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BMI-Based Radiation Dose Reduction in CT Colonography

Posted Mar 31, 2013 Updated Apr 10, 2023
By Michael Ginsburg MD , Piotr Obara MD , Leon Wise MD , Kristen Wroblewski MS , Michael W. Vannier MD , Abraham H. Dachman MD
8 min read

Rationale and Objectives

There is potential for x-ray dose reduction in computed tomography colonography (CTC) relative to body mass index (BMI). We evaluated the association between BMI and three-dimensional (3D) CTC image quality to assess the potential utility of BMI as the basis for radiation dose reduction in CTC.

Materials and Methods

Ninety-six consecutive patients underwent CTC and were randomized for scanning at 15 or 30 mAs. Extremely obese patients (BMI > 50) were excluded. Each patient was scanned supine and prone on a multidetector CT scanner. Postprocessing CTC visualization was performed on a dedicated workstation. Three independent observers assessed 3D image quality using a four-point scale. Image noise was measured in both the abdomen and pelvis. The association between BMI and image noise was examined using random-effects linear regression models. Logistic regression was used to examine the relationship between BMI, mAs, and conspicuity scores.

Results

Statistically significant differences in image noise were observed between 15 and 30 mAs in both the abdomen and pelvis, and the difference was greater with increasing BMI. A positive relationship was detected between BMI and noise in the abdomen ( P < .001) and pelvis ( P < .001). Inverse correlation was identified between BMI and conspicuity scores in the abdomen ( P = .01) and pelvis ( P < .001). Overall conspicuity scores were reduced for both 15 and 30 mAs groups as BMI increased.

Conclusion

The radiation dose for CTC can be reduced by 40% and 70% below commonly employed doses for overweight and normal BMI patients, respectively, by using a BMI-adjusted dose reduction approach. Conspicuity scores dropped in obese patients with reduced dose suggesting that standard accepted doses should be utilized in that group.

Because computed tomography colonography (CTC) has gained acceptance as an appropriate method to screen for colorectal cancer , the risk of ionizing radiation exposure for patients 50 years of age or older has been evaluated, particularly if multiple follow-up exams are needed . Recently, the potential risks of CT radiation associated with CT heightened the concerns even in comparatively low-dose examinations such as CTC. It is important to realize that the risks of “low”-dose CT are controversial. The Health Physics Society position statement updated in 2010 states that for low-dose exposure (50-100 mSv) the associated health risks are “either too small to be observed or are nonexistent” . In a recent survey of research institutions performing CTC, Liedenbaum et al reported a median effective dose of only 5.7 mSv (2.5-2.8 mSv per series) , or approximately half of the dose administered for a diagnostic single-phase CT examination of abdomen and pelvis.

The radiation dose for CTC exams performed at our institution is much lower than the average dose in a survey reported by Liedenbaum et al , and is in the range of the lowest doses reported previously. The purpose of this study was to determine the potential of radiation dose reduction relative to the body mass index (BMI). After random assignment to one of two doses (15 mAs or 30 mAs), we evaluated three-dimensional (3D) image quality subjectively and image noise quantitatively. These findings were compared based on BMI to determine the best dose for patients with different body habitus. This information could serve as a guide for further reduction of patient radiation dose in CTC, using BMI as a parameter for individualized dose adjustment.

Materials and methods

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Quantitative Assessment: Noise

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Qualitative Assessment: Conspicuity Score

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Figure 1, Conspicuity scale. Three-dimensional endoluminal images showing a typical example of each conspicuity score. (a) Zero score defined as no mucosal detail with excessive floaters, (b) score of “1” defined as poor mucosal detail and/or many floater, (c) score of “2” defined as moderate mucosal detail and minimal floater, and (d) score of “3” defined as excellent mucosal detail.

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Statistical Methods

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Results

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Quantitative Assessment: Noise Analysis

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Figure 2, Box plot showing the distribution of noise in the abdomen based on mAs and body mass index. Data are calculated by combining the supine and prone data.

Figure 3, Box plot showing the distribution of noise in the pelvis based on mAs and body mass index. Data are calculated by combining the supine and prone data.

Table 1

Results from Random-Effects Linear Regression Models Fit to Image Noise

Variable Regression Coefficient 95% CI_P_ Value PELVIS BMI (per 1 SD increase) ∗ 29.3 (22.8, 35.7) <.001 mAs <.001 15 Referent 30 −24.6 (−30.8, −18.3) BMI × mAs interaction −13.9 (−21.9, −6.0) .001 Position Prone Referent Supine 6.5 (3.9, 9.0) <.001 Constant 95.2 (90.2, 100.3) <.001 ABDOMEN BMI (per 1 SD increase) ∗ 26.0 (19.5, 32.5) <.001 mAs <.001 15 Referent 30 −30.3 (−38.2, −22.3) BMI × mAs −9.7 (−17.4, −2.0) .01 Position <.001 Prone Referent Supine 9.6 (6.4, 12.8) Constant 98.3 (92.3, 104.4) <.001

CI, confidence interval.

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Figure 4, Image noise in abdomen. (a) Predicted image noise values from model in Table 1 . X-axis is standardized body mass index (BMI) values (ie, z-scores), a value of 0 is equal to the mean BMI, which is about 27; a value of 1 is equal to 1 SD above the mean, which is about 33; and a value of −1 is equal to 1 SD below the mean, which is about 21. (b) Predicted differences in image noise between mAs 30 versus 15 at various values of BMI.

Figure 5, Image noise in pelvis. (a) Predicted values from model in Table 1 . X-axis is standardized body mass index (BMI) values (ie, z-scores); a value of 0 is equal to the mean BMI, which is about 27; a value of 1 is equal to 1 SD above the mean, which is about 33; and a value of −1 is equal to 1 SD below the mean, which is about 21. (b) Predicted differences in image noise between mAs 30 versus 15 at various values of BMI.

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Qualitative Assessment: Conspicuity Score Analysis

Per-Patient analysis (using maximum of supine and prone for each patient)

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Figure 6, Conspicuity scores distribution in the abdomen based on mAs and body mass index. The maximum of each patient's scores across the supine and prone positions are displayed.

Figure 7, Conspicuity scores distribution in the pelvis based on mAs and body mass index. The maximum of each patient's scores across the supine and prone positions are displayed.

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Per-CT scan analysis

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Figure 8, Conspicuity scores distribution based on mAs, location, and body mass index. Each patient's supine and prone studies are considered separately.

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Discussion

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Academic Radiology, Volume 20, Issue 4
Journals General Radiology
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