Laser reflectance imaging of human organs and comparison with perfusion images

Based on the principle of backscattering of laser radiation from tissues, a non-invasive PC-AT based reflectance imaging technique is developed. The laser beam from a semiconductor laser operating at 670 nm is guided to the tissue site by an optical fibre. The backscattered radiation is collected by another fibre placed in the same probe, and is detected by a photodiode-amplifier assembly. This probe is moved manually over the organs under observation, and the data after the ADC, interpolation and median filtering are displayed in the form of reflectance image of the organ along with grey scale. By this technique images of the human hands and forearms are obtained, which depend on the variations in their colour, composition and blood flow. A comparison is made with perfusion images, obtained by a Periflux laser Doppler flowmeter. These show that the reflectance images provide greater details of the tissue structure than the perfusion images.     

Ultrasonic flowmeter with implantable miniature sensors

An ultrasonic flowmeter incorporated with implantable miniature sensors was developed. Two sets of miniature ultrasonic sensors are attached to the surface of a blood vessel. The outer diameter of the vessel and the blood velocity are measured simultaneously by means of the ultrasonic transit-time technique and the ultrasonic Doppler method, respectively. Instantaneous blood flow is calculated by multiplying the flow velocity by the cross-sectional area calculated from the diameter Stroke volume and cardiac output can also be obtained, when the system is applied to the ascending aorta. After preliminary benchtop tests, acutein vivo studies were carried out to evaluate the accuracy and performance of the system. The blood flow in the descending aortas of dogs and a goat were measured simultaneously by the ultrasonic system and an electromagnetic flowmeter. The correlation coefficient between the data obtained by the two methods was 0·968. The system was applied to measure the blood flow in the ascending aortas of goats which had had heart failures induced and were implanted with left ventricular assist devices.     

Laser Doppler flowmetry: characteristics of a modified single-fibre technique

The single-fibre percutaneous laser Doppler technique has been used in previous studies of intramuscular blood flow. This method facilitates studies of blood flow in deep tissue volumes and minimises the tissue trauma. The technique has been further developed with the aim of improving the signal quality. This has been accomplished by modifying the geometry of the fibre tip. By melting the fibre core material, lenses of different shapes are formed. Flat, spherical and ‘pears-type tips have been manufactured and are evaluated theoretically and experimentally. The paraxial theory cannot accurately predict the position of zones of highest irradiance. Therefore, a ray-tracing program has been developed in the C language, by means of which some of the optical properties of the modified fibre tips can be simulated. Isoirradiance graphs and beam profiles are calculated for the three different fibre tips. Measured and calculated irradiance curves are used for evaluation of the properties of the ray-tracing model. The three types of fibre tips are also evaluated and compared in flow models. The sphere and pear-type probes show a higher flow sensitivity than the flat-end type. These improvements in flow sensitivity are interpreted as being related to the larger, strongly irradiated tissue volumes in front of the fibres. Intramuscular measurements with the pear-type probe show high sensitivities to induced blood flow changes.     

Comparative study of two laser Doppler blood flowmeters

A new infrared laser Doppler blood flow instrument (Moor MBF3D) was evaluated using an in vitro model allowing measurements over a range of flow velocities and concentrations. The responses correlated well (r = 0·96, p < 0·01) with those obtained simultaneously using a Perimed PF3 laser Doppler instrument. The different processing bandwidths of the instruments were investigated and the wideband mode of operation is recommended for flow measurements where there may be fast moving red blood cells (rbcs). The infrared instrument is capable of dual-channel operation, and the two channels are shown to respond almost identically for similar changes in blood flow through the in vitro model (r = 0·999, p < 0·01). The main advantage of the dual-channel instrument is that continuous measurements may be made simultaneously at two different skin sites allowing dynamic flow responses to be compared.     

