1.
Ultrasound transducer and
interaction of
ultrasound with tissues.
Doppler USG
Pradeep Kumar

2.
Basic Component
Basic components of US comprises of:
• Transducer
• Control panel
• Pulse generator
• Amplifier
• Scan converter
• Image memory and
• Image display.

3.
Transducer
Convert an electrical energy to
ultrasonic energy and vice versa.
Constituents
1. Piezoelectric Crystal
2. Two electrodes
3. Backing block
4. Matching layer
5. Plastic Housing

4.
Piezoelectric Crystal
 It is the functional component of the transducer
Characterized by well defined molecular arrangement of electrical
dipoles
An electrical dipole is molecular entity containing positive and
negative electrical charge that has no net charge .
Natural : Quartz
Ceramics: PZT, PbTiO3, PLZTio3
Polymers: PVDF
Latest: PMN-PT

5.
An external voltage source applied to the element surfaces causes compression or
expansion from equilibrium by realignment of the dipoles in response to the electrical
attraction or repulsion force.

6.
Matching layer
The matching layer provides the interface between the
transducer element and tissue and minimize the acoustic
impedance difference b/w transducer and patient
 made from epoxy resin ,perspex .
It reduce the acoustic impedance difference between soft
tissue and PZT so that more ultrasound is transmitted and
less is reflected .
 thickness 1/4th the wavelength of the ultrasound
produced .

7.
Backing block
Aka damping material: layered on the back of the piezoelectric
element ,
Damping material used is mixture of metal powder (tungsten or
aluminum), plastic or epoxy resin
It absorb the backward directed ultrasound energy and
attenuates stray ultrasound signal from the housing .

8.
Outer casing
Made by a strong plastic ,and it prevent from passing into the
housing
Acts for housing for active material
Insulate from electrical noise

9.
Electrodes
The surface of the piezoelectric crystal are placed with gold or silver electrodes
The outside electrode is grounded to protect the patient from electrical shock and
its outside is coated with a watertight electrical insulator
The inside electrodes abuts against a thick backing layer .

10.
Types of Transducer
1 Mechanical Scanner(Transducer is mechanically moved to form
images in real time.)
2. Electronic Scanner

11.
Electronic Array Transducer
Current using technology
Basically are of two types-
- Linear array- produces rectangular scan
- Phased/steer array- produces sector scans.
Both are composed of many small rectangular transducer elements ( 2x10mm)
arranged adjacent to each other.
Unlike mechanical, the transducers in electronic array system do not move. US
beam is moved by electronic controls.

12.
Linear Array
 Individual elements arranged in linear fashion
 256 to 512 small rectangular elements each 2mmx10mm arranged in a line
 By firing the elements in sequence either individual or in group a series of
parallel pulse is generated
 In linear array each time only a group of elements work together to transmit or
receive
 Size of FOV is equal in both
the far field and near field
USES: superficial structure such
as vessels , neck , liver texture

13.
Linear array transducer is operated by applying voltage pulses
to groups of elements in succession . Each group of element
acts as a larger transducer element .
The width of the image is approx. equal to the length of the
array

14.
Curved Array
Linear array that have been shaped into convex curves
Gives sector display with relatively larger field of view.
High frequency curved array transducers are often used in
transvaginal and transrectal probes and for pediatric imaging.
It is constructed as a curved line of elements rather than a
straight line.
Operation is similar to that of linear array
Applications:
Abdominal
Obstetric
Transabdominal pelvic scan

15.
Focused array
Focused transducer restrict beam width & improve lateral resolution
but they are designed to focus at a specific depth so that it can pass
through space between structures & permit them to appear as
separate entities
Focusing is done by-
i)The lens material use to focus the beam are
polystyrene or epoxy resin
ii)By crystal with a concave face
ii)By electronic focusing(steering)

16.
Contd..
A focused ultrasound transducer
produces a beam that is narrower
at some distance from the
transducer face than its dimension
at the face of the transducer.
In the region where the beam
narrows (termed the focal zone of
the transducer), the ultrasound
intensity may be heightened by
100 times or more compared with
the intensity outside of the focal
zone.

