Download Adult patient doses in interventional neuroradiology

Adult patient doses in interventional neuroradiology
Nikolaos A. Gkanatsios, Walter Huda, and Keith R. Peters
Department of Radiology, University of Florida, P.O. Box 100374, Gainesville, Florida 32610 a)
and Department of Radiology, SUNY Upstate Medical University, 750 E. Adams Street, Syracuse,
New York 13210
共Received 9 April 2001; accepted for publication 5 February 2002; published 11 April 2002兲
We investigated radiation doses to 149 adult patients who underwent interventional neuroradiologic
procedures, consisting of 132 patients who had diagnostic imaging examinations and 17 patients
who had therapeutic procedures. The interventional procedures were carried out on a biplane
system capable of performing fluoroscopy and digital subtraction angiography 共DSA兲. The x-ray
imaging system was interfaced to a patient dosimetry system, which computed surface 共skin兲 doses
based on the selected radiographic technique factors in each of the radiographic and fluoroscopic
imaging modes. For each patient, an assessment was made of the maximum surface dose received
during the procedure, which predicts the possibility of inducing deterministic effects. Knowledge of
the surface doses, beam quality and x-ray cross sectional area permitted the computation of the total
energy imparted to each patient. Energy imparted values were converted to effective dose, which
provides an estimate of the stochastic radiation risk to the patient. The median surface dose for the
frontal plane during diagnostic imaging examinations was 1.3 Gy, with a maximum surface dose of
5.1 Gy. The median surface dose for the frontal plane during therapeutic procedures was 2.8 Gy
with a maximum surface dose of 5.0 Gy. Ratios of the lateral to frontal median surface doses were
0.47 for diagnostic examinations and 0.68 for interventional procedures. The median energy imparted was 1.8 J during fluoroscopy, and 4.3 J during radiography, showing that on average, 66% of
the patient exposure comes from radiographic imaging 共DSA兲. For diagnostic examinations, the
median patient effective dose was 33 mSv, with a maximum of 152 mSv. For therapeutic procedures, the median patient effective dose was 74 mSv, with a maximum of 156 mSv. In interventional neuroradiology, surface doses could induce deterministic effects, and the corresponding effective doses are noticeably higher than those normally encountered in diagnostic
radiology. © 2002 American Association of Physicists in Medicine. 关DOI: 10.1118/1.1470206兴
Key words: dosimetry, surface dose, energy imparted, effective dose, neuroradiology
INTRODUCTION
Interventional neuroradiology studies the vasculature and potential pathologies of the brain by means of catherization
performed with the transfemoral artery technique.1,2 Most
interventional neuroradiologic procedures are considered to
be of a diagnostic nature where the objective is to identify a
neurologic problem. An increasing proportion of interventional neuroradiologic procedures may be categorized as
therapeutic, where the procedure is undertaken to treat a neurologic condition. Examples of therapeutic procedures include brain aneurysm obliteration using detachable coils, arteriovenous malformation embolization using liquid acrylics,
and arteriovenous fistula closure with detachable balloons.3
Imaging in interventional neuroradiology is normally accomplished by use of fluoroscopy and digital subtraction angiography 共DSA兲. In general, neuroradiologic procedures require
good image quality, long fluoroscopic times and a significant
number of angiographic images to visualize and evaluate any
vascular pathology.4,5
Interventional neuroradiologic procedures have the potential to result in high patient doses to the head region.6,7 The
surface 共skin兲 dose is an important dosimetric parameter, as
it can be used to predict the possibility of inducing determin717
Med. Phys. 29 „5…, May 2002
istic radiation injuries, such as skin erythema or epilation.8,9
Surface doses may be calculated from measured x-ray tube
output data, or may be measured using radiation detectors
such as thermoluminescent dosimeters. Another important
dose parameter is the effective dose to the patient. The effective dose takes into account the individual dose and radiosensitivity of all the irradiated organs and tissues.10 The magnitude of the effective dose is taken to be a measure of the
stochastic risks to patients undergoing the specified radiologic examination.10,11 The effective dose is generally the
most important dosimetric parameter when patient surface
doses do not exceed the threshold dose dictating the induction of deterministic effects.