Design analysis of parallel plate and hollow fibres haemofilters

A theoretical model of mass transfer by ultrafiltration (u.f.) at large transmembranes pressures in parallel plate and hollow fibres haemofilters is presented. The analysis assumes that the maximum u.f. flow Q_FM is limited by protein concentration polarisation and that the concentration boundary layer is thin. It takes into account the decrease of local blood flow along the membrane and therefore remains valid when the u.f. flow rate is a large fraction of the incoming blood flow Q_bi. It is found that the ratio Q_FM/Q_bi increases nonlinearly with S/(Q_bid_h) where S is the membrane area and d_h the hydraulic diameter of the blood film. A parallel-plate haemofilter will have 20% more ultrafiltration than one of hollow fibre of the same membrane area and blood volume. However, if the two units have the same u.f. flow rate, the hollow fibre one will have the smaller blood volume. For a fibre type haemofilter at a given blood flow, Q_FM is a function of the total fibre length and is independent of fibre diameter. By using a blood film thickness in the vicinity of 150 μ, adequate clearances (0·4Q_bi) can be obtained with membranes areas of 0·6 to 0·7 m², significantly less than with present haemodialysers.     

Non-invasive monitoring of muscle blood perfusion by photoplethysmography: evaluation of a new application

AIM: To evaluate a specially developed photoplethysmographic (PPG) technique, using green and near-infrared light sources, for simultaneous non-invasive monitoring of skin and muscle perfusion. METHODS: Evaluation was based on assessments of changes in blood perfusion to various provocations, such as post-exercise hyperaemia and hyperaemia following the application of liniment. The deep penetrating feature of PPG was investigated by measurement of optical radiation inside the muscle. Simultaneous measurements using ultrasound Doppler and the new PPG application were performed to elucidate differences between the two methods. Specific problems related to the influence of skin temperature on blood flow were highlightened, as well. RESULTS: Following static and dynamic contractions an immediate increase in muscle perfusion was shown, without increase in skin perfusion. Liniment application to the skin induced a rapid increase in skin perfusion, but not in muscle. Both similarities and differences in blood flow measured by Ultrasound Doppler and PPG were demonstrated. The radiant power measured inside the muscle, by use of an optical fibre, showed that the near-infrared light penetrates down to the vascular depth inside the muscle. CONCLUSIONS: The results of this study indicate the potentiality of the method for non-invasive measurement of local muscle perfusion, although some considerations still have to be accounted for, such as influence of temperature on blood perfusion.     

Comparison of laser speckle and laser Doppler perfusion imaging: Measurement in human skin and rabbit articular tissue

Laser Doppler perfusion imaging (LDI) is currently used in a variety of clinical applications, however, LDI instruments produce images of low resolution and have long scan times. A new optical perfusion imager using a laser speckle measurement technique and its use for in vivo blood flow measurements are described. Measurements of human skin and surgically exposed rabbit tissue made using this instrument were compared with a commercial laser Doppler perfusion imaging instrument. Results from blood flow measurements showed that the laser speckle imager measured an 11–67% decrease in blood flow under arterial occlusion. Under similar conditions, the laser Doppler imager measured blood flow decreases of 21–63%. In comparison with LDI, it was observed that the higher temporal resolution of the laser speckle imager was more sensitive to measuring the hyperaemic response immediately following occlusion. This in vivo study demonstrated some of the several advantages laser speckle imaging has over conventional LDI, making the new instrument more versatile in a clinical environment.     

Non‐invasive monitoring of muscle blood perfusion by photoplethysmography: evaluation of a new application

Aim: To evaluate a specially developed photoplethysmographic (PPG) technique, using green and near‐infrared light sources, for simultaneous non‐invasive monitoring of skin and muscle perfusion. Methods: Evaluation was based on assessments of changes in blood perfusion to various provocations, such as post‐exercise hyperaemia and hyperaemia following the application of liniment. The deep penetrating feature of PPG was investigated by measurement of optical radiation inside the muscle. Simultaneous measurements using ultrasound Doppler and the new PPG application were performed to elucidate differences between the two methods. Specific problems related to the influence of skin temperature on blood flow were highlightened, as well. Results: Following static and dynamic contractions an immediate increase in muscle perfusion was shown, without increase in skin perfusion. Liniment application to the skin induced a rapid increase in skin perfusion, but not in muscle. Both similarities and differences in blood flow measured by Ultrasound Doppler and PPG were demonstrated. The radiant power measured inside the muscle, by use of an optical fibre, showed that the near‐infrared light penetrates down to the vascular depth inside the muscle. Conclusions: The results of this study indicate the potentiality of the method for non‐invasive measurement of local muscle perfusion, although some considerations still have to be accounted for, such as influence of temperature on blood perfusion.     

