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DOI: 10.1148/radiol.2362041327
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(Radiology 2005;236:565-571.)
© RSNA, 2005

Medical Physics

Comparison of Different Body Size Parameters for Individual Dose Adaptation in Body CT of Adults1

Jan Menke, MD

1 From the Department of Diagnostic Radiology, University Hospital, Rosenwinkel 5, 37081 Goettingen, Germany. Received July 29, 2004; revision requested October 7; revision received October 17; accepted November 15. Address correspondence to the author (e-mail: Menke_J{at}T-Online.de).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 APPENDIX A
 APPENDIX B
 APPENDIX C
 References
 
PURPOSE: To investigate prospectively which of several body size parameters are suitable for individual dose adaptation in body computed tomography (CT) of adults.

MATERIALS AND METHODS: Three body regions (thorax, abdomen, pelvis) were scanned exclusively for clinical reasons, with institutional ethical approval and informed consent. For each of the three regions, 50 men and 50 women (aged 18–87 years) were studied (300 scans total). Individual x-ray properties for each scan were summarized with a water-equivalent diameter (Dw). Different body size parameters, based on weight, height, and shape, were correlated with Dw by using regression analysis. This resulted in Dw estimation errors of different magnitudes, indicated with 95% prediction intervals. The errors from weight were compared with those from each of the other body parameters by using comparison of variance in paired samples (P < .05). In addition, a topogram-based estimate for Dw was studied, which simulated an automated body size measurement.

RESULTS: For the thorax, abdomen, and pelvis, mean Dw was 28.0, 29.1, and 29.3 cm, and estimation of Dw from weight enabled 95% prediction intervals of ±2.5, ±2.4, and ±2.6 cm, respectively. Combinations of height and weight were only slightly more or even less exact than were measurements from only weight. Diameter-related parameters such as body circumference were similar to or better than weight. However, the topogram-based estimate was significantly more exact.

CONCLUSION: Body weight and circumference enable suitable estimates for individual dose adaptation in body CT of adults if automated dose adaptation is not available.

© RSNA, 2005


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 APPENDIX A
 APPENDIX B
 APPENDIX C
 References
 
Over the past decades, computed tomography (CT) (1) has become an important imaging technique, but it is also a major contributor to individual and collective radiation dose (2,3). For patient protection, CT dose is recommended to be as low as reasonably achievable to meet clinical needs (3). For conventional projection radiography, technical developments including automated exposure control help to optimize the relationship between image noise and radiation dose. Correspondingly, automated exposure control with tube current modulation has been developed for CT (4,5), but many installed CT scanners are not equipped with this facility and these scanners are likely to remain in use for some time. With these CT scanners the user has to estimate the tube current–time product, and consequently the weighted CT dose index (CTDIw) (6,7), which is necessary to acquire images with the desired level of image noise among patients of different body sizes (8). This necessary individual CT dose can be estimated by the user by means of personal experience or by means of look-up tables with reference to a suitable body size parameter (9).

Several authors have suggested the use of body weight for individual adaptation of CT dose (8,10–14), but the literature also indicates that other body size parameters, such as body mass index, cross-sectional diameter, or a combination of weight and height, might be comparable or better parameters (8,12–17). Thus, the aim of this study was to prospectively investigate which of several body size parameters are suitable for individual dose adaptation in body CT of adults.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 APPENDIX A
 APPENDIX B
 APPENDIX C
 References
 
Patients and Body Regions
For this prospective study, three body regions—thorax, abdomen, and pelvis—were scanned in patients, exclusively for clinical reasons, with institutional ethical approval and informed patient consent. One hundred CT scans obtained in 50 men and 50 women were included for each of the three body regions, for a total of 300 CT scans. The patients were enrolled in the order of their admittance to undergo CT; thus, they represent a random sample from the regional CT patient collective. A brief clinical history was obtained, and each patient was asked for body weight and height, which were usually well known by the patient. Only patients who knew their weight and height were included in the study. The CT scans were acquired between July 2001 and December 2002 and were evaluated mainly from April to June 2003 by the author.

