Physics & Instrumentation of

Diagnostic Ultrasound

Dr. Stavros Tsantis

Department of Biomedical Engineering

Technological Educational Institute of Athens

email: stsantis@teiath.gr

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

Frequency vs. Penetration Depth

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

Convex Array

Frequency: 1.5 – 4 ΜHz Field of View: ~ 900 Abdominal, Ob/Gyn, Pathology

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

Contemporary Ultrasound Systems

‒ Α – Mode

‒ B – Mode

‒ Μ – Mοde

‒ Doppler Imaging

‒ Harmonic Imaging

‒ Compound Imaging

‒ Elastography

‒ 4D Imaging

Imaging Modes

Α-Mode

B- Mode

Doppler Mode

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

Tissue Harmonic Imaging

Contrast Enhanced Harmonic Imaging

Intravenous Injection of targeted Contrast Agents (<0.5 μm)

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

Without Contrast Agents With Contrast Agents

Contrast Enhanced Harmonic Imaging

4D Ultrasound Imaging

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.

Thank You…