A methodologic description of functional near-infrared spectroscopy during bladder filling

Objectives: Cortical functional near infrared spectroscopy (fNIRS) measures changes in neuroexcitation based on the chromophores of deoxy and oxyhemoglobin (O2Hb) to a depth of 1-2 cm. fNIRS is a versatile, noninvasive tool alternative to costly fMRI; however the sensitive measurement can be overlay with signal disruption. This study aimed to provide a carefully outlined, repeatable methodology that can be utilized in future studies to improve collection and processing of the raw signal to better identify differences in cortical excitation patterns during bladder filling.

Methods: Healthy controls without known urologic diagnoses or symptoms of urinary urgency completed a validated oral hydration protocol. They reported “first desire” to void and real time sensation of bladder fullness (0-100%) using a Sensation Meter. A 24-channel fNIRS head cap simultaneously recorded prefrontal cortical O₂Hb. Raw optical density signals were processed in Matlab with low pass filtering, mono-exponential curve fitting, and interpolation to standardize for different fill times (figure 1). Data was divided into low sensation and high sensation regions at the point of first desire. In addition to account for head motion artifacts, segmentation was completed utilizing the headcap’s inertial measurement unit (IMU) or accelerometer which captures spatial orientation. A custom algorithm visually-identified segments of motion artifact then spliced the pre- and post-artifact tracings together (figure 1).

Results: A total of 8 participants (male: n=4, female: n=4) were enrolled with mean age 39.4± 20 years and BMI of 25± 3.9 kg/m², and no significant differences in age, BMI, race, fill time, time to first desire, or ICIQ-OAB scores. Signal acquisition improved with postural support to reduce head motion and headcap optimization to increase contact between sensors and the scalp. Although readings were more irregular during low urgency, O₂Hb patterns generally increased in the frontal cortex during the elevated urgency period (first desire to 100% sensation) (figure 2).

Conclusions: This protocol demonstrates the ability to noninvasively measure changes in O₂Hb through natural bladder filling in a physiologic seated position. We highlight some troubleshooting and technical considerations to improve data collection, processing and analysis for application in similar urologic protocols.