The Food and Drug Administration approved magnetic resonance imaging, or MRI, machines for commercial sale in the 1980s, and by the end of the 1990s, more than 8,000 machines had conducted more than 20 million scans. While MRI machines are a noninvasive method for capturing 3-dimensional images of internal body tissue, electromagnetic interference, or EMI, can cause problems with the machines and alter images.
By understanding how the sources of EMI impact MRI machines, healthcare facilities can take steps with the MRI room design and through shielding methods to limit interference.
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A Look Into Magnetic Resonance Imaging
Magnetic resonance imaging is a noninvasive medical procedure that takes detailed images of the body. During an MRI, a patient is placed in a large magnetic device, as machines create a strong magnetic field that aligns with the body’s protons. Then, the patient’s body is pulsed through with a radiofrequency current, which stimulates the protons, making them spin out of equilibrium and struggle against the magnetic field.
MRI sensors can then detect the energy that protons release as they realign with the magnetic field after the radiofrequency field is disabled. The chemical nature of the molecules and the environment impact how long this process takes and how much energy the protons release. Different tissues have varying magnetic properties, so medical professionals can tell the difference between tissue types in the images.
Because MRI doesn’t use X-ray radiation like computed tomography, or CT, it’s best for capturing images of soft tissue like the brain, muscles, ligaments, tendons and nerves. Doctors use MRI to see the difference between white and gray brain matter and diagnose conditions like tumors or aneurysms. Patients who need routine imaging also typically choose MRIs, as they won’t be frequently exposed to the radiation from X-rays. However, patients must be very still during the imaging process, and MRIs cost more than other imaging methods.
Exploring the Effects of EMI Emissions on MRI Equipment
Electromagnetic interference happens when unintentional electrical interference disrupts the electrical currents that an electronic device, such as an MRI, is supposed to receive. The two types of EMI include DC, which DC voltage sources generate, and electrical disturbances. EMI interference can either be continuous, which results from a prolonged signal like a broadcast tower, or sporadic, which is caused by temporary disturbances.
MRI machines are especially susceptible to EMI, as they use a strong magnetic field to create 3-dimensional images of the body. EMI interferes with the machine’s images, affecting how the equipment works and wearing down the device over time.
In addition to EMI causing interference with and damage to machines, they can also harm patient health. EMI may cause headaches and sleeping disorders and can interfere with pacemakers. Patients with metal implants should not get an MRI, as the strong magnetic field can impact these devices and interfere with imaging. Additionally, patients should remove items and apparel made of metal before entering the MRI room. Before conducting an MRI, physicians should also remove magnetic or potentially magnetic materials like oxygen tanks, unless they are clearly labeled “non-magnetic,” from the examination room.
Uncovering EMI Sources Affecting MRI Scanners
Sources of EMI can be difficult to detect. MRI technicians might struggle to establish whether a disturbance is due to EMI or another source of interference. Both the MRI machine itself and external sources can cause EMI, even when the source of the disturbance is 200 feet away.
Emissions that come from the actual MRI chamber itself could lead to conduction through electrical or signal cables or even through the air, disturbing lab equipment and potentially affecting patients and technicians. EMI from the main polarizing magnet, shim coils and gradient systems can impact how other parts of the MRI function. However, both natural and human-made industrial EMI sources can also affect MRI scanner functioning.
Natural sources that can cause interference include:
- Lightning strikes.
- Static electricity.
- Magnetic and electrical storms.
- Solar flares.
- Cosmic noise.
Human-made equipment that causes EMI includes:
- Subways, trains and other transportation that operates with strong electrical currents.
- Cell phone towers.
- Radio and television broadcast towers.
- Geomagnetic field disturbances from passing automobiles and non-electric trains.
- Transformers, switchgear and electrical generators.
- Grid power disruptions like voltage dips or spikes when equipment is connected to that grid.
- Heavy construction equipment.
- High-voltage power and electric distribution lines.
- Other medical equipment like X-rays and life support.
While some wireless devices like cell phones, Bluetooth speakers or baby monitors can cause EMI, industrial devices typically create more widespread interference that can impact medical devices or military equipment.
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Meeting MRI Compliance Standards With EMI Solutions
Specific MRI room requirements exist to mitigate EMI. The FDA-recognized IEC 60601-1-2 standard specifies requirements and immunity tests for limiting EMI with MRI machines and improves the safety of medical equipment. Manufacturers must design MRI machines and other medical equipment in a way that complies with the IEC 60601-1-2.
The FDA Center for Devices and Radiological Health suggests healthcare facilities create a plan to mitigate interference. Healthcare facilities should:
- Assess the facility’s electromagnetic environment by completing tasks like mapping radio transmitter locations and identifying sections of the hospital where critical devices are used.
- Create a plan and written policies to manage the electromagnetic environment, electronic equipment and medical devices, including coordinating the purchase, installation and servicing of electrical and electronic equipment.
- Consider EMI and how to create electromagnetic compatibility, or EMC, when designing the layout of healthcare facilities. You can ensure MRI room design for safety by placing devices that create EMI away from the MRI scan room. If possible, the device causing the interference should be turned off.
- Educate healthcare facility staff, patients and all visitors in minimizing EMI. However, biomedical engineers and the organization’s safety committee should lead plans in preventing EMI. Some facilities might consider working with an electromagnetic compatibility (EMC) specialist.
- Conduct immunity testing whenever EMI is suspected, when radiofrequency transmitters must operate nearby medical devices and before purchasing new radiofrequency transmitters or medical devices. Transmitters should also have the lowest possible electrical output power rating.
- Report any EMI problems to the FDA MedWatch program and the equipment manufacturer.
EMI shields block electromagnetic noise. While most MRI machines are designed with protective shielding to minimize the risks associated with EMI, older machines may require additional shielding. Shielding made from high-conductivity metals for high levels of interference or high-permeability metals for low interference levels helps to mitigate EMI. Attenuation is the difference between an electromagnetic signal’s intensity before and after shielding and measures a shield’s strength. Shielding ranges between 60 and 90 decibels provide strong protection. However, ranges between 90 to 100 decibels are exceptional.
Shielding medical devices’ cable systems is a crucial part of protecting medical devices from EMI. Medical equipment like MRI machines and mobile MRI trucks require specialized conduit solutions for cable protection. Lightweight, polyamide conduit systems with EMI shielding protects data transmission and reduces EMI with critical circuits. Specialized cable glands, like the Progress MS EMC cable gland line, also protect cables from EMI.
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