Clinical assessment of near-infrared spectroscopy for noninvasive diabetes screening.

BACKGROUND: Current diabetes screening techniques comprise the fasting plasma glucose (FPG) and oral glucose tolerance tests. Both tests demand patient compliance, and neither test has ideal performance. Near-infrared (NIR) spectroscopy is a noninvasive means of interrogating characteristics of a sample and is evaluated as a novel screening method for type 2 diabetes. METHODS: One hundred fifty-four patients with and without type 2 diabetes were recruited. Their forearm skin was measured with the NIR spectroscopic system, and a capillary blood glucose measurement was also taken. Sixty-six patients returned for a second visit at a later date. A multivariate model, generated from a separate training study, was employed to produce a quantitative risk marker of disease for each NIR spectrum. Sensitivity and specificity (the probabilities that the NIR method will correctly identify a subject as having diabetes or as not having diabetes, respectively) were calculated. As the NIR method produces a continuous rather than categorical classification, various thresholds were evaluated to give several sensitivity and specificity pairs. Test reproducibility was also determined. RESULTS: At a false-positive rate of 70%, the NIR test had a sensitivity of 77.7%, which is comparable to the 77.3% sensitivity for the FPG test as reported for the Third National Health and Nutrition Examination Survey (NHANES III) study. The reproducibility of the NIR test was also similar to the FPG test (inter-day agreement rates of 84.2% and 79.2%, respectively). CONCLUSIONS: A noninvasive NIR spectroscopic measurement of the volar forearm was shown to have comparable performance characteristics with the FPG test. The source of the spectroscopic signal is still uncertain and is the subject of ongoing research.

Porphyrin bleaching and PDT-induced spectral changes are irradiance dependent in ALA-sensitized normal rat skin in vivo.

Photobleaching kinetics of aminolevulinic acid-induced protoporphyrin IX (PpIX) were measured in the normal skin of rats in vivo using a technique in which fluorescence spectra were corrected for the effects of tissue optical properties in the emission spectral window through division by reflectance spectra acquired in the same geometry and wavelength interval and for changes in excitation wavelength optical properties using diffuse reflectance measured at the excitation wavelength. Loss of PpIX fluorescence was monitored during photodynamic therapy (PDT) performed using 514 nm irradiation. Bleaching in response to irradiances of 1, 5 and 100 mW cm-2 was evaluated. The results demonstrate an irradiance dependence to the rate of photobleaching vs irradiation fluence, with the lowest irradiance leading to the most efficient loss of fluorescence. The kinetics for the accumulation of the primary fluorescent photoproduct of PpIX also exhibit an irradiance dependence, with greater peak accumulation at higher irradiance. These findings are consistent with a predominantly oxygen-dependent photobleaching reaction mechanism in vivo, and they provide spectroscopic evidence that PDT delivered at low irradiance deposits greater photodynamic dose for a given irradiation fluence. We also observed an irradiance dependence to the appearance of a fluorescence emission peak near 620 nm, consistent with accumulation of uroporphyrin/coproporphyrin in response to mitochondrial damage.

Quantitative broadband near-infrared spectroscopy of tissue-simulating phantoms containing erythrocytes.

We report the use of steady-state diffuse reflectance spectroscopy (SSDRS) to measure the near-infrared absorption spectrum of liquid phantoms containing human erythrocytes in aqueous suspensions of polystyrene spheres which simulate the scattering properties of tissue. The absorption spectra obtained from these SSDRS measurements of intact red cells under oxygenated and deoxygenated conditions are compared with several published spectra of 'stripped' haemoglobin prepared from lysed cells. Two fitting algorithms (nonlinear least squares and singular value decomposition) which exploit the broad spectral range provided by these measurements (170 data points spanning 164 nm in a single acquisition) are used to determine haemoglobin oxygen saturation (SO2) from SSDR spectra collected over a wide range of measured oxygen partial pressures. The validity of these algorithms is assessed by comparing literature values of p50 (the oxygen tension at which haemoglobin is 50% saturated) and the Hill coefficient to values of these parameters determined from the SO2 estimates. The singular value decomposition algorithm can also be used to reconstruct the non-haemoglobin background absorption spectrum without a priori assumptions regarding its constituent chromophores or their concentrations. Using this technique, the absorption spectrum of a small amount of India ink (maximum absorption coefficient (mu(a max)) approximately 0.0006 mm(-1)) added to a phantom containing red cells (mu(a max) approximately 0.026 mm(-1)) was reconstructed over a full range of oxygen saturations. The implications of these measurements for detection of weakly absorbing chromophores (such as cytochrome aa3) in the presence of haemoglobin are discussed.

