Optimisation in fluoroscopy
Department of Medical Physics and Biomedical Engineering, Central Hospital, V�xj�, Sweden
Optimisation of radiation protection in fluoroscopy is
important since the procedure could lead to relatively high absorbed doses both
in patients and personnel resulting in acute radiation injury. Optimisation
procedures include adjustment of the fluoroscopy equipment such as exposure
factors as well as proper use of automatic brightness control and pulsed
fluoroscopy. It is also important to gain the benefits of image processing and
the higher sensitivity of flat panel detectors as compared to image intensifier-TV
Proper positioning of the patient with respect to detector
and X-ray tube is of fundamental importance to image quality and radiation dose
to the patient. Both image quality and radiation dose are also affected by the
methodology used with parameters such as magnification factor, increased
filtration, use of last-image-hold and the use of a grid.
There is a direct relation between patient dose and the
absorbed dose to the personnel since this is mostly due to scattered radiation
from the patient. If the correct methodology and the correct radiation
protection devices are used, the absorbed dose to the personnel could be
minimised to acceptable levels even for those working with complex procedures.
In order to have an organised review of all aspects of
optimisation, it is recommendable to have an active quality system at the
department. This system should define responsibilities and tasks for persons
involved. � 2007 Biomedical Imaging and Intervention Journal. All rights reserved.
Keywords: Radiation protection, fluoroscopy, patient dose,
Fluoroscopy is being used not only by radiologists but
also by an increasing number of clinicians, for instance in interventional
radiology. To obtain optimal benefit from the use of fluoroscopy without undue
risk to the patient, it is important that the personnel have a thorough
knowledge about the functioning and performance of the equipment and also
adequate training in radiation protection and an awareness of the potential for
injury both to the patient and personnel. It is widely known that there may be
substantial differences in image quality and radiation dose among different
institutions for the same type of procedure depending on the level of training
and methodology [1, 2]. Some aspects on optimisation of image quality and
radiation protection are discussed in this presentation.
Radiation dose and biological effects
Radiation exposure to the patient could be characterised
by the dose-area product (DAP) or the entrance skin dose (ESD). DAP is the
product of the absorbed dose in air by the area of the beam and is a measure of
the total amount of radiation emitted from the equipment towards the patient.
DAP could be used to calculate the effective dose, which characterises
stochastic risk such as radiation-induced cancer. Cardiac catheterisation
procedures, for instance, have been reported to deliver DAP of about 60 Gy/cm2,
which would result in an effective dose of about 12 mSv . ESD is used to
evaluate the risk for deterministic effects such as skin lesions. ESD of 1-2.5
Gy has been reported for coronary interventions . In recent years, there
were also several reports on radiation-induced deterministic effects on
patients as a result of complex interventional procedures [5, 6,]. For
procedures where the ESD is estimated at or above 3 Gy (1Gy for repeated
procedures), there should be a system to establish the maximum skin dose. These
calculations should be indicated in the patients� notes and the patients should
also be reviewed between 10 and 14 days after the treatment.
Radiation exposure to the personnel is characterised by
the absorbed dose to organs of interest such as hands or eye lens, or by the
effective dose. High radiation doses to the hands and to the eye lens as well
as deterministic effects have been reported with some procedures .
Modern fluoroscopy equipment gives the user opportunities
to adjust the image quality and the radiation exposure according to the needs
for the actual examination. Automatic brightness control (ABC) is used to
ensure that the brightness of the image at the monitor is constant. This is
accomplished with automatic adjustment of tube voltage and current to
accommodate the varying attenuation of the patient. There are at least two dose
levels available and in most examinations adequate image quality is obtained
using the low-dose mode [8, 9]. Low-dose technique is also applicable for
cine-runs. It is advisable to always start fluoroscopy in low-dose mode and
then switching to a higher dose level if necessary. In examinations of the peripheral
parts of the patient, the ABC might not work satisfactorily and cause �image
flare�. In these cases manual selection of exposure parameters or
technique-lock of the ABC to a preferred setting is recommended. Use of
technique-lock is also needed if radiation-opaque objects have to be inserted
into the image field.