Non-invasive measurement of hepatic oxygenation by an oxygen electrode in human orthotopic liver transplantation

Precise evaluation of graft reperfusion is difficult in clinical liver transplantation. The oxygen electrode (OE) is a novel technique to detect blood flow indirectly by measuring the quantity of oxygen which can diffuse from the hepatic tissue to the surface electrode. Application of the surface OE does not influence the liver blood flow or parenchymal perfusion. Adequate graft oxygenation is essential to the outcome of organ transplantation and has not previously been analysed intra-operatively in liver transplant recipients. The OE was applied to the surface of the graft intra-operatively in 22 human liver grafts after restoring portal vein and hepatic artery inflow. OE readings were compared with liver blood flow using an electromagnetic flowmeter (EMF). Intra-operative haemodynamics and donor organ parameters known to influence graft function were correlated with the OE readings. There was a significant correlation (r=0.89; p<0.001, n=14) between tissue oxygenation using the OE and total liver blood flow measured by EMF. The tissue oxygenation measurements were reproducible with a coefficient of variation of 5%. The hepatic tissue oxygenation increased significantly from baseline following venous reperfusion of the graft (282+/-23 vs 3107+/-288 (+/-SE) nA, p<0.001). Hepatic arterial revascularisation resulted in a significant (p<0.001) increase of 41+/-7% in liver oxygen perfusion. There was significant negative correlation (r=0.80, p<0.001, n=22) between cold ischaemic time and graft tissue oxygenation. The OE provides a reliable, cheap and non-invasive method of monitoring liver graft oxygenation and perfusion during transplantation.     

Analytical study of electrical impedance plethysmography in peripheral blood flow

The impedance Z, the derivative of Z with respect to time t, dZ/dt, and blood flow F have been measured simultaneously in the central caudal artery of a series of anaesthetised rats. The results of the experiments were expressed in the relationship derived from the Windkessel theory where f(t)=adz(t)/dt+βz(t), in which α and β are constants and f(t) and z(t) express the pulsating fractions in the blood flow F and the impedance Z, respectively. The correlation between the peak-to-peak values of f(t) and of adz(t)/dt+βz(t) was 0·96. The correlation between the mean flow f_o and the peak-to-peak value of f(t) was 0·74. Furthermore, even when the mean flow was zero, adz(t) still existed. The theoretical model was tested against volume flow measurements made using an electromagnetic flowmeter and was found to correlate well. Theoretical estimates of vessel calibre also accord with measured values.     

Leg blood flow during exercise in man in relation to muscle fibre composition

Fibre composition in the vastus lateralis muscle, leg blood flow, oxygen uptake and respiratory exchange ratio were determined in 12 healthy male volunteers during submaximal exercise (50% of V_O2 max). The percentage of slow-twitch fibres varied from 26 to 66. Mean leg blood flow during exercise was 4.68 ± 0.191 · min^-1. The blood flow and respiratory exchange ratio correlated positively to the percentage of slow-twitch fibres in the vastus muscles. No correlation was found between the muscle fibre composition and either oxygen uptake heart rate or mechanical efficiency. The results with a dependence of muscle blood flow and carbon dioxide release to muscle fibre composition support the view that the arrangement of the vascular bed and blood supply differ between fast-twitch and slow-twitch muscle fibres in humans.     

Effects of alternating magnetic fields and low-frequency electric currents on human skin blood flow

The influence of alternating magnetic fields on human skin blood flow was investigated. The hands of volunteers were fixed just above an induction heater and the regional skin blood flow of a fingertip was measured by a laser Doppler flowmeter. The hands were exposed to fields with flux densities of 16 mT, 32 mT and 48 mT at a frequency of 3·8 kHz. Two parameters of the laser Doppler blood flow recording were studied: the dynamic and DC components. The dynamic component did not show any particular changes following magnetic stimulation. The DC component showed a rapid decrease in blood flow in the first period after field-on at 48 mT and 32 mT. A pain sensation with muscle contraction was recognised (48 mT and 32 mT). The effect of electrical stimulation on skin blood flow was also investigated. The differences between the magnetic and electrical effects are discussed. The results suggest that the human body responds to magnetic stimulation with an escape reaction which results in an elevated vasoconstrictor tone. The change in microvascular blood flow recorded with the laser Doppler method can probably be explained by the influence of the eddy currents on the sensory receptors of the skin.     

