Investig Magn Reson Imaging.  2016 Mar;20(1):36-43. 10.13104/imri.2016.20.1.36.

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

• Choi N
• Kim MO
• Shin J
• Lee J
• Kim DH

• KMID: 2161368
• DOI: http://doi.org/10.13104/imri.2016.20.1.36

Abstract

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.

Keyword

Dielectric property imaging; Loss tangent imaging; MREPT; Hyperthermia; Thermal sensitivity

MeSH Terms

Fever
Hyperthermia, Induced
Magnetic Resonance Imaging
Magnets

Figure

• 

  Fig. 1 Simulation results obtained at 915 MHz: (a) Phantom design, (b) B1+ magnitude (yellow ring denotes the tissue boundary, and the ablation antenna is placed in the center), (c) Ez (εr = 40, σ = 0.5 S/m), and (d) SAR values (Electrical properties are noted on each figure).

• 

  Fig. 2 Simulation results showing peak local SAR for ablation performed at four different frequencies. At lower frequencies, the peak local SAR (and thus temperature rise) was directly proportional to the conductivity (a, b). However, at the higher frequency ranges, used in microwave ablation, temperature elevation was more proportional to the loss tangent (c, d). The direction of the white arrow indicates increase of the loss tangent.

• 

  Fig. 3 Results of the liver model simulation: (a) Liver model (from outside to inside: muscle, liver, cancer, antenna), (b) Loss tangent map (cancer σ: 1.2 S/m, εr : 40, frequency: 1 GHz), (c) SAR map, (d) Average SAR in the cancer region, with increase of the loss tangent in the direction of the black arrow, and (e) Average SAR of the liver.

• 

  Fig. 4 Results of the phantom study: (a) MR image and the concentrations of CuSO4 and NaCl for each chamber, (b) conductivity image, (c) relative permittivity image, and (d) loss tangent image. The loss tangent image shows distinct contrast between each specific chamber. Since the permittivity processing is more sensitive to noise, the estimated permittivity map has larger standard deviation compared to the conductivity map.

• 

  Fig. 5 Ex-vivo experiment results (shoulder of a pig). (a) MR image, with delineation of fat (yellow) and non-fat (red) regions. (b) Conductivity image. (c) Relative permittivity image. (d) Loss tangent image. Conductivity and permittivity of the fat showed low intensity compared to the non-fat. The loss tangent image, however, showed different contrast. Artifacts (yellow arrows) appeared due to the complex structure of the tissues.

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