Test #1 (Unit 1)

What is the principle of Piezoelectricity?

What is the principle of Piezoelectricity?

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

What principle do ultrasound transducers operate according to?

Principle of Piezoelectricity

How do transducers create a voltage using the principle of Piezoelectricity?

1. Convert electric voltages into ultrasound pulses

2. Convert returning echoes into voltages

What are other names for Piezoelectric elements?

1. Crystal

2. Active element

3. Transducer element

What are some examples of natural Piezoelectric elements?

1. Quartz

2. Tourmaline

3. Rochelle salt

What are some examples of synthetic Piezoelectric elements?

Lead zirconate titanate (PZT)

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

If a Piezoelectric element is not natural, how do they become Piezoelectric?

1. Heat material to Curie point

   1. Material loses magnetic properties

2. Polarize dipoles with a strong electric current

   1. Alignment circuit

3. Cool crystal below Curie point

4. Remove from alignment circuit

5. Aligned dipoles are fixed parallel to each other

6. Material can

   1. Exhibit piezoelectric properties

   2. Generate sound waves

What temperature is Curie point?

365° C

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

Destroys all Piezoelectric properties

What is Piezoelectricity?

When applied pressure produces a voltage

What does fo mean?

Operating frequency

What is the fo of a crystal determined by?

1. Crystal

   1. Propogation speed

   2. Thickness

Propogation Speed of Crystal (cPZT) Formula

cPZT

Crystal Thickness (cth) Formula

Wavelength / 2

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 principle do ultrasound waves follow?

Huygens’ Principle

What is Huygens’ Principle?

Any point on a spherical wave can become origin for a new spherical wave

What is the sound beam a combination of?

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

What is the beam profile formed by?

Constructive and destructive interference as wavelets collide within beam

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

Natural focusing (narrowing)

What shape is the sound beam?

Three-dimensional

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 occurs when additional beams travel out in directions not included in the main beam path?

Side lobes

When do side lobes occur?

ONLY in single element transducers

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

How are beam width and distance related in the near zone?

1. Inversely

   1. Beam width decreases = increasing distance

   2. Beam width increases = decreasing distance

Far Zone

1. AKA far field, Fraunhofer zone

2. Region that lies beyond min beam width

How are beam width and distance related in the far zone?

1. Directly

   1. Beam width increases = increasing distance

   2. Beam width decreases = decreasing distance

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

What is the formula for the beam width at natural focus?

Crystal D / 2

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

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)

Where can focusing be achieved?

ONLY in the near zone

What transducers can only be focused mechanically?

1. Single element

   1. Curved crystal

   2. Acoustic lens

   3. Mirrors

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 are the pros of damping?

1. Improves resolution

2. Broadens bandwidth

What are the cons of damping?

1. Reduces ultrasound amplitude

2. Decreases sensitivity of system

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?

1. Improves sound transmission into body

2. Reduces reflection

Matching Layer Thickness Formula

Wavelength / 4

What does coupling gel do?

1. Eliminates air between transducer and skin

2. Eliminate strong reflection caused by air

3. Improves transmission of sound into and out of body

What is the DMU frequency range?

1-20 MHz

Which frequencies provide inadequate axial resolution?

Lower than 1 MHz

Which frequencies cannot penetrate deep enough for ultrasound?

Above 20 MHz

What frequency transducer should be used for abdomen, pelvis, and OB?

1-5 MHz

What frequency transducer should be used for adult echo, abdomen, and abdominal Doppler?

1-5 MHz

What frequency transducer should be used for vascular, thyroid, scrotum, and MSK?

5-12 MHz

What frequency transducer should be used for breast?

10-20 MHz

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

Sequencing

Pulses are applied to small groups of elements in rapid succession

What is the time delay between pulses determined by?

Depth (time it takes for all echoes to return)

What does sequencing allow to happen?

1. Fast acquisition of images and frame rates

2. Real-time scanning

Phasing

Pulses are applied to elements in rapid succession

What does phasing allow to happen?

1. Sweeping

   1. Real-time scanning

2. Steering direction

3. Focusing scanning plane and perpendicular to scanning plane

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

What planes does focusing occur in?

All 3 planes