics of displays and monitors can usually be checked by displaying digital test patterns.
Most systems for displaying digital images provide the capability for windowing. This allows the user to adjust and optimize the contrast in the displayed image.
The first step in obtaining optimum contrast in an x-ray image is to adjust the x-ray beam spectrum for the specific anatomy and clinical purpose. The penetration and the resulting contrast of a specific object or structure in the body generally depends on the photon energy spectrum.
Effect of X-ray Beam Penetration on Contrast, Body Penetration, and Dose
Contrast is not the only thing that must be considered in selecting the spectrum for a specific procedure. The spectrum also affects the penetration through the body section being imaged. This has a significant effect on the radiation dose to the patient. Also, as the penetration through a body section is reduced, the amount of radiation required from the x-ray tube is increased with a resulting increase in x-ray tube heating. We will see examples as we consider specific procedures.
In radiography, especially mammography, the objective is to select an x-ray beam spectrum that provides the optimum balance between contrast and dose. Both of these factors depend on photon energy as illustrated below.
The General Relationship of Contrast and Dose to Photon Energy in Mammography.
If we consider the contrast-to-dose "ratio" we find that it changes as we move along the photon energy scale. At very low energies the contrast is high (that is good) but the body section penetration is very low resulting in a high dose to the patient (that is bad). At the higher photon energies the body section penetration is increased and the dose is reduced (that is good), but the contrast goes down (that is bad).
Now for the very important point......for every radiographic procedure and specific anatomical environment (breast thickness and density) there is probably an x-ray photon energy (spectrum) that is optimum in that it produces the best contrast to dose relationship. The task is setting up the imaging technique factors to produce that optimum spectrum.
We recall that the spectrum is determined by three factors: (1) x-ray tube anode material, (2) x-ray beam filtration, and (3) KV. Since most x-ray examinations are performed with tungsten anode tubes, the first factor cannot be used to adjust contrast. The exception is the use of molybdenum and rhodium anode tubes in mammography. Most x-ray machines for general radiography and fluoroscopy have essentially the same amount of filtration, which is a few millimeters of aluminum. Two exceptions are molybdenum and rhodium filters used with molybdenum anode tubes in mammography and copper or brass filters, sometimes used in chest radiography.
In most procedures, KV is the only spectrum controlling factor that can be changed by the operator to alter contrast. Radiographic examinations are performed with KV values ranging from a low of approximately 24 kV, in mammography, to a high of approximately 140 kV, in chest imaging. The selection of a KV for a specific imaging procedure is generally governed by the contrast requirement, but other factors, such as patient exposure must be considered.
Both photoelectric and Compton interactions contribute to the formation of image contrast. It was shown in Interaction of Radiation with Matter that the rate of Compton interactions is primarily determined by tissue density and depends very little on either tissue atomic number or photon energy. On the other hand, the rate of photoelectric interactions is very dependent on the atomic number of the material and the energy of the x-ray photons. This means that when contrast is produced by a difference in the atomic numbers of an object and the surrounding tissue, the amount of contrast is very dependent on the photon energy spectrum and the selected KV. If the contrast is produced by a difference in density (Compton interactions), it will be relatively independent of photon energy. Changing KV produces a significant change in contrast when the conditions are favorable for photoelectric interactions. In materials with relatively low atomic numbers (i.e., soft tissue and body fluids), this change is limited to relatively low KV values. However, the contrast produced by higher atomic number materials, such as calcium, iodine, and barium, has a KV dependence over a much wider range of KV values.
Compared to other anatomical regions, the breast has very low physical contrast because it is composed completely of soft tissues. It is generally a background of fat surrounding the slightly more dense glandular structures and pathologic tissues or cysts if they are present. Typical breast calcifications are very small and thin and produce low physical contrast even though calcium is somewhat more dense and has a higher atomic number than soft tissues.
Two basic factors tend to limit the amount of contrast that can be produced between types of soft tissue and between soft tissue and fluid. One factor is the small difference in the physical characteristics (density and atomic number) among these materials, as shown in the table above titled, "Physical Characteristics of Contrast-Producing Materials," and the second factor is the relatively low number of photoelectric interactions because of the low atomic numbers.
