Non-Invasive in vivo Loss Tangent Imaging: Thermal Sensitivity Estimation at the Larmor Frequency

Visualization of the tissue loss tangent property can provide distinct contrast and offer new information related to tissue electrical properties. A method for non-invasive imaging of the electrical loss tangent of tissue using magnetic resonance imaging (MRI) was demonstrated, and the effect of loss tangent was observed through simulations assuming a hyperthermia procedure. For measurement of tissue loss tangent, radiofrequency field maps (B1+ complex map) were acquired using a double-angle actual flip angle imaging MRI sequence. The conductivity and permittivity were estimated from the complex valued B1+ map using Helmholtz equations. Phantom and ex-vivo experiments were then performed. Electromagnetic simulations of hyperthermia were carried out for observation of temperature elevation with respect to loss tangent. Non-invasive imaging of tissue loss tangent via complex valued B1+ mapping using MRI was successfully conducted. Simulation results indicated that loss tangent is a dominant factor in temperature elevation in the high frequency range during hyperthermia. Knowledge of the tissue loss tangent value can be a useful marker for thermotherapy applications.

Evaluation of myocardial viability during cold storage by measurement of myocardial dielectric properties tanδm in radio frequency / 中华器官移植杂志

ObjectiveTo study the mechanism of myocardial dielectric property changes in radio frequency during hypothermic preservation and explore myocardial viability evaluative method. Methods Hybrid young pigs (20-30 kg) were used in the experiment. Heart arrest was induced with GIK solution. According to preservative temperature, the animals were divided into three groups: group A (4 ℃),group B (15 ℃) and group C (25 C). The heart was preserved in saline for 12,6 and 4 h respectively. Myocardial dielectric properties and ATP content were tested every 5 and 30 min during hypothermic preservation respectively. The relationship between tanoδm and ATP content was analyzed. ResultsTanδm of three groups was decreased significantly at the beginning, most slightly in group A and most obviously in group C. There was no significant difference in Tanδm between group B and group C with group A at 1 h,but there was obvious difference at 2,3 and 4 h (P＜0. 05,P＜0. 05,P＜0. 01 ). There was correlation between tanδm and ATP in the three groups. ConclusionTanδm in three groups is dropped with preservation time and temperature, and tanδm had a correlation with ATP content. Detection of myocardial dielectric property in radio frequency is a kind of quick, accurate and noninvasive method.

Effects of Tail Suspension on Dielectric Properties of Cells in Isolated Rat Sural Muscle / 航天医学与医学工程

Objective To study changes of dielectric properties of gastrocnemius caused by simulated weightlessness.Methods Male Sprague-Dawley rats tail suspended for 10 wk were observed together with its normal control.AC impedance of isolated gastrocnemius was measured with an impedance analyzer(Agilent 4294A).Effects of simulated weightlessness on electric properties of cells in isolated rat sural muscle were also observed by analyzing the dielectric numerical characters of the dielectric spectra,the Cole-Cole plots,spectrum of loss factor and loss tangent,as well as spectrum of conductivity imaginary part.Results It was found that simulated weightlessness could induce certain changes:both relative permittivity at low frequency ?L and relative permittivity increment ??,as well as conductivity at high frequency ?h and conductivity increment ?? were all reduced.The first characteristic frequency fC1 decreased,while the second characteristic frequency fC2 increased.Peak of loss factor,peak of loss tangent and the maximum of conductivity imaginary part were also reduced.Conclusion Changes of dielectric property of gastrocnemius induced by simulated weightlessness can be observed effectively by frequency domain impedance technique.