シイタケ栽培では、樹木を切って乾燥させた原木にシイタケ菌種を打ち込み、シイタケ菌糸を原木内に成長させる。菌糸が原木全体に広がるほど、栄養の吸収が促進され、良質なシイタケが収穫できる。現状のシイタケ栽培は農家の勘に頼った栽培が行われており、生産量のばらつきが大きい。勘に頼る理由は、原木内での菌糸の成長状態を把握する計測手法がなく、良質なシイタケを生育させる原木の内部状況が定量的に把握できないためである。そこで、本研究では、原木内の菌糸の空間分布をMRI で計測し、水分状態と菌糸分布を定量的に評価することを目的とする。
原木としてコナラ（石川県産）を用いた。切断した原木の寸法は直径が48 mm、長さが110 mm、室温乾燥時の質量が 216.15 g、含水量βが11 .6 %であった。原木の片側端面から２日間に渡って吸水させた。その面にシイタケ菌（森産業XR1）を2.5 gだけ植菌した。原木を恒温槽（21℃、湿度70％）に入れてシイタケ菌を育成した。約一週間ごとに給水し、４日間隔で約２か月間に渡りMRI計測をした。
MRI計測は1 T 110 mm Air-Gapの永久磁石で構成される自作マルチスライス用計測装置を用いた。MR画像は単純スピンエコー法によるマルチスライスシーケンスを用いて計測した。繰り返し時間TRは1500 ms、エコー時間TEは18 ms、画素数は256×128 pixels、空間分解能は0.34 mm×0.68 mm、スライス厚さは3.8 mmとした。T1緩和時定数はTRを変えてT1強調画像を得ることで算出した。
原木内にシイタケ菌を成長させ、原木内のMR画像を取得した。シイタケ菌糸のT1緩和時定数は原木内の水よりも長いためにシイタケ菌糸と水を区別することが可能であることが分かった。これを基にしてT1緩和時定数の空間分布からシイタケ菌糸の空間分布を得ることができた。
In shiitake mushroom cultivation, the inoculated shiitake mushroom mycelium is grown in a log. As the mycelium spreads over the entire log, good quality shiitake can be harvested because the mycelium absorbs more nutrients. Currently, shiitake mushroom cultivation relies on the intuition of farmers, so that the amount of shiitake mushrooms varies greatly. If there is a method for measuring the growth state of mycelia in a log, a farmer can adjust the environment for growing good quality shiitake mushrooms, and as a result, the production of shiitake mushrooms will increase. The purpose of this study is to measure the spatial distribution of hyphae in the log by MRI and quantitatively evaluate the water status and hyphae distribution.
Quercus serrata (produced in Ishikawa Prefecture) was used as log. The dimensions of the log were 48 mm in diameter, 110 mm in length, 216.15 g in mass when dried at room temperature, and 11.6% in water content. Water was absorbed from one end of the log for two days. The surface was inoculated with Shiitake mushroom (Mori Sangyo XR1) in an amount of 2.5 g. Logs were placed in a constant temperature room (temperature 21%, humidity 70%) to grow Shiitake mushrooms. Water was supplied about once a week, and MRI measurement was performed for about two months at four-day intervals.
For the MRI measurement, an apparatus composed of a 1 T 110 mm Air-Gap permanent magnet was used. MR images were measured using a multi-slice sequence by the simple spin echo method. The repetition time TR was set to 1500 ms, the echo time TE was set to 18 ms, the number of pixels was set to 256 × 128 pixels, the spatial resolution was set to 0.34 mm × 0.68 mm, and the slice thickness was set to 3.8 mm. The T1 relaxation time constant of water was calculated by changing TR and obtaining a T1-weighted image.
Shiitake mushrooms were grown in the log and MR images of the log were acquired. The T1 relaxation time constant of Shiitake mushroom mycelium was longer than the water in the log, indicating that it was possible to distinguish Shiitake mushroom hypha from water. Based on this, the spatial distribution of shiitake mushroom mycelium was obtained from the spatial distribution of the T1 relaxation time constant.