Radiation dose and image quality with abdominal computed tomography with automated dose-optimized tube voltage selection

Abstract

Objectives

This study assessed image quality and radiation dose of multidetector computed tomography (CT) examination using a standard protocol and a low-voltage protocol.

Methods

Patients requiring contrast-enhanced abdominal CT examination were randomly assigned to two groups with different voltage protocols: (i) 120 kV; (ii) an automated attenuation-based tube potential optimization mode (CARE kV). The volume CT dose index (CTDI_vol) and dose length product (DLP) were recorded. Image quality was semiquantitatively assessed by two blinded radiologists using a five-point scale.

Results

There were 39 patients in the 120 kV group and 50 patients in the CARE kV group. There was no obvious difference in image quality score between the groups. CARE kV resulted in a voltage reduction to 100 kV in 45 patients and to 80 kV in five patients. CTDI_vol and DLP were significantly lower with CARE kV than with the 120 kV protocol.

Conclusions

The use of CARE kV reduces radiation dose with no loss of image quality compared with a standard 120 kV protocol.

Keywords

CARE kV computed tomography automated dose-optimized tube voltage selection radiation dose image quality

Introduction

Computed tomography (CT) is useful for both diagnostic and therapeutic purposes and is one of the most important radiological examinations undertaken worldwide.¹ The frequency of CT examinations has increased rapidly, from 2% of all radiological examinations in year 1990 to 10–15% in year 2000.¹ But, unlike ultrasound and magnetic resonance imaging, CT exposes patients to radiation levels close to the annual dose received from natural sources of radiation; therefore, the increasing use of diagnostic CT examinations² has contributed to an increase in overall radiation doses.^1,3 In addition, the increased use of CT may have resulted in an increased risk of cancer: one study based on CT usage data between 1991 and 1996 suggested that as much as 0.4% of all cancers in the USA may be due to CT radiation.³ Some organizations, such as the International Commission on Radiation Protection⁴ and the US Food and Drug Administration,⁵ have published reports on reducing unnecessary radiation exposure; the International Commission on Radiation Protection⁴ clearly stated that CT examinations must be performed using doses that are as low as reasonably achievable to maximize patient benefit and minimize potential harm. Despite such advice, lack of awareness and lack of effective quality assurance among medical radiologists⁶ may be contributing to the increased levels of radiation exposure observed.

Great efforts have been made to minimize CT doses and optimize image quality. One parameter that affects radiation dose is the electrical current,⁷ and manual modulation of the current according to patient size is the traditional approach to reducing the radiation dose.⁶ Concurrently, technologies such as automatic exposure control^8,9 have been developed and made available for all CT systems now available. Automatic exposure control automatically adapts the tube current according to patient attenuation and has demonstrated dose reductions in the region of 20–40% without loss of image quality.^1,6,10–12 However, tube voltage has a larger influence on radiation dose than current, and some studies have indicated that low voltage is useful for facilitating reductions in radiation dose^6,13–19 and quantity of contrast material.¹⁹ Because the patients’ sizes and examination sites differ between individual patients, however, it is difficult for a physician to choose the best voltage for any particular patient.⁶

An automated attenuation-based tube potential optimization mode (CARE kV, Somatom Definition Flash; Siemens Healthcare, Forchheim, Germany) has been developed and made available for some types of CT instruments. In one study, the radiation dose with CARE kV was significantly reduced (by 25%) compared with a standard protocol with 120 kV for contrast CT scan.²⁰ However, this study included a relatively low number of patients and did not assess the intraindividual variability.²⁰

In the present study, we compared image quality and radiation dose using CARE kV and a standard 120 kV protocol.

Patients and methods

Patient population

This study included patients assigned to clinically indicated contrast-enhanced abdominal CT at the Tongde Hospital of Zhejiang Province, Hangzhou, China between May 2011 and July 2012. Inclusion criteria were age 18–80 years and body mass index (BMI) 20–23 kg/m². Patients were recruited to the study when they attended the clinic and were assigned (using a randomly using computer-generated code) to one of two treatment groups with different voltage protocols: (i) 120 kV; (ii) CARE kV. This study was approved by the local Ethics Committee of Tongde Hospital of Zhejiang Province. Informed verbal consent was obtained from each individual and participation was on a completely voluntary basis. Glomerular filtration rate (GFR) was estimated based on the equation described by Levey et al.²¹ and those patients with low GFR (<90 ml min⁻¹ per 1.73 m²) were excluded from the study.

