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Transducers and Beams > Chapter 5-7, Physics > Flashcards

Chapter 5-7, Physics Flashcards

1
Q

Discovered in the 1880’s by Pierre and Jacques Curie

When a mechanical force is applied to certain materials they create a voltage or……

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

Conversely, these same materials produce a pressure wave when when an applied voltage deforms the materials

When voltage is applied to these materials, they change shape and vibrate

A

Piezoelectric Effect

Piezoelectric is also called ferroelectric

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2
Q

Refers to any device that converts one form of energy into another

A

Transducer

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3
Q

Active elements (cristals) in transducers are heated to a certain point to make them piezoelectric. What is this critical temperature called?

A

Curie Point

Transducer crystals lose their piezoelectric properties if they are heated above the Curie Point or Curie Temperature
The material becomes depolarized if heated to this point

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4
Q

Main component of a transducer is this with piezoelectric properties

A

Crystal

Natural materials: Quartz, Rochelle salts, tourmaline

Man-made: Lead zirconate titanate or PZT (most often used in diagnostic US transducers), barium titanate, lead metaniobate, lead titanate

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5
Q

Bonded to the back of the active element;

Shortens SPL and pulse duration

Improves image quality

Increases bandwidth (range of frequencies within the pulse)

Decreases the Quality factor (Q)

Decreases the transducers sensitivity to reflected echoes

A

Damping Element

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6
Q

Reduces reflections at transducer - tissue interface

Usually ¼ the wavelength of the ultrasound beam

Used to reduce the impedance difference between the transducer element and the skin itself

Helps send more US energy into the body rather than reflecting it because of the impedance difference

(Gel also helps to reduce the impedance difference)

A

Matching Layer or Impedance Matching Layer

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7
Q

Acoustic impedance of composite piezoelectric elements is closer to that of soft tissue, so matching is easier and more efficient with these transducers

A

Composites

Man Made

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8
Q

The operating frequency of a transducer

Also called natural frequency

Transducer frequency depends on:

    - The thickness of the crystal (indirectly related), and 
    - Speed of sound in the crystal (directly related)
A

Resonant Frequency

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9
Q

Pulsed ultrasound transducers emit not a single ultrasonic frequency, but a spectrum of frequencies

IT describes the difference between the highest and the lowest frequency in a pulse

A

Bandwidth

The shorter the pulse, the wider the bandwidth

Resonant Frequency is at the center of the bandwidth

Advantages of Wide Bandwidth
Fewer cycles per pulse give longer listening time (allows acquisition of more echoes)

Wider bandwidth can receive a wider range of frequencies

Disadvantage of wider bandwidth

Decreased probe sensitivity

Element is not as responsive to the returning sound waves (pressure)

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10
Q

In pulsed ultrasound, a description of the width of the pulse as it travels away from the transducer

Width varies with distance away from the transducer

A

Beam

Sound beams are usually shaped like an hourglass; starts the size of the transducer, then gets smaller, then it diverges

Beam width at any location depends on:

Frequency
Aperture (size of the source)
Distance from the transducer

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11
Q

Region between the transducer and the focus

Also called the Fresnel Zone

Determined by the size and the operating frequency of the element

Much variation in beam intensity here because wavefronts are still coming together

Increases (longer) with increasing frequency, element size, or diameter squared

A

Near Zone (field)

Larger crystal diameter, longer focal length or near zone

Higher frequency, longer focal length

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12
Q

IT is the area of highest, most uniform beam intensity (anatomy of interest should lie here)

A

Focal Zone

Larger the crystal diameter, the farther or deeper the beam focus

Smaller the crystal diameter, the shallower or nearer the focus or focal depth

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13
Q

Near Zone Length or differently called…

A

Focal Length

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14
Q

The surface of the transducer face where ultrasound is transmitted and received.

A

Aperture

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15
Q

Also called the Fraunhofer zone

The region that lies beyond the distance of one near zone length

Place where the beam begins to diverge

Beam intensity tends to drop off here but is also more homogenous

A

Far Zone

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16
Q

Near Zone Length vs Frequency-Diameter

A

Directly proportional to near zone length

increasing frequency or diameter, increases near zone length

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17
Q

Divergence vs. Frequency and Diameter

A

Inversely proportional to divergence

smaller frequency or diameter, = more divergence

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18
Q
  1. Improves the accuracy of the ultrasound image
  2. Causes the focus of the beam (the waist) to become narrower
  3. Shortens the entire focal zone
  4. Creates a shallower focal depth
    (near zone length is shorter)
  5. Causes more divergence in the far field
A

Focusing

19
Q

Describes the machine’s ability to image structures with accuracy

A

Resolution

As the numerical values of both types of resolution get smaller, the images are getting better (we’re distinguishing smaller structures)

Axial resolution is always better than lateral resolution because pulses are always wider than they are long (remember, smaller is better!)