As we have already seen, contrast is not the only consideration, especially in mammography. The effect of the spectrum on dose must also be taken into account. The relationship of contrast to dose leading to an optimum photon energy spectrum depends on the size and density of the breast as illustrated below.
The Optimum Photon Energy Increases with Breast Size and Density
Mammography is performed with a spectrum containing photons within a relatively narrow energy range (19keV - 21kev) in an attempt to optimize the contrast to dose relationship.. This spectrum is produced using the characteristic radiation from a molybdenum anode x-ray tube and filtered by either a molybdenum or rhodium filter. Some equipment has a dual track anode so that either molybdenum or rhodium can be selected.
We will now look at each of these spectra.
Spectrum Produced with Molybdenum Anode and Molybdenum Filter
The "moly-moly" spectrum is the most frequently used for mammography. The molybdenum anode produces two peaks of characteristic radiation at 17.6 keV and 19.7 keV as shown above. Let's notice that this is very close to the optimum spectrum, especially for the smaller and less dense breasts. However, the x-ray beam will also contain the usual bremsstrahlung spectrum with energies extending up to the set KV value which will be in the range of 24 kV to 32 kV. This part of the spectrum is undesirable because of its increased penetration which reduces the contrast. That problem is solved by using a molybdenum filter that works on the K-edge principle, that is it attenuates photons with energies above the molybdenum K edge energy of 20 keV.
With this combination a significant portion of the spectrum is in the range from 17.6 to 20 keV which is quite good for general mammography.
With mammography equipment with only the "moly-moly" combination (the standard for many years) the only adjustable factor for changing the spectrum is the KV. As the KV is increased within the 24 - 32 kV range, the x-ray beam becomes more penetrating. Increasing the KV increases the amount (but not the photon energy) of the characteristic radiation and also increases the amount of bremsstrahlung just below the filter K edge cut-off.
Increasing the KV also increases the efficiency of x-ray production in the tube so that there is more radiation per MAS and per unit of heat. It is the combination of these factors (higher penetration and increased x-ray tube output) that makes the higher KV values necessary for larger and more dense breast, not only to achieve the necessary receptor exposure within a reasonable exposure time
Spectrum Produced with Molybdenum Anode and Rhodium Filter
Many mammography machines give the operator the opportunity of selecting between two filters, molybdenum or rhodium. Rhodium has a slightly higher atomic number (Z) that molybdenum and therefore its K-edge energy is higher, 23.22 keV. When the rhodium filter is selected the x-ray spectrum is now extended up to that energy and becomes more penetrating.
The rhodium filter is useful when imaging dense breast where additional penetration improves visulation within the dense areas.
Spectrum Produced with Rhodium Anode and Rhodium Filter
Some equipment have dual-track x-ray tubes so that either molybdenum or rhodium can be selected as the active anode material Because of its higher atomic number (Z) rhodium produces characteristic x-radiation with higher energies than molybdenum as shown above.
When the rhodium anode is selected (always with the rhodium filter) the beam penetration is increased and generally is optimum for imaging dense breast.
We have seen how the use of rhodium, both as a filter and anode material, extends the spectrum and makes it more penetrating. This does improve contrast and visibility in the more dense breast by making it possible to "see through" some of the dense areas. However, the increased penetration can reduce contrast in other breast environments.
The figure below shows the relationship between calcium penetration (contrast) and photon energy. In principle, the optimum photon energy range (KV) for imaging calcium depends, to some extent, on the thickness of the object. When imaging very small (thin) calcifications, as in mammography, a low photon energy must be used or the contrast will be too low for visibility. When the objective is to see through a large calcified structure (bone), relatively high photon energies (KV) must be used to achieve adequate object penetration.
Relationship of Calcium Penetration and Contrast to Photon Energy
Calcium is a significant source of contrast not only in bones, but in the form of calcifications that form with some pathologic conditions. Calcium produces contrast relative to soft tissue because it differs in both density and atomic number. Because of its higher atomic number, photoelectric interactions predominate over Compton interactions up to a photon energy of approximately 85 keV. Above this energy, the photoelectric interactions contribute less to image contrast. This means that calcium contrast is dependent on the spectrum of the x-ray beam.
