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Detection of Coronary Calcifications: Feasibility of Dose Reduction with a Body Weight–Adapted Examination Protocol

¹ Department of Diagnostic Radiology, University of Technology, Aachen, University Hospital, Pauwelsstr. 30, D-52074 Aachen, Germany.
² Siemens Medical Solutions, Siemensstr., D-91301 Forchheim, Germany.
³ Medical Clinic I, University of Technology, Aachen, University Hospital, D-52074 Aachen, Germany.

Received November 14, 2002; accepted after revision January 28, 2003.

 
Address correspondence to A. H. Mahnken.

Abstract

Top Abstract Introduction Subjects and Methods Results Discussion References

 
OBJECTIVE. The purpose of this study was to assess the applicability of individual body weight–adapted tube current time settings in multidetector CT for detection of coronary calcifications and to evaluate the effect of reducing the radiation dose on the coronary calcium score.

SUBJECTS AND METHODS. One hundred patients underwent retrospectively ECG-gated MDCT for detection of coronary calcifications. First, fixed tube current time settings were used in 50 patients. Second, image noise corresponding to body weight–adapted tube current time settings was added to these images. Finally, body weight–adapted tube current time settings were applied to another 50 patients. For each patient group, the radiation dose was calculated. Coronary calcium scores were compared for the patient groups with the fixed tube current time settings with and without artificially added image noise. In all image series, image noise was assessed by a region-of-interest methodology. Image noise was analyzed using a regression analysis.

RESULTS. The effective radiation dose was reduced by 11.6% for men and 24.8% for women using the body weight–adapted tube current time settings. There were no statistically significant changes in the coronary calcium score after the addition of artificial image noise (p = 0.84). Adaptation of the tube current time settings did not lead to a relevant increase in image noise. The radiation doses for the plotted noise-to-body weight (slope, 0.081) and noise-to-body mass index (slope, 0.378) ratios for the standard protocol proved relatively high for patients of lower weight. An improved noise-to-body weight (slope, 0.054) and noise-to-body mass index (slope, 0.190) ratio was achieved by application of the body weight–adapted tube current time settings, resulting in nearly constant image noise related to body weight.

CONCLUSION. Individual body weight–adapted current time settings are applicable for coronary calcium scoring without a change of the coronary calcium score or relevant increase of the image noise.

Introduction

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Coronary calcifications are known to be a constituent of arteriosclerosis. They are regularly found in coronary artery disease. Therefore, detection and quantification of coronary calcifications were accepted as a diagnostic tool in the diagnosis of coronary artery disease. Although electron beam CT is accepted as a reference standard for detection and quantification of coronary calcifications, single-detector helical CT and multidetector CT (MDCT) are widely used to detect coronary calcifications [1–3]. Especially for fast subsecond MDCT scanners, a good correlation with electron beam CT was proven for detection and quantification of coronary calcifications [3, 4]. In particular, retrospectively ECG-gated MDCT with overlapping image reconstruction showed good reliability with high reproducibility [5]. However, ECG-gated MDCT requires a higher radiation dose when compared with electron beam CT [6]. Because the detection of coronary calcifications is considered a screening tool, radiation dose has to be kept as low as reasonably achievable [7].

Tube current modulation is the most recent approach for dose reduction in cardiac MDCT [8, 9]. However, body weight–adapted tube current time settings proved useful in abdominal and thoracic CT [10, 11]. To the best of our knowledge, body weight–adapted tube current time settings has not yet been evaluated for ECG-gated MDCT of the heart. Therefore, the aim of this study was to assess the feasibility of a simple directive for individual adjustment of the tube current time settings for the detection and quantification of coronary calcifications and to evaluate the effect of reducing the radiation dose on the coronary calcium score.

Subjects and Methods

Top Abstract Introduction Subjects and Methods Results Discussion References

 
A total of 100 consecutive patients (66 men, 34 women; mean age, 63.7 ± 9.9 years) referred for detection and quantification of coronary calcifications was entered into a prospective trial. Patients with a body weight greater than 100 kg were excluded from the study. All the patients included in this study underwent retrospectively ECG-gated MDCT of the heart because of suspected coronary artery disease. Personal data including body weight and body height were recorded.

