FINAL

What is the principle of Piezoelectricity?

“Some materials, when deformed by an applied pressure, produce a voltage”

What happens when a voltage is applied to a Piezoelectric element? What does this depend on?

1. Thickness of element increases or decreases

2. Depends on polarity of voltage

What would happen if a synthetic Piezoelectric element was created and reheated to the Curie point?

Destroys all Piezoelectric properties

What is the fo (operating freq) of a crystal determined by?

1. Crystal

   1. Propogation speed

   2. Thickness

Operating Frequency (fo) Formula

fo = cPZT / 2 * cth

How are thickness and operating frequency related?

1. Indirectly

   1. Thin elements = high freq

   2. Thick elements = low freq

What is the sound beam a combination of?

All sound arising from different point-like sources (wavelets) on transducer crystal face

What occurs due to the superposition of all sound waves in the beam?

Natural focusing (narrowing)

How is the shape of the sound beam determined?

Crystals

Axial Plane

1. Along direction of sound travel

2. Parallel

Lateral Plane

Perpendicular to direction of sound travel

Elevational Plane

Thickness of sound beam

What produces the width of a sound beam? How is the width determined?

1. Transducer

2. The distance from the transducers face

Is intensity uniform throughout a beam? Why?

1. No

2. Area varies (intensity = power/area)

How are beam diameter and resolution related?

1. Inversely

   1. Small beam = good resolution

   2. Large beam = bad resolution

Near Zone

1. AKA Fresnel zone, near field

2. Region extending from transducer to minimum beam width

Focal Point

1. Smallest beam

2. Maximum intensity

When does a beam have the best resolution?

At the focal point

Focal Zone

1. Where beam is focused on each side of focal point

2. Maximum

   1. Sensitivity

   2. Intensity

When does a beam have the best lateral resolution?

At the focal zone

How are diameter and intensity related in the focal zone?

1. Inversely

   1. Diameter decreases = intensity increases

   2. Diameter increases = intensity decreases

Near Zone Length (NZL)

1. Distance from transducer face to where the beam has the smallest diameter

2. Additional focusing can be added

How are diameter and NZL related?

1. Directly

   1. Increase diameter = increase NZL

   2. Decrease diameter = decrease NZL

How are frequency and NZL related?

1. Directly

   1. Increase diameter = increase NZL

   2. Decrease diameter = decrease NZL

Far Field Divergence (FFD)

When the beam diameter increases after natural focus

How are diameter and far field divergence related?

1. Indirectly

   1. Increased diameter = Low divergence

   2. Decreased diameter = High divergence

How are frequency and far field divergence related?

1. Indirectly

   1. Increased frequency = Low divergence

   2. Decreased frequency = High divergence

At a distance of one near zone length the diameter of the beam is…?

½ the crystal diameter

At a distance of 2 near zone lengths the diameter of the beam is…?

The crystal diameter

What two things does focusing contribute to?

1. Better resolution (narrow beam)

2. Stronger beam (decreased area)

A-mode (Amplitude Mode)

1. Displayed on graph

   1. X-axis = depth

   2. Y-axis = strength

B-mode (Brightness Mode)

1. 2D images, B-scans, displayed on a matrix

2. Displayed dots with brightness

What does brightness on B-mode show?

1. Strength

2. Location

M-mode (Motion Mode)

1. Displayed on a graph

   1. X-axis = time

   2. Y-axis = depth

What imaging mode is used most used today?

B-mode

What imaging mode is used for cardiac and fetal cardiac?

M-mode

Transducer (Probe)

Device that converts one form of energy to another

Bandwidth (BW)

Range of frequencies produced by the transducer

How are pulse length and bandwidth diameter related?

1. Inversely

   1. Short pulses = broad bandwidth

   2. Long pulses = narrow bandwidth

Damping (Backing) Material

Epoxy resin attached to back of element

What does damping material do?

1. Absorbs vibrations

2. Reduces #cycles/pulse

How are pulse duration (PD) and spatial pulse length (SPL) related to resolution?

1. Low PD and SPL = Improved resolution

2. High PD and SPL = Reduced resolution

How is bandwidth with damping related to quality factor, efficiency, and sensitivity?

1. Inversely

   1. Increased bandwidth and damping = Decreased QF, efficiency and sensitivity

   2. Decreased bandwidth and damping = Increased QF, efficiency and sensitivity

How are diagnostic imaging transducers damped? How many cycles per pulse does this produce?

1. Highly damped

2. 2-3 cycles per pulse

How are pulsed-wave Doppler transducers damped? How many cycles per pulse does this produce?

