Chapter 12: Two-Dimensional Imaging

Two characteristics of sound make 2D images challenging

-Sound travels in a straight line

-Sound mist be narrowed to optimize lateral resolution

The use of what overcomes the difficulites of 2D imaging?

Multiple pulses

Process of 2D imaging

1. The US system sends a narrow pulse to the body which receives, processes, and stores the data

2. The system directs a second sound beam into the body at a slightly different direction than the first. The data is processed

3. The system directs a third pulse into the body at another direction

4. Repeat

Mechanical Transducer

contains a single, circular, disc-shaped active element that is physically moved. This creates a scan plane

Crystals in a mechanical transducer

Only have one active element. The elemebt resembles a coin

Image shape of a Mechanical Transducer

fan or sector-shaped. Scan lines are spread apart creating gaps at greater depths.

beam steering of mechanical transducer

mechanical steering

beam focusing for mechanical transducer

conventional, mechanical, or fixed focusing

Two methods of fixed focusing are:

-Internal focusing

-External focusing

internal focusing

with the use of a curved active element

external focusing

with the use of an acoustic lens

What occurs if a crystal is damaged with a mechanical transducer

the entire image is lost

Transducer Arrays

arrays contain multiple active elements

array comprised a single slab of PZT cut into a collection of separate pieces called elements (each element is connected by a wire to its own electronic circuitry)

a channel is the combo of the active element, wire & systems electronics

Since each element has its own electrical connection, what is the system able to do?

Can excite a single or multiple elements in various ways during transmission

linear phased array

the transducer that uses phasing, or small time differences, to steer and focus the beam

Crystals in a linear phased array

100-300 elements in a rectangular and narrow manner

Image shape in a linear phased array

fan or sector shaped

Beam steering in a linear phased array

The 2D image is built up with an electronic steering process called phasing.

The sound beams are electronically transmitted in different directions without the use of moving parts and are directed in a pattern similar to the Mechanical Sector.

beam focusing of linear phased array

1. Sound beams are electronically focused that allow the sonographer to modify the depth and amount of focusing.

2. Whether the region of clinical interest is superficial or deep, the sonographer can adjust the focal length accordingly.

3. Multiple beams can also be transmitted along the same scan line providing multi-focusing capability.

What occurs if there is a damaged PZT crystal in a phased array?

Inconsistent or erratic beam steering and focusing

How many elements in a phased array probe are used to create a single sound pulse?

All active elements are fired to create each sound pulse. The wavelets interfere constructively and destructively creating a single sound pulse with particular characteristics

In a phased array system, how does the pattern of electrical spikes from the ultrasound system steer the sound beam in different directions?

The spikes travel down the wires and excite the active elements at the end of the wires. The electrical signals are lined up and will arrive at each of their respective active elements at the same time. Each active element creates a wavelet. The wavelets intere, forming a single beam.

When a spike pattern is a straight line, an ___________ sound beam is created.

unfocused

Phase delays

time differences between many wavelets that create a single, unified sound beam (~10 nanoseconds)

beam former

the part of the machine that shapes and steers the beam on the transmit end

The curved pattern in a linear array creates a??

focused sound beam during transmission, since there is no predominant slope to this pattern, the beam will be directed straight down

How does a phased array system create images with multiple transmit foci?

Each electrical spike pattern has a different degree of curvature; the pattern with the greatest curvature creates the beam with the shallowest focus. The least curved pattern produces the beam with the deepest focus.

dynamic receive focusing

time delays to electrical signals upon reception that focus the image

variable time delays

delay of electrical signals during reception

creates more accurate imaging and is dependent of depth

Annular Phased Array

elements are arranged as circular rings with a common center

Beam Steering:Annular Phased Array

MECHANICAL steering is used by the Annular Array. A 2D image is built up by physically rotating the ringed element array so that it transmits sound beams in different directions.

Beam Focusing:Annular Phased Array

1. The primary advantage of annular phased arrays is that MULTIPLE transmit FOCAL ZONES create each scan line of the image.

2. ecall that smaller diameter PZT crystals create shallow focus beams, whereas larger diameter crystals create deeper focus beams.

3. The inner crystal of an annular array creates a shallow focus beam and collects information only from the shallowest depths; all other information from the inner crystal's sound pulse is ignored. The next larger crystal is fired down the same scan line, and information only from its focal zone is stored. This process is repeated for each of the rings.