In vivo evaluation of signal processors for laser Doppler tissue flowmeters

The performance of different signal processors for laser Doppler tissue flowmeters was evaluated by the use of a well defined flow model comprising a segment of the feline intestinal wall. The processor that, apart from being based on the calculation of the first moment of the power spectral density, also takes into account the effect of multiple scattering in a number of blood cells gave an output signal that was linearly related to the intestinal wall perfusion as recorded independently by a drop-counting technique. At a recording bandwidth of 12 kHz, this linear relationship was valid for the entire flow range 0–300 ml min⁻¹ 100 g⁻¹ (r=0·98). The processor based on the first moment of the power spectral density alone under-estimated the highest flow rates by about 35 per cent, while within the flow range 0–100 ml min⁻¹ 100 g⁻¹ this processor also gave an output signal linearly related to flow at a recording bandwidth of 12 kHz (r=0·96). When the bandwidth was limited to 4 kHz, the output signals from both processors were linearly related to flow only within the range 0–100 ml min⁻¹ 100 g⁻¹ (r=0·90). The output signals recorded with the 4 kHz systems were, however, generally only about 65 per cent of those recorded with the 12 kHz systems.     

Renal blood flow in isolated kidneys measured with an electromagnetic flowmeter and by xenon-133 and krypton-85 wash-out techniques

Summary By use of an electromagnetic flowmeter and the inert gas desaturation method 43 comparative measurements of the blood flow were performed in 6 isolated dog kidneys. For the measurements by use of the latter method krypton-85 and xenon-133 were employed, for a fast, and respectively, a slow injection of the indicator into the renal artery. Calculation of the blood flow from the wash-out curves was made by use of the formulas advanced by Kety (1951) and by Zierler (1965).A good accordance was found between the flowmeter values and those obtained with the desaturation methods. Recirculation and resorption of indicator in fat tissue surrounding the kidney explain that previous examinations in kidneys in situ have resulted in very low perfusion values, when Zierler's formula has been applied. Extrapolation of the final slope to obtain the area of the curve may be a major source of error by calculation of the blood flow with Zierler's formula.     

Microdialysis of rat skeletal muscle and adipose tissue: dynamics of the interstitial glucose pool

Microdialysis was evaluated as a method for studying glucose metabolism in skeletal muscle. Dialysis probes (0.5 × 10 mm) were perfused at 0.5 or 1.0 μl min^-1. Based upon perfusion with glucose, the muscle interstitial glucose concentration was estimated to be 6.9 ± 0.3 mM (w = 14), which was not significantly different from the blood glucose level. With insulin infusion (1200 mU kg^-1 body wt i.v.), the insulin-induced change in the glucose concentration of the interstitial space of muscle was of equal magnitude to that of blood and adipose tissue. In spite of this, when the perfusion medium was not supplemented with glucose, the glucose concentration decreased more in skeletal muscle dialysates (to 36.7 ± 4.9% of the initial level) than in blood (to 29.7±5.0%) but less than in adipose tissue (to 17.7 ± 4.9% of the initial level) (P < 0.05). The results indicate that these differences are due to tissue-specific differences in the dynamic balance between the supply to, and removal from, the interstitial glucose pool. This balance is revealed as a result of the constant glucose drainage by the microdialysis probe. The present results show that, in skeletal muscle, increases in glucose uptake occur with a concomitant increase in tissue blood flow as revealed by the microdialysis ethanol technique, whereas in adipose tissue the glucose uptake increases in the absence of a corresponding increase in blood flow.     

Quantification of aortic regurgitation by Doppler echocardiography: a new method evaluated in pigs

We have developed a method to quantify aortic regurgitant orifice and volume, based on measurements of the velocity of the regurgitant jet, aortic systolic flow, the systolic and diastolic arterial pressures, a Windkessel arterial model, and a parameter estimation technique. In six pigs we produced aortic regurgitant flows between 2·1 and 17·8 ml per beat, i.e. regurgitant fractions from 0·06 to 0·58. Pulmonary and aortic flows were measured with electromagnetic flow probes, aortic pressure was measured invasively, and the regurgitant jet velocity was obtained with continuous-wave Doppler. The parameter estimation procedure was based on the Kalman filter principle, resulting primarily in an estimate of the regurgitant orifice area. The area was multiplied by the velocity integral of the regurgitant jet to estimate regurgitant volume. A strong correlation was found between the regurgitant volumes obtained by parameter estimation and the electromagnetic flow measurement. These results from our study in pigs suggest that it may be possible to quantify regurgitant orifice and volume in patients completely noninvasively from Doppler and blood pressure measurements.     