1. Less damped

2. 5-30 cycles per pulse

How are continuous wave Doppler transducers damped? Why?

1. Not damped

2. Reflects all energy into patient

What does quality factor determine?

Sensitivity

What does quality factor detect?

Weak echoes

Quality Factor (QF) Formula

fo / BW

Matching Layer

1. Located on the transducer face

2. Has impedance value between crystal and tissue

What does the matching layer do?

Reduces reflection

How are frequency and penetration related?

1. Low frequency = Improved penetration

2. High frequency = Reduced penetration

How are penetration and resolution related?

1. Improved penetration = Reduced resolution

2. Reduced penetration = Improved resolution

What is a complete scan of the ultrasound beam called?

Frame

What is required for real-time scanning?

Transducer arrays

What means are used for sweeping, steering, and focusing the beam? What is this accomplished by?

1. Electronic means involving constructive interference

2. Accomplished by

   1. Sequencing

   2. Phasing

What is the time delay between pulses determined by?

Depth (time it takes for all echoes to return)

Beam Steering

1. Sweeping the beam

2. Accomplished with phasing

What does beam steering produce?

Automatic scanning

How are time and beam steering related?

1. Directly

   1. Increased delay = Increased steering

   2. Decreased delay = Decreased steering

How do you know what direction a beam is going during beam steering?

1. Beam goes toward side activated last

2. Right to left = steered left

3. Left to right = steered right

How are time and focus related during electronic (transmit) focusing?

1. Inversely

   1. Increased delay of curvature = Closer focus

   2. Decreased delay of curvature = Deeper focus

Linear Array

1. Straight line of elements

2. Rectangular image

3. Vertical, parallel scan lines

What can be done during linear array to produce a parallelogram?

Can be steered to right or left (in Doppler)

Curved (Convex) Array

1. Curved line of elements

2. Produces sector image

3. Pulses travel in different directions from different origins

Phased (Sector) Array

1. Compact line of elements

2. Produced pie shaped image

3. Pulses travel in different directions from same origin

What occurs during phased array?

Voltage pulses applied to entire group of elements with varying time delays = sweeping of beam

Vector Array

Parallelogram-shaped display

What is a 2D array? What can they do?

1. Matrix of elements

2. Ability to steer and focus in two dimensions

3. Allows for focusing in elevational plane by phasing

Name the three spatial categories of resolution

1. Axial

2. Lateral

3. Elevational

What does spatial resolution give us the ability to see?

Detail on an image

What is spatial resolution related to?

1. Directly related to # of scan lines

2. Related to # of pixels in a monitor

What size resolution is always better?

1. Smaller

2. Less distance between reflectors to be displayed as separate objects

What is axial resolution?

The minimum reflector separation necessary to resolve reflectors parallel to sound beam

What is axial resolution determined by?

SPL

What does LARD stand for? (Has to do with AR)

Longitudinal, Axial, Range, Depth

To improve AR, SPL must be _____

Reduced

As SPL decreases, AR ____

Decreases

As frequency increases, AR _____

Decreases

Does axial resolution change with depth? Why?

1. No

2. AR is constant along beam path

What is lateral resolution?

Minimum reflector separation necessary to resolve reflectors perpendicular to beam

What is lateral resolution determined by?

Beam width

Does lateral resolution vary with depth? Why?

1. Yes

2. Because of sound beam shape

What does LATA stand for? (Has to do with LR)

Lateral, Angular, Transverse, Azimuthal

Elevation resolution varies with ____ because of the shape of the sound beam

Depth

What is contrast resolution?

The ability of gray-scale display to distinguish subtle differences in echogenicity, or brightness, of adjacent tissues

What is contrast resolution determined by?

1. Number of pixels in an image

2. Number of shades of gray displayed in each pixel

What is contrast resolution controlled by?

1. System’s memory

2. Dynamic range settings

What is temporal resolution?

Ability to follow moving structures in temporal detail

What is frame rate?

Number of images displayed per second

Faster FR = _____ temporal resolution

Improved

Frame Rate is dependent on… (5 things)

1. Line density

2. Lines per frame

3. Depth & PRF

4. Sector Width

5. Number of foci

What is line density?

1. # of scan lines per degree of sector

2. # of scan lines per centimeter

What is lines per frame?

# of lines in each frame

More lines = ____ spatial resolution

Improved

How are line density and lines per frame related to FR?

Inversely

____ sector width improves image quaility

Low

Why does a low sector width improve image quaility?

1. Less tissue interrogated

2. Less artifacts

3. Improved signal to noise ratio

4. Improves contrast resolution

Depth and PRF are ____ related

Inversely

Depth is ____ related to FR

Inversely