It is sometimes also possible to choose the mode of
operation of the ABC. If low dose is a priority, the tube voltage is increased
more than the current as the patient thickness increases. The increase in tube
voltage will result in a slight decrease in image contrast especially for soft
tissue. In situations where image contrast is crucial, the tube current could
instead be increased more than the tube voltage (Figure 1). For paediatric use,
it is desirable to have a low dose and therefore paediatric mode (if available)
will provide a slightly higher tube voltage for thin patients. Theoretical
studies  have shown that there is a potential for dose reduction in
paediatric examinations by using a combination of low tube voltage and
increased filtration (0.2 mm Cu). It is however difficult to accomplish this
with present generators.
Proper algorithms for ABC function and use of the suitable
mode is thus important both for patient dose (reduction of factor 2) and image
quality . Since it is not certain that these factors could be adjusted
easily on the equipment, it is important to consider them during commissioning
of the equipment.
A useful way of decreasing patient dose while maintaining
image quality is to use pulsed fluoroscopy [9, 11], which produces radiation in
short pulses, opposite to continuous mode. Pulse rates as low as single pulses
per second can be chosen. Lower pulse rates will result in larger dose savings
(Figure 2). A digital image memory and gap filling is used to obtain a
continuous flicker-free video display on the monitor. The disadvantage of
pulsed fluoroscopy is the loss of temporal resolution. With some training, this
is however not a major problem.
Another method to reduce patient dose is to use frame
averaging. In this case a series of frames produced by the detector are
averaged before presentation on the monitor. This will reduce the noise in the
presented image and therefore give the possibility to reduce the dose rate used
without loss of image quality. A disadvantage using substantial frame averaging
is the noticeable image lag.
Flat panel detectors have also been introduced for
fluoroscopy. Characteristics of these detectors, such as high sensitivity to X-rays,
large dynamic range and good contrast resolution, give the opportunity to
optimize the examination technique with respect to absorbed dose and image
quality. When introducing these systems, it is essential to explore the
possibilities of reducing patient dose while maintaining adequate image
quality, and not to improve image quality when it is not necessary . It has
been reported that the patient dose could be reduced by 30% using flat panel
detectors. A prerequisite to capitalize on these possibilities is that the
function of the equipment and the methodology is thoroughly reviewed .
Even though careful considerations on the functioning of
the equipment will give the possibility to perform procedures with low patient
doses, the main factor deciding the patient dose is the methodology used by the
operator. Important factors in this respect are the fluoroscopy time,
restriction of the radiation field and positioning of the patient.
Use of last-image-hold (LIH), which enables the last live
image to be displayed continuously when the radiation is terminated, could
reduce the fluoroscopy time to half compared to when it is not used. It enables
the operator to examine the image as long as necessary without the use of
radiation. Many types of equipment also have the possibility to see, on the LIH
image, the effect of adjustment of the collimators on the image field. This
further decreases the beam-on time. It has been shown that equally large dose
savings can be obtained if appropriate restriction of the radiation field is
employed [3, 14]. Reduction of a circular radiation field size from 20 cm to 16
cm will reduce the amount of radiation to the patient by about 40 percent.
Images needed for documentation should preferably be
exposed using the fluoroscopy system since the absorbed dose needed is less
compared to radiography.
For procedures employing cine runs it is equally important
to limit the number of frames to what is essential for the examination. Short
cine loops viewed repeatedly usually provide adequate information . It is
not uncommon for the length of the cine runs to increase when shifting from
film to digital techniques . This is probably because long runs no longer
present a handling problem for the personnel.
There is a possibility to increase detail resolution by
using magnification mode of the image intensifier. This will however decrease
the brightness gain of the intensifier and the generator will compensate for
this by increasing the exposure by the square of the magnification factor.
Magnification mode should therefore not be used unless it is necessary to
perform the procedure.
