Rationale and Objectives
To evaluate image quality and radiation exposure of portal venous–phase thoracoabdominal third-generation 192-slice dual-source computed tomography (DSCT) with automated tube voltage adaptation (TVA) in combination with advanced modeled iterative reconstruction (ADMIRE).
Materials and Methods
Fifty-one patients underwent oncologic portal venous–phase thoracoabdominal follow-up CT twice within 7 months. The initial examination was performed on second-generation 128-slice DSCT with fixed tube voltage of 120 kV in combination with filtered back projection reconstruction. The second examination was performed on a third-generation 192-slice DSCT using automated TVA in combination with ADMIRE. Attenuation and image noise of liver, spleen, renal cortex, aorta, vena cava inferior, portal vein, psoas muscle, and perinephric fat were measured. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were calculated. Radiation dose was assessed as size-specific dose estimates (SSDE). Subjective image quality was assessed by two observers using five-point Likert scales. Interobserver agreement was calculated using intraclass correlation coefficients (ICC).
Results
Automated TVA set tube voltage to 90 kV ( n = 8), 100 kV ( n = 31), 110 kV ( n = 11), or 120 kV ( n = 1). Average SSDE was decreased by 34.9% using 192-slice DSCT compared to 128-slice 120-kV DSCT (7.8 ± 2.4 vs. 12.1 ± 3.2 mGy; P < .001). Image noise was substantially lower; SNR and CNR were significantly increased in 192-slice DSCT compared to 128-slice DSCT (all P < .005). Image quality was voted excellent for both acquisition techniques (5.00 vs. 4.93; P = .083).
Conclusions
Automated TVA in combination with ADMIRE on third-generation 192-slice DSCT in portal venous–phase thoracoabdominal CT provides excellent image quality with reduced image noise and increased SNR and CNR, whereas average radiation dose is reduced by 34.9% compared to 128-slice DSCT.
Contrast-enhanced computed tomography (CT) is a well-established cross-sectional imaging technique in oncologic patients . Because CT may account for majority substantial amount of radiation exposure in oncologic patients , new technical innovations have focused on improvement of image quality and further radiation dose reduction to keep radiation dose “as low as reasonably achievable” .
Several techniques for radiation dose reduction have been proposed . Tube current modulation (TCM) adapts tube current to the body’s anatomy in real-time and is activated on most currently available CT systems . Tube voltage reduction is another promising technique for radiation dose reduction and can be performed using automated attenuation-based software . A potential drawback of low–tube-voltage acquisition is an increase in image noise, which may impair diagnostic image quality. Therefore, manual adjustment of tube voltage on an individual basis may be challenging. Automated tube voltage adaptation (TVA) can adapt tube voltage to the patient body based on attenuation measurements, maintaining sufficient image quality, even in low-kilovoltage examinations. Furthermore, the application of iterative reconstruction (IR) techniques can substantially reduce image noise .
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Materials and methods
Patient Population
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Examination Protocol
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Radiation Dose Estimation
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SSDE(mGy)=CTDIvol×conversionfactor S
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Objective Image Quality Analysis
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CNR=HUsofttissuestructure–HUmuscleimagenoise(muscle) C
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Subjective Image Quality Analysis
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Statistical Analysis
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Results
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Radiation Dose
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Table 1
Radiation Dose Comparisons of Examination A (Second-Generation 128-Slice DSCT) and Examination B (Third-Generation 192-Slice DSCT)
Parameter Examination A Examination B_n_ = 51 All; n = 51P Value 90 kV; n = 8 100 kV; n = 31 110 kV; n = 11 120 kV; n = 1 Effective diameter (cm) 28.5 ± 4.9 28.8 ± 3.4 .243 25.9 ± 1.7 29.0 ± 3.4 30.4 ± 3.1 30.0 CTDI vol (mGy) 12.1 ± 3.2 7.8 ± 2.4 <.001 5.2 ± 0.5 7.3 ± 1.3 10.7 ± 1.9 13.6 DLP (mGy × cm) 831.8 ± 236.9 541.0 ± 182.2 <.001 334.0 ± 45.7 500.8 ± 96.2 776.0 ± 163.0 860.5 SSDE (mGy) 15.2 ± 2.8 9.9 ± 2.4 <.001 7.4 ± 0.4 9.3 ± 1.4 12.8 ± 1.3 16.7
CTDI vol , CT dose index volume; DLP, dose–length product; DSCT, dual-source computed tomography; SSDE, size-specific dose estimates.
