|Hand X-ray of Mrs.
Roentgen, spouse of Wilhelm Conrad Roentgen, the German
physicist who accidentally discovered unknown rays on November 8,
1895 and called them... "X-rays".
This tutorial is intended to familiarize
you with the dose concept. The purpose is to give you a quick overview
of the whole topic of x-ray dose through a simple explanation of the
technology and to expose the different techniques available to reduce
Determining Dose Parameters
When an X-ray tube is in operation,
so-called X-ray beams, a type of radiation, are released. Using these
beams, the technician can create images of whatever is being examined.
This radiation penetrates objects and human bodies, passes through them,
and is weakened in the process.
In simple terms, this weakening is equivalent to a reduction in the
number of individual radioactive particles. A statement concerning the
amount of radiation, which is measured at a site, produces the concept
Because not all the radiation particles generated during an X-ray are
used to produce the resulting images, and because radiation can cause
damage to the human body, we try to achieve the greatest possible
effect, that is the best possible image with the smallest possible dose
In general, the concept of "dose" can mean different things according to
the circumstance, for example according to the site where the dose is
measured. For this reason, the dose concepts most commonly used in
radiology will be explained here.
The incident dose is the dose measured in
the middle of a radiation field on the surface of a body or a phantom.
However, it is only measured at this point if there is no body in the
path of the x-ray beam. Thus, there is no scatter radiation from the
body during this measurement. When radiation strikes a substance, there
is always a certain scattering of radioactive particles. This is
comparable to light striking a glass surface; a certain portion of the
light is always reflected.
The unit used to measure the incident dose is joules per kilogram, and
is known as "Gray" where 1 Gray (Gy) = 1 J/kg. The former unit used to
measure the incident dose was the "Rad," and using this unit, 1 Rad (rd)
= 0.01 Gy, or 1 Gy = 100 rd. But because today's doses are generally
very small, they are usually described using the unit "uGy", that is,
Incident dose = the dose measured on the intended surface of the
patient, but without the presence of the patient
The System International unit (SI unit) used to measure the incident
dose is the Gray, where 1 Gy = 1 J/kg
The surface dose is measured with the body in
the path of the beam. Because of the scattered radiation that results on
the surface and in the depths of the body, the surface dose differs from
the incident dose by including the amount of scattered radiation.
Thus we can say:
Surface dose = incident dose + scattered radiation from the body
The SI unit used to measure the surface dose is the Gray (Gy)
The exit dose serves in the evaluation of the
X-ray image. It is measured in the radiation field in immediate
proximity to the surface of the body where the beams exit from the body.
On the basis of the exit dose and the surface dose, we can calculate how
much radiation must have remained in the patient's body.
Radiation in the body = surface dose - exit dose
The SI unit used to measure the exit dose is the Gray (Gy)
Image receptor dose
The image Receptor dose is measured at the
film cassette, X-ray system's image intensifier assembly or Digital
Detector. The image receptor dose is generally smaller than the exit
dose, because the radiation weakens before it reaches the image
receptor, for example by encountering objects behind the patient's body
such as the radiation protection grid, anti-scatter grid or the table.
Image receptor dose <= exit dose
The SI unit used to measure the image receptor dose is the Gray (Gy)
Dose rate at image receptor
In order to measure a dose, the beam must
operate for a certain period of time. The dose rate therefore represents
the measured dose for the amount of time required to complete the dose
measurement. If the image receptor dose is measured in the process, then
the dose rate is the image receptor dose rate. If the dose is measured
at a different site, then the dose rate is determined using one of the
previously mentioned dose parameters.
Dose rate = -------------------------
The SI unit used to measure the dose rate is Gray per second: (Gy/s)
The dose-area product is a measurement of the
amount of radiation that the patient absorbs. It is usually measured
behind the multi-leaf collimator, that is, on the side of the patient
where the radiation enters the body, by attaching a measuring device in
front of the X-ray tube and passing a beam through it. The dose-area
product is independent of the distance between the X-ray tube and the
measuring device because the further away from the X-ray tube this
measurement is taken, the more the size of the device increases, and the
dose itself decreases (see diagram). The dose to the patient can be
calculated from the dose-area product, the size of the measuring device,
and the distance to the X-ray tube and the patient.
