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Use of an Automatic Exposure Control Mechanism for Dose Optimization in Multi–Detector Row CT Examinations: Clinical Evaluation¹

¹ From the Department of Radiology, Heilig Hart Hospital, Kolveniersvest 20, B-2500 Lier, Belgium. Received February 12, 2004; revision requested September 14; revision received September 29; accepted November 4. Address correspondence to T.H.M. (e-mail: tom.mulkens{at}hhzhlier.be).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION References

 
PURPOSE: To prospectively compare dose reduction and image quality achieved with an automatic exposure control system that is based on both angular (x-y axis) and z-axis tube current modulation with dose reduction and image quality achieved with an angular modulation system for multi–detector row computed tomography (CT).

MATERIALS AND METHODS: The study protocol was approved by the institutional review board, and oral informed consent was obtained. In two groups of 200 patients, five anatomic regions (ie, the thorax, abdomen-pelvis, abdomen-liver, lumbar spine, and cervical spine) were examined with this modulation system and a six-section multi–detector row CT scanner. Data from these patients were compared with data from 200 patients who were examined with an angular modulation system. Dose reduction by means of reduction of the mean effective tube current in 600 examinations, image noise in 200 examinations performed with each modulation system, and subjective image quality scores in 100 examinations per-formed with each modulation system were compared with Wilcoxon signed rank tests.

RESULTS: Mean dose reduction for the angular and z-axis tube current modulation system and for the angular modulation system was as follows: thorax, 20% and 14%, respectively; abdomen-liver, 38% and 18%, respectively; abdomen-pelvis, 32% and 26%, respectively; lumbar spine, 37% and 10%, respectively; and cervical spine, 68% and 16%, respectively. These differences were statistically significant (P < .05). There was no significant difference in image noise and mean image quality scores between modulation systems, with the exception of cervical spinal examinations (P < .001 for both), where the examinations with angular modulation resulted in better scores. There is good correlation between the mean effective tube current level and the body mass index of patients with the new modulation system. Correlation was as follows: thorax, 0.77; abdomen-pelvis, 0.83; abdomen-liver, 0.84; lumbar spine, 0.8; and cervical spine, 0.6. This correlation was not observed with the angular modulation system.

CONCLUSION: An automatic exposure control mechanism that is based on real-time anatomy-dependent tube current modulation delivers good image quality with a significantly reduced radiation dose.

© RSNA, 2005

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION References

 
Recent advances have enhanced the clinical applications of computed tomography (CT), especially the use of spiral scanning techniques and the development of multi–detector row CT technology (1,2). During the past 10 years, there has been a corresponding increase in the number of CT examinations performed around the world. Surveys performed in the United States reveal that the annual number of CT examinations has increased almost 10-fold in less than 2 decades, from 3.6 million in 1980 to 33 million in 1998 (2,3). An estimated 2.7 million CT studies were performed in children younger than 15 years of age in 2000 (4).

While CT accounts for only 11%–13% of x-ray–based examinations performed in the United States, it delivers over two-thirds of the total radiation dose associated with medical imaging and about one-third of the collective population radiation dose in the United States (1,2).

While the clinical value and benefits of CT are unquestioned and exceed the potential harmful effects of radiation exposure in patients, increasing radiation doses to the population have raised a compelling case for reduction of radiation exposure from CT (1–3).

The International Commission on Radiological Protection stated that the radiation doses from CT are relatively high and that technologic developments and advances in CT generally have not led to reductions in patient radiation dose per examination, which stands in contrast to the trend in conventional radiography (5). This is because of the "uncoupling effect" of CT. The final image is divorced from the radiation dose because of digital and electronic manipulation. The tube current used in image acquisition does not affect the attenuation. Even if we use too great a radiation dose, image quality is not compromised (6,7).

CT dose reduction will require a combination of approaches, including the education of radiologists and technologists and the development of technique charts and automatic exposure control devices by medical physicists and manufacturers, respectively. CT dose reduction will also require the creation of a climate of attention to dose reduction (1,2,5).