17.
Phased Array
Also called electronic sector Transducer
Generally 32 elements in the array
Operates at frequency of 2-3 MHz
Sector scan obtained
Transducer does not move during imaging.
Beam can be directed or steered at the desired angle by choosing appropriate
time delay between stimulation of individual elements. In the similar way beam
can be focused as well. Steering and focusing can be achieved at the same time.
 it steer the beam by applying different time delay on each elements.
Provide very broad imaging field at larger depth

18.
Types of phased array
• Convex phased array
• Linear phased array
• Focused phased array

19.
Convex phased array
It is operated by applying voltage pulse to all elements(not a small
group) but with small time differences (phasing) so that the
resulting sound pulse may be sent over a specific path
If the same time difference are used, then the process is repeated
and the sound pulse is produced from the same direction
However the time differences ( phasing) are changed with each
successive repetition so that the beam direction is continually
changed. This can then result sweeping of the beam
Used in spectral Doppler & color flow imaging

20.
Linear phased array
Phasing can be applied to the group in a linear array transducer to steer pulses in
a direction other than perpendicular to the array or in various directions
Also called vector array
Image is similar to that of convex phased array except that the contact surface is
smaller & the top of the display is flat
More elements can be used at one time thus provide larger aperture compared
to convex phased array

21.
Focused phased array
Phasing provides electronic control of the location or depth of the focus
Multiple focuses can be employed. However a pulse can be focused at only one
depth therefore multiple focuses require multiple pulses
Use of multiple pulses per scan line takes more time & the frame rate is reduced
thus decreasing temporal resolution but increasing detail resolution at the same
time.

22.
Annular Array
A series of concentric elements nested within one another in a
circular pieces of piezoelectric crystals
Use of multiple concentric elements enables precise focusing with
a cone shaped beam reducing the beam width in the scan plane &
perpendicular to it
Reduces section thickness artefact

23.
Some other transducers
Capacitive Micromachined US Transducers
Wireless transducer
Dual element transducers

24.
Dual element transducers

25.
TRANSDUCERS FOR SPECIFIC PURPOSE
Trans-oesophageal Transducer
Trans-rectal Transducer
Trans-vaginal Transducer
Endovascular Transducer
Endoscopic Transducer
Aspiration Transducer

26.
Transducer used for special purpose

27.
SELECTION OF TRANSDUCER
For General purpose-convex with 3.5 MHz
For obstetric purpose- convex or linear with 3.5 MHz
For Superficial structures-linear with 5 MHz
For pediatric or in thin people-5 MHz

28.
A short-duration voltage spike causes the resonance piezoelectric element to vibrate at
its natural frequency, which is determined by the thickness of the transducer equal to ½
lambda. Low-frequency oscillation is produced with a thicker piezoelectric element. The
spatial pulse length (SPL) is a function of the operating frequency and the adjacent
damping block.

29.
Beam focusing
For a single transducer or a linear array , the focal distance is the
function of the transducer diameter ,the center operating
frequency and presence of any acoustic lenses attached to the
element surface.
 Phase array and many linear array transducers allows a selectable
focal distance by applying specific timing delays between
transducer elements that cause the beam to converge at a
specified distance
A shallow focal zone (close to transducer surface ) is produced by
firing outer element in array before the inner element in a
symmetrical pattern
Greater focal distances are achieved by reducing the delay time
difference among the transducer elements , resulting in more
distal beam convergence.

30.
The transducer crystal are fired simultaneously with programmable delay time for transmit
focusing

31.
In the figure we can figure out variable timing circuitry that will differ the focal zone. Firing
the crystal at different time changes the position of the focal plane.

32.
A-mode
Also called amplitude mode ,echoes are
displayed as spike projecting from a base line
It is the graphical display of measurement of
tissue interface
The base line identifies the central axis of the
beam
The spikes height is proportional to echo
intensity
Uses: ophthalmology, EEC, ECHO, midline
shift in brain and together with B mode
when accurate measurement is required.

33.
B-mode
B-mode (B for brightness) is the electronic conversion of the A-mode and A-line
information into brightness-modulated dots on a display screen
In general, the brightness of the dot is proportional to the echo signal amplitude
The position of a dot along the time base is a measure of the distance of the
associated reflector from the transducer.
Used for M-mode and 2D gray-scale imaging

34.
M-Mode
M-mode (“motion” mode) or T-M mode (“time-motion” mode): displays time
evolution vs. depth
M-mode is valuable for studying rapid movement, such as mitral valve leaflets
Sequential US pulse lines are displayed adjacent to each other, allowing
visualization of interface motion
The transducer is placed in one fixed position in relation to the moving structure.
Returning echoes are displayed in the form of dots of varying intensity along a
time base as in B mode.

