There are two basic ways to create images with x-radiation. One method is to pass an x-ray beam through the body section and project a shadow image onto the receptor. The second method, used in
computed tomography (CT), employs a digital computer to calculate (reconstruct) an image from x-ray penetration data
obtained by scanning a relatively thin beam over the patient's body.
At this time, we consider only projection imaging, which is the basic
process employed in radiography and fluoroscopy.
The contrast and visibility of objects that ultimately appears in the image is developed in steps
and determined by many factors, as illustrated below.
Factors That Affect Radiographic Contrast
In addition to the
x-ray penetration characteristics to be considered in this chapter, image contrast is significantly affected by scattered radiation
(Scattered Radiation and Contrast) and the contrast characteristics
of the receptor and display system.
(Film Contrast Characteristics). Also, the contrast of small
objects within the body and anatomical detail is reduced by image blurring
(Blur, Resolution, and Visibility of Detail and Radiographic
As the x-ray beam emerges from the patient's body it contains an image in the form of variations in exposure across the image area.
This is formed by variations in the attenuation through different parts of
the body. In this attenuation image contrast is represented by the amount of variation in x-ray exposure between points within the image; the amount of contrast produced in a specific examination is determined by both the physical characteristics of the body section and the penetrating characteristics of the x-ray beam.
In this chapter we explore the characteristics of both the objects within a body and the x-ray beam and show how optimum image contrast can be achieved.
Several types of contrast are encountered during the process of x-ray imaging.
The formation of a visible image involves the transformation of one type
of contrast to another at the different stages in the image-forming
process, as shown in the illustration above.
For an object or anatomical structure to be visible in an x-ray image, it must have
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
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
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.
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
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
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.
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.
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.
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
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
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.
systems for displaying digital images provide the capability for
windowing. This allows the user to adjust and optimize the contrast
in the displayed image.
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
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
The General Relationship of Contrast and Dose to Photon
Energy in Mammography.
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.
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
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.
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
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.
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.
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
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.
will now look at each of these spectra.
Spectrum Produced with Molybdenum Anode and Molybdenum
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
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
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
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
The figure below shows the relationship between calcium penetration
(contrast) and photon energy. In principle, the optimum photon energy
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
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
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
Physical Conditions That Produce
High Area Contrast
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.