CT protocols and data reconstruction

Scans were performed using a 64-slice CT machine (Somatom Definition AS, Siemens Healthcare) equipped with CARE kV. Iodinated contrast material (iopamidol, 370 mg iodine/ml) was injected at a flow rate of 2.6 ml/s into an antecubital vein, followed with a 60-ml saline flush. All scans were performed using the following parameters: beam collimation 64 × 0.625 mm; section thickness 5.0 mm; helical pitch 0.7; scanning field of view 40 cm. Patients were asked to hold their breath during scanning. Scans in the standard voltage group were performed using 120 kV and automatically adjusted current. Scans in the CARE kV group were performed using CARE kV according to the attenuation profile of the topogram (from 70 to 140 kV) and the attenuation-based tube current modulation (CARE Dose4D).

All data were reconstructed using a section thickness of 2 mm, increment of 1.7 mm and a soft-tissue convolution kernel (B30f). Further analysis was performed with an external workstation (Multi-Modality Workplace; Siemens Healthcare).

Radiation dose estimation

The volume CT dose index (CTDI_vol; mGy) and dose length product (DLP; mGy cm) provided by the CT scanner were recorded at the portal phase for each patient. The effective dose (ED; mSv) was estimated by multiplying DLP by a conversion factor of 0.015.²²

CT image quality assay

Two independent radiologists (H.H. and F.W.) with 3 and 4 years of experience in cardiovascular radiology, respectively, assessed the image quality of the aortoiliac arterial system. The assessors were blinded to all clinical information. The assessment protocol allowed the adjusting of window widths and levels. Overall image quality was graded using a diagnostic readability score²³ based on a five-point scale: 5, excellent; 4, good; 3, acceptable; 2, poor; 1, nondiagnostic. The criteria for image grading were consensually established before the beginning of image assessment.

Statistical analyses

Database management and analysis was performed using SPSS® version 11.5 (SPSS Inc., Chicago, IL, USA). Data were presented as the mean ± SD. Normally distributed data were compared using independent-samples t-test. Nonparametric data were tested using Wilcoxon’s signed-ranked test. Relationships between radiation dose and age, section number and BMI were tested using Spearman’s correlation coefficient. The significance level was set at P < 0.05.

Results

This study recruited 50 patients to each treatment group, but on analysis excluded 11 participants from the control group on the basis of age (<18 or >80 years of age) and/or estimated GFR <90 ml min⁻¹ per 1.73 m². Therefore, the study population comprised 89 patients and their characteristic are presented in Table 1. There were no significant differences in age or BMI between the groups. The currents used were 145–286 mA in the 120 kV group and 135–473 mA in the CARE kV group. The use of CARE kV resulted in a reduction in voltage from the standard 120 kV to 100 kV in 45/50 (90%) patients and to 80 kV in the remaining five (10%) patients.

Table 1.

Characteristics of 89 patients requiring contrast-enhanced abdominal CT examination, assigned to two different voltage protocols: 120 kV or CARE kV.

Characteristic	120 kV, n = 39	CARE kV, n = 50
Age, years	55.7 ± 15.1	53.1 ± 17.2
Sex, female/male	10/29	30/20
BMI, kg/m²	21.6 ± 1.01	21.6 ± 0.99

Data presented as mean ± SD or n.

BMI, body mass index.

No statistically significant differences observed (P ≥ 0.05; independent-samples t-test).

The CTDI_vol, DLP and ED were decreased by 14, 17 and 17%, respectively, in the CARE kV group compared with the 120 kV group (all P < 0.05; Table 2).

Table 2.

Radiation dose parameters in 89 patients requiring contrast-enhanced abdominal CT examination assigned to two different voltage protocols: 120 kV or CARE kV.

Dose parameter	120 kV, n = 39	CARE kV, n = 50
CTDI_vol, mGy	11.2 ± 2.0	9.6 ± 1.5*
DLP, mGy cm	497.9 ± 133.9	411.4 ± 69.3*
ED, mSv	7.5 ± 2.0	6.2 ± 1.0*

Data presented as mean ± SD.

P < 0.05; independent-samples t-test.

CTDI_vol, volume CT dose index; DLP, dose length product; ED, effective dose.