Recent articles have focused on the estimated risk of cancer development due to the use of diagnostic x-rays, both in adults (3,8) and in children (4). The radiation doses of CT examinations can approach and sometimes exceed levels that may increase the probability of cancer, and they can add a small risk to the lifetime cancer mortality risk of the natural background cancer rate.

The purpose of this study was to prospectively compare radiation dose reduction and image quality achieved with an automatic exposure control system that is based on both angular (x-y axis) and z-axis tube current modulation with radiation dose reduction and image quality achieved with an angular modulation system for multi–detector row CT.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION References

 
Study Design and Modulation System
A prospective study was conducted in two successive phases to evaluate dose reduction and image quality with a new online tube current modulation system for CT. This system was part of a test version of the new software (VB 10; Siemens, Erlangen, Germany), which was installed in December 2003, on a six-section multi–detector row CT system (Somatom Emotion 6; Siemens). This x-ray tube current modulation system (Care Dose 4D; Siemens) is based on both angular modulation and z-axis modulation. Study approval was received from the ethics committee of our institution. Because use of this modulation system in our scanning protocol yields an overall reduction of radiation dose in comparison with the radiation dose delivered with our previous conventional scanning protocols and because this modulation system was the only parameter we changed, our ethics committee did not require individual patient informed consent. Nevertheless, before the start of the examination, the CT technologist obtained oral informed consent and informed patients of the use of this dose modulation program in our study. At the same time, patient weight and height were noted.

This online tube current modulation system is used to combine two forms of modulation: one in the z-axis and one in the x-y axis, which is referred to hereafter as angular modulation. Tube current modulation in the z axis is determined from the attenuation values and shape obtained by refined analysis of a single anteroposterior or lateral projection radiogram (topogram) obtained at the start of the examination. By default, one anteroposterior or lateral topogram is used; however, if two topograms are selected by the user in the scanning protocol, the online tube current modulation system will use the measured attenuation of both. In a review, Kalra et al (2) wrote, "In z-axis modulation, tube current is adjusted to maintain a user-selected image quality level in the image data. Noise is regulated on the final image to a level desired by the user. In this sense, z-axis modulation is the CT equivalent of the autoexposure control systems used for many years with conventional x-ray systems. Z-axis modulation is an attempt to render all images with similar noise, independent of patient size and anatomy."

Angular tube current modulation has a different objective than does z-axis tube current modulation. In angular modulation, the tube current is adjusted to minimize x-rays in projections that are of less importance in the reduction of overall image noise content (2,9–11). Noise in CT scans is dominated by projections in which attenuation is highest. For a homogeneous object with a circular cross section, attenuation is constant over all projections, and all measured values contribute equally. For a nonhomogeneous object (like the human body) with a noncircular cross section, attenuation varies greatly, sometimes by more than three orders of magnitude (9–11). Because noise in the data from high-attenuation projections (ie, the lateral direction) greatly influences noise level on CT scans, the radiation dose for projections with relatively low attenuation (ie, the anteroposterior direction) can be reduced substantially without a measurable increase in image noise (9–11). The angular tube current modulation 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 each section position, the CT system calculates the arithmetic average tube current, which is expressed as the average effective tube current, throughout the duration of exposure. As described by Mahesh et al (12), the effective tube current is determined by dividing the product of tube current and rotation time by the pitch, which Silverman et al (13) reported is the ratio between the table feed per rotation and the x-ray beam width or collimation. This mean effective tube current is displayed at the scanning console at the end of the examination and was recorded in our study.

Study Population and CT Examinations
We started to evaluate the x-ray tube current modulation system, beginning with a limited number of scanning protocols that we frequently use and that constitute the majority of our daily CT work, with the exception of examinations performed in the skull, for the following regions: the thorax (routine protocol), the abdomen (abdominal-pelvic protocol and multiphase abdomen-liver protocol), and the spine. Spiral examinations were performed in the lumbar spine and separated into those performed in normal-sized patients and those performed in large or obese patients. Spiral examinations were performed in the cervical spine in all patients. The scanning protocols used are summarized in Table 1.

