Characterisation of apple microstructure using quantitative MRI

International audienceIntroduction Quantitative Magnetic Resonance Imaging is an appropriate non-destructive tool to study plants microstructure. Recent developments allow accessing multi-exponential relaxation of water protons, furnishing data about water repartition inside cells. Microporosity in fruit tissues can also be estimated by MRI, providing additional information about gas distribution. The present study aims to provide a better understanding of the microstructure and micro-heterogeneities of apple, for the first time by applying multi-exponential relaxation models to spatially resolved MRI data. Experiments were performed on fruits of different size combining MRI multi-exponential T2 and microporosity quantification with histological measurements. Methods Experiments were performed on 7 small (~130g) and 7 big (~300g) apples, variety Jonagored. MRI measurements were carried out on a 1.5T MRI scanner (Avanto, Siemens). The 5mm median planes of fruit were imaged with a pixel size=1.19mm² and a TR=10s. T2 was obtained from a 512echo MSE sequence with ΔTE=7.1ms. T2*, used with T2 for porosity estimation, was obtained from GE sequences with TE1=2.77ms and ΔTE=1.61ms. Analyses were focused on the inner cortex, near the core and on the outer cortex, near the cuticle. Cell size distribution was estimated on same samples from macrovision images using an erosion/dilation method. Results Apple T2 decay was well fitted by a tri-exponential decay curve: T2 were about 50, 150 and 450ms and relative intensities about 5, 20 and 75% respectively. T2 measurements in outer cortex of small and big apples showed variations only for the longest T2 component from 423±25ms to 469±20ms respectively; relative intensities being similar (76±1%). Cell size distributions, obtained by macrovision, showed larger cells in big apples thus linking cell size to T2 relaxation time. Microporosity also increased from 22±4% for small fruits to 28±1% for big apples. Variations between inner and outer cortex were also studied. Microporosity increased from 25±3% (inner) to 34±4% (outer) and the longest T2 relaxation time was 454±21ms in the inner cortex and 512±30ms in the outer cortex. Using size-contrasted fruits permitted to highlight cell size influence on multi-T2 relaxation time. In apple cortex, longest T2 relaxation time was found to increase with distance to the core. Unexpectedly, T2 time increased while microporosity increased showing other factors affecting T2 remain to be investigated