20
Q

The machine’s ability to see and differentiate small structures and represent them (anatomically) correctly on the ultrasound image

A

Spatial or Detail Resolution

21
Q

Along the axis of the ultrasound beam

The minimum distance 2 structures are separated from front to back or anterior to posterior, and still be distinguished as separate by the ultrasound machine

A

Axial Resolution

Determined by spatial pulse length

Shorter pulse lengths give better axial resolution

Increasing frequency shortens SPL, improves axial resolution

22
Q

Also called transverse resolution

The minimum distance that two side by side structures can be separated and still show 2 separate echoes on the screen

Approximately equal to beam diameter

A

Lateral Resolution

Beam diameter varies with depth, so does lateral resolution

Lateral resolution is best at the focus of the beam because diameter is smallest there

Focusing improves lateral resolution

Scan plane width or beam diameter
Determines lateral resolution

23
Q

A collection of active elements within a transducer housing

A single slab of piezoelectric material is cut into many separate elements

Each element has its own electrical connection to the US machine (a channel)

Elements can be activated or “excited” individually or in groups

A

Transducer Arrays
Linear, Curved (convex), Phased, Annular

Two Advantages:
Enable electronic beam steering (beam is swept across the imaged field with no mechanical motion of parts)

Enable electronic focusing and beam forming (this allows control of focal distance and beam width throughout the entire imaged field)

24
Q

Commonly called Phased array or electronic sector

Elements are still arranged in a line but the array is very small

Beam steering and focusing is electronic

Multiple electronic signals are used to create a single pulse

Image is fan or sector shaped

Scans the beam in sector format with short time delays

Beam focusing – done by electronic curvature of the beam

Beam steering – done by electronic slope

Voltage is applied to groups of elements in such a way that allows steering or focusing of the beam

The number of crystals excited at once determines beam shape or lateral resolution

A

Linear Phased

25
Q

Concentric rings cut from the same circular slab of piezoelectric material

Steering – done mechanically

Focusing – done electronically

A

Phased Array

Advantages
Superior image quality at all depths
Small footprint good for small windows

Disadvantages
Longer time to form image
Lower frame rates (reduced temporal resolution)
Mechanical steering (moving parts - can break)

26
Q 
Multiple elements arranged in a line
Elements are fired in sequence 
Image consists of parallel scan lines
Rectangular image shape
Conventional focusing
NO Beam Steering
A

Linear Sequential

Advantage: Beam is parallel at all depths

Disadvantage: Large footprint (bad for small windows)

27
Q

Crystals arranged in an arc; sector shape format

NO Beam Steering

Elements are fired in a sequence

Upper region does not reach a point as in a sector or vector scan

Focusing is by conventional (lens or internal)

A

Convex Sequential (switched)

Advantages
Natural sector image with wider field of view superficially

Electronic – no moving parts

Disadvantages
Large footprint, hard to use on small windows such as rib spaces

Sound beams tend to separate from each other leaving gaps as you move farther out from the probe

28
Q

Axial or Anterior to Posterior is one dimension

Lateral or side to side is a second dimension

Slice, Section thickness, or elevation is the third dimension

A

Third Dimension

Takes an odd number of rows of elements (at least 3) to be able to focus the third dimension electronically

29
Q

Combination of phased array electronics and linear sequential array probes to provide electronic steering and multiple focal zones

A

Vector Array

30
Q

Man Made;

Most often used in diagnostic US transducers

A

Lead zirconate titanate or PZT

31
Q

Some of the energy from the transducer radiates at various angles to the transducer face known as…

A

Side Lobes

32
Q

The non-linear excitation of crystal elements.

A

Apodization

33
Q

Along the beam path

A

Axial

34
Q

Perpendicular to the beam path

A

Lateral

35
Q

Groups of piezoelectric material working singly or in groups

A

Electronic Arrays

36
Q

crystals are placed parallel or in concentric rings
– transducer face is curved
– produces sector or pie-shaped image

A

Sector Array

37
Q

crystals are placed parallel
– transducer face is flat
– produces rectangular image

A

Linear Array

38
Q

The ability to select focal zones at different depths throughout the image. As the number of focal zones increases, the frame rate decreases.

A

Dynamic focusing

39
Q

A small piece of pieozelectric material in a transducer assembly

A

Element

40
Q

Additonal minor beams of sound traveling out in directions different from the primary beam. These result for the multielement structure of transducer arrays

A

Grating Lobes

41
Q

“Slice Thickness”
- Thickness of the sound beam.
Determined by the construction of the transducer and can not be controlled by the sonographer

A

Elevational Resolution

42
Q

Ultrasound beam from a flat aperture will get narrow and then spread out within and angle range. The depth where beam is most narrow is this….. of the aperture.

A

Natural Focus

43
Q

Ultrasound transducers are referred to by:

A

Operating, Resonant, or Main Frequency

Transducers and Beams (1 decks)

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