In the first phase of the study (February through April 2004), we included 200 patients (92 men and 108 women; mean age, 54.7 years; age range, 18–92 years) who underwent scanning with the online tube current modulation system and the following types of examinations: thoracic (n = 40), abdominal-pelvic (n = 50), abdominal-liver (n = 30), cervical spinal (n = 30), and lumbar spinal in normal-sized (n = 40) and large or obese (n = 10) patients. With these consecutive examinations, we collected patient data such as weight and height, and we recorded the following examination parameters: tube potential (ie, peak voltage, measured in kilovolts), mean effective tube current (measured in milliampere-seconds), volume CT dose index (measured in milligrays), and dose length product (measured in milligrays-centimeters). These values are displayed at the scanner console and archived in a separate patient dose protocol at the end of the examination. These data were compared with the data of 200 consecutive patients (93 men and 107 women; mean age, 55.8 years; age range, 18–91 years) who were examined with the previous dose modulation system, which was based on angular tube current modulation (Care Dose; Siemens) in the same regions with the same scanning protocols (Table 1) and the same number of examinations. For the multiphase CT examinations of the abdomen superior (abdomen-liver protocol), only the values of the venous phase were used for comparison.

For the thoracic and abdominal-pelvic examinations, scanning began 15–20 seconds after monophasic intravenous bolus injection of 100 mL of iobitridol, which contained 300 mg of iodine per milliliter (Xenetix 300; Laboratoire Guerbet, Aulnay-sous-Bois, France), at a rate of 2 mL per second with a power injector (Stellant Dual; Medrad, Indianola, Pa). The arterial phase of the multiphase CT examination of the upper abdomen (ie, liver) was started with a bolus tracking system after monophasic injection of 100 mL of iobitridol at a rate of 2.5 or 3.0 mL per second (Care Bolus; Siemens) with the same power injector. The venous phase of the examination was started 30 seconds after the end of the arterial phase.

After official release of the new software (VB 10; Siemens), this new version was installed on our scanner on May 15, 2004. In the second phase of our study, we examined a third group of 200 consecutive patients with the x-ray tube current modulation system (91 male and 109 female patients; mean age, 59.1 years; age range, 17–89 years), and we collected the same data with the same scanning protocols, regions of interest, and numbers of examinations per region. We compared the level of dose reduction, which is expressed as reduction of mean effective tube current in comparison with the reference tube current and the CT dose index volume, in the same anatomic region for each study group. We observed no obvious difference in the use of both the beta version and the definitive version of the software, especially regarding the use of the tube current modulation system.

An estimation of the effective radiation dose (measured in millisieverts) was calculated from the dose length product values by using conversion coefficients proposed by the European Commission (14).

The scanning range of the liver examination extended from the top of the diaphragm to the level of the iliac crests. For the abdominal-pelvic examination, the entire abdomen was scanned from the top of the diaphragm to the level of the symphysis pubis. For the thoracic examination, the whole thoracic region was scanned. For the lumbar spinal examination, the area from the top of the first lumbar corpus (ie, L1) to the sacrum (ie, the level of the bottom of S1) was scanned. For the cervical spinal examination, the area from the foramen magnum to the bottom of the first thoracic vertebral corpus (ie, T1) was scanned. One anteroposterior topogram was used for the abdominal and thoracic examinations, and one lateral topogram was used for the spinal examinations.

The correlation between mean effective tube current and body weight (measured in kilograms) and body mass index (BMI) (measured in kilograms per square meter) was calculated in 200 patients who were examined with the angular tube current modulation system and 200 patients who were examined in phase 1 with the angular and z-axis tube current modulation system. The BMI was calculated by dividing body weight by height (2), and it was used to classify patients into subgroups, according to the recommendations of the National Institutes of Health. Patients were classified as (a) underweight if their BMI was less than 18.5 kg/m², (b) normal if their BMI was 18.5–24.9 kg/m², (c) overweight if their BMI was 25.0–29.9 kg/m², (d) obese if their BMI was 30.0–39.9 kg/m², and (e) extremely obese if their BMI was more than 40.0 kg/m² (15).

