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Systematic Comparison of Reduced Tube Current Protocols for High-pitch and Standard-pitch Pulmonary CT Angiography in a Large Single-center Population

Posted Apr 30, 2016 Updated Apr 10, 2023
By Andreas M. Bucher MD , Matthias J. Kerl MD , Moritz H. Albrecht MD , Martin Beeres MD , Hanns Ackermann PhD , Julian L. Wichmann MD , Thomas J. Vogl MD , Ralf W. Bauer MD , T. Lehnert MD
14 min read

Rationale and Objectives

Benefits of iterative reconstruction (IR) algorithms combined with dose-reduction techniques have been shown at computed tomography pulmonary angiography (CTPA) in several medium to small patient collectives. In this study, we performed a systematic comparison of image quality to combinations of reduced tube current (RC) and IR for both standard-pitch (SP) single-source and high-pitch (HP) dual-source CTPA in a large, single-center population.

Materials and Methods

Three hundred eighty-two consecutive patients (October 2010 through December 2012) received clinically indicated CTPA with one of four consecutively changed protocols: (1) HP SC : 180 mAs, weighted filtered back projection, pitch = 3; (2) HP RC : 90 mAs, IR, pitch = 3; (3) SP SC : 180 mAs, weighted filtered back projection, pitch = 1.2; and (4) HP RC : 90 mAs, IR, pitch = 1.2. Tube potential was 100 kV. Vascular attenuation and standardized signal-to-noise ratio (sSNR) were measured in the pulmonary trunk (sSNR PT ) and on segmental artery level (sSNR S1 , sSNR S10 ). Dose-length-product was recorded per series. Two independent investigators rated image quality. Kolmogorov-Smirnov test, Kruskal-Wallis test, and kappa statistics were used for statistical analysis. Median values are presented per group.

Results

Image quality was consistent between all groups (observer 1: P = 0.118; observer 2: P = 0.122). Inter-reader consistency was very good (κ = 0.866, P < 0.001). Dose-length-product was significantly reduced in HP and RC groups ( P < 0.001 for each; SP SC : 139.5 mGycm; HP RC : 92 mGycm; SP SC : 211 mGycm; HP RC : 137 mGycm). sSNR was comparable (sSNR PT overall: P = 0.052; sSNR S1 overall: P = 0.161; and sSNR S10 overall: P = 0.259).

Conclusions

Substantial dose reduction can be within a routine clinical setting without quantifiable loss of image quality either by HP pulmonary angiography or by a combination of IR and RC in either HP or SP acquisition.

Introduction

Pulmonary embolism (PE) is considered the third most common cause of acute cardiovascular disease, accounting for 5–10% of hospital deaths in the United States . Computed tomography (CT) pulmonary angiography (CTPA) has been established as the first-line imaging modality and the standard of care imaging test for the diagnosis of PE . CTPA is noninvasive, has a high sensitivity and specificity, is widely available, cost-effective, and can simultaneously evaluate a variety of differential diagnoses . Depending on successful evaluation of pretest likelihood, incidence of PE on CT angiography (CTA) is 5–15% . Thus, the number of patients receiving ionizing radiation in CTPA for suspected PE with a negative test result is far in excess of the true-positive cases. In addition, several studies have been suggesting that in clinical routine, inadequate pretest screening negatively impacts this percentage . Increasing use of CTPA has therefore led to continuous efforts to optimize standard CTPA protocols and thus reduce the radiation dose (RD) of CTPA while maintaining image quality .

In recent years, several technical advances at CT have enabled significant reduction of RD at CTPA . Among these, tube voltage reduction from 120 kV to 100 kV can lower radiation exposure by 50% alone . High-pitch (HP) protocols with pitch values up to 3.4 are available on dual-source CT scanners through use of simultaneous data acquisition from both x-ray tubes and detectors. This can reduce RD on the order of 30% . Model-based iterative reconstruction algorithms substantially reduce image noise and are therefore ideal for use in combination with low-dose acquisitions . Because of the increasing use of CTPA, implementation of these dose-saving techniques in standard clinical protocols has become a relevant factor to reduce medical radiation exposure . Benefits of individual protocol changes have been well demonstrated in medium- and small-size patient populations. However, a systematic confirmation of a combination of these dose-reduction techniques against standard dose protocols in both dual-source and single-source acquisitions on a large sample population in a routine clinical setting has not yet been reported.

