Quantitative Aerosol Deposition in Mechanically-Ventilated Healthy and Asthmatic Rats using UTE-MRI
Hongchen Wang1, Georges Willoquet1, Catherine Sebrié1, Sébastien Judé2, Anne Maurin2, Rose-Marie Dubuisson1, Luc Darrasse1, Geneviève Guillot1, Ludovic de Rochefort1, and Xavier Maître1

1Imagerie par Résonance Magnétique Médicale et Multi-Modalités (UMR8081) IR4M, CNRS, Univ. Paris-Sud, Orsay, France, 2Centre de Recherches Biologiques, CERB, Baugy, France

Synopsis

Asthma is the most common chronic respiratory disease treated with inhaled therapy. However, aerosol deposition patterns are complex and imaging methods are needed to improve delivery efficiency. 3D UTE-MRI combined with aerosolized Gd-DOTA had been formerly applied onto spontaneous nose-only-breathing animals. Here, a mechanical administration system was developed to ventilate and nebulize rats. Resulting aerosol distribution and kinetics were compared with free-breathing in healthy and asthmatic animals.

PURPOSE

Asthma is the most common chronic respiratory disease treated with inhaled therapy1. However, aerosol deposition patterns are complex and imaging methods are needed to improve delivery efficiency. 3D UTE-MRI combined with aerosolized Gd-DOTA had been formerly applied onto spontaneous nose-only-breathing animals2. Here, a mechanical administration system was developed to ventilate and nebulize rats. Resulting aerosol distribution and kinetics were compared with free-breathing in healthy and asthmatic animals.

METHODS

Animal model: Four female Wistar rats (262±2g) were sensitized to ovalbumin (OVA; 1mg/rat)3. Sensitization was verified by plethysmography following OVA-challenge with an increase of the enhanced pause indicator (PenH) correlated to airway resistance4. Before imaging, asthmatic rats were challenged by 20-minute OVA-nebulization. Asthmatic symptoms appeared 2h-post and lasted >2h. Three healthy rats (277±4g) were used as controls.

Administration protocol was carried out with a Small Animal Gas Administration System (SAGAS) (Fig.1), compatible with MRI, micro-SPECT, and micro-CT. Two glass syringes (2–6mL/s), actuated by piezomotors, yielded accurate volume control. Chronograms of mouth pressure and expiration mass flow rate were recorded with a Matlab® interface. Breathing cycles and ventilation volumes were set through the same interface. Mechanical ventilation was tuned to match individual free-breathing patterns (45-60 breath/min). Nebulization was synchronized to the animal inspiration.

Imaging protocol was implemented on a clinical 1.5T [Philips Achieva] with a 47mm-diameter receiver coil2. Rats, in supine position, were tracheotomized and anaesthetized (isoflurane-O2) through SAGAS with an intratracheal catheter. Two 3D radial acquisitions (TE1/TE2=0.4/1.4ms) were used for R2* mapping (pre- and 1h post-nebulization); 3D isotropic T1W-UTE radial acquisitions (TR/TE=14/0.4ms, α=30º, (0.5mm)3 voxel, Tacq=7.5min) were performed once before, then continuously repeated during nebulization and up to ~1h post-nebulization. Gd-DOTA solution [Dotarem; Guerbet] was nebulized on line [Aeroneb Solo; Aerogen] during ~35min to the rats. A respiration-synchronized cine sequence was used to estimate tidal volume to verify ventilated volume.

Image analysis: Lungs were segmented by a histogram-based method2. Relative signal enhancement (RSE) was extracted at each time point and RSE at 15min and at the end of nebulization was converted into concentration maps5, with r1=3.7mM-1∙s-1 and baseline T1,0=1.1s for rat lung at 1.5T. Aerosol distribution was evaluated in the lungs using relative dispersion RD (ratio of standard deviation to mean). Regional distribution at 15min nebulization was assessed by relative concentration deposited in 2 equal volumes (V1=V2) divided along 4 anatomical directions (Fig.3). Group analysis was expressed as mean ± standard error of the mean (SEM). Unpaired two-tail t-tests with a significance level of 0.05 were performed.

RESULTS

Baseline signal-to-noise-ratio (SNR) under mechanical ventilation was reduced ~27% in the lung and ~11% in the surrounding tissues. After same nebulization duration (15min), compared to free-breathing, mechanically-ventilated healthy rats had comparable pre-R2* (525.63±1.40)s-1, higher RSE (67.86±28.23)% vs. ~50%2, higher deposited aerosol concentration (0.26±0.10)mM vs. 0.15mM2. RSE dynamics showed important inter-subject variability but similar trend: continuous increase during and even after nebulization whereas clearance could be monitored during free-breathing. Higher RD was observed in 2 healthy rats, unlike more homogeneous distribution under free-breathing. The effective administered Gd dose over the same duration showed a five-fold increase with SAGAS versus free-breathing.

