Physics Chapters 12

Two dimensional images display

SLICES of anatomy for physician interpretation

Two challenges of 2D imaging in ultrasound

1. Sound travels in a straight line

2. Sound beam must be narrow for optimal lateral resolution

How do you overcome the challenges of 2D imaging?

by using multiple pulses

Steps for Creating an Image

1. Ultrasound system sends out a pulse in a straight line (with a narrow beam width)

2. The pulse returns to the machine for processing and reflections are stored

3. Another pulse is then sent out in slightly different direction

Real Time Imaging

Steps for creating an image happen about 100 times in 1/100th of a second

How can we create a 2D image that maps out the anatomy we are looking at?

by creating multiple sound beams, very quickly, and keeping track of all the returning information from each beam

• real time

Characteristics of the Different Types of 2D Transducers

• shape and number of active elements

• automatic redirection of sound beam for image creation

• focusing of each sound beam

• shape of 2D image

• issue that occurs from damaged/nonfunctioning active element

Array

a transducer assembly containing several piezoelectric elements

Phased

time delays that allow electronic steering and focusing

Mechanical Steering

• Refers to changing the ultrasound beam direction through mechanical means (such as attaching a crystal to motor) which physically points the crystal in different direction.

  • When a transducer is referred to as mechanical, it means the steering is mechanical.

Mechanical Transducer Active Element Characteristics

single, circular, disc shaped

Mechanical Transducer….

physically moves, creating a sector shaped image

In Mechanical Transducers, the crystal is rotated

through an arc by a motor

• mechanically steered, usually by a motor

Mechanical Transducer Crystals

Number: only one crystal

Shape: resembles a coin

Mechanical Transducer Image Shape

• fan or sector shaped (spokes of a bicycle)

• Greater depths = larger gaps

Mechanical Transducer Beam Steering

• crystal rotates with a motor, emitting sound beams as it goes

  • called mechanical steering

Mechanical Transducer Beam Focusing

• fixed focal depth (AKA conventional, mechanical, fixed focusing)

  • internal focusing (curved active element)

  • external focusing (acoustic lens)

Mechanical Transducer Damaged Crystal

• entire image is lost (only one crystal, so if it breaks, entire transducer breaks)

• least expensive type of transducer

• some vaginal and rectal still use this

Transducer Arrays Active Element Characteristics

more sophisticated, multiple active elements

Transducer Arrays…

slab of PZT cut into multiplee seperate pieces called elements

In transducer arrays, each element has

its own wire connecting to it allowing it to work independently of the others

One element or multiple elements can be excited at one time, each producing

its own reflection and electronic signal.

Channel

the combination of the active element, wire, and system electronics

Linear

active elements are 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 (AKA curved, curvilinear)

Multidimensional

Multiple elements arranged in a series of rows within the transducer

• 1.5D or 2D array

Linear Phased Arrays Characteristics

• small footprint

• no moving parts

• focused by phasing

Linear Phased Array Crystals

Number: 100-300 elements

Shape: each element is rectangular and narrow; arranged in a single row

Linear Phased Array Image Shape

fan or sector shaped (similar to mechanical)

Linear Phased Array Beam Steering

• electronically steered (phasing)

• electronically transmits in different directions without any moving parts

Linear Phased Array Beam Focusing

• electronically focused

• controls that allow the sonographer to change depth and amount of focusing

• todays equipment allows multi-focusing with multiple beams down the same scan line each with differing focal depths

Linear Phased Array Damaged Crystal

• inconsistent or erratic beam steering - cannot always appreciated damage when looking at image

Are all elements excited in phased array?

yes

Individual wavelets are created from

each element

The individual wavelets interfere and form a

single sound beam

wavelets interfere _____ and ______ to determine the sound beam ___and ____

constructively and destructively, direction and focus

In linear phased arrays, Electrical Signals fire ______ of a second apart from ___ to ___ or ____ to ____

ten billionths

right to left or left to right

The tiny fraction of time allows the beam to

steer one way or another

Phase Delays

the name for these time differences

if the line has a slope

the beam is steered

if the spike is in a straight line

the beam is not steered

Beam Former

the electronics within the ultrasound system that create the patterns that allow steering

Beam steering to the right happens when the crystals are excited from ___ to ____

left to right

Beam steering to the left happens when the crystals are excited ____ to ___

right to left

What will create a focused sound beam?

creating a curved pattern with the excitation of the crystals where the outer crystals are excited first

if the line is straight

the beam is unfocused

if the line is curved

the beam is focused

SHORTER time delays between firing elements from outside in = _____curve

LESS

Less severe curve = _____ focus

deeper

describe this beam

this beam is being focused and steered to the left

Slope

pertains to steering

Curvature

pertains to transmit time

Multiple transmit focuses are accomplished by

sending multiple sound beams down the same scan line

Each of the sound beams in a given scan line will have a different

• curvature pattern of excitation

  • produces different focal depths

Greatest curvature pattern = ______ focus

shallowest

Least curved pattern = _____ focus

deepest

can focusing occur during reception

yes

Dynamic Receive Focus is performed

automatically by the system, sonographer has no control

When sound returns to the transducer,

the returned sound excites neighboring elements in the probe

Variable time delays

change continuously throughout scanning

depend on depth of the reflector

Reflected Sound is focused at ____ depths

many

Transmit focus is limited to a

single focal point

Annular Phased Arrays

inner rings acquire data from shallow

outer rings acquire data from deep

Annular Phased Arrays Crystals

Number: varies

Shape: shaped like a disc, multiple rings with common center (bullseye)

