ULTRASOUND. Ultrasound physical phenomenon properties basics of medical applications, History. History

Transcription

1 Ultrasound physical phenomenon properties basics of medical applications, ULTRASOUND History History Dr. Leopold Auenbrugger 76 - medical doctor first suggests the method of percussion in diagnostics Dr. Leopold Auenbrugger 76 - son of an innkeeper in Graz, Austria Percussion from barrels to human body 3 4

2 Ultrasound a physical phenomenon Sound is a Radiation, a Wave Reference to remember Electromagnetic wave: * Harmonic change of E and B field vectors propagates Harmonic change of a physical parameter propagates in space Described by a wave function Radiation: energy propagation * Propagation does not require a medium * Energy propagating : electric and magnetic 5 6 Sound is a mechanical wave: vibration of particles propagates in a medium What is characterized by a wave function? Sound is a mechanical wave.. whistle Density change of gas molecules Deflection from equilibrium Pressure change Sound waves are described as pressurewaves or The change of the characteristic parameter is periodic in space () and in time (t) or * The energy that is propagating: mechanical energy Energy : mv * Propagation requires a medium 7 8

3 Reminder Waves can be longitudinal : or transverse: direction of harmonic change in the physical parameter is parallel with the direction of propagation direction of harmonic change in the physical parameter is perpendicular to the direction of propagation e.g. electromagnetic wave is a transverse wave r r E c 9 Robe or string: transverse wave (not sound) sound in liquids (tissues) and gases: longitudinal wave sound in solid materials (bones): transverse or longitudinal wave 0 Density change in air wave motion Description of a sound wave in a medium p t p hydrostat + Δp pressure change due to sound wave Φ Sine-wave type point-like sound source amplitude Δp( t, ) Δpma sin π t T +phase Φ + π c T, c f f/t : frequency speed of sound wave propagation, not the speed of light!!!!

4 Sound Ultrasound f>0 khz, c does not depend on f c air 343 m/s therapy diagnostics Ultrasound physical parameters Δp( t, ) Δpma sin π t T Intensity of US (is an important parameter in practical applications) Typical value: W/cm 0 4 W/m Above the pain threshold in the audible range! I or ΔE W Δt A m flu or energy-density denoted now by 3 4 Ultrasound physical parameters Ultrasound physical parameters Δp( t, ) Δpma sin π t T Δp( t, ) Δpma sin π t T Intensity of US (is an important parameter in practical applications) I or ΔE W Δt A m flu or energy-density denoted now by Intensity P el el el ΔE W Δt A m U eff analogy Δ p eff acoustic impedance (Δp eff Δp ma /) Power - application for sound? Δp Δp eff ma 5 High intensity means large Δp ma! 6

5 Ultrasound physical parameters The Intensity of Ultrasound must be limited The Intensity of Ultrasound must be limited Δp ma Δp ma distance of p ma and p min distance of p ma and p min Therapy: f 0.5 MHz? 7 8 The Intensity of Ultrasound must be limited The Intensity of Ultrasound must be limited Δp ma Δp ma Therapy: f 0.5 MHz c muscle 600m/s distance of p ma and p min Therapy: f 0.5 MHz distance of p ma and p min c muscle 600m/s c/f 3..6 mm / mm c/f 3..6 mm / mm 9 - very small distance between ma and min of p! - pressure change Δp ma within a distance of / 0

6 Therapy: f 0.5 MHz suggested limiting value of average W/cm ( in practice it may go up to 3 W/cm ) The Intensity of Ultrasound must be limited Diagnostics: f () 0 MHz Δp Δp eff ma / μm in soft tissue cellular and subcellular size! Δp ma ~ 3. atmospheric!!! within about mm muscle in practice may be high : 0 W/cm??? danger for cavitation and chemical reactions The Intensity of Ultrasound must be limited Why pulses? Diagnostics: f () 0 MHz / μm in soft tissue cellular and subcellular size! in practice may be high : 0 W/cm Diagnostic applications are based on registering the time span between the emission and return of ultrasound pulses from a reflecting surface BUT: in most cases, pulse-mode is used Pulse Echo - techniques 3 4

7 Features of pulsed ultrasound Ultrasound physical parameters role of the medium c ρκ Speed of propagation depends on the ρ density and κ compressibility of the medium understood in the pulse with Δt~ μs and ms pause average ~ 0mW/cm Low value! 5 κ Δ V V Δp compressibility κ epresses the negative relative volume change (decrease) induced by pressure change Δp (increase) 6 Role of the medium in the propagation. Role of the medium in the propagation. Δ p ma Δ p ma acoustic impedance determines how large pressure fluctuations will be generated by US flu acoustic impedance determines how large pressure fluctuations will be generated by US flu Relation to the tissue properties? 7 8

