Patients can get dizzy inside hospital scanners because the machines upset their sense of balance, a study has found. The radio waves and magnetic fields used by MRI scanners to look inside the body often induce feelings of vertigo. This is because a strong magnetic field pushes on fluid in the inner ear's 'labyrinth', which controls balance, scientists revealed.
This pressure on the tube-like labyrinth causes a feeling of unexpected or unsteady movement. The U.S. study compared the experiences of 10 healthy volunteers and two who did not have a functioning labyrinths. The researchers found that the healthy volunteers having MRI scans produced an involuntary eye movement called nystagmus, which reflects a detection of motion by the brain. Those with no labyrinth did not react. In higher magnetic field strengths, the eye movement from the healthy volunteers was even faster, showing a clear link between the scan and the impression of movement.
The nystagmus even changed direction depending on which way volunteers entered the machines. The scientists, who published their findings today in the online journal Current Biology, said the scanner could be distorting results by showing previously unnoticed brain activity related to movement and balance. MRI vertigo is caused by interplay between the magnetic field and the salty fluid that fills labyrinth canals, the scientists claim. The magnetic field is thought to push on the current of electrically charged particles circulating within the tubes. This in turn exerts a force on the cells which use the fluid's flow to detect motion.
The discovery has implications for MRI-based brain research, say the scientists. They point out that the scanner could itself be inducing previously unnoticed brain activity related to movement and balance. Researcher Dale Roberts, from Johns Hopkins Medical Institutions, said: 'We've shown that even when you think there's nothing happening in the brain while volunteers are in the scanner, there's actually a lot happening because the MRI itself is causing some effect. 'These effects must be taken into account in the way we interpret functional imaging.'The MRI effect on balance could in future be used to diagnose and treat inner ear disorders by stimulating the labyrinth, said the researchers.