Evaluation of Image Quality
Objective evaluation of image quality was based on the evaluation of image noise by using the standard deviation of the attenuation (measured in Hounsfield units) for pixels in a standard 1-cm² circular region of interest, which was placed in the same location for each anatomic region. For thoracic imaging, this region of interest included the distal trachea, cranial to the carina. For abdominal-pelvic imaging, the region of interest included the left psoas muscle at the level of L5. For lumbar spinal imaging, the region of interest included the left psoas muscle at the level of L5. For cervical spinal imaging, the region of interest included the spinal medulla at the level of C5. For abdominal-liver imaging, the region of interest included the left psoas muscle at the level of L5. This process was performed by one radiologist (T.H.M.) in the 200 examinations performed with the angular and z-axis tube current modulation system in the first phase of the study and in the 200 examinations performed with the angular tube current modulation system.

Subjective evaluation of image quality was performed by using a scoring system that enabled us to assess the level of image noise. Images were ranked on a scale of 0 to 4. A score of 4 indicated excellent image quality, with no or very minimal image noise. A score of 3 indicated good image quality, with minimal image noise. A score of 2 indicated moderate image quality, with moderate image noise that did not affect the diagnostic quality of the image. A score of 1 indicated poor image quality, with too much image noise that may affect the diagnostic quality of the image. A score of 0 indicated very poor image quality, with a great deal of image noise. Images with a score of 0 were not useful for diagnostic imaging.

At random, 20 studies of each anatomic region that were obtained with each modulation system were selected and loaded on a separate workstation (Leonardo; Siemens), whereby the readers were blinded to the modulation system used. The scanning parameters were removed in the display. In this way, a total of 200 studies (100 examinations were performed with angular and z-axis modulation and 100 were performed with only angular modulation) were reviewed by eight radiologists. Two radiologists had more than 10 years experience in CT (T.H.M. and P.B., with 11 and 15 years of experience, respectively). Two radiologists had experience in magnetic resonance imaging (D.G. and M.B., with 6 and 13 years of experience, respectively). Two general radiologists had experience with angiography (J.L.T., 21 years of experience) and ultrasonography (X.V.D., 20 years of experience). There were two residents who also participated in this review (E.M. and C.V., with 4 and 2 years of residency, respectively). Diagnostic image quality was considered sufficient when the score was 2 or higher.

Statistical Analysis
Statistical analysis was performed with commercially available statistical software (InStat, version 3.0; GraphPad Software, San Diego, Calif). Comparison of mean dose reduction, which is expressed in level of reduction of mean effective tube current (in comparison with the reference tube current value), was performed between the examinations performed with only angular modulation and those performed with both angular and z-axis modulation and between the phase 1 and phase 2 examinations performed with both angular and z-axis modulation; Wilcoxon signed rank tests were performed for each anatomic region. The three groups of 200 patients were compared for differences in age and sex. A Kruskal-Wallis test (nonparametric analysis of variance test) with comparative post hoc Dunn tests was used for comparison of age difference, and a ² test was used for comparison of sex.

Objective image quality scores (ie, the standard deviation of region of interest measurements) of images obtained with only angular modulation and those obtained with both angular and z-axis modulation in phase 1 were compared with the Wilcoxon signed rank test for each anatomic region.

Kruskal-Wallis tests with comparative post hoc Dunn tests were used to compare the subjective image quality scores: The mean scores assigned by each radiologist in each anatomic region were compared for all examinations together and for the examinations performed with each modulation system separately. The mean scores of all radiologists together for each anatomic region were compared with Wilcoxon signed rank tests between the modulation systems. A P value of less than .05 indicated a statistically significant difference.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION References

 
Dose Reduction
The use of a tube current modulation system in spiral multi–detector row CT examinations results in substantial dose reduction in comparison with examinations performed with a fixed tube current (Table 2). In our study, dose reduction varied from 14% for thoracic examinations to 26% for pelvic examinations performed with angular modulation. Dose reduction varied from 20% for thoracic examinations (Fig 1) to 68% for cervical spinal examinations (Fig 2) performed with angular and z-axis modulation.

