Application Note

Precision Temperature Measurements with Fiber Optic Probes in MRI Applications

Techniques for calibration, verification, and precision measurements.

Introduction

Accurately measuring temperature is crucial in medical care and device safety—especially in high-stakes environments such as Magnetic Resonance Imaging (MRI). Traditional temperature probes, which often contain metal, cannot be used in MRIs due to powerful magnetic fields that may cause dangerous interactions or image distortion. For patient groups such as infants, or when testing medical implants, the demand for precise and MRI-safe temperature measurement is even greater. In response, fiber optic temperature probes have emerged as a reliable, accurate, high-resolution, low noise, and safe solution for these challenging settings. Unlike traditional applications for fiber optic probes, such as power, switchboard, and transformer, the medical market requires much higher accuracies for absolute and relative temperature measurements. This market often requires accuracies of 0.1°C with resolutions of 0.02°C

The Challenge of Temperature Measurement in MRI

MRI scanners generate intense magnetic fields and radiofrequency (RF) pulses, making most electronic thermometers and metal-based probes unsuitable because they:

Pose safety hazards by attracting magnetic forces or inducing electrical currents.
Cause distortions or artifacts in MRI images.
Provide inaccurate readings due to electromagnetic interference.

Given these challenges, clinicians and researchers require alternatives that offer both safety and accurate data within the MRI suite.

Why Temperature Monitoring Is Critical

Monitoring temperature plays several crucial roles in MRI:

Patient Safety: Especially for neonates and infants, keeping body temperature within a narrow range is vital. Babies are highly sensitive to both low and high temperatures, which can result in serious health issues.
Implant Safety: Medical devices like pacemakers, stents, or Neurostimulators may heat up during MRI scans, potentially endangering patients. Regulatory bodies typically set a maximum temperature rise (often 2°C) to ensure patient safety.
Device Development and Validation: Companies developing MRI-compatible devices must rigorously test their products’ temperature performance in MRI conditions.
Body and Transmit Receive Coil Temperature Monitoring

Limitations of Conventional Temperature Probes

Traditional sensors made with metal wires or thermocouples can present problems inside MRI machines:

They may heat up and cause tissue burns.
Magnetic forces can move or displace them.
They can introduce artifacts into MRI images, undermining diagnoses and research.
Electromagnetic interference can affect their accuracy.

Moreover, many commercial fiber optic temperature systems are not approved for MRI use as an instrument or device. For example, the Osensa system itself is not FDA-approved; however, it is used as a component within FDA-approved and agency-approved systems, such as anesthesia or monitoring equipment.

Fiber Optic Temperature Probes: Principles and Advantages

Fiber optic probes have become the preferred technology for measuring temperature in MRI and other settings with strong electromagnetic fields. They use thin, flexible glass or plastic fibers that transmit light instead of electricity. Temperature readings are obtained by detecting changes in light, usually through fluorescence decay.

Technology Behind Osensa Innovations Probes

Fluorescence Decay: The probe is based on a fluorescent material that emits light when excited by a specific wavelength. The decay time of this fluorescence varies with temperature. Osensa’s system uses ultra-low-noise electronics and advanced algorithms to deliver quick, high-resolution readings. Accurate measurement depends on correct calibration, applying probe-specific offsets, and proper placement of the probe.

Key Advantages

Non-Metallic Build: No metal, so there’s no risk of MRI-related hazards or artifacts.
Immune to Electromagnetic Interference: Light transmission is unaffected by MRI’s strong electromagnetic fields.
Miniature and Flexible: The slender, flexible design is ideal for infants and other vulnerable patients.
High Accuracy: Top-quality probes deliver sub-degree precision (as fine as 0.1°C).
High Resolution: Can detect temperature changes as small as 0.01°C.
No Magnetic Temperature Shift: Fluorescent probes maintain accuracy regardless of magnetic field strength.
Low Artifacts: Osensa’s probes are designed for minimal MRI image disruption.

Applications in Neonatal MRI

Infants, especially those born prematurely, are highly vulnerable to temperature shifts—even minor deviations can cause apnea, metabolic issues, or affect development. MRI scans expose babies to cool environments and limit the use of standard warming equipment for safety reasons. Babies who have cardiac implant leads improperly installed can suffer potentially from burns in MRI. Babies and also animals can’t describe in language that they are getting a burning or warming sensation.

