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. 2013 Sep;48(9):629-37.
doi: 10.1097/RLI.0b013e31828ad647.

Optimization of dual-energy xenon-computed tomography for quantitative assessment of regional pulmonary ventilation

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Optimization of dual-energy xenon-computed tomography for quantitative assessment of regional pulmonary ventilation

Matthew K Fuld et al. Invest Radiol. 2013 Sep.

Abstract

Objective: Dual-energy x-ray computed tomography (DECT) offers visualization of the airways and quantitation of regional pulmonary ventilation using a single breath of inhaled xenon gas. In this study, we sought to optimize scanning protocols for DECT xenon gas ventilation imaging of the airways and lung parenchyma and to characterize the quantitative nature of the developed protocols through a series of test-object and animal studies.

Materials and methods: The Institutional Animal Care and Use Committee approved all animal studies reported here. A range of xenon/oxygen gas mixtures (0%, 20%, 25%, 33%, 50%, 66%, 100%; balance oxygen) were scanned in syringes and balloon test-objects to optimize the delivered gas mixture for assessment of regional ventilation while allowing for the development of improved 3-material decomposition calibration parameters. In addition, to alleviate gravitational effects on xenon gas distribution, we replaced a portion of the oxygen in the xenon/oxygen gas mixture with helium and compared gas distributions in a rapid-prototyped human central-airway test-object. Additional syringe tests were performed to determine if the introduction of helium had any effect on xenon quantitation. Xenon gas mixtures were delivered to anesthetized swine to assess airway and lung parenchymal opacification while evaluating various DECT scan acquisition settings.

Results: Attenuation curves for xenon were obtained from the syringe test-objects and were used to develop improved 3-material decomposition parameters (Hounsfield unit enhancement per percentage xenon: within the chest phantom, 2.25 at 80 kVp, 1.7 at 100 kVp, and 0.76 at 140 kVp with tin filtration; in open air, 2.5 at 80 kVp, 1.95 at 100 kVp, and 0.81 at 140 kVp with tin filtration). The addition of helium improved the distribution of xenon gas to the gravitationally nondependent portion of the airway tree test-object, while not affecting the quantitation of xenon in the 3-material decomposition DECT. The mixture 40% Xe/40% He/20% O2 provided good signal-to-noise ratio (SNR), greater than the Rose criterion (SNR > 5), while avoiding gravitational effects of similar concentrations of xenon in a 60% O2 mixture. Compared with 100/140 Sn kVp, 80/140 Sn kVp (Sn = tin filtered) provided improved SNR in a swine with an equivalent thoracic transverse density to a human subject with a body mass index of 33 kg/m. Airways were brighter in the 80/140 Sn kVp scan (80/140 Sn, 31.6%; 100/140 Sn, 25.1%) with considerably lower noise (80/140 Sn, coefficient of variation of 0.140; 100/140 Sn, coefficient of variation of 0.216).

Conclusion: To provide a truly quantitative measure of regional lung function with xenon-DECT, the basic protocols and parameter calibrations need to be better understood and quantified. It is critically important to understand the fundamentals of new techniques to allow for proper implementation and interpretation of their results before widespread usage. With the use of an in-house derived xenon calibration curve for 3-material decomposition rather than the scanner supplied calibration and a xenon/helium/oxygen mixture, we demonstrate highly accurate quantitation of xenon gas volumes and avoid gravitational effects on gas distribution. This study provides a foundation for other researchers to use and test these methods with the goal of clinical translation.