35.
Coupling Agent
• The coupling agent prevents air
pockets forming between the skin and
the transducer.
• Composition:
Carbomer : 10.0 gm Propylene
Glycol : 7.5 gm
Theolamine : 12.5 gm
EDTA(edetic acid) : 0.25 gm
Distilled water : upto 500 ml

36.
• One of the problem that must be considered is the attenuation of the
ultrasonic beam with depth.
• Equally reflective interface produce different signal levels depending
on their relative distance from the transducer.
• It is often advantageous to display reflectors of similar size, shape and
equal reflection coefficient with equal signal strength or brightness
levels.
• Exponential amplification is used to correct the signal for attenuation
known as Time Gain Compensation(TGC)

37.
Time Gain Compensation
It is a way to overcome ultrasound attenuation in which signal
gain is increased as time passes from the emitted wave pulse.
This correction makes equally echogenic tissue look the same
even if they are located in different depths.
Attenuation: decrease in intensity of sound as they pass through
the tissues measured in db/cm.
Gain: amplification of the reflected ultrasound waves by the
ultrasound unit. Deeper tissues need more gain.
Acoustic impedence: resistence offered by the tissues to the
movement of particles caused by ultrasound waves. Density x
velocity

38.
Pulser
Provides the electrical voltage for excitation of Piezoelectric element and controls
the output transmit power by adjustment of applied voltage.
Increase of transducer amplitude results in the increase of sound intensity so
higher will be the SNR.
Output power is usually indicated in terms of thermal index (TI) and mechanical
index(MI).

39.
Ultrasound waves when strike the medium, cause expansion and
compression of the medium. Various types of interactions
Reflection
Scattering
Refraction
Absorption
Attenuation.

40.
• Reflection: when us is deflected towards the transducer. The major factors affecting the amount
of reflection are :
• Angle of incidence.
• Acoustic impedence mismatch
• Width of tissue boundry . If less than wavelength wont be reflected
• Angle of tissue boundry.

41.
• Scattering : occurs when the width or lateral dimension of the tissue boundary is
less than one wavelength. RBC
• Refraction: when ultrasound beam is deflected from a straight path and angle of
deflection is away from the transducer. Enhanced image quality by using acoustic
lenses. Can result in ultrasound double image artefact.
• Attenuation: depends upon 1.transducer frequency low freq low attenuation.
2. Acoustic impedance
3.Acoustic impedance mismatch

42.
• Absorption:
1. Dissipation of sound energy into the medium.
2. Energy transformed into other form (heat).
3. Medical application of therapeutic US.

43.
Doppler ultrasound
Christian Doppler
(1803-1853)

44.
Doppler Effect
• Change in the perceived frequency of sound produced by a moving
source.
• Basis of Doppler Ultrasonography: Reflected/scattered ultrasonic
waves from a moving interface will undergo a frequency shift

45.
• In diagnostic ultrasound, the Doppler effect is used to measure blood
flow velocity.
• When emitted US beam strikes moving blood cells, the latter reflect
the pulse with a specific Doppler Shift frequency that depends upon
the velocity and direction of the blood flow.

46.
Factors affecting Doppler signal
• Blood velocity
• Ultrasound frequency
• Angle of insonation

47.
• Choice of frequency is a compromise between better sensitivity to
flow or better penetration;
• Doppler ultrasound equipment : filters to cut out the high amplitude,
low-frequency Doppler signals from tissue movement, (e. g. due to
vessel wall motion). Filter frequency can usually be altered by the
user, for example, to exclude frequencies below 50, 100 or 200 Hz
(limits the minimum measureable flow velocities).

48.
Doppler techniques
1. Continuous wave Doppler
2. Pulsed Doppler: Color, Power and Spectral Doppler

49.
Continuous Doppler
• Uses two crystals, one to send and one to receive.
• Uses continuous transmission and reception of ultrasound.
• Doppler signals are obtained from all the vessels in the path of US
beam (until it gets sufficiently attenuated due to depth).

50.
• Unable to determine the specific location of velocities within the
beam and cannot be used to produce color flow images.
• Used in adult cardiac scanners to investigate the high velocities in the
aorta.

51.
Pulsed Wave Doppler
• Same transducer as transmitter & receiver.
• The sensitive volume from which flow are sampled can be controlled
in terms of shape, depth and position.
• Transmitter mode → short duration bursts of pulses at precise rate of
repetition (Pulse repetition frequency or PRF) → receiver turned on
(gating on) for short period at specified time → returning signals
(echoes).
• Used in general and obstetric Doppler ultrasound scanner. Provides
data for Doppler sonograms and color flow images.

