A transient heating technique for the measurement of thermal properties of perfused biological tissue

In Vivo Ischemia Detection by Luminescent Nanothermometers

There is an urgent need to develop new diagnosis tools for real in vivo detection of first stages of ischemia for the early treatment of cardiovascular diseases and accidents. However, traditional approaches show low sensitivity and a limited penetration into tissues, so they are only applicable for the detection of surface lesions. Here, it is shown how the superior thermal sensing capabilities of near infrared-emitting quantum dots (NIR-QDs) can be efficiently used for in vivo detection of subcutaneous ischemic tissues. In particular, NIR-QDs make possible ischemia detection by high penetration transient thermometry studies in a murine ischemic hindlimb model. NIR-QDs nanothermometers are able to identify ischemic tissues by means of their faster thermal dynamics. In addition, they have shown to be capable of monitoring both the revascularization and damage recovery processes of ischemic tissues. This work demonstrates the applicability of fluorescence nanothermometry for ischemia detection and treatment, as well as a tool for early diagnosis of cardiovascular disease.

Noninvasive measurement of local thermal diffusivity using backscattered ultrasound and focused ultrasound heating

Previously, noninvasive methods of estimating local tissue thermal and acoustic properties using backscattered ultrasound have been proposed in the literature. In this article, a noninvasive method of estimating local thermal diffusivity in situ during focused ultrasound heating using beamformed acoustic backscatter data and applying novel signal processing techniques is developed. A high intensity focused ultrasound (HIFU) transducer operating at subablative intensities is employed to create a brief local temperature rise of no more than 10 degrees C. Beamformed radio-frequency (RF) data are collected during heating and cooling using a clinical ultrasound scanner. Measurements of the time-varying "acoustic strain", that is, spatiotemporal variations in the RF echo shifts induced by the temperature related sound speed changes, are related to a solution of the heat transfer equation to estimate the thermal diffusivity in the heated zone. Numerical simulations and experiments performed in vitro in tissue mimicking phantoms and excised turkey breast muscle tissue demonstrate agreement between the ultrasound derived thermal diffusivity estimates and independent estimates made by a traditional hot-wire technique. The new noninvasive ultrasonic method has potential applications in thermal therapy planning and monitoring, physiological monitoring and as a means of noninvasive tissue characterization.

A novel model of the pulse decay method for measurement of local tissue blood perfusion

The blood perfusion of biological tissue is a basic parameter of physiology and medical engineering. A novel average temperature model (ATM) is presented in this paper to measure the blood perfusion of tissue based on the thermal pulse-decay method. Differing from the existing point source model (PSM) and spherical source model (SSM), the probe bead average temperature analytical solution is derived and used to estimate the blood perfusion. The blood perfusion prediction errors caused by the approximate assumptions used in each model are studied using the numerical experiment method. Contributions of the tissue parameters, probe thermistor bead parameters, and measurement parameters, such as tissue thermal conductivity, tissue thermal diffusivity, blood perfusion rate, bead thermal conductivity, bead radius, measurement time, and thermal pulse length are discussed. The predicting accuracy of the ATM model is compared with the PSM model and SSM model. The results show that, for all the cases tested, the ATM model is better than the other two.

A general model for the propagation of uncertainty in measurements into heat transfer simulations and its application to cryosurgery

This report presents a technique for estimating the propagation of uncertainty in measurements into mathematical simulations of heat transfer. The motivation for this report is to show the dramatic uncertainty associated with estimating the value of the so-called "lethal temperature," even in a case where a perfect correlation appears to exist between histo-pathologic observations and a corresponding heat transfer simulation. Although the example presented in this report relates to cryosurgery, the technique proposed in this report is rather general and can be applied to any heat transfer problem. The uncertainty analysis presented in this report can be considered as an extension of the well-known concept of the rule of the square root of the sum of the square errors. A comparison of the new technique with the worst case scenario concept is also presented. In conclusion, it is recommended that the proposed technique be routinely applied when presenting simulated results, whether as a part of a theoretical study, or in comparison with experimental data.

Estimation of blood perfusion using ultrasound

Ways to measure blood perfusion using ultrasound techniques such as continuous-wave Doppler, pulsed Doppler, colour Doppler and power Doppler will be reviewed. From a certain standpoint, blood perfusion may be defined as the difference between arterial inflow and arterial outflow from a considered volume, i.e. capillary flow. The low velocities and small blood volumes involved make the signal-to-noise ratio, dynamic range and frequency resolution critical factors in the detection system. Another limiting factor is tissue motion which obscures the blood signal. Perfusion may still under certain conditions be estimated with the first moment of the Doppler power spectrum, as obtained with any Doppler ultrasound method. Modern flow mapping techniques also make it possible to estimate perfusion by counting the number of pixels that indicate flow, but low flow velocities cannot be included in the estimate. Future high-frequency systems may, however, provide very detailed images of minute flow distributions in superficial layers. Contrast agents are widely used today to enhance the blood signal, and a technique named harmonic imaging can suppress movement artefacts from surrounding tissue. Transient signals from disrupting contrast agent particles in an ultrasound field can potentially be used for perfusion quantification. Future developments to extract the blood flow signal from its noisy environment, aside from contrast agents, may be multiple sample volumes, frequency compounding and/or improved signal processing. The lack of an adequate perfusion phantom for verification of measurements of microcirculatory flow becomes, however, more apparent with improved detectability of slow flows.

