Background Pulmonary fibrosis, either idiopathic or secondary to diseases such as systemic sclerosis, is a devastating and life threatening disorder for which effective treatment is still lacking. The bleomycin-induced pulmonary fibrosis model is well-characterized and the most widely used mouse model. The resulting fibrosis is routinely quantified by labor-intensive end-stage histological assessments, requiring many animals and lack the ability to follow-up on disease progression and potential therapeutic effects in the individual animal. At present, imaging tools for the evaluation of lung disease with good temporal and spatial resolution in vivo are limited.
Objectives To optimize and evaluate lung MRI protocols to visualize disease onset and progression in the bleomycin-induced model of lung fibrosis. We compared prospectively and retrospectively gated MRI sequences and validated our results with established CT imaging of lung fibrosis and histochemical techniques.
Methods Animal Model: Male C57Bl/6 mice were intratracheally instilled with bleomycin (0.05U in 50 µl of PBS) or sham. The mice were scanned with MRI and CT at baseline and weekly until 3 weeks after instillation. After the last imaging time point, mice were sacrificed, ex vivo CT data were acquired and the lungs were isolated for histological analysis and quantification as described before 3. MRI images were acquired at 9.4T (Bruker Biospin, 20 cm) in combination with a 7.5cm quadrature coil, using the following sequences: (1) a respiratory triggered RARE sequence (TR 6000ms TEeff=15.9ms, 50slices of 0.5mm thick), (2) a respiratory triggered ultra short echo (UTE) sequence (FID mode, TR= 20ms, TE=0.4ms, 8 slices, 0.6mm slice thickness) and (3) a retrospectively gated FLASH sequence IntraGate ( TR/TE = 30/1.26 ms, 17 deg flip angle, 5 slices covering the lung, slice thickness 1 mm). For reconstruction, 70% of the respiration and ECG period was used (Paravision 5.1, Bruker)). MRI data were quantified using ImageJ. CT methods: retrospectively gated CT images were acquired on a small animal µCT scanner (SkyScan 1076, Bruker microCT) and quantified.
Results The prospectively gated UTE and RARE protocols as well as retrospectively gated IntraGate-FLASH imaging were able to visualize an increase of hyperintense focal spots over time, corresponding to progression of lung fibrosis as corroborated by lung CT images. Quantification of the mean lung signal intensity shows an increase over time, which was confirmed by the decrease in aerated lung volume quantified from the CT data and by histology. UTE, RARE and IntraGate-FLASH images of control animals confirmed the absence of contrast without fibrosis induction.
Conclusions The evaluated MRI protocols were all able to non-invasively visualize and quantify lung disease progression. Moreover, the IntraGate-FLASH protocol does not need setup of respiratory triggering for lung imaging, making it an easy to use and efficient alternative to more conventional sequences. Where CT provides poor soft tissue contrast, MRI has the potential to provide contrast differences between vasculature, fibrotic areas and inflammation, without concerns for radiotoxicity when scanning the same animal repeatedly.
Disclosure of Interest None Declared