High -resolution imaging of esophageal pressure waves based on fiber gratings

The esophagus is a hollow tube that transports food from the mouth to the stomach. Anatomically, it is a muscle tube with sphincters at both ends. The function of the sphincters is to keep the tube empty of external (food) and internal (acid) intrusions. Swallowing and transport of food from the mouth to the stomach is a highly coordinated neuromuscular event.¹ Dysfunction of reflex pumping caused by the contraction and relaxation of muscles causes a variety of disorders, such as gastro-esophageal reflux, esophageal adenocarcinoma, heartburn or pyrosis, achalasia (failure of the lower esophageal sphincter to relax properly), and ‘nutcracker esophagus,’ where swallowing contractions are overpowerful. In cases of clinical uncertainty of diagnosis, the functionality of the esophagus must be examined with sensors. A clearly defined goal is to make these examinations faster and more comfortable for the patient. Here, we describe a catheter equipped with sufficient pressure sensors in close proximity that a physician can obtain semicontinuous images of pressure distribution. Several concepts for HRM (high-resolution manometry, or pressure measurement in a medical context) have been proposed,^2–4 but many are limited with respect to size or the number of measurement positions.

Figure 1. System for measuring peristaltic waves. FBG: Fiber Bragg grating.

Our proposed sensor concept combines an elastic tube and embedded fiber Bragg gratings (FBGs, see Figure 1). Applying local pressure to the tube causes it to deform. Volume conservation of the tube results in elongation of the fiber.^{5, 6} The device has a lateral homogeneous architecture and contains no movable parts. An increase in local pressure reduces the cross-sectional area of the sensing cell. The elongation of the cell applies strain to the fiber, which alters the resulting center wavelength of the Bragg grating. FBGs are periodic index modulations inside optical fibers that work as wavelength-selective mirrors. They are affected by temperature and strain and therefore are often used as sensors of these parameters. Their small dimensions, multiplexing capability, and insensitivity to electromagnetic interference make FBGs ideally suited for structural monitoring of large engineering systems.^{7, 8} In particular, the small size of FBGs is highly promising for medical measurement.⁹

Figure 2. Realized catheter and sensitivity measurement. Numbers 18–21 specify the corresponding sensor.

FBGs can be inscribed during the drawing process of the fiber, which is advantageous in several ways.¹⁰ Cladding occurs after FBG inscription, resulting in uniform coverage with high mechanical stability compared to standard FBGs with local recoating.¹¹ In addition, grating arrays are made in a single writing process. We have developed a spectrometer-based measurement system for FBG reflection.⁷ Light comes from a superluminescent LED covering a wavelength range from 800 to 860nm. It is reflected by the FBG array and monitored with a grating spectrometer and a CCD array. The measurement system has a scan repetition rate of 1kHz, which is more than sufficient to measure swallow events. One peristaltic wave takes around 20 seconds,² and must be resolved with a sampling frequency of 10Hz. The wavelength spacing of the sensors is limited to 1.3nm to prevent overlap of the sensing channels. Currently, 32 channels can be measured simultaneously without further multiplexing techniques such as time-resolved detection or switching between several sensing fibers.

The catheters containing the fibers with the sensor arrays are fabricated using an extruder and thermoplastic elastomers. These materials combine the properties of olefin-based elastomers and thermoplastics. They are biocompatible and suitable for sterilization and disinfection.

A measurement system for medical applications has to satisfy various constraints. Not only should it be compatible with the manufacturing process and suitable for disinfection and sterilization. It must also show maximum sensitivity. Optimal performance is achieved with catheter materials having a high Poisson number ν, a fiber with an outer diameter as small as possible, and a catheter diameter as large as possible. The catheter diameter is limited by application conditions and should not exceed 5mm. The minimum fiber diameter is restricted to 80μm by the currently available manufacturing technology.

To simulate medical conditions, we introduce our device into a pressure chamber through steel tubes. Air is supplied with a pressure-regulating valve, and internal pressure is controlled with a pressure-measurement gauge. Hydrostatic pressure is transmitted to the catheter using a soft tubular membrane that adapts easily to the test device and prevents air flow to the environment. The soft catheter reaches a sensitivity of more than the required 1μ∊/mbar, which is the target specification (see Figure 2).

In summary, we have reported a pressure-sensing tube with high spatial resolution. Our experiments show that FBG arrays in soft polymer tubes meet the sensitivity demands of medical applications. FBG-based HRM is not limited to the diagnosis of gastrointestinal motility disorders. Indeed, the number of sensors can be increased, which offers the promise of novel uses, for example, colonoscopic measurements,¹² where good spatial resolution prevents artifacts in diagnostics.¹³As next steps, we plan to improve the cross-sensitivity of the sensors (e.g., to temperature) and to take into account increased functionalities, such as acidity detection.

The OESOPHAGUS project was funded by the German Ministry of Economy through the Federation of Industrial Research Association under project 15529BR.