Dose-area product = dose * surface area of the measuring device
The SI unit used to measure the dose-area product is the Gray *
Fig. 2: Dose-area product
The dose-area product at 50 cm from X-ray tube is just as great as
dose-area for 100 cm or 200 cm, because the size of the measuring device
increases with greater distance to the X-ray tube. But the dose itself
decreases with greater distance to the tube. Thus the dose-area product
is the same at each position if the size of the measuring device enables
it to detect all of the radiation.
Body dose and effective dose
The body dose is the comprehensive concept
for the organ or partial-body dose equivalent and the effective dose. In
the practical application of radiation protection, however, local and
individual doses are monitored, because body doses cannot be measured
directly. The Radiation Protection Regulations therefore use the concept
of effective dose, in which all the individual doses to the irradiated
organs or parts of the body are multiplied by a factor and then added
together. The resulting value may not exceed the dose limit for the
effective dose that a patient is allowed to receive.
Body dose = sum of all organ or partial-body doses
Effective dose <= patient dose limit
The SI unit used to measure the body dose and the effective dose is
the sievert, where 1 sievert = 1 Sv = 1 Joule/kilogram = 1 Gray
In many countries or states, by means
of rules, guidelines or regulations, lawmakers have contributed to
improving radiation protection for patients and medical personnel; after
all, medical exposure to radiation is the single largest source of
radiation exposure among the general population. On an international
level, guidelines are laid down by the International Commission on
Radiological Protection (ICRP). Many of the rules, guidelines or
regulations are governed by the ALARA concept (As Low As
Reasonably Achievable), meaning the production of a
diagnostically relevant image at minimum possible dose.
Because children have a greater sensitivity
to radiation than adults, special conditions apply to pediatric
radiology. In particular, an attempt should be made to avoid X-ray
examinations altogether and to use alternative procedures instead, such
as nuclear spin tomography or sonography. Erroneous X-ray exams should
be avoided and the dose measurement should be taken with special
pediatric measuring devices. Special X-ray intensifying screens should
be used as well to reduce additional dose. And because children have a
lower body thickness, the operator generally does not use an
anti-scatter grid, which is used when adults are X-rayed. Of additional
benefit is a more precise collimation of the area through which the beam
will pass; this also reduces the dose. Beam filtration is performed with
a pediatric filter consisting of 0.1 mm copper and 1 mm aluminum. It is
especially important to use a gonad shield and to time a child's
inhalations exactly when making thoracic X-ray images.
Cardiology is another special case. The
generator output must reach at least 100 kW, and there must be an
additional filter of 0.1 mm copper available for fluoroscopy as well as
an appropriate system of collimators for the radiation area. This should
include an iris diaphragm, and rectangular and semi-transparent
collimators. Calculation of the dose-area product during application has
been prescribed by some laws.
A bundle of X-rays corresponds to the shape of a cone, with the tube at
its tip. The intensity or dose of the radiation emitted from the source
of the X-ray beam diminishes with the square of its distance from the
source. If you double the distance x, the dose changes by a factor of
1/(2²), and if you triple it, the dose changes by a factor of 1/(3²).
|Fig. 3: Inverse
In general, the dose amounts to 1/x².
Therefore, if you double the film-to-target distance, you will need four
times as much radiation to achieve the same image blackening. If you did
not change the patient's position, this would lead to radiation stress
in the patient; thus, increasing the distance between X-ray tube and
patient helps to reduce the dose.
Collimation at the site of the film cassette does not result in any dose
reduction, because the radiation is not collimated to the appropriate
film format until it has passed through the patient. It merely serves to
improve image quality by reducing scattered radiation and thereby
Collimation at target
Collimation at the target brings about a
genuine dose reduction and also produces better image quality.
Collimation is performed using cones and collimators (multi-leaf
collimators or iris diaphragms) that are attached directly in front of
the X-ray tube. Collimation at the target is the most effective
radiation protection for the patient and personnel, because it narrows
the area that the radiation can strike.
