A Design Space Exploration (DSE) on Non-Invasive Sensing of Bladder Filling Using Near Infrared Spectroscopy (NIRS)

A Design Space Exploration (DSE) on Non-Invasive Sensing of Bladder Filling Using Near Infrared Spectroscopy (NIRS) M. Saarpour msa@ucdavis.edu S. Ghiasi ghiasi@ucdavis.edu ABSTRA CT Urinary Incontinence (UI) is a widespread medical condition that af- fects one person from every three or four Americans. Near-Infrared Spectroscopy (NIRS) is a non-invasive under-study method for blad- der lling sensation that can enhance the life quality of UI patients by nding the optimal voiding time. Howev er , the application of NIRS to bladder volume sensing can be quite challenging due to three major obstacles: non-adequate traversal depth of NIR wave- lengths, robustness and p ower eciency requirements of the appli- cation, and low power transmission rate of NIR wavelengths. is work provides a Design Space Exploration (DSE) through the ee ct of various design parameters on NIRS applicability for bladder vol- ume sensing. W e investigate the impact of 7 dierent wavelengths from 650-950 nm, 16 possible detector-source distances, and 6 dif- ferent sensation depths. e results of our work can b e use d as a guideline through optimal design and implementation of NIRS for bladder lling sensation. KEY W ORDS UI, NIRS, non-invasive, DSE 1 IN TRODUCTION NIRS can be a promising method for non-invasive bladder moni- toring of patients with UI symptoms. e loss of bladder control, known as UI, is a world-wide prevalent me dical condition that af- fects 25-33% of Americans [ 6 ]. NIRS te chnique is projecte d to be a useful tool for sensing the bladder lling which can help in nding the optimal voiding time in short-term, while providing valuable information for the long-term treatment process. Although NIRS technique has been teste d for bladder volume sensing in some clinical trials [ 2 ][ 1 ][ 3 ], its applicability for practical usage faces several challenges. First of all, we show that due to high absorption and scaering characteristics of tissue layers in ab dominal area for NIR wave- lengths, the traversal depth may not b e adequate to sense the blad- der . is problem becomes more challenging when bladder depth is higher as a result of patient’s ob esity . Secondly , robustness and power eciency of the nal probe is a requisite. is device should not b e sensitive to misplacements and small movements during usage and should be low p ower in order to b e powered by baer y in a daily basis. On the other hand, the intensity of detected photons should be high enough to provide a reasonable power range to the detection module. Since the input p ower is limite d by system’s power budget, providing the minimum sensational power for dete ction module can be an obstacle. erefore, the optimal wavelength should b e able to maximize the power transmission ratio from input to output, and as a result, minimize the absorption and scaering power losses in the transmission process. ese challenges necessitate the need for a comprehensive de- sign space exploration (DSE) to evaluate the ee ct of parameters such as wavelength and source-detector distances on penetration depth, sensitivity , and power transmission ratio. In this work, we employed monte carlo simulation to p erform the aforementioned DSE while considering following parameters: • 7 wavelengths in range of 650-950 nm • 16 p ossible detector-source spacings • 6 dierent tissue thicknesses in range of 15-40 mm In what follows, rst the power transmission ratio calculation has been explained (section 2). en, section 3 and 4 cover the sim- ulation setup and the results of this project, consecutively . Finally , Section 5 is de dicated to conclusion of this work. 2 PO WER TRANSMISSION RA TIO e p ower transmission ratio is the ratio of dete cted photons power to the input photons’ power . In this work, we assume that the detection of all photons happen at the same time. As a result, the power transmission ratio would be equal to energy transmission ratio. e energy of N photons with wavelength λ can be calculated by e quation 1 where h and c are the Plank constant and speed of light, conse cutively . E = N ∗ hc λ (1) erefore, the overall energy transmission ratio would be equal to the numb er of detected photons to the numb er of input photons. 