CT Examinations
After clinical history was obtained, the patient was placed in the supine position on the table of a four–detector row CT scanner (Mx8000; Philips Medical Systems). A topogram was acquired with a posteroanterior projection. Then a spiral CT scan was acquired with the following parameters: collimation of 4 x 2.5 mm (four detectors with 2.5-mm section thickness), tube voltage of 120 kV, rotation time of 0.5 second, pitch of 1, and tube current selected between 150 and 250 mAs per section (CTDIw of 11–19 mGy) according to clinical indications. The CT scan was reconstructed with a 5-mm effective section thickness and was digitally stored together with the topogram. The three CT body regions of this study were defined as follows: The thorax was evaluated from the lung apex to the costodiaphragmatic angle; the abdomen, from the dome of the liver to the iliac crest; and the pelvis, from the iliac crest to the symphysis.

Body Size Parameters
This study was performed to analyze several body size parameters that were based to different extents on the weight, height, and shape of the body (Table 1). For comparison, an x-ray attenuation–based estimate was calculated from the topogram (topogram-based estimate [Etopo]).2 These body size parameters were correlated with the Dw (water-equivalent diameter). Dw is used to summarize the individual x-ray attenuation properties of the scanned body region by means of a single number and thus may be useful for adaptation of individual CT dose. Dw is introduced as follows.


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  		TABLE 1. Body Size Parameters

 


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  		Figure 1. Posteroanterior topogram shows calculation of the Etopo. A rectangular region of interest is placed to determine the Etopo of the abdomen in a 65-year-old man undergoing CT for follow-up of renal cancer (weight, 84 kg; height, 173 cm; circumference, 98 cm). Width of the region of interest is 50 cm and its mean topogram attenuation reading is 233.8 arbitrary units. With the formula given in footnote 2, Etopo is calculated as 61.7 cm. With the corresponding regression equation for the abdomen from Table 2 (Dw = 12.4 + 0.33 Etopo; 95% prediction limit, ±1.2 cm), Dw is estimated from the topogram as 32.8 cm ± 1.2. The true Dw from the corresponding CT sections is 32.5 cm. For comparison, estimation of Dw from body weight is 31.1 cm ± 2.4, and estimation of Dw from body circumference is 30.8 cm ± 2.6.

 

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  		TABLE 2. Regression Analysis between Body Size Parameters and Dw

 
CT Dose, Image Noise, and Body Size
The relationship between CT dose and image noise depends greatly on individual body size (6,8,19). With standard CT protocols, technical factors like section thickness and filter kernel are often kept constant among adults of various body sizes. If only the tube current (and from this, the applied CT dose) is changed to obtain images with a similar level of noise, the formula used by Nagel (6), who referred to Brooks and Di Chiro (19), can be reduced to describe the relationship between CT dose, image noise, and body size: CTDIw·{sigma}2 {propto} e(µ·D), where CTDIw is measured in milligrays as displayed on the CT monitor (6,7), {sigma} is the image noise equal to the standard deviation of CT numbers in a homogeneous region, e is the base of natural logarithms (e = 2.718...), µ is the mean attenuation coefficient of the body region at a defined tube voltage (in cm–1), and D is the diameter of the scanned body region in centimeters.

The CTDIw is related to the patient dose associated with CT examination of the trunk (20), and it is used to describe reference radiation dose to the patient for different CT protocols (7). CTDIw can be estimated from the CT dose index of the Food and Drug Administration, or CTDIFDA (6). In this study, image noise was described in Hounsfield units. The exponential term on the right side of the equation, e(µ·D), describes the x-ray attenuation within the body. Besides tube voltage, this attenuation is dependent on diameter and tissue composition of the scanned body region. The CT dose-noise relationship can be studied with a conic water phantom. Figure 2 shows a nomogram of image noise ({sigma}) at different circular diameters (Dw) for different CT doses (CTDIw) at 120 kV.