Because radiation scatters in a body exposed
to X-rays, compression of the body is another way to reduce the
radiation dose. Scattered radiation also produces an undesirable
reduction in contrast in the X-ray image. With compression, the
thickness of the body is reduced, and so a lower dose is absorbed by the
body. Additionally, compression ensures that less scattered radiation
Anti scatter grid
|Fig. 4: Cross
section of an anti scatter grid
The anti-scatter grid is located between
the patient and the image intensifier, or Cassette or Digital Detector.
It is the most effective method of reducing scattered radiation. The
grid absorbs a portion of the scattered radiation in its lead plates.
This absorbed dose therefore does not reach the image intensifier,
Cassette or Digital Detector, even though it has already passed through
the patient. Thus, the use of an anti-scatter grid leads to an increase
in the dose, because the amount of radiation that reaches the image
intensifier, cassette or Digital Detector, is not reduced until it has
passed through the patient: if the anti-scatter grid is used, the
patient must be exposed to a higher dose of radiation in order for the
minimum dose to reach the Image intensifier, Cassette or Digital
We can differentiate between individual anti-scatter grids using their
grid ratios. This is the relationship between the height of the plates
to their distance from each other.
The greater the grid ratio, the greater the grid's effect. Thus, the
required dose increases with the grid ratio. The typical grid ratio is
8:1 for Radiography and 5:1 for Mammography.
The adjustment of the kilovolt values at the
operating console also has an important effect on the dose, because if a
high kilovolt setting is chosen, the radiation is "harder," that is,
richer in energy and more able to pass through the body. High
kilovoltage and strong filtration are therefore similar in their dose
reduction effects, except that image contrast decreases with high
|Fig. 5: Rhodium
and Molybdenum energy spectrums for GE Mammography X-ray tube.
For Mammography, the traditional X-ray
tube target material is molybdenum, but some equipment feature an
additional tube target material, Rhodium or Tungsten, in order to
slightly harden the X-ray beam to better penetrate dense breast without
compromising image quality or contrast.
Radiation filtration / hardening
The quality of the X-rays also plays a great
role in the size of the administered dose. X-ray radiation normally has
so-called "hard" and "soft" particles, that is, particles with a lot of
energy and particles with little energy. Hard particles are better for
the patient, because they pass through the body. Soft particles, by
contrast, get caught inside the body because they are too weak to pass
through and out of it. Therefore, it is primarily soft radiation that
creates unnecessary exposure to the patient. For this reason, copper and
aluminum (Molybdenum and Rhodium in the case of Mammography) are used as
filters in front of the X-ray tube. The soft radiation is caught in the
filter plates, and the remaining radiation emerging from the filter is
"harder." This additional filtration can also reduce the dose to the
patient without diminishing image quality, because in any case only the
"hard" rays reach the image intensifier, film cassette, or Digital
Target/filtration materials impact on dose
Because the GE Senographe DMR+ and 2000D
(Mammography Systems) feature a double Molybdenum / Rhodium X-ray tube
tracks as well as two different filters, they provide a good example of
the impact of different X-ray target/filtration materials on dose (fig.
Choosing the right film/screen combination
can greatly influence the required dose. In general, the dose is a
function of the sensitivity of the combination. This sensitivity is the
quotient of 1000 uGy and the required dose in uGy.
Sensitivity = -------------------------
Required dose in uGy
Required dose in uGy = -------------------
For example, a combination with a sensitivity of 400 requires 2.5 uGy in
dose. The sensitivity is greatly dependent on the intensifying screen
that is used, because the screen is the principal component in image
blackening. We differentiate mainly between screens using traditional
fluorescent materials such as calcium tungstate, and screens made of
so-called rare earths. These rare earths intensify better, which means
they transform more X-ray beams into light. They can therefore reduce
the dose by up to 50%, because the operator can select a lower dose and
still get the same image quality that would be attainable using
traditional screens. These screens are stipulated in pediatrics, for
Image intensifier input screen
At the input screen of the image intensifier
there is a situation similar to that of the screen-film combinations.
The input fluorescent screen substantially determines the
intensification, that is, the transformation of the X-rays into light.
In conjunction with the X-ray image intensifier, the age of the X-ray
system plays a role, because the properties of intensification decrease
considerably with age. In addition, the radiation field adapts
automatically to the format of the image intensifier, which also lowers
the dose, because only small portions of the patient are irradiated
instead of the patient's entire body. The choice of a small input screen
causes collimation, and this too leads to a reduction in the dose.