8 Role of the medium in the propagation. ~340 m/s Speed of propagation in various media Δp ma acoustic impedance determines how large pressure fluctuations will be generated by US flu cρ ρ κ is determined by the properties of the medium 9 Speed in average soft tissue540 m/s 30 c(m/s) ρ(kg/m 3 ) ρ c D(m) R 0kHz MHz air * water * few ~ can be muscle * *0 - significantly ~0.3 bone * different ~0-3 brain * Another feature of tissues Absorption of sound waves Described by the eponential law of radiation attenuation Damping coefficient α I I I/ I/e 0 D /μ I I 0 e - μ 0 e μ 0 lg lg e μ 0 α 0 lg db α 0 μ lge db α const μ Absorption is a disadvantage in US diagnostics! 3 3

9 The attenuation of sound waves depends on the frequency diagnostics α 0 μ lge db Introducing α spec α spec α f α const μ μ μ const f Specific damping coefficient ~ linear dependence on the frequency in the high f range characterizes solely the tissue α spec will not depend on the frequency of radiation and on the thickness of tissue Linear dependence: straight line at 45 o e.g. soft tissues ~db/(cm. MHz) D μ c(m/s) ρ(kg/m 3 ) ρ c D(m) R 0kHz MHz air * water * few muscle * *0 - bone * ~0-3 brain * Half value thickness in function of frequency: Radiation with higher f is absorbed more ~ ~0.3 too high absorption 35 Air Lung Fat Water Water Brain Soft tissue Liver Kidney Spleen Muscle Blood Eye lens Bone marrow Bone-porous Bone-solid Al Contact gel Lead-irconate- Titanate Quartz anyag ρ sűrűség κ kompresszibilitás c terjedési sebesség akusztikus impedancia α/(f ) Specific fajlagos csillapítás damping [kg/m 3 ] [/GPa] [m/s] [kg/(m s)] [db/(cm MHz)] levegő, , , tüdő 400 5, ,6 0 6 zsír 95 0,5 470, ,63 víz, 0 C , ,00 víz, 36 C , agy , ,85 lágy szövet , ,3,7 máj 060 0, , ,94 vese 040 0,40 560,6 0 6,0 lép , izom ,63 0 6,3 3,3 vér 060 0,38 570,6, ,8 szemlencse 60,84 0 6,0 csontvelő , csont, porózus 380 0, ,,9 0 6 csont, tömör 700 0, , 0 6 0,0 aluminium 700 0, ,8 0 6 csatoló gél 6,5 0 6 ólom-cirkonáttitanát kvarc ,

10 Basics of Pulse Echo techniques US is reflected and refracted at the boundary of media with different ρ and κ. Incident beam is to the boundary total refl Incidence at right angle to the boundary reflection and transmission total refl + Incident beam hits the boundary at α angle total α α c c refl Incidence at α angle Snell s law is valid US optics with mirrors, lenses β sinα c sin β c 37 Pulse echo: US diagnostics is based on the reflection of radiation at internal media-boundaries Reflectivity R R reflected incident + Total reflection: R <<, R US is totally reflected at boundaries of media of very different impedances 38 reflection of radiation at internal media-boundaries. R reflected incident Source Ceramics Crystal Solid state material + AIR Total reflection! Total reflection: R <<, R Body Skin Coupling medium to avoid total reflection c(m/s) ρ(kg/m 3 ) ρ c D(m) R 0kHz MHz air * water * few muscle * *0 - bone * ~0-3 brain * ~ ~0.3 Partial reflection is good condition for diagnostics coupling source skin (e.g. Pb-r-Ti source see Table) 39 40

11 Sources of Ultrasound radiation: Piezoelectric crystal - transducer ELECTRIC MECHANICAL Eamples for piezoelectric crystals: quartz (SiO ) Rochelle salt (KNaC 4 H 4 O 6 *4H O) Center of weight of positive and negative charges overlaps. Ceramics based on electro/magneto-striction Effect of alternating compression (b) and dilatation (c) Alternating charge separation Materials built up from magnetic or electric dipole elements Eternal field induces orientation deformation Direct piezoelectric effect receiver Inverse piezoelectric effect source MECH EL Case b and c EL MECH The same unit can be used both as receiver and source 4 Comparison with piezoelectric transducers: - lower frequencies are possible - mechanically more endurable materials Dental applications: 0-40 khz US transducer in direct contact with dental deposits-> disintegration ->cleaning 4 ESWL (etracorporeal schockwave lithotripsy) Not an Ultrasound technique! End of the first part Non invasive etermination of stones in organs (kidney, bile, etc.) Body is brought in contact with a conatiner filled with water Two electrodes of ~0 kv, immersed in water produce a spark that generates a pressure pulse in water Surrounding elliptical mirrors focus the pulse/shock wave to the site of the stone that becomes destroyed. The location of the stone is controlled by X ray or US imaging 43 44

12 45 46 US therapy Read in the tetbook End of Ultrasound part I 47 48