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How protect MRI Machine

The purpose of the driver section of the CPD board is to individually select the different system

coils and to provide the distinction between transmit and receive modes on these coils by

biasing PIN diodes located at the coils to either pass or prohibit the flow of RF energy.

Pulsed drive signals (amplifier unblank signals) are directed to the various drive elements:

Multicoils, TRs, DD, and body coil drivers as specified by the communications section of the

CPD board. Additionally, open-circuit and short-circuit error detection signals (faults) for each

of the drivers are stored in fault buffers which are read via select lines originating from the

CPD microprocessor. The CPD board reports any faults to the remainder of the system where

appropriate actions are taken.

The multicoil drivers consist of four separate drivers that each output a voltage of –5 volts at

nominally 0.5 amp of current. Each driver receives its own drive signal from the EPLD. The

drive signal goes through a level shift circuit consisting of a comparator and dual rail precision

line driver to obtain +5V and –5V levels. A high power operational amplifier (op-amp)

accurately converts the low-current –5V drive level to a high-current –5V level. External

circuitry around the op-amp determines its gain and frequency response and sets current limits

to 1 amp.

A low value precision current sense resistor and high gain instrumentation amplifier accurately

measure the output current of the op-amp. A window comparator arrangement triggers a fault

signal if the measured value of current does not fall within a small, precise range.

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