Physical problems in therapeutic focused ultrasound

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1 Physical problems in therapeutic focused ultrasound Alexander Kenis Insightec LTD. Offer your patients cutting edge technology.without the cutting 1

2 Outline Vocabulary What is therapeutic focused ultrasound and how the treatment looks Physical problems Sound propagation and absorption Transducer modeling Cavitation 2

3 Vocabulary Ablation - surgical removal MI NM for pedestrians US - Ultrasound MgFUS MI guided focused ultrasound Transducer wave emitter Hydrophone microphone for water 3

4 What is focused ultrasound Ultrasound sound waves (above 20kHz) Sound waves can be focused like light 4

5 What is M guided Focused Ultrasound (MgFUS)? Technology which combines: High intensity focused ultrasound that heats and destroys targeted tissue, non-invasively Magnetic resonance imaging system (MI) which visualizes patient anatomy, and controls the treatment by monitoring the tissue temperature in real time 5

6 How does Focused Ultrasound work? Treatment illustration Parallel to beam Tumor Perpendicular to beam Water Bath Transducer Focused Ultrasound Beam Focused ultrasound generates heat, ablating tissue only at the focal point. The effect is similar to a magnifying glass used to focus the sun s energy on a single point. 6

7 New bone setup Strappable Bone Applicator* Access to more anatomical locations Patient comfort High density steerable transducer Pelvis ibs Shoulder 7

8 Clinical applications Uterine fibroids Pain palliation of bone metastases Breast cancer Liver tumors Prostate cancer Brain tumors and blood clots Abdominal tumors 8

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10 Physical problems Sound propagation and absorption in Water Solids Transducer modeling Cavitation 10

11 Sound propagation - Liquids Wave (Helmholtz) equation in liquids and gases (p-pressure,( pressure, c- sound speed): 2 p = c 2 2 t 2 p Taking into account the density (ρ)) changes: ρ 1, c 1 ρ 2, c 2 1 p = c 2 t 2 ρ 2 1 p ρ eflection defined by impedance (Z)) and refraction by c : Z = ρ*c 11

12 Sound propagation ups and downs Ups: ayleigh integral calculates pressure in homogeneous liquids Downs: Air in lungs and intestines (air reflects US) Bones are everywhere (bones absorb US) Skull is a bone Tissue that absorbs ultrasound gets heated. 12

13 Sound propagation - Solids, Elastic wave equation (Navier s s equation). U-displacement (vector), ρu λ, µ Lame coefficients. = ( λ + 2µ ) U µ U First term longitudinal waves c l = λ + 2µ ρ Second term shear waves c s = µ ρ 13

14 Elasticity and Sound absorption F - force, U - displacement (Vectors) σ - stress, u - strain) (Tensors) Vectors Tensors For water stress pressure (scalar) strain expansion (scalar) Energy flow Vector Poynting u P k ik = = 1 Ui 2 xk F i U + x = σ x ik k i ik 1 / 2σ U k i Energy absorption Q = P 14

15 Bioheat equation, Bioheat equation: ρ C where T t T is the temperature, C is the tissue specific heat, ρ the tissue density, W b blood perfusion rate, K is the thermal conductivity, T 0 is the body temperature. ( ) = ( K T ) + Q+ W C T T b b 0 15

16 Focused Field Simulation tool finite elements program (Comsol Multiphysics) 16

17 Wave propagation and absorption Setup 4 Pressure Amplitude Temperature Water Bone 3 Water

18 Lower frequency, closer focus Setup 4 Pressure Amplitude Temperature

19 Challenges in Trans Cranial Focusing: De-focusing Skull effect on the ultrasound beam path Variable thickness/density Variable incidence angles Defocusing Ultra Sound Array De-Focusing effect CT data is used to reproduce skull structure and regain sharp focus CT corrected focus 19

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21 Transducer Modeling Electrical behavior of the material: E electric field, D electric displacement, ε permittivity Hooke's Law: u strain, σ stress. uik Coupled equations, in the strain-charge form u = µ σ + γ ik D i = ε iklm ik E l lm D i =µ + 4πγ = ε ik iklm l, ik i, kl E σ E σ l kl k lm 21

22 Transducer Simulation Piezoceramics shadow lines Color displacement in substrate 22

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24 Cavitation. Definition Cavitation behavior of voids or bubbles in a liquid. Cavitation is divided into two types: inertial (or transient) cavitation void or bubble in a liquid rapidly collapses, producing a shock wave non-inertial cavitation. bubble oscillates in size or shape due to some form of energy input 24

25 Applications of cavitation Sonoluminesence Sonochemistry Water purification Surface cleaning Homogenization of solutions 25

26 Cavitation - Background First discovered in propellers Seen in submarines 26

27 Supercavitation Shkval Torpedo Torpedo in one huge bubble Speed up to 370 km/h 27

28 Early ages of cavitation First identified and reported in 1895 by Sir John Thornycroft and Sidney Barnaby Caused inexplicably poor performance of a newly built destroyer HMS Daring. The rapid motion of the blades through water was found to tear the water structure apart. Studied by lord ayleigh 28

29 Problems connected to cavitation Cavitation very violent phenomenon Cavitation damage of a Francis turbine. 29

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31 Sonoluminesence 31

32 Equilibrium Bubble Equilibrium Bubble conditions p v, p g, p 0, - vapor, gas, hydrostatic pressures, 0 Bubble initial radius σ surface tension p g + p v = p 0 + 2σ 0 adius 0 κ - polytropic index p g = p g0 0 3κ = p 0 2σ + p 0 v 0 3κ 32

33 Blake Threshold Pressure Quasistatic Bubble conditions p L pressure in the liquid, on the bubble border 2σ 0 = p0+ pv + pv 0 For p L < 0, if bubble larger than some critical radius, 2σ/ 2 cannot compensate it, bubble grows explosively After some derivation and neglects Blake threshold is obtained p L P B 3κ 2σ σ = p

34 34 ayleigh ayleigh Plesset Plesset equation equation Kinetic energy = Work Kinetic energy = Work Integrating and deriving with respect to : Integrating and deriving with respect to : ayleigh ayleigh Plesset Plesset: η viscosity viscosity + + = + P t p p p p v v η σ σ ρ κ 4 ) ( ( ) ρ π π d p p L = ρ = + p p L 2 3 2

35 Bjerknes Forces: Bjerknes Forces Primary: adiation force on a bubble in a standing wave field FB = V (t) p(x, t) Secondary: attraction between two bubbles in the incident sound field T 35

36 What else is there? Behavior of multiple bubbles Development of bubble cloud Energy absorption by cavitation Energy scattering by cavitation Focus shape deterioration and drift 36

37 Cons of Cavitation Cavitation stops the energy flow Tissue Treatment No Cavitation Cavitation Bone Treatment No Cavitation Cavitation Bone Ablated Tissue 37

38 Pros of cavitaton Cavitation enhances energy absorption No Cavitation Brain Treatment Cavitation 38

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Antonio Rampoldi Struttura Complessa di Radiologia Interventistica Azienda Ospedale Niguarda Ca Granda, Milano. Terminology

Antonio Rampoldi Struttura Complessa di Radiologia Interventistica Azienda Ospedale Niguarda Ca Granda, Milano. Terminology Biophysical principles and clinical applications of MRgFUS Alberto Torresin Struttura Complessa di Fisica Sanitaria Azienda, Milano Università degli Studi di Milano Dip. di Fisica Antonio Rampoldi Struttura