Time domain diffuse Raman spectrometer based on a TCSPC camera for the depth analysis of diffusive media.

We present a time domain diffuse Raman spectrometer for depth probing of highly scattering media. The system is based on, to the best of our knowledge, a novel time-correlated single-photon counting (TCSPC) camera that simultaneously acquires both spectral and temporal information of Raman photons. A dedicated non-contact probe was built, and time domain Raman measurements were performed on a tissue mimicking bilayer phantom. The fluorescence contamination of the Raman signal was eliminated by early time gating (0-212 ps) the Raman photons. Depth sensitivity is achieved by time gating Raman photons at different delays with a gate width of 106 ps. Importantly, the time domain can provide time-dependent depth sensitivity leading to a high contrast between two layers of Raman signal. As a result, an enhancement factor of 2170 was found for our bilayer phantom which is much higher than the values obtained by spatial offset Raman spectroscopy (SORS), frequency offset Raman spectroscopy (FORS), or hybrid FORS-SORS on a similar phantom.

Full-field laser-Doppler imaging and its physiological significance for tissue blood perfusion.

Using Monte Carlo simulations for a semi-infinite medium representing a skeletal muscle tissue, it is demonstrated that the zero- and first-order moments of the power spectrum for a representative pixel of a full-field laser-Doppler imager behave differently from classical laser-Doppler flowmetry. In particular, the zero-order moment has a very low sensitivity to tissue blood volume changes, and it becomes completely insensitive if the probability for a photon to interact with a moving red blood cell is above 0.05. It is shown that the loss in sensitivity is due to the strong forward scatter of the propagating photons in biological tissues (i.e., anisotropy factor g = 0.9). The first-order moment is linearly related to the root mean square of the red blood cell velocity (the Brownian component), and there is also a positive relationship with tissue blood volume. The most common physiological interpretation of the first-order moment is as tissue blood volume times expectation of the blood velocity (in probabilistic terms). In this sense, the use of the first-order moment appears to be a reasonable approach for qualitative real-time blood flow monitoring, but it does not allow us to obtain information on blood velocity or volume independently. Finally, it is shown that the spatial and temporal resolution trade-off imposed by the CMOS detectors, used in full-field laser-Doppler hardware, may lead to measurements that vary oppositely with the underlying physiological quantities. Further improvements on detectors' sampling rate will overcome this limitation.

Detection limits of multi-spectral optical imaging under the skin surface.

The present work shows that the optical/biological information contained in a typical spectral image mainly reflects the properties of a small (conic like) volume of tissue situated vertically under each individual pixel. The objects appearing on a spectral image reasonably reproduce the correct geometrical shape and size (like a non-deformed shadow) of underlying inclusions of pathological tissue. The information contained in a spectral image comes from a depth that does not exceed approximately 2-3 mm. The number of photons that visit a given tissue voxel situated at a depth larger than approximately 2 mm represents less than the 1% of the total number of photons reaching the corresponding detection pixel (forming the image). A pathological inclusion (e.g. a pool of blood or vascular tumor) situated at a depth of approximately 0.5 mm with a thickness of 0.5 mm produces an image intensity contrast of approximately 5% (for images taken at wavelengths in the 600-1000 nm range) when compared to the normal skin background. The same inclusion at a depth of 20 microm provides a contrast decreasing from 55 to 20% with respect to an increase in wavelength. The dermis/hypodermis interface behaves as a partial barrier for the photons, limiting their access to deeper skin regions. The image contrast depends on the depth and the type of chromophore contained in the inclusion. An increase in the concentration of a given molecule may produce different contrast, independently of the depth, depending on the characteristics of the skin layer where this change occurs.

Light transport in tissue by 3D Monte Carlo: influence of boundary voxelization.

Monte Carlo (MC) based simulations of photon transport in living tissues have become the "gold standard" technique in biomedical optics. Three-dimensional (3D) voxel-based images are the natural way to represent human (and animal) tissues. It is generally believed that the combination of 3D images and MC based algorithms allows one to produce the most realistic models of photon propagation. In the present work, it is shown that this approach may lead to large errors in the MC data due to the "roughness" of the geometrical boundaries generated by the presence of the voxels. In particular, the computed intensity of the light detected on the tissue surface of a simple cubic tissue phantom may display errors from -80% to 120%. It is also shown that these errors depend in a complex manner on optical and geometrical parameters such as the interoptode distance, scattering coefficient, refractive index, etc. and on the degree of voxelization ("roughness") of the boundaries. It is concluded that if one wants to perform reliable 3D Monte Carlo simulations on complex geometries, such as human brain, skin or trabecular bone, it is necessary to introduce boundary meshing techniques or other equivalent procedures in the MC code to eliminate the deleterious effect of voxelization.

