Ultrasound Physics and Instrumentation Ravi Managuli, PhD, RDMS Ultrasound from a Radiologists perspective ! Have a good understanding of ultrasound and what it is capable of ! Know machine ! How to optimize the image for the specific ! Application ! Have a thorough knowledge of the relevant anatomy Ultrasound Machine Ultrasound Machine Features ! For a given transducer and an application ! System is optimized to give acceptable results for an average patient ! Main optimization sonographer performs ! BW ! TGC à Adjust for depth of penetration ! Compound imaging à Enhance shadows ! Harmonic imaging à Enhance anechoic area ! Advanced image processing à Better contrast ! Gain à enhance certain structures ! Color Doppler ! Optimization is very very difficult!! Cons Ultrasound ! It is generally not useful in imaging bony structures ! The ultrasound waves cannot penetrate bone well so brain imaging is not useful with Sonography ! It can be hard to visualize large patients and pockets of gas can cause distortion ! Ultrasound exams can also be invasive and uncomfortable Thus understanding of ultrasound physics becomes quiet critical : More so in ultrasound imaging Topics ! Ultrasound wave characteristics ! Ultrasound interaction with the media ! Transducers ! Beamforming ! Ultrasound modes ! Color Spectral Doppler ! Artifacts/Harmonic imaging ! Sources of information on Ultrasound Bushberg Chatper 16 ! AAPM/RSNA web modules ! Basic US Imaging and Display ! Image Quality – Artifacts – Doppler ! US – Concepts and Transducers ! AAPM/RSNA Physics Tutorials ! This course website ! AAPM/RSNA Physics Curriculum – Module 15 Outline © UW and Renée Dickinson, MS Ultrasound Waves • Imaging with reflected acoustic signal • Short ultrasound pulse of specific frequency is used for imaging Acoustic signal generator and receiver (transducer) Ultrasound machine Acoustic signal Tissue Ultrasound Waves Transverse ! Two types of ultrasound waves ! ! Classified based upon how they move and how they transfer energy Transverse wave ! Consists of oscillations occurring perpendicular to the direction of energy transfer ! Longitudinal waves ! Particles move to and fro and parallel to the wave ! This is the wave utilized to from the ultrasound images Longitudinal Longitudinal Wave : Sound Movement of particles and propagation of wave are in the same direction Movement of particle Propagation of wave Half Value Layer (depth) ! Half-value layer thickness ! Distance at which the intensity is ½ the original intensity ! 3dB attenuation ! Units are centimeter Half value layer is thin: For tissues that attenuates the sound highest : Lung or bone Half value layer is thick: For tissues that attenuates the sound least : fluids Thin Half value Thick half value High frequency sound Low frequency sound Media with high attenuation rate Media with low attenuation rate Attenuation Medium Attenuation Water Extremely low Blood, Urine, Fluids Low Fat Low Soft tissue Intermediate Muscle Higher Bone and lung Even higher Air Extremely high Attenuation in Various Tissues Absorption, Scattering, Reflection ! Air-absorption ! Bone-absorption ! Lung-absorption, scatter ! Muscle-absorption ! Soft Tissue- all three ! Fat-absorb & scatter ! Biologic Fluids- C1 ! Φt > Φi sin(ϕt ) C2 = sin(ϕi ) C1 Reflection Ii Φi Φr Transmission Φt It Ir ! If C2 < C1 ! Φt < Φi C1, Z1 Reflection Ii Φi Φr ! If Φi = or Φi = 90 ! No refraction C2, Z2 Transmission Φt It Ir C1, Z1 C2, Z2 Refraction : only if normal oblique incidence and speeds are different Speed in the Body ! Sound in a medium is Tissue type Lung Fat Soft tisue (average) Liver Blood Muscle Tendon Bone Speed (m/s) 500 1,450 1,540 1,560 1,560 1,600 1,700 3,500 sin(ϕt ) C2 = sin(ϕi ) C1 Refract small degree at soft-tissue fat interface Refract greater extent between soft tissue bone interface For simplicity : 1540 m/s is used while forming the image Refraction : Many artifacts Summary ! Reflection, Transmission and Refraction Event Requirement Reflection with normal incidence Different impedance required Reflection with oblique incidence We cannot predict, it is complex Transmission Laws of conservation Refraction Oblique incidence with different speed required [...]