Kv / Kvp
Potential difference between film and anode
The energy (you can consider
this the penetrating power) of the x-ray beam is controlled by the
voltage adjustment. This control usually is labelled in keV (thousand
electron volts) and sometimes the level is referred to as kVp (kilovoltage
potential). Do not be confused by the different terminology, just
remember there is a control by which the difference in potential
between the cathode and anode can be controlled. The higher the
voltage setting, the more energetic will be the beam of x-ray. A
more penetrating beam will result in a lower contrast radiograph
than one made with an x-ray beam having less penetrating power. It
is probably obvious that the more energetic the beam, the less
effect different levels of tissue density will have in attenuating
that beam.
The generator waveform if is not constant potential (medium
frequency etc) will affect the effective Kv.
mA Tube Current
The second control of the output of the x-ray tube is called the mA
(milliamperage) control. This control determines how much current is
allowed to flow through the filament which is the cathode side of
the tube. If more current (and therefore more heating) is allowed to
pass through the filament, more electrons will be available in the
"space charge" for acceleration to the target and this will result
in a greater flux of photons when the high voltage circuit is
energized. The effect of the mA circuit is quite linear. If you want
to double the number of "x" photons produced by the tube, you can do
that by simply doubling the mA. Changing the number of photons
produced will affect the blackness of the film but will not affect
the film contrast.
S Time
The third control of the x-ray tube which is used for medical
imaging is the exposure timer. This is usually denoted as an "S"
(exposure time in seconds) and is combined with the mA control. The
combined function is usually referred to as mAs or milliampere
seconds so, if you wanted to give an exposure using 10 milliampere
seconds you could use a 10 mA current with a 1.0 second exposure or
a 20 mA current for a 0.5 second exposure or any combination of the
two which would result in the number 10. Both of these factors and
their combination affect the film in a linear way. That is, if you
want to double film blackness you could just double the mAs.
The X-Ray beam
The x-ray beam has two main properties you need to
understand. 1) Beam QUALITY is the ability of the beam to
penetrate an object, its all about the penetrating power of the
x-ray photons, this is controlled by the KV control. 2) Beam
INTENSITY this is the number of x-ray photons in the beam and is
principally controlled by the mAS But note as you increase the KV
not only does the QUALITY harden (more penetrating) but you do
actually get more photons so INTENSITY increases too.
Putting it all together the exposure
Any radiographic subject has a minimum Kv required for the x-ray
photons penetrate the most dense part of the subject, the most
radiographicaly dense part of the subject will depend upon what the
part is chemicaly composed of (Atomic number) and its thickness
(remember linear attenuation coefficients and HVL!?) The thicker
the subject the more absorption of x-rays so the thicker the part
the more mAS you require. In theory the more Kv you use the less
the contrast of the image will have
However in practice film screen / processing conditions affect
contrast much more In practice it is not as simple as this as
scatter is produced which is not image forming but adds density to
the film and needs to be controlled, if you remember all those
complex diagrams about interactions of x-rays with matter you will
realise the amount and direction of scatter depends on the Kv and
the material absorbing the x-rays.
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Image 1 The Kv is too low the femoral
condyle is under pentrated you cannot see the bone trabecualr
patterns. the contrast is too high to demonstrate all the soft
tissues. |
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Image 2 Much better the all the subject is
penetrated and all the soft tissues are visible |
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A well exposed abdomen image demonstrating all
the soft tissue structures. |
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A good chest image the mediastinum is pentrated
the image is exposed well demonstrating the bones and soft
tissues. |
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Under penetrated |
 |
OK |
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Under penetrated |
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Too much mAS |
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Too Little mAS |
A few myths
Changing the Kv by 2 or 3 makes almost no
perceptable image change!
Adding 10 Kv does not double the image density
Exposure factors are an exact science !
(the image you produce must satisfy the radiologist who interprets
the image - not all radiologists like the same penetration / density
/ contrast for the same body part)
Image Contrast
Here, we need to spend a little more time discussing the issue of
radiographic contrast. This is an important concept because image
contrast plays a critical part in the interpreter's ability to
detect abnormalities which are only slightly different from the
density of the surrounding material. It is not possible to say what
is the optimal contrast (or the optimal radiographic technique) for
all situations. Different body parts have different inherent tissue
contrast. This can be illustrated by using the extreme examples of
the chest and the breast. In the chest, there is good inherent
tissue contrast with densities ranging all the way from bone at the
high end to air at the low end. On the other hand, the breast is
inherently very low in tissue contrast only containing structures
which are water density (glandular material or tumor) or fat
density. For the moment, we will disregard small calcifications
which are really not normal structures. Because of this difference
in inherent tissue contrast, we would be likely to use a very low
contrast radiographic technique for the chest because we have good
tissue contrast. Conversely we would be likely to use a very high
contrast technique for the breast because the breast has minimal,
inherent tissue contrast.
