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Systems for Lung Volume Standardization during Static and Dynamic MDCT-based Quantitative Assessment of Pulmonary Structure and Function

Posted Jul 31, 2012 Updated Apr 10, 2023
By Matthew K. Fuld BS , Rall W. Grout BS , Junfeng Guo PhD , John H. Morgan BS , Eric A. Hoffman PhD
11 min read

Rationale and Objectives

Multidetector-row computed tomography (MDCT) has emerged as a tool for quantitative assessment of parenchymal destruction, air trapping (density metrics), and airway remodeling (metrics relating airway wall and lumen geometry) in chronic obstructive pulmonary disease (COPD) and asthma. Critical to the accuracy and interpretability of these MDCT-derived metrics is the assurance that the lungs are scanned during a breathhold at a standardized volume.

Materials and Methods

A computer monitored turbine-based flow meter system was developed to control patient breathholds and facilitate static imaging at fixed percentages of the vital capacity. Because of calibration challenges with gas density changes during multibreath xenon CT, an alternative system was required. The design incorporated dual rolling seal pistons. Both systems were tested in a laboratory environment and human subject trials.

Results

The turbine-based system successfully controlled lung volumes in 32/37 subjects, having a linear relationship for CT measured air volume between repeated scans: for all scans, the mean and confidence interval of the differences (scan1-scan2) was −9 mL (−169, 151); for total lung capacity alone 6 mL (−164, 177); for functional residual capacity alone, −23 mL (−172, 126). The dual-piston system successfully controlled lung volume in 31/41 subjects. Study failures related largely to subject noncompliance with verbal instruction and gas leaks around the mouthpiece.

Conclusion

We demonstrate the successful use of a turbine-based system for static lung volume control and demonstrate its inadequacies for dynamic xenon CT studies. Implementation of a dual-rolling seal spirometer has been shown to adequately control lung volume for multibreath wash-in xenon CT studies. These systems coupled with proper patient coaching provide the tools for the use of CT to quantitate regional lung structure and function. The wash-in xenon CT method for assessing regional lung function, although not necessarily practical for routine clinical studies, provides for a dynamic protocol against which newly emerging single breath, dual-energy xenon CT measures can be validated.

Introduction

Multidetector-row computed tomography (MDCT) has emerged as a tool for quantitation of parenchymal destruction, air trapping, and airway remodeling in chronic obstructive pulmonary disease and asthma . Critical to the accuracy and interpretability of these metrics is the assurance that the lungs are scanned during a breathhold at a standardized volume . Methods have been reported in which correction factors are proposed for inconsistent lung volumes . However, there is no replacement for accurate control of lung volume at the time of scanning.

In addition to structural-based measurements, wash-in xenon CT enables the measurement of regional ventilation by using the increase in measured density of a region of interest caused by the wash-in of radiodense xenon gas. This method has proved reliable in animal studies , and because the animals were anesthetized the anesthetic properties of xenon were not a problem . Animals were mechanically ventilated, thus it was very straightforward to scan at a repeatable set of respiratory pauses as xenon gas was washed into and out of the lungs. The goal for measuring ventilation in humans is to capture the dynamic nature of ventilation in awake, free-breathing subjects. This therefore necessitates the use of a lower concentration of xenon gas and identifying repeatable pause points in sequential respiratory cycles when axial scans can be acquired. Currently a 30% xenon/70% oxygen mixture is used for safety purposes, though a mixture that also includes krypton would be preferred . However, the introduction of gases of varying densities complicates the tracking of gas flow at the mouth with standard respiratory gas flow meters. With the emergence of dual-energy CT methods for single breath xenon-based assessment of regional lung function have been reported . However, there have been no reports validating these single-breath techniques against a method that assesses true regional ventilation. The dynamic volume control approach presented here provides for such a comparator.

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Materials and methods

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Breathhold Lung Volume Control

Turbine-based breathhold system design

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Figure 1, Components of the turbine-based breathhold lung volume controller: overall system ( top panel ); Interface USA, VMM-400 turbine-based flow meter ( bottom-left panel ); Hans Rudolph two-way balloon occlusion valve ( bottom-right panel ).

