Troubleshooting electromagnetic interference issues with the Hitachi Jirou Shinsai ultrasound cardiac probe

With the advancement of science and technology, the types and quantities of equipment used in hospitals are increasing day by day. The emergence and application of new technologies such as wireless networks in the surrounding environment have led to a more complex electromagnetic environment, and the sources of interference to ultrasound instruments are increasing. Therefore, this paper analyzes a fault case of a color ultrasound machine suffering from electromagnetic interference and provides a solution.

Fault Phenomenon

During the clinical trial after the installation of the Hitachi color ultrasound Hi Vision Preirus (Erlang Shen), interference problems were found in the phased array cardiac probe EUP-S70. Specifically, the imaging of other probes equipped with the device was basically normal, but when using the cardiac probe, dense small white spots of interference artifacts appeared on the fan-shaped area image, greatly affecting the image quality and making it impossible for the operating doctor to make a correct diagnosis.

Fault Solution

Initially, the problem was suspected to be with the cardiac probe itself, but after testing, it was found that the probe was in good condition. It was considered to be caused by electromagnetic field interference. The ways in which the interfering electromagnetic field enters the color ultrasound equipment are:

① Introduced through the power line of the color ultrasound equipment;

② Entering from the probe, poorly shielded wires, connectors, and casing.

This color ultrasound machine is placed in a dedicated ultrasound examination room on the first floor of a building. There are already multiple ultrasound devices working simultaneously inside. Observing by sequentially turning off the power of the ultrasound workstation and other equipment directly connected to it in the examination room, interference still exists in the ultrasound image, indicating that the interference is not caused by the operation of the electrical equipment directly connected to it. Although this examination room is far from equipment that can emit strong electromagnetic interference waves, such as X-ray machines in the radiology department and high-frequency electrosurgical knives in the operating room, there are electronic gastrointestinal endoscopy equipment, electrocardiogram and electroencephalogram diagnostic equipment on the same floor, and elevators a little further away. All of these devices may generate spatial stray electromagnetic field interference. Therefore, it is necessary to solve the problem of stray field interference.

The existing basic steel-reinforced concrete shielding of the building cannot be changed. Considering strengthening the shielding measures, a 1m × 0.5m iron sheet is placed under the equipment to suppress the interference signal entering from the underground pipeline. Subsequently, the interference phenomenon is tested, but it cannot be solved; a dedicated ground wire is found, and the iron sheet is reliably grounded. Subsequently, the interference phenomenon is tested, but it still cannot be solved: adjusting the various placement directions of the ultrasound equipment to avoid the possible direction of electromagnetic wave interference, and then testing the interference phenomenon, it cannot be solved; using a dedicated ground wire to ground the ground terminal of the ultrasound equipment and then testing, the interference phenomenon still exists. It is considered necessary to bury a separate standard ground wire outdoors to try to solve the interference problem, but this process is relatively complex, time-consuming, and costly, and requires cooperation with other departments.

Further attempts are made with easier methods. It is suspected that it may be interference introduced by the power line. Recalling that all ultrasound equipment in the hospital is equipped with AC power purifiers, which greatly filters out the interference introduced by the power line, and these devices have been working well. Since this device was newly introduced and has not yet been equipped with an AC power purifier, the AC power purifier of the ultrasound equipment in use is connected and tested, and it is found that the cardiac probe interference artifact problem is solved.

Fault Cause Analysis

The above fault shows that the interference of the cardiac probe image is caused by electromagnetic interference from the power supply, the specific source of interference is unknown, and the newly introduced HiVision Preirus color ultrasound has slightly worse anti-interference performance compared to other color ultrasound equipment in use. By using an oscilloscope to detect the changes in the 220V, 50Hz power supply before and after the installation of the AC power purifier, it can be found that many high and low frequency burr-like interferences are superimposed on the sine wave before installation, while after installation, the sine wave becomes clean and many noises are filtered out.

By consulting information, it is understood that: the coupling methods of electromagnetic interference include conductive coupling and radiation coupling. Conductive coupling is the fluctuation from the power grid or external interference voltage through the power line, ground line coupling, input and output coupling, causing interference to the equipment. This fault is a typical case of power line conductive coupling. Radiation coupling refers to the radio frequency energy generated by the equipment radiating outward in the form of electromagnetic waves through the air medium. This radio frequency electromagnetic wave will interfere with the receiver's reception and affect normal communication broadcasting, or when the radiated radio frequency electromagnetic wave encounters the shell, power line, or signal cable of other equipment around the equipment, it will interfere with the normal operation of the equipment. In the wall power output circuit, there are high-current tip pulses or high-order harmonics contained in the waveform, which generate electromagnetic fields in space through the output lines.

Such harmonics will cause the input voltage waveform and current waveform to be distorted, causing electromagnetic interference. At the same time, the voltage and current on the input and output sides contain rich high-order harmonics, becoming a powerful interference source, causing serious harmful interference to other electronic equipment [4]. These interferences couple with the power supply and internal circuit of the ultrasound machine to form high-frequency noise, which forms a signal near 3.0MHz and is superimposed on the video line. Therefore, by using a dedicated AC power purifier, the electromagnetic wave interference introduced from the power supply network is greatly suppressed and attenuated, and the interference electromagnetic wave from the power line is effectively blocked.

Summary

When installing ultrasound equipment, a suitable site should be selected first. If the electromagnetic field environment is suspected to be unsuitable, a dedicated detection instrument can be used to test the electromagnetic field environment. Generally, it should be kept away from large equipment such as X-ray machines, CT, MRI, and elevators that are prone to interfering with other equipment. When interference is found in the ultrasound, the equipment's own faults should be ruled out first, and it should be determined that it is electromagnetic interference from the environment. Then, the three major technologies of grounding, filtering, and shielding are used to solve the electromagnetic compatibility problems of medical equipment. If necessary, the installation site of the ultrasound should be changed. When encountering electromagnetic interference of medical equipment during work, careful analysis is needed, and information should be searched from multiple sources and communicated with factory engineers.

As the user of medical equipment, hospitals should attach importance to the necessary electromagnetic compatibility knowledge training for relevant medical and nursing personnel, select products that meet EMC requirements according to the electromagnetic environment of the application site, and install and operate them according to the instruction manual or technical specification to ensure that medical equipment operates normally under the best conditions as intended and without interference.

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