| Abstract |
Purpose
The aim of the present study was:
a) to provide a comprehensive evaluation of the dosimetric characteristics of a 16-
slice scanner.
b) to provide a method and required data for the estimation of effective dose (E)
values to adult and pediatric patients from computed tomography CT scans of the
head, chest abdomen, and pelvis, performed on multi-slice scanners.
c) to provide normalized effective doses to adult and pediatric patients from
examinations of the head and trunk, corresponding doses from typical protocols.
d) to investigate the effect of acquisition and reconstruction parameters on the patient
radiation burden in order to facilitate the optimization of acquisition toward the
minimization of radiation doses.
Materials and Methods
a) Standard CT dose measurements were performed on a Siemens Sensation 16
scanner. Specifically, we have performed an extensive series of standardized CT
measurements and analyzed the effect of technique factors on fundamental CT
radiation dose parameters. CT dose indices, free-in-air (CTDIF) and weighted
(CTDIW), were measured in all available axial and helical beam collimations of the
head and body scanning modes. The effect of tube current, high voltage, rotation time,
beam collimation and pitch on CT doses was investigated.
b) Mean section radiation dose (dm) to cylindrical water phantoms of varying radius
normalized over CT dose index free-in-air (CTDIF) were calculated for the head and
body scanning modes of the multi-slice scanner with use of Monte Carlo techniques.
Patients were modelled as equivalent water phantoms and the energy imparted (ε) to
simulated pediatric and adult patients was calculated. Body region specific energy
imparted to effective dose conversion coefficients (Ε/ε) for adult male and female
patients were generated. Effective doses normalized over tube load for adult and
pediatric patients were derived for all available axial beam collimations, on the basis
of measured CTDIF values.
c) effective dose values normalized to computed tomography dose index measured
free-in-air were calculated for adult, newborn, 1, 5, 10 and 15 year old patients
regarding helical scans of the head, chest, abdomen, pelvis, abdomen and pelvis, and
trunk, using the energy imparted method. The effect of z-overscanning on patient
doses was accounted for, and normalized doses are provided for varying beam
collimation, pitch and reconstruction slice width values.
Results
a) CT doses increased as a power function of high voltage: CTDI = C (kVp)n. The
kVp exponent n varied with beam collimation from 2.7 to 3.1 for CTDIW, and from to
2.4 to 2.6 for CTDIF. Automatic change of the focal spot size increased radiation
doses up to a factor of 1.18 Measured small-focus CTDIW values differed from those
displayed at the console from –24% to 14%. Peripheral doses in the head phantom
were higher compared to the body phantom by a factor of 1.5 to 2. Central doses were
2.7 to 4.1 times higher. Due to the overbeaming effect, differences in beam
collimation resulted in 50% variation in the CTDIW in the body phantom and 60% in
the head phantom.
b) Depending on high voltage, body region and patient sex, E/ε values ranged from
0.011 mSv/mJ for head scans to 0.025 mSv/mJ for scans of the pelvis. When scanned
with the same technique factors as the adults, pediatric patients absorb as little as 5%
of the energy imparted to adults, but corresponding effective dose values are up to a
factor of 1.6 higher. On average, pediatric patients absorb 44 % less energy per
examination but have a 24 % higher effective dose, compared with adults. In clinical
practice, effective dose values to pediatric patients are 2.5 to 10 times lower than in
adults due to the adaptation of tube current.
c) The contribution of overscanning depends on patient age, anatomic region imaged,
acquisition and reconstruction settings. For a head scan it constitutes 15% of the
adult effective dose and 24% of the effective dose to a newborn but for an abdomen
scan it may be as high as 58% for a newborn and 31% for an adult. The ratios of
normalized pediatric doses relative to that for adults for helical scans depend not only
on age but also on acquisition and reconstruction parameters, because of variations in
the relative distance between the primary beam and the radiosensitive tissues/organs
of the body. Regarding scans of the trunk, pediatric doses are up to a factor of 2.5
times higher compared to adult doses (abdominal scans), whereas for scans of the
head up to a factor of 1.5. Increasing the pitch value of helical scans while
maintaining the same effective mAs setting, and hence noise levels, leads to an
increase in patient doses which depends on age, body region, scan and reconstruction
parameters. The % difference between doses at pitch 1.5 and pitch 1 is more
pronounced in the abdominal region (14% increase for adults) and in young patients
(31% in a newborn and 18% in a 10 year old patient) and it is minimal in head scans
(4% increase in newborns and 1% in adults). If multiple body regions are to be
imaged, doses to adults can be reduced by up to 15% and 36% to children by
performing single long-range scans. Scanning adult patients at 100 kVp instead of 120
kVp, results in a 32% reduction in effective dose from head scans and 38% for scans
of the torso. The corresponding reduction for a 5 year old patient is 31% for the head
and 37% for the trunk. Due to the combined overbeaming and overscanning effect the
24 mm collimation is more dose effective in the head mode and the 12 mm
collimation in the body mode.
Conclusions
a) Οur study has confirmed the great impact of technique factors and acquisition
parameters on CT doses. Derived data elucidate the relative effects of scanning
parameters on administered doses and enable the comparison of CT scanners and
scanning protocols.
b) A method is provided for the calculation of effective dose to adult and pediatric
patients on the basis of individual patient characteristics such as sex, mass,
dimensions and density of imaged anatomy, and the technical features of modern
multi-slice scanners.
c) Provided comprehensive dosimetric data will facilitate the dose effective use of the
scanner studied. It allows the optimum selection of scanning parameters regarding
patient doses at CT. Provided data enable informed design of examination protocols,
calculation of effective dose values and familiarization with the technical features of
multi-detector technology.
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