Organ dose and inherent uncertainty in helical CT dosimetry due to quasiperiodic dose distributions

Purpose : The purpose of this study was to characterize relative magnitudes of peripheral dose modulations present in helical MDCT resulting from variation in x-ray tube starting position and phantom position relative to isocenter using a novel methodology that employs direct dosimetric measurements...

Full description

Saved in:
Bibliographic Details
Published in:Medical physics (Lancaster) 2011-06, Vol.38 (6), p.3177-3185
Main Authors: Winslow, J. F., Tien, C. J., Hintenlang, D. E.
Format: Article
Language:English
Subjects:
Anatomy
anthropometry
anthropomorphic phantom
biological tissues
Collimation
Computed tomography
computerised tomography
diagnostic radiography
diagnostic radiology
Distribution theory and Monte Carlo studies
dose aliasing
dose reduction
dosimetry
Dosimetry/exposure assessment
eye
eye lens
fiber-optic-coupled dosimetry
helical computed tomography
isocenter
Medical image quality
Medical imaging
Medical X‐ray imaging
Monte Carlo methods
pediatric
periodic dose distribution
phantoms
Physiological optics
pitch
point dosimetry
Radiation Dosage
Radiometry
solid scintillation detectors
thyroid
Time Factors
Tissues
Tomography, X-Ray Computed - methods
Uncertainty
Vacuum tubes
X‐ray imaging
Citations: Items that this one cites
Items that cite this one
Online Access:Get full text
Tags: Add Tag
No Tags, Be the first to tag this record!
Description
Summary:Purpose : The purpose of this study was to characterize relative magnitudes of peripheral dose modulations present in helical MDCT resulting from variation in x-ray tube starting position and phantom position relative to isocenter using a novel methodology that employs direct dosimetric measurements and knowledge of scan geometry. The magnitudes of potential dose savings to specific radiosensitive tissues related to the phase of the quasiperiodic dose distribution are also quantified and compared to similar Monte Carlo based studies. Methods : For this study, a Siemens SOMATOM Sensation 16 helical MDCT scanner and a tomographic adult anthropomorphic phantom from the University of Florida phantom series were used for all scans. In addition, a plastic scintillator-based fiber-optic-coupled dosimetry system was used to record real-time axial dosimetric measurements. These direct measurements were used to derive cumulative point doses and tissue doses for helical MDCT using different pitch values and surface to isocenter distances. Results : Cumulative point doses and doses for the lens of the eye and thyroid showed strong variation with both the phase of the dose distribution and phantom positioning relative to isocenter. Depending on the phantom positioning relative to isocenter, individual in-phantom cumulative point dose values were shown to vary anywhere from 0 to 25% lower than the maximum value for scans of pitch 1, and greater than 60% for scans of pitch 1.5. Reduction in total tissue dose to the lens of the eye (thyroid) varied from 0 to 20% (4%) lower than the maximum value for pitch 1, and from 59 (14%) to 71% (19%) lower than the maximum value for scans using pitch 1.5. These values are similar to those found in previous Monte Carlo based studies. The reduction in average total tissue dose between the extremes (±3 cm from nominal) of phantom positioning relative to isocenter for the eye (thyroid) was 16% (13%) for pitch 1, and 14% (12%) for pitch 1.5. Conclusions : As recent Monte Carlo simulations have shown, there exists an inherent uncertainty when performing dose measurements within a phantom during helical MDCT scans. The periodic dose distributions in helical MDCT means that low resolution sampling of local phantom doses could result in dose measurement aliasing. For reliable results, these considerations should be accounted for in helical MDCT phantom dosimetry studies. The variability in surface dose has a strong dependence on phantom posit
ISSN:0094-2405
2473-4209
DOI:10.1118/1.3590367