Clinical Observations
CT Imaging
March 2006

Comparison of Angular and Combined Automatic Tube Current Modulation Techniques with Constant Tube Current CT of the Abdomen and Pelvis

Abstract

OBJECTIVE. The objective of our study was to compare image quality and radiation dose associated with abdominopelvic CT using combined modulation, angular modulation, and constant tube current.
CONCLUSION. Compared with using a constant tube current to scan the abdomen and pelvis, the use of a combined modulation technique results in a substantial reduction (42-44%) in radiation dose with acceptable image noise and diagnostic acceptability.

Introduction

Automatic tube current modulation in CT is analogous to the automatic exposure control or photo timing technique, which has been used for automatically terminating radiographic exposure in conventional radiography once the predetermined radiographic density has been obtained. Automatic tube current modulation is based on the principle that X-ray attenuation and quantum image noise are determined by the size of the object and its tissue density [1]. Thus, tube current can be adjusted with changing regional attenuation to maintain image quality and increase radiation dose efficiency. In other words, tube current can be decreased automatically for regions with lower attenuation while maintaining an acceptable level of image noise and improving radiation dose efficiency.
In 1994, the concept of localizer radiograph-based automatic tube current modulation in CT was introduced to adjust tube current according to changing regional attenuation to obtain a constant prespecified image quality [1]. Real-time online attenuation-adapted automatic tube current modulation for CT was introduced in 1998 [2]. Because these techniques automatically adjust the tube current at different X-ray beam projection angles in each slice location, they have been collectively labeled “angular modulation” [1]. Angular or xy-axes modulation techniques decrease the selected tube current in projections (in the xy plane) that cause less attenuation—for example, lower tube current is used in the anteroposterior projection versus the lateral projection. The z-axis modulation techniques automatically select a tube current for each slice position in the scanning direction (the z-axis) to maintain a constant image quality. Thus, z-axis modulation adjusts tube current from slice to slice depending on regional body anatomy to maintain a constant user-specified quantum image noise level with improved radiation dose efficiency. Combined, or xyz-axes, automatic tube current modulation techniques, which merge the complementary techniques of angular and z-axis modulation, have been recently introduced on state-of-the-art MDCT scanners.
The purpose of our study was to compare image quality and radiation dose associated with CT of the abdomen and pelvis using xyz-axes modulation, angular modulation, and constant tube current techniques.

Materials and Methods

Patients and Scanning Techniques

The human research committee of our institutional review board approved the research study with a waiver of informed consent. The study was performed in compliance with the guidelines established by the Health Insurance Privacy and Portability Act (HIPPA). The study cohort comprised 152 patients (male-female ratio, 78:74; mean age, 60 years; age range, 25-101 years) who underwent contrast-enhanced CT examination of the abdomen and pelvis with a 16-MDCT scanner (Somatom Sensation 16, Siemens Medical Solutions). All studies were clinically indicated for assessing abdominal and pelvic diseases. Of these 152 patients, 79 consecutive patients were scanned with the combined or xyz-axes modulation technique (CARE Dose 4D, Siemens) using either weak decrease (slim)-strong increase (obese) type (n = 42; male-female ratio, 21:21; mean age, 61 years; age range, 25-84 years) or average decrease (slim)-average increase (obese) type (n = 37; male-female ratio, 20:17; mean age, 63 years; age range, 35-83 years). An image quality reference of 160 mAs was used for scanning the patients in the groups. The effective milliampere-second setting can be defined as the tube current-time product divided by the pitch factor.
Fig. 1 —Line graph shows relationship between attenuation profile and effective milliampere-second (mAs) setting. Adaptation strength can be adjusted separately for left branch (slim patients) and right branch (obese patients) of curve. White line indicates theoretic limits of adaptation for constant image noise. Absolute effective mAs value is scaled with image quality reference mAs value selected for performing CT study.
Of the remaining 73 patients, 42 consecutive patients (male-female ratio, 23:19; mean age, 61 years; age range, 25-94 years) were scanned using only the angular modulation technique (CARE Dose 4D, Siemens) with an effective milliampere-second setting of 160-200. Thirty-one consecutive patients were scanned with a fixed or constant tube current (effective mAs, 160-200; male-female ratio, 14:17; mean age, 60 years; age range, 25-101 years). Whereas the user needs to specify a milliampere value for scanning on CT systems manufactured by other vendors, the effective milliampere-second value is used to specify a desired tube current to perform scanning with the constant tube current or angular modulation technique on the CT scanner used in our study. “Effective mAs” is a vendor-specific term that has been defined as the tube current-gantry rotation time product (mAs) divided by pitch. Weights of all patients were recorded before the CT examinations. The mean weight of the study cohort was 77.2 kg, with a range of 41-136 kg.
To facilitate comparison of the radiation dose, the remaining scanning parameters were kept identical for each patient regardless of the tube current technique used. These scanning parameters included 140 kVp, 0.5-sec gantry rotation time, 16 × 1.5 mm detector configuration, 24-mm table feed per gantry rotation, 5-mm reconstructed slice thickness, 5-mm slice interval, and B31 medium soft-tissue reconstruction algorithm. In addition, the CT radiation dose descriptors—that is, CT dose index volume (CTDIvol) and dose-length product (DLP)—were recorded for each study. Because tube current changes during each gantry rotation and at each slice position with automatic tube current modulation techniques, the system takes into account the modulation of exposure and automatically provides the average CTDIvol and aggregate DLP values on the user interface for studies performed with modulation techniques.

