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Dose Reduction in Multidetector CT Using Attenuation-Based Online Tube Current Modulation

¹ Department of Radiology, Centre Hospitalier Universitaire de Charleroi, Boulevard Janson, 92, B-6000-Charleroi, Belgium.
² Statistical Unit, Institut de Recherche Interdisciplinaire en Biologie Humaine et Moléculaire, Université Libre de Bruxelles, Route de Lennik, 808, Brussels, B1070 Belgium.
³ Department of Radiology, Hôpital Erasme, Université Libre de Bruxelles, Brussels, B1070 Belgium.

Received September 18, 2002; accepted after revision February 26, 2003.

 
Address correspondence to D. Tack.

Abstract

Top Abstract Introduction Subjects and Methods Results Discussion References

 
OBJECTIVE. This study was designed to quantify the radiation dose saved by attenuation-based online tube current modulation applied to multidetector CT (MDCT) of the adult trunk as a function of effective milliampere-second (mAs) presets, sex, and body habitus.

SUBJECTS AND METHODS. One hundred twenty patients underwent MDCT of the trunk (60 thoracic, 60 abdominal) with an attenuation-based online tube current modulation. Consecutive acquisitions at standard and two lower effective mAs presets were obtained in each patient. Mean percentage effective mAs reductions were compared for each effective mAs preset, taking into account sex and body mass index.

RESULTS. Mean effective mAs reduction was 16.9% and 20.0% for the chest and the abdomen, respectively. Mean percentage effective mAs reductions were found to be significantly different for sex (chest, p = 0.003; abdomen, p = 0.002) but not significantly different for the different effective mAs presets or body mass index.

CONCLUSION. Attenuation-based online tube current modulation used with MDCT should be considered as a secondary tool of radiation dose reduction because it saves as much as 20% of the radiation dose on the adult trunk, regardless of initial mAs preset. However, initial decreases of mAs presets by the physician should be considered the primary tool for radiation dose reduction.

Introduction

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Approximately 60% of the radiation exposure from imaging modalities is attributable to CT, despite the fact that CT examinations represent only 15% of all X-ray–based imaging studies [1]. As clinicians develop familiarity with helical CT, the number of examinations with this particular modality is increasing [2, 3]. Unlike single-detector helical CT, multidetector CT (MDCT) is able to cover larger ranges within shorter acquisition times and with thinner X-ray beam collimations. These factors contribute to an increase in radiation dose delivered to the population [1].

Various strategies for decreasing the dose, including increased pitch and lowered milliampere-second (mAs) presets (i.e., low-dose CT), have been recommended [4, 5]. Because attenuation of X-rays in the trunk is inhomogeneous and varies with the projection angle, the tube current can be modulated during tube rotation as a function of the projection angle [6]. Applied to standard-dose single-detector CT, attenuation-based online tube current modulation provides approximately a 25% dose reduction with equal or even improved image quality, more homogeneous noise, and fewer attenuation artifacts than with constant tube current [7–10].

Because this modulation has not yet been investigated using MDCT or low-dose CT, the aim of our study was to quantify the relative reduction in radiation dose achieved by applying this algorithm to MDCT and as a function of effective mAs presets. Because the image quality and the diagnostic accuracy of low-dose CT are also determined by the patient's body habitus [4, 5], we investigated the possible influence of body mass index (BMI) on dose reduction [11]. Finally, because the distribution of body mass may depend on sex, we also investigated the possible influence of BMI on dose reduction.

Subjects and Methods

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From February to March 2002, 120 consecutive adults (69 men and 51 women) from 48 to 87 years old (mean age, 67 years) with stage IV malignancies were included in this study. Sixty patients underwent MDCT of the chest (41 men and 19 women) and 60 of the abdomen (28 men and 32 women). The BMI was calculated from data available in the patients' medical charts. For the entire study group, the mean BMI was 26.2 kg/m² (range, 18.0–48.7 kg/m²). The classification of patients into subgroups according to the recommendations of the National Institutes of Health [11] is shown in Figure 1. The study protocol was approved by our institutional review board, and oral informed consent was obtained from all patients.

View larger version (13K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1. —Bar chart shows classification of patients into groups according to body mass index (BMI), as defined by National Institutes of Health [11]. Classifications of patients into BMI subgroups were similar for multidetector CT (MDCT) of chest and abdomen. Underweight = < 18.5 kg/m2, normal weight = 18.5–24.9 kg/m2, overweight = 25.0–29.9 kg/m2, obese = 30.0–39.9 kg/m2, extremely obese = > 40.0 kg/m2. Black bars = patients who underwent MDCT of chest, white bars = patients who underwent MDCT of abdomen.