52.
Advantages:
1. Selection of site & sample volume for Doppler.
2. No overlap of information from other
vessels/structures.
Disadvantages:
1. Angle restriction if same transducer for imaging &
Doppler.
2. Flow information from one site in image.

53.
Flow imaging Modes
• Color Flow
• Power/energy/amplitude flow
• Spectral Doppler

54.
Color Doppler
 to identify vessels for examination,
 to identify the presence and direction of flow,
 to highlight gross circulation anomalies, throughout the entire color
flow image,
 to provide beam/vessel angle correction for velocity measurements.

55.
Advantage of DCFI
1. Technical efficiency → small vessel detection.
2. Global Doppler sampling.
3. Vascular evaluation: entire luminal evaluation, vascular vs non
vascular structures, stenotic measurements, severe stenosis vs
occlusion, flow direction (reversal /reflux), turbulence at high
velocity gates.
4. Rapid acquisition of data.

56.
Limitations
1. Qualitative flow information (mean velocity at each
point).
2. Limited PRF → limited Doppler shift frequency →
aliasing.
3. No accurate information of abnormal flow like range
gated Doppler.
4. Angle dependent flow information → stenosis vs
occlusion.
5. Obscured vascular pathology d/t flow artifacts.
6. Color out by other moving structures (peristalsis).
7. High cost.

57.
Power Doppler
(Energy Doppler/ Amplitude Doppler/ Doppler angiography)
Features:
a. The magnitude of the color flow output is displayed
& not Doppler frequency signal.
b. Does not display flow direction or different
velocities (no directional information)
c. Sensitive to low flows (used in conjunction with
frame averaging to increase sensitivity to low flows
and velocities). Hybrid color flow modes
(incorporating power and velocity data )are also
available from some manufacturers. These can also
have improved sensitivity to low flow Complements
other two modes

58.
Duplex Scanning
• Real time B-mode scanners with built in Doppler Capabilities.
• B-mode: Outlines anatomic structures
• Pulsed Doppler: Flow and movement patterns
Triplex mode: When combined with spectral analysis

59.
Spectral Doppler
• Display in Graphic form as time varying plot of the frequency
spectrum of the returning signal, Provides Quantitative information.
• Fast Fourier transformation used to perform frequency analysis.
• Gate: Used to determine the interval after emission when returning
signals are received and therefore the depth from which sample is
taken.

60.
Doppler Spectrum Assessment
• Presence of flow
• Direction of flow
• Amplitude
• Window
• Pulsatility

61.
How to improve the sensitivity?
• Increase power or gain
• Decrease velocity scale
• Decreasing reject or filter
• Slowly increasing the SV size.

62.
Direction of flow
• Monophasic
• Biphasic
• Triphasic
• Bidirectional

63.
Amplitude
• Displayed as the brightness of display
• Determined by:
• Echo intensity
• Power
• Gain
• Dynamic range

64.
Window
• The received shift consists of a range of frequencies.
• Narrow range of frequencies will result in a narrow display line.
• The clear underneath the spectrum is called window.

65.
Spectral Widening
Loss of the spectral window
Occurs:
• As the blood decelerates in diastole
• SV placed close to vessel wall
• In small vessels (parabolic velocity profile)
• Tortuous vessels
• Low flow states
• Excessive gain/power/dynamic range

66.
Pulsatility
• Measures the difference between maximum and minimum velocities
within the cardiac cycle.
• Unitless
• Increase in pulsatility means increase in all the values.
• No need of knowledge of Doppler angle.

67.
Doppler Indices
• PI = S-D/Vm (Gosling index)
• RI= S-D/S (Pourcelot index)
• S/D ratio
• Acceleration time (AI) and Acceleration Index (AI).
• Spectral broadening

68.
Doppler artifacts
Artifacts related to
inappropriate settings
Anatomically related
artifacts
Instrument and
processor related
artifacts
1. Doppler gain setting
errors
2. Velocity settings
errors and aliasing
3. Inappropriate wall
filter settings
1. Mirror imaging
2. Vascular motion
artifact
3. Color in
neurovascular
structures
1. Directional
ambiguity
2. Grating or side lobe
artifact
3. Spectral broadening

69.
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