View larger version (173K): [in this window] [in a new window] [Download PPT slide]   Figure 1a. Images obtained in a 67-year-old woman (height, 1.56 m; weight, 48 kg; BMI, 19.72). Multi–detector row CT thoracic examination was performed with angular and z-axis modulation for evaluation of a mass, which was seen on a standard radiograph. Mean effective tube current of 48 mAs was used, which means there was approximately 33% reduction in comparison with the reference tube current of 70 mAs. Calculated effective dose for the whole examination was 2.75 mSv. (a) Topogram shows mass (white arrows) on the right side of the thorax. (b) A 5-mm transverse CT image obtained with the lung window setting shows tumor mass (arrow) in the superior segment of the right lower lobe.

View larger version (160K): [in this window] [in a new window] [Download PPT slide]   Figure 1b. Images obtained in a 67-year-old woman (height, 1.56 m; weight, 48 kg; BMI, 19.72). Multi–detector row CT thoracic examination was performed with angular and z-axis modulation for evaluation of a mass, which was seen on a standard radiograph. Mean effective tube current of 48 mAs was used, which means there was approximately 33% reduction in comparison with the reference tube current of 70 mAs. Calculated effective dose for the whole examination was 2.75 mSv. (a) Topogram shows mass (white arrows) on the right side of the thorax. (b) A 5-mm transverse CT image obtained with the lung window setting shows tumor mass (arrow) in the superior segment of the right lower lobe.

View larger version (95K): [in this window] [in a new window] [Download PPT slide]   Figure 2. CT scan of the cervical spine obtained in a 35-year-old-woman (height, 1.74 m; weight, 67 kg; BMI, 24.57) with angular and z-axis modulation for evaluation of cervical pain with left-side irradiation. The mean effective tube current was 45 mAs, which was a dose reduction of nearly 75% in comparison with the reference tube current of 175 mAs. Calculated effective dose for the whole examination was only 0.54 mSv. Sagittal 3-mm multiplanar reconstruction image shows small median disk protrusion at the level of the C5-6 disk (white arrow) and large herniation at the level of the C6-7 disk (black arrow).

View larger version (131K): [in this window] [in a new window] [Download PPT slide]   Figure 3a. Images obtained in a 63-year-old man (height, 1.70 m; weight, 84 kg; BMI, 27.74) with a history of left-sided nephrectomy for renal carcinoma 2 years earlier. CT examination of the pelvis was performed with angular and z-axis modulation for evaluation of low abdominal pain. Mean effective tube current was 110 mAs, which was 5% greater than the reference tube current of 105 mAs. Calculated effective dose of the examination was 6.61 mSv. (a) Topogram shows surgical clips after left-sided nephrectomy (arrows). (b) A 5-mm transverse CT image shows metastatic osteolytic bone lesions in the left side of the ilium and sacrum (arrows).

View larger version (124K): [in this window] [in a new window] [Download PPT slide]   Figure 3b. Images obtained in a 63-year-old man (height, 1.70 m; weight, 84 kg; BMI, 27.74) with a history of left-sided nephrectomy for renal carcinoma 2 years earlier. CT examination of the pelvis was performed with angular and z-axis modulation for evaluation of low abdominal pain. Mean effective tube current was 110 mAs, which was 5% greater than the reference tube current of 105 mAs. Calculated effective dose of the examination was 6.61 mSv. (a) Topogram shows surgical clips after left-sided nephrectomy (arrows). (b) A 5-mm transverse CT image shows metastatic osteolytic bone lesions in the left side of the ilium and sacrum (arrows).

View larger version (122K): [in this window] [in a new window] [Download PPT slide]   Figure 4. Image obtained in a 65-year-old woman (height, 1.65 m; weight, 66 kg; BMI, 24.24). A CT multiphase image of the superior abdomen was obtained with angular and z-axis modulation for clinical history of recurrent pancreatitis. A mean effective tube current of 58 mAs was used in the arterial phase, and a mean effective tube current of 59 mAs was used in venous phase. This means there was a 35% and 38% reduction in tube current compared with the reference tube currents of 90 mAs and 95 mAs, respectively. Calculated effective dose for the whole examination was 14.3 mSv. Transverse 5-mm CT image obtained in the arterial phase shows multiple small pancreas pseudocysts (arrows).