Baseline signal intensity (SI) was comparable and pre-R2* was higher (810.02±1.09)s-1 in mechanically-ventilated asthmatic rats versus controls. After 15min post-nebulization, asthmatic rats had lower RSE (47.36±15.05)% and lower deposited aerosol (0.19±0.05)mM. After ~30min post-nebulization, SI or concentration differences between two groups were not significant. R2* at 1h-post did not present significant differences. Asthmatic rats showed highly heterogeneous enhancement, while controls had relatively lower heterogeneity (Fig.2). A positive correlation was observed between concentration RD and PenH in the asthmatic group at 15min post-nebulization (R2=0.66, p<0.05) and at 35min post-nebulization (R2=0.63, p<0.05), which corresponds to the free-breathing case6. Healthy rats showed higher heterogeneity not only H/F (same as free-breathing) but also Ce/Pe direction, while asthmatic rats showed mainly Ce/Pe heterogeneity (Fig.3).

DISCUSSION AND CONCLUSION

Contrast-enhanced 3D UTE was applied to mechanically-ventilated healthy and asthmatic rats. The administration efficiency was improved by inspiration-synchronized intratracheal nebulization. The correlation between aerosol heterogeneity in asthmatic rats and airway resistance PenH was further confirmed in both breathing modalities, suggesting a pathophysiological marker for asthma: heterogeneous aerosol distribution. The rather low-dose and highly-heterogeneous aerosol deposition observed in asthmatic lungs could be related to the induced bronchoconstrictions7. SNR reduction was caused by electronic noise and more controlled breathing. Continuous RSE after nebulization may be due to accidental instillations from accumulated aerosol droplets in the catheter during administration. The method here at 1.5T on animals provides an efficient tool to map aerosol deposition in the lungs. It relies on minimal Gd-DOTA doses, which suggests potential clinical applications.

Acknowledgements

This work is part of the OxHelease project. It was funded by the grant ANR-11-TecSan-006-02.

References

1. Capstick TG, Clifton IJ. Inhaler technique and training in people with chronic obstructive pulmonary disease and asthma. Expert Rev Respir Med 2012;6:91-101; quiz 102-103.

2. Wang H, Sebrié C, de Rochefort L, et al. Aerosol deposition in the lungs of spontaneously breathing rats using Gd-DOTA-based contrast agents and ultra-short echo time MRI at 1.5 Tesla. Magnetic Resonance in Medicine 2015; doi: 10.1002/mrm.25617.

3. Raza Asim MB, Shahzad M, Yang X, et al. Suppressive effects of black seed oil on ovalbumin induced acute lung remodeling in E3 rats. Swiss Med Wkly 2010;140:w13128.

4. Chong BT, Agrawal DK, Romero FA, et al. Measurement of bronchoconstriction using whole-body plethysmograph: comparison of freely moving versus restrained guinea pigs. J Pharmacol Toxicol Methods 1998;39:163-168.

5. Schabel MC, Parker DL. Uncertainty and bias in contrast concentration measurements using spoiled gradient echo pulse sequences. Phys Med Biol 2008;53:2345-2373.

6. Wang H, Sebrié C, de Rochefort L, et al. Quantitative Gd-DOTA-based Aerosol Deposition in Asthmatic and Emphysematous Rats using UTE-MRI. ISMRM 2015; Toronto, Canada. p 3980.

7. Rottier BL, Rubin BK. Asthma medication delivery: mists and myths. Paediatric respiratory reviews 2013;14:112-118; quiz 118, 137-118.

Figures

Figure 1: Small Animal Gas Administration System (SAGAS) and corresponding Matlab® interface. The system allows controlling the breathing pattern by the following parameters: inspiration and expiration duration, ventilation volume, apnea duration and nebulization rate.

Figure 2: Pre- (a,d), post- (b,e) nebulization (TE=0.4ms) and concentration maps (c,f: color; 0-2 mM) superposed on anatomic image (grey; TE=1.4ms) after 15 min-nebulization with lower heterogeneous distribution in a healthy rat (a,b,c) and highly-heterogeneous distribution in an asthmatic rat (d,e,f). Blue arrowheads indicate highly-concentrated deposition.

Figure 3: Regional aerosol distribution in equal volumes (V1 = V2) with its associated SEM along 4 anatomical directions: left/right (L/R), anterior/posterior (A/P), head/feet (H/F), and central/peripheral (Ce/Pe), in control (blue) and asthmatic (red) rats. Regional homogeneity is considered at ~50%.




Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
1610