Theoretical analysis of the transient thermal clearance method for regional blood flow measurement

In the noninvasive transient thermal clearance method a metal plate at room temperature is attached to the investigated skin. The plate is thermally insulated from the environment, and so the tissue temperature, after an initial decrease, is increased at a rate which depends both on heat convection by blood and on tissue thermal conductivity. The corresponding bioheat conductivity equation is solved and the dependence of plate temperature on time and on blood flow discussed. It is shown that, for an appropriate choice of metal and plate thickness, regional blood flow can be derived from temperature/time curves.     

Transient heat clearance method for regional blood flow measurements

A method for assessing regional blood flow by transient heat clearance is described. A probe at room temperature is attached to the tissue which is to be investigated and the temperature of the probe is thereby decreased. The time constants which describe the increases in tissue temperature as the temperature approaches equilibrium temperature are related to blood flow. Regional blood flow can be determined by the method of transient heat clearance without calibration.     

Pharmacological zero for electromagnetic renal blood flow measurement

Measurement of blood flow in chronically instrumented laboratory animals using electromagnetic flow probes depends on a reference zero flow value. The latter is usually obtained by mechanical vessel occlusion. We tested an alternative method: a bolus of angiotensin II was injected in the abdominal aorta resulting in an immediate decrease of renal blood flow. Diastolic blood flow at this stage appeared to be identical to the zero level obtained by mechanical occlusion. Thus, pharmacological interruption of renal blood flow may serve as a zero reference in the dog.     

Continuous measurement of renal cortical blood flow and renal arterial blood flow during stimulation of the renal nerve

Stimulation of the renal sympathetic nerve in young pigs with biphasic pulses of current (3 mA, 800 microseconds per phase, 5, 10 and 50 Hz) produced decreases in arterial and cortical blood flow in the kidney, with the greatest decreases occurring at the highest stimulus frequencies. The decrease in cortical flow lagged that in arterial flow by 1.53-1.99 s; the delay increased with decreasing frequency but was unaffected by captopril, an angiotensin-converting enzyme-blocking agent. This result was consistent with the hypothesis that stimulation of the sympathetic nerve causes constriction first in the afferent arteriole and then in the efferent arteriole. Systemic arterial pressure increased during stimulation of the nerve; the increase was greater in intact nerves than in nerves that had been crushed proximal to the point of the stimulus, indicating that pigs do have renal afferent nerves. Pressure increased after the stimulus ended, but the increase abated or changed to a decrease after administration of captopril. The changes in flow were unaffected by administration of captopril, but were markedly reduced by the blocking agent labetalol (renal arterial flow, 77 +/- 14 per cent; cortical flow, 70 +/- 12 per cent). Thus, the observed changes in flow resulted from direct stimulation of the sympathetic nerves and not from stimulation of the renin-angiotensin system, which affects the pressure response after the stimulus.     

Freeze-dissection analysis of 133Xe distribution to measure regional renal blood flow

To verify the freeze-dissection technique for measuring renal blood flow (RBF) distribution in anesthetized dogs we compared the sum of all compartment flow rates with total RBF, compared compartment flow distribution with intercepts, and determined recirculated 133Xe. By dissection, we found that the cortex, delineated by its granular brownish red appearance, comprised 69%, whereas the outer medulla (the reddish portion) was 18% of the total kidney weight. Average cortical flow was 3.77 ml X g-1 X min-1 and the y-intercept for 133Xe washout was 85% of the initial radioactivity distribution. Outer medullary flow was 2.01 ml X g-1 X min-1 with a y-intercept of 13%. In a 100-g kidney, total cortical flow would be 259.8 ml/min (3.77 ml X g-1 X min-1 X 68.9 g) and total outer medullary flow would be 36.0 ml/min. These calculations indicate that 86% of the total flow is distributed to the cortex and 13% to the outer medulla, as the intercept-calculated percentages indicated. Summing whole cortical and medullary flows results in a flow that agrees with the electromagnetically measured flow of 292 ml/min for a 100-g kidney.     

Comparison of methods for instantaneous angiographic blood flow measurement.

Several different algorithms have been reported for measurement of blood flow rates and velocities from digital x-ray angiograms. We compare four videodensitometric methods: (1) distance-density curve matching (DDCM), (2) distance-density curve matching with curve-fitting (DDCM-F), (3) bolus mass tracking with curve-fitting (BMT-F) and (4) fluid continuity method (FCM). We tested the flow algorithms with simulated angiograms and with images obtained from a programmable flow phantom under clinically realistic flow and contrast injection conditions including imperfect mixing. All methods perform well for simulated angiograms. On phantom angiograms with constant flow, all methods tended to underestimate flow velocities by at least 7% and demonstrate high variability between consecutive measurements. The FCM demonstrated relatively low variability, but a large negative bias. The DDCM method was moderately biased and had the highest variability. The BMT-F method demonstrated the lowest bias (-7.1%) and the lowest variability both within (27%) and between (27%) studies. No method yields reliable measurements near the peak contrast opacification, when little or no gradient of contrast is present. The extrapolating version of the BMT-F method was also the most robust for estimation of interframe displacements longer than the field of view.     

