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Low-dose Unenhanced CT for IV Contrast Bolus Timing

Posted Jun 30, 2011 Updated Apr 10, 2023
By Daniela B. Husarik MD , Daniel T. Boll MD , Rendon C. Nelson MD , Elmar M. Merkle MD
9 min read

Rationale and Objectives

To determine whether an unenhanced low-dose image acquired during automated contrast bolus timing can be used to assess hepatic steatosis.

Materials and Methods

Fifty subjects (29 male, 21 female; 26–92 years; mean body mass index (BMI; 26.9) with abdominal multiphasic computed tomography were included. Abdominal diameters and circumferences were derived from anteroposterior and lateral scout radiographs. Hepatic attenuation (HA) was measured on unenhanced low-dose images (120 kV; 40 mA; 0.5 seconds’ rotation time) and corresponding unenhanced standard-dose images (120 kV, z-axis automatic tube current modulation, noise index 11.5). Noise estimates were measured in surrounding air. Pearson correlation was calculated between abdominal circumference and BMI. Mean HA assessed on low-dose images and standard-dose images was compared using a paired Student’s t -test and Bland Altman plots.

Results

Abdominal circumference (mean, 142.8cm) correlated well with BMI ( r = 0.83). No significant difference was found for HA on low-dose images (mean +57.7 HU) compared to HA on standard-dose images (+56.0 HU) ( P = .077). Image noise (+11.5 HU) was significantly higher on low-dose images compared to image noise (+8.1 HU) on standard-dose images ( P < .05). For HA mean difference comparing low- and standard-dose images was −1.7 HU (limits of agreement: −14.6, 11.2).

Conclusion

In all subjects, hepatic attenuation can be correctly assessed on unenhanced low-dose images.

Macrovesicular hepatic steatosis is the most common cause of low-attenuating liver at computed tomography (CT) . It can have a variety of causes including obesity, hyperlipidemia, and alcohol, steroid, and chemotherapeutic drug use. Hepatic core biopsy is currently the standard method for accurate detection and quantification of macrovesicular steatosis. It is, however, invasive, associated with risk, and contributes to overall cost and morbidity. Various methods based on unenhanced as well as contrast-enhanced CT have been proposed to determine the degree of hepatic steatosis noninvasively . Although multidetector CT (MDCT) has become invaluable in the evaluation of a wide spectrum of disease processes, numerous abdominal CT protocols do not include an unenhanced phase. Unfortunately, many of the CT-based methods for detection and quantification of hepatic steatosis rely on such an unenhanced phase.

To achieve a late arterial phase on abdominal CT, a bolus monitoring scan is routinely used. This technology allows for an automated monitoring of the vascular contrast inflow phase using a series of low-dose images . When a desired level of enhancement is reached for a particular structure (eg, the suprarenal abdominal aorta), the routine diagnostic scan will be automatically initiated. In our practice, all low-dose monitoring series for arterial phase abdominal CT include a single unenhanced CT image at the level of the suprarenal abdominal aorta which also includes the liver.

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Materials and methods

Patient Population

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CT Imaging

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Quantification of Body Habitus

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Attenuation Value Assessment

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Statistical Analysis

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Results

Quantification of Body Habitus

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Attenuation Value Assessment

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Figure 1, (a) Bland-Altman analysis plotting the difference against the means for hepatic attenuation determined on unenhanced standard-dose (SD) and unenhanced low-dose (LD) series showing a mean variation of −1.7 HU with 95% confidence intervals at −14.6 HU and 11.2 HU. (b) Bland-Altman analysis plotting the difference against the means for splenic attenuation determined on unenhanced standard-dose (SD) and unenhanced low-dose (LD) series, showing a mean variation of −6.4 HU with 95% confidence intervals at −25.2 HU and 12.4 HU.

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Figure 2, Scatter plot with linear regression line and 95% confidence intervals for the difference between splenic attenuation determined on unenhanced standard-dose (SRD) and unenhanced low-dose (LD) series plotted against abdominal circumference in all 50 patients. Note the substantial variation between splenic attenuation values determined on unenhanced standard-dose (SD) and unenhanced LD series in patients with an abdominal circumference of ≥140 cm.

Table 1

Quantitative Results of Entire Study Population

Min Max Mean SD Abdominal circumference 103.0 203.6 142.8 22.6 HA unenhanced low-dose images 5.7 73.9 57.7 11.4 HA unenhanced standard-dose images 7.3 76.7 56.0 13.3 SA unenhanced low-dose images 38.0 71.3 52.0 6.0 SA unenhanced standard-dose images 29.7 55.7 45.6 6.9 CT L/S unenhanced low-dose images 0.1 1.8 1.1 0.3 CT L/S unenhanced standard-dose images 0.1 1.9 1.3 0.3 SD noise unenhanced low-dose images 4.8 39.6 11.5 6.7 SD noise unenhanced standard-dose images 4.6 18.9 8.1 2.7

CT L/S , liver-to-spleen attenuation ratio; HA, hepatic attenuation; Min, minimum; Max, maximum; SA, splenic attenuation; SD, standard deviation.

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Discussion

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Figure 3, Images obtained in a 65-year-old female subject with a body mass index of 17.3 and a circumference of 103.0 cm. (a) Unenhanced low-dose image (120 kV, 40 mA) shows three 150 mm 2 region of interest ( white circles ) each in the liver (mean, 70.7 HU) and the spleen (mean, 49.3 HU). (b) Mean hepatic attenuation and splenic attenuation on the corresponding unenhanced standard-dose image (120 kV, varying mA, noise index 11.5) are 73.3 HU and 50.7 HU, respectively.

Figure 4, Images obtained in a 69-year-old male subject with a body mass index of 41 and a circumference of 203.6 cm. (a) Unenhanced low-dose image (120 kV, 40 mA) with high image noise (standard deviation, 21.6 HU) shows three 150 mm 2 regions of interest ( white circles ) each in the liver (mean, 51.3 HU) and the spleen (mean, 60.3 HU). Please note streak artifacts ( arrows ). (b) Corresponding unenhanced standard-dose image (120 kV, varying mA, noise index 11.5) with lower image noise (standard deviation, 8.5 HU) and a mean HA of 43.3 HU and mean SA of 33.3 HU.

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Limitations

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Conclusion

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Acknowledgements

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Academic Radiology, Volume 18, Issue 7
Journals General Radiology
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