Combined Modulation Technique (CARE Dose 4D)

The online real-time, anatomy-adapted, attenuation-based angular modulation technique (CARE Dose 4D) does not need information from a radiographic localizer to adapt the tube current. In noncircular cross-sectional regions, the X-ray beam attenuation varies in different projections, and in these settings, image noise in high beam attenuation projection angles determines the overall image noise content. Without angular modulation, the tube current remains constant over 360° rotation regardless of regional attenuation. With angular modulation, the tube current is reduced as a function of the projection angles for low-attenuation projections (reduced tube current in anteroposterior projection compared with that in lateral direction). The angular modulation technique assessed in our study estimates the modulation function data from the real-time regional attenuation profile. These data are analyzed and relayed to the generator control for adapting tube current with a 180° delay from the X-ray generation angle. In asymmetric regions, such as the shoulders, with significantly less beam attenuation in the anteroposterior direction than in the lateral direction, a substantial reduction in tube current occurs in the anteroposterior or posteroanterior direction with the use of angular modulation.
As stated in the previous section, the combined modulation technique used in the present study includes automatic tube current adaptation in the scanning direction (z-axis modulation, which determines a tube current value for each slice position in the z-axis) and online tube current modulation for each tube rotation (angular modulation, which uses a different tube current value for different beam projections at each slice position). For the z-axis modulation component of the combined modulation technique, an attenuation profile along the patient's long axis (z-axis) is measured in the direction of the projection and is estimated for the perpendicular direction with a mathematic algorithm on the basis of a single localizer radiograph. The attenuation profile consists of information regarding the patient's size, anatomic shape, and density at each position in the z-axis. On the basis of these attenuation profiles, axial tube current values are calculated to adapt tube current for z-axis modulation. An analytic function defines the correlation between attenuation profile and tube current for slice position in the z-axis and adapts the tube current to patient size and attenuation changes. Tube current adjustment is based on a user-defined image quality reference milliampere-second setting to maintain the desired image quality in all images along the scanning direction (z-axis modulation component).
On the basis of these levels, the technique also modulates the tube current online during each tube rotation according to the patient's angular attenuation profile (angular modulation component). The image quality reference mAs value is selected according to the diagnostic requirements and the preference of the radiologist. For a given scanning protocol, this value reflects the effective mAs that is used for a reference patient defined as a typical adult weighing 70-80 kg (for adult protocols) or a typical child weighing 20 kg (for pediatric protocols).
The combined modulation technique adapts tube current to the size of the individual patient on the basis of the image quality reference mAs value, which is changed only if an adjustment to image quality is required and not for individual patient size (Fig. 1). The use of different modulation settings for slim and obese patients, as seen in Figure 1, is supported by prior studies of the z-axis modulation technique (not assessed in the present study) that have reported a substantial difference in image quality with the use of similar tube current modulation settings in patients of different sizes [3]. The technique decreases tube current for slim patients and increases tube current for obese patients. However, the extent of change in tube current can be controlled with the use of the appropriate modulation strengths, which may be set as weak, average, or strong. Thus, the strong modulation setting for obese patients results in a larger increase in radiation dose and in reduced image noise than the preset image quality reference mAs and average modulation setting. The weak modulation setting for obese patients results in more image noise and lower radiation dose than the preset image quality reference mAs and average modulation setting. Conversely, compared with the preset mAs, a strong modulation setting for slim patients results in more image noise and lower radiation dose. Likewise, a weak modulation setting for slim patients results in less image noise and higher radiation dose. These modulation types for slim and obese patients were determined from preclinical studies conducted by the vendor (unpublished data, Siemens Medical Solutions).
To use the combined modulation technique, the modulation strength for slim and obese patients must be selected before acquisition of the localizer radiograph; the modulation strength can be constant for all patients or can be changed on the basis of the diagnostic requirement for image quality. The software determines whether a patient is slim or obese from the localizer radiograph and modulates the dose based on the preselected modulation strength for these patients. Image quality and radiation dose can be controlled by selecting an appropriate setting of combined modulation and image quality reference mAs value.