Remember, image contrast is controlled by the energy of the "x"
photon beam. Therefore, high kV techniques result in low contrast
images (the assumption is always made that the image will have
approximately the same average film density so if kV is increased,
there must be a compensation in mAs to keep film density constant).
To increase image contrast in situations where there is low tissue
contrast, a low kV, high mAs technique should be used. This is
obvious for mammography but you should also remember this
possibility for other special situations such as looking for
low-density foreign bodies embedded in soft tissue. To improve film
contrast for mammograms we would need to use a very low energy x-ray
beam. Mammograms are frequently done with beams in the 25 keV range.
For the chest x-ray, we would like to use a low contrast technique
which requires a relatively high-energy beam. Chest x-rays are
frequently done with beam energies above 100 keV. You should
understand that for similar film densities, the high KV technique
usually results in lower patient radiation exposure. Think about
this long enough to clearly understand why less radiation is
absorbed in the patient when a high-energy beam is used.
Grids
One of the problems in getting a sharply defined image in clinical
radiology is the presence of scattered or secondary radiation. These
photons are created in the body of the patient or closely
surrounding objects by the interaction of that material and the
primary "x" photons coming from the x-ray tube. Several possible
interactions occur in the diagnostic energy range. At relatively low
energies, the photoelectric effect is probable. The photoelectric
effect is actually the desirable, photon/tissue interaction because
there is complete absorption of the photon with no production of a
secondary photon. The more common tissue interaction at the photon
energies used for the majority of clinical procedures is called the
Compton effect or coherent scattering. In this interaction, a
secondary photon is produced at the site of interaction. The
secondary photon will always have lower energy than the primary
photon and will be going in an altered direction. These secondary
photons, if allowed to reach the film, will actually produce
erroneous information by recording gray tone variation (and
therefore indicating relative tissue densities) at some distance
from the site at which the photon/tissue interaction actually
occurred. The net result of allowing a significant number of
secondary photons to reach the film is a reduction in image
sharpness. There will always be a loss of spatial resolution.
Several methods have been devised to reduce the problem of scattered
radiation. The simplest and most direct is to simply limit the field
of exposure. If a small image area is adequate to make the clinical
diagnosis, the image area should be "coned down" to that small size.
For instance, if you want to image the gallbladder, you will get a
much sharper picture if you bring the shutters down to include an
area only the size of the gallbladder instead of including the
entire upper abdomen on the image. Just remember that the smaller
the area of the x-ray beam the fewer scattered photons you will
produce.
In the typical clinical imaging situation, the most common method of
reducing scatter is to use a radiographic grid. The grid looks like
a flat metallic plate the size of the x-ray film if you look at it
directly. However, it is more complicated than that. It actually is
composed of alternating radiopaque (lead) and radiolucent (aluminum)
strips. These are arranged on edge, sort of like looking at the
strips of a venetian blind which is arranged to let light come
between the strips. The edge of these strips is turned towards the
source of x-rays and in the most commonly used grid, the focused
grid, the anglulation of the strips is arranged to match the
divergence of the x-ray beam.
This arrangement of the radiographic grid will give the highest
probability for primary "x" photons passing between the lead grid
strips and reaching the film, while the off-focus or secondary
photons are likely to interact in the lead strips and never reach
the film.
The use of this radiographic grid will greatly improve image
sharpness when a relatively thick body part is being imaged.
Unfortunately, there is always a trade off. Since the grid does stop
some of the photons which would contribute to film blackening, if
you just add a radiographic grid without changing the tube settings,
the film will be greatly underexposed. If you decide to use a grid,
you will have to increase the number of photons produced by the
x-ray tube in order to get the correct film exposure. This will
result in giving the patient increased radiation exposure. Remember,
the position of the grid is between the patient and the film.
The third method of reducing scatter or at least reducing the
probability that scattered photons will reach the film is to use an
air gap. This is infrequently used in clinical radiography but can
still, sometimes be used to an advantage particularly when
magnification of the image might be helpful. Ordinarily we would
have the film positioned as close to the patient's body as possible
for the radiography of any body part. With an air gap technique, the
film is moved several inches away from the patient's body. That
separation, (because secondary photons are likely to be lower energy
and moving at a greater angle than primary photons) will result in a
decreased probability of the secondary photon hitting the film. From
the diagram below, you will be able to understand that creating the
air gap will also result in magnifying the radiographic image.
Remember the x-ray beam is produced from almost a point source and
it diverges as it goes towards the patient.
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