Heterogeneous blood flow response in the foot on dependency,assessed by laser Doppler perfusion imaging

The exact nature of the decrease in foot skin blood flow seen after a change in posture remains unsettled. This mechanism has previously been examined by non-invasive techniques such as the laser Doppler perfusion monitor (laser Doppler flowmetry). Taking into account the shortcomings of laser Doppler perfusion monitoring when applied to the determination of skin blood flow, which normally shows substantial heterogeneity, we have applied an emerging technology, the laser Doppler perfusion imager (LDPI). This technique provides a more comprehensive picture of the blood flow distribution in the skin, as it maps skin blood flow over a surface area (120×120 mm, 4096 measurement sites). It was used to examine if the reduction in tissue perfusion or the alterations in flow distributions seen after a change in posture (supine to dependency) could be fully explained by an increase in venous pressure (venous stasis of 50 mmHg) or if the data suggest a complementary mechanism. Skin blood flow of the forefoot decreased from 0.60 V (volt) (median) during rest to 0.40 and 0.38 V during venous stasis and dependency, respectively. Although almost identical median values were obtained during stasis and dependency, the flow distributions were different, with a loss of high flow values during venous stasis. Biological zero was 0.24 V. As the LDPI technique readily records skin perfusion during variations in venous stasis and posture, as well as information on flow distribution changes, it appears promising for future application in stimuli-response studies of skin blood flow. The difference in flow distribution seen between increased venous pressure and dependency suggests an additive regulatory mechanism to the veni-vasomotor reflex during a change in posture.     

Influence of illumination-collection geometry on fluorescence spectroscopy in multilayer tissue

Device-tissue interface geometry influences both the intensity of detected fluorescence and the extent of tissue sampled. Previous modelling studies have often investigated fluorescent light propagation using generalised tissue and illumination-collection geometries. However, the implementation of approaches that incorporate a greater degree of realism may provide more accurate estimates of light propagation. In this study, Monte Carlo modelling was performed to predict how illumination-collection parameters affect signal detection in multilayer tissue. Using the geometry and optical properties of normal and atherosclerotic aortas, results for realistic probe designs and a semi-infinite source-detection scheme were generated and compared. As illumination-collection parameters, including single-fibre probe diameter and fibre separation distance in multifibre probes, were varied, the signal origin deviated significantly from that predicted using the semi-infinite geometry. The semi-infinite case under-predicted the fraction of fluorescence originating from the superficial layer by up to 23% for a 0.2 mm diameter single-fibre probe and over-predicted by 10% for a multifibre probe. These results demonstrate the importance of specifying realistic illumination-collection parameters in theoretical studies and indicate that targeting of specific tissue regions may be achievable through customisation of the illumination-collection interface. The device- and tissue-specific approach presented has the potential to facilitate the optimisation of minimally invasive optical systems for a wide variety of applications.     

Comparison of the histochemical and contractile properties of human triceps surae

The purpose of the study was to compare the contractile properties determined from an electrically stimulated twitch with histochemically determined fibre type parameters of the human triceps surae. Muscle samples were obtained from the medial head of the gastrocnemius of ten male athletes. Ages ranged from 20 to 29 years. Muscle samples from the belly of the medial gastrocnemius muscle were obtained using the needle biopsy technique. The samples were treated histochemically for myosin ATPase to classify the fibres as either slow twitch (ST) or fast twitch (FT) and to determine fibre areas. Surface electrical stimulation was used to determine muscle twitch parameters. The contractile variables of the muscle twitch were latency (L), time to peak force (TPF), peak force (PF), half-contraction time (1/2 CT) and half-relaxation time (1/2 RT). Backward elimination procedures for dependent variables were used to determine which contractile properties best represented the histochemical profile of the muscles. Prediction formulas were developed for FT and ST percentages (R²=0·98, p<0·001), relative area percentage (R²=0·87, p<0·001), and ST area (R²=0·85, p<0·01). It was concluded that the use of the electrotensiometer (ETM) protocol was a valid testing procedure when studying physiological relationships of histochemical properties in intact human skeletal muscle. Protected by patent no. 4 688 581.