Ultrasound Imaging System

1. Ultrasound Imaging
System
Lecture 9

2. Introduction
• Medical uses of ultrasound came about shortly after the close of World
War II, derived from underwater sonar research.
• Ultrasound rapidly progressed through the 1960s using analog electronics.
• Advances in equipment design, data acquisition techniques, and data
processing capabilities have led to electronic transducer arrays, digital
electronics, and real-time image display.
• ultrasound provides
• two-dimensional (2D) tomographic imaging
• anatomical distance and volume measurements
• motion studies
• blood velocity measurements
• 3D and 4D (3D with real time) imaging.

3. Basics Physics
• Sound is mechanical energy that propagates longitudinally(antonym
for transverse) through a continuous, elastic medium by the
compression and rarefaction of “particles” that comprise it.
• The wavelength (ʎ) of the ultrasound energy is the distance (usually
expressed in units of mm or um) between compressions or
rarefactions.
• The frequency(f) is the number of times the wave oscillates through
one cycle each second (s).

5. • Sound waves with frequencies less than 15 cycles per second (Hz) are
called infrasound
• sound waves with frequencies below the lower limit of human audibility.
• the range between 15 Hz and 20 kHz
• comprises the audible acoustic spectrum.
• Ultrasound represents the frequency range above 20 kHz.
• Medical ultrasound uses frequencies in the range of 2 to 10 MHz,
with specialized ultrasound applications up to 50 MHz.
• The speed of sound is the distance traveled by the wave per unit time
and is equal to the wavelength divided by the period.
Basics Physics

6. • The speed of sound is dependent on the propagation medium and
varies widely in different materials.
• The wave speed is determined by
• B is the ratio of the bulk modulus
• a measure of the stiffness of a medium and its resistance to being compressed)
• ρ is the density of the medium
Basics Physics

7. • A highly compressible medium, such
as air, has a low speed of sound,
while a less compressible medium,
such as bone, has a higher speed of
sound.
• A less dense medium has a higher
speed of sound than a denser
medium (e.g., dry air versus humid
air).
• The speeds of sound in materials
encountered in medical ultrasound
are listed in the table.
Basics Physics

8. • The difference in the speed of sound at tissue boundaries is a fundamental property that generates
echoes (and contrast) in an ultrasound image.
• The ultrasound frequency is unaffected by changes in sound speed as the acoustic beam propagates through different
media.
• Thus, the ultrasound wavelength is dependent on the medium.
• A high-frequency ultrasound beam (small wavelength) provides better resolution and image detail
than a low-frequency beam; however, the depth of beam penetration is significantly reduced at
higher frequency.
• For body parts requiring greater travel distance of the sound waves (e.g., abdominal imaging), lower frequency
ultrasound is used (3.5 to 5 MHz) to image structures at significant depths.
• For small body parts or organs close to the skin surface (e.g., thyroid, breast), higher frequency ultrasound is selected
(7.5 to 10 MHz).
Basics Physics

9. Interactions of Ultrasound with Matter
• Ultrasound interactions are determined by the acoustic properties of
matter.
• As ultrasound energy propagates through a medium, interactions
include reflection, refraction, scattering, attenuation, and absorption.
• The acoustic impedance (Z) of a material is defined as
• Z= ρ c
• ρ is the density in kg/m3
• c is the speed of sound in m/s.
• The SI unit for acoustic impedance is kg/(m2s), with the special name the rayl
• 1 rayl is equal to 1 kg/(m2s).

11. Reflection
• The reflection of ultrasound energy at a boundary between two
tissues occurs because of the differences in the acoustic impedances
of the two tissues.
• The reflection coefficient describes the fraction of sound intensity
incident on an interface that is reflected.
• For perpendicular incidence the intensity reflection coefficient, Ri, is
expressed as

12. • The intensity transmission coefficient, Ti, is defined as the fraction of
the incident intensity that is transmitted across an interface.
• The intensity transmission coefficient is Ti = 1 − Ri
• For a fat-muscle interface, the intensity reflection and transmission
coefficients are calculated as
Reflection

13. Ultrasound Transducers
• Ultrasound is produced and detected with a transducer, comprised of
one or more ceramic elements with electromechanical properties and
peripheral components.
• A piezoelectric material (often a crystal or ceramic) is the functional
component of the transducer.
• Piezoelectric materials are available from natural and synthetic materials.
• An example of a natural piezoelectric material is quartz crystal and of
synthetic materials is lead zirconate titanate (PZT)
• It converts electrical energy into mechanical (sound) energy by physical
deformation of the crystal structure.
• Conversely, mechanical pressure applied to its surface creates electrical
energy.

15. • The majority of ultrasound systems employ transducers with many
individual rectangular piezoelectric elements arranged in linear or
curvilinear arrays.
• Typically, 128 to 512 individual rectangular elements comprise the transducer
assembly
• The thickness of the PZT layer should be equal to the half of the intended
wavelength.
• Because resonance frequency of the PZT is determined at this thickness.
• The matching layer provides the interface between the raw transducer
element and the tissue and minimizes the acoustic impedance differences
between the transducer and the patient.
• The thickness of each layer is equal to ¼ wavelength.
Ultrasound Transducers

16. • In addition to the matching layer, acoustic coupling gel (with acoustic
impedance similar to soft tissue) is used between the transducer and
the skin of the patient to eliminate air pockets that could attenuate
and reflect the ultrasound beam.
• Modern transducer design coupled with digital signal processing
enables “multifrequency” or “multihertz” transducer operation,
whereby the center frequency can be adjusted in the transmit mode.
• Broadband multifrequency transducers have bandwidths that exceed
80% of the center frequency.
Ultrasound Transducers

18. Transducer Arrays
• Two modes of activation are used to produce a beam.
• The “linear” (sequential).
• Linear array transducers typically contain 256 to 512 elements
• In operation, the simultaneous firing of a small group of approximately 20
adjacent elements produces the ultrasound beam.
• Echoes are detected in the receive mode by acquiring signals from most of
the transducer elements.
• A rectangular field of view (FOV) is produced with linear transducer
arrangement.
• For a curvilinear array, a trapezoidal FOV is produced

19. • Two modes of activation are used to produce a beam.
• The “phased” activation/receive.
• A phased-array transducer is usually comprised of 64 to 128 individual
elements in a smaller package than a linear array transducer.
• All transducer elements are activated nearly simultaneously to produce a
single ultrasound beam.
• During ultrasound signal reception, all of the transducer elements detect
the returning echoes from the beam path, and sophisticated detection
algorithms synthesize the data to form the image.
Transducer Arrays

21. Up to Date Transducer
• Another method of producing high-frequency ultrasound is with the use of
capacitive micromachined ultrasound transducers (CMUT).
• The basic element of a CMUT is a capacitor cell with a fixed electrode (backplate) and a free
electrode (membrane).
• The principle of operation is electrostatic transduction
• Whereby an alternating voltage is applied between the membrane and the backplate
• The modulation of the electrostatic force results in membrane vibration with the generation of
ultrasound.
• Conversely, when the membrane is subject to an incident ultrasound wave, the capacitance
change can be detected as a current or a voltage signal.
• The main advantages of CMUT arrays compared to PZT are better acoustic
matching with the propagation medium, which allows wider bandwidth
capabilities, improved resolution, potentially lower costs with easier fabrication,
and the ability to have integrated circuits on the same “wafer.”