Assessing lung transplantation ischemia-reperfusion injury by microcomputed tomography and ultrashort echo-time magnetic resonance imaging in a mouse model.

PURPOSE Ischemia-reperfusion injury (I/R) is a common early complication after lung transplantation. The purpose of this study was to compare ultrashort echo-time (UTE) sequences in magnetic resonance imaging (MRI) with a microcomputed tomography (micro-CT) reference standard for detection of I/R injury in a lung transplantation mouse model. MATERIALS AND METHODS Six mice (C57BL/6) underwent orthotopic lung transplantation using donor grafts that were exposed to 6-hour cold ischemia. Imaging was performed within 24 hours after the transplantation with high-resolution micro-CT (tube voltage, 50 kV; current, 500 mA; aluminum filter, 0.5 mm; voxel size, 35 × 35 × 35 μm³) and small-animal MRI at 4.7 T with a linearly polarized whole-body mouse coil. The imaging protocol comprised radial 3-dimensional UTE sequences with different echo times (repetition time, 8 milliseconds; echo time, 50/75/100/500/1500/3000/4000/5000 μs; voxel size, 350 × 350 × 350 μm³). Images were assessed visually and through calculation of contrast-to-noise ratio (CNR) values. Calculated S0 values and T2* transverse relaxation times (MRI) of lung parenchyma were compared with Hounsfield unit (HU) density in micro-CT images. Receiver operating characteristic curves and area under the curve values were calculated for comparison of diagnostic power. All samples underwent a histologic examination. RESULTS The results of both UTE MRI and micro-CT showed an excellent depiction of pulmonary infiltration due to I/R injury, with MRI exhibiting a significantly higher CNR (mean [SD] CNR MRI, 19.7 [8.0]; mean [SD] CNR micro-CT, 10.3 [2.5]; P < 0.001). Measured parametrical values were as follows: mean (SD) HU, -416 (120); mean (SD) S0 value, 1655 (440); mean (SD) T2*, 895 (870) μs for the non-transplanted right lung and mean (SD) HU, 29 (35); mean (SD) S0 value, 2310 (300); and mean (SD) T2*, 4550 (3230) μs for the transplanted left lung. Slight infiltration could be better discriminated with micro-CT, whereas, in strong infiltration, a better contrast was provided by UTE MRI. The area under the curve values resulting from the receiver operating characteristic curve analysis were 0.99 for HU density, 0.89 for S₀, 0.96 for T2*, and 0.98 for the combination of S₀ and T2*. CONCLUSIONS Results show that MRI of the lung has a similar diagnostic power compared with that of micro-CT regarding the detection of I/R injury after experimental lung transplantation. Both modalities provide complementary information in the assessment of dense and slight infiltration in the early phase after lung transplantation. Therefore, UTE MRI seems to be a promising addition to computed tomographic imaging in the assessment of I/R injury after lung transplantation.