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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
ENDEND