Comment on 'the use of the Henyey-Greenstein phase function in Monte Carlo simulations in biomedical optics'.

In this letter the authors highlight the presence of an error appearing in the discussion of the note 'The use of the Henyey-Greenstein phase function in Monte Carlo simulations in biomedical optics' previously published by them (Binzoni et al 2006 Phys. Med. Biol. 51 N313). In the light of this error, the discussion and conclusions in the original paper are revised in this letter and the role of the use of the phase functions in MC simulations, interpreted in probabilistic terms, is better clarified. The exact definition for the probability density function for the deflection angle, in the case of the Henyey-Greenstein model, is also given.

The use of the Henyey-Greenstein phase function in Monte Carlo simulations in biomedical optics.

Monte Carlo (MC) simulations are often at the heart of the testing procedure in biomedical optics. One of the critical points in MC simulations is to define the new photon direction after each scattering event. One of the most popular solutions is to use the Henyey-Greenstein phase function or some linear combinations of it. In this note, we demonstrate that randomly generating the angle defining the new direction of a photon after a collision, by means of the Henyey-Greenstein phase function, is not equivalent to generating the cosine of this angle, as is classically done. In practice, it is demonstrated that for a nearly isotropic medium (asymmetry parameter g approximately 0) this discrepancy is not large, however for an anisotropic medium as is typically found in vivo (e.g. g = 0.98) the two methods give completely different results.

Anisotropic photon migration in human skeletal muscle.

It is demonstrated in the short head of the human biceps brachii of 16 healthy subjects (12 males and 4 females) that near infrared photon migration is anisotropic. The probability for a photon to travel along the direction of the muscle fibres is higher (approximately 0.4) than that of travelling along a perpendicular axis (approximately 0.3) while in the adipose tissue the probability is the same (approximately 0.33) in all directions. Considering that the muscle fibre orientation is different depending on the type of muscle considered, and that inside a given skeletal muscle the orientation may change, the present findings in part might explain the intrasubject variability observed in the physiological parameters measured by near infrared spectroscopy techniques. In other words, the observed regional differences might not only be physiological differences but also optical artefacts.

Absorption and scattering coefficient dependence of laser-Doppler flowmetry models for large tissue volumes.

Based on quasi-elastic scattering theory (and random walk on a lattice approach), a model of laser-Doppler flowmetry (LDF) has been derived which can be applied to measurements in large tissue volumes (e.g. when the interoptode distance is >30 mm). The model holds for a semi-infinite medium and takes into account the transport-corrected scattering coefficient and the absorption coefficient of the tissue, and the scattering coefficient of the red blood cells. The model holds for anisotropic scattering and for multiple scattering of the photons by the moving scatterers of finite size. In particular, it has also been possible to take into account the simultaneous presence of both Brownian and pure translational movements. An analytical and simplified version of the model has also been derived and its validity investigated, for the case of measurements in human skeletal muscle tissue. It is shown that at large optode spacing it is possible to use the simplified model, taking into account only a 'mean' light pathlength, to predict the blood flow related parameters. It is also demonstrated that the 'classical' blood volume parameter, derived from LDF instruments, may not represent the actual blood volume variations when the investigated tissue volume is large. The simplified model does not need knowledge of the tissue optical parameters and thus should allow the development of very simple and cost-effective LDF hardware.

A new method of thermoablation with hot water vapour for localized tumours.

BACKGROUND: A new method of thermoablation with hot water vapour based on a new type of microtube was developed. This approach allows tumours, with volume and anatomical positions not accessible to other techniques (cryoablation, radiofrequency ablation, laser ablation) to be treated. MATERIALS AND METHODS: The method was tested on a human colon carcinoma grafted subcutaneously in Swiss nude mice and the experiment monitored under magnetic resonance imaging. RESULTS: It was found that 2.52 cal s(-1) per cm3 of tumour were necessary to reduce tumour size. The microtube is built to withstand a large range of temperatures and pressures and is biocompatible. CONCLUSION: A specific feature of this technique is that, besides hot vapour, several types of drugs can be delivered through the same microtube depending of the location, type or size of the tumour. These properties make it a unique device for multi-therapeutic treatments.

Mapping human skeletal muscle perforator vessels using a quantum well infrared photodetector (QWIP) might explain the variability of NIRS and LDF measurements.

