Active Compensation
MRI installations, not only those with exposed low-field magnets, are sensitive to fluctuating spurious magnetic fields. These can originate from electric DC trains, elevators, large moving ferromagnetic masses etc., and in cities with underground train and tram services can exceed the levels permitted for MRI equipment.
Since passive magnetic shielding is less effective at low frequencies, other ways have to be found to reduce field fluctuations to an acceptable level, otherwise it may be impossible to use a proposed site. One such way is to actively compensate them.
Operating principle
An active compensation system measures the fluctuations of the magnetic field. Based on the measurements, a current flows through so-called “Helmholtz” coils and generates an equivalent opposing magnetic field. The vectors of the spurious field and the opposing field are added and will cancel.
The IMEDCO active compensation system consists of a magnetic field sensor, the controller and the “Helmholtz” coils. The sensor and the coils are placed inside the RF enclosure and the controller outside. The interconnecting cables pass through filters to the inside of the RF enclosure.
To facilitate setting the controller, the magnetic field at the isocentre of the MRI has to be measured. This measurement is necessary to determine the transfer function and check the adjustment of the controller at the end of the installation.
During operation, the active compensation system updates itself. Therefore, no maintenance or recalibration is necessary as long as the positions of the sensor and coils remain unchanged.
Magnetic field lines for Helmholtz coils
Performance
Low frequency magnetic field fluctuations up to 20 µTPP (200 mGaussPP) can be reduced to less than 0.1 µTPP or 100 nTPP (1 mGaussPP). Magnetic fluctuations below 100 nTPP (1 mGaussPP) are permissible according to the specifications of most MRI equipment manufacturers.