View larger version (136K): [in this window] [in a new window] [Download PPT slide]   Figure 5. CT image obtained in a 45-year-old woman (height, 1.71 m; weight, 63 kg; BMI, 21.55) with severe low back pain and left-sided ischiatiform pain. CT lumbar spinal image obtained with angular and z-axis modulation and a mean effective tube current of 92 mAs (mean reduction of 54% in comparison with a reference tube current of 200 mAs). Calculated effective dose for the whole examination is 5.56 mSv. Sagittal 3-mm multiplanar reconstruction image shows large descending disk herniation (black arrow).

Comparison of age distribution with the Kruskal-Wallis test results in a significant difference (P = .031) between the three groups. In patients who were examined with angular and z-axis modulation, the comparative post hoc Dunn test shows a significant difference in age between patients examined in phase 1 and those examined in phase 2 (P < .05); however, there was no significant difference between patients in the phase 1 or phase 2 group and those who were examined with only angular modulation (P > .05). A ² test showed no difference in sex between the three groups of patients (P = .98).

Objective Image Quality
Objective image quality analysis, which was performed with the standard deviation of attenuation, shows no significant difference in image noise with Wilcoxon signed rank tests between the scanners for each anatomic region (Table 4). Cervical spinal images (P < .001) obtained with angular and z-axis modulation, however, showed more image noise than did images obtained with only angular modulation.

View larger version (28K): [in this window] [in a new window] [Download PPT slide]   Figure 6a. Graphs show correlation between mean effective tube current and BMI of patients who underwent CT scanning with angular and z-axis modulation in the (a) thorax, (b) pelvis, (c) lumbar spine (normal-sized patients), and (d) lumbar spine (large or obese patients).

View larger version (31K): [in this window] [in a new window] [Download PPT slide]   Figure 6b. Graphs show correlation between mean effective tube current and BMI of patients who underwent CT scanning with angular and z-axis modulation in the (a) thorax, (b) pelvis, (c) lumbar spine (normal-sized patients), and (d) lumbar spine (large or obese patients).

View larger version (30K): [in this window] [in a new window] [Download PPT slide]   Figure 6c. Graphs show correlation between mean effective tube current and BMI of patients who underwent CT scanning with angular and z-axis modulation in the (a) thorax, (b) pelvis, (c) lumbar spine (normal-sized patients), and (d) lumbar spine (large or obese patients).

View larger version (29K): [in this window] [in a new window] [Download PPT slide]   Figure 6d. Graphs show correlation between mean effective tube current and BMI of patients who underwent CT scanning with angular and z-axis modulation in the (a) thorax, (b) pelvis, (c) lumbar spine (normal-sized patients), and (d) lumbar spine (large or obese patients).

View larger version (27K): [in this window] [in a new window] [Download PPT slide]   Figure 7a. Graphs show correlation between mean effective tube current and BMI in patients who underwent CT scanning with angular and z-axis modulation in the (a) cervical spine and (b) abdomen-liver. Graphs also show correlation between mean effective tube current and weight in patients who underwent CT scanning with angular and z-axis modulation in the (c) cervical spine and (d) abdomen-liver.

View larger version (29K): [in this window] [in a new window] [Download PPT slide]   Figure 7b. Graphs show correlation between mean effective tube current and BMI in patients who underwent CT scanning with angular and z-axis modulation in the (a) cervical spine and (b) abdomen-liver. Graphs also show correlation between mean effective tube current and weight in patients who underwent CT scanning with angular and z-axis modulation in the (c) cervical spine and (d) abdomen-liver.

View larger version (26K): [in this window] [in a new window] [Download PPT slide]   Figure 7c. Graphs show correlation between mean effective tube current and BMI in patients who underwent CT scanning with angular and z-axis modulation in the (a) cervical spine and (b) abdomen-liver. Graphs also show correlation between mean effective tube current and weight in patients who underwent CT scanning with angular and z-axis modulation in the (c) cervical spine and (d) abdomen-liver.

