A generic bioheat transfer thermal model for a perfused tissue.
ABSTRACT A thermal model was needed to predict temperatures in a perfused tissue, which satisfied the following three criteria. One, the model satisfied conservation of energy. Two, the heat transfer rate from blood vessels to tissue was modeled without following a vessel path. Three, the model applied to any unheated and heated tissue. To meet these criteria, a generic bioheat transfer model (BHTM) was derived here by conserving thermal energy in a heated vascularized finite tissue and by making a few simplifying assumptions. Two linear coupled differential equations were obtained with the following two variables: tissue volume averaged temperature and blood volume averaged temperature. The generic model was compared with the widely employed empirical Pennes' BHTM. The comparison showed that the Pennes' perfusion term wC(p)(1-epsilon) should be interpreted as a local vasculature dependent heat transfer coefficient term. Suggestions are presented for further adaptations of the general BHTM for specific tissues using imaging techniques and numerical simulations.
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ABSTRACT: Quantitative PET with (15)O provides absolute values for cerebral blood flow (CBF), cerebral blood volume (CBV), cerebral metabolic rate of oxygen (CMRO(2)), and oxygen extraction fraction (OEF), which are used for assessment of brain pathophysiology. Absolute quantification relies on physically accurate measurement, which, thus far, has been achieved by 2-dimensional PET (2D PET), the current gold standard for measurement of CBF and oxygen metabolism. We investigated whether quantitative (15)O study with 3-dimensional PET (3D PET) shows the same degree of accuracy as 2D PET. 2D PET and 3D PET measurements were obtained on the same day on 8 healthy men (age, 21-24 y). 2D PET was performed using a PET scanner with bismuth germanate (BGO) detectors and a 150-mm axial field of view (FOV). For 3D PET, a 3D-only tomograph with gadolinium oxyorthosilicate (GSO) detectors and a 156-mm axial FOV was used. A hybrid scatter-correction method based on acquisition in the dual-energy window (hybrid dual-energy window [HDE] method) was applied in the 3D PET study. Each PET study included 3 sequential PET scans for C(15)O, (15)O(2), and H(2)(15)O (3-step method). The inhaled (or injected) dose for 3D PET was approximately one fourth of that for 2D PET. In the 2D PET study, average gray matter values (mean +/- SD) of CBF, CBV, CMRO(2), and OEF were 53 +/- 12 (mL/100 mL/min), 3.6 +/- 0.3 (mL/100 mL), 3.5 +/- 0.5 (mL/100 mL/min), and 0.35 +/- 0.06, respectively. In the 3D PET study, scatter correction strongly affected the results. Without scatter correction, average values were 44 +/- 6 (mL/100 mL/min), 5.2 +/- 0.6 (mL/100 mL), 3.3 +/- 0.4 (mL/100 mL/min), and 0.39 +/- 0.05, respectively. With the exception of OEF, values differed between 2D PET and 3D PET. However, average gray matter values of scatter-corrected 3D PET were comparable to those of 2D PET: 55 +/- 11 (mL/100 mL/min), 3.7 +/- 0.5 (mL/100 mL), 3.8 +/- 0.7 (mL/100 mL/min), and 0.36 +/- 0.06, respectively. Even though the 2 PET scanners with different crystal materials, data acquisition systems, spatial resolution, and attenuation-correction methods were used, the agreement of the results between 2D PET and scatter-corrected 3D PET was excellent. Scatter coincidence is a problem in 3D PET for quantitative (15)O study. The combination of both the present PET/CT device and the HDE scatter correction permits quantitative 3D PET with the same degree of accuracy as 2D PET and with a lower radiation dose. The present scanner is also applicable to conventional steady-state (15)O gas inhalation if inhaled doses are adjusted appropriately.Journal of Nuclear Medicine 02/2008; 49(1):50-9. · 5.77 Impact Factor
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ABSTRACT: During hyperthermia the presence of a large vessel entering the heated volume and carrying blood at the systemic temperature can be an important source of temperature non-uniformity and possible underdosage. The minimal tumour temperature near a large vessel is determined by the vessel wall temperature: a number of factors influencing the vessel wall temperature are considered--effective tissue conductivity, flow type, vessel size, entrance effects and counter-current flow. In some specific cases, especially when tissue perfusion is high, the vessel wall temperature may reach therapeutic levels when the mean blood temperature is still low. In general, well perfused tumours have a better chance of being heated uniformly. Regional heating improves temperature uniformity by reducing entrance and equilibration effects as blood is heated before entering the tumour. Raising the core temperature also reduces temperature inhomogeneity. Spatial SAR resolution should preferably be of the order of magnitude of a centimetre or better.Physics in Medicine and Biology 07/1992; 37(6):1321-37. · 2.70 Impact Factor
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ABSTRACT: Large blood vessels can produce steep temperature gradients in heated tissues leading to inadequate tissue temperatures during hyperthermia. This paper utilizes a finite difference scheme to solve the basic equations of heat transfer and fluid flow to model blood vessel cooling. Unlike previous formulations, heat transfer coefficients were not used to calculate heat transfer to large blood vessels. Instead, the conservation form of the finite difference equations implicitly modelled this process. Temperature profiles of heated tissues near thermally significant vessels were calculated. Microvascular heat transfer was modelled either as an effective conductivity or a heat sink. An increase in perfusion in both microvascular models results in a reduction of the cooling effects of large vessels. For equivalent perfusion values, the effective conductivity model predicted more effective heating of the blood and adjacent tissue. Furthermore, it was found that optimal vessel heating strategies depend on the microvascular heat transfer model adopted; localized deposition of heat near vessels could produce higher temperature profiles when microvascular heat transfer was modelled according to the bioheat transfer equation (BHTE) but not the effective thermal conductivity equation (ETCE). Reduction of the blood flow through thermally significant vessels was found to be the most effective way of reducing localized cooling.Physics in Medicine and Biology 05/1995; 40(4):477-94. · 2.70 Impact Factor