In this study, values of surface dose, energy imparted and
effective dose were determined for 149 adult patients undergoing interventional neuroradiologic procedures. Patient
doses were quantified to investigate how patient surface
doses compare with the threshold for the induction of deterministic effects, and how the corresponding effective doses
compare with the effective doses of other types of radiologic
examinations. Our analysis included an investigation regarding the importance of fluoroscopy relative to radiographic
image acquisitions, as well as the relative importance of the
frontal versus the lateral imaging projection plane. In addi-
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Gkanatsios, Huda, and Peters: Interventional neuroradiology
tion, we investigated how patient doses during diagnostic
neuroradiologic procedures compared to those encountered
during therapeutic interventional procedures.
718
TABLE I. Values of water equivalent thickness and x-ray beam area of
exposure.
Body region
Head PA
Head LAT
Chest/Neck PA
Abdomen/Pelvis PA
METHOD
Interventional neuroradiologic imaging
The x-ray imaging system used in this study was a biplane, Toshiba 共Toshiba America Medical Systems, Tustin,
CA兲 KXO-80 x-ray generator and the Toshiba DFP2000A/A3 digital fluorography system configured to perform
interventional neuroradiologic procedures. Radiographic acquisitions were performed using digital subtraction angiography 共DSA兲 where a mask frame obtained prior to the administration of iodinated contrast was subtracted digitally
from subsequent images of the vasculature containing iodinated contrast. For radiographic acquisitions, the frame rate
ranged between 1 and 3 frames per second, with the total
number of frames for a given run ranging between 10 and 50
frames. Biplane fluoroscopy of the head region was used for
target position verification. In diagnostic procedures, the acquisition of radiographic images was done in biplane mode
almost exclusively. In therapeutic procedures, both biplane
and single plane imaging, either frontal or lateral were used
during different stages of the evaluation of the embolization
progress.
A total of 149 consecutive adult patients were considered
in this study, including 132 patients who underwent diagnostic examinations, and 17 patients who underwent therapeutic
procedures. The transfemoral artery technique was used to
guide the catheter to the vertebral or carotid arteries. A limited amount of fluoroscopy was performed over the
abdomen–pelvis and thoracic regions of each patient. Based
on a study of 10 patients, about 30 seconds of frontal plane
fluoroscopy were spent on the abdomen–pelvis region with
an additional two minutes at the upper thoracic, lower neck
region. The amount of fluoroscopy performed over the
abdomen–pelvis and thoracic regions was found to be independent of the patient and the type of the interventional neuroradiologic procedure.12 Computation of surface doses to
the head region excluded fluoroscopy performed on the
abdomen–pelvis and thoracic region of the patient. Fluoroscopy of the body region, however, was explicitly included
when computing patient effective doses.
a
Water thickness
共cm兲
Exposure areaa
(cm2 )
22.2
17.8
15.0
20.4
279
335
175
175
At entrance to the patient.
Surface doses from fluoroscopy and radiography were combined to obtain the total surface dose to each patient for the
frontal and lateral planes, respectively.
The on-line patient exposure meter computes patient skin
exposures by using the x-ray tube radiation output at the
selected technique factors 共kV and mA兲 as measured during
calibration of the system. The dosimetry system also utilizes
additional information about the patient location relative to
the x-ray tube as well as measured exposure times.5,12 All
skin exposures included the contribution of backscatter radiation. An exposure of 2.58⫻10⫺4 C/kg 共1R兲 was taken to
correspond to 9.3 mGy absorbed dose for muscle tissue in
converting entrance skin exposure values into patient surface
doses.13
Energy imparted and effective dose
The energy imparted, ␧, to a patient undergoing any radiologic x-ray examination can be estimated by modeling
the patient as a slab of water with thickness z using the
expression
␧⫽ ␻ 共 z 兲 ⫻ESE⫻A,
J
共1兲
where ␻ is the energy imparted per entrance exposure-area
product, ESE is the exposure measured free-in-air at the
beam entrance plane of the patient, and A is the area of
exposure also measured at the entrance plane.14,15 The pa-
Surface dose
A patient dosimetry system 共PEMNET兲 共Clinical Microsystems Inc., Arlington, VA兲 was operational on each of the
two x-ray-imaging planes. The PEMNET unit is a
microprocessor-based system running its own on-board software, which is passively hardwired to the x-ray generator
where it samples a multitude of signals 共i.e., kVp, mA,
C-arm and table location, etc.兲.5 These input signals permit
the computation of surface doses to patients, assuming that
the same skin area is continually exposed to the x-ray beam.