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  		Figure 2. Nomogram for CT dose-noise relationship. A conic water phantom was scanned at different doses at multi–detector row CT (120 kV, 500-mm field of view, 4 x 2.5-mm collimation, 5-mm effective section thickness, pitch of 1, standard body filter "C"). Tube current was varied between 50 and 250 mAs per section. The numbers at the upper part of the exponential curves give the corresponding CTDIw (3.8–19.1 mGy) as displayed on the CT monitor. X-axis shows the Dw of the 5-mm-thick circular section at actual scanning position. Y-axis shows corresponding image noise ({sigma}) as standard deviation (SD) of CT numbers.

 
Estimation of Dw
Water is a major component of body tissues (21), and the x-ray attenuation coefficient for water is approximately 0.19 cm–1 at 120 kV (1). For CT dose estimation, the scanned body region may be virtually modeled as a cylindrical water phantom (10,22). Its Dw is chosen so that this phantom has the same average x-ray attenuation as the scanned body region (Fig 3, B). Dw is used to summarize the x-ray attenuation information from all voxels within the scanned body region as a single parameter. Dw can be calculated from the x-ray attenuation information, which is inherent in CT scans (Appendixes A, B) (6,10,22,23). Figure 3 shows this calculation for the abdomen of a 65-year-old man, with a Dw of 32.5 cm. According to the CT dose-noise relationship, the individual's x-ray attenuation property (summarized as Dw) is relevant for the necessary CT dose (as CTDIw) if a predefined image noise should be achieved. However, Dw is calculated from the CT scan and is thus unknown until scanning is completed. To adapt the necessary CT dose to individual body size, the individual's attenuation properties (summarized as Dw) must be estimated prior to the examination by a user with personal experience or from references of suitable body size parameters. In this study, a body size parameter was regarded as suitable for estimation of Dw if the corresponding estimation error was small; this was analyzed as follows in the next section.



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  		Figure 3. Example of calculation of Dw in the same patient as in Figure 1. A, Shaded surface display of abdomen (ascending anterior-posterior view). B, Abdomen modeled as cylindrical water phantom with Dw. C, Transverse CT scan of abdomen, averaged to one section. A region of interest has been drawn around the body contours, with an area of 985 cm2 and an average CT number of –160 HU. D, Calculation according to Appendix A produces a Dw of 32.5 cm.

 
Statistical Analysis
Correlation between Dw and body size parameters.—For each of the three body regions, the correlation between Dw and the different body size parameters was studied by using simple linear regression analysis. In addition, body height and weight were simultaneously correlated with Dw by means of multiple linear regression analysis. The strength of the correlation was measured by using the correlation coefficient r. With the regression equation, Dw can be estimated from the corresponding body size parameter. The standard deviation of the regression residuals corresponds to the standard error of the estimate, or SEE. The 95% prediction interval was calculated from ±2 SEE and quantified the usual range of estimation errors for Dw (24).

Comparison of body size parameters regarding their suitability for CT dose estimation.—In this study, a body size parameter with a large estimation error for Dw was regarded as less suitable, and a body size parameter with a smaller estimation error for Dw was regarded as more suitable for CT dose estimation. In the literature, CT dose is often related to body weight (8,10–14), and so the estimation error of body weight versus Dw was used as the reference standard in this study. The estimation errors from each of the other body size parameters were compared with the estimation error from body weight by using the comparison of variance in paired samples according to Armitage and Berry (Appendix C) (24). The null hypothesis of identical variances was rejected at P < .05.