Image Quality Assessment

Quantitative image noise was recorded for each examination in the liver parenchyma at the level of the porta hepatis with a region of interest of constant rectangular shape and 30 square pixels. Each region of interest was placed in a homogeneous region in the liver parenchyma without obvious vessels or focal liver lesions.
Two subspecialty radiologists, one with 8 years' experience and the other with 3 years' experience, blinded to the scanning techniques, independently reviewed each study on a digital PACS diagnostic workstation (AGFA Impax RS 3000 1K review station, AGFA Technical Imaging Systems). All CT examinations were assessed at the same window level and width level of 40 and 400 H, respectively. Each study was assessed independently for image noise, diagnostic acceptability, presence of streak artifacts, and visibility of small structures at three different levels in the abdomen and pelvis. These subjective image quality parameters were selected on the basis of prior studies [4] and European Guidelines on Quality Criteria for CT recommendations [5].
Each radiologist graded image noise using the following 5-point scale: 1, very little noise; 2, better than average noise; 3, acceptable noise; 4, more than acceptable noise; and 5, too much noise. Image noise was categorized as acceptable if there was average mottle or graininess with acceptable visualization of anatomic structures and interfaces between structures of different attenuation. Too much noise was defined as graininess or mottle that interfered with visualization of these structures, and very little noise was graded on the basis of minimal image graininess.
Diagnostic acceptability was graded using the following 4-point scale: 1, fully acceptable; 2, probably acceptable: 3, only acceptable in limited conditions; and 4, unacceptable. Fully acceptable diagnostic acceptability was defined as acceptable contrast, sharpness of different structures, and lesion visualization. Unacceptable diagnostic acceptability was defined as completely unsatisfactory visualization of these image attributes. Visibility of small structures, such as small blood vessels, lymph nodes, or adrenal glands, was graded on a 5-point scale: 1, excellent; 2, above average; 3, acceptable; 4, suboptimal; and 5, very poor. Streak artifacts were graded using a 3-point scale: 1, absent; 2, present without affecting visibility; and 3, present and affecting visibility.

