Aktive Kompensationssysteme (ACS) für MRT
Der MRT-Standort ist nicht nur mit Hochfrequenzen, sondern auch mit niederfrequenten Magnetfeldstörungen belastet. Die übliche HF-Abschirmung der Kabine reicht oft nicht aus, denn durch hohe Energieströme, beispielsweise Transformatoren, Starkstromtrassen, Straßen-, Eisen- und U-Bahnen, entstehen große Magnetfeldfluktuationen, was zu Artefakten in den Untersuchungsbildern führt. Um diese Störungen abzusondern wird üblicherweise eine zusätzliche magnetische Abschirmung eingebaut. (Baustahl, Trafostahl, µ-Blech etc.) Allerdings sind diese Maßnahmen oft sehr sperrig und bei fertiggestellten Systemen ist diese Methode nur mit großem Aufwand realisierbar. Ein Aktives Kompensationssystem (engl. Active Compensation System, ACS) kann häufig die innovativere und kostengünstigere Alternative sein. Hierbei werden mit dem ACS Gegenfelder erzeugt. Dies ermöglicht eine aktive Magnetfeldschirmung, welche DC- und AC-Störungen reduziert.
Active Compensation System ACS-MR
Single axis compensation of low frequency magnetic field disturbances
Feature
magnetic field measurement with high resolution fluxgate sensors
meas. range 200 µT, DC to 1 kHz
selective compensation of different frequencies through switchable filters
modular system equipped with up to three magnetometers and filters
integrated power amplifier for direct connection of compensation coils
designed for permanent operation
Application
reduces magnetic field disturbances in MRI applications
compensation of power line frequencies 50/60 Hz
compensation of 16 2/3 Hz (railway) or other custom frequency
suppression of slow or stepped mag-netic field changes caused by vehicles, moved magnetic objects, elevators, etc.
Specifications of frame unit with power amplifier
• supply voltage 230/110 V, AC 50/60 Hz
• current consumtion 0.5 A
• enclosure Aluminium 19 inch, 3HU, 320 mm depth
• operating temperature 0 to 40’ C
• preamplifier gain 0 to 11, adjustable in steps
• gain of poer amplifier 0.5 A/V
• max. output current 2.5 A
• min. load for permanent operation ≥3 Ω
• compliance at 2.5 A typ. ±7 V
Specifications of
magnetometer
• Display 3 1/2 digits LC display with sign
• switchable measurement ranges ±19.99 µT, ±199.9 µT
• display resolution 10 nT
• noise 0,7 nT RMS
• accuracy 0.5 % ± 25 nT ± 1 digit
• offset stability < 0,1 nT/K
• scale factor stability < 40 ppm/K
• offset adjustment ±500µT coarse and fine
• stability of offset setting < 40 ppm/K
• analog output 0.03 V/µT, short circuit protected, BNC connector
• output voltage range typ. −6 V to +6 V
• bandwidth 0 to 1kHz (-3 dB)
• sensor cable 10 m long, shielded
• sensordimensions 30 mm x 30 mm x 15 mm
Specifications of filter
uinit
• filter channels 1 low pass, 2 band pass filters
• band pass filter type biquad active filter, 2nd order
• bandwidth (−3 dB) ∼2.6 Hz
• frequency corners 16 2/3 Hz (or custom) and 50 Hz (or 60 Hz)
• adjustment range of phase -90`to + 30’
• shift > 20 db
• channel separation
• Changes without notice
Active Compensation System A C S-MS-3
Three axis automatic compensation of magnetic field disturbances from DC to 10 kHz
Feature
Continuous real time compensation of magnetic field disturbances from DC to ∼10 kHz
Typical 50 dB attenuation from DC to 200 Hz, 13 dB @ 9 kHz
Magnetic field measurement with high resolution sensors
Rugged analog design, no tedious programming
Integrated power amplifiers for direct connection of compensation coils
Field monitor and alarm functions
Application
Improvement of electron microscope images (SEM and TEM)
Biomagnetic and paleomagnetic applications
Compensation of power line frequencies (50/60 Hz) and harmonics
Attenuation of slow or stepped mag- netic field changes caused by vehicles, moved magnetic objects, elevators, etc.
Attenuation of 9 kHz magnetic field from robotic wafer transports
Description
The magnetic field compensation system A C S - MS-3 is a new version of our proven MS-3 system. Typical applications are noise reduction in electron microscopy, electron and ion beam experiments, magnetic resonance imaging (MRI), biomagnetic investi- gations, SQUID operation, and paleomagnetic research. The frequency range has been increased to 10 kHz to cancel high frequency noise from robotic wafer transport in modern semiconductor wafer fabs.
The A C S - MS-3 is a feedback compensation system. The magnetic field noise is reduced by establishing a compensating magnetic field in opposite direction. The A C S - MS-3 continuously compensates magnetic field disturbances in the frequency range from DC to ∼10 kHz. The magnetic field is measured with a low noise triaxial fluxgate magnetometer combined with a triaxial induction coil sensor. Analog control electronics generate compensation signals which are fed to the built-in power amplifiers for direct connection of compensation coils. Typically, such coils simply consist of a set of cable rings with a low number ofwires installed in the edges of the laboratory.
Specifications
• Magnetic field sensor triaxial fluxgate sensor and triaxial induction coil sensor
• Zero drift < 0.1 nT/K
• Noise < 1 nT RMS (0.1 Hz < f < 10 kHz)
• Dynamic compensation range > 6 µTP P (60 mGP P )
• Analog outputs Bandwidth 1 V/µT, BNC connectors for X, Y, Z - 0 to 10 kHz (−3 dB)
• Digital displays show incremental DC or true rms AC, magnetic field for X, Y, and Z Resolution 1 nT
• Measurement accuracy ±1%
• Analog meters show coil current, range ±3 A
• Attenuation DC to 200 Hz typ. 50 dB at sensor position
• Attenuation @ 9 kHz typ. 13 dB at sensor position
• Subject to alterations.