An effective method for skin blood flow measurement using local heat combined with electrical stimulation

Electrical stimulation (ES) is a modality used to increase skin blood flow (SBF) and to aid in wound healing. A greater SBF in non wounded skin is induced if ES is used in a warm environment compared to a thermoneutral environment, where ES is usually applied. Therefore, in this paper, a method to investigate the effect of local heating and ES on the SBF is developed. A total of 33 males (18–40 years) were divided into group G (n?=?15) who received the ES during a global heating protocol and group L (n?=?18) who received ES during a local heating protocol. In the global heating protocol, ES (30?Hz, 250?μs) was applied for 15?min on the subject's thigh in thermoneutral (25?±?0.5°C) and warm (35?±?0.5°C) environments. In the local heating protocol, ES was applied for 15 minutes at 25°C, 35°C and 40°C local skin temperatures. A laser Doppler imager measured the SBF in both protocols pre, during, and post ES. The results of the experiment showed the significant differences in the SBFs were found at pre, during, and post ES in a thermoneutral environment or when the skin was locally cooled to 25°C. The SBFs were significantly increased during and post ES after global heating or during local heating at 35°C and 40°C. There were no significant differences in SBFs between the warm environment and at 35°C of local heating. However, the SBF response to ES was the highest at 40°C of local heating. Thus, ES during local heating of the skin, as well as during global heating is an effective method to increase SBF.     

Theoretical analysis of regional blood flow studies

Regional blood flow measurements were analyzed using a two-compartment model. The distribution of injected tracer between blood and tissue was assumed to be in equilibrium only at the beginning of the experiment. Equations for the determination of the partition coefficient of injected radionuclides from in vivo measurements were derived. These equations can also be used for estimation of the relative blood content of a tumor or an organ. The results were applied to experiments, where three diffusible radionuclides were injected simultaneously into patients with diagnosed tumors.     

Finite element analysis of blood flow and heat transfer in an image-based human finger

The human finger is said to be the extension of the brain and can convey the information on mechanical, thermal, and tissue damaging. The quantitative prediction of blood flow rate and heat generation are of great importance for diagnosing blood circulation illness and for the noninvasive measurement of blood glucose. In this study, we developed a coupled thermofluid model to simulate blood flow in large vessels and living tissue. The finite element (FE) model to analyze the blood perfusion and heat transport in the human finger was developed based on the transport theory in porous media. With regard to the blood flow in the large arteries and veins, the systemic blood circulation in the upper limb was modeled based on the one-dimensional flow in an elastic tube. The blood pressure and velocity in each vessel were first computed and the corresponding values for the large vessels in the finger were subsequently transferred to the FE model as the boundary conditions. The realistic geometric model for the human finger was constructed based on the MRI image data. After computing the capillary pressure and blood velocity in the tissue, the temperatures in the large vessels and the tissue of the finger were computed simultaneously by numerically solving the energy equation in porous media. The computed blood flow in tissues is in agreement with the anatomical structure and the measurement. It is believed that this analysis model will have extensive applications in the prediction of peripheral blood flow, temperature variation, and mass transport.     

A PC-based neural network for on-line measurement of regional cerebral blood flow.

The measurement of regional cerebral blood flow has been used in diagnoses for many cerebral disorders. However, the complexity of data processing makes the technique only possible for off-line study. This paper explores a PC-based neural net technique for on-line determination of the blood flow rate. Experimental results justify the efficiency of the proposed methodology and raise the possibility of designing a portable system for time-critical applications.     

Proposed reference values for nine methods for measurement of free thyroxin

In connection with a comparative study of nine kits for the measurement of free thyroxin, we determined reference values in a adult control group of 81 women and 73 men. The correlations observed between the kits are associated with very large differences in the results obtained. The reference ranges are more or less broad according to the kits, but narrower than those offered by the manufacturers.     

Error analysis of tumor blood flow measurement using dynamic contrast-enhanced data and model-independent deconvolution analysis

We performed error analysis of tumor blood flow (TBF) measurement using dynamic contrast-enhanced data and model-independent deconvolution analysis, based on computer simulations. For analysis, we generated a time-dependent concentration of the contrast agent in the volume of interest (VOI) from the arterial input function (AIF) consisting of gamma-variate functions using an adiabatic approximation to the tissue homogeneity model under various plasma flow (F(p)), mean capillary transit time (T(c)), permeability-surface area product (PS) and signal-to-noise ratio (SNR) values. Deconvolution analyses based on truncated singular value decomposition with a fixed threshold value (TSVD-F), with an adaptive threshold value (TSVD-A) and with the threshold value determined by generalized cross validation (TSVD-G) were used to estimate F(p) values from the simulated concentration-time curves in the VOI and AIF. First, we investigated the relationship between the optimal threshold value and SNR in TSVD-F, and then derived the equation describing the relationship between the threshold value and SNR for TSVD-A. Second, we investigated the dependences of the estimated F(p) values on T(c), PS, the total duration for data acquisition and the shape of AIF. Although TSVD-F with a threshold value of 0.025, TSVD-A with the threshold value determined by the equation derived in this study and TSVD-G could estimate the F(p) values in a similar manner, the standard deviation of the estimates was the smallest and largest for TSVD-A and TSVD-G, respectively. PS did not largely affect the estimates, while T(c) did in all methods. Increasing the total duration significantly improved the variations in the estimates in all methods. TSVD-G was most sensitive to the shape of AIF, especially when the total duration was short. In conclusion, this study will be useful for understanding the reliability and limitation of model-independent deconvolution analysis when applied to TBF measurement using an extravascular contrast agent.     