All examinations were performed with a four-channel MDCT scanner (Somatom Volume Zoom, Siemens, Forchheim, Germany) during a single breath-hold of 11.1–22.6 sec (median, 15.8 sec). We used a standardized examination protocol with a collimation of 4 x 2.5 mm, a table feed of 3.8 mm per rotation (pitch, 0.38), and a tube rotation time of 500 msec. For image reconstruction, a field of view of 180 x 180 mm with a 512 x 512 reconstruction matrix and a medium smooth convolution kernel (B35f) were chosen. Temporal resolution of the image reconstruction algorithm was 125–250 msec, depending on the patient's heart rate. In patients with a heart rate of 65 beats per minute or less, the temporal resolution was 250 msec fixed. In patients with a higher heart rate, the temporal resolution ranged between 125 and 250 msec depending on the heart rate of the patient at the time of imaging [12]. In every patient, the axial images with and without overlap were reconstructed at 60% of the R-R wave interval [13]. For the first image series, an effective slice thickness of 3 mm and a reconstruction increment of 3 mm were applied, whereas for the second image series, a reconstruction increment of 2 mm was used resulting in a 33% overlap. Raw data sets of all examinations were stored on a CD-ROM.

The initial 50 patients (group 1.0) were examined with a standard protocol (tube voltage, 140 kV; tube current effective, 133 mAs_eff) on the basis of the recommendations of the supplier (Somatom Volume Zoom Application guide, Special Protocols Software version A20, Siemens, 1999:17). To test the applicability of body weight–adapted tube current time settings and to rule out a variation of the coronary calcium score caused by changes in the tube current time settings, we reconstructed all images of group 1.0 twice. After the standard reconstructions (group 1.0), random quantum noise was added to the data [14], simulating a reduction of the current time product to a value equal to the body weight measured in kilograms plus 33 mAs_eff. These data sets were used for a second reconstruction (group 1.1). For these operations, the raw data were transferred to a commercially available PC equipped with an image reconstruction tool (CardioRecon 6, Siemens). All other image reconstruction parameters were kept constant.

For quantification of coronary artery calcifications, we transferred all reconstructed images to an external workstation (3D-Virtuoso, Siemens). For each image series, the coronary calcium score was determined by applying the method described by Agatston et al. [15], using a threshold of 130 H.

After successful simulation of body weight–adapted tube current time settings, another 50 patients were examined using the same tube voltage as in group 1.0, but with body weight–adapted tube current time settings (kilograms plus 33 mAs_eff) (group 2.0). Both groups were comparable with respect to patient distribution (Table 1). The effective radiation dose according to the International Commission on Radiological Protection [7] was calculated using a commercially available software tool (WinDose 2.1, IMP, Erlangen, Germany) [16].

To measure noise levels, we placed regions of interest (area, 2 cm²) in the midsection of the left ventricle at constant table positions. Noise levels were measured on four consecutive axial slices. The mean of the CT number (mean Hounsfield units) and the SD were determined for each region of interest, with the SD of the attenuation measurements ascribed to the image noise. For each image, the series' mean values of the four measurements were calculated and used for further analysis.

The coronary calcium scores in group 1.0 and group 1.1 were compared using a two-tailed paired Student's t test. The noise levels in the patient groups were compared either using a two-tailed paired (group 1.0 vs group 1.1) or a two-tailed unpaired Student's t test (group 1.0 vs group 2.0) to find statistically significant differences between the patient groups. A p value of 0.05 or less was considered statistically significant. The noise levels were plotted versus the body weight and the body mass index, as described previously [10]. All figures are given as mean value ± SD.

Results

Top Abstract Introduction Subjects and Methods Results Discussion References

 
For the nonoverlapping image reconstruction, the mean Agatston score ranged from 0 to 3422.3 (mean, 588.6 ± 762.5) in group 1.0 and from 0 to 3456.7 (mean, 589.6 ± 763.6) in group 1.1. Using overlapping image reconstruction, we found that the coronary calcium score ranged from 0 to 2522.8 (mean, 508.4 ± 601.1) in group 1.0 and from 0 to 2562.9 (mean, 508.9 ± 605.5) in group 1.1. On average, overlapping image reconstruction resulted in lower Agatston scores than that of nonoverlapping image reconstruction. The coronary calcium score did not vary significantly between groups 1.0 and 2.0 (p = 0.84). In group 1.0, the mean of the calculated effective radiation dose according to the International Commission on Radiological Protection [7] was 4.44 mSv (range, 3.28–5.88 mSv) for women and 3.01 mSv (range, 2.52–4.18 mSv) for men, whereas in group 2.0, the mean of the calculated radiation dose was 3.34 mSv (range, 2.39–3.83 mSv) for women and 2.66 mSv (range, 2.09–3.53) for men. The mean of the simulated radiation dose in group 1.1 was 3.41 mSv (range, 2.18–4.45 mSv) for women and 2.61 mSv (range, 1.95–3.67 mSv) for men. Simulated dose reduction led only to a slight increase in image noise. After we applied the body weight–adapted tube current time settings, the image noise remained in comparable ranges. No statistically significant differences were observed between group 1.0 and group 2.0 (nonoverlapping image reconstruction, p = 0.21; overlapping image reconstruction, p = 0.72). In Table 2, the results are given in detail.