Medical Physics, Vol. 29, No. 5, May 2002
FIG. 1. Histogram distribution of surface doses from use of fluoroscopy for
149 patients undergoing interventional neuroradiologic procedures. Black
bars correspond to the frontal imaging plane and gray bars correspond to the
lateral plane. Each histogram bin on the x axis corresponds to 0.15 Gy.
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Gkanatsios, Huda, and Peters: Interventional neuroradiology
FIG. 2. Histogram distribution of surface doses from use of radiography
共DSA兲 for 149 patients undergoing interventional neuroradiologic procedures. Black bars correspond to the frontal imaging plane and gray bars
correspond to the lateral plane. Each histogram bin on the x axis corresponds to 0.30 Gy.
rameter ␻ depends on the patient water equivalent thickness,
z, the x-ray tube voltage and x-ray beam half-value layer.14,15
The free-in-air entrance exposure was obtained from the patient exposure data recorded by the dosimetry system for
each plane, corrected for backscattered radiation. The area of
exposure was estimated using the geometry and the image
receptor size for an average patient. Table 1 shows the water
equivalent thickness and x-ray beam exposure areas for the
principal body regions and projections that were used to
compute energy imparted.
For any given projection, patient effective doses can be
estimated from a knowledge of the energy imparted and
x-ray beam quality.16 In this paper, values of energy imparted, ␧, were converted into corresponding values of patient effective dose, E, by using E/␧ conversion factors that
accounted for the body region being irradiated, the specific
projection and the x-ray beam quality. The average effective
dose per unit energy imparted for the head projections was
approximately 5 mSv J⫺1 . Corresponding effective dose per
unit energy imparted values were approximately 15 mSv J⫺1
719
FIG. 3. Histogram distribution of the total surface doses for 149 patients
undergoing interventional neuroradiologic procedures. Black bars correspond to the frontal plane and gray bars correspond to the lateral plane. Each
histogram bin on the x axis corresponds to 0.30 Gy.
for the chest and approximately 14 mSv J⫺1 for the
abdomen.
RESULTS
Entrance surface doses
Figure 1 shows the histogram distribution of surface doses
to the head region during fluoroscopy for all 149 patients
included in this study. The median values of the fluoroscopic
surface doses were 0.45 Gy and 0.12 Gy for the frontal and
lateral projection planes, respectively. Maximum surface
doses were 2.6 Gy for the frontal plane and 2.7 Gy for lateral
plane. Figure 2 shows the histogram distribution of the surface doses to the head region received from radiography. The
median values of the radiographic surface doses were 0.83
Gy and 0.50 Gy for the frontal and lateral planes, respectively. Maximum radiographic surface doses of 4.8 Gy and
3.8 Gy were recorded in the frontal and lateral planes, respectively.
Figure 3 shows the histogram distributions of the combined surface doses to the patients’ head region from use of
TABLE II. Surface doses in Gy for diagnostic angiographic and therapeutic embolization interventional neuroradiologic procedures.
Diagnostic procedures
Therapeutic procedures
Projection
Median
dose
Maximum
dose
Median
dose
Maximum
dose
Fluoroscopy
Frontal
Lateral
0.43
0.11
2.6
1.6
0.74
0.77
2.3
2.7
Radiography
Frontal
Lateral
0.73
0.49
3.4
3.8
1.8
1.3
4.8
2.6
Fluoroscopy⫹Radiography
Frontal
Lateral
1.3
0.61
5.1
4.1
2.8
1.9
5.0
5.2
Imaging procedure
Medical Physics, Vol. 29, No. 5, May 2002
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Gkanatsios, Huda, and Peters: Interventional neuroradiology
720
TABLE III. Energy imparted (J) statistics for interventional neuroradiologic
procedures.
Imaging mode
Fluoroscopy
Radiography
Fluoroscopy⫹Radiography
25th
Median
75th
Maximum
percentile value percentile
value
1.2
2.4
3.2
1.8
4.3
6.7
3.2
6.5
9.0
13
21
27
fluoroscopy and radiography during an interventional neuroradiologic procedure. The medians of the combined surface
dose were 1.4 Gy and 0.70 Gy for the frontal and lateral
plane, respectively. The maximum surface dose received by a
patient was of the order of 5 Gy for both imaging planes.