Data are averages ± standard deviations.
Examination B is further subdivided into 90, 100, 110, and 120 kV.
P value between examination A and examination B (all).
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Objective Image Quality Analysis
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Table 2
Results of Objective Image Analysis
Parameter Examination A Examination B_n_ = 51 All, n = 51P Value 90 kV, n = 8 100 kV, n = 31 110 kV, n = 11 120 kV, n = 1 CT values (Hounsfield unit [HU]) Liver 96.9 113.7 <.001 138.9 114.3 98.3 62.4 Spleen 102.1 127.4 <.001 157.9 126.7 108.6 113.9 Renal cortex 154.3 197.8 <.001 261.1 198.7 153.1 154.0 Aorta 139.4 186.8 <.001 237.3 186.5 155.6 133.7 Inferior vena cava 105.9 132.6 <.001 162.1 138.6 98.3 90.4 Portal vein 140.0 181.8 <.001 236.2 181.6 147.7 128.4 Psoas muscle 61.1 63.6 .043 69.1 62.1 63.8 62.9 Perinephric fat −95.5 −102.1 <.001 −102.7 −101-6 −103.3 −97.9 Image noise (HU) Liver 12.4 9.6 <.001 9.1 9.7 9.7 8.9 Spleen 13.0 9.7 <.001 9.7 9.5 10.1 8.7 Renal cortex 13.9 11.9 .001 11.1 12.0 12.2 10.4 Aorta 14.2 10.6 <.001 11.2 10.3 10.9 10.7 Inferior vena cava 14.6 10.9 <.001 10.3 11.2 10.7 9.7 Portal vein 15.0 11.9 <.001 13.3 11.5 11.7 11.7 Psoas muscle 13.4 10.4 <.001 10.4 10.5 10.4 9.1 Perinephric fat 12.9 10.1 <.001 9.1 10.7 9.3 8.5 Signal-to-noise ratio Liver 8.3 12.1 <.001 15.4 12.0 10.4 7.0 Spleen 8.3 13.5 <.001 16.6 13.6 11.0 13.1 Renal cortex 11.9 17.3 <.001 24.2 17.1 12.9 14.8 Aorta 10.5 18.2 <.001 21.2 18.7 14.5 12.4 Inferior vena cava 7.5 12.4 <.001 15.9 12.6 9.4 9.3 Portal vein 9.7 15.9 <.001 19.2 16.2 13.0 11.0 Psoas muscle 4.8 6.3 <.001 6.9 6.2 6.4 6.9 Perinephric fat −7.8 −10.8 <.001 −12.1 −10.3 −11.3 −11.5 Contrast-to-noise ratio Liver muscle 3.2 5.0 <.001 6.8 5.1 3.6 −0.1 Spleen muscle 3.4 6.3 <.001 8.8 6.3 4.6 9.1 Renal cortex muscle 7.7 13.3 <.001 18.8 13.5 9.1 10.1 Aorta muscle 6.5 12.3 <.001 16.5 12.4 9.3 7.8 Inferior vena cava muscle 3.7 6.8 <.001 9.2 7.5 3.5 3.0 Vena porta muscle 6.5 11.7 <.001 16.4 11.7 8.6 7.2
Data are mean ± standard deviation.
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Subjective Image Quality Analysis
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Table 3
Results of Subjective Image Quality Assessment
Examination Overall Image Quality Delineation of Soft Tissue Structures Image Sharpness Image Noise Examination A 4.93 (0.79) 4.84 (0.31) 4.98 (0.63) 4.52 (0.26) Examination B 5.00 (1.00) 4.93 (0.79) 4.86 (0.40) 4.98 (0.71)P value .083 .160 .580 <.001
Data are means (intraclass correlation coefficients) based on ratings from two observers.
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Discussion
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