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Figures

Figure 1
Figure 1
Top Panel: Groups of three 60cc plastic syringes were sequentially placed inside an Alderson RS-320 Lung/Chest Phantom (Radiology Support Devices, Long Beach, CA) to establish radiodensity enhancement scales for xenon gas at different photon energies (80, 100, 140 kV). A volume rendering of the setup is show in the upper-left top-panel. A resultant xenon intensity image from dual-energy three-material decomposition is shown in the upper-right top-panel using the established scales. This result is also presented as an image fusion with the greyscale CT image, seen in the bottom-right top-panel, and rendered in 3D in the bottom-left top-panel. Bottom Panel: A hollow plastic airway phantom positioned along the imaging plane of the CT scanner is used to characterize inspired gas distribution to the central airways. The syringe manifold consisted of four 60cc syringes linked together to allow controlled delivery of gas to the phantom.
Figure 2
Figure 2
Mean Hu measurements from the syringe studies, inside the Alderson RS-320 Lung/Chest Phantom and in open air, plotted as a function of xenon concentration.
Figure 3
Figure 3
A 3-L respiratory balloon, place inside the Alderson chest phantom (middle-left panel) and in open air (middle-right panel), was filled with an initial amount of xenon gas and scanned via DECT. Small amounts of room air were then sequentially added to increase the volume of gas in the balloon while keeping the xenon content the same. The balloon was rescanned (bottom-row) after each increase in volume. The goal was to determine the quantitative accuracy of the assessment of total xenon content within the balloon as the concentration of xenon gas decreased while the xenon gas volume remained constant. Plots of xenon volume inside the phantom (top-left panel) and in open air (top-right panel) compare the default calibration with a custom calibration based on previous syringe tests.
Figure 4
Figure 4
Three xenon gas mixtures (80%Xe/20%O2; 40%Xe/60%O2; 40%Xe/20%O2/40%He) were delivered during time-series MDCT axial image acquisition, at fast and slow flow rates, to a hollow plastic airway phantom positioned along the imaging plane of the CT scanner (Figure 1, bottom panel). Gravitationally non-dependent and dependent ROIs were placed as shown in the flow profile column, row 1. Intensity vs. time plots for both ROIs are shown in the middle and right columns respectively, with dashed lines on each plot indicating the expected level of density enhancement from baseline for each gas mixture. With a fast delivery, density measurements for non-dependent and dependent ROIs reach their expected levels in all cases. However, when delivered slowly, gravity plays a larger role in the gas distribution. Thus, all of the dependent ROI reach expected levels; while the only non-dependent ROI to reach it's expected level is the one that includes helium.
Figure 5
Figure 5
Helium gas does not affect the radiodensity of xenon gas. Columns represent the calculated dual-energy xenon signal from plastic syringes placed inside an artificial chest phantom (see Figure 1, top panel) filled with 40% xenon gas and various percentages of helium gas, balance oxygen. The xenon signal is the output of the three-material decomposition algorithm within Siemen's “xenon” module. (Error bars reflect ± standard deviation)
Figure 6
Figure 6
Deviations from the suggested default values by raising the pitch from 0.55 to 1.00, increasing the number of channels from 64 to 128, or doing both yielded considerable visual artifacts (left panel). The mean xenon value (upper-right panel) stays relatively consistent when the number of channels is changed from 64 to 128, however, the standard deviation (lower-right panel) increases in many of the ROIs. Changing the pitch from 0.55 to 1.00 causes variations in the mean xenon value and considerable increases in standard deviation.
Figure 7
Figure 7
Scanning with 80/140Sn kVp yields more accurate dual-energy xenon-CT images compared to those using 100/140Sn kVp. Transverse (left column), sagittal (middle column), and coronal (right column) views are shown from scans of a swine following a large single breath inhalation of 40%Xe/40%He/20%O2 gas mixture. The increased density separation between 80 and 140 kVp (bottom row) over 100 and 140 kVp (top row) allows for better calculation of xenon content. Airways are clearer and brighter in the 80/140 kVp results in addition to an overall smoother image.
Figure 8
Figure 8
Volume rendered image of the three-material decomposition derived xenon imaged of a swine. Xenon-DECT using a 40%Xe/40%He/20%O2 gas mixture was successful in imaging all of the swine's central airways and peripheral airways out to those with an approximately 2-mm diameter.

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