Positioning of the patient with respect to the X-ray tube
and the detector is very important not only for the possibility of visualising
the anatomy but also for the image quality and to restrict the radiation dose
to the patient. Tube angulations influence the exposure significantly due to
the large effect on the projected path through the patient. Orientations giving
rise to high dose rates should not be used more than absolutely necessary .
Integral to good practice is to position the patient as close as possible to
the detector. ESD increases dramatically as the patient is moved towards the
X-ray tube. If combined with thick patients, a short distance between X-ray
tube and the patient will lead to very high dose rates (Figure 3). This could
lead to infliction of radiation injury to the patient even with modest
fluoroscopy time. In extensive interventional procedures it is advisable to reposition
the equipment with respect to the patient during some occasions to avoid
irradiation of the same part of the skin.
Transmission of the radiation through the patient can be
increased if additional filtration of the beam is used. This has been applied
for several types of examination [14, 16, 17] and substantial dose reductions
(about 50%) have been reported. Usually, about 0.2 mm Cu is added to the
original filtration of the radiation beam. Another way of reducing the patient
dose is to remove the antiscatter grid. The grid not only removes scattered
radiation but also a part of the primary radiation. The dose rate has to be
increased by approximately a factor 2 when the grid is used. In small-sized
patients such as small children, the amount of scattered radiation is also
small and no grid is needed. It is therefore important that the grid is easily
removable in equipments used for paediatric examinations. For medium-sized
objects, an air gap could be used for scatter rejection instead of a grid. The
reduction of radiation dose is also large for air gap technique but care has to
be taken to avoid small distance between patient and X-ray tube . For
large-sized patients a grid is necessary to avoid deterioration of image
Radiation protection of the personnel
It is important to remember that the radiation dose to the
personnel is directly related to the dose to the patient since the major
contribution is scattered radiation from the patient. Intensity of the
radiation is highest at the tube side of the patient and therefore the amount
of scattered radiation is largest at this side. It is therefore advantageous to
perform examinations with an undercouch tube whenever possible since this will
reduce the amount of scattered radiation towards the head and chest. Staff
working with fluoroscopy should have adequate radiation protection. This
includes well designed aprons or vest and skirt, and thyroid protection if
deemed necessary. Attenuation equivalent to 0.35 mm lead provides substantial
protection even for those working with complex interventional procedures .
Light-weight aprons manufactured from non-lead materials provide adequate
protection for those who do not have a heavy workload in the fluoroscopy room
and will at the same time spare the spine and shoulders from the heavy weight
of lead aprons. Ceiling mounted lead acrylic viewing screens will provide very
good protection for the head and neck . They are recommended for rooms
where angiography and interventional work is performed.
For those performing interventional procedures it is very
important to keep the hands out of the radiation field. This is especially so
when working on the tube side of the patient.
As discussed above, image quality and radiation dose are greatly
influenced by technical and procedural factors. Image quality and dose are also
linked and the optimisation of the procedures is not trivial. There should be a
comprehensive quality system established involving physicians, staff and
medical physicists to review both existing procedures and the introduction of
new methods. A quality system should cover all aspects from procurement and
quality control of the equipment, evaluation of methods and measurement of dose
to patients and personnel to a program to ensure that everybody working with
the radiological procedures have adequate knowledge on radiation protection and
dose control techniques.
Figure 1 Different modes of operation for the regulation of tube voltage and tube current using automatic brightness control.
Figure 2 Example of dose levels for continuous and pulsed fluoroscopy.
Figure 3 Entrance skin dose level dependent on patient thickness, tube voltage and focus-skin distance. For tube voltage 70 kV and focus-skin distance 70 cm (?), 100kV, 70cm (�), 70 kV,40cm (?). This figure gives an example. The actual dose rate depends on setting of the ABC.
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|Received 30 October 2006; accepted 12 November 2006
Correspondence: Department of Medical Physics and Biomedical Engineering, Central Hospital, S 35185 V�xj�, Sweden. E-mail: firstname.lastname@example.org (Bertil Axelsson).
Please cite as: Axelsson B,
Optimisation in fluoroscopy, Biomed Imaging Interv J 2007; 3(2):e47
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