Troubleshooting a Case of Electromagnetic Interference in a Hi Vision Preirus Ultrasound Machine's 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.

01

During the clinical trial after the installation of the Hitachi color ultrasound Hi Vision Preirus, an interference problem was found in the phased array cardiac probe EUP-S70.

Specifically, the imaging of other probes equipped with the device is basically normal, but when using the cardiac probe, dense small white spots interference artifacts appear on the fan-shaped area image, which greatly affects the image quality and makes it impossible for the operating doctor to make a correct diagnosis.

02

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, and it was considered to be caused by electromagnetic field interference.

The ways in which interfering electromagnetic fields enter 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 color ultrasound examination room on the first floor of the building. There are already multiple color ultrasound machines working simultaneously in the room. Observing the ultrasound image after sequentially turning off the power of the ultrasound workstation and other equipment directly connected to the examination room, the interference still exists, 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 the X-ray machine in the radiology department and the high-frequency electrosurgical knife 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. These devices may all generate spatial stray electromagnetic field interference. Therefore, it is necessary to solve the problem of stray field interference.

The existing basic steel bar and cement 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; the various placement directions of the color ultrasound equipment are adjusted to avoid the possible direction of electromagnetic wave interference. Subsequently, the interference phenomenon is tested, but it cannot be solved; after grounding the ground terminal of the color ultrasound equipment with a dedicated ground wire and 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 communication and cooperation with other departments.

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

03

The above fault situation shows that the interference of the cardiac probe image is caused by the electromagnetic interference of the power supply, the specific source of interference is unknown, and the newly introduced color ultrasound Hi Vision Preirus has slightly worse anti-interference performance compared to other color ultrasound machines 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, and 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 RF energy generated by the equipment radiating outward in the form of electromagnetic waves through the air medium. This RF electromagnetic wave will interfere with the receiver's reception and affect normal communication broadcasting, or when the radiated RF 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 connection.

These 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. 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, using a dedicated AC power purifier to supply power greatly suppresses and attenuates the electromagnetic wave interference introduced from the power supply network, and effectively blocks the introduction of interfering electromagnetic waves from the power line.

04

When installing color ultrasound equipment, the appropriate 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 color 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 color 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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