Setting up computed tomography automatic tube current modulation systems

Automatic tube current modulation (ATCM) on CT scanners can yield significant reductions in patient doses. Modulation is based on x-ray beam attenuation in body tissues obtained from scan projection radiographs (SPRs) and aims to maintain the same level of image quality throughout a scan. Noise level is important in judging image quality, but tissues in larger patients exhibit higher contrast resulting from the presence of fat. CT scanner manufacturers use different metrics to assess image quality. Some employ a simple measure of image noise, while others adopt a measure related to a reference image that accepts higher noise levels in more attenuating parts with higher contrast. At the present time there is no standard method for testing ATCM. This paper reviews the operation of different ATCM systems, considers options for testing, and sets out a framework that could be used for optimizing clinical protocols. If dose and image quality can be established for a reference phantom, the modulation performed by ATCM systems can be characterised using anatomical phantoms or geometrical elliptical phantoms which may be conical or include sections of varying dimension. For scanners using a reference image or mAs, selection of the image quality reference determines other factors. However, for scanners using a noise reference, a higher noise level should be selected for larger patients to avoid high doses, and the operator should ensure that appropriate limits are set for mA modulation. Other factors that need to be considered include the SPRs used to plan the ATCM and image thickness. Users should be aware of the mode of operation of the ATCM system on their CT scanner, and be familiar with the effects of changing different protocol parameters. The behaviour of ATCM systems should be established through testing of each CT scanner with suitable phantoms during commissioning

Influence of CT automatic tube current modulation on uncertainty in effective dose

Computed tomography (CT) scanners are equipped with automatic tube current modulation (ATCM) systems that adjust the current to compensate for variations in patient attenuation. CT dosimetry variables are not defined for ATCM situations and, thus, only the averaged values are displayed and analysed. The patient effective dose (E), which is derived from a weighted sum of organ equivalent doses, will be modified by the ATCM. Values for E for chest–abdomen–pelvis CT scans have been calculated using the ImPACT spreadsheet for patients on five CT scanners. Values for E resulting from the z-axis modulation under ATCM have been compared with results assessed using the same effective mAs values with constant tube currents. Mean values for E under ATCM were within ±10 % of those for fixed tube currents for all scanners. Cumulative dose distributions under ATCM have been simulated for two patient scans using single-slice dose profiles measured in elliptical and cylindrical phantoms on one scanner. Contributions to the effective dose from organs in the upper thorax under ATCM are 30–35 % lower for superficial tissues (e.g. breast) and 15–20 % lower for deeper organs (e.g. lungs). The effect on doses to organs in the abdomen depends on body shape, and they can be 10–22 % higher for larger patients. Results indicate that scan dosimetry parameters, dose–length product and effective mAs averaged over the whole scan can provide an assessment in terms of E that is sufficiently accurate to quantify relative risk for routine patient exposures under ATCM

Application of Gafchromic film in the study of dosimetry methods in CT phantoms

Gafchromic film has been used for measurement of computed tomography (CT) dose distributions within phantoms. The film was calibrated in the beam from a superficial therapy unit and the accuracy confirmed by comparison with measurements with a 20 mm long ionisation chamber. The results have been used to investigate approaches to CT dosimetry. Dose profiles were recorded within standard CT head and body phantoms and scatter tail data fitted to exponential functions and extrapolated to predict dose levels in longer phantoms. The data have been used to simulate both CT dose index (CTDI) measurements with ionisation chambers of differing length and measurements of cumulative doses with a 20 mm chamber for scans of varying length. The results show that the length of a pencil ionisation chamber is the most significant factor affecting measurements of weighted CTDI (CTDIw) and a 100 mm chamber would record 50–61% of the dose measured with a 450 mm one. The cumulative dose measured at the centre of a 150 mm long body phantom records over 70% of the equilibrium dose from a helical scan of a longer phantom. For routine CT dosimetry tests, the determination of correction factors could allow measurements with a 100 mm chamber to be used to derive the CTDI that would be recorded with a longer chamber, and cumulative doses measured with a 20 mm chamber in shorter phantoms to be used to calculate equilibrium doses for helical scans

Relationships between patient size, dose and image noise under automatic tube current modulation systems

Automatic tube current modulation (ATCM) systems are now used for the majority of CT scans. The principles of ATCM operation are different in CT scanners from different manufacturers. Toshiba and GE scanners base the current modulation on a target noise setting, while Philips and Siemens scanners use reference image and reference mAs concepts respectively. Knowledge of the relationships between patient size, dose and image noise are important for CT patient dose optimisation. In this study, the CT patient doses were surveyed for 14 CT scanners from four different CT scanner manufacturers. The patient cross sectional area, the tube current modulation and the image noise from the CT images were analysed using in-house software. The Toshiba and GE scanner results showed that noise levels are relatively constant but tube currents are dependent on patient size. As a result of this there is a wide range in tube current values across different patient sizes, and doses for large patients are significantly higher in these scanners. In contrast, in the Philips and Siemens scanners, tube currents are less dependent on patient size, the range in tube current is narrower, and the doses for larger patients are not as high. Image noise is more dependent on the patient size