Ratios between body weight and body mass index versus image noise showed a distinct variability. These ratios were similar for overlapping and nonoverlapping image reconstruction (Tables 3 and 4). The plotted noise-to-body weight (slope, 0.081; confidence interval [CI], 0.007–0.155) and noise-to-body mass index (slope, 0.378; CI, 0.099–0.356) ratios for the standard protocol suggested relatively higher radiation doses for patients of lower weight (Figs. 1 and 2) compared with the body weight–adapted examination protocol. An improved noise-to-body weight (slope, 0.054; CI, –0.006 to 0.109) and noise-to-body mass index (slope, 0.190; CI, –0.005 to 0.385) ratio was achieved by application of the body weight–adapted current time settings (Figs. 3 and 4), indicating a tendency toward more homogenous distribution of the image noise and therefore more efficient dose distribution in this patient group. The ideal slope of the graphs is zero, indicating a balanced image noise in the patient population. A slope greater than zero indicates an increased image noise in heavier patients compared with that of lower weight patients and consequently a relative overexposure in lower weight patients. However, there was no statistically significant difference between these slopes. The same finding was observed for the patient group with a simulated adaptation of the tube current time settings (Table 3).

View larger version (9K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1. —Scatterplot shows ratio between body weight (kg) and image noise (H) for nonoverlapping image reconstruction at 133 mAseff. r2 = 0.301, slope = 0.081 H/kg, intersection = 11.956 H.

View larger version (10K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2. —Scatterplot shows ratio between body mass index (kg/m2) and image noise (H) for nonoverlapping image reconstruction at 133 mAseff. r2 = 0.370, slope = 0.378 H/kg per millimeter squared, intersection = 8.163 H.

View larger version (9K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3. —Scatterplot shows ratio between body weight (kg) and image noise (H) for nonoverlapping image reconstruction with body weight–adapted tube current time setting. r2 = 0.225, slope = 0.054 H/kg, intersection = 14.528 H.

View larger version (9K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4. —Scatterplot shows ratio between body mass index (kg/m2) and image noise (H) for nonoverlapping image reconstruction with body weight–adapted tube current time setting. r2 = 0.275, slope = 0.190 H/kg per millimeter squared, intersection = 14.210 H.

Discussion

Top Abstract Introduction Subjects and Methods Results Discussion References

 
CT accounts for approximately 4% of all radiology examinations but resulted in 35% of the effective cumulative dose between 1990 and 1992 [17]. Consequently, the European community classified CT as a high-dose procedure [18]. However, improvements in CT technique have led to better performance of the CT scanners, and further software developments are giving way to new applications such as ECG-gated MDCT of the heart. Traditionally, electron beam CT was the predominant method for motionless X-ray–based imaging of the heart [19]. Especially for quantification of coronary calcifications, electron beam CT is widely accepted [2], but over the last few years MDCT has proven to be a powerful tool in cardiac imaging, acquiring four or more slices per rotation with a gantry rotation time down to 420 msec [20]. For the first time, this technique allows a gapless helical CT scan of the entire heart, with reasonable slice width within a single breath-hold. The new technique has proven as reliable as electron beam CT for the detection and quantification of coronary calcifications [2–4]. Moreover, the use of overlapping image reconstruction from retrospectively ECG-gated MDCT data sets achieved a significant reduction of the interscan variability of the coronary calcium score [5]. However, this development resulted in an increase in radiation dose compared with that of electron beam CT. Although the effective radiation dose using electron beam CT with prospective ECG-triggering was approximately 0.8 mSv [6], the measured effective radiation dose in a phantom study using retrospectively ECG-gated MDCT increased to 6.2 mSv in women and 3.6 mSv in men [21].

