Application Summary
Before Philips Medical Systems, Cleveland, Ohio, installs an MRI (magnetic resonance imaging) machine in a hospital, clinic, or other facility, its engineers and technicians perform an extensive environmental study of the site. This is a procedure for measuring all magnetic disturbances or stray electromagnetic fields and mechanical vibrations that might be present. The disturbances can be generated by any number of different sources, such as cars, trucks, trains, or any similar “chunk” of metal or iron, either stationary or moving in the vicinity.

These types of large metal objects can produce a magnetic field disturbance with an intensity that depends on its distance from the site where the MRI machine will be installed. Even small fields, as little as 400 nanotesla, are considered large magnetic disturbances because they interact with the magnetic fields generated by the MRI for imaging and manifest in either AC or DC interference or a combination. Another ac magnetic disturbance comes from ac fields generated by high-power transformers, utility lines, or a variety of machines or devices running large amounts of ac current.

In addition to magnetic field disturbances, technicians measure mechanical vibrations in both vertical and horizontal directions with respect to the MRI machine axes. In order for the site to be approved, the floor must be perfectly steady, no extraneous vibrations are allowed. The technicians look for machinery near by that could conduct vibrations through the floor, such as those that receive power from utility lines to drive motors and move machine components, as well as air conditioning units, fans, and similar equipment.

Potential Solution
The test gear that Philips initially used to make these measurements consisted of a collection of individual instruments including large analyzers, oscillators, and charge amplifiers for sensors. The wide variety of boxes was relatively expensive, somewhat time-consuming to set up, and cumbersome for the engineers and technicians to transport.

IOtech’s Solution
When it came time to purchase additional test equipment, Vishnu Srivastava, Staff Engineer at Philips decided to take a long, hard look at possible alternative instruments that could do the job for less cost and be more portable. He found a solution in the IOtech WavePort/PE8 data acquisition system. The WavePort/PE8 is a rugged, portable package based on the WaveBook/516 digitizer architecture for high-speed waveform acquisition and analysis. It contains eight channels of voltage input plus eight channels of IEPE (accelerometer or microphone) input. “Considering these three variables to be measured,” says Srivastava, “the WavePort/PE8 really satisfies our purpose. We eliminated all the separate pieces, and now have everything we need all in one IOtech WavePort/PE8.” The WavePort is specifically designed for harsh environments and for being transported frequently between multiple locations. The case is made of ABS plastic with space available for mounting a notebook PC. It comes with single and multiple-channel analog triggers that are set with programmable level and slope.

Srivastava programs his own special software modules, which are used with the companion DASYLab^® software package, tailored specifically for measuring ac and dc fields and mechanical vibration. After setting up the modules in the WavePort/PE8, he needed less than four hours to make it run under the control of one laptop computer.

Srivastava uses special magnetic sensors to measure the electromagnetic field variations in the X, Y, and Z-axes that range from 0 to 400 nanotesla. The maximum that can be tolerated is 500 nanotesla for both ac and dc disturbances, and they must be measured with high accuracy. He uses three channels each for ac and dc field measurements and two channels of vibration in the horizontal and vertical directions.

Conclusion
Philips Medical Systems engineers use an IOtech WavePort to measure magnetic disturbances and physical vibrations in a prospective site before installing an MRI machine. The environmental study lets them know where electrostatic and electromagnetic shielding should be placed, and the type of vibration isolation needed. The WavePort’s portability and small size makes it ideal for the engineers to carry safely from one site to another.

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