Most of the surface dose occurs as a result of radiography,
which accounts for 65% of the combined surface dose in the
frontal plane and 81% of the surface dose in the lateral plane.
Table II provides a breakdown of the measured surface
doses between diagnostic and therapeutic interventional neuroradiologic procedures. Ratios of the lateral to frontal median surface doses were 0.47 for diagnostic examinations and
0.68 for interventional procedures. In general, diagnostic
procedures resulted in surface doses that were a factor of 2 to
3 times smaller than surface doses accounted for therapeutic
procedures. Maximum surface doses for diagnostic examinations and therapeutic procedures, however, were similar.
Energy imparted and effective doses
Table III shows data for energy imparted resulted from
use of fluoroscopy and radiography. The median value of
energy imparted for a complete neuroradiologic procedure
was computed to be 6.7 J, with 66% on average due to radiographic acquisitions. An average of 70% of the energy
imparted resulted from use of the frontal projection plane.
Figure 4 shows the histogram distribution of the effective
FIG. 5. Histogram distribution of the total effective dose for 149 patients
undergoing interventional neuroradiologic procedures. Each histogram bin
on the x axis corresponds to 10 mSv.
dose to the patients from use of fluoroscopy and radiography.
The median values of effective dose were 10 mSv and 23
mSv for fluoroscopy and radiography, respectively. The
maximum computed values of effective dose were 76 mSv
for fluoroscopy and 114 mSv for radiography. Figure 5
shows the histogram distribution of the effective dose to patients from complete interventional neuroradiologic procedures. Computed effective doses for interventional neuroradiologic procedures had a median value of 36 mSv and a
maximum of 156 mSv.
Table IV provides a breakdown for the computed effective doses for diagnostic and therapeutic interventional procedures. The maximum effective doses computed for diagnostic and therapeutic procedures were similar. The median
effective doses, however, were two to three times larger for
therapeutic procedures in comparison to diagnostic examinations.
DISCUSSION
The accuracy and precision of measuring surface doses
with the patient dosimetry system used in this study was
estimated to be within 5% of the true surface dose by simulating a full diagnostic neuroradiologic examination.5,12 This
level of accuracy is satisfactory for the purpose of determining surface doses to patients undergoing interventional neuroradiologic procedures. Surface doses depicted in Figs. 1–3
and Table II were based on the assumption that the same skin
TABLE IV. Patient effective doses in mSv for diagnostic angiographic and
therapeutic embolization interventional neuroradiologic procedures.
Diagnostic
procedures
FIG. 4. Histogram distribution of the effective dose for 149 patients undergoing interventional neuroradiologic procedures. Black bars correspond
to effective doses from fluoroscopy and gray bars correspond to effective
doses from radiography. Each histogram bin on the x axis corresponds
to 10 mSv.
Medical Physics, Vol. 29, No. 5, May 2002
Imaging mode
Fluoroscopy
Radiography
Fluoroscopy⫹Radiography
Therapeutic
procedures
Median Maximum Median Maximum
dose
dose
dose
dose
9.8
19
33
75
76
152
22
41
74
76
114
156
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Gkanatsios, Huda, and Peters: Interventional neuroradiology
area was continually exposed to the x-ray beam. During interventional neuroradiologic procedures, a 20° to 30° rotation of the x-ray source in the sagittal plane of the patient
may be used when acquiring radiographic images. Although
the central axis of the x-ray beam changes position on the
surface of the head with rotation of the x-ray source, there
may be parts of the skin surface, which are continually exposed to the x-ray beam. Surface dose data presented in this
study should be considered as an upper limit of the patient
surface dose since they assume that there was a skin region
that was always in the direct x-ray beam.