Evaluation of xenon-133 renal blood flow measurement in the intact rat

The xenon-133 method for measuring renal blood flow in the intact rat was evaluated by direct measurement using a nonhemolyzing pump to perfuse kidneys in situ with the rat's own blood. Flows were calculated from the xenon data by means of four commonly used types of analysis: compartmental analysis using the weighted arithmetic mean (WAM), compartmental analysis using the weighted harmonic mean (WHM), stochastic analysis (SA), and initial slope analysis (ISA). WHM and SA estimate actual blood flows, whereas WAM and ISA provide only an index of mean renal flow. All results correlated well with the pumped flows (r values ranged from 0.79 to 0.98). However, the various types of analysis gave a wide range of calculated flows. This may explain some of the variation found in mean renal flow values reported in the literature. The method of choice was WHM, using only the first two compartments; the regression line between this (y) and direct measurement (x) was y = 0.98x + 0.17, r = 0.96.     

Total and regional renal blood flow during complete unilateral ureteral obstruction

Regional renal blood flow was investigated after 1, 2, 12 and 24 h of unilateral ureteral obstruction in rats. Total renal and cortical blood flow rates were determined by the microsphere technique and regional medullary blood flow by the 86-Rb extraction method. The kidneys were microdissected into cortex and medulla, which was further divided into the outer stripe and the inner stripe of the outer zone, and the inner zone. In the obstructed kidneys cortical blood flow increased by 33% after 1 h of obstruction, but was normalized after 2 h. After 12 and 24 h this flow was significantly decreased (55% and 65% of control respectively). The pattern of total renal blood flow was similar to that of cortical blood flow. In the outer stripe there was an initial increase (44%) after 1 h, but later no significant deviations from control values were found. After 1 h of obstruction the blood flows in the inner stripe and the inner zone were not significantly different from control values. A significantly decreased blood flow was seen in the inner stripe after 2 h (57% of control) and in the inner zone after 12 h (58% of control). A significant recovery of the blood flow had occurred in the inner stripe and the inner zone after 24 h of obstruction. In the contralateral, non-obstructed kidney no significant changes were found, besides an increased innerstripe blood flow after 24 h of UUO.     

Rapid measurement of regional cerebral blood flow in the baboon using15O-labelled water and dynamic positron emission tomography

The sensitivity and reproducibility of rapid measurements of regional cerebral flow (rCBF) using a bolus injection of H₂ ¹⁵O and dynamic positron emission tomography (PET) were investigated in anaesthetised baboons. The cerebrovascular reactivity to changes in arterial pCO₂ was used as an experimental support. PET data were acquired over 4 min following a single bolus intravenous injection of H₂ ¹⁵O, while arterial blood was withdrawn for continuous activity counting. Images were reconstructed with a dynamic sequence of 45×2s+15×10s, including a correction for decay. Regional values of CBF were derived from non-linear least-squares fits of the time activity curves using a four-parameter two-compartment model. The results obtained with a four-parameter fitting method were compared with those obtained with two other rapid estimation methods, first fitting two parameters only, CBF and partition coefficient (p), and secondly autoradiography (with p fixed at 0·95 ml brain ml blood⁻¹). Twelve regions of interest were analysed. The values for the basal CBF obtained from 13 measurements in two baboons were close to published values obtained with other techniques. Reproducibility checks showed a mean variation of 9·7 per cent. The CBF measurements performed in hypercapnic conditions gave results similar to published data in other animal species, showing a 4·5±0·9 per cent increase in CBF per mm Hg p_aCO₂. The results obtained with the three estimation techniques were closely correlated. The dynamic bolus H₂ ¹⁵O method appeared to be suitable for high blood flow measurements.     

Flat crossed-coil detector for blood flow measurement using nuclear magnetic resonance

A new flat crossed-coil nuclear magnetic resonance (n.m.r.) detector can be used to measure flood flow at many regions of the body, is introduced. It is compared, bothin vitro andin vivo, with the already proven cylindrical crossed-coil n.m.r. detector. Both detectors use a single high-field homogenous magnet. A single-sideband receiver reduces the effect of transmitter leakage into the receiver coil to allow the use of a high-level transmitter field. Preliminary application of the flat crossed-coil detector to the detection of cerebral blood flow is presented.