Fiber optic sensors have become a critical technology enabler behind the latest functional MRI (magnetic resonance imaging) suite upgrades and new MRI equipment designs.

1. Fiber optic sensors enable new MRI applications | ISweek - Industry sourcing
Fiber optic sensors have become a critical technology enabler behind the latest
functional MRI (magnetic resonance imaging) suite upgrades and new MRI equipment
designs. It is increasingly desirable to synchronize certain patient activity with the MRI
imaging system. The incredible high magnetic field strengths are increasing with each
generation (3.0 Tesla being the top of the line norm today) so that the electromagnetic
transparency of components become more important with each succeeding generation
and new application. The intrinsic passiveness and electromagnetic immunity of optical
sensors plus the all-dielectric nature of optical fiber is ideal for both sensor design and
optical signal transmission in and out of Zone 4 (MRI Scanner location) of the MRI suite.
Designing equipment that can operate within the extreme electromagnetic fields present
in an MRI suite is extremely challenging. The MRI suite precludes the use of
conventional components and structures fabricated from ferrous-based materials, nickel
alloys and most stainless steel materials – including electronics, electric motors and other
electrical and electromechanical devices commonly used in the industrial world.
Magnetically attracted metals – small or large - can become harmful projectiles and either
damage the machine or affect patient/operator safety. Also improper materials can
create undesirable artifacts or distortions which affect the quality of the imaging results.
Our central focus is the development and application of MRI compatible fiber optic
sensors necessary for closing the loop - specifically for measuring position, speed and
limits. In this article we present three MRI-based motion control applications which
demonstrate the operation and use of recently developed, commercially available MRI
safe fiber optic-based feedback sensors.
Mythbuster - fiber optics is not fragile
Although made of glass, fiber optics is not fragile! Optical fiber and cabling is designed to
be strong and resistant to physical abuse – especially excess bending and high tensile
loads. The military uses optical fiber in the harshest applications , including aircraft,
missiles, satellites and the most hostile environments - from the desert to the arctic, from
undersea to space.
It’s essentially just another type of wire – a glass wire.
What is a fiber optic sensor?
As shown in Figure 1, a fiber optic sensor is a device that alters the properties of the light
passing through the device based on a physical quantity imparted on the device. In this
sense, the fiber optic sensor is not a true transducer - it does not convert one form of
energy into another - but is instead a “sensing element” which changes a characteristic
parameter of the light injected into the sensor. Hence, a typical fiber optic sensor system

2. consists of three parts – the fiber coupled “passive” optical sensor, the “active”
interrogator or system interface, and the fiber optic light path or link that connects them.
Because of its low loss and ability to transmit interference-free over long distances, the
fiber optic link provides the means of locating the active interrogator/system interface
outside the MRI Scanner (Zone 4) Area.
Figure 1. Block diagram of
a fiber optic sensor systemHow does a fiber optic position sensor work?
Typically optical power (light) is sent to the sensor where the light is being altered or
changed in amplitude, wavelength, polarization, etc. Other sensors measure the time of
flight of the light while the physical property changes the optical path length.
The simplest form of a fiber optic sensor is an optic limit switch where the presence or
absence of an object in the light path must be determined. In this case evaluating the
ON-OFF state of light is sufficient and works reliably. To the fiber optic designer it is an
unfortunate reality that optical amplitude within a fiber optic link is not stable and cannot
be relied on for making absolute measurements. Long term source degradation, fiber
bending and fiber optic connector non-repeatability all affect optical transmission over
time and environmental factors severely affect measurement accuracy. Fiber optic
communication links are reliable because they transmit digital information and all
receivers incorporate an automatic gain control (AGC) amplifier.
Thus, position sensors that depend on light amplitude modulation have proven to be
unstable, inaccurate and unreliable. Spectral-based techniques are much more reliable
because they are not affected by light intensity. Whether the light level is low or high, the
spectral light distribution in the fiber remains the same. For instance, Fiber Bragg
Gratings are one such technology which alter the spectral behavior but are affected by
temperature – making for a poor position sensor. The key optical innovation of the
Micronor MR330 series MRI position sensor is that the position information is embedded
into the optical spectrum and provides accurate, high resolution position information
unaffected by varying losses or degradation in the fiber optic link. Utilizing the optical
spectrum as the information carrier rather than amplitude assures reliable accuracy, even
when the fiber link installation is degraded.
Figure 2. Diagram of the MR338 MRI safe fiber optic absolute position
sensor

3. As shown in Figure 3, the interrogator/controller transmits a broadband light pulse to the
sensor via the input fiber. Based on the position of the rotary code wheel, the internal
optics passively convert this light pulse source into a return signal transmitted over the
output fiber, in which the spectral pattern is essentially a unique binary representation of
the rotary encoder’s angular position. Internally, the interrogator functions like a spectral
analysis system in which the optical return signal is imaged onto a CCD and the resultant
spectral signature analyzed and converted to an angular position code.
Figure 3. How the MR338 fiber optic position sensor works
The second innovation of the MR338 MRI Safe Position Sensor is its fabrication from
non-metallic materials so to be completely RF transparent. This was not a simple
substitution of non-metallic materials versus the original MR332 “Metallic” industrial
sensor design. Due to the accuracy required, the materials must be extremely stable over
temperature, humidity and time. Internally the sensor accurately resolves down to 4µm
thus any shift of the material introduces an error in position reading. There are numerous
plastic materials that have a suitable low temperature coefficient, however, as is typical
for plastics, they exhibit hygroscopic property which means they change size based on
moisture content. A suitable ceramic-like material is used for alignment of the
dimensionally critical optics. This part is fabricated using high precision stereo
lithographic fabrication technology.

4. The resulting MR338 MRI position sensor system offers 13-bit (8192 counts or 0.044°)
single turn resolution and 12-bit (4096 count) multiturn tracking. The same optical
technique is also applied to a fiber optic linear position sensing system.
ISweek（http://www.isweek.com/）- Industry sourcing & Wholesale industrial products