Transducer arrays and spatial resolution

Transducers do what?

emit ultrasound pulses, receive echoes, and send pulses through many anatomic pathways to create an image

the ways transducers emit ultrasound, receive echoes, and send pulse through multiple anatomic pathways

scanning sweeping or steering the beam

a complete scan of the ultrasound beam

a frame

why is scanning done rapidly?

so that many images, frames, can be presented in rapid sequence ( real- time sonography)

automatic scanning of sound beam is performed with arrays

electronic scanning

transducer assemblies with several transducer elements

arrays

elements in arrays are in an ____ or ___ line

straight or curved

arrays operate in 2 ways

sequencing

phasing

explain how arrays work

each crystal has its own electrical current being sent to it. 100-300 piezoelectric elements that each have their one electrical circuitry.

small group of elements fire to create sound beam

will be rectangular

lines are straight

linear sequenced array

how do linear sequenced arrays work?

120-250 elements, each 1 in wavelengths width

there will be small groups of elements (like 4 at a time) firing to create each sound; currents sent in a way that allow pulses to make shape

curved line of elements that operates similar to the linear sequenced array except pulses travel out in different directions from different points across the curved surface; large footprint but will go out in all diff directions instead of a straight line

convex sequenced array

most or all elements involved in creation of the sound beam, this allows steering of the sound beam and will create a sector shaped image and have a small acoustic footprint

phased array

phased array will have a small acoustic footprint to?

see in btwn ribs while also having a widening that will allow for scanning large tissue.

is applied to some linear and convex arrays to steer the beam in several directions; echo info from these multiple views makes an improved quality image

phasing

if echo probe is linear why dont the beams give rect shape like other linear probe

due to the phasing

what is electronic compoundiing

the use of phasing to steer the beam in several directions allowing for multiple echo views that make a single improved quality image

linear phased array is at different angles called sloping for what?

to make the scan as parallel as possible for color

some currents can have time delays (phasing) in stimulation of elements

the focus will be shallower and the beam will be narrower in near field

electronic focusing

a greater electrical current curvature will allow for

closer focus to the transducer

a smaller electrical current curvature will allow for

a deeper further image focus from the transducer

multiple foci require what?

multiple pulse/ scan lines focused at different depths

multifocal zones do what to detail resolution and temporal resolution

it will decrease detail resolution and increase temporal resolution

a combination of linear sequential and phase array and uses phasing to allow for steering of sound beam. This transducer array will allow for small groups of elements to fire to create sound beam. The image format is similar to convex array but footprint is smaller and flat.

vector array

how to get 3d or volume images

allows focusing in elevational dimension with phasing of element stimulation

1.5-D array

over 3000 elements firing simultaneously and creates 3D volume set

will have electronic focusing in lateral and elevational dimensions

matrix (2D) array

what you put out; machine delays firing of individual elements to create a focused beam going out; can NOT be changed once the pulse leaves

transmit focusing

what you put in; machine continuously adjusts how it "listens" to echoes coming back in; focusing will change in real time as echoes come back

receive focusing

echoes from a reflector arrive back at different times; the individual signals must be lined up into phase before summing; time delays on returning echoes to line them up

to sum up: delays to create receives focus to turn it back into linear info

dynamic receive focus

improves lateral resolution; what we want to happen

variable aperture

T/F all elements of phased are used to generate all pulses

false

smaller groups are used for

short focal lengths (to see more superficial)

larger groups are used for foci located at

increasing depths (deeper)

To maintain the same beam width at the focus for increasing focal lengths, the aperture must also be

increased

what determines the beam width at the focus

Aperture, focal length, and wavelength

active aperture is increased as echoes arrive from deeper structures, keeping effective beam width narrow at all depths; what's creating change for crystals; either grabbing more or less crystals to change how superficial or deep we want to see an image.

dynamic aperture

when the focal zone is directed toward the object you want to see

the image will be much clearer than if it were focused deeper in the same area

similar to side lobes; can be reduced by driving the elements in a group nonuniformly (different voltage amplitudes)

grating lobes

grating lobes are

additional weak beams that array transducers demonstrate

outer elements are driven at ___ amplitudes than inner elements

lower

what weakens grating lobes and reduces interelement interaction for improved electric focusing?

sub-dicing of each element into a group of small crystals

sub-elements are tied together _____ to function as one element

electrically

stronger on the inside than outside; reduces grating lobes;

stronger electrical signals are used to excite the inner elements, and progressively weaker signals excite the outer elements

echo reception sensitivity is less for outer elements

apodization

optimal apodization changes?

continually with focusing and steering

relates to transducer

spatial resolution

types of spatial resolution

axial (superficial-deep)

lateral

elevational

based on intergrating height portion; not just single element

elevational resolution (slice thickness)

relates to instrument

contrast resolution and temporal resolution (frame rate)

the minimum distance by which two structures are separated along the scan line that will still produce two distinct echo lines

axial resolution

axial resolution equals

1/2 spatial pulse length

axial resolution is improved with

higher frequency

the higher the frequency the _____ the axial res

better

decreasing spatial pulse length?

decreasing axial res

do you want a higher or lower number for a better axial reso

lower

why do we need to change axial resolution?

if echoes overlap they will be received as one structure (instead of 2 separate spatial pulses you will be seeing 1)

to increase axial resolution?

decrease spatial pulse length

increase frequency

increase damping

an increased frequency will

decrease penetration

the minimum distance by which two structures are separated side to side that will produce two distinct echoes

lateral resolution

lateral resolution equal

beam width in scan plane

what improves lateral resolution in the near field

focusing the beam

when the beam is not focused for lateral res what happens?

You aren't able to tell the difference btwn two separate structures; they will look like one large one

if the beam is narrowed in diameter what happens in lateral res?

you are now able to distinct btwn 2 structures and are able to see that there is space between them.

why measure LVOT diameter in PLAX and not apical 5-C?

bc axial res is better than lateral; ap 5= lateral so less accurate

determines elevational res; ultrasound machine superimposes all echoes into one thin plane

slice thickness

the thicker the slice?

the less details youll see

the best slice thickness resolution is?

thin and narrow; allows to separate RBC's from tissues

uses a weakly focused or nonfocused beam and computed reception "beams" to yield images that are in focus throughout;

used in operating principle 2

virtual beam forming

what happens to all resolutions when virtual beam forming is used

lateral and elevational resolutions are greatly improved; axial resolution similar to operating principle 1

lateral resolution with virtual beam forming

lateral resolution is as if a laser thin ultrasound beam had been transmitted

enhances reception focus of the image is precisely located retrospectively

section thickness is reduced, improving elevational resolution

for diagnostic applications, the useful frequency range is?

2-20 MHz

higher frequencies increase and decrease what?

Resolution; the effective imaging depth

in cases of opthalmologic, dermatologic, and intravascular imaging frequencies up to ___ may be used

50 MHz