physics & instrumentation of diagnostic...
TRANSCRIPT
Physics & Instrumentation of
Diagnostic Ultrasound
Dr. Stavros Tsantis
Department of Biomedical Engineering
Technological Educational Institute of Athens
email: [email protected]
Transmit Mode
Ultrasound Basics
5 MHz
Sound waves are transmitted by a transducer
Pulse – Echo Imaging Technique
Pulse Phase
Receive Mode
Ultrasound Basics
5 MHz
c=s/t, c=1540 m/sec
Several reflection waves are produced within tissue interfaces creating echo
signals
These signals are recorded and visualized according to their origin and
intensity
Pulse – Echo Imaging
Echo Phase
Waves Physics & Ultrasound
Wave is a mechanism towards energy transition from one point to an other
Mechanical Waves
Electromagnetic Waves
Need a medium to propagate
They can propagate in void
1. Light
2. Heat
3. Χ – Rays
4. γ – Rays
5. ΤV,RF signals
Transverse Waves
Longitudinal Waves
Ultrasound ? (Shear Waves)
How Ultrasound Waves are produced?
They are produced from PZT (Lead Zirconate
Titanate) Crystal Oscillation via electric
stimulation
Mechanical Model of Sound Propagation
Oscillating Crystal Surface
Medium Molecules
Compression - Rarefaction
Basic Conclusions
1. Ultrasound waves are mechanical longitudinal waves
2. Need a medium to propagate
3. A PZT crystal transforms electrical energy into mechanical and vice versa
4. Contemporary transducers employ compound materials towards maximum efficiency
5. In diagnostic Ultrasonography the transmit frequency range is within 1.5 – 15 MHz
What Happens when Ultrasound Propagates Through Tissues?
‒ Reflection
‒ Refraction
‒ Scattering
‒ Absorption and Attenuation
Specular Reflection
Transducer
Reflected Wave
Propagated Wave
Medium 1 (Ζ1)
Medium 2
(Ζ2)
Interface
Incident Wave
‒ If Ζ1=Ζ2 then Zero Reflection
‒ If Ζ1<Ζ2 or Ζ1 >Ζ2 then Partial Reflection in interface
‒ If Ζ2>>Ζ1 then Total Reflection
Interface Reflectivity Reflection(%)
Soft tissue – Water 0,0025 0,25
Fat – Kidneys 0,0064 0,64
Muscles – Blood 0,007 0,74
Fat – Muscles 0,01 1,08
Bones – Muscles 0,410 41,00
Bones – Fat 0,476 47,60
Soft tissue – Crystal PZT 0,792 79,20
Soft tissue – Air 0,999 99,90
Specular Reflection (2)
Water – ‘Friend’ of Ultrasound
Air – ‘Enemy’ of Ultrasound
Refraction
The beam bending occurs due to the fact that the portion of the wavefront in
the second medium travels at different velocity from that of the first medium
θr
θt
θi Incident wave
Reflected Wave
Refracted Wave
Interface
c1
c2
Scattering
1. Scattering due to diffuse reflection in a rough interface
2. Rayleigh Scattering in heterogeneous media
Scattering
Scattering occurs due to random redirection of sound waves to multiple
directions in rough surfaces
Incident beam
Scattering waves
Rayleigh Scattering
Particles with comparable diameter with the US wavelength create
multiple wavelets
Transducer
Scattering waves
Incident beam
Scattering – Speckle
The several echoes generated simultaneously interact with each other:
1. They may arrive in the transducer reinforced (constructive
interference)
2. They may arrive in the transducer canceled (destructive
interference)
Scattering
Reflection, Refraction
Reflection, Refraction, Scattering
The displayed granular – dot pattern is
called speckle
All US system manufactures have embedded
various anti-speckle algorithms towards
imaging optimization
Absorption and Attenuation
Ultrasonic energy is transformed into other energy forms, primarily
heat
It is related to the beam’s frequency and the medium’s viscosity and
relaxation time
Ultrasound amplitude reduction due to absorption, scattering,
reflection and refraction
The initial intensity of an ultrasonic wave is decreased approximately
by 100.000 times propagating in soft tissue for 10 cm
Absorption
Attenuation
Transducers
Phased Array Convex Array Linear Array
Transesophageal Transvaginal - Endorectal Μatrix Arrays
Τ1 Τ2 Τ3 ΤΝ
ΚρύσταλλοιPZT
How US beam is formatted? Sequencing
Number of PZT crystal within a transducer: ~200
Each time ~9 of them are triggered with a sequencing step of 1 transducer (1 to 9, then 2 to 10 …)
1000 times/sec
Linear Array
Muscoloceletal, Circulatory_system (arterial – venous), Endocrinology, Breast
Frequency: 5 – 15 ΜHz Field of View: ~ 900
Phased Array
Lesser Number of crystals
Simultaneous triggering of all crystal with electronic delays
Small acoustic window - Large Field of View – Cardiac applications
Ultrasound Field
Large Number of crystals – 64x64
Block by block triggering 8x8 – 16x16
Electronic focusing in both lateral and elevation planes
Matrix Arrays
‒ Α – Mode
‒ B – Mode
‒ Μ – Mοde
‒ Doppler Imaging
‒ Harmonic Imaging
‒ Compound Imaging
‒ Elastography
‒ 4D Imaging
Imaging Modes
1. Tissue Harmonic Imaging
2. Contrast Enhanced Harmonic Imaging
Tissue Harmonic Imaging
• During the pulse phase – nominal frequency is transmitted
• During the echo phase, beside the reflected waves with the same
frequency, additional waves are produced with frequency multiplied
by a factor 2,3,… (2nd harmonics, 3rd harmonics,…)
Second Harmonic Imaging
LinearOscillation
Non - Linear Oscillation
Dissruption
NominalFrequency
Enhancement
HarmonicsEnhancement
Massive backscattering
Power - MI
Contrast Enhanced Harmonic Imaging
low MI power oscillates the micro-bubbles at the insonation frequency
At higher power levels the amplitude oscillations are no longer synchronized
Further increase of the MI will results to the disruption of the micro-bubble
References
• WR Hedrick, DL Hykes, DE Starchman, Ultrasound Physics and Instrumentation (Mosby, New York,
1995) pp. 7, 19, 98±109.
• Angelsen, B.A.J., 2000a. Ultrasound Imaging: Waves, Signals and Signal Processing, vol. I. Emantec
AS.
• Hill CR , Physical Principles of Medical Ultrasonics (Ellis Horwood Ltd. from John Wiley and Sons, New
York, 1986) pp. 225±257.
• Bureau, J.-M., Steichen, W., Lebail, G., 1998. A two-dimensional transducer array for real-time 3D
medical ultrasound imaging. In: Proceedings of the IEEE Ultrasonics Symposium.
• Reid J.M., “Doppler Ultrasound,” IEEE Engineering in Medicine and Biology Magazine, Vol. 6, No. 4,
pp. 14-17, December 1987.
• Averkiou, M.A., 2000. Tissue harmonic imaging. In: Proceedings of the IEEE Ultrasonics Symposium,
vol. 2, pp. 1563–1572. Beyer, R.T., 1974. Nonlinear Acoustics. Department of the Navy.