Statistical Analysis

The Student's t test (Excel, Microsoft) was performed to compare the ages and weights of patients who were scanned with any of the three techniques (combined modulation, angular modulation, and constant tube current). The Wilcoxon's signed rank test (MedCalc software, MedCalc) was used to compare image noise, diagnostic acceptability, presence of streak artifacts, and visibility of small structures for examinations performed with the different techniques. To minimize chance occurrence of significant statistical difference with multiple comparisons (four qualitative parameters × three levels = 12 comparisons), Bonferroni correction [6] was applied to find the appropriate level of alpha or p value for statistical significance. The alpha level for significant statistical difference for each test was lowered to 0.0042 to bring the alpha level overall back to 0.05 for multiple comparisons performed in the present study.
Means and SDs of quantitative noise, CTDIvol, and DLP were determined for examinations performed with the combined modulation, angular modulation, and constant tube current techniques (Excel). Quantitative image noise values for examinations performed with different techniques were compared with the analysis of variance (MedCalc software). Likewise, CTDIvol and DLP for examinations performed with the aforementioned techniques were also compared using the analysis of variance test. Linear statistical correlations between patient weight and CTDIvol for examinations performed with the different techniques were calculated using MedCalc software. The degree of interobserver concordance was determined using the kappa test. Kappa coefficient values for interobserver agreement were considered as slight if less than 0.2; fair, 0.21-0.40; moderate, 0.41-0.60; substantial, 0.61-0.80; or almost perfect, 0.80-1.00.

Results

Study Cohort Homogeneity

There was no significant statistical difference between the weights of the patients included in the studies performed with the different techniques including angular modulation (mean weight, 74 kg; range, 44-120 kg), combined modulation (weak decrease [slim]-strong increase [obese] type: mean weight, 75 kg and range, 40-136 kg; average decrease [slim]-average increase [obese] type: mean weight, 78 kg and range, 34-127 kg), or constant tube current (mean weight, 82 kg; range, 44-117 kg) (p = 0.06-0.3). Likewise, no significant difference among the ages of the patients included in the studies performed with the different techniques was found (p = 0.2-0.5).

Image Quality

The average values and SDs for subjective image noise, diagnostic acceptability, and conspicuity of small structures for studies performed with the different techniques are summarized in Table 1. There was no significant difference in image quality parameters (subjective noise, diagnostic acceptability, and conspicuity of small structures) for the examinations performed with constant tube current, angular modulation, and the weak decrease-strong increase type of combined modulation technique (p = 0.01-0.9, Bonferroni's correction) (Figs. 2A, 2B, 2C, and 2D). However, subjective image quality scores for studies performed with the average decrease (slim)-average increase (obese) type of combined modulation were significantly lower than the corresponding scores for the studies performed with the other techniques (p < 0.0001). Among examinations performed with the average decrease (slim)-average increase (obese) type of combined modulation technique, six of nine examinations with more than acceptable subjective noise were not fully acceptable to both radiologists.
TABLE 1: Average Subjective Image Noise, Diagnostic Acceptability and Conspicuity of Small Structures for Examinations Performed Using Different Techniques
Average ± SD
TechniqueSubjective Image NoiseaDiagnostic AcceptabilitybConspicuity of Small Structuresc
Constant tube current2.3 ± 0.41.0 ± 0.42.0 ± 0.4
Angular modulation2.3 ± 0.41.0 ± 0.12.3 ± 0.4
Combined modulation   
   Weak decrease–strong increase2.6 ± 0.41.0 ± 0.22.5 ± 0.4
   Average decrease–average increase
3.0 ± 0.4d
1.3 ± 0.1d
2.7 ± 0.5d
Note—Data are average ± SD ratings of two radiologists. With the exception of the average decrease–average increase settings, there was no substantial difference between the quality of images acquired with the remaining techniques.
a
On a 5-point scale, with 1 being very little noise
b
On a 4-point scale, with 1 being fully acceptable
c
On a 5-point scale, with 1 being excellent
d
p < 0.0001
There was no significant difference between patient weight for examinations with unacceptable noise and patient weight for examinations with acceptable noise (p = 0.9). All examinations performed with the techniques, including the weak decrease (slim)-strong increase (obese) type of combined modulation, were deemed fully acceptable.
There was no significant difference in subjective image noise, diagnostic acceptability, and conspicuity of small structures at different levels in the abdomen and pelvis assessed in the present study for examinations performed with different techniques (p > 0.1). However, regardless of the technique used for CT, significantly greater noise streaking artifacts were noted at the level of the confluence of the hepatic veins compared with the porta hepatis and upper margin of the acetabulum (p < 0.0001). There was almost perfect interobserver agreement between the two radiologists (κ =0.9, p < 0.05).
The objective image noise values (SD of the mean attenuation value) for the examinations performed with the different techniques are summarized in Table 2. There was no significant difference in objective image noise values for examinations performed with the constant tube current technique, angular modulation, and weak decrease (slim)-strong increase (obese) type of combined modulation (p > 0.1). However, objective image noise was significantly higher in examinations performed with the average decrease (slim)-average increase (obese) type of combined modulation technique when it was compared with the other techniques assessed in the present study.
TABLE 2: Average Objective Image Noise, CT Dose Index Volume (CTDIvol) and Dose–Length Product (DLP) for Examinations Performed Using Different Techniques
Average ± SD
TechniqueObjective Image Noise (H)CTDIvol (mGy)DLP (mGy × cm)
Constant tube current7.9 ± 2.019.0 ± 2.2814.3 ± 94.0
Angular modulation8.5 ± 2.815.4 ± 2.0a658.3 ± 87.5a
Combined modulation   
   Weak decrease–strong increase9.9 ± 2.011.0 ± 2.5a473.0 ± 105.4a
   Average decrease–average increase
11.9 ± 2.2a
10.6 ± 1.9a
456.1 ± 80.0a
a
p < 0.0001