There were no cases of epilation or skin erythema in this
series of 149 adult patients who underwent interventional
neuroradiologic procedures. The data shown in Fig. 3 共total
surface dose兲, however, indicate that up to 30% of the patients in this study exceeded the nominal threshold skin dose
for the induction of deterministic effects 共2 Gy兲. Observable
effects such as total epilation are more likely to occur at
doses in excess of 6 Gy.6 At lower doses, signs of erythema
would be fleeting and faint, which would make detection
difficult. Additional factors which could affect the threshold
radiation doses for the induction of deterministic effects include the anatomic location and size of the irradiated region,
tissue vascularity and oxygenation, as well as patient age,
genetic background and hormonal status.9,17,18 In addition, in
neuroradiologic examinations radiation doses are delivered
over an extended time period, which may be as long as several hours. This is likely to reduce the effectiveness of the
radiation due to antagonistic cellular repair processes.19
The median surface dose reported in this study may be
compared to the data available in the scientific literature.
Bergeron et al.20 measured a range of maximum surface
doses between 0.13 and 1.3 Gy in eight patients, with a mean
value of 0.62 Gy. Norbash et al.21 studied 12 typical interventional neuroradiologic procedures and measured a range
of 0.31–2.7 Gy to the skin surface of the head with a mean
value of 1.5 Gy, and showed that rotation of the x-ray tube
was effective in reducing patient surface doses. O’Dea
et al.22 reported the mean and maximum surface doses from
94 therapeutic embolization procedures to be 2.1 Gy and 8.8
Gy, respectively. The same study reported a mean of 0.94 Gy
and a maximum of 6.0 Gy for 160 cerebral angiographic
procedures. Detailed comparison with these studies is problematic due to differences in clinical cases and settings, imaging protocols and radiographic equipment. Nevertheless,
surface doses in interventional neuroradiologic examinations
are high enough to cause concern about induction of deterministic effects. Furthermore, patient doses appear higher
than in the past, which is likely a result of use of more
complicated and sophisticated protocols in therapeutic
neuroradiology.3
For most patients undergoing interventional neuroradiologic procedures, their maximum surface dose was below
the threshold dose for induction of deterministic effects. In
these cases, surface dose has major limitations as an indicator of radiation risk to the patient since surface dose does not
take into account either the x-ray beam quality 共i.e., penetrating power兲 or the size of the x-ray beam incident on the
Medical Physics, Vol. 29, No. 5, May 2002
721
patient. An alternative dose parameter to surface dose is energy imparted, or integral dose, which is straightforward to
measure or calculate.23,24 Energy imparted is a measure of
the total energy deposited in a volume 共i.e., head, chest, abdomen, etc.兲 from x-ray exposures and is generally taken to
be proportional to patient stochastic risk for a given type of
radiographic examination. The principal limitation of energy
imparted as a dosimetric parameter is the fact that it does not
account for radiosensitivity of the organs and tissues that are
being irradiated. The ease with which energy imparted can
be converted into a corresponding value of patient effective
dose, however, makes the latter ideal for quantifying patient
stochastic risks.
The median effective dose of 36 mSv reported in this
study may be compared with those in the scientific literature.
Feygelman et al.25 studied ten cases and reported values
ranging from 1.6 –14 mSv with a mean of 6.2 mSv. Bergeron
et al.20 reported an average of 1.8 mSv with a range of 0.44 –
3.4 mSv for a limited number of eight patients undergoing
similar procedures. McParland26 reported a median of 7.0
mSv with a range of 2.1–20 mSv when he computed effective doses to patients undergoing cerebral angiography. A
wider range was reported by Berthelson and Cederblad4 who
computed effective doses between 3.5 mSv and 25 mSv. As
in the case of surface doses, the most likely reason for higher
effective doses recorded in this study is the use of a biplane
imaging system, and more extensive and complicated procedures. Different imaging equipment, setup and imaging protocols can play a major role to how different effective doses
may be found among institutions. The fact that this study
was done at an academic institution that trains new neurointerventional radiologists may also account for some of the
differences in the effective doses recorded between this work
and others. Most of these examinations were performed by
three radiologists while training radiology residents and
some of the examinations were carried out by two fellows.
There were no significant differences between radiologists
and fellows that will indicate any obvious trends with years
of experience. However, there were some differences between the radiologists themselves that could indicate personal preferences in practice.