Fiber optic probes offer continuous, non-invasive temperature monitoring for these patients. They can be gently attached to the skin or used in incubator ports, providing real-time data without introducing any metal components, thus ensuring both comfort and safety.

Clinical Workflow Benefits

Quick, safe setup before imaging.
Continuous, real-time monitoring during scans, allowing for rapid intervention if needed.
No disruption to MRI images, preserving diagnostic clarity.

Applications in MRI Implant Safety

Medical implants must be evaluated for their behavior in MRI fields, with heating due to RF energy as a primary concern. Regulators like the FDA mandate that the temperature near an implant must not rise by more than 2°C during MRI.

Role of Fiber Optic Probes in Safety Testing

Positioned directly at the implant-tissue interface to track local heating accurately.
Used in multiples to create a temperature map around the implant.
Essential for regulatory submissions and ensuring devices are properly labeled as MRI-safe.

Research and Development

Medical device makers rely on fiber optic temperature probes to create safer, more MRI-compatible products and to satisfy international safety standards—fostering ongoing innovation in the field.

Setting up the Osensa System for Accurate Measurements

To obtain the most accurate readings, each probe’s calibration table must be uploaded to the FTX-300 or FTX-020-OEM temperature transmitters.

Access the Calibration Table via the Calibration Menu, select the MODBUS ID and channel, then pick the correct table for your specific probe and transmitter model.

Click the write icon and enter the password; on the FTX-300, lights will turn red during writing.
Under the configuration tab, access the OsensaView Device Configuration

Click Read

Probe offsets for the table are found on the probe label or in the factory calibration report. Otherwise, Osensa can provide an offset if the serial number is provided.

Insert the offset value in the offset box for each probe (found on the probe label or calibration report; Osensa can provide it if given the serial number) and click write.

Each probe’s calibration offset must be entered for the system to recognize its unique response. These calibration details are critical for minimizing measurement error and ensuring high-fidelity, repeatable results essential for both compliance and research.

To ensure the reliability and accuracy of temperature measurements, test labs, internal quality departments, and calibration teams routinely verify both probe and system calibration. Utilizing a temperature-controlled dry well and a precision temperature meter equipped with a platinum resistance thermometer (PRT) probe, such as the Fluke 5615, these teams can establish a traceable reference point. By running the probe under controlled conditions and comparing its output to the reference standard, users may generate their own calibration offsets tailored to their laboratory setup.

This critical calibration phase guarantees that the system accurately recognizes the unique response profile of each fiber optic probe. By meticulously matching each probe to its corresponding calibration data, the margin of error in measurement is minimized, ensuring maximum fidelity. This enables the collection of precise, repeatable temperature readings—an essential requirement for maintaining regulatory compliance and upholding the integrity of both safety testing and advanced research applications. Only after this foundational calibration process is complete should any safety assessment or clinical temperature monitoring be initiated.

Precision Temperature Meter – Fluke 1502A- For Drywell Verification

Fluke 5615 Secondary Reference Probe

For organizations lacking access to specialized temperature wells and secondary reference devices, alternative methods for probe verification can ensure coarse accuracy in routine checks. A simple comparative method involves arranging fiber optic probes together—such as pressing them within the pages of a thick book and placing them in an insulated cooler. After allowing sufficient time for thermal stabilization, all probes should register comparable temperatures, indicating consistent baseline performance.

However, attempting this verification in open air often yields unreliable results due to fluctuating currents and HVAC influences; thus, calibrations in uncontrolled air environments are discouraged. Submerging the probes in water that has equilibrated to ambient temperature offers a more reliable approach. Ensuring the probes occupy the same region within the container promotes uniform readings. Cross-checking with a platinum resistance thermometer (PRT) or a trusted secondary reference ensures that the water itself provides an accurate measurement environment. Using an insulated container, such as a cooler, helps maintain thermal stability and minimize external temperature fluctuations.

Conclusion

Fiber optic probes have revolutionized temperature measurement in MRI, enhancing patient safety and device validation—especially for vulnerable groups like infants. In implant testing, these probes provide reliable data that meets regulatory standards and drives innovation. As MRI use continues to expand, fiber optic temperature monitoring will remain vital to modern medicine.

This overview outlines procedures for setting up, measuring, and verifying fiber optic temperature systems and related equipment.