Annular Phased Arrays Image Shape

fan or sector shaped

Annular Phased Arrays Beam Steering

• mechanical steering

• a motor physically moves the elements allowing the beam to be transmitted in different directions

Annular Phased Arrays Beam Focusing

• multiple transmit focal zones create each scan line

• smaller diameter = shallower focus

• image created from focal zone of each crystal, the rest is ignored

Annular Phased Arrays Damaged Crystals

• When one ring malfunctions, a portion of the image is lost in that “zone” of the image

• horizontal or side to side band is missing information

  • innermost ring —> band in most superficial aspect

  • outermost ring —> deeper band is missing information

Linear Sequential Arrays are AKA

linear switched arrays

Linear Sequential Arrays Crystals

Number: 120-250

Shape: rectangular shaped strips of PZT

• active elements are larger than phased array

• each crystal = 1 wavelength in width

• transducer face can be up to 10 cm long

Linear Sequential Arrays Image Shape

• rectangular

• image is never wider than transducer face, but can be narrower

Linear Sequential Array Beam Steering

• a group of crystals fires simultaneously for each sound beam

• beams are parallel and directed straight ahead

• a group of ten crystals will be excited and create a single sound beam, then another group of ten at slightly differing times at a different location

Linear Sequential Array Beam Focusing

Electronically focus on transmit and receive (modern transducers)

Linear Sequential Array Transmit Focus

curved excitation pattern (like linear phased array)

• this changes the near zone length and therefore changes to the focus

Linear Sequential Array Receive Focus

electrical delays in the signals returning

• phase delays change continuously during reception, thereby focusing the beam at many depths

Linear Sequential Arrays Damaged Crystals

• the portion of the image extending directly below the damaged crystal will be affected

• only the scan lines that would have been created by the damaged element are missing

In linear sequential arrays, elements are excited in ___ to allow

groups

allows wavelet interference to create better resolution

In linear sequential arrays, a single element firing at a time would create

divergence too quickly, degrading image quality

electronic sound beam steering is possible with linear sequentials by

using sloped patterns of electrical excitation

• this creates a parallelogram rather than a rectangle

• used when we steer for doppler

Convex, Curved, Curvilinear Array Transducers

similar to linear arrays, but made bowed shaped to increase the clinical utility

Convex, Curved, Curvilinear Array Transducers Crystals

Number: 120-250

Shape: rectangular shaped strips of PZT

• active elements are similar in size to linear array

• each crystal = 1 wavelength in width

• transducer face can be up to 10cm long

Convex, Curved, Curvilinear Array Transducers Image Shape

• blunted sector shape — convex, but does not come to a point in the near field

• curvature at the top of the image corresponds directly to the arc of transducer face

Convex, Curved, Curvilinear Array Transducers Beam Steering

• Some, but not all, of the crystals fire simultaneously, creating each beam.

• The beams are directed straight out from the crystal

• Remember, the elements are arranged in an arc, therefore the beams travel in difference directions

• The sound beams are not parallel to one another

Convex, Curved, Curvilinear Array Transducers Beam Focusing

Electronically transmitting and receiving (modern transducers)

Dynamic Receive Focusing

varying phase delays in returning echoes

Convex, Curved, Curvilinear Array Transducers Damaged Crystal

• The portion of the image extending directly below the damaged crystal will be affected.

• Only the scan lines that would have been created by the damaged element are missing

Vector, Vector Arrays or Virtual Sector

combination of linear sequential and linear phased array

• sloped phase delay patterns can steer the beam in different directions

Vector, Vector Arrays or Virtual Sector Crystals

Number: 120-250

Shape: rectangular shaped strips of PZT

• small transducer face/footprint

Vector, Vector Arrays or Virtual Sector Image Shape

• trapezoid

• at mid and deep depths, appears as sector but the top is flat

Vector, Vector Arrays or Virtual Sector Beam Steering

• Some but not all of the crystals fire simultaneously, creating each beam (similar to linear sequential).

• The beams radiate out in different directions from the transducer face after excitation in a slope pattern (similar to phased array)

Vector, Vector Arrays or Virtual Sector Beam Focusing

• electronically transmitting and receiving (modern transducers)

• dynamic receive focusing

Vector, Vector Arrays or Virtual Sector Damaged Crystal

drop out

Resolution

accuracy in imaging

Slice Thickness AKA Elevation Resolution contributes to

image quality and accuracy

Slice Thickness AKA Elevation Resolution answers the question…

are the reflections on the image created from structures directly in the imaging plane or from structures above or below the imaging plane?

Slice Thickness AKA Elevation Resolution is measured

perpendicular to the imaging place or above-to-below the imaging plane

is there slight thickness that varies with depth on an imaging plane ?

yes, not razor thin

THICK slice —> structures on either side of the razor thin image will

create reflections that appear on the image

What has the best elevational resolution?

mechanical and annular phased array

because they have disc shaped crystals and disc shaped crystals provide the thinnest slices