An integrated magnetic resonance plant imager for mobile use in greenhouse and field

In this contribution we demonstrate a mobile, integrated MR plant imager that can be handled by one single person and used in the field. Key to the construction of it was a small and lightweight gradient amplifier, specifically tailored to our combination of magnet, gradient coils and the requirements of the desired pulse sequences. To allow imaging of branches and stems, an open C-shaped permanent magnet was used. In the design of the magnet, pole gap width, low weight and robustness were prioritized over homogeneity and field strength. To overcome the adverse effects of short T₂*, multi-spin echo imaging was employed, using short echo times and high spectral widths. To achieve microscopic resolution under these constraints requires fast switching field gradients, driven by strong and fast gradient amplifiers. While small-scale spectrometers and RF amplifiers are readily available, appropriate small-scale gradient amplifiers or designs thereof currently are not. We thus constructed a small, 3-channel gradient amplifier on the basis of a conventional current-controlled AB amplifier design, using cheap and well-known parts. The finished device weighs 5 kg and is capable of delivering 40 A gradient pulses of >6 ms in duration. With all components built onto an aluminum hand trolley, the imaging setup weighs 45 kg and is small enough to fit into a car. We demonstrate the mobility and utility of the device imaging quantitative water content and T₂, first of an apple tree in an orchard; second, of a beech tree during spring leaf flushing in a greenhouse. The latter experiment ran for a continuous period of 62 days, acquiring more than 6000 images.