ultrasound physics in the operating room i 972012

Ultrasound Ultrasound Physics in the Physics in the

Operating RoomOperating RoomTamas Seres, M.D.Tamas Seres, M.D.

UC DenverUC Denver20122012

UltrasoundUltrasound Ultrasound (US) represents a Ultrasound (US) represents a

mechanical pressure disturbance mechanical pressure disturbance propagating as wave through propagating as wave through materials dense enough to transmit materials dense enough to transmit the fast oscillations imparted on the fast oscillations imparted on molecules. molecules.

2-12 MHz in medical practice.2-12 MHz in medical practice.

Piezoelectric CrystalPiezoelectric Crystal

Piezoelectric Crystal as a Piezoelectric Crystal as a SenderSender

Piezoelectric Crystal as a Piezoelectric Crystal as a ReceiverReceiver

Propagation of the Propagation of the SoundSound

Sound is a traveling variation of acustic variables.

Acustic variables: particle motion, density and pressure.

Description of US wavesDescription of US waves Amplitude - decibels (dB) Amplitude - decibels (dB) Frequency (f) - Cycles/sec – Hz Frequency (f) - Cycles/sec – Hz Wavelength (Wavelength () - mm) - mm Velocity of propagation (c)- depends Velocity of propagation (c)- depends

on each carrying medium - on each carrying medium - approximately 1540 m/s in blood and approximately 1540 m/s in blood and soft tissuesoft tissue

Amplitude of the SoundAmplitude of the Sound Amplitude: the difference between Amplitude: the difference between

the reference value (1x10the reference value (1x10-12-12 W/m W/m22 ) ) and the maximum value of an and the maximum value of an acoustic variable (dB). acoustic variable (dB).

Sound scale:Sound scale: 0 dB hearing treashold0 dB hearing treashold 130 dB painful130 dB painful

AmplitudeAmplitude Decibels (dB) are logarithmic units Decibels (dB) are logarithmic units

based on a ratio of the measured based on a ratio of the measured value V of acoustic pressure to a value V of acoustic pressure to a reference value R ((1*10reference value R ((1*10-12-12 W/m W/m22) :) :dB = 20 log (V/R)dB = 20 log (V/R)For example: ratio of 2For example: ratio of 220 x log 2 = 20 x 0.3 = 6 dB20 x log 2 = 20 x 0.3 = 6 dBRatio of 1000 = 60 dBRatio of 1000 = 60 dB

Decibel Scale of SoundDecibel Scale of Sound

Amplitude Change in dB Amplitude Change in dB and %and %

Propagation Velocity, Propagation Velocity, Frequency and Wavelength Frequency and Wavelength

c = f x

c= m/s, mm/s = mmf = cycles/s (Hz), cycles/s (MHz)

Propagation velocity of Propagation velocity of US in different tissues US in different tissues

(m/s or mm/µs )(m/s or mm/µs ) m/sm/s mm/µs mm/µs

AirAir 330330 0.330.33 LungLung 500500 0.50.5 FatFat 1450 1450 1.451.45 Soft tissueSoft tissue 1540 1540 1.541.54 BoneBone 4000 4000 44

C (human soft tissue) = 1540 m/s or 1.54 mm/µs

Frequency and Frequency and WavelengthWavelength

Propagation Velocity, Propagation Velocity, Frequency and Wavelength Frequency and Wavelength

c = x f

(mm) = 1.54 (mm/µs)/ f (cycles/µs)

f = 5 MHz = 0.3 mm

f = 7.5 MHz = 0.2 mm

Air Bubbles in the Blood during TMR

Ultrasound PulsesUltrasound Pulses

Ultrasound PulsesUltrasound Pulses

Ultrasound PulseUltrasound Pulse

Ultrasound PulseUltrasound Pulse

Ultrasound PulseUltrasound Pulse

Ultrasound PulseUltrasound Pulse

Ultrasound PulseUltrasound Pulse

Axial ResolutionAxial Resolution

For better reflection from a particle: particle diameter > λFor axial resolution: Pulse Length/2 < Distance between 2 particles

ImpedanceImpedance Impedance is the acoustic resistance Impedance is the acoustic resistance

to sound traveling through a to sound traveling through a medium (Z).medium (Z).

Units: RAYLUnits: RAYL Characteristic of the medium only.Characteristic of the medium only. Z=density(kg/mZ=density(kg/m33) x propagation ) x propagation

speed(m/s)speed(m/s)

ImpedanceImpedanceBody Tissue Acoustic Impedance

(106 Rayls) Air 0.0004

Lung 0.18Fat 1.34

Liver 1.65Blood 1.65Kidney 1.63Muscle 1.71Bone 7.8

ReflectionReflection

Z transducer >>> Z air <<<<Z tissue

TransmissionTransmission

Z transducer = Z jelly = Z tissue

ReflectionReflection

Z tissue <<< Z needle

ReflectionReflection

IRC = Intensity Reflection Coefficient

RefractionRefraction

RefractionRefraction

Ultrasound in the Human Ultrasound in the Human BodyBody

Can be oriented like beams.Can be oriented like beams. Follow the physical laws of Follow the physical laws of

reflection, refraction and scattering.reflection, refraction and scattering. Propagate freely in liquids, but very Propagate freely in liquids, but very

poorly through air. poorly through air.

Air Bubbles in the Blood during TMR

AttenuationAttenuation As US penetrates into the body, As US penetrates into the body,

signal strength is attenuated due to signal strength is attenuated due to absorption of the US energy by absorption of the US energy by conversion to heat, as well as by conversion to heat, as well as by reflection and scattering.reflection and scattering.

AttenuationAttenuation

Amplitude Change in dB Amplitude Change in dB and %and %

AttenuationAttenuation

Body Tissue Attenuation Coefficient (dB/cm at 1MHz)

Water 0.002

Blood 0.18

Fat 0.63

Liver 0.5-0.94

Kidney 1.0

Muscle 1.3-3.3

Bone 5.0

Half-power distances for Half-power distances for a frequency of 2 MHza frequency of 2 MHz

WaterWater 380 cm380 cm BloodBlood 15 cm 15 cm Soft tissueSoft tissue 1-5 cm1-5 cm MuscleMuscle 0.6-1 cm0.6-1 cm BoneBone 0.2-0.7 cm0.2-0.7 cm AirAir 0.08 cm0.08 cm LungLung 0.05 cm0.05 cm

AttenuationAttenuation

US: 7.2 MHz

US: 5 MHz

US: 3.5 MHz

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