Near-infrared spectroscopy (NIRS) and laser Doppler flowmetry (LDF) have become the techniques of choice allowing the non-invasive study of local human skeletal muscle metabolism and blood perfusion on a small tissue volume (a few cm3). However, it has been shown that both NIRS and LDF measurements may show a large spatial variability depending on the position of the optodes over the investigated muscle. This variability may be due to local morphologic and/or metabolic characteristics of the muscle and makes the data interpretation and comparison difficult. In the present work, we use a third method to investigate this problem which permits fast, non-invasive mapping of the intramuscular vessel distribution in the human vastus latelralis muscle. This method uses an advanced, passive, infrared imaging sensor called a QWIP (quantum well infrared photodetector). We demonstrate, using a recovery-enhanced infrared imaging technique, that there is a significant presence of perforator vessels in the region of interest of approximately 30 x 18 cm (the number of vessels being: 14, 9, 8, 33, 17 and 18 for each subject, respectively). The presence of these vessels makes the skeletal muscle highly inhomogeneous, and may explain the observed NIRS and LDF spatial variability. We conclude that accurate comparison of the metabolic activity of two different muscle regions is not possible without reliable maps of vascular 'singularities' such as the perforator vessels, and that the QWIP-based imaging system is one method to obtain this information.

Translational and Brownian motion in laser-Doppler flowmetry of large tissue volumes.

This study reports the derivation of a precise mathematical relationship existing between the different p-moments of the power spectrum of the photoelectric current, obtained from a laser-Doppler flowmeter (LDF), and the red blood cell speed. The main purpose is that both the Brownian (defining the 'biological zero') and the translational movements are taken into account, clarifying in this way what the exact contribution of each parameter is to the LDF derived signals. The derivation of the equations is based on the quasi-elastic scattering theory and holds for multiple scattering (i.e. measurements in large tissue volumes and/or very high red blood cell concentration). The paper also discusses why experimentally there exists a range in which the relationship between the first moment of the power spectrum and the average red blood cells speed may be considered as 'linear' and what are the physiological determinants that can result in nonlinearity. A correct way to subtract the biological zero from the LDF data is also proposed. The findings should help in the design of improved LDF instruments and in the interpretation of experimental data.

Non-invasive laser Doppler perfusion measurements of large tissue volumes and human skeletal muscle blood RMS velocity.

This study proposes the implementation of an algorithm allowing one to derive absolute blood root-mean-square (RMS) velocity values from laser Doppler perfusion meter (LDP) data. The algorithm is based on the quasi-elastic light scattering theory and holds for multiple scattering. While standard LDP measurements are normally applicable to a small region of interest (approximately 1 mm2), the present method allows the analysis of both small and large tissue volumes with small and large interoptode spacings (e.g., 1.5 cm). The applicability and the limits of the method are demonstrated with measurements on human skeletal muscle using a custom-built near-infrared LDP meter. Human brachioradialis muscle RMS velocity values of 9.99 +/- 0.01 and 5.58 +/- 0.03 mm s(-10 at 1.5 cm and of 5.18 +/- 0.01 and 2.54 +/- 0.09 mm s(-1) at 2 cm were found when the arm was (a) at rest and (b) occluded, respectively. At very large optode spacings or very high moving particle densities, the theory developed here would need to be amended to take into account second-order effects.

Human tibia bone marrow blood perfusion by non-invasive near infrared spectroscopy: a new tool for studies on microgravity.

Human tibia bone marrow (BM) and tibialis anterior muscle (TA) perfusion index (PI) was assessed non-invasively by near infrared spectroscopy. A decrease in the postis-chaemic reperfusion capability of the human tibia BM and TA muscle was observed for increasing age i.e., PI increases linearly as a function of age, starting from 30 years, both for BM (0.062 %/year, from -4.185 to -0.967 %/s) and TA muscle (0.046 %/year, from -5.760 to -3.883 %/s). The results define a "normal" baseline and demonstrate the sensitivity of the method to PI changes. The present technique should allow one to investigate physio-pathological effects induced by microgravity on tibia BM blood perfusion.

Local temperature changes and human skeletal muscle metabolism.

The aim of this review is to describe the effects induced by local temperature changes on human skeletal muscle metabolism. More specifically, we will consider the influence of temperature on the mechanical properties of muscle contraction, on aerobic metabolism, anaerobic metabolism and on the Lohmann reaction. The text has been voluntarily organized on the basis of a simple bioenergetic model describing the different energy fluxes appearing in the muscle system. This approach should better highlight some of the points that still need to be investigated. Although it was not always possible to restrict the discussion to human muscle, the references report mainly data obtained directly on humans or on isolated human fibres. A short comment on skeletal muscle temperature measurement techniques, on humans, is also included.

Age dependence of human gastrocnemius Mg2+: fitting 31P-NMR spectra using quantum mechanics-based prior knowledge.

