Fiber optic shape sensing generally requires three key components; a fiber sensing data acquisition system, a fiber-based 3D shape sensor, and an algorithm which translates measured optical data to the shape of the fiber sensor.
Shape is most often obtained through the measurement and integration of a multitude of strain measurements along the length of the fiber sensor.
Because of this, 3D coordinate errors will always grow as a function of sensor length, and how quickly this error grows is driven by the amount of information the system obtains between the proximal and distal ends. Thus, it is critical for the data acquisition component of 3D solutions to maximize the number of measurements along the length of a 3D shape sensor.
There are two types of systems available today being utilized in 3D shape sensing solutions.
The most common type, known as traditional fiber Bragg grating (FBG) or Wavelength Division Multiplexing (WDM) systems, provide a few distributed strain measurements through a series of spectrally unique FBGs inscribed into the fiber.
This technique typically offers ten to thirty data points between the proximal and distal ends of a 3D shape sensor. This is a significant limitation for a few reasons.
First, these systems are limited in the length of sensor they can interrogate accurately. Since each FBG is typically about one centimeter long, the sensors used with these systems are on the order of ten to thirty centimeters.
This may be sufficient for biopsy needles and relatively short medical devices, but is insufficient for catheters, endoscopes and other similar devices which are often more than a meter in length.
A second limitation is the lack of spatial granularity in the quasi-distributed measurement.
The FBGs may be spaced apart along the fiber to cover a longer 3D sensor length, but then information is lost between consecutive FBGs. Suppliers and users of such solutions are forced to mathematically interpolate between them to fill in the missing information.
This process will always reduce the accuracy of 3D measurements. Even if a subtle bend or twist occurs between two FBGs, this information is lost and can cause centimeters of error in the output shape.
On top of this, FBGs are most commonly one centimeter in length, with some providers offering as small as three millimeters. The strain measurement obtained from these FBGs is effectively averaged over the FBG length.
While three millimeters may sound small, 3D shape sensors are often placed into shapes with significant bend, twist, and strain gradients. In these cases, averaging leads to additional lost information.
This loss may be somewhat small for an individual FBG location, but often results in centimeters of error after the integration performed by shape sensing algorithms where the error associated with each individual FBG compounds.
The final and most significant limitation of traditional FBG systems is their inability to measure the twist of a 3D sensor.
Twist measurements are an essential component of accurate 3D shape sensing, particularly for tortuous and complex shapes. The small number of FBG sensors and lack of granularity makes it impossible for such systems to perform twist measurements.
This limitation can easily lead to several centimeters of error, even along short 3D sensor lengths. When using traditional FBG systems, end users are often forced to integrate 3D fibers into their devices in such a way that guarantees they do not twist – a nearly impossible task for catheters and endoscopes which are expected to pass through tortuous paths.
Test results published by proponents of these solutions include tight restrictions on twist and various corrections to account for lost information – these are straightforward to implement in laboratory testing but are not feasible for medical device deployment and general, unrestricted use.
These limitations are why the FBG/WDM solution is most often restricted to biopsy needles and other stiff, short medical devices which do not see significant deformation or complex shapes.
The alternative to the FBG/WDM approach is the use of Optical Frequency Domain Reflectometry (OFDR). Instead of measuring strain at a few tens of locations, OFDR provides thousands of measurements along the length of a 3D shape sensor.
The solution offered by The Shape Sensing Company (TSSC) is based on OFDR technology and is specifically designed and optimized for 3D shape sensing applications. TSSC’s Pathfinder product line provides more than eight thousand measurements along a 1.25 meter 3D shape sensor.
This yields extremely granular information – down to 150 micron resolution – along the sensor length such that not even the most subtle bend or gradient is missed.
The resolution offered by OFDR enables sub-centimeter accuracy to be maintained over lengths longer than one meter, even if the sensor or device is being twisted.
Since twist is a simple measurement for OFDR systems, users have far more freedom in how the 3D fibers are integrated into their devices and they do not have to restrict twist or perform corrections to fill in missing information.
It is for these reasons that 3D shape sensing solutions built around OFDR are the optimal choice for medical devices which navigate tortuous and complex anatomies.