3 SIMULA TION SET UP W e have use d Monte Carlo Extreme simulator in order to quantify the ee ct of wavelength and source-detector spacing on traversal depth. In order to get track of photons’ traversal depth in the simula- tion, we have compared the number of detected photons with and without a super-absorb ent layer (SAL) at the depth of interest. SAL has signicantly high absorption and zero scaering characteris- tic which would swallow and absorb photons reaching its surface. e de creased number of detected photons, as a result of applying SAL, indicates the numb er of photons reaching the SAL depth. e simulation mo del has b een presented in gure 1. W e utilized Monte Carlo simulation with 500 million input pho- tons at 7 dierent NIR wavelengths in range of 650nm to 950nm. e optical properties of tissue layers used in this model has been adopted from [5]. For all the wavelengths under study , we have swept the SAL from 10 mm to 40 mm with a step-size of 5 mm and colle cted the Figure 1: e simulation model which is capable of perform- ing p enetration depth analysis by using a sweeping SAL. penetration depth information. e simulation contains 16 dete c- tors which are placed every 5 mm in range of 10-85 mm distance to light source. All these 16 detectors have the similar radius of 1.41 mm. Finally , the power transmission ratio has been calculated p er wavelength for each source-detector distance using equation 1 and number of detected photons at each detector . en we back- calculated the minimum input power required for receiving the minimum sensible output power (detector characteristics provided in [4]) considering these transmission ratio values. 4 RESULTS AND DISCUSSION Figure 2 and 3 illustrate the source detector distance and wave- length eect on penetration depth. Figure 2 focuses on the ratio of photons reaching the depth of interest to the overall numb er of input photons. On the other hand, gure 3 provides a measure of sensitivity by presenting the p ercentage of dete cted photons which are reaching the depth of interest to overall detected photons at the same dete ctor . e comparison of gure 2 and gure 3 oers a beer under- standing of optimal source-detector spacing. Although a higher number of photons can be detecte d at low spacings, a stronger signal to noise ratio is achievable using higher separation distances. W e present the minimum sensible output power and the resulted minimum input p ower in gure 4. 5 CONCLUSION In this work we p erforme d DSE to analyze the eect of parameters such as wavelength and source-detector spacing on the penetration depth of NIR photons for a bladder volume spectroscopy application. W e used monte carlo simulation using 7 dierent wavelengths and 16 p ossible source-detector spacings for 6 penetration depths. W e have also illustrated the minimum input p ower for a sp ecic choice of detector at these wavelengths and spacings. e results of our simulation can be used to direct the optimal design and implementation of a NIRS probe for bladder lling sensation. REFERENCES [1] Macnab, Andrew J. ”e evolution of near infrared spectroscopy in urology . ” Biomedical Spectroscopy and Imaging 3.4 (2014): 311-344. [2] Macnab, A. J., R. E. Gagnon, and L. Stothers. ”Clinical NIRS of the urinary bladderA demonstration case report. ” Journal of Spectroscopy 19.4 (2005): 207-212. [3] Molavi, Behnam, et al. ”Noninvasive optical monitoring of bladder lling to capacity using a wireless Near Infrared Spectroscopy device. ” IEEE transactions on biomedical circuits and systems 8.3 (2014): 325-333. [4] FairChild. ”QSB34GR / QSB34ZR / QSB34CGR / QSB34CZR Surface-Mount Sili- con Pin Photo diode. ” QSB34GR / QSB34ZR / QSB34CGR / QSB34CZR datasheet, Sep. 2016. [5] Simpson, C. Rebecca, et al. ”Near-infrared optical properties of ex vivo human skin and subcutaneous tissues measured using the Monte Carlo inversion tech- nique. ” P hysics in Medicine & Biology 43.9 (1998): 2465. [6] Urology care foundation. ”What is Urinar y Incontinence?” , hp://www.ur ologyhealth.org/urologic-conditions/urinary- incontinence/printable-version. Accessed 23 June. 2018. 2 Figure 2: e ratio of photons reaching depth of interest to overall number of input photons at each source-detector distance. Figure 3: e percentage of photons reaching depth of interest from the total number of photons detected at each detector 3 Figure 4: Minimum input power calculation using mini- mum sensible output power and power transmission ratio. 4