Biotransport Education: Thermal Therapies

John Pearce,

Temple Foundation Professor

Electrical & Computer Engineering

The University of Texas at Austin

Acknowledgment & Objective

• Partial funding: the T.L.L. Temple Foundation,• Motivation: Dr. Leslie Geddes (1928 - 2009),• ECE Department allows me to teach my course (even years):

EE 385J Topic 26 / BME 381J Topic 5

“Therapeutic Heating”

Objective: Motivate coursework in this arena

because prediction of tissue effects is a more appropriate endpoint than temperature fields alone.

Approach

• Topics range from low temperature diathermy and tumor hyperthermia to ablation and electro-surgery,

• Heavy reliance on purpose-generated course notes (lack of a textbook),

• Primary heating modalities are electromagnetic,• Use FEM numerical models (Comsol) to compare

and contrast the development of irreversible thermal alterations in tissues.

tcTt

kT qgen qmet wtcb Ta T Bioheat Eqn.

Now for something completely different ...

“It is interesting to listen to an Electrical Engineer explain Chemical Engineering to

Mechanical Engineers.”Gene Wissler, ASME 1991 Annual Conference, Dallas TX.

I’m very grateful that he didn’t say “mess up”.

Theory of Relative Reaction Rates:

CEM43 Rcem43 T t

0

dt

S

N0 t

e

t t0D0 T e

CEM43

D0 43 e C t C 0

Theory of Absolute Reaction Rates:

lnC t C 0

A e

ERT

0

dt

Rcem 2

1e

E

RT1T2

Comsol FEM software

• Highly flexible and adaptable,• Excellent drawing features,• Multiphysics modes include:

—Bioheat equation,—Quasi-static electromagnetic modes (complex

properties),—Full wave equation solutions.

• Supports thermal damage process modeling:—Additional general Partial Differential Equation

modes may be superposed as desired.

Diathermy Models

Capacitive Applicator Inductive Coil (w/ perfusion)

Qgen: 0 - 1000 (W/m3)

T@14 m: 32 - 46 (C)

T@30 m: 37 - 42 (C)

Qgen: 0 - 250000 (W/m3)

Hyperthermia models

CHO cells Disk electrode, T = 37 - 62C

Damage: 10, 63.2, 90% CEM43: 30, 60, 90 min.

CEM43 Rcem43 T t

0

dtP % 100 1 e

Sq. CAUnheated Heated

Cardiac Ablation Models

Cooled bipolar elliptical Atricure Squares

20s

T: 35 - 90 C Damage: 10, 63.2, 90%

90s90s

20s

Moving monopolar electrode 1,0 u exp ln A Eact

RT ln Ux

Electrosurgery models

Vessel sealing:Bipolar forceps electrodes

2 mm x 6 mmCartesian model

Qgen x 109

(W/m3)

Problem: Comsol can’t model equilibrium boiling processes.

log10{Qgen}

Corneal shrinkage:Needle electrode

0.5 mm x 2 mmCylindrical model

Conclusions

• Course notes provide an effective basis for the course,

• A disappointing fraction of students are facile in electromagnetics and/or thermodynamics,

• The Comsol commercial FEM package has adequate power to effectively support the many diverse aspects of the class,

• The FEM numerical models are an outstanding positive feature of the overall pedagogy.