Modeling bronchial circulation with application to soluble gas exchange: description and sensitivity analysis

The steady-state exchange of inert gases across an in situ canine trachea has recently been shown to be limited equally by diffusion and perfusion over a wide range (0.01-350) of blood solubilities (betablood; ml . ml-1 . atm-1). Hence, we hypothesize that the exchange of ethanol (betablood = 1,756 at 37 degrees C) in the airways depends on the blood flow rate from the bronchial circulation. To test this hypothesis, the dynamics of the bronchial circulation were incorporated into an existing model that describes the simultaneous exchange of heat, water, and a soluble gas in the airways. A detailed sensitivity analysis of key model parameters was performed by using the method of Latin hypercube sampling. The model accurately predicted a previously reported experimental exhalation profile of ethanol (R2 = 0.991) as well as the end-exhalation airstream temperature (34.6 degrees C). The model predicts that 27, 29, and 44% of exhaled ethanol in a single exhalation are derived from the tissues of the mucosa and submucosa, the bronchial circulation, and the tissue exterior to the submucosa (which would include the pulmonary circulation), respectively. Although the concentration of ethanol in the bronchial capillary decreased during inspiration, the three key model outputs (end-exhaled ethanol concentration, the slope of phase III, and end-exhaled temperature) were all statistically insensitive (P > 0.05) to the parameters describing the bronchial circulation. In contrast, the model outputs were all sensitive (P < 0.05) to the thickness of tissue separating the core body conditions from the bronchial smooth muscle. We conclude that both the bronchial circulation and the pulmonary circulation impact soluble gas exchange when the entire conducting airway tree is considered.

Modeling the concentration of ethanol in the exhaled breath following pretest breathing maneuvers

A previously developed mathematical model that describes the relationship between blood alcohol (ethanol) concentration and the concentration of alcohol in the exhaled breath at end-exhalation (BrAC) has been used to quantitate the effect of pretest breathing conditions on BrAC. The model was first used to "condition" the airways with different breathing maneuvers prior to simulating a single exhalation maneuver, the maneuver used in standard breath alcohol testing. On inspiration, the alcohol in the air reaches local equilibrium with the alcohol in the bronchial capillary bed prior to entering the alveolar region. On expiration, approximately 50% of the alcohol absorbed on inspiration is desorbed back to the airways. BrAC correlates with the amount of alcohol that is desorbed to the airways. The six pretest breathing conditions and the percent change in BrAC relative to the control maneuver were: hyperventilation (-4.4%), hypoventilation (3.7%), hot-humid air (-2.9%), hot-dry air (0.66%), cold-humid air (0.13%), and cold-dry air (0.53%). The mechanism underlying these responses is not due to changes in breath temperature, but, rather to changes in the axial profile of alcohol content in the mucous lining of the airways.

Self-heated thermistor measurements of perfusion

A microcomputer-based control system applies a combination of steady state and sinusoidal power to a thermistor probe which is inserted into the tissue of interest. The steady-state temperature response is an indication of the effective thermal conductivity (keff), which includes a component due to intrinsic conduction plus a convective component due to the tissue blood flow near the probe. By careful choice of the excitation frequency, the sinusoidal temperature response can be used to measure intrinsic thermal conductivity (km) in the presence of blood flow. Optimal sinusoidal heating frequency depends on the thermistor size. Experimental results in the alcohol-fixed canine kidney cortex show that perfusion is linearly related to the difference keff minus km. The instrument can measure tissue thermal conductivity with an accuracy of 2%. The instrument can resolve changes in perfusion of 10 mL/100g-min with a Thermometrics P60DA102M thermistor. The maximum error in measured perfusion is about 30%. When tissue trauma due to probe insertion is minimized, the self-heated thermistor method gives a reliable indication of local tissue blood flow.

Estimation of tissue blood flow from hyperthermia treatment data

During hyperthermia treatment of patients the delivered heat and the temperatures at several points in the tissue are often measured and recorded. These data contain information about thermal tissue parameters. A method for extracting this information, i.e. estimating the tissue parameters--in particular the blood perfusion rate--is described. The method applies a system identification technique, adjusting the unknown parameters in a thermal tissue model, until the predicted model output (temperature) coincides well with the measured temperature. Data from a number of patient treatments have been used to test the method, and although the accuracy of the method remains to be established conclusively it appears to give a good estimate of the model parameter representing blood flow. The obvious advantage of the method is that it requires no special transducers or experiments. The weak aspect is that it depends on the correctness of a thermal model of the perfused tissue.