Image quality in the 120 kV group was rated as excellent for 14 images, good for 23 images and acceptable for two images. Image quality in the CARE kV group was rated as excellent for 17 images, good for 30 images and acceptable for three images. The mean image scores are shown in Figure 1. There was no statistically significant difference in the diagnostic readability score between the CARE kV group (3.72 ± 0.57) and the 120 kV group (3.69 ± 0.51).

Figure 1.

Image quality scores of the aortoiliac arterial system in a study of contrast-enhanced abdominal CT examination, assessing two different voltage protocols: 120 kV (n = 39) and CARE kV (n = 50). Overall image quality was graded using a diagnostic readability score²³ based on a five-point scale: 5, excellent; 0, nondiagnostic. Data analysed using Wilcoxon’s signed-ranked test.

Table 3 displays the correlations between CTDI_vol, DLP, ED, age and BMI. There were significant positive correlations between BMI and DLP (P < 0.05), and between BMI and ED (P < 0.05).

Table 3.

Analysis of correlations between radiation dose parameters, age and body mass index (BMI); 89 patients requiring contrast-enhanced abdominal CT examination were assigned to two different voltage protocols: 120 kV or CARE kV.

Parameter	Age	CTDI_vol	DLP	ED
CTDI_vol	−0.0189	–	–	–
DLP	−0.322*	0.928**	–	–
ED	−0.0322*	0.928**	1.000**	–
BMI	−0.079	0.158	0.290*	0.290*

P < 0.05, **P < 0.01; independent-samples t-test.

CTDI_vol, volume CT dose index; DLP, dose length product; ED, effective dose.

Discussion

The increasing use of CT examinations has contributed to an increase in radiation doses that has been observed in the general population.^1–3 In the present study, we found that CTDI_vol, DLP and ED in CT examination were decreased by 14–17% using CARE kV compared with a standard 120 kV approach. This study indicated that CARE kV has the potential to reduce radiation dose without incurring loss of image quality in CT procedures.

The principle of CARE kV was introduced by Winklehner et al.²⁴ CARE kV is a fully automated feature integrated into the workflow at the CT scanner, which optimizes the tube potential within the range 70–140 kV on the basis of the individual patient (i.e. attenuation profile, size and size changes along the patient’s axis), system capabilities and clinical task. The tube current is adapted with CARE Dose4D simultaneously. To determine the lowest radiation dose possible, the system first calculates the tube current required to reach a defined image quality for different voltages, then the CT dose index is calculated for the different combinations of current and voltage. The scan is performed using the current–voltage combination with the lowest radiation dose. Winklehner et al.²⁴ indicated that radiation dose with CARE kV could be substantially reduced (by 25%) while image quality was maintained, compared with a standard protocol with fixed 120 kV; however, the study included a relatively low number of patients and did not assess intraindividual variability.²⁴ In the present study, using CARE kV resulted in a 14–17% decrease in radiation dose. The reduction in this study is smaller than that observed by Winklehner et al.,²⁴ possibly because the variation in BMI was smaller in the present study.

For contrast-enhanced CT of the abdomen, it has been indicated that increased noise at low voltage and high current settings may result in a substantial deterioration in image quality.²⁵ It is difficult to explain why CARE kV could reduce the radiation dose without leading to a loss of image quality, which is what was observed in the present study. The CARE kV technique selects the voltage according to the patient’s size and the diagnostic task. Nakaura et al.¹⁴ speculated that small patient size may decrease noise with low voltage, and that a low contrast-agent dose protocol may decrease noise with low-voltage techniques. In the current study, the BMI of patients was 20–23 kg/m², which indicates that the patients were of normal size.

There are some limitations to the present study. First, only a relatively small number of patients was included. Secondly, the difference in each patient’s radiation dose between the protocols, which may have contributed to differences observed between the protocols, was not evaluated. Thirdly, images were evaluated semiqualitatively; no full quantitative analysis (such as contrast-to-noise ratio or attenuation) was undertaken. In addition, the CARE kV protocol is only used in a few CT applications, which has restricted our ability to review other findings related to this approach.

In conclusion, the present study findings suggest that CARE kV has the potential to offer abdominal CT with a reduced radiation dose compared with standard 120 kV, without affecting image quality. The study findings also indicate that, in particular, decreasing the tube voltage may benefit those patients who need to undergo multiple CT examinations. Further studies are needed to evaluate CARE kV in larger populations and for other diagnostic tasks.

Footnotes

Declaration of conflicting interest

The authors declare that there are no conflicts of interest.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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