The pensioner was undergoing treatment in the hospital in the west German town, with the machine hanging over her.
The machine - a gamma camera - was examining the woman for changes in her tissue when part of it came loose and it fell on top of her, Spiegel reports.
Prosecutors believe there was a technical defect in the machine and that its operator bears no responsibility.
The accident happened on Sunday, although it was only made public on Friday.
Only a week before the accident, the manufacturer came to the office to service the machine, a spokesperson for the hospital told Spiegel.
No other similar incidents in Germany are known, she said, adding that the examination room has now been closed down and the machine will be exchanged.