Statistical program.—Statistics were calculated by using Statgraphics Plus for Windows (version 2.1; Manugistics, Rockville, Md).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 APPENDIX A
 APPENDIX B
 APPENDIX C
 References
 
Patients and Body Regions
For each of the three body regions (thorax, abdomen, and pelvis), 100 CT scans from 50 women and 50 men were studied, which resulted in 300 different body region scans from 168 adults (Table 3). The patients ranged in age from 18 to 87 years, in weight from 46 to 108 kg, and in height from 153 to 200 cm, thus covering a broad range of different body sizes in adults. A total of 19 (11%) of 168 patients weighed less than 60 kg, and 58 (35%) of 168 patients weighed more than 80 kg. Within the study group, body weight showed only a relatively weak correlation with body height (r = 0.47).


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  		TABLE 3. Descriptive Statistics for Patients and Scanned Body Regions

 
Relationship between Dw and Body Size Parameters
Regression analysis of Dw versus the different body size parameters resulted in 11 different regression equations for each body region (Table 2); examples of this analysis can be seen in Figure 4. Estimation of Dw from weight was used for reference and resulted in 95% prediction intervals of ±2.5 cm for the thorax, ±2.4 cm for the abdomen, and ±2.6 cm for the pelvis. For all three body regions, estimation of Dw solely from height caused significantly larger estimation errors. Estimation of Dw from body mass index or body surface area was also generally less accurate. For the thorax and the abdomen, the fixed ratio of weight divided by height and the independent combination of height and weight resulted in Dw estimates that were similar to those determined by using only weight. For the pelvis only, the estimates from the weight-height ratio and from independent combination of height and weight were slightly more accurate. Estimation of Dw from only one body diameter (either coronal or sagittal) resulted in Dw estimates that were generally less accurate than were those determined from weight, even though these body diameters are region-specific measurements while weight is a global body size measurement. However, combining the information from the sagittal and coronal diameters by using the mean diameter or the body circumference resulted in Dw estimates that were generally similar to (ie, for the abdomen) or more accurate than (ie, for the thorax and pelvis) those determined by using body weight. For all body regions, estimation of Dw from the topogram (ie, from Etopo) was significantly more accurate than estimation of Dw from body weight.



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  		Figure 4. Graphs show correlation between Dw and body weight, body circumference, and Etopo for the abdomen based on the data of 100 CT scans. The central line represents the regression equations, and outer lines represent the 95% prediction limits of the fitted linear model. Numeric results of each regression analysis are given in Table 2.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
APPENDIX A
 APPENDIX B
 APPENDIX C
 References
 
Since the invention of CT, corresponding advances in diagnostic imaging have been accompanied by considerations about radiation dose (1,25,26). The applied CT dose should be as low as reasonably achievable to reduce the stochastic risk for radiation-induced cancer (3). However, reduction of CT dose is limited because of the clinical requirements for image quality.

If CT is clinically indicated then the CT scans acquired are recommended to meet different quality criteria that necessitate a minimum of spatial resolution and a maximum of image noise, depending on the organs and clinical problem under investigation (7). In using standard CT protocols for adults, the factors for spatial resolution (eg, filter kernel and section thickness) are often kept constant among patients of different body size, so that image noise appears to be the variable that determines image quality among patients of different body size. With a half-value thickness for tissues of approximately 4 cm (6), image noise increases or decreases by more than a factor of 1.4 if thick (arbitrary definition: Dw, >33 cm; weight, >95 kg) or tall (arbitrary definition: Dw, <25 cm; weight, <55 kg) adults are scanned with the same CT dose (as CTDIw) as are standard-sized adults (Dw, approximately 29 cm; weight, approximately 75 kg) (Table 3). To achieve in thick patients the same level of image noise that is achieved in standard-sized patients, the CTDIw would have to be approximately doubled (6). Therefore, in thick patients, some reduction of image quality because of a higher level of image noise or reduced spatial resolution (smoothing filter or larger voxel size) is often accepted to restrict radiation dose. On the other hand, tall patients are often scanned with a CTDIw similar to that used in standard-sized patients. Those CT images are usually of fine quality with little noise. However, dose savings of up to 50% are possible in tall adults if a level of image noise similar to that used in standard-sized patients is accepted (6).

