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Ha, Jung, Chang, Park, and Shim: Effects of Iterative Reconstruction Algorithm, Automatic Exposure Control on Image Quality, and Radiation Dose: Phantom Experiments with Coronary CT Angiography Protocols
Original Article

Progress in Medical Physics 2015;26(1):28-35.

Published online: 20 March 2015

DOI: https://doi.org/10.14316/pmp.2015.26.1.28

Effects of Iterative Reconstruction Algorithm, Automatic Exposure Control on Image Quality, and Radiation Dose: Phantom Experiments with Coronary CT Angiography Protocols

Seongmin Ha∗, Sunghee Jung†, Hyuk-Jae Chang†, Eun-Ah Park‡, Hackjoon Shim§

∗Yonsei – Cedars-Sinai, Integrative Cardiovascular Imaging Research Center, Seoul, Korea

†Graduate School of Medical Sciences, College of Medicine, Yonsei University, Seoul, Korea

‡Department of Radiology, Seoul National University Hospital,§Toshiba Medical Systems Korea, Seoul, Korea

Correspondence: HackjoonShim (hackjoon.shim@toshiba-medical.co.kr) Tel:82-2-860-8046, Fax:82-2-855-8052

Received 3 February 2015       Revised 6 March 2015       Accepted 16 March 2015

Copyright © 2015 Korean Society of Medical Physics

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

In this study, we investigated the effects of an iterative reconstruction algorithm and an automatic exposure control (AEC) technique on image quality and radiation dose through phantom experiments with coronary computed tomography (CT) angiography protocols. We scanned the AAPM CT performance phantom using 320 multi-detector-row CT. At the tube voltages of 80, 100, and 120 kVp, the scanning was repeated with two settings of the AEC technique, i.e., with the target standard deviations (SD) values of 33 (the higher tube current) and 44 (the lower tube current). The scanned projection data were reconstructed also in two ways, with the filtered back projection (FBP) and with the iterative reconstruction technique (AIDR-3D). The image quality was evaluated quantitatively with the noise standard deviation, modulation transfer function, and the contrast to noise ratio (CNR). More specifically, we analyzed the influences of selection of a tube voltage and a reconstruction algorithm on tube current modulation and consequently on radiation dose. Reduction of image noise by the iterative reconstruction algorithm compared with the FBP was revealed eminently, especially with the lower tube current protocols, i.e., it was decreased by 46% and 38%, when the AEC was established with the lower dose (the target SD=44) and the higher dose (the target SD=33), respectively. As a side effect of iterative reconstruction, the spatial resolution was decreased by a degree that could not mar the remarkable gains in terms of noise reduction. Consequently, if coronary CT angiogprahy is scanned and reconstructed using both the automatic exposure control and iterative reconstruction techniques, it is anticipated that, in comparison with a conventional acquisition method, image noise can be reduced significantly with slight decrease in spatial resolution, implying clinical advantages of radiation dose reduction, still being faithful to the ALARA principle.

Keywords: Iterative reconstruction, AEC, Low-dose CT, CCTA, Phantom experiment

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Fig. 1.
Region of interest of phantom module. (a) spatial resolution: (a-1) thin wire part, (a-2) qualitative part, (a-3) water part, (b) (b-1) low-contrast part, (b-2) water part.
pmp-26-28f1.tif
Fig. 2.
Comparisons of tube current from AEC was adjusted dose. According to rise 80 kVp to 120 kVp, tube current was increased the tube current in order to adjust the optimal dose.
pmp-26-28f2.tif
Fig. 3.
Tube voltage according to applied AEC was compared with MTF of FBP and AIDR3D algorithm. (a) In MTF of 100% to 50%, the more right side be located, the more image was contained noise. (b) In MTF of 50% to 10%, the more right side be located, the more image was sharp.
pmp-26-28f3.tif
Fig. 5.
Noise rates of automatic exposure control options: (SD44/ SD33, reference: (44/33)≒1.33).
pmp-26-28f4.tif
Fig. 4.
Spatial resolutions of hole pattern with (a). Filtered back projection (FBP) and (b). Iterative reconstruction (AIDR 3D).
pmp-26-28f5.tif
Table 2.
Standard deviations and CNRs in phantom image of FBP and IR at each kVp.
Voltage Target SD Measured SD MeasuredCNR
FBP IR FBP IR
80 kVp SD33 10.2 6.8 8.8 11.0
  SD44 14.2 8.2 8.2 13.3
100 kVp SD33 10.8 6.2 11.2 16.4
  SD44 16.2 8.3 7.1 13.9
120 kVp SD33 9.9 6.2 9.1 14.2
  SD44 14.1 7.5 8.1 11.7

SD: standard deviation, CNR: contrast-to-noise ratio, FBP: filtered back projection IR: iterative reconstruction.

Table 1.
Radiation doses (CTDIvol, DLP and effective dose) at each kVp.
Voltage Target SD Tube current (mA) CTDIvol (mGy) DLP (mGy) Effective dose (mSv), k=0.014
80 kVp SD33 222 5.0 105.8 1.5
SD44 136 3.6 75.3 1.1
100 kVp SD33 96 4.5 95.4 1.3
SD44 53 2.6 55.4 0.8
120 kVp SD33 56 4.4 91.7 1.3
SD44 33 2.4 51.2 0.7

SD: standard deviation, CTDI: computed tomography dose index, DLP: dose length product.

Table 3.
Noise and CNR reduction rates of IR compared with FBP.
Voltage Target Noise reduction CNR improvement
SD from FBP from FBP
80 kVp SD33 33.3% 25.0%
  SD44 42.3% 62.2%
100 kVp SD33 42.6% 46.4%
  SD44 48.8% 95.8%
120kVp SD33 37.4% 56.0%
  SD44 46.8% 44.4%

SD: standard deviation, CNR: contrast-to-noise ratio, FBP: filterd back projection.

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