Effective doses for interventional neuroradiologic procedures may be compared to effective doses for other common
radiologic examinations that also use ionizing radiation. Effective doses for chest x rays are of the order of 0.05 mSv,
for skull radiographic examinations of the order of 0.2 mSv,
for abdominal radiographic examinations between 0.5 and
1.5 mSv, and for excretory urogram examinations between
2.5 and 5.0 mSv. Patients undergoing barium enemas receive
doses between 3.0 and 7.0 mSv, head CT scans measure
effective dose between 1 and 2 mSv, and body CT scans
about 5 mSv. A routine coronary angiographic procedure
measures between 3.0 and 6.0 mSv. The average nuclear
medicine procedure has an effective dose of 5 mSv. Interventional neuroradiologic effective doses are therefore markedly
higher than those normally encountered in diagnostic radiology. Patient doses in interventional neuroradiology can also
be compared with natural background in the United States 共3
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Gkanatsios, Huda, and Peters: Interventional neuroradiology
mSv/year兲, as well as regulatory dose limits for radiation
workers 共50 mSv/year兲 and for members of the public 共1
mSv/year兲.27–33
The data in Tables II and IV demonstrate that radiation
doses associated with therapeutic procedures are markedly
higher than those associated with diagnostic procedures. Median surface doses encountered in therapeutic procedures
were approximately a factor of 3 higher than those encountered in diagnostic procedures, and effective doses in therapeutic procedures were a factor of 2 higher when compared
to those in diagnostic procedures. Diagnostic neuroradiologic
procedures usually require the practitioner to catherize the
vessels to the brain only up to the level of the middle neck
and use a limited number of DSA acquisitions. Therapeutic
procedures require the catheters to be migrated to the specific
site of abnormal vasculature within the brain, which increases the complexity and procedure duration. The diagnostic complexity of therapeutic procedures requires far more
DSA acquisitions increasing the total dose to the patient
quickly. Therapeutic procedures also utilize much smaller
catheters 共0.59 mm diameter for therapeutic versus 1.65 mm
diameter for diagnostic procedures兲, which may require use
of magnification radiography and will result in elevated surface doses.34 As shown in Fig. 4, as the dose to the patient
increases implying a more complex procedure, most of the
dose comes as a result of radiographic 共DSA兲 exposures and
less of it is fluoroscopy related.
Quantitative radiation risk may be obtained for a given
value of effective dose, although the risk will be modified by
factors such as age and sex of the exposed individual. A risk
coefficient of 5⫻10⫺5 cancers and genetic abnormalities per
mSv of radiation dose was derived from the ICRP10 attempt
to estimate absolute stochastic risks from whole-body irradiation. The radiation risk associated with the median effective dose of 36 mSv is therefore of the order of 0.2%. The
risk from a neuroradiologic interventional procedure can be
compared to other every day risks. An effective dose of 36
mSv is comparable to the risk of dying from lung cancer
when smoking ⬃600 packs of cigarettes, or the risk of dying
in an automobile when driving a distance of
⬃30 000 miles. 19 These risk factors, however, need to be
treated with great caution. Since patients undergoing interventional neuroradiologic procedure are generally older, the
radiation risk could be significantly lower than the one obtained using the nominal ICRP risk coefficients.35 Of greater
significance is the uncertainty associated with the extrapolation of radiation risks from high doses to those normally
encountered in diagnostic radiology.36,37
It is important to note that the nominal radiation risk is
expected to be very small in comparison to the benefit to the
individual undergoing the interventional procedure, given
that the latter is performed to decrease the chance of future
or further neurologic damage, to improve the quality of life
of the patient or literally to save lives. As such, the risk
benefit ratio to the exposed individual is likely to be small.
The primary goal of the interventional neuroradiologist is to
ensure that the image quality associated with the fluoroscopic and radiographic exposures are adequate for the imMedical Physics, Vol. 29, No. 5, May 2002
722
aging task at hand. Diagnostic imaging information should
not compromise optimal patient care. Secondary objectives
should be to try to prevent the incidence of deterministic
effects of radiation and to minimize the resultant patient stochastic risk. These secondary objectives require that surface
doses do not exceed the threshold doses for the induction of
deterministic effects, and the elimination of all unnecessary
patient exposures. Application of these radiation protection
principles will ensure that patient risks are minimized without compromising the undoubted clinical benefits to the patient that can be achieved from interventional neuroradiology.
ACKNOWLEDGMENTS
The authors would like to acknowledge Jim Freeman and
Anita Loyd for overseeing the recording of data by the dosimetry system.
a兲
LORAD, A Hologic Company, 36 Apple Ridge Road, Danbury,
CT 06810.
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