在 + 脑海 + 中
The noun 脑海 (nǎohǎi) literally means "brain sea" and refers to one's mind or consciousness, especially regarding thoughts and memories. It is commonly used in the phrase 在脑海中, meaning "in one's mind." For example, 一直在他的脑海中 means "always in his mind."
那首歌的旋律一直在我脑海中回荡。
Nà shǒu gē de xuánlǜ yīzhí zài wǒ nǎohǎi zhōng huídàng.
The melody of that song keeps echoing in my mind.
小时候的画面突然浮现在她的脑海中。
Xiǎoshíhòu de huàmiàn tūrán fúxiàn zài tā de nǎohǎi zhōng.
Scenes from her childhood suddenly flashed through her mind.
这个想法在他脑海里转了好几天。
Zhège xiǎngfǎ zài tā nǎohǎi lǐ zhuǎn le hǎo jǐ tiān.
This idea has been turning over in his mind for days.