Radiation Dose

The average and SD values of CTDIvol and DLP for CT examinations performed with different tube current techniques assessed in the present study are summarized in Table 2. A significant reduction in radiation dose was noted with the combined modulation and angular modulation techniques compared with the constant tube current technique (p < 0.0001). Compared with the constant tube current technique, there was a 19% reduction (from 19.0 to 15.4 mGy) in radiation dose for angular modulation, a 42% reduction (from 19.0 to 11.0 mGy) with weak decrease (slim)-strong increase (obese) type of combined modulation, and a 44% reduction (from 19.0 to 10.6 mGy) with average decrease (slim)-average increase (obese) type of combined modulation. The average reduction in radiation dose (p < 0.0001) with combined modulation techniques was 43%, whereas use of the angular modulation technique resulted in a 19% reduction (p < 0.0001).
A significant linear statistical correlation was found between patient weight and CT-DIvol for studies performed with weak decrease (slim)-strong increase (obese) type (r = 0.84, p < 0.0001) and average decrease (slim)-average increase (obese) type of combined modulation (r = 0.65, p < 0.0001). However, no significant statistical correlation was found between patient weight and CTDIvol for examinations performed with the constant tube current technique (r = 0.01, p = 0.2) or angular modulation technique (r = 0.19, p > 0.05).

