Synopsis

The success of lung volume reduction with endobronchial valves in patients with Chronic Obstructive Pulmonary Disease (COPD) depends on the absence of collateral ventilation into the target lung segment. Collateral ventilation can be assessed using time-resolved ³He MRI, but providing this clinical application with ¹²⁹Xe MRI would be desirable. Two patients with severe COPD were scanned with time-resolved ³He and ¹²⁹Xe MRI. In both patients, collateral ventilation visualised using ³He MRI was not observed with ¹²⁹Xe MRI. Although the small sample size is a limitation, this suggests that ¹²⁹Xe MRI may not be suitable for the assessment of collateral ventilation.

Introduction

The assessment of collateral ventilation in patients with Chronic Obstructive Pulmonary Disease (COPD) is key to the success of lung volume reduction (LVR) with endobronchial valves, as collateral ventilation from adjacent lung regions can prevent collapse of the target lung segment¹. The gold standard assessment of collateral ventilation is gas catheter bronchoscopy. Assessment of lobar fissure integrity with CT can also assist in patient selection, but is not recommended as a means of including or excluding patients for LVR due to considerable inter-operator variability¹.

Time-resolved ³He imaging at breath-hold can be used to directly visualise collateral ventilation in patients with COPD², however, the shortage of ³He³ has motivated a shift towards ¹²⁹Xe MRI for most applications. The aim of this work was to compare ³He and ¹²⁹Xe time-resolved imaging for the detection of collateral ventilation in patients with severe COPD.

Methods

Two patients with advanced COPD were scanned using a 1.5T whole body MRI system (GE HDx).

³He imaging: Patients were positioned supine in a ³He transmit-receive vest coil (CMRS). A mix of 100ml hyperpolarized ³He (25% polarization) and 900ml N₂ was inhaled from functional residual capacity (FRC), and dynamic time-series ventilation images were acquired during breath-hold. Sequence parameters were as follows; 3D coronal steady state free precession (SSFP), full lung coverage, voxel size=6x13x10mm, θ=8.5⁰, TE=0.5ms, TR=1.6ms, BW=167kHz, Cartesian centric phase encoding, and six dynamic volumes acquired at 0, 4, 7, 11, 15 and 19 seconds.

¹²⁹Xe imaging: Patients were positioned supine in a ¹²⁹Xe transmit-receive vest coil (CMRS). A mix of 350ml hyperpolarized ¹²⁹Xe (30% polarization) and 650ml N₂ was inhaled from FRC, and dynamic time-series ventilation images were acquired during breath-hold. Sequence parameters were as follows; 3D coronal SSFP, full lung coverage, voxel size=6x13x10mm, θ=6.5⁰, TE=1.4ms, TR=4.5ms, BW=16kHz, Cartesian centric phase encoding, and six dynamic volumes acquired at 0, 4, 8, 12, 16 and 20 seconds.

Whole lung ventilated volume (VV) was calculated for each time point using automated spatial fuzzy C-means segmentation⁴.

Results and Discussion

Figures 1 and 2 show ³He and ¹²⁹Xe images of gas distribution within the lungs at the first and last time-points during breath-hold for the two patients. Arrows highlight initially non-ventilated lung regions where signal increased over time in the ³He images, but ¹²⁹Xe signal did not. This behaviour is highlighted by the maps of signal increase and region of interest signal plots (figure 3). Some evidence of delayed ventilation was observed with ¹²⁹Xe but only within lung regions which were ventilated at t=0s with ³He. Whole lung ventilated volume increased over time for both gases (figure 4), but the ratio VV_3He/VV_129Xe was greater at the end of the breath-holds than at t=0s.

The ratio of ¹²⁹Xe diffusivity to ³He diffusivity within the hyperinflated lungs of a patient with a FRC of 7L (as measured by body plethysmography in patient 2) was calculated⁵ as 0.15 for the gas mixtures used (D_{0(129Xe-air,lungs)}=0.13cm² s^-1, D_{0(3He-air,lungs)}=0.87cm² s^-1). This was associated with a mean free diffusion path length (z_rms=√(2DΔt)) of 2.0cm for ¹²⁹Xe and 5.1cm for ³He on the time-course of the time-resolved experiment (Δt~15s). The visualization of collateral ventilation with ³He but not ¹²⁹Xe, and the increased VV_3He/VV_129Xe ratio at the end of the breath-holds compared to t=0s, are likely due to this large difference in diffusivity between the gas mixtures used.

The observation of reduced ventilated volume in ¹²⁹Xe images when compared to ³He images acquired from the same patients with COPD has been reported before for single time-point ventilation imaging⁶. The diffusion coefficient of ¹²⁹Xe in air (0.14cm²s^-1)⁵ is closer to that of air alone (0.22cm²s^-1)⁷ than ³He in air (0.86cm²s^-1)⁵. However, in the case of LVR, where the aim is to remove all air from the target lung compartment to enable collapse, the higher diffusivity of ³He highlights delayed ventilation which would take place on a longer time-scale for pure air rather than the ³He-air mixture used for imaging (60s for z_rms=5.1cm for pure air).

A limitation of this study is the small sample size. However, in the context of detection of collateral ventilation for LVR, any instances where collateral ventilation was present but not detected indicate a limitation of time-resolved ¹²⁹Xe MRI for that application.

Conclusions

Acknowledgements

References

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