Effective dose and image quality evaluations of an automatic CT tube current modulation system with an anthropomorphic phantom

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Abstract

The purpose of this study was to evaluate the consequences of different choices of acquisition parameters on the actual image noise and on the patient dose with an automatic tube current modulation system. The CT investigated was a GE Lightspeed 16-slice and an anthropomorphic phantom was used to simulate the patient. Several acquisitions were made varying noise index (NI), kilovoltage and pitch values. Tube current values were compared for the different acquisitions. Patient dose was evaluated in terms of volumetric computed tomography dose index (CTDIvol) and also as effective dose. The noise actually present in the images was analyzed by a region of interest analysis considering representatively phantom sections in the regions of the shoulders, of the lungs and of the abdomen. The obtained results generally evidenced a good agreement between the noise index and the measured noise for the abdomen sections, whereas for the shoulders and the lungs sections the measured noise was respectively greater and lower of the NI. Varying the kV the automatic current modulation system provided images with a substantially constancy of the actual noise and of the patient dose. An increase of the pitch generally decreased the patient dose, whereas the noise was slightly greater for the lowest pitch and almost constant for the other pitch values. This study outlines some important relationships between an automatic tube current modulation system and other CT acquisition parameters, providing useful informations for the choice requested by radiologists in the task of optimization of the CT acquisition protocols. Unless there are other considerations in place, pixel pitches below 1.375 should be avoided, and kVp settings can be changed with no real impact on dose or image noise.

Introduction

The rapid technological evolution of the computed tomography (CT) equipments challenges the radiologists community to continuously optimize acquisition protocols, with the goal of obtaining diagnostic images with the lowest achievable patient dose. In the last years, one of the most important improvements was the implementation of automatic tube current modulation systems [1], [2]. With this system it is possible to set in the acquisition protocol the desired level of image quality. Based on this parameter the tube current is automatically defined in order to obtain a constant level of noise over all the scanned regions. As a consequence one of the most important parameter is now the value of the “desired noise” that is specified in different ways for the different CT manufacturers. Depending on the software, the modulation system varies the current along the Z-axis direction or in the XY plane keeping into account the different thickness and attenuation property of the patient in the antero-posterior and lateral directions. Several studies on both phantoms and patients have demonstrated the efficacy of these systems in reducing patient dose [3], [4], [5], [6], [7], [8].

However, few published works investigated the behaviour of these automatic current modulation systems when other acquisition parameters are changed. With a current modulation system the dose and image quality dependences on other acquisition parameters, like the kilovoltage or the pitch value, are different from the case of constant current and investigation of different acquisition protocols should keep into account this fact. Goo and Suh [9] explored the consequence of using different kV values and found differences in terms of volumetric computed tomography dose index (CTDIvol) and mAs between the unmodulated and the modulated scans. Analysis of how the beam width and the pitch factor selection affect the efficacy of the tube current modulation has been done by Papadakis et al. [10]. However, as outlined in Section 4 of these studies, the results are strictly valid for the CT used and the associated tube current modulation systems. The different manufacturers adopted different solutions to modulate the tube current and, as a consequence, it is important to study how the different acquisition parameters influence the dose reduction achievable by tube current modulation for the system in use.

The purpose of this study was to evaluate the effective dose and the actual noise present in the CT images of an anthropomorphic phantom when an automatic tube current modulation system is used with different values of the other acquisition parameters.

Section snippets

Materials and methods

CT scans were performed with a multislice CT (Lightspeed 16Pro, GE, Milwakee, WI, USA). The “AutomA” option was set for all the scan in order to have the longitudinal tube current modulation. This modulation adjusts tube current to maintain an operator-defined noise level in the reconstructed images that is predominately independent of patient size and anatomy. The parameter used to specify the desired image quality was the noise index (NI). The NI value is specified by the vendor as

Results

Part of the figures presented in this study shows the tube current time product (mAs) versus the longitudinal coordinate Z. The rotational time is obviously constant, so the trend shown corresponds to the tube current trend. We choice to represent the mAs values and not the mA values in order to facilitate comparisons between acquisition performed with a different choice of the rotational time.

Fig. 2(a) shows the mAs trend along the longitudinal axis for the standard protocol. The highest tube

Discussion

Tube current modulation systems are an effective tool to reduce patient dose without compromising the image quality. However, also other acquisition parameters affects the dose and the image quality and the optimization of their values should be investigated. In this study, the consequences of different tube voltages and pitch selections on the patient dose and image quality were explored for a particular tube current modulation system.

Results showed that for an anthropomorphic adult phantom

Acknowledgements

The study was supported, in part, by grants from the project “Riduzione del rischio associato all’esposizione a radiazioni ionizzanti per fini medici - Compagnia San Paolo di Torino”.

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