Discussion

Concerns about the increasing use of CT and the rapid evolution of MDCT technology have inspired the search for an automatic technique of controlling scanning parameters so that a desired image quality can be obtained at the optimal radiation dose. Technologic developments for CT radiation dose reduction include prepatient collimation of X-ray beams, use of better image-processing algorithms, noise reduction filters, efficient detector configuration, and automatic tube current modulation [7]. These techniques assume added importance in view of several publications that have documented substantial variations in radiation dose associated with current clinical CT protocols and acceptable image quality with a substantial reduction in tube current [7, 8]. Of these techniques, automatic tube current modulation represents the most promising technologic innovation for optimization of radiation dose in patients undergoing CT [7].
In previous studies, investigators have reported a substantial reduction in radiation dose with the angular automatic tube current modulation technique. The initial study reported a 38% reduction in average effective milliampere-seconds (mAs) with the use of an online angular modulation technique in the shoulder region [9]. A recent pediatric CT study performed with an online angular modulation technique reported a 26-43% (mean, 36%) reduction in radiation dose compared with a fixed tube current technique [10]. In adult patients, use of the angular modulation technique for scanning six anatomic regions—including the head, shoulder, thorax, abdomen, pelvis, and extremities (knees)—resulted in a 15-50% reduction in mAs [11].
Likewise, the effects of z-axis modulation on image quality and radiation dose have recently been reported. Compared with the fixed tube current technique, the z-axis modulation technique (Auto mA, GE Healthcare) resulted in a 37.5% reduction in the average mAs value (1-87.5%) in 87% of the patients undergoing CT of the abdomen and pelvis [12]. The study also reported a slight increase in the average mAs (11.6%) when the technique was used for scanning heavier patients. However, compared with manual selection of a fixed tube current, CT examinations of the abdomen and pelvis that are performed with the technique provide images with similar image noise, diagnostic acceptability, and lesion detection. Although several publications have documented the effect of separate angular and z-axis modulation techniques on image quality and the associated radiation dose, to the best of our knowledge the use of a combined modulation technique for routine abdominopelvic CT has not been reported before in the medical literature [9-12].
Our study shows that use of the combined modulation technique with weak decrease (slim)-strong increase (obese) settings for CT of the abdomen and pelvis results in a 42% radiation dose reduction (compared with constant tube current scanning) with acceptable subjective image quality and objective image noise. Although the combined modulation technique was associated with a higher reduction in radiation dose (44%) than constant tube current scanning, images acquired at those settings were not satisfactory for routine abdominopelvic CT. Compared with the angular modulation technique in age- and weight-matched patients, the average decrease (slim)-average increase (obese) and weak decrease (slim)-strong increase (obese) settings resulted in 31% (from 15.4 to 10.6 mGy) and 29% decreases (from 15.6 to 11.0 mGy) in radiation dose, respectively. Higher radiation dose savings with the combined modulation technique in comparison with radiation dose reductions reported with independent use of the angular and z-axis modulation techniques confirm the value of combining the two techniques to obtain maximum benefit [9-12].
A strong correlation between patient weight and CTDIvol with use of the combined modulation technique proves that the technique adapts tube current on the basis of patient size and validates its use in abdominopelvic CT. It is interesting to note that the lack of statistical correlation between patient weight and CTDIvol for examinations performed with the fixed tube current technique and angular modulation technique corroborates the findings of other studies [3, 12]. These findings may justify the use of the combined automatic tube current modulation technique for standard CT of the abdomen and pelvis.
Fig. 2A —Transverse contrast-enhanced CT images at level of porta hepatis acquired with different techniques show acceptable image quality. CT image of 58-year-old woman scanned at fixed tube current of 200 effective mAs (CT dose index volume [CTDIvol] = 20.4 mGy, noise = 8 H).
Fig. 2B —Transverse contrast-enhanced CT images at level of porta hepatis acquired with different techniques show acceptable image quality. CT image of 67-year-old woman scanned with angular modulation at 200 effective mAs (CTDIvol = 15.4 mGy, noise = 8 H).
Fig. 2C —Transverse contrast-enhanced CT images at level of porta hepatis acquired with different techniques show acceptable image quality. CT image of 47-year-old woman scanned with combined modulation at weak decrease (slim)-strong increase (obese) settings (CTDIvol = 11.12 mGy, noise = 9 H).
Fig. 2D —Transverse contrast-enhanced CT images at level of porta hepatis acquired with different techniques show acceptable image quality. CT image of 64-year-old woman scanned with combined modulation at average decrease (slim)-average increase (obese) settings (CTDIvol = 8.8 mGy, noise = 10 H).
Interestingly, we did not find any difference between patient weight for studies with acceptable and unacceptable image quality when using combined modulation. However, a recent study reported that patients with unacceptable image quality using the z-axis modulation technique, which was not assessed in the present study, had substantially lower weights and regional transverse diameters than patients with acceptable image quality [3]. The study recommended the use of different strengths of z-axis modulation (with less reduction in radiation dose) for “slimmer” patients compared with “larger” patients. Thus, the lack of difference in the weight of patients with acceptable and those with unacceptable image quality noted in our study can be explained by the different curves of tube current adaptation used for the combined modulation technique in slim and obese patients (Fig. 1).
The present study highlights pertinent considerations that relate to the use of combined modulation techniques. It is important to remember that the image quality reference mAs setting and the size of the patient undergoing CT examination determine the image quality and radiation dose savings with the combined modulation technique assessed in our study. Thus, the results of our study may not reflect actual image quality and radiation dose with use of the combined modulation technique in different circumstances. For example, a higher image quality reference mAs may be associated with greater radiation dose. This raises an interesting question not addressed in the present study: What is the ideal image quality reference mAs for an average-size patient for a given indication? Until that value is determined, imaging centers or radiologists must select an image quality reference mAs value that meets their preference or adopt the level recommended by the vendor. This method introduces an element of arbitrary selection and will affect image quality and radiation dose associated with use of this technique.
Similarly, although the radiologists in our study did not accept the average decrease (slim)-average increase (obese) settings of combined modulation technique, which was associated with a greater reduction in radiation dose, some radiologists and CT centers may find “noisier” images obtained at these settings acceptable for diagnostic interpretation. Another consideration with the combined modulation technique is the need to determine an individual image quality reference mAs or modulation setting (strong, average, or weak) for different body regions and indications including standard- and low-dose CT protocols. Additional studies must be performed to evaluate the use of average decrease (slim)-average increase (obese) and strong decrease-weak increase settings of combined modulation technique for low radiation dose indications, such as follow-up CT, the kidney stone imaging protocol, and CT colonography.
There are limitations to our study. We did not perform statistical analysis to determine the appropriate number of patients required to assess the combined modulation technique. Different patients were assessed with combined modulation, angular modulation, and constant tube current techniques because it is difficult to evaluate the same group of patients with the different techniques due to radiation concerns. However, statistical analysis showed that there was no difference between the weights and ages of the patients who underwent CT with the different techniques. Although we did not compare combined modulation techniques of different vendors, the underlying basis of automatic tube current modulation techniques is similar regardless of the vendor.
The use of the strong decrease-weak increase type of combined modulation was not assessed in the present study because it would have resulted in further reduction of radiation dose compared with the average decrease (slim)-average increase (obese) type of combined modulation technique and further deterioration in image noise and diagnostic acceptability. A lower image quality reference mAs value—for example, less than 160 mAs—for combined modulation may be useful in situations in which greater image noise may be more acceptable and is less likely to affect diagnostic acceptability, such as with a kidney stone CT protocol and CT angiography. Likewise, although the average decrease (slim)-average increase (obese) type of combined modulation was associated with compromised image quality for clinical abdominopelvic CT, we did not assess its application in other indications for low-dose CT.
In summary, compared with constant tube current scanning of the abdomen and pelvis, the combined modulation technique results in a substantial reduction in radiation dose—up to 42%—with acceptable image noise and diagnostic information for abdominopelvic CT.

