Ultrasound Physics Transducers Ii

Front 1

What is sequencing?

Back 1

Exciting groups of elements in a specific patern to scan a region in a linear fashion.
Generally done with large linear or curved linear array

Front 2

What are the limitation of pencil probes?

Back 2

no image

Front 3

Frame rate

Back 3

- the ability to create multiple frames per second
- HZ (images per second)

Front 4

temporal resolution

Back 4

ability to position moving structures from one instant to another

Front 5

relation between Frame rate and temporal resolution

Back 5

direct proportional
frame rate increase = temporal resolution increase

Front 6

What influence the frame rate?

Back 6

- speed of sound in medium
- depth of imaging

Front 7

frame time

Back 7

the time it takes to generate one frame

Front 8

relation between frame rate and frame time

Back 8

frame rate is reciprocal of frame time

Front 9

Calculate the frame time for an image at the depth of 10cm and 200 lines.

Back 9

13microsec (cm/Hz) * 200 lines = 26 ms/frame

Front 10

If the frame rate is 1/26 msec, what is the frame rate in Hz?

Back 10

0.038 KHz = 38 Hz

Front 11

Relation between
depth
frame time
frame rate

Back 11

debth increase
frame time incrrease
frame rate decrease

Front 12

Calculate the frame time for an image at 10 cm debth and 400 lines in msec and in Hz.
What is PRF ? PRP ?

Back 12

13microsec * 400 lines = 52 ms/frame
frame rate is 1/52=19Hz
PRF = debth * 13microsec/cm = .13ms
PRP = 1/PRF =1/0.1ms=10Hz

Front 13

What is line density?

Back 13

ability to alter the spacing between sound beams or scan lines

Front 14

Lateral resolution

Back 14

- lateral resolution is defined by the beam width
- best lateral resolution results from a narrow beam

Front 15

Electronic focusing

Back 15

- can only be done if there is more than one crystal in the tx
- also called array (multiple crystals)

Front 16

How can we change the focusing?

Back 16

1. lenses
2. curved elements
3. electronic focusing
4. mirrors
We can only affect the beamwidth in the near field.
If we need a deeper focus, use a larger diameter crystal

Front 17

Lenses used to change the focus

Back 17

advantage:
-sound can be converged more rapidly
-produce a shallower focus
disadvantage:
-creates a supl. acoustic impedance mismatch
-heating of tx

Front 18

Curved surface focusing

Back 18

disadvantage:
- PZT very brittle, difficult to construct
advantage:
-newer materials are more flexible
-eliminates the acoust. imped.mismatch and absorbtion from lens

Front 19

Focus and intensity

Back 19

By focusing the beam:
- intensity is increased
- power is increased
- area diminish
intensity = power / area

Front 20

A mode

Back 20

amplitude mode
- can only look straight ahead, creating a one dimensional scaning technique

Front 21

B mode

Back 21

- Brightness mode
- 2 dimensions, can see more than one region simultaneously

Front 22

1D arrays tx

Back 22

- can not focus in elevational plane
- can not acquire data in the 3rd dimension

Front 23

1.5 D arrays tx

Back 23

- developed in 90'
- 32 elements in elevational plane
64 elem. in lateral dimension
- allow focusing at two different focal depths in elevation

Front 24

Relation between line density, lateral resolution, color and frame rate

Back 24

line density increased
= better lateral resolution

line density decreased
= better color
= better frame rate

Front 25

If we change the sector from 90 degrees to 45 degrees, what will happen to the scan lines? what about the frame rate ?

Back 25

=scan lines * 2 (will double)
= frame rate * 2 (will double)

Front 26

Non imaging modalities (CW Doppler, Non imaging PW, blind M mode) are governed by PRF

Back 26

PRF = round trip travel time
pulser drive voltage
Since there is no image formation, temporal resolution is high

Front 27

Imaging modalities are governed by the frame rate or frame frequency

Back 27

that makes the temporal resolution less than for nonimaging modalities

Front 28

What influence beam divergency ?

Back 28

- tx diameter
- frequency of sound

OBS: they also affect natural focus

Front 29

Diameter of a crystal and the beam divergence

Back 29

Crystal Diameter and beam divergence are inversely related
Larger crystal diverge less, therefore have a better resolution in Fraunhofer zone

Front 30

Frequency, Attenuation, Divergence

Back 30

low frequency:
- attenuates less
- diverge more
- lower lateral resolution in far field
higher frequency
- diverge less in fraunhofer zone
- create a better resolution in the far field

Front 31

Larger diameter crystals and higher frequency will create less divergence

Back 31

Smaller diameter crystals and lower frequency will create more divergence

Front 32

Huygens wavelets

Back 32

Spherical waves
diffraction patterns

Front 33

Huygens' Principle

Back 33

A large element can be thought as a milion of tiny sound sources. Each of those sources is than a huygen source and creates a huygens wavelet (v shaped)

Front 34

Linear Switched Array Tx

Back 34

- has a group of elements that can be switched on off electronically
- no steering possibilities
- rectangular image (wider than thicker)
advantage:
- create a wider image in near field (in their time)
disadvantage:
- we can not change focus
- no focus
- expensive in comparison with single element.

