We appreciate the comments of Mr. Jayasekera and Mr. Roach and acknowledge
that our methodology does not reflect a number of conventional techniques that
have been employed in the past during the routine use of mobile C-arm
fluoroscopy.
In our paper, we make note of several dose-reducing measures that have been
studied over the years and have enabled mobile C-arm operators to produce
high-quality images while optimizing the overall safety to the patient and
operating room staff. These measures include minimizing exposure time,
reducing exposure factors, manipulating the x-ray beam with collimation,
maximizing distance from the beam, using protective shielding, and imaging
with the C-arm in an inverted orientation relative to the specimen.
Positioning the phantom limb directly on the platform of the image
intensifier increases the distance from the radiation source to the specimen,
subsequently reducing the amount of scatter produced. Although many of these
measures have been studied with use of a standard large C-arm unit, the
literature regarding similar parameters with the mini C-arm unit is limited.
In our experimental design, we attempted to create a "best-case
scenario" by utilizing known dose-reducing techniques to quantify
radiation exposure just as a surgeon would likely strive to achieve in a true
operating room setting.
With regard to the second portion of the correspondents' comments, we point
out that at positions of 15 and 25 cm from a focal point on the phantom hand,
we found minimal radiation exposure (1 to 2 mrem) as measured with our
dosimeters. These measurements were made in the plane of the image
intensifier. In contrast, when the radiation dosimeter was placed directly in
the phantom hand, substantial exposure levels (181 to 272 mrem) were recorded.
We did not collect data points between these two locations.
We concur with Jayasekera and Roach that many orthopaedic trainees and, for
that matter, a great number of mini or large C-arm operators, have a poor
understanding of the science behind image intensifier usage. This may lead
them to grossly underestimate the potential for high-dose radiation exposure
if these mobile fluoroscopy units are not used judiciously and with proper
intent.
A common error made by novice trainees is the use of the mini C-arm to
image larger body parts such as the tibia, femur, humerus, elbow, or shoulder.
As the tissue density and cross sectional area of the imaging subject
increase, technique factors automatically adjust, in the normal mode, to
produce an image with optimal penetration and visual quality. To accommodate
for the increased tissue density of a larger body part, technique factors
increase by a substantial margin, leading to a much higher radiation exposure
rate than may have been encountered when using a large C-arm.
We appreciate the interest in our paper and strive to advance science
safety with the commonly used fluoroscopy units.