View larger version (29K): [in this window] [in a new window] [Download PPT slide]   Figure 7d. Graphs show correlation between mean effective tube current and BMI in patients who underwent CT scanning with angular and z-axis modulation in the (a) cervical spine and (b) abdomen-liver. Graphs also show correlation between mean effective tube current and weight in patients who underwent CT scanning with angular and z-axis modulation in the (c) cervical spine and (d) abdomen-liver.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION References

The program, which worked consistently, is very simple and easy to use. This program works as an automatic exposure control mechanism once it is activated in the scanning protocol. The results for each scanning protocol in different anatomic regions were consistent in 400 patients. Comparison of the two study groups of 200 patients who were examined with angular and z-axis modulation yields comparable results for mean effective tube current and mean volume CT dose index. Schmidt and Kalender (16) used Monte Carlo dose simulations to show that reduction of effective tube current with a tube current modulation mechanism results in underestimation of the actual reduction in radiation dose by approximately 20%. The actual patient dose is reduced more markedly. In this way, the percentage of effective tube current reduction in tube current modulation represents the minimum dose saving.

We found no reports in the medical literature in which a tube current modulation system based on both angular and z-axis modulation was used. In our study, we compared the results obtained with this tube current modulation system with those obtained with the previous system, which was based on angular modulation of tube current. Previous studies performed with angular tube current modulation showed similar results. Greess et al (11,17) reported dose reduction of 15% and 25% in abdominal and pelvic single-section CT examinations, which is comparable with the 18% and 26% dose reduction we report in the abdominal-liver and abdominal-pelvic examinations. Greess et al (11,17) reported a 22% dose reduction in the thoracic region, which is greater than the 14% dose reduction we achieved with angular modulation only. Tack et al (18) used a four–detector row CT scanner and reported comparable results of dose reduction of 16.9% in the thoracic region and 20% in the abdominal region. In the shoulder region, larger dose reduction is achieved. A dose reduction of 53% was achieved with single-section CT (11), and a dose reduction of 47.0%–50.5% was achieved with four–detector row CT in the evaluation of the thoracic outlet with CT angiography (19). We found no reports that mentioned the use of a Care Dose scanner (Siemens) in spinal examinations. In children, angular modulation yields comparable results than in adults (20). The tube current product was typically reduced by 10%–60%; the amount of reduction depended on patient geometry and anatomic regions, where the mean tube current reduction was 20% for the neck, 23% for the thorax, 23% for the abdomen, and 22% for the thorax and abdomen together. The effective tube current reduction obtained with angular tube current modulation is independent of the effective tube current preset (18), which we also found. When looking for the best preset tube current level for each protocol before the start of our study, we observed the same average reduction with different presets, both with angular modulation and with angular and z-axis modulation. The extra dose reduction with angular and z-axis modulation can be explained by modulation of the tube current as a function of the table position (z-axis) because there is supplementary modulation according to differences in absorption along the region examined. Itoh et al (21) showed that detection of lung nodules requires a tube current that may vary by 50% from the shoulders to the lung bases, and they concluded that an ideal CT protocol for the lung should permit the tube current to be changed during spiral scanning at different positions in the thorax. The broader dynamic range in tube current modulation and the good correlation with the BMI of the patients with angular and z-axis modulation in every anatomic region also reflects better anatomy-based modulation.

Although angular and z-axis modulation yields an overall reduction in mean tube current in our study, the mean effective tube current goes above the reference tube current level for overweight and obese patients, and there is an excellent correlation with the BMI of these patients. The same correlation is not found with angular modulation, with the exception of lumbar spinal examinations performed in large or obese patients. The differences in mean tube current modulation in lumbar spinal examinations in large or obese patients between examinations performed with angular modulation and those performed with angular and z-axis modulation were not, however, statistically significant in our study, perhaps because there was a small amount of these patients. Indeed, Tack et al (18) found no correlation between BMI and mean effective tube current in a larger group of overweight and obese patients who were examined with angular modulation.

On the other hand, mean effective tube current is significantly reduced in slim underweight patients examined with both angular and z-axis modulation. In thoracic examinations, mean effective tube current was 29 mAs (or 20 mAs), which corresponds to the setting used in low-dose CT and screening protocols (22,23). Also in the abdomen, a low mean tube current is reached with examinations performed with angular and z-axis modulation in slim patients, with the lowest mean values being 36 mAs (effective) and 29 mAs (effective) (or 24 mAs and 32 mAs) for abdomen-pelvic and abdomen-liver examinations, respectively. This causes increased image noise when compared with conventional CT images, which can result in problems in the abdominal region, where the noise level may impair the visibility of low-contrast lesions (24). The software of the Care Dose 4D scanner (Siemens) allows for fine-tuning of the modulation settings; thus, the power of the tube current adaptation is reduced for slim patients.