Automatic exposure timing
The automatic exposure timer or Automatic
Exposure Control (AEC) measures the dose of radiation that strikes the
X-ray film behind the patient, and turns the X-ray system off when the
predetermined dose for that screen-film combination has been reached.
This assures that only the smallest required dose is administered. The
resulting images all show a uniform blackening, and the danger is
reduced that the X-ray examination might have to be repeated owing to an
error in the image. In this way, automatic exposure timing also
indirectly reduces the dose.
Automatic dose rate adjustment
The dose rate is the dose over the total time
in which the patient is exposed to radiation. If the radiation exposure
time to the body can be reduced, this leads to a decrease in the total
dose to the patient. By automatic dose rate adjustment, the operator
tries to reduce the time during which the dose rate is measured at the
input of the image intensifier and the kilovolt and milliamp values are,
in turn, adjusted at the generator. In the process, the dose rate should
be kept as low as possible. Automatic dose rate adjustment is comparable
to automatic exposure timing for images made using a screen-film
The material from which the tabletop is
constructed is also significant for the required dose, because the
tabletop is penetrated by the radiation and weakens it before it reaches
the image intensifier. Therefore, if at all possible the tabletop should
not contain any material that strongly weakens the radiation or absorbs
it well, such as lead or metals in general. Carbon fiber has proven to
be the best material for X-ray system tabletops because its radiation
absorption is minimal and the tabletop can take a great amount of
stress; today a tabletop is expected to be able to support a patient
weighing 120 - 150 kg (up to 330 lbs).
Fluoroscopy using a reduced dose has become
possible primarily through digital technology. Principally, parts of the
body with low levels of spontaneous movement are well suited to this
method of examination. A few digital fluoroscopy procedures will be
described in the following paragraphs.
In pulsed fluoroscopy, X-rays are no longer
delivered continuously; they are delivered in pulses that follow in
rapid succession. This reduces the amount of time during which radiation
is released. The resulting radiation-free gaps in the imaging process
are filled with the last stored digital image until a new and more
current image is available. The short X-ray pulses mean that the dose is
significantly reduced; additionally, image definition is increased.
Pulsing can take place either by using a pulse control at the X-ray
generator, or with a grid-controlled X-ray tube; however, the grid
control leads to a lower level of radiation exposure.
|Fig. 7a: Pulse
The illustration shows the advantages of grid control of the X-ray
tube (top) compared to control at the generator (bottom).
Fig. 7.a Top, we can clearly see the exact
pulses through the grid control; moreover, they allow rapid switching
times. Fig. 7a Bottom in contrast, we can see that during pulsing
controlled at the generator, the kilovolt values move more slowly toward
the correct value and away from it again, resulting in the patient
receiving an unnecessary dose of pulses with a low kilovolt value. Low
kilovolt values contribute to radiation exposure but do not result in
|Fig. 7b: Pulse
The Diagram shows the constant pulse width of the selected frequency
during pulse frequency control (top), and the constant frequency
with variable pulse width using pulse width control (bottom).
The Revolution XQ/i is designed to improve clinical effectiveness
and productivity of Chest Exams
The Revolution XR/d includes a four-way float-top elevating table.
Grid control can itself be subdivided into pulse frequency control and
pulse width control (Fig 7b). The frequency of pulse frequency control
can be varied for example, 12 b/s or 3 b/s, and it controls the X-ray
tube continuously. Pulse width control, on the other hand, changes the
duration of the individual pulses while at a constant frequency, for
example, 25 b/s.
Image integration means adding together two
or more individual images to create a single image. The dose rate can
either be kept the same, resulting in images that are clearer because
there is less noise in the image, or the dose can be reduced. This is
accomplished by reducing the dose rate and adding individual images to
each other until the same image quality is achieved without image
integration, but with a reduction in total dose. Combining individual
images does result, however, in fewer finished images for viewing at the
monitor. The gaps that occur with this method are filled by outputting
the same image twice in a row, similar to the method used for pulsed
fluoroscopy. The reduction in dose with this method is approximately
50%. A disadvantage of the method is the stroboscopic impression that
can arise with fast moving objects.
