45
5.3.2.2 MMA (Compensation for MRI magnet interaction with interfering fields):
• Adjust Principle Axis (Z-axis) Gamma ± value for optimum isocenter interfering signal cancellation. If large
X-Y plane interference exists, adjust Secondary Axes (Y- and Z-axes) Gamma ± values for possible isocenter
cancellation improvement.
• Finally, repeat the gain adjustment on all 3 axes a second time, to insure convergence on the optimal Output
Gain setpoints.
5.4 AMPLIFIER CURRENT CHECK AND ATTENUATION FACTOR CHECKS
5.4.1 Amplifier Current Check:
In the absence of large interfering signals, the displayed CDA idle current value should be in the range of 0.3
- 0.5 A
rms
. If a greater value is observed for longer than approximately 30 seconds, an abnormal condition may
exist. Check the X Drive, Y Drive and Z Drive Trace page displays for excessive channel drive offsets or other
aberrations.
5.4.2 Attenuation Factor Checks:
Casual on-site attenuation factor evaluation is difficult due to the generally random nature of predominant low-
frequency ambient interfering fields. Nonetheless, either the survey system or the MACS/D itself can be used to
verify correct basic system functionality.
• For EMFC (MEA) installations, to make an approximate attenuation factor determination using only the installed
system, perform the following measurements and calculation: take the ratio of the average of a 3 or 4 one-
minute peak-to-peak records of the Main X display (or Y or Z, whichever axis has the largest interference) with
the system DISABLED to the average of a similar series of records taken with the system in the OPERATE
state. This process will yield a system attenuation factor usually within 20% of the actual value. Overall
interfering field attenuation determined by this method should be greater than 40 dB (factor of 100).
• For MRI (MMA) installations, direct measurement of isocenter stability improvement is made directly using
either the MRI instrument’s diagnostic modes or an exotic low noise high dynamic range NMR probe. The
advantage of the latter approach is good time correlation of the NMR response with an external survey probe
by recording both signals simultaneously. Measured isocenter attenuation, with proper adjustment of the Z-axis
(principal channel) ACR parameter, should be greater than 46 dB (factor of 200).
• In the event lower than expected site attenuation factors are encountered, it is useful to check (with survey
probe and oscilloscope) the frequency of the oscillation occurring in each individual channel when the Output
Gain is set above the axial closed-loop stability point. The normal range of oscillation for either MEA or MMI
controller configuration is1.4±0.3 KHz. Lower values indicate the presence of problematic eddy currents due to
conductive surfaces too near the compensation coils or too near the sensing probe.
5.4.3 Chime Loudness Adjustment:
• Press the RESET pushbutton, adjust rear panel Chime aperture for desired loudness (
Figure 5, (17)
).
