Evaluation of the effect of inappropriate patient centering in MDCT of neck on patient dose and image diagnostic quality using combined angular and z-axis automatic tube current modulation system : a phantom study

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Evaluation of the effect of inappropriate patient centering in MDCT of neck on patient dose and image diagnostic quality using combined angular and z-axis automatic tube current modulation system : a phantom study

 

Author: Lam, Siu-yan Ivan
Title: Evaluation of the effect of inappropriate patient centering in MDCT of neck on patient dose and image diagnostic quality using combined angular and z-axis automatic tube current modulation system : a phantom study
Degree: M.Sc.
Year: 2011
Subject: Hong Kong Polytechnic University -- Dissertations
Neck -- Tomography
Department: Dept. of Health Technology and Informatics
Pages: xiv, 113 leaves : ill. (some col.) ; 30 cm.
InnoPac Record: http://library.polyu.edu.hk/record=b2411441
URI: http://theses.lib.polyu.edu.hk/handle/200/5884
Abstract: An awareness of Multi-detector Computed Tomography (MDCT) radiation dose and its potential complications has led to the development of several dose reduction strategies in MDCT of neck. Two of these strategies are automatic tube current modulation (ATCM) systems and radioprotective shields for superficial radiosensitive organs, such as the thyroid gland. Inappropriate patient centering leads to misoperation of ATCM system which controls the tube current on the basis of information obtained from scanograms and causes increase in patient dose and image noise in abdomen and pelvis MDCT. The aim of our study was to evaluate the effectiveness of the automatic tube current modulation (ATCM) systems, use of thyroid shield and manipulation of other scanning parameter such as the preset noise level and reconstruction thickness of image on dose reduction and image quality of MDCT of neck in different extent of off centerings. A RANDO phantom was used which is built on a human skeleton with materials equivalent to human soft tissue. The thermoluminescent dosimetry (TLD) chips were placed in the phantom neck at anterior thyroid region. Scans 1-8 were done in 5 times for different inappropriate phantom centerings (-30, -10, isocenter, +10 and +30 cm). All other scanning parameters were kept unchanged. In turn, the three outcome variables include the CT numbers (Hounsfield units [HU]), quantitative image noise (standard deviation of the CT numbers), and the radiation dose as measured by the TLD were evaluated. Our result showed that the increase of radiation dose due to inappropriate phantom centering might not improve image quality, but led to the misoperation ATCM system. The result suggested that as the preset noise level increased, the dose delivered to patient decreased we found that as the preset noise level changed from 7.5 to 10 (33.3 % increase) at isocenter position, thyroid dose decreased 30 %. Results also suggested that the use of thyroid shielding affected the calculation of CT number by the scanner and interrupted the correlation with the different phantom positions as well. It showed that as the centering of phantom elevated above the isocenter, the CT number decreased, as the centering of phantom lowered below the isocenter, the CT number increased where there were no thyroid shield applications. There were significances of the correlation of the CT number with different phantom positions, the Spearman’s rho ranged from -0.749 to -0.901 (p= 0.00). Therefore, use of thyroid shield in examination which stress on the importance of accurate CT number measurement is not recommended. Moreover, correct and consistent positioning of patient in MDCT neck examination is important to facilitate proper ATCM operation. Vendor specific scanner geometry, bowtie filter and reconstruction algorithms also restrict scan parameters to change too much, such as Scan field of view (SFOV) and phantom position in y-axis direction. In addition, appropriate scanning parameters selection, such as reconstruction thickness, preset noise level SD and reconstruction algorithms help controlling radiation dose delivered to patient minimum.

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