Digital filtering / SMART Fluoro
With digital filtering, also known as
recursive filtering, a fluoroscopy image is mixed or overlaid with one
or more previously stored images. The proportion of the previous images
is smaller the farther back or longer ago that they were acquired. On
the whole, there is flexibility in choosing the proportional mixture of
images, and the process represents a compromise between dose reduction
and the lag effects that result when mixing images. However, the dose
rate can be significantly lowered since the images that are produced are
only new, or newly made, to a certain degree; that is, combining
individual images means that less overall dose is required. The image
mixture proportion can be monitored using a motion detector that lowers
the image mixture proportion in the event of a strong shift in the gray
scale values in the image, for example when there is movement, so that
the output predominantly reflects the current gray scale value.
Last image hold
Last image hold means that the last image
obtained during a fluoroscopy is stored until a new image is produced.
The physician can then study the image without further radiation
exposure. This can lead to a reduction in radiation exposure since the
total fluoroscopy time is reduced, and with it, the total dose.
Frame grabbing means that the physician can
"grab" or extract and view a chosen image from a fluoroscopic series
without the necessity of additional radiation. Additionally, spot film
radiography can be reduced because the physician can use the "frozen"
images from the fluoroscopic series. This dose reduction is particularly
well suited to pediatrics.
Roadmapping is the overlaying of two images.
A stored image is superimposed upon a current fluoroscopic image, or a
current image can be copied for storage and later used in roadmapping.
This is useful primarily in viewing blood vessels, because an existing
image of a blood vessel filled with contrast medium can be superimposed
on a catheter image made during fluoroscopy. This can save time and
contrast medium, and reduce the radiation dose.
Digital Fluoro imaging techniques
Due to digital correction functions and the
superior quality of modern X-ray image intensifiers, it is possible to
produce spot films from previously captured fluoroscopy images instead
of making new images with screen-film systems. Thus, the additional dose
that would be needed for new spot film radiography can be completely
eliminated, which means a significant reduction in dose. The somewhat
lower quality of the fluoroscopy-generated images, which stems from the
lower dose used for fluoroscopy, is usually accepted as a reasonable
The subdivision of the dose into individual
levels permits finer gradation. With this practice, the minimum dose for
optimum image quality can be selected for every kind of examination. In
addition, the standard examination protocols can be individually adapted
to the patient.
Virtual collimation is a term used to
describe the possibility of positioning the collimators via a display or
at the monitor. Pre-setting the collimators this way does not require an
X-ray beam, and thus reduces the time that the patient is exposed to
radiation and consequently, the entire administered dose.
Solid State detector
Another possibility for dose reduction is
provided by the use of an electronic flat bed detector, also known as a
solid state detector. These silicon-base detectors have a higher degree
of effectiveness than traditional detectors, which is expressed in
Detective Quantum Efficiency (DQE). The patient dose is in direct
proportion to this quantum efficiency:
Patient Dose proportional to
On the basis of this equation, we can see that the greater the Detective
Quantum Efficiency, the smaller the dose for the patient, yet with the
same image quality. An electronic flat bed detector therefore means that
a larger amount of the released radiation is actually used, so that from
the outset a smaller dose of radiation is needed to produce a comparable
Also, because Solid State detectors feature a high dynamic range, they
can accommodate for less X-ray and compensate for the lack of film
blackening through appropriate Brightness and Contrast adjustment
techniques. For some Solid State detectors, the Image Quality does not
suffer from a higher energy X-ray beam, thus contributing to a decrease
of the overall patient dose.
Revolution XQ/i and XR/d Digital X-ray Systems
The digital radiography systems from GE are
designed to meet your clinical requirements today and to address the
trends that will make digital x-ray a logical imperative over the next
The Revolution XQ/i and XR/d feature a high DQE that can contribute to
reduction of dose.
Senographe 2000D is the new Digital
Mammography System from GE Medical Systems. It is a complete, modular
system that eliminates the need for film cassettes and takes full
advantage of digital technology from on-screen image display to
Networking, Filming and Archiving.
The Senographe DMR+ employs a unique, patented bi-metal mammography tube
with a Rhodium track for superior imaging of the most challenging breast
The Senographe 2000D feature a Solid State Detector with High DQE in
addition to the dose reduction measures already incorporated into the
Additional Rhodium target X-ray tube