Technical approaches to the optimisation of CT

Abstract

This paper reviews current technical approaches to the optimisation of CT practice, i.e. approaches to reduce patient dose to the necessary minimum. The most important step towards this goal appears to be the technology of tube current modulation (TCM), which came into practice in the early 2000s and has become the standard approach recently. Anatomy- or attenuation-based TCM allows for a dose reduction between 10 and 60% as compared to scans with constant tube current. Automatic exposure control (AEC) approaches are the next step; based on TCM technology, AEC adapts the tube current both with the rotation angle α (α-modulation) and along the z-axis (z-modulation) to achieve a pre-selected image quality level at minimal dose. To pre-select the image quality level, i.e. primarily the pixel noise level, tools for simulation are important to investigate the necessary noise levels pro- and retrospectively for given cases and diagnostic tasks. Respective “dose tutor” approaches have become available recently and are presented. The most recent technical innovation which may lead to substantial dose reduction is the investigation of optimal spectra taking the type of contrast and 3D dose distributions into account. A high potential has been shown especially for pediatric CT and for thoracic CT where dose reduction of a factor of 2 and more is possible when using reduced tube voltages.

Introduction

In the 1980s, X-ray computed tomography (CT) was considered a mature technique which had reached its peak. And it was expected at the time that it would soon be replaced completely by magnetic resonance imaging (MRI). This expectation also reflected the general desire that patient exposure to ionizing radiation should be reduced or omitted completely whenever diagnostic alternatives were present. This followed the ALARA (“as low as reasonably achievable”) principle [1] which is the background and motivation for the work reported below.

As is well known today, CT is still around. The introduction of spiral CT in the late 1980s [2] led to a renaissance of CT and to renewed investments in CT technology, in particular to the development of array detectors. Four-slice acquisition systems became available in 1998, followed by 16-slice and 64-slice systems in 2001 and 2004, respectively. The combination with rotation times being reduced from 1 s in 1990 to 0.3–0.4 s today have led to a remarkable increase in technical performance [3]. Whole body scanning in 10–30 s at isotropic resolution levels of 0.4–0.6 mm, CT angiography in a few seconds and cardiac CT and coronary CT angiography with temporal resolution of 0.1–0.3 s are routine today. This resulted in the re-establishment of CT as currently the most important diagnostic modality in radiology and, in consequence, the frequency of CT examinations increased continuously over the past years [3], [4], [5]. Not surprisingly, concern about increasing patient dose grew in synchrony with the increasing use of CT.

It is a fact that CT today is the single modality which contributes 50% and more to the per capita exposure of the population from medical use of ionizing radiation [6], [7]. This is due to the increasing number of examinations. It is not true, however, that the CT dose per examination increased. Quite the contrary, it has stayed relatively stable or, as shown in several examinations and studies, the average dose decreased. For Germany the average effective dose per CT examination today is quoted at 10 mSv [6].

Nevertheless, there is high motivation and great endeavour to reduce doses resulting from CT examinations further on, and there are many organisations and agencies which encourage respective efforts. The European Union, for example, supported and still supports these goals and initiated respective research within their framework program 6 (FP6). Essential parts of the work presented in this paper were supported by a major FP6 grant as a multi-center project under the title “Safety and efficacy of computed tomography: a broad perspective” [8]. A complete overview on the project was given in a special symposium at the X European Congress of Medical Physics in Castelvecchio, Italy, in September 2007 and can be reviewed on the consortium's website www.msct.eu.

One selected symposium presentation reviewed technical approaches to the optimisation of patient dose in CT and is summarized in this article. The focus is on four recent developments which are considered the most significant and presented in the next four subchapters. Fortunately there is no limitation to further creative or simply professional developments such as optimisation of X-ray filtration, the development of better collimation schemes or more efficient detector systems with respect to both absorption and geometric efficiency. It is the authors' conviction that, based on all these developments, the dose per patient examination can be reduced significantly further on.