...Simplified Overview Ultrasound System Front-end Transducer Ultrasound Wave ADC Digital Delay Back-end Summer Σ Clinical Signal/Image Processor Display Ultrasound Waves Short pulse transmitted by the ultrasound transducer That propagate into the body 1. Frequency, Period, Wavelength, Velocity 2. Power, Energy, Intensity 3. Pulse duration, pulse length, Bandwidth ! This wave: ! ! ... responsible for signal to noise ratio in an image Ultrasound: Characteristics ! ! Frequency (f) [cycles per second = Hertz (Hz)] : Medical ultrasound is 1-20 MHz ! Infrasound – sound w : f < 15 Hz ! Audible – 15 Hz < f < 20 kHz ! Ultrasound – f > 20 kHz Period (1/f) [seconds] – time duration of one wave cycle ! Reciprocal of frequency : 1 microsec to 0.05 microsec Ultrasound: Characteristics ! ! Wavelength... ! ! ! Low ~1400 m/s (fat) Middle ~1500 m/s (most soft tissues) High ~1700m/s (muscle, cartilage, tendon) Speed, Frequency (period), and wave length are all related by: c [m/sec] = λ[ m] period [sec] c [m/sec] = λ[m] ⋅ f [ Hz ] = λ[ m] 1 f [ Hz ] Ultrasound Physics Frequency range ( f ) : 1Mhz – 15Mhz Human Hearing Range: ~20hz - 20, 000hz Wavelength range ( l ) : 1.5 mm – 0.1mm Relationships... Reciprocal of frequency : 1 microsec to 0.05 microsec Ultrasound: Characteristics ! ! Wavelength (λ) [mm or μm] – distance between compression and refraction ! 0.1 to 1.5 mm Stiffness Speed = Speed (velocity) of sound [m/sec] Density ! Depends upon the density and stiffness of the medium ! ~1400 m/s (fat) to ~1500 m/s (most soft tissues) to ~1700m/s (muscle, cartilage, tendon) ! Soft tissue is... color-mode ! Pulse length (mm) ! Length of the pulse : Wave length * number of cycles ! For BW : (wavelength typically is 0.3mm for 5 MHz) ! (0.3/2) * 2 = 0 3 mm à Resolution Pulsed Wave Ultrasound ! Transmit AND Receive capability ! Pulsing voltage to crystals, produces pulses of sound energy : ! ! During on time transducer is transmitting Transducer listens to the reflecting signal : off... Speed Wavelength = Frequency Wavelength = Speed * Period Period = 1/Frequency Distance = Speed * time Magnitude parameter Magnitude of Sound ! Amplitude ! Difference between maximum positive value and average ! Units of pressure (pascal), ! Typical values in clinical imaging : 1 MPa to 3 Mpa ! Power ! ! Rate of energy transfer Typical values: 4mw-90mw ! Intensity is Power per unit area... ∞ Amplitude2 Peak to peak amplitude Next US Parameters 1 Frequency, speed, wavelength, Intensity Beamformer Transducer ADC Digital Delay Summer Σ Signal/ Image Processor Display 2 All about the shape and size of this waveform Clinical Pulsed Wave ! Pulse wave sequence is: 1. Transmit sound wave 2. Wait for signal to come back 3. Receive the signal ! How long do we wait? ! Depends upon maximum... Parameters Spatial pulse length Pulsed Duration Ability to resolve Structure (Spatial resolution) Spatial pulse length Pulse duration Pulsed repetition period (PRP) Pulse repetition frequency (PRF) Duty factor Ultrasound imaging depth Frame rate (Temporal resolution) Resolution : Pulse Length ! Axial resolution : Ability to resolve two structures closely placed along the longitudinal direction ! Perpendicular... ! Parameters of waves ! Speed ! Frequency ! Period ! Wavelength Speed Wavelength = Frequency Wavelength = Speed * Period Period = 1/Frequency Distance = Speed * time Electromagnetic spectrum Ultrasound: Characteristics ! Duration : Number of cycles in a pulse ! Typically 2 to 3 cycles for BW-mode ! About 8 to 10 cycles for color-mode ! Pulse length (mm) ! Length of the pulse : Wave... V5 V2 2nd vector V3 V4 PRF ! Deep imaging ! Shallow imaging PRP (=1/PRF) ! PRP (or PRF) plays in important role for determining the frame rate (frames/s) ! Each vector is PRF apart Transducer Ultrasound Display Frame Time = # of vectors ! PRP # of vectors Frame Time = PRF V1 V5 V2 V3 V4 V1 V2 V3 V4 Frame Time = # of vectors ! PRP Frame Time = ! For 10 cm depth, 300 vectors ! Frame time