When using a CT scanner without automated dose adaptation, look-up tables with reference to a suitable body size parameter can be used to obtain images with a similar level of quality and noise among patients of various body sizes (8,9,12,13,16,27). When using a CT scanner with automated dose adaptation, images of a similar level of noise and quality are expected to be generated automatically. But even when using these improved CT scanners, the user remains responsible for the patient's radiation exposure and should compare the automatically applied CT dose with reference doses of CTDIw. However, reference doses are usually given only for standard-sized patients (ie, 60–80 kg) (7) and thus need modification with respect to individual body size if thick or tall patients are scanned.

For such an individual CT dose adaptation, different body size parameters are available, which are less or more suitable. Height and weight are body size parameters that can be easily taken from the patient's history in most cases. Several authors have shown that body weight is suitable for individual adaptation of the CT dose (8,10–14). Therefore, weight may be looked on as the reference standard of body size parameters, as it was used in this study. In contrast, height as a stand-alone parameter is not a suitable CT dose predictor; this is indicated by the results of this study and by those of others (12,13). However, Starck et al (16) discussed that scanning parameter selection based only on weight might lead to large variations in image quality between tall and short persons who have the same body weight.

The information from height and weight can either be combined by using a fixed ratio like division of weight by height, body mass index, and body surface area or be entered as independent variables (height and weight) in a formula. Wildberger et al (12) and Coppenrath et al (13) found no major difference between weight and body mass index in relation to image noise. Our study also showed that fixed ratios of weight and height are not much more accurate, or, for some body regions, are even less accurate than using only weight for estimation of Dw. The use of weight and height as independent variables is only slightly more exact than using only weight. These results indicate that it is sufficient to use body weight without the additional information from body height for individual dose adaptation in body CT of adults.

According to Haaga (8), the patient's diameter may be a better predictor of the tube current requirement than is body weight because the diameter better correlates with the x-ray beam attenuation in the patient (8,27). This is in correspondence with the CT dose-noise relationship (given in Materials and Methods), which incorporates an exponential relationship between body diameter and image noise. Our study results for the thorax and pelvis show that the mean body diameter was significantly more accurate for estimation of Dw than was body weight (P < .05). For the abdomen only, there was no significant difference. However, using solely the coronal or the sagittal diameter is generally less accurate than is using body weight. According to the results of this study, the coronal and sagittal diameters should both be used. This can be done in different ways, such as by using mean cross-sectional area (13), mean body diameter, or body circumference. Among these diameter-related body size parameters, body circumference is the most easily measured. However, in medicine, body weight is a more standardized parameter than is mean body diameter or body circumference, so it seems more practical to conduct CT dose optimization programs with respect to body weight even if this may be, for some body regions, less exact than using diameter-related body size parameters.

In this study, the Etopo used for the body regions under investigation showed the most exact estimates for Dw. If implemented on CT systems, the Etopo could be used as a diameterlike and region-specific automated body size measurement. In CT dose optimization programs, such an automated estimate for Dw might be an alternative to body weight.

In summary, body weight and body circumference enable suitable estimates for individual dose adaptation in body CT of adults if automated dose adaptation is not available.


   APPENDIX A
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 APPENDIX A
APPENDIX B
 APPENDIX C
 References
 
Calculation of Dw: Dw can be calculated from the CT scan by using a simple equation (see end of paragraph). The algorithm is as follows: The CT sections of the scanned body region are averaged into one combined section (Fig 3, C). A region of interest, ROI, is drawn around the body contours. Its area, AROI, is read in square centimeters. Its mean CT number, CTROI, is rescaled to a relative attenuation coefficient, RACROI, by using the equation RACROI = (CTROI + 1000)/1000, as shown in Appendix B. For dosimetry purposes, a body region can be modeled as a cylindrical water-equivalent phantom (10,22). Its cross-sectional area is calculated as Aw = RACROI · AROI, where Aw is the water-equivalent area. Its Dw is then calculated by using the equation Dw = 2(Aw/{pi})0.5. For this algorithm, the complete scanned body region must be included in the region of interest. However, it is not relevant how much air around the body contours is included in the region of interest. If less surrounding air is included, the mean CT number of the region of interest—and therefore also the rescaled RACROI—increase, but AROI decreases proportionally so that the product Aw = RACROI · AROI remains unchanged. In summary, Dw is calculated by using the following equation: Dw = 2(AROI · RACROI/{pi})0.5, where RACROI = (CTROI + 1000)/1000.