CT Examinations
CT scans were obtained using a commercially available helical scanner (Somatom Plus Volume Zoom, Siemens Medical Systems, Forchheim, Germany) with four rows of detectors. Patients were examined while in the supine position. A 52-cm scout view was first obtained at 80 kV and 50 mA, followed by three acquisitions at different effective mAs presets that are summarized in Table 1. As defined by Mahesh et al. [12], effective mAs corresponds to mAs divided by the pitch, whereas the pitch is defined by Silverman et al. [13] as the ratio between the table feed per rotation and the X-ray beam width.

For the three consecutive acquisitions, the scanned region ranged from the pulmonary apex to the costodiaphragmatic sulci for the chest and from the top of the liver to the symphysis pubis for the abdomen. The effective mAs presets were chosen to reach a total absorbed dose—expressed in weighted CT dose index—not higher than the reference doses recommended by the European Guidelines on Quality Criteria for Computed Tomography [14] and not exceeding 15.7 and 17.0 mGy for thoracic and abdominal examinations, respectively. The weighted CT dose index used in this study has already been corrected for the pitch in helical and axial scanning. It is equivalent to the new volume CT dose index, as recently introduced by the International Electrotechnical Commission [15].

Chest examinations were not enhanced, whereas abdominal examinations were performed after IV administration of 120 mL of contrast medium with 350 mg of iodine per milliliter (iobitridol, Xenetix 350, Guerbet, Roissy, France) injected at 2 mL/sec after a 70-sec start delay.

Tube Current Modulation
Noise in CT scans varies proportionately to the square root of the applied dose—that is, proportionately to the square root of the mAs product if the tube current is kept constant. Noise in CT scans is dominated by those projections in which attenuation is the highest. For a homogeneous object with a circular cross section, attenuation is constant over all projections, and all measured values contribute equally. However, for a nonhomogeneous object with a noncircular cross section, attenuation varies strongly—sometimes by more than three orders of magnitude [6, 10]. Noise in the data measured from high-attenuation projections (i.e., lateral direction) greatly influences noise level in the CT scans. This means that the dose for projections with relatively low attenuation (i.e., anteroposterior direction) can be reduced substantially without a measurable increase in image noise [6, 10]. The tube current should thus be decreased as a function of rotation angle whenever attenuation is low.

The commercially available current modulation software (Care Dose, Siemens Medical Systems) used during all acquisitions in this study is characterized by online monitoring of the attenuation and subsequent tuning of the tube current as a function of the projection angle with a delay of 360°. For projections with low attenuation, the maximal reduction of the tube current is 90%. For each acquisition, the CT unit calculates the arithmetic average effective mAs throughout the duration of the exposure. The mean effective mAs, as displayed on the CT scans, was recorded for further calculations.

Statistical Analysis
Mean percentage effective mAs reductions were compared for the three effective mAs presets taking into account sex and BMI for the thoracic and abdominal examinations, respectively. Analyses of variance for repeated measures, corresponding to each of the three different effective mAs presets, were performed with sex as the intersubjects factor and BMI as the covariate. The two-way interactions between dose, sex, and BMI were also investigated. Statistical significance for all tests was set at a p value of less than 0.05.

Results

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Effective mAs values and mean percentage effective mAs reductions obtained using the attenuation-based online tube current modulation are listed in Tables 2 and 3 for thoracic and abdominal examinations, respectively. The only statistically significant different mean percentage effective mAs reductions were obtained for sex (chest, p = 0.003; abdomen, p = 0.002) but not for the BMI (chest, p = 0.105; abdomen, p = 0.432) or the effective mAs preset (chest, p = 0.308; abdomen, p = 0.405).

When we compared the sex of patients, mean percentage effective mAs reductions were significantly higher in men than in women for the three effective mAs presets applied to the chest (p = 0.018, 0.001, and 0.004 for 20, 40, and 80 effective mAs presets, respectively) and were significantly lower in men than in women for the three mAs presets applied to the abdomen (p = 0.001, 0.001, and 0.004 for 30, 50, and 100 effective mAs presets, respectively).

This difference according to sex was found to be dependent on BMI for the chest (p = 0.001, 0.015, and 0.004 for 20, 40, and 80 effective mAs presets, respectively) but not for the abdomen (p = 0.388, 0.171, and 0.259 for 30, 50, and 100 effective mAs presets, respectively). The difference according to sex in thoracic examinations reached high statistical significance in the normal BMI subgroup of patients only (p = 0.001, < 0.001, and < 0.001 for 20, 40, and 80 effective mAs presets, respectively).

