Các loại đầu dò Siêu Âm (Ultrasound Transducers)

Last WeekUltrasound Wave Parameters Frequency, Wavelength, Period Intensity, Power Pulse length, Pulse duration, Axial resolution Ultrasound wave propagation Reflection, Scattering, Abso

Ultrasound Transducers

Ravi Managuli

Last Week

Ultrasound Wave Parameters

Frequency, Wavelength, Period

Intensity, Power

Pulse length, Pulse duration, Axial resolution

Ultrasound wave propagation

Reflection, Scattering, Absorption

Ultrasound interaction with the media

Reflection, Transmission Refraction

Spatial pulse length Spatial pulse duration

Beamformer

Clinical Media

(organ system)

Transducer : Many types

A device that converts one form of energy

into another form of energy

Ultrasound Transducers

Major parts of a transducer

Piezoelectric (PZT) elements – functional component of the transducer : Frequency

Matching layer – reduces acoustic impedance b/w PZT & tissue Backing (damping) block – absorbs backwards directed (stray) U/S from housing

Bandwidth (pulse length)

c.f Bushberg, et al The Essential Physics

of Medical Imaging, 2nd ed., p 484.

Ultrasound Transducer

Lateral direction (x) Elevation direction (y)

Axial direction (z)

Axial Resolution

Axial resolution: Half of spatial pulse length

• Inversely proportion to the frequency

• Axial resolution of 12 MHz probe is better than 5 MHz

Spatial pulse length

Cannot resolve : Reflections overlap

Bandwidth is the useful range of contiguous frequencies over which

transducer can operate.

Ultrasound transducer typically have multiple frequencies

For example

C5-1 transducer means : It has frequencies ranging from 1 to 5 MHz L12-5 means frequencies ranging from 5 to 12 MHz

Bandwidth : Inversely proportional to number of cycles present in a pulse

Bandwidth is related to spatial pulse length

Large bandwidth  Small pulse length  Better axial resolution BW-mode

Narrow bandwidth  Long pulse length  Worse axial resolution Color-Doppler

(little dampening) (heavy dampening)

Lateral Resolution

Ability to resolve two structure laterally

Best in the near zone Depends upon the transducer width

•

Focal point

Elevation Resolution (Slice Thickness)

Elevation resolution is the dimension perpendicular to the image plane

• Depends on the transducer element height

• Axial resolution is better than lateral resolution

• Later resolution is better than elevation resolution

Ultrasound Transducer

thickness

transducer in

speed Sound

Frequency  2

wavelength = Thickness * 2 Smaller the thickness  Higher frequency

Connector

Aluminum-filled epoxy

Backing (Damping) Material

Reduces oscillations of the transducer element

Reduces the number of cycles

Reduced spatial pulse length

Ultrasound TransducerSignal strength

Matching Layer

Matching layer sits between PZT and skin

Reduces the impedance mismatch between transducer and skin

Otherwise most of the ultrasound would be reflected back from skin

Increases the efficiency of sound energy transfer

Impedance of PZT > Matching layer > Gel Impedance > SkinThickness of this layer: wavelength/4

Housing

Absorber PZT

Lens Matching

Electrical

Connector

Ultrasound Transducer

Lateral direction (x)?

Elevational direction (y)?

Axial resolution (z) : Depends upon thickness of transducer,

Lateral Resolution

Lateral resolution depends transducer diameter  Decides the beam path

f D D

lateral

• Lateral resolution depends upon the beam diameter

• Best at the focal point

• Diameter/2

2 2

D c

f

D z

zone



D c

f

D z

zone



Fixed Focusing

External focusing

Curved acoustic lens

Increased curvature increases

focusing

Internal focusing

Curved crystal

Increased curvatureIncreases focusing

Focused

Unfocused

Still limitations of fixed focus

Cannot be controlled for different depth Curved Lens

Electronic Focusing

Electronically focus the beam

User can control the location of the focus

Only possible with the multiple element transducer

For both transmit focusing and Receive focusing

Delays Target

Delays Target

Delays Target

Delays Target

Delays Target

Delays Target

Delays Target

Delays Target

Delays Target

Delays Target

Delays Target

Electornic Focusing

Multiple-focus?

Apply different delays for different location

Electronic Focusing

Multiple-focus : Different delays

Update Steering For Different Lateral Resn

Normal connecting the majority delays indicate the beam direction

© UW and Renée Dickinson, MS

Ultrasound Image Quality

Side lobes – off-axis energy emission directed

away from the main beam

Unavoidable; remapped along the main

beam in receive mode

Wide broad bandwidth– reduces emission of

side lobe energy

Grating lobes – energy emitted far off-axis (large

angles) from the main beam by multi-element

arrays

Consequence of a non-continuous

transducer surface

Low amplitude; appears as highly reflective,

off-axis objects in the main beam

Two techniques are employed:

Types of Transducers

Types of transducers and their focusing technique

Curved/Curvilinear/convex array

Linear Phased array

Linear sequential array

Linear phased array

Annular phase array

Vector array/Linear sequential phased array

Linear Array Transducer

High frequency

Group of elements are fired in sequence

Electronically focused

Width of each element = 1 wavelength

Wide near filed of view

Excellent superficial imaging

Disadvantages

Difficult to use in small acoustic windows

Limited far field of view

1 2 3 4 5 6 7 8 9 10,,

Rectangular field of view

Convex Array Transducer

Convex/Curved Array Transducer

96 to 128 elements

Width of each element = 1 wavelength

Elements are arranged along arc

Only few elements are used for steering

Wide far field of view

Wider near-filed than phased array

Electronic focusing

Disadvantages

Difficult to use in small acoustic windows

Degradation of resolution in the far field Blunted sector shape

Ultrasound Transducer

Microconvex or transviginal Array Transducer

High frequency transducer

Wide near and far field

Phased Array Transducer

Transducer

128 elements : All elements are fired simultaneously

Width of each element

¼ to ½ of wavelength

Electronic beam steering

Advantages

Small footprint for tight acoustic windows

Rapid frame rates

Wide far filed of view

Disadvantages

Complex and expensive to manufacture

Ultrasound Transducer

Array Transducer for 3D imaging

Mechanically steering 1D array transducer

Voluson 730 (GE) / SonoAce 9900 (Medison)

Realtime 3D (or 4D)

About 4 frames / sec

Ultrasound Transducer

Array Transducer for 3D imaging

Fully 2D Array Transducer

About 15 frames/sec,

Limited FOV  Cardiac application