Image Shape:Annular Phased Array

fan or sector shaped

Damaged PZT Crystal: Annular Phased Array

When one ring of an annular array is damaged, only a portion of the image is lost. This will appear as a HORIZONTALor side to side band of dropout at a particular depth. If the inner most ring is damaged then superficial imaging will be sacrificed; if an outer ring is damaged then a deeper horizontal ban will dropout.

linear sequential array

- Rectangular elements

- 120 to 250 rectangular shaped strips of PZT

- Image shape: rectangle

- No moving parts

- Steered electronically

- Fixed electrical focusing

- Portion of image is lost when PZT is damaged

Beam Steering in Linear Sequential Arrays

a small group of the crystals are fired simultaneously

Beam Focusing in Linear Sequential Arrays

Focusing is achieved electronically through both transmit and receive focusing that incorporate electronic time delays.

electronic transmit focusing

achieved with a curved excitation pattern of the active elements

receive focusing

achieved dynamically by introducing electrical delays in the signals returning from the transducer

Damaged PZT in linear sequential array:

if one element is damaged, only the portion of the image directly below the damaged element is affected

Can linear sequential array transducers steer sound beams electronically?

Yes, this form of sterring create paralleogram-shaped images rather than rectangular images

slope-patterned phased delays

the electrical excitation spikes of the elements steer the beam

Convex, Curved, or Curvilinear Array Transducers

•bowed or curved shape to increase clinical utility of linear array probes

•operational characteristics are similar to linear arrays

•120-250 rectangular shaped strips of piezoelectric material arranged side by side in a bowed line, the array is large.

•acoustic footprint may be as long as 10 cm

•each crystal is about 1 wavelength in width

•blunted sector shaped images

Beam steering of convex, curved, or curvilinear array

some, but not all of the crystals are fired simultaneously to create each sound of the crystals, but since they are arranged in an arc, the pulses travel in different directions as they radiate from the transducer.

Beam Focusing in Curvilinear Array

Electronically and is achieved by exciting the active elements with a curved spike line pattern

Image Shape in Curvilinear Array

blunted sector shape

Damaged PZT crystal of convex, curved, or curvilinear array transducers

Only a portion is damaged

Vector Arrays

combination of linear sequential and linear phased array

Crystals number and Shape: Vector Arrays

120-250 rectangular-shaped strips

Beam Steering:Vector Array

1. Steering combines PHASEDand SEQUENTIAL technologies. As in sequential arrays, some but not all of the crystals are fired simultaneously.

2. In manner similar to phased arrays, the electrical signals from the beam former are delayed in a slope spike-line pattern as they excite a group of elements in a vector array. As a result, the beams radiate out in different directions from the face of the transducer.

Beam focusing (Vector Array Transducers)

electronic

Image Shape:Vector Array

trapezoidal

Slice Thickness Resolution

measured in a direction perpendicular to the imaging plane or above-to-below the imaging plane

Image resolution deals with

1. shallow-to-deep

2. side-to-side

3.above-to-below the imaging plabe

When the slice is thick, structures above and beneath the plane create _________________ that appear in the image.

reflections

Disc-shaped crystals are found in?

Mechanical and ammular phased array

Disc-shaped crystals

creates the best elevational resolution?

What determines elevational resolution in phased array, linear array, and convex array transducers?

-Rectangular shaped active elements

-With phasing, the beam is focused or narrowed only side to side

-Improves lateral resolution

One limitation of linear arrays is the inability to dynamically create thin slices. What newer types of transducers have improved slice thickness resolution?

One-and one-half dimenstional arrays

3D Imaging

From a volume data set, static 3D images can display height, width, and depth of anatomy (three dimensions) from any orthogonal plane

4D Imaging

Adds motion to static 3D imaging, allowing for real-time 3D imaging of anatomy in motion

side lobes

an artifact caused by extraneous sound that is not found along primary beam path; occurs with single-element transducers, degrade lateral resolution

grating lobes

an artifact caused by extraneous sound that is not located along primary beam path; occurs with arrays; reduced or eliminated by apodization, subdicing, and tissue harmonics

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

Apodization

Apodization

the technique that varies the voltage to the individual elements to reduce grating lobes

subdicing

dividing the piezoelectric elements into very small pieces to reduce grating lobes

Modern transducers are designed to keep the sound beam narrow over a substantial depth range. How is this accomplished?

dynamic aperture

dynamic aperature

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