Understanding Ultrasound Physics Chapter 12

Mechanical Transducer

- Disc shaped

- One active element

- Fan or sector shaped

- Crystal moves with motor

- Steered mechanically

- Fixed focusing

- No image when PZT is damaged

What is the channel a combination of?

Combination of the active element, wire, and system electronics.

Types of Array Transducers

Linear

Annular

Convex

Linear

Active elements are arranged in a straight line

Annular

Elements are arranged as circular rings with a common center

Convex

Active elements are arranged in a bowed or arched line. The transducer may also be called a curved, or a curviliner array.

Linear Phased Array

- compact with a small footprint

- 100 to 300 active elements on face

- square shaped

- No moving parts

- Steered and focused with phasing

- Adjustable focus (electronic focusing)

How many elements are excited in a phased array probe during a single sound pulse?

All elements

What happens when the spike line is straight?

An unfocused sound beam is created

What determines the direction the sound beam is steered?

The slope of the electrical spike line

How is a focused beam created?

If the electrical pattern is curved

What determines the depth of the focus?

Altering the electricity (changing the curved pattern) coming from the beam former

What happens when the spike line is curved and has a slope?

It creates a focused beam that is steered

What happens when the spike curve is curved outward or in a "D" shape?

Defocused sound beam, has no application in diagnostic imaging

In Multifocusing three beams are created, what will the pattern with the greatest curvature beam create, and the pattern with the least curve produce?

Shallowest focus, and deepest focus

When the sonographer adjusts the focus what happens?

The curve changes

With dynamic receive focusing, focusing may also occur during....

Reception

When the ultrasound system introduces time delays to some of the electrical signals during the reception what happens?

A more accurate image can be created

As the transducer listens for reflections, what happens to the delay patterns during receive-focusing?

They are continuously changing

Can the sonographer control the dynamic receive focusing?

No the system does it automatically

Annular Phased Array

- Disc shaped elements

- Multiple rings with common centers (bulls eye)

- Image shape, sector or fan shaped

- Motor moving parts

- Mechanical steering

- Electrical focusing, multiple transmit focal zones

Linear Sequential Array

- Rectangular elements

- 120 to 250 rectangular shaped strips of PZT

- Image shape: rectangle

- No moving parts

- Steered electronically

- Fixed electrical focusing

Can linear sequential array transducers steer sound beams electronically?

Yes. It creates a parallelogram shaped image instead of rectangle shaped

Convex, curved, curvilinear array

- Rectangle elements

- 120 to 250 rectangle shaped PZT

- Image shape: blunt sector

- No moving parts

- No slope excitation, no all parts fire at once

- Focused electronically

Vector Array

- Rectangle elements

- 120 to 250 rectangle shaped PZT

- Image shape: Trapezoidal

- No moving parts

- Electrical steering and focusing

- Poor steering and focusing when PZT is damaged

Slice thickness resolutions is also called...

Elevational resolution

Elevational resolution is dependent on what?

Transducer style

What is created by elevational resolution?

Structures that lie above or beneath the image plane

What shape of active element creates the best elevational resolution?

Disc shaped crystals

Which transducers have the best elevational resolution?

Mechanical and annular phased array

What newer types of transducers have improved slice thickness resolution?

1 1/2-Dimensional Array transducers

2-D Arrays have how many crystals?

May contain thousands of crystals, Same number of elements in the up and down as they do in the side to side

Single elements transducers create what kind of lobes? Grated or Side

Side lobes

What type of transducer creates grated lobes?

Array transducers

Modern transducers are designed to reduce the strength of side and grating lobes. How is this accomplished?

By a process called apodization. Inner crystals are excited with stronger signals and gradually get weaker moving to the outer crystals, lobes become diminished.

Where are more side lobes created?

Far field

What do side lobes bring back?

Reflective information

Where are grating lobes created?

Near and far field

What is degraded with grating lobes?

lateral and elevational resolution degrade

How do modern transducers keep the sound beam narrower over a substantial depth range?

Dynamic Aperture. It makes sound beam narrower over a greater range of depths and optimize lateral resolution.

When the ultrasound system changes the number of crystals along the face of the probe, that is being used to transmit pulses and receive reflections, is called?

Variable, aperture, or dynamic aperture

What is an aperture?

Opening or hole

Dynamic aperture improves?

Lateral resolution at wide range of depths

With a Linear Phased Array, what shape is the image

fan or sector shaped

What does the beam former do?

it creates the electronic firing patterns for a phased array

If the firing pattern is curved then the beam is __________

focused

If the firing pattern is curved and has sloped than the beam is ________________and __________________

steered and focused

Smaller diameter crystals create ________________focus beams

shallow

Larger diameter crystals create _____________focus beams

deeper

In a mechanical transducer what is the image shape

sector

In a mechanical transducer what is the steering technique? Focusing Technique?

Mechanical and fixed

In a mechanical transducer what is happens if the crystal is damaged?

image loss

In a linear phased array transducer what is the image shape?

fan or sector shaped

In a linear phased array transducer what is the steering technique? focusing technique?

electronic and electronic

In a linear phased array transducer what happens if there is a crystal defect?

there is poor steering and focusing

In an annular phased transducer what is the image shape

sector

In an annular phased transducer what is the steering technique? focusing technique?

mechanical and electronic

In an annular phased transducer what happens if there is a crystal defect?

horizontal line dropout

In a convex sequential transducer what is the image shape

blunted sector

In a convex sequential transducer what is the steering technique? focusing technique?

electronic and electronic

In a convex sequential transducer what happens if there is a crystal defect?

vertical line dropout

In a convex phased transducer what is the image shape

blunted sector

In a convex phased transducer what is the steering technique? focusing technique?

electronic and electronic

In a convex phased transducer, what happens if there is a crystal defect?

poor steering and focusing

In a vector transducer, what happens if there is a crystal defect?

poor steering and focusing

In a vector transducer, what is the image shape

trapezoid

In a vector transducer, what is the image shape?

trapezoid

In a vector transducer, what is the steering technique? Focusing technique?

electronic and electronic

Conventional ultrasound has a _______________line of sight

single

SonoCT Real-time Compound imaging has _____________lines of sight

multiple

What do matrix array transducers improve

spatial and contrast resolution

The elements are arranged in rows AND_____________. Which enables beam forming in 3 dimensions

columns

Beam focusing was originally done with a lens in front of the elements, modern equipment is now ________________________focused

electronically

In a linear sequential array the image shape is __________________and never wider the the _________________of the transducer

rectangular, footprint

Linear sequential arrays are often used in ___________ imaging

vascular

With a curvilinear array the beams are directed straight out but are not parallel due to the ____________

arc