The two chemical elements iodine and barium produce high contrast with respect to soft tissue because of their densities and atomic numbers. The significance of their atomic numbers (Z = 53 for iodine, Z = 56 for barium) is that the K-absorption edge is located at very favorable energies relative to the typical x-ray energy spectrum. The K edge for iodine is at 33 keV and is at 37 keV for barium. Maximum contrast is produced when the x-ray photon energy is slightly above the K-edge energy of the material. This is illustrated for iodine in the figure below. A similar relationship exists for barium but is shifted up to slightly higher photon energies.
Relationship of Iodine Penetration and Contrast to Photon Energy; the Values Shown Are for a 1-mm Thickness of Iodine Contrast Medium
Since the typical x-ray beam contains a rather broad spectrum of photon energies, all of the energies do not produce the same level of contrast. In practice, maximum contrast is achieved by adjusting the KV so that a major part of the spectrum falls just above the K-edge energy. For iodine, this generally occurs when the KV is set in the range of 60-70.
We have considered a single object embedded in tissue. In this simple case an increase in contrast generally increases the visibility of the object. However, in most clinical applications one image contains many objects or anatomical structures. A problem arises when the different objects are located in different areas of the body and the thickness or density of the different areas is significantly different. A chest image that contains lung and mediastinal areas is a good example; a simple representation is shown in the figure below. Because of the large difference in tissue density between the lungs and the mediastinum, the contrast is significant between these two areas in the image. In this typical radiograph, the area of the mediastinum is very light (low film density), and the lung areas are much darker. Any objects within the mediastinum are imaged on a light background, and objects within the lung areas are imaged on dark backgrounds.
Physical Conditions That Produce High Area Contrast
High anatomical area contrast, as in the chest, is a challenge especially with film radiography. One of the advantages of digital radiography is the ability to overcome some of the problems of high area contrast through a combination of receptors with a wide exposure dynamic range, digital image processing, and digital image windowing.
A characteristic of radiographic film is that its ability to display object contrast is reduced in areas that are either very light (mediastinum) or relatively dark (lungs). If there is a relatively high level of contrast between areas within an image, then the contrast of objects within these areas can be reduced because of film limitations. Two actions can be taken to minimize the problem. One is to use a wide latitude film that reduces area contrast and improves visibility within the individual areas in many situations; this is described in the chapter titled, "Film Contrast Characteristics." The other action is to use a very penetrating x-ray beam produced by high KV and more filtration than is used for other types of radiography.
- The x-ray imaging process transfers physical contrast from within a body (produced by differences in densities and atomic numbers) to visible contrast displayed in an image.
This occurs in several steps and is affected by several factors, beginning with the difference in x-ray attenuation by the objects and structures within the body, the effects of scattered radiation, the contrast limiting characteristics of the receptor, image processing, and the contrast characteristics and adjustments of the display.
The formation of the x-ray image within the body by differences in attenuation is controlled by the "matching" of the x-ray beam spectrum to the characteristics of the tissues or contrast media and the body sections being imaged.
The selection of an optimum spectrum for a specific clinical procedure must take into consideration not only the requirements for contrast but also produce the necessary penetration through the body section and limit the radiation dose to the patient.
The spectrum of an x-ray beam is determined by combinations of the anode material, the filter material and thickness, and the selected KV for the procedure.
Optimum spectra in mammography for various breast sizes and densities are obtained with combinations of molybdenum and rhodium anodes, molybdenum and rhodium filters, and KV values in the range of 24 kV to 32 kV.
Maximum contrast with iodine and barium is obtained with an x-ray spectrum that has many photons with energies just above the K-edge energy of the contrast materials (33 keV for iodine and 37 keV for barium). This is generally achieved by operating with KV values in the range of 60 kV to 70 kV.
The chest is an anatomical region with very high physical contrast because of the lungs that form a low-density background for most of the other anatomical structures and pathologic tissues.
The large difference in density between the lungs and other regions produces high contrast between the lungs and other areas. This high area contrast can reduce object and structure contrast within the very dark areas (the lungs) and the very light areas ( the mediastinum) for images recorded on film.
Area contrast is reduced and visibility through the ribs is increased by using a very penetrating x-ray beam produced with high KV values (120 kV) and added filtration in the beam.