Tomographic Probe for Perfusion Analysis in Deep Layer Tissue Melissa Berthelot, Guang-Zhong Yang and Benny Lo Abstract— Continuous buried soft tissue free flap postopera- tive monitoring is crucial to detect flap failure and enable early intervention. In this case, clinical assessments is challenging as the flap is buried and only implantable or hand held devices can be used for regular monitoring. These devices have limitations in their price, usability and specificity. Near- infrared spectroscopy (NIRS) has shown promising results for superficial free flap postoperative monitoring but it has not been considered for buried free flap mainly due to the limited penetration depth of conventional approaches. A wearable wireless tomographic probe has been developed for continuous monitoring of tissue perfusion at different depths. Using the NIRS method, blood flow can be continuously measured at different tissue depths. This device has been designed following conclusions of extensive computerised simulations and validated with a vascular phantom. I. INTRODUCTION Being able to monitor the perfusion of an organ at different depth is crucial to prevent tissue death or organ failure. In particular, buried soft tissue free flap surgery is a common operation for reconstruction. Flap failure, usually due to improper blood vessel anastomosis, is morbid and leads to additional surgery. Venous thrombosis is reported to be the most common cause of flap failure [1] for both superficial and buried flaps. In general, flap salvage rate following thrombosis is directly correlated to the time of flap failure detection. Most of flap failure happens in 24 to 48 hours post- surgery, with more cases happening within the 5 hours post- surgery [2]. In the case of buried flap, clinical assessment is not possible [3]. In some cases, buried fasciocutaneous or myocutaneous flaps can clinically be monitored by resurfac- ing or temporally externalising a segment of the flap sharing the same pedicles [4]. However, when it is not possible to implement these methods, other approaches are needed to ensure early detection of flap failure. Implantable devices such as implantable Doppler, tissue oxygen tension or pres- sure and microdialysis have shown to be suitable methods for continuous monitoring of both superficial and buried free flaps [3] [5] [6] [7]. These assessment methods can be combined with confirmatory test, such as the duplex Doppler sonography or the hand-held Doppler flowmetry [8] [9], to prevent unnecessary additional surgery in the case of false positive outcome. The duplex Doppler sonography device is a hand held device that measures and displays the velocity blood flow at the blood vessels. Similar to the Doppler flowmetry device, it requires expert handling to ensure the reliability of the measurements. This device can be used with Melissa Berthelot, Guang-Zhong Yang and Benny Lo are with The Hamlyn Centre, Imperial College London, South Kensington, London UK. email: {meb14,g.z.yang,benny.lo}@imperial.ac.uk the pulsatility index which relies on the waveform of the flow velocity at the pedicles for postoperative monitoring of the flap [10]. This method has shown promising results; however, the point of entry of the perforator into the flap needs to be marked during the operation to ensure reli- able measurements. Previous studies have shown that near- infrared spectroscopy (NIRS) is a reliable method to early detect superficial free flap failure, with particular detection of the origin of flap failure (venous/arterial thrombosis) [3]. However, this method has not been applied for continuous monitoring of buried free flap. This is particularly due to the light penetration depth which particularly depends on the distance between the photodetector and the light source, the light intensity, the wavelength of the light source and the penetrated medium (extinction and reflection coefficients). Tuning these parameters should allow the sensor to target specific tissue depths. Although the measurements would be fused with perfusion in the superficial layers, in the case of buried free flaps, the oxygenation saturation (StO 2 ) in the superficial tissue is usually constant (i.e. healthy tissue), while StO 2 can increase or decrease in the buried flap (i.e.successful or failed flap). This paper introduces a new wearable wireless tomographic probe for continuous StO 2 monitoring. The probe uses NIRS to measure StO 2 at different penetration depths. The probe has been designed through an extensive computerised simu- lations and has been validated using a vascular phantom. II. SIMULATION & HARDWARE DESIGN Computerised simulations were performed using the Monte Carlo simulation for multi-layered environment [11] [12]. They were run on a i7-6700 CPU at 3.4GHz with 16GB RAM. The simulation code was firstly adapted for light transport of different wavelengths (577nm, 633nm, 700nm and 800nm) into the first 5 layers of the skin with and without blood vessels of different diameters and oxygen saturations and at different depths [13] [14]. A benchmark of light intensity (10-100-1000 which are respectively 1E7, 1E8 and 1E9 photons at the light source at t=0 of the simulation) and penetration angle (0-45-90 degrees of penetration of the photons at t=0 of the simulation) parameters have been performed to observe their influence on the light path (see Fig. 1 and 2). The fluence (Φ) is defined as the normalised light extinction (A) according to the refractive index of the filler medium for the given wavelength (n w ) following Φ = A/n w ; the use of the base 10 logarithmic function allows to linearise the fluence. Therefore, the overall equation describing the behaviour of the fluence encapsulates the wavelength (w),