Ultrasound Imaging System Ultrasound physics

1. Lecture 10
Ultrasound Imaging
System

2. Ultrasound Beam Properties
• The ultrasound beam propagates as a longitudinal wave from
the transducer surface into the propagation medium.
• It exhibits two distinct beam patterns:
• The near field
a slightly converging beam out to a distance determined by the geometry
and frequency of the transducer.
• The far field
a diverging beam beyond the end point of converging.

3. The formulas are
Applicable only for
a single-element,
transducer .
Ultrasound Beam Properties

4. Transducer Array Beam Focusing
• Transmit Focus
• For a single transducer or group of simultaneously fired transducers in a linear
array, the focal distance is a function of the transducer diameter, the center
operating frequency, and the presence of any acoustic lenses attached to the
element surface.
• This focal depth is unchangeable.
• Phased array transducers and many linear array transducers allow a selectable
focal distance by applying specific timing delays between transducer elements
that cause the beam to converge at a specified distance.
• Greater focal distances are achieved by reducing the delay time differences
amongst the transducer elements, resulting in more distal beam convergence.

6. • Receive Focus
• Dynamic receive focusing is a method to
rephase the signals by dynamically
introducing electronic delays as function
of depth (time).
• The phase delay circuitry for the receiver
varies as a function of echo listening time.
• In addition to phased-array transducers,
many linear array transducers permit
dynamic receive focusing amongst the
active element group.
Transducer Array Beam Focusing

7. Spatial Resolution
• The major factor that limits the spatial resolution and
visibility of detail is the volume of the acoustic pulse.
• The axial, lateral, and elevational (slice thickness)
dimensions determine the minimal volume element.
• Axial resolution
• refers to the ability to discern two closely spaced objects in
the direction of the beam.
• Achieving good axial resolution requires that the returning
echoes be distinct without overlap.

8. Axial Resolution
Separation
just greater
than half the
spatial
pulse length
Gap;
Separate
Echoes
Axial Resolution = Spatial Pulse Length / 2

9. Axial Resolution
Separation
just less
than half the
spatial
pulse length
Overlap;
No Gap;
No Separate
Echoes
Axial Resolution = Spatial Pulse Length / 2

10. • Lateral resolution
• refers to the ability to discern as separate two closely spaced objects
perpendicular to the beam direction.
• For both single-element transducers and multielement array transducers, the
beam diameter determines the lateral resolution
• Since the beam diameter varies with distance from the transducer in the near and
far field, the lateral resolution is depth dependent.
• The lateral resolution of linear and curvilinear array transducers can be varied.
• For the phased-array transducer, focusing to a specific depth is achieved by both
beam steering and transmit/receive focusing to reduce the effective beam width
and improve lateral resolution
Spatial Resolution

13. • Elevational Resolution
• Elevational resolution is dependent on the
transducer element height.
• Multiple linear array transducers with five to
seven rows, known as 1.5D transducer arrays,
have the ability to steer and focus the beam in
the elevational dimension.
• Elevational focusing is implemented with phased
excitation of the outer to inner arrays to minimize
the slice-thickness dimension at a given depth.
• By using subsequent excitations with different
focusing distances, multiple transmit focusing can
produce smaller slice thickness over a range of
tissue depths.
Spatial Resolution

14. Image Data Acquisition
• Images are created using a pulse-echo mode format of ultrasound
production and detection.
• Each pulse is directionally transmitted into the patient and experiences
partial reflections from tissue interfaces that create echoes, which return to
the transducer.
• Image formation using the pulse-echo approach requires a number of
hardware components:
• The beam former
• Pulser.
• Receiver.
• Amplifier.
• Scan converter/image memory.
• Display system.

17. • Beam Former
• The beam former is responsible for generating the electronic delays for individual transducer
elements in an array to achieve transmit and receive focusing and, in phased arrays, beam
steering.
• Most modern, high-end ultrasound equipment incorporates a digital beam former and digital
electronics for both transmit and receive functions.
• Pulser(Also known as transmitter)
• provides the electrical voltage for exciting the piezoelectric transducer elements and controls
the output transmit power by adjustment of the applied voltage.
• In digital beam former systems, a digital-to-analog converter (DAC) determines the amplitude
of the voltage.
• An increase in transmit amplitude creates higher intensity sound and improves echo
detection from weaker reflectors.
• User controls of the output power are labeled “output,” “power,” “dB,” or “transmit” by the
manufacturer.
Image Data Acquisition

18. • Transmit/Receive Switch
• The transmit/receive switch, synchronized with the pulser, isolates the high
voltage associated with pulsing (~150 V) from the sensitive amplification
stages during receive mode, with induced voltages ranging from
approximately 1 V to 2 μV from the returning echoes.
• Preamplification and Analog-to-Digital Conversion
• In multielement array transducers, all preprocessing steps are performed in
parallel.
• Each transducer element produces a small voltage proportional to the pressure
amplitude of the returning echoes.
• An initial preamplification increases the detected voltages to useful signal levels.
Image Data Acquisition

19. • Preamplification and Analog-to-Digital Conversion
• In state-of-the-art ultrasound units, each piezoelectric element has its
own preamplifier and ADC.
• A typical sampling rate of 20 to 40 MHz with 8 to 12 bits of precision is
used.
• ADCs with larger bit depths and sampling rates are necessary for systems
that digitize the signals directly from the preamplification stage.
• In systems where digitization of the signal occurs after analog beam
formation and summing, a single ADC with less demanding requirements
is typically employed.
Image Data Acquisition

20. • Beam Steering, Dynamic Focusing, and Signal Summation
• Echo reception includes electronic delays to adjust for beam direction and
dynamic receive focusing to align the phase of detected echoes from the
individual elements in the array as a function of echo depth.
• In digital beam former systems, this is accomplished with digital processing
algorithms.
• The output signal represents the acoustic information which is sent to the
receiver for further processing before rendering into a 2D image.
Image Data Acquisition

22. • Receiver
• Subsequent signal processing occurs in the
following sequence
• Gain adjustments and dynamic frequency tuning.
• Dynamic range (logarithmic) compression
• Rectification, demodulation, and envelope
detection.
• Rejection
• viewing on monitors
Image Data Acquisition

23. Echo Display Modes
•A-mode(Amplitude Mode)
• The A mode is the simplest form of ultrasound imaging and is not
frequently used. The ultrasound wave that comes out of the probe travels
in a narrow pencil like straight path.
• One use of the A scan is to measure length. For example, ophthalmologists
can use it to measure the diameter of the eye ball. Imagine that the red
circle below is the eye ball and you want to measure its diameter.
• An ultrasound wave is sent from the probe and at the same instance, a line
from the left of the screen starts to be drawn. This line moves horizontally
measuring time

25. •B Mode (Brightness mode)
• The B scan mode is very similar to the A scan mode.
• The strength of the returning wave is recorded by a bright dot. The brightness
of the dot represents the strength of the returning wave. The brighter the dot,
the stronger is the returning wave. The letter “B” of “B scan” represents
Brightness.
• In real life, the process happens very quickly. The structures are scanned and
the image redrawn many times a second.
Echo Display Modes

27. •M Mode (Motion Mode)
• M-Mode ultrasound displays the position of structures with respect
to time. Therefore, M-Mode is a good indicator of the velocity of
structures that move within the body. A typical example would be the
heart valves. The heart valves can be identified using the B-Mode,
then the M-Mode could be used to determine the rate at which the
valves open and close. The slope of the changing position of the valve
flaps with respect to time (as displayed by the M-Mode) can be
calculated as a valve velocity.
Echo Display Modes