Single breath washout (SBW) is a whole lung pulmonary function test that has been shown to be sensitive to early changes in lung disease¹. Of particular clinical interest has been the Phase III slope (S_III) as the concentration decay between 25-75% of the exhaled volume. Commonly nitrogen is used as tracer gas, but SBW can also be performed with Helium. In this work rapid lung imaging of exhaled hyperpolarized gas is used to acquire 2D images of SBW of subjects expiring to residual lung volume. We demonstrate the ability to measure regional SIII slopes from time resolved images.

Single breath washout imaging (SBW-I) was performed on a 1.5T scanner with a quadrature flex coil tuned to Helium-3 frequency. A gas mixture of Helium-3 (~30% polarisation,100ml) topped up to one liter with N₂ was inhaled. Sequence parameters of the time resolved 2D spoiled gradient echo sequence were: 64x64 Matrix, a single slice with thickness = 300mm, FOV= 38cm, TR/TE = 2.7/0.8ms, receiver bandwidth=62.5kHz, phase FOV = 0.82, flip angle = 1°. A total of 80 images were acquired with no delay between acquisitions, resulting in a total breath-hold of 15s. Subjects were instructed to exhale to functional residual capacity before inhaling the 1l of tracer gas mix as per SBW lung function test^2-4. Upon inhalation data acquisition was started and subjects aimed for a constant flow of 400ml/s breathing through a restricted flow meter⁵ (RS 100 Pneumotachograph, Hans Rudolph Shawnee, KS, USA). Images were corrected for RF depolarisation and registered for lung motion6. Flow recordings were interpolated and aligned with the timings of the MRI acquisition (Figure1). MRI images were converted into maps of regional gas fraction⁷. This allowed quantification of SIII slope in %/liters as commonly expressed for the SBW test in the pulmonary function lab. SIII was calculated as the linear slope of fractional signal decay per liter expired volume between 25%-75% of the exhaled volume⁵. Single breath washout imaging (SBW-I) was performed in two healthy subjects. One subject was imaged twice to show repeatability of the imaging method using a coronal and a sagittal whole lung projection.A representative time series of exhaled gas SBW-I is shown in Figure 2. Single breath washout integrated over the whole lung alongside signal from the trachea is shown in Figure 3. In addition Figure 4 shows maps of S_III slopes from both subjects SBW-I. Data were shown to be highly reproducible (Subject 2: S_III mean(standard deviation) (1) -0.142(0.084) (2) -0.148(0.084)). The whole lung average values from both volunteers (Subject 1 S_III= -0.15; Subject 2 S_III= -0.22) are in the range of those in the literature for the SBW lung function test for sitting healthy subjects using a similar breathing manoeuvre albeit with slightly lower Helium concentration: 0.2(0.06)3. In the sagittal S_III map from subject 2 a gravitational effect with an increase of slope from anterior to posterior can be observed which reflects the postural gradient in ventilation observed in static ventilation imaging and multi breath washout imaging⁸.The mean average S_III values from both volunteers matches the literature and a good repeatability was demonstrated when imaging the same subject twice. The well-known gravitational effect⁹ was also found in the S_III indicating a faster concentration decay in the posterior part of the lung compared to the anterior one. Expiration was shorter than 8s in both cases, with a T₁ in the range of 25-30s signal depolarisation from T₁ decay was therefore neglected in this preliminary work. Rapid signal decay in the trachea shows quick washout of the dead space gas similar to SBW in the PFT (Figure 3). However the dead space signal (Figure 1,DS) is difficult to fully assess using SBW-imaging, as only parts of the trachea were visible at the edge of the FOV. This could be addressed with a larger FOV and a coil that covers the upper airways. In the rest of the lung signal decay is dominated by gas washout from the respiratory zone of the lung (small airways and alveolar gas space; airway generation >16) rather than the conducting zone. In future work acceleration techniques like parallel imaging and compressed sensing could be used to allow imaging with a higher temporal resolution or to increase the spatial resolution of images to 3D.Regional Phase III slope (S_III) mapping from single breath washout imaging of hyperpolarized gas is demonstrated. Future work could investigate how regional distribution in Phase III slope changes in disease and compare it to ventilation imaging and fractional ventilation from multiple breath washout.