   APPENDIX B
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 APPENDIX A
 APPENDIX B
APPENDIX C
 References
 
Attenuation coefficient scales for CT: Each voxel of a CT scan is a measurement for the x-ray attenuation of the tissue that is represented by this voxel (6). Its value is determined by using the calculation CTvox = 1000(µtis – µwtr)/µwtr, where CTvox is the CT number of the voxel and µtis and µwtr are the linear x-ray attenuation coefficients for the tissue and for water, respectively (6,23). For computations in this study, CT numbers were rescaled to a relative attenuation coefficient, RAC, with RAC = µtis/µwtr. Air has a value of 0 and water has a value of 1 on this dimensionless RAC scale. CT numbers were converted with RAC = (CTvox + 1000)/1000. For example, if CTvox = –160 HU then RAC = 0.84.


   APPENDIX C
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 APPENDIX A
 APPENDIX B
 APPENDIX C
References
 
Comparison of variances of paired regression residuals: Each regression analysis of a body size parameter versus Dw resulted in a regression line and additional residuals. The residuals from body weight, res(BW), versus Dw were used for reference in this study. Their magnitude was compared with that of the residuals from each other body size parameter res(BSP) by means of comparison of variances in two paired samples. According to Armitage and Berry (24), this is done by means of simple linear regression analysis of Xi versus Yi, where i = 1,...,n; n = 100; Xi = res(BSP)i + res(BW)i is the sum; and Yi = res(BSP)i – res(BW)i is the difference of the paired residuals for the ith CT scan. Equality of variances was tested by means of the null hypothesis, that is, the correlation coefficient of X versus Y is 0 (24). The difference was considered significant at P < .05. When compared with the estimation of Dw from body weight, body size parameters with significantly smaller residuals produce more accurate Dw estimates, and body size parameters with significantly larger residuals produce less accurate Dw estimates.


   FOOTNOTES
 

Abbreviations: CTDIw = weighted CT dose index • Dw = water-equivalent diameter • Etopo = topogram-based estimate

Author stated no financial relationship to disclose.

See also Science to Practice in this issue.

Author contribution: Guarantor of integrity of entire study, J.M.

2 The Etopo for a water-equivalent diameter (Dw) was calculated by using a simple equation, derived as follows: The posteroanterior topograms acquired with the CT system used in this study present air with a pixel value of –1000 arbitrary units. Parts of the body are presented with higher pixel values depending on the total x-ray attenuation along the line of projection through the body. In this way, the topogram comprises information about the attenuation of the scanned body region, which can be used to calculate an estimate for Dw by means of the following algorithm: A rectangular region of interest was placed on the topogram, with a constant width of 50 cm and a length determined according to the investigated body region (Fig 1). The mean topogram density, Topomean, was transformed with (Topomean + 1000)/1000 to a scale on which air has the value 0. This rescaled topogram density value was multiplied by the width of the region of interest (50 cm). In this way, Etopo = [(Topomean + 1000)/1000] · 50 cm Back


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 APPENDIX A
 APPENDIX B
 APPENDIX C
 References
 

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What Parameters Are Most Accurate in Predicting Appropriate Technique Factors for CT Scanning?
John M. Boone
Radiology 2005 236: 377-378. [Full Text] [PDF]



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J. M. Boone
What Parameters Are Most Accurate in Predicting Appropriate Technique Factors for CT Scanning?
Radiology, August 1, 2005; 236(2): 377 - 378.
[Full Text] [PDF]

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