Front 35

Mechanical steered tx

Back 35

- make a sector image
- single crystal, wobbled by a motor rotating the crystal
- symetrical beam in elevation and lateral planes
- broad depth of field for a deep focus
- cardiac use

Front 36

Mechanical steered tx

Back 36

- advantage over phased array: less expensive (only two crystals)
- disadvantage;
- fixed focus for transmit and receive
- parts wear out
- motion artifact
- air pockets in gel creates artefact

Front 37

Mechanical annular array

Back 37

- sector image with curved top
- multiple concentric elements

Front 38

lateral plane
synonims

Back 38

azimuthal
side by side
transverse
angular

Front 39

axial plane
synonims

Back 39

radial
depth
longitudinal
range

Front 40

fo (pulsed mode)

Back 40

fo = c/ 2*thickness

Front 41

fo(continuous mode)

Back 41

determined by the frequency of the drive (trasmit) voltage

Front 42

Blind (Doppler only) tx have a broad depth of field
True / False

Back 42

True

Front 43

A phased array tx must have multiple elements.
True / False

Back 43

True

Front 44

A mechanical system can steer and focus the beam electronically
True / False

Back 44

False

Front 45

An annular array can be focused electronically but must be steered mechanically.
True / False

Back 45

True

Front 46

An annular tx has the distinct benefit of variable focus in both elevation and lateral planes. A disadvantege with respect to phased tx is that annular tx:
a. cannot image deeper than 10 cm
b. is inherently less sensitive
c. must be steered mechanicaly
d. can not perform Doppler or color flow

Back 46

c. must be steered mechanicaly

Front 47

Which of the following has a rectangular image?
a. annular
b. sector
c. linear switched array
d. more than one of the above

Back 47

c. linear switched array

Front 48

A fundam. difference between a linear swithced array and a linear phased array is that a linear switched array:
a. has a fixed focus and cannot steer
b. has a variable focus and can steer
c. has a lower maximum PRF

Back 48

a. has a fixed focus and cannot steer

Front 49

Sector phased array principally create a sector image
a. for rib acces
b.to allow for more lines per scan
c. for cost effectiveness
d. to minimize the time it takes to shoot a frame

Back 49

a. for rib acces

Front 50

The foot print of a tx has 2 dimensions. The beamwidth is associated with the......... dimension and the slice thickness is determined by the ..... dimension.
a. axial, elevation
b. elevation, lateral
c. elevation, axial
d. lateral, elevation

Back 50

d. lateral, elevation

Front 51

Which of the following does not belong?
a. axial
b.longitudinal
c. depth
d. radial
e. azimuthal

Back 51

e. azimuthal

Front 52

Which of the following is not the same as lateral resolution?
a. radial
b. angular
c. Azimuthal
d. Transverse

Back 52

a. radial

Front 53

Which focusing technique best improves the focus in the far field?
a. lens
b.mirrors
c.electroning focusong
d.curved elements
e. none, one can only improve focus in the near field

Back 53

e. none, one can only improve focus in the near field

Front 54

radial resolution equals

Back 54

SPL/2

Front 55

Lateral resolution equals the ...... and varies with depth since the .... varies with depth

Back 55

beamwidth, beamwidth

Front 56

When a tx is refered to as mechanical, it means that the ... is performed mechanically

Back 56

steering

Front 57

A..... is used to help focus a beam in elevation plane

Back 57

lens

Front 58

The ..... plane coresponds to the beam thickness

Back 58

elevation

Front 59

the ..... ...... is used to minimize the acoustic impedence mismatch between the PZT and tissue

Back 59

matching layer

Front 60

The ........ ........is used to shorten SPL , improuving longitudinal resolution

Back 60

backing material

Front 61

the focus is where beam reaches its minimum..... and maximum........

Back 61

diameter, intensity

Front 62

the focal region is also called ..... of field and refers to area of the beam which is most tightly......

Back 62

depth, focused

Front 63

...... is the ability to distinguish between 2 objects

Back 63

resolution

Front 64

....... resolution is also called axial, radial, and depth resolution

Back 64

longitudinal

Front 65

..... resolution is the ability to distinguish between objects along the beam direction

Back 65

longitudinal

Front 66

...... resolution is also called angular, azimuthal, or transverse resolution

Back 66

lateral

Front 67

...... resolutin is the ability to distinguish between structures that are side by side

Back 67

lateral

Front 68

In .... the operating freq. is determined by the speed of sound in the crystal and the crystal thickness

Back 68

pulsed mode

Front 69

In pulse mode a thicker crystal will have a .... operating freq

Back 69

lower

Front 70

In .......... a thicker crystal will not change the operating freq.

Back 70

continuous mode

Front 71

the .... appears at a distance of one NZL

Back 71

focus

Front 72

A phased array tx uses ...... focusing in the lateral dimension and a ...... for focusing in the elevation dimension

Back 72

electronic, lens

Front 73

Beyond Fresnel zone, the beam is said to be ...... limited

Back 73

diffraction

Front 74

Bmode is an example of ..... wave mode

Back 74

pulsed

Front 75

color flow is an example of ....wave mode

Back 75

pulsed

Front 76

CW Doppler is an example of ...... wave mode

Back 76

continuos

Front 77

The use of the name mechanical for a tx implies the means by which it is:
a. focused
b. steered
c. assembled
d. all of abave

Back 77

b. steered

Front 78

Which tx style has a variable focus in both lateral and elevation plane?

Back 78

annular array (almost extinct)
2-D arrays

Front 79

Which of following tx can be electronically steered?
a. sector phased array
b. annular array
c. phased linear array
d. linear switched array

Back 79

a. sector phased array
c. phased linear array

Front 80

What factors can influence the lateral resolution ?

Back 80

Any factor that can affect the beamwidth; frequency, aperture, focus, multiple transmit foci, parallel procesing

Front 81

What factors can influence the longitudinal resolution ?

Back 81

Any factor that can influence SPL such as: frequency, wavelenght, propagation speed, backing or dumping material, excitation pulse driving, crystal...etc