An image optimization process should include a careful evaluation of image quality (2,24,25). A limitation of our study is that we use the image-noise parameter as the only objective parameter in this respect, whereas other factors, like contrast-to-noise ratio, might have been useful (25).

A large reduction in mean tube current was achieved with the cervical spinal examinations performed with angular and z-axis modulation. A mean tube current reduction of 68% (range, 47%–75%) was achieved, but there was significantly more noise in the images obtained with angular and z-axis modulation (Table 3). The subjective image quality scores (Table 4) of cervical spinal images obtained with both angular and z-axis modulation were significantly worse than those of images obtained with only angular modulation; however, the mean score for every radiologist was obviously higher than 2, which was the threshold of diagnostic image quality. Perhaps the reference tube current of 175 mAs (effective) was set too low, and with a higher tube current (eg, 200 or 225 mAs [effective]), the noise level can be brought to the same level as that of cervical spinal images obtained with angular modulation. The choice of the reference tube current level and, therefore, the image quality and noise level, is user-dependent with tube current modulation systems. In our study, the reference tube current level was based on subjective "good image quality" in normal-sized patients by consensus of two radiologists who were experienced in CT examinations before the start of the study. This is a weakness of the study and the modulation program because statistical analysis of the subjective image quality scores of eight radiologists after completion of the study showed a large variation of image quality appreciation. The adequate reference tube current level should be based on consensus of all radiologists in the study or on the best level of tube current for a normal-sized patient in different scanning protocols in different anatomic regions with the same CT system.

Another limitation of the study is that we did not evaluate the angular and z-axis modulation program with low-dose protocols. Since the effective tube current reduction is independent of the preset reference tube current, an additional dose reduction can be expected, even for low-dose protocols in this program, as shown by Tack et al (18). We have some preliminary experience with the use of angular and z-axis modulation in children, but the data are too limited to draw conclusions. A final limitation is that we compared two tube current modulation systems in our study, but we did not compare them with examinations performed with a constant tube current. The reason for this is because we used angular tube current modulation on both of our CT systems (ie, a six-section and a 16-section multi–detector row CT scanner) for more than 1 year, and we were satisfied with the image quality obtained with a lower dose. It seemed inappropriate to examine a group of 200 patients with fixed tube current at higher dose levels to compare the angular and z-axis modulation protocol with the conventional constant tube current protocol.

Methods other than tube current modulation (eg, use of lower tube potential) can be used to reduce the radiation dose in CT examinations; however, these methods were not considered in our study. Sigal-Cinqualbre et al (26) showed that the use of a weight-adapted low-tube-voltage setting (eg, 80 kV) in thoracic multi–detector CT examinations yields good image quality and substantially reduces both the radiation dose to patients and the amount of contrast material needed.

In conclusion, the use of a tube current modulation system that is based on both angular and z-axis modulation is simple and clear: Once the standard reference tube current level of each scanning protocol is adapted to a level of adequate image quality for a normal-sized patient, it works as an automatic exposure control mechanism and adapts the dose to the individual patient size, without need for further modifications. Use of angular and z-axis modulation with real-time tube current adjustment ensures good image quality at a lower dose.

	ACKNOWLEDGMENTS

 
We thank the CT technologists of our department for their substantial help in the instruction of the patients, collection of patient data, and performance of the examinations. We thank Steffen Fieuws, MD, for his assistance in statistical evaluation of the data and Jean-Luc Berrier and Christoph Suess, PhD, for their technical support of this study.

	FOOTNOTES

 

Abbreviations: BMI = body mass index

Authors stated no financial relationship to disclose.

Author contributions: Guarantor of integrity of entire study, T.H.M., J.L.T.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; literature research, T.H.M., E.M., C.V.; clinical studies, T.H.M., P.B., M.B., D.G.; statistical analysis, T.H.M.; and manuscript editing, T.H.M., X.V.D.

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