Footnotes

S. Rizzo was supported by a research grant from Siemens Medical Solutions, Forchheim, Germany.
Address correspondence to M. K. Kalra ([email protected]).

References

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Kalra MK, Maher MM, Kamath RS, et al. Sixteen-detector row CT of abdomen and pelvis: a study for optimization of Z-axis modulation technique performed in 153 patients. Radiology 2004; 233:241-249
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Information & Authors

Information

Published In

American Journal of Roentgenology
Pages: 673 - 679
PubMed: 16498094

History

Submitted: September 24, 2004
Accepted: January 31, 2005

Keywords

  1. abdomen
  2. CT technique
  3. pelvis
  4. radiation dose
  5. tube current modulation

Authors

Affiliations

Stefania Rizzo
Department of Radiology, Massachusetts General Hospital, White 270, Boston, MA 02114.
Mannudeep Kalra
Department of Radiology, Massachusetts General Hospital, White 270, Boston, MA 02114.
Present address: Department of Radiology, Emory University Hospital, 1364 Clifton Rd. NE, Atlanta, GA 30322.
Bernhard Schmidt
Siemens Medical Solutions, Forchheim, Germany.
Tejas Dalal
Department of Radiology, Massachusetts General Hospital, White 270, Boston, MA 02114.
Present address: University of Vermont, Burlington, VT 05405.
Christoph Suess
Siemens Medical Solutions, Forchheim, Germany.
Thomas Flohr
Siemens Medical Solutions, Forchheim, Germany.
Michael Blake
Department of Radiology, Massachusetts General Hospital, White 270, Boston, MA 02114.
Sanjay Saini
Present address: Department of Radiology, Emory University Hospital, 1364 Clifton Rd. NE, Atlanta, GA 30322.

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