The age dependence of human gastrocnemius Mg2+ concentration is demonstrated. To quantitate Mg2+ concentration, an original and accurate fitting algorithm using quantum mechanics-based prior knowledge is detailed. In a group of 28 volunteers (14 females) in the age range 5-80 years, pH, PCr/ATP and Pi/ATP values in the gastrocnemius were 7.02 +/- 0.02 pH, 4.16 +/- 0.33 and 0.13 +/- 0.02, respectively and independent of age and sex. By contrast, intracellular Mg2+ concentration (mM) decreased linearly (p < 0.05) with age (Mg2+ = 0.7803 +/- 0.0247-0.0027 +/- 0.0005 * age). No difference was found between sexes. From these results, it follows that care must be taken when comparing muscle Mg2+ data from subjects of different age. The hypothesis can be put forward that during aging insufficient intake and/or increased depletion of Mg2+ (e.g., intestinal hypoabsorption or urinary leakage) may affect the musculoskeletal system.

Human gastrocnemius medialis pennation angle as a function of age: from newborn to the elderly.

The aim of the present study was to quantify changes in human skeletal muscle pennation angle (F theta) values during growth and adult life. The human gastrocnemius medialis muscle of 162 subjects (96 males and 66 females) in the age range 0-70 years was scanned with ultrasonography. The subjects were laying prone, at rest, with the ankle maintained at 90 degrees with all muscles relaxed. F theta increased monotonically starting from birth (0 years) and reached a stable value after the adolescent growth spurt. There was a significant (p < 0.05) linear relationship between F theta and muscle thickness (TK). F theta = 0.84 (+/- 0.09) * TK + 3.15 (+/- 1.13). Human gastrocnemius medialis F theta and TK data found in the literature seem to fit the F theta-TK plot in a coherent manner, independent of the physiological or anatomical characteristics of the subject. The present findings indicate that F theta is not a constant parameter but evolves, as is the case for bone length and height, as a function of age.

Temperature dependence of human gastrocnemius pH and high-energy phosphate concentration by noninvasive techniques.

It is well established that ADP is an important regulator of the oxidative phosphorylation in the mitochondria. Thus, by means of noninvasive techniques it is demonstrated that the relationship between O(2) consumption of the human gastrocnemius at rest and its temperature is likely determined by at least two factors: 1) the modulation of the rate of the chemical reactions imposed by the "physical" temperature-effect; 2) the influence of temperature-induced ADP concentration changes ( approximately 0.83 microM degrees C(-1)) on oxidative phosphorylation. ADP was assessed by applying the temperature-corrected Lohmann equilibrium equation. PCr and ATP were found to increase, with decreasing temperature (-0.54+/-0.05 and -0.17+/-mM degrees C(-1), respectively), while pH varies following the alpha-stat hypothesis (-0.016+/-0.001 pH degrees C(-1)). These findings should be of value when dealing with muscle physiology in extreme environments or clinical applications of hypothermia.

Human calf microvascular compliance measured by near-infrared spectroscopy.

The purpose of this study is to develop a new method for the measurement in humans of the compliance of the microvascular superficial venous system of the lower limb by near-infrared spectroscopy (NIRS). This method is complementary to strain-gauge plethysmography, which does not allow compliance between deep and superficial venous or between venous and arterial compartments to be distinguished. In practice, hydrostatic pressure (P) changes were induced in a calf region of interest by head-up tilt of the subject from alpha = -10 to 75 degrees. For P < or = 24 mmHg, the measured compliance [0.086 +/- 0.005 (SD) ml. l(-1). mmHg(-1)] based on NIRS data of total, deoxygenated, and oxygenated hemoglobin, reflects essentially that of the superficial venous system. For P > or = 24 mmHg, no distinction can be made between arterial and venous volumes changes. However, by following the changes in oxy- and deoxyhemoglobin in the P range -16 to 100 mmHg, it appears to be possible to assess the characteristics of the vasomotor response of the arteriolar system.

Muscle O(2) consumption by NIRS: a theoretical model.

In the past, the measurement of O(2) consumption ((2)) by the muscle could be carried out noninvasively by near-infrared spectroscopy from oxyhemoglobin and/or deoxyhemoglobin measurements only at rest or during steady isometric contractions. In the present study, a mathematical model is developed allowing calculation, together with steady-state levels of (2), of the kinetics of (2) readjustment in the muscle from the onset of ischemic but aerobic constant-load isotonic exercises. The model, which is based on the known sequence of exoergonic metabolic pathways involved in muscle energetics, allows simultaneous fitting of batched data obtained during exercises performed at different workloads. A Monte Carlo simulation has been carried out to test the quality of the model and to define the most appropriate experimental approach to obtain the best results. The use of a series of experimental protocols obtained at different levels of mechanical power, rather than repetitions of the same load, appears to be the most suitable procedure.