Fokko Pieter Wieringa

Contactless multiple wavelength photoplethysmographic imaging: a first step toward "SpO2 camera" technology.

We describe a route toward contactless imaging of arterial oxygen saturation (SpO2) distribution within tissue, based upon detection of a two-dimensional matrix of spatially resolved optical plethysmographic signals at different wavelengths. As a first step toward SpO2-imaging we built a monochrome CMOS-camera with apochromatic lens and 3λ-LED-ringlight (λ1 = 660 nm, λ2 = 810 nm, λ3 = 940 nm; 100 LEDs λ−1). We acquired movies at three wavelengths while simultaneously recording ECG and respiration for seven volunteers. We repeated this experiment for one volunteer at increased frame rate, additionally recording the pulse wave of a pulse oximeter. Movies were processed by dividing each image frame into discrete Regions of Interest (ROIs), averaging 10 × 10 raw pixels each. For each ROI, pulsatile variation over time was assigned to a matrix of ROI-pixel time traces with individual Fourier spectra. Photoplethysmograms correlated well with respiration reference traces at three wavelengths. Increased frame rates revealed weaker pulsations (main frequency components 0.95 and 1.9 Hz) superimposed upon respiration-correlated photoplethysmograms, which were heartbeat-related at three wavelengths. We acquired spatially resolved heartbeat-related photoplethysmograms at multiple wavelengths using a remote camera. This feasibility study shows potential for non-contact 2-D imaging reflection-mode pulse oximetry. Clinical devices, however, require further development.

Differentiation between nerve and adipose tissue using wide-band (350–1,830 nm) in vivo diffuse reflectance spectroscopy

Intraoperative nerve localization is of great importance in surgery. In certain procedures, where nerves show visual resemblance to surrounding adipose tissue, this can be particularly challenging for the human eye. An example of such a delicate procedure is thyroid and parathyroid surgery, where iatrogenic injury of the recurrent laryngeal nerve can result in transient or permanent vocal problems (0.5–2.0% reported incidence). A camera system, enabling nerve‐specific image enhancement, would be useful in preventing such complications. This might be realized with hyperspectral camera technology using silicon (Si) or indium gallium arsenide (InGaAs) sensor chips.

Automated spectroscopic tissue classification in colorectal surgery

Background. In colorectal surgery, detecting ureters and mesenteric arteries is of utmost importance to prevent iatrogenic injury and to facilitate intraoperative decision making. A tool enabling ureter- and artery-specific image enhancement within (and possibly through) surrounding adipose tissue would facilitate this need, especially during laparoscopy. To evaluate the potential of hyperspectral imaging in colorectal surgery, we explored spectral tissue signatures using single-spot diffuse reflectance spectroscopy (DRS). As hyperspectral cameras with silicon (Si) and indium gallium arsenide (InGaAs) sensor chips are becoming available, we investigated spectral distinctive features for both sensor ranges. Methods. In vivo wide-band (wavelength range 350-1830 nm) DRS was performed during open colorectal surgery. From the recorded spectra, 36 features were extracted at predefined wavelengths: 18 gradients and 18 amplitude differences. For classification of respectively ureter and artery in relation to surrounding adipose tissue, the best distinctive feature was selected using binary logistic regression for Si- and InGaAs-sensor spectral ranges separately. Classification performance was evaluated by leave-one-out cross-validation. Results. In 10 consecutive patients, 253 spectra were recorded on 53 tissue sites (including colon, adipose tissue, muscle, artery, vein, ureter). Classification of ureter versus adipose tissue revealed accuracy of 100% for both Si range and InGaAs range. Classification of artery versus surrounding adipose tissue revealed accuracies of 95% (Si) and 89% (InGaAs). Conclusions. Intraoperative DRS showed that Si and InGaAs sensors are equally suited for automated classification of ureter versus surrounding adipose tissue. Si sensors seem better suited for classifying artery versus mesenteric adipose tissue. Progress toward hyperspectral imaging within this field is promising.

In vitro demonstration of an SpO2-camera

Feasibility of pulse oxigraphy is examined using an in-vitro phantom, perfused with human blood by a heart-lung machine. 29 different oxygen saturation levels, measured with our experimental camera and a clinical pulse oximeter, were matched against laboratory blood gas analysis. Discrete transfer functions between measurements & laboratory values were derived for camera (fCAM-LAB) & pulse oximeter (fPULSE-LAB). 87 additional independent pulse oximeter & camera measurements were subjected to these functions to check reproducibility (R2 = 0.99). To demonstrate imaging capacities, a dual reservoir was applied to image venous and arterial samples from 2 patients. Camera-derived vs laboratory arterial values were: 97.0% vs 99.5% (pat. 1, pH=7.41) & 97.5% vs 99.5% (pat. 2, pH=7.37). Venous resulst were: 86.5% vs 74.4% (pat. 1, pH=7.39) & 89.1% vs 86.2% (pat. 2, pH=7.34). In vitro pulse oxigraphy can visualize regions with different blood oxygenation.