Discussion

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Our study shows that attenuation-based online tube current modulation applied to standard-dose MDCT reduces total effective mAs by 15–20%. Because Schmidt et al. [16] have shown in phantom studies that the reduction in effective mAs underestimates the real reduction in radiation dose by approximately 20%, the percentage of effective mAs reduction provided by the online tube current modulation represents the minimum dose saving. The mAs reduction obtained with MDCT is approximately 5% lower than that reported with single-detector CT [7–9]. With single-detector CT, the mAs reduction obtained with attenuation-based online tube current modulation ranges from 19% to 27% and from 11% to 24% for scans obtained through the whole chest and the upper abdomen, respectively [8]. Because only MDCT was used in the present study protocol, we could not compare the efficacy of online tube current modulation used with single-detector CT on the same patients. Nor were we able to verify whether this apparent 5% difference between the two modalities would have reached statistical significance. Nevertheless, lower effective mAs reductions obtained with MDCT may be related to a higher delay in adapting the tube current (360° with MDCT vs 180° with single-detector CT), narrower X-ray beam collimations (1–3 mm with MDCT vs 5–8 mm with single-detector CT), shorter rotation time, and differences in the cone-beam geometry. The effective mAs reduction could be even higher for scans focused on regions with larger differences in attenuation between anteroposterior and lateral projections such as the shoulder and the pelvis. With single-detector CT, the mAs reduction obtained with attenuation-based online tube current modulation ranges from 22% to 56% and from 21% to 30% for scans taken of the shoulders [10] and the pelvis [8], respectively.

In our study, we considered 80 and 100 effective mAs as standard presets [17–19], corresponding to mAs values of 160 and 150 mAs and to tube currents of 320 and 300 mA for the chest and the abdomen, respectively. These presets were set at levels in which no degradation of image quality had been observed in comparison with doubled values of mAs pressets as shown by Ravenel et al. [17] in the chest and by Kalra et al. [18] in the abdomen. The lowest effective mAs presets tested in our study are the ones currently used in screening programs [20, 21]. We have also considered the middle effective mAs preset used in our study as a possible low-dose preset suitable for patients who are overweight.

The effective mAs reduction obtained with attenuation-based online tube current modulation is independent of the effective mAs preset. Because this modulation saves up to only 20% of the dose, it cannot by itself save up to 90% of the dose as the so-called "low-dose" CT protocols do by lowering the effective mAs preset [4, 5, 19–21]. Nevertheless, when applied to low-dose presets, the attenuation-based online tube current modulation provides a 15–20% supplementary dose reduction [5, 20, 21].

The effective mAs reduction obtained with attenuation-based online tube current modulation is independent of BMI, indicating that normal weight and obese patients have similar ratios in attenuation between anteroposterior and lateral projections. In fact, tube current modulation does not adapt effective mAs settings to the body's diameter nor to the total absorption of X-rays by the patient's body. If CT parameters are maintained constant, independently of body size, the energy delivered to small individuals is lower than that delivered to larger individuals, but the effective dose delivered to sensitive organs is higher in smaller individuals [22, 23]. As suggested by Kalra et al. [18], the effective mAs preset should thus be adapted to the patient's body size (i.e., BMI) before applying attenuation-based online tube current modulation. This recommendation applies to pediatric patients as well.

The effective mAs reduction obtained with attenuation-based online tube current modulation varies also with sex. The mAs reduction was found to be 2% higher in men than in women for the chest and 3% higher in women than in men for the abdomen. However, even if statistically significant, these differences are clinically small. They indicate that men and women have subtle differences in attenuation between anteroposterior and lateral projections. In the chest CT only, this difference is related to BMI, probably as a result of a woman's breasts. Interestingly, sex differences (because of breasts) were seen only in patients of normal weight.

The software release used in our study to modulate online tube current was designed only as a function of the projection angle with a delay of 360°. It was not designed to tune the mAs as a function of the table position (z-axis). Therefore, the tube current was not modulated according to differences in absorption along the scanned region. Because Itoh et al. [24] have reported that the detection of lung nodules requires a tube current that may vary by 50% from the shoulders to the mid lung zones, the attenuation-based online tube current modulation along the cephalocaudal axis could be a technical advance able to reduce the radiation dose more adequately [25].

In conclusion, attenuation-based online tube current modulation used with MDCT reduces radiation dose by 15–20% in all patients, regardless of the initial effective mAs preset. As a consequence, attenuation-based online tube current modulation does not replace the reduction in mAs presets but should be considered as a supplementary tool to decrease the radiation dose.

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