The radiation dose must be kept as low as reasonably achievable when screening for the detection of coronary artery calcifications [7]. Several approaches are applicable to achieve this goal. The tube voltage and the tube current can be modified. Although various scanning protocols are used, a systematic comparison of different tube current time settings has not been performed yet. We analyzed two different examination protocols with fixed and body weight–adapted tube current time settings. The formula we used to calculate body weight–adapted tube current time settings was chosen to reduce the patient dose by approximately 25% for men weighing an average of 70 kg. To keep this formula as simple as possible for implementation in a clinical examination, we found that the easiest way to achieve this goal was by adding 33 mAs_eff to the patient's body weight. Previously, a similar approach was successfully introduced for abdominal and thoracic CT [10, 11]. Recently, an ECG-gated tube current modulation was successfully introduced into the clinical routine [8]. These techniques can be combined because they represent different approaches to dose reduction. ECG-gated tube current modulation is especially well-suited for use with individual adaptation of tube current time settings because the technique of tube modulation reduces the tube current only during systole, whereas diastolic images are used for image analysis only. Using individually body weight–adapted tube current time settings, we found that the diastolic images are also affected, resulting in an optimized radiation dose.

The influence of the body mass index on image noise in electron beam CT of coronary calcifications is well described [22]. However, in comparison with a body mass index–based adaptation of the tube current time settings a body weight–adapted examination protocol is easier to handle in a clinical examination because it depends on only a single parameter, whereas a body mass index–adapted examination protocol is influenced by at least two patient parameters. Therefore, we decided to assess the feasibility of a simple directive for individually body weight–adapted tube current time settings.

Overlapping image reconstruction is known to be advantageous for quantification of coronary calcifications. The interscan variability can be reduced using overlapping image reconstruction because the overlapping image reconstruction reduces the partial volume effect [23]. However, in the clinical examination, detection and quantification of coronary calcifications are commonly performed using nonoverlapping images. For this reason, both reconstruction techniques were compared.

Using single-detector CT, Takahashi et al. [24] showed that there were no differences in the detection of mediastinal and lung abnormalities for low-dose examination protocols, despite increased noise levels and reduced low contrast resolution. However, reduced tube current inevitably leads to increased image noise, which was identified as an important factor for interscan variability of the coronary calcium score [25].

To rule out statistically significant influences in the patient–adapted tube current time settings for the image noise and subsequently the coronary calcium score, we simulated body weight–adapted tube current time settings (group 1.1). In comparing groups 1.0 and 1.1, we found that the pixel noise increased but remained in comparable ranges. The coronary calcium score did not show a statistically significant change either. Similar findings were reported from a recent phantom study [26].

In our study, even in small calcifications, no significant change of the coronary calcium score occurred when we compared the different tube current time settings. This finding is of particular interest because it has been reported in the literature that elevated pixel noise can affect the coronary calcium score [27]. Especially for small calcifications revealed on electron beam CT, pixel noise is a known problem [25]. Because we observed only a slight increase in the pixel noise and MDCT presents with less noise when compared with electron beam CT, pixel noise did not affect the coronary calcium score using the evaluated examination protocol [28]. According to a risk stratification schema that was established for electron beam CT examinations by Rumberger et al. [29], none of our patients had to be assigned to a different risk group using the individually body weight–adapted examination protocol.

Applying the individually body weight–adapted tube current time settings to group 2.0, we found that the average image noise only slightly increased for overlapping and nonoverlapping image reconstruction when compared with group 1.0. The plotted ratios between body weight and body mass index versus image noise showed a reduced dependency when the individual body weight–adapted examination protocol was applied compared with the examination protocol with fixed tube current time settings. Ideally, the slope of the graphs and the correlation (r²) between body weight and image noise are zero, indicating a balanced image noise and therefore an optimized dose distribution in the patient population. Application of the body weight–adapted tube current time settings led to a reduced slope of the graphs for overlapping and nonoverlapping images, representing a reduction of overexposure in lower weight patients compared with heavier patients. Nevertheless, the tube current time settings can be reduced even more until the slope of the graphs equals zero.

On average, the effective radiation dose was reduced by 11.6% in men and 24.8% in women when the body weight–adjusted examination protocol was applied. The obvious differences between men and women are mainly due to the following facts. First, in women, the mammary gland, which is highly sensitive to radiation exposure, is located within the scan range. Second, women had a lower body weight compared with men. Consequently, the tube current time settings were reduced more. However, even using individual body weight–adapted examination protocols with reduced radiation dose, we found that the application of retrospectively ECG-gated MDCT still leads to a radiation exposure that is up to four times higher than that in electron beam CT. However, in combination with tube current modulation, comparable ranges of radiation exposure will be achievable [9].

From the presented data, we conclude that individual body weight–adapted tube current time settings are applicable for coronary calcium scoring without affecting the coronary calcium score or the clinically relevant increase of the image noise. Although the presented examination protocol allows a reduction in the effective radiation dose of approximately 11.6–24.8%, further reduction of the tube current time settings is still possible. A combination of other techniques for reduction of the radiation dose such as the ECG-gated tube current modulation is recommended.

References

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