FORT MYERS, Fla. -- Twins right-hander Boof Bonser will undergo an MRI on Monday after his right shoulder did not show any improvement in his latest throwing session.

Bonser has been battling tendinitis in his shoulder, and he received a cortisone shot on Thursday in the hope of lessening the discomfort. Bonser threw on Wednesday with no improvement, and the hope was that another throwing session on Thursday might allow him to work through the discomfort.

But that was not the case.

"Nothing different," Bonser said when asked about his throwing session.

Bonser has already undergone an MRI and X-rays on the shoulder, and no structural damage was found at the time. Now, doctors will try another MRI to see if they can figure out the problem that has been plaguing Bonser since late last season.

"It's obviously not going very good," Twins manager Ron Gardenhire said. "This is his second [MRI] in not very long."

Bonser's hope is just that doctors can determine why his shoulder is not improving.

"It's extremely frustrating right now," Bonser said.