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Figure 2, Screenshots of the LabVIEW control interface, during a functional residual capacity scan ( top panel ), during slow vital capacity calibration ( bottom left panel ), and during tidal breathing ( bottom right panel ).

Figure 3, Flow diagram of turbine-based breathhold volume controller usage.

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Turbine-based breathhold system calibration and testing

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Dynamic Lung Volume Control

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Turbine-based dynamic system design

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Figure 4, Adaptation of the turbine-based breathhold volume controller for dynamic xenon multidetector-row computed tomography (MDCT). CIVCO's “Imaging Overlay,” which is made of carbon fiber with a foam core, is positioned onto the scanner table that in turn allows for the attachment of CIVCO multiarticulated arms to custom fit the volume control's patient interface so that the subject can comfortably bite down on the system mouthpiece ( right panel ). A second multiarticulated arm is used to hold the display screen used to help guide the subject regarding inspiratory timing and depth of breathing ( left panel ).

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Dual piston-based dynamic system design

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Figure 5, Photograph and schematic diagram of the dual-piston rolling seal volume controller. During a wash-in xenon-computed tomography study, the system is situated next the to scanner table with the subject positioned inside the gantry for axial image acquisition ( left panel ). The lower portion of the system casing ( middle top panel ) is composed of Plexiglas to allow visualization of piston motion by the system operator. Through the Plexiglas casing, the separate Plexiglas rolling seal pistons can be seen with their linear variable differential transformers (LVDTs) positioned along the side. The upper casing has been removed ( middle bottom panel ) to reveal the system's internal components including electronics and solenoid valves. A simplified schematic diagram illustrates individual component connectivity ( right panel ).

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Dynamic systems calibration and testing

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Results

Turbine-based Breathhold System

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Table 1

Results from a Linear Model Indicate That None of the Listed Parameters Statistically Influence the Difference between Volume Difference Recorded from the Volume Controller and Computed Tomography–measured Air Volume Difference

Parameter_P_ Value Height .876 Weight .336 Age .398 Lung capacity .807 Smoking status .925 Gender .590 Computed tomography technician .351 Volume Controller operator .771

Figure 6, Multidetector row computed tomography (CT) measured air volume correlates well between repeated scans using the turbine-based breathhold lung volume controller. Plots of CT-measured air volume comparing scan 1 versus scan 2 for total lung capacity (TLC) and functional residual capacity (FRC) combined ( left top panel ), TLC ( left middle panel ), and FRC ( left bottom panel ). The middle and right columns show corresponding Bland-Altman and difference value histogram plots for each group.

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Figure 7, The computed tomography (CT)-measured air volume difference (CTVD) between repeated scans correlates well with the analogous turbine-based breathhold volume controller-measured air volume difference (VCVD). Plots of CTVD versus VCVD for total lung capacity (TLC) and functional residual capacity (FRC) combined ( left panel ), Bland-Altman ( middle panel ), and difference value histogram ( right panel ).

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Turbine-based Dynamic System

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Figure 8, Wash-in xenon computed tomography results. The turbine-based dynamic system fails to maintain a consistent end-expiratory lung volume yielding noisy time versus density curves ( top middle panel ) and incorrect and incomplete color-map data ( top right panel ). The dual-piston system more reliably controls end-expiratory lung volume yielding cleaner time versus density curves ( bottom middle panel ) and accurate and complete color-map data ( bottom right panel ). Note the lower density and slower wash-in time constants ( blue curve, bottom middle panel ) in the nondependent versus dependent ( orange curve, bottom middle panel ) representing a greater lung expansion at functional residual capacity (FRC) and reduced ventilation to this same region in the nondependent lung region. This is in agreement with well-recognized heterogeneity of lung function. Despite the inconsistent lung volumes at FRC achieved by the turbine-based system, the same vertically oriented relationship in lung density is observed within the early time points.

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Dual Piston–based Dynamic System

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Discussion

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Acknowledgments

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Academic Radiology, Volume 19, Issue 8
Journals General Radiology
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