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Materials and Methods

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Patient Population

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TABLE 1

Acquisition Parameters and Reconstruction Algorithm Corresponding to Group Number

Parameter_HP SC__HP RC__SP SC__SP RC_ Tube activation Single source Dual source Single source Dual source Collimation 128 × 0.6 mm 128 × 0.6 mm 128 × 0.6 mm 128 × 0.6 mm Rotation time (s) 0.5 0.28 0.5 0.28 Pitch 3.0 3.0 1.2 1.2 Reference tube current time product (mAs) 180 90 180 90 Tube voltage (kV) 100 100 100 100 Reconstruction algorithm wFBP SAFIRE wFBP SAFIRE

HP RC , high pitch, reduced tube current; HP SC , high pitch, standard tube current; SAFIRE, sinogram-affirmed iterative reconstruction; SP RC , standard pitch, reduced tube current; SP SC , standard pitch, standard tube current; wFBP, weighted filtered back projection.

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CT Protocols

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Contrast Medium Application

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Image Reconstruction

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Figure 1, Example image section of each reconstruction algorithm. Reconstructions were performed either in weighted filtered back projection ( a ) or with an iterative reconstruction algorithm ( b ). This reconstruction of a high-pitch, reduced tube current dataset ( HP RC protocol) illustrates the more homogenized visual appearance of iteratively reconstructed images. Removal of statistical image noise reduces interference of grainy noise patterns down to small image structures, as is noticeable within the subsegmental pulmonary vasculature.

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Quantitative Analysis/Objective Image Analysis

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Figure 2, Qualitative image measurements. An axial slice at the height of the pulmonary bifurcation was used for measurement of background noise ( a ) and for measurements of vascular attenuation in the pulmonary trunk. Horizontal ( b ) and vertical ( c ) thorax diameters were recorded on the same height.

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Qualitative Analysis/Subjective Image Analysis

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TABLE 2

Five-point Likert Scales for Image Quality and Overall Image Noise

Image Quality of Pulmonary Vasculature 1 Excellent to the very peripheral branches beyond the subsegmental level, i.e. arteries of the fifth order or lower 2 Good, exclusion of PE to the subsegmental level still possible 3 Moderate, exclusion of PE up to the segmental level is possible, with uncertainties beyond 4 Still diagnostic to the lobar level, but significantly reduced confidence beyond 5 Not fully diagnostic, only central PE could be excluded

Overall Image Noise 1 Excellent, even large homogenous areas are low in noise 2 Good, without degradation of the pulmonary vessel margin delineation due to image noise 3 Moderate, image noise is limiting the evaluation of the pulmonary parenchyma 4 Excessive noise, poor delineation of the bronchiolar vessel margins, considerable image noise in the chest wall and mediastinal structures, yet still diagnostic image 5 Diagnostic evaluation impracticable because of noise artifacts

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RD Measurements

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Parameter Calculation

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Statistical Analysis

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Results

Population

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TABLE 3

Group Demographics, Thorax Diameters, and Radiation Dose

Group_N_ Age (years) Gender (m/w) TD hor /TD ver CTDI vol (mGy) DLP (mGycm)HP SC \* 92 60.7 ± 18.5 45/47 36.9 ± 3.9/23.4 ± 15.4 3.99 (3.89–4.05) 139.5 (133–143)HP RC \\ 100 63.5 ± 15.7 52/48 36.4 ± 3.9/22.3 ± 6.4 2.53 (2.33–2.65) 92 (82–96)SP SC \\\* 96 59.5 ± 16.8 64/32 37.1 ± 4.3/23.0 ± 9.5 7.12 (6.66–7.30) 211 (197–220)SP RC \\\\ 94 64.2 ± 15.0 55/39 37.0 ± 4.3/23.3 ± 8.8 4.01 (3.86–4.06) 137 (116–147)

CTDI vol , CT dose index volume; DLP, dose-length-product; TD hor , horizontal thorax diameter; TD ver , vertical thorax diameter.

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Quantitative Analysis

Measurements of Patient Habitus

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Vascular Attenuation

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Figure 3, Vascular enhancement per group. Box and whisker plot of mean vascular enhancement on segmental artery level showing median, 25th, and 75th percentiles; there were no statistically significant differences in vascular attenuation for groups of equal pitch ( P = nonsignificant). Vascular attenuation of the S10 segmental artery was significantly higher with high-pitch protocols ( P ≤ 0.023). Vascular attenuation of the S1 segmental artery was significantly higher with high-pitch protocols only in comparison to SP SC ( P ≤ 0.042). HU, Hounsfield units; ROI, region of interest; S1, pulmonary artery of segment 1; S10, pulmonary artery of segment 10; TP, pulmonary trunk.

