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頭部用PETとAI応用の研究
頭部固定計測によるストレスの影響と体動補正技術
数 mmの体動でも得られる画像の画質が大きく損なわれてしまうため、従来のPET撮影ではマスクなどの器具による頭部の固定が必要不可欠となっています。しかし、多くのPET計測は数十分から数時間に及ぶため、このような頭部の固定を伴うPET計測は、高齢者や認知症の患者にとっては困難であり、非常にストレス負荷のかかるものでした。
海馬傍回や内側前頭葉など、ストレスと関連する領域で頭部の固定時に高い活動がみられた（参考文献1）
そこで当社は、PET装置に当社製高速カメラを組み合わせて体動を同時計測して特別な画像再構成を行うことにより、頭部の固定を必要としない新しい体動補正技術を開発しました。この技術を搭載したPET装置を用い、頭部固定時と非固定時の脳活動を計測・比較した結果、頭部固定によるストレスに起因する脳血流増加や神経伝達物質の分泌が低減されることが分かりました。
体動検出のしくみ
頭部用PET装置：HIAS-29000の開発
体動補正技術の有用性を基に、当社はその技術を組み込んだ頭部用PET装置HIAS-29000を開発しました（参考文献2）。HIAS-29000は、仰臥位・座位・立位の各姿勢での計測が可能であり、どの姿勢に対しても体動を補正する機能をもっています。脳ファントムを用いた計測により、頭部が前後左右に動いた場合でも鮮明な画像が得られることが示されました。
HIAS-29000
頭部を固定せず鮮明な画像を取得可能
HIAS-29000に搭載されている検出器には、当社のPET用MPPCモジュールが使用されています。当社製MPPCモジュールは280 ps未満という高い時間分解能を有し、HIAS-29000で画質評価ファントムを撮像した結果、良好な画像が得られることを確認できています。
MPPCモジュール
画質評価ファントムでの結果
| 販売名 | 頭部用PET装置 HIAS-29000 |
| 一般名称 | 核医学診断用ポジトロンCT装置 管理医療機器 特定保守管理医療機器/設備管理医療機器 |
| 製造販売業者 | 浜松ホトニクス株式会社 |
| 連絡先 | 中央研究所 主幹 TEL：053-586-7111（代表） |
| 承認番号 | 30400BZX00209000 |
PET診断をより身近にするための取り組み
当社では、AIを用いたさまざまな研究を行っており、将来的なPET診断での活用を目指しています。
■AI技術の実践応用(PET診断の信頼性向上、PETシステムのコストダウン)を目指した研究
脳PET検査においては、被写体によるガンマ線の吸収を補正する吸収補正が必須となります。通常は、外部線源機構やX線CT装置を用いて撮像された吸収マップをもとに吸収補正を行いますが、被ばく量、装置コストが増加するといったデメリットが存在します。
当社は、これらのデメリットを解決するため、ディープラーニングにより吸収補正なしのエミッション画像から吸収マップを推定することで、被ばく量、装置コストをおさえる研究を行っています（参考文献3）。これらの研究には、浜松PET診断センターにて20年以上蓄積した臨床研究データを活用しています。
E2T (Emis2Trans) : Emission To Transmission
各患者のPET用吸収補正データをAIが推定
[¹⁸F] FDG (糖代謝)の結果
[¹¹C] Raclopride (ドーパミンD2受容体)の結果
■短時間・低投与量PETの開発による患者負担の軽減の実現を目指した研究
PET画像は、被ばく低減や短時間の収集を繰り返すダイナミック撮像により、フレーム当たりのカウント数が制限されるため、ノイズが増加します。ディープラーニングによるノイズ除去には、ノイズの多い・少ないPET画像のペアを大量に用意する必要があります。畳み込みニューラルネットワーク（CNN）には本来、画像の構造は早く、ノイズは遅く学習する性質があります。初期状態のCNNにMRI画像を１例入力し、出力が劣化PET画像1例に近づくようパラメータの微調整を繰り返します。適切なタイミングで学習を止めるとノイズの少ない復元PET画像を得ることができます。当社は、データセットが不要なディープラーニング（教師なしディープラーニング）の応用を通して人にやさしいPET検査の実現を目指しています（参考文献4-11）。
なお、本研究はJSPS科研費JP22K07762の助成を受けて実施されました。
教師なしディープラーニングによるPET画像ノイズ除去
参考文献
1. Inubushi T et al. Neuroimage. 2021. PMID :33039616
2. Onishi Y et al. Ann. Nucl. Med. 2022.
3. Hashimoto F. et al. Ann. Nucl. Med. 2021.
4. Hashimoto F. et al. IEEE Access. 2019.
5. Onishi Y. et al. Med. Image Anal. 2021.
6. Hashimoto F. et al. Phys. Med. Biol. 2021.
7. Hashimoto F. et al. IEEE Trans. Radiat. Plasma. Med. Sci. 2022.
8. Onishi Y et al. IEEE Trans. Radiat. Plasma. Med. Sci. 2023.
9. Hashimoto F. et al. Phys. Med. Biol. 2023.
10. Ote K. et al. IEEE Trans. Med. Imaging. 2023.
11. Hashimoto F. ey al. Radiol. Phys. Technol. 2024.
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