An evaluation of near infrared spectroscopy and cryospectrophotometry estimates of haemoglobin oxygen saturation in a rodent mammary tumour model.

Haemoglobin oxygen saturation in subcutaneous rat mammary tumours was measured using near infrared spectroscopy (NIRS) in vivo and in rapidly frozen sections from the same tumours using cryospectrophotometry, which reports oxygen saturation in individual blood vessels to depths of 4 mm from the tissue surface. Measurements were performed on tumours while animals breathed either room air or carbogen. In five of nine tumours, the average saturation calculated from cryospectrophotometric measurements agreed with that determined from NIRS to within 13%, and in four of these five tumours agreement was 5% or better. In the remaining four of nine tumours, where agreement was poor, the volume-averaged saturations estimated from NIRS were consistently higher than those calculated from cryospectrophotometry. Monte Carlo simulations demonstrated that the depth of tissue probed by NIRS was significantly greater than that sampled by cryospectrophotometry. Analysis of the frequency of severely hypoxic vessels showed that when NIRS reported a saturation of approximately 70% or higher, the fraction of tumour vessels with saturations less than 10% was limited to 0.06 or less. Sensitivity and specificity analysis suggests that NIRS and NIRS imaging may identify clinically relevant hypoxia, even when its spatial extent is below the resolution limit of the NIRS technique.

Design and testing of a white-light, steady-state diffuse reflectance spectrometer for determination of optical properties of highly scattering systems.

We present a steady-state radially resolved diffuse reflectance spectrometer capable of measuring the absorption and transport scattering spectra of tissue-simulating phantoms over an adjustable 170-nm wavelength interval in the visible and near infrared. Measurements in a variety of phantoms are demonstrated over the relevant range of tissue optical properties, and the accuracy of the instrument is found to be approximately 10% in both scattering and absorption. Monte Carlo simulations designed to test the accuracy of the instrument are presented that support the experimental findings.

Localization of Luminescent Inhomogeneities in Turbid Media with Spatially Resolved Measurements of cw Diffuse Luminescence Emittance.

We present a steady-state method for localizing a source ofluminescence (i.e., fluorescence or phosphorescence) buried in asemi-infinite turbid medium with unknown optical properties. Adiffusion theory expression describing the emittance of an isotropicpoint source is fit to spatially resolved surface measurements of thediffuse emittance from the luminescent source. The techniquereports the location of the center of a 6.0-mm-diameter, fluorophore-containing spherical bulb embedded in a liquid phantom withan accuracy of 1.0 mm or better for source depths as great as 40.0mm. Monte Carlo data are analyzed to investigate the range and thepossible sources of error in the reconstructed source depth.

Carbogen-induced changes in rat mammary tumour oxygenation reported by near infrared spectroscopy.

We have evaluated the ability of steady-state, radially-resolved, broad-band near infrared diffuse reflectance spectroscopy to measure carbogen-induced changes in haemoglobin oxygen saturation (SO2) and total haemoglobin concentration in a rat R3230 mammary adenocarcinoma model in vivo. Detectable shifts toward higher saturations were evident in all tumours (n = 16) immediately after the onset of carbogen breathing. The SO2 reached a new equilibrium within 1 min and remained approximately constant during 200-300 s of administration. The return to baseline saturation was more gradual when carbogen delivery was stopped. The degree to which carbogen increased SO2 was variable among tumours, with a tendency for tumours with lower initial SO2 to exhibit larger changes. Tumour haemoglobin concentrations at the time of peak enhancement were also variable. In the majority of cases, haemoglobin concentration decreased in response to carbogen, indicating that increased tumour blood volume was not responsible for the observed elevation in SO2. We observed no apparent relationship between the extent of the change in tumour haemoglobin concentration and the magnitude of the change in the saturation. Near infrared diffuse reflectance spectroscopy provides a rapid, non-invasive means of monitoring spatially averaged changes in tumour haemoglobin oxygen saturation induced by oxygen modifiers.