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BN

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Image Noise

SD PT

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Figure 4, Image noise per group. Box and whisker plot of mean image noise in vascular ROI measurements showing median, 25th, and 75th percentiles; image noise within vascular ROIs was lower with iterative reconstruction algorithm, except for measurements of the pulmonary trunk. SD TP in HP RC was significantly higher than SD TP in both standard-pitch reconstructions ( P = 0.001). HU, Hounsfield units; ROI, region of interest; S1, pulmonary artery of segment 1; S10, pulmonary artery of segment 10; TP, pulmonary trunk.

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SD S1 /SD S10

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SNR

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TABLE 4

Quantitative Image Noise per Group

HP SC__HP RC__SP SC__SP RC SNR PT 36.40 (26.91–45.36) 38.78 (34.67–51.94) 41.95 (29.94–52.22) 44.04 (31.90–57.69) SNR S1 41.16 (34.67–51.94) 46.48 (36.38–54.94) 45.51 (31.28–60.75) 51.29 (35.73–68.36) SNR S10 41.43 (34.11–52.21) 44.89 (36.26–53.14) 44.17 (28.38–58.96) 46.03 (33.87–61.19) sSNR PT 0.027 (0.026–0.032) 0.031 (0.031–0.036) 0.033 (0.032–0.040) 0.035 (0.033–0.041) sSNR S1 0.032 (0.031–0.037) 0.037 (0.036–0.042) 0.037 (0.035–0.044) 0.038 (0.038–0.048) sSNR S10 0.030 (0.030–0.036) 0.037 (0.035–0.041) 0.035 (0.033–0.043) 0.036 (0.036–0.045)

CTDI vol , CT dose index volume; DLP, dose-length-product; HP RC , high pitch, reduced tube current; HP SC , high pitch, standard tube current; SNR, signal-to-noise ratio; SP RC , standard pitch, reduced tube current; SP SC , standard pitch, standard tube current; sSNR, standardized signal-to-noise ratio; TD hor , horizontal thorax diameter; TD ver , vertical thorax diameter.

Values are presented as median (25th–75th quartiles).

Figure 5, Signal-to-noise ratio per group. Box and whisker plot of mean signal-to-noise ratio showing median, 25th, and 75th percentiles; standardized signal-to-noise ratio (sSNR) values were comparable between all groups ( P = nonsignificant). HU, Hounsfield units.

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Qualitative Analysis

Image Quality per Reader

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TABLE 5

Subjective Rating for Each Group

HP SC__HP RC__SP SC__SP RC Observer 1 Observer 2 Observer 1 Observer 2 Observer 1 Observer 2 Observer 1 Observer 2 “excellent” (1/5) to “good”(2/5) image quality 47 (94%) 47 (94%) 43 (86%) 44 (88%) 40 (80%) 41 (83%) 43 (86%) 43 (86%) “moderate”(3/5) image quality 3 (6%) 3 (6%) 7 (14%) 6 (12%) 9 (18%) 9 (18%) 6 (12%) 6 (12%) “very low” (1/5) to “low” (2/5) image noise 1 31 (64%) 32 (64%) 37 (74%) 38 (76%) 43 (86%) 44 (88%) 49 (98%) 50 (100%) “medium” (3/5) image noise a 17 (34%) 16 (32%) 13 (26%) 12 (24%) 7 (14%) 6 (12%) 1 (2%) 0 (0%) Movement artifacts \* 0 (0%) 0 (0%) 1 (2%) 1 (2%) 7 (14%) 6 (12%) 12 (24%) 13 (26%) Contrast artifacts \\ 1 (2%) 1 (2%) 2 (4%) 2 (4%) 6 (12%) 7 (14%) 2 (4%) 2 (4%)

HP RC , high pitch, reduced tube current; HP SC , high pitch, standard tube current; SP RC , standard pitch, reduced tube current; SP SC , standard pitch, standard tube current.

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Interobserver Agreement

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RD

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Discussion

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Conclusion

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Academic Radiology, Volume 23, Issue 5
Journals General Radiology
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