Resolution, Acoustic Windows and Real-Time Transducers

Question 1

What are the types of resolution in clinical ultrasound?

Answer 1

• Spatial resolution
• Temporal resolution
• Contrast resolution
• Colour resolution

Question 2

What does spatial resolution refer to, and what are the two types?

Answer 2

The ability to differentiate small structures on a B-Mode image. The type will determine the smallest structure which can be visualised on the image.

Axial and Lateral

Question 3

What are four factors that spatial resolution can be affected by

Answer 3

• Beam characteristics
• Line density (specifically for L.R)
• Storage decide (scan converter)
• Resolution of viewing monitor

Question 4

Define axial resolution

Answer 4

The closest distance two structures can be along the axis of the beam and which can still be at different entities (clearly resolved on monitor)

Question 5

Axial resolution is directly related to _______________

Answer 5

Spatial pulse length

A.R = 1/2 SPL
(Remember SPL = wavelength x n)

Question 6

List the factors affecting axial resolution

Answer 6

Factors specifically affecting the SPL
- Frequency (higher freq means
shorter wavelength = shorter SPL)
- Transducer design (better
dampening = fewer cycles =
shorter SPL

• Gain / Power
  If increased, the SPL will increase because the very low aptitude ends of the pulse/echo will be larger and therefore detectable.
• Beam width - in the case of a beam passing through oblique laying interfaces.
• Pixel resolution
• Monitor resolution

Question 7

If an interface is further from the transducer, what happens to axial resolution?

Answer 7

Nothing as depth/ distance from the transducer does not affect axial resolution

Question 8

Define lateral resolution

Answer 8

The closest distance two structures can be at 90 degrees across the axis of the beam at the same depth, and which can still be seen as seperate entities

Question 9

The main factor affecting lateral resolution is _________

Answer 9

Beam width

L.R approx = BW

Question 10

If beam width is reduced, will lateral resolution will ____

Answer 10

Improve

Question 11

List the factors that determine lateral resolution?

Answer 11

1. Transducer crystal diameter
2. Transducer frequency
3. Focussing
4. distance from the transducer (depth)
5. Output power/ gain settings
6. Line density
7. Scan converter pixel size
8. Monitor resolution

Question 12

How does beam focussing affect lateral resolution?

Answer 12

Beam focussing reduced beam width and therefore improves lateral resolution. So, at the focal zone there is improved lateral resolution, whereas in the near and far field will be worse.

Question 13

Discuss how lateral resolution is affected by line density. What is the limit to which this is affected?

Answer 13

The closer together the beam paths are (increased line density), the better potential resolution. However, this cannot be improved beyond the beam width limitation.

Question 14

Define contrast resolution

Answer 14

The ability to differentiate tissues of different echogenicity

(sometimes a structure can be quite large, but because it is almost identical in appearance to the surrounding tissue, it is very difficult to differentiate it as seperate)

Question 15

List the factors that affect contrast resolution

Answer 15

• Backscatter interference/
  background noise
• Slice thickness
• Inherent electronic characteristics
  of the machine and transducer
  connection

Question 16

How does backscatter interference affect contrast resolution

Answer 16

• Echoes returning to the transducer will interact with each other and cause complex, low intensity interference patterns. This will cause a “haze” over the image - the use of harmonics can reduce this

Question 17

How does slice thickness affect contrast resolution

Answer 17

If a beam passes through a tissue in front of or behind a leison, echoes will be returned from the surrounding tissue and overwrite the leison, thus reducing contrast separation between the two. Thin slices will improve contrast resolution.

Question 18

Define temporal resolution. How is it optimised?

Answer 18

The ability to resolve rapidly moving structures - this is dependent on the frame rate. High frame rates are required to optimise temporal resolution.

Question 19

Define colour resolution

Answer 19

A term used to describe the spatial resolution of the doppler colour when defining moving structures (usually blood - aka how well the colour actually represents an area of blood flow). This is always much worse than B-Mode spatial resolution.

Question 20

What are acoustic windows?

Answer 20

A term referring to a structure or anatomical configuration that allows deeper anatomy to be visualised by ultrasound. They will cause little beam attenuation and used to view anatomy from as many views as possible.

Question 21

Provide some examples of good acoustic windows

Answer 21

Full bladder (to image uterus/ovaries/prostate)

Liquid filled stomach (to image pancrease)

Amniotic fluid around baby (to image foetus)

Liver/spleen (to image kidneys)

Anatomical configurations such as bone free fontanelle and rib spaces

Question 22

What are real time transducers? What are the four principle types?

Answer 22

Real time transducers automatically perform repeated sweeping of the U/S beam across the anatomy to form a single frame. The four basic types are:
1. Sector (phased array)
2. Linear
3. Convex (sam as linear, but curved transducer head)
4. Radial

Question 23

What is the difference between a phased array and linear array transducer?

Answer 23

Phased array involves all crystals of the transducer firing in a sweeping motion, whereas linear array is groups of crystal elements firing and focussing.

Question 24

Which type of array transducer has higher frame rates

Answer 24

Phased array (sector) (cardiac)

Question 25

Discussed sector (phased array) transducers and how they work

Answer 25

Transducer is composed of a number of narrow crystal elements. The beam can be made to transmit at an angle from the probe by altering the firing sequence - altering time delays to adjust beam direction. This is known as ELECTRONIC STEERING.
These very short time delays allow the elements to be energised from one direction to the other in a sequence, performing a sweeping motion. The direction of the sound beam will always travel at 90 degrees to the wavefront.
The time delays can also allow for beam focussing.

Question 26

What are the advantages and disadvantages of sector transducers?

Answer 26

Advantages
- Very small probe footprint
- No moving parts - reliable/robust
- Adjustable focussing
- Multiple focal zones can be used
- High frame rate capability
- Ideal for CARDIAC

Disadvantages
- Expensive
- Electronic steering means more
susceptible to grating lobe artefact
- Orthogonal plane focus is fixed (although some modern can overcome)

Question 27

Describe a linear array transducer

Answer 27

Composed of a large number of small PZT crystals arranged in a linear format, each individually wired and acoustically insulated.
GROUPS of elements are fired for each beam path to improve focussing capabilities and beam profile. When a group of crystals is pulse, the effective crystal diameter is much larger and creates a longer near field and focussing.

Question 28

Discuss the crystal spacing in a linear transducer and how this can affect the number of scan lines

Answer 28

To produce multiple beam paths, groups of elements are stepped ONE CRYSTAL SPACING across the transducer face. It is possible to double the number of scan lines by moving the group one half a crystal spacing - this is done by adding one crystal to the next group (increasing by 1) therefore moving the centre of the group one half.

Question 29

What are the advantages and disadvantages of linear array transducers

Answer 29

Advantages
- No moving parts - robust/reliable
- Wide near field view
- Adjustable focussing
- Multiple focal zones
- Colour doppler flow capability

Disadvantages
- Large probe footprint - causes problems with small acoustic windows
- Relatively narrow far field of view (doesn’t have the sector capabilities like phased array/ curved transducer)

Question 30

Describe the convex array transducer

Answer 30

Similar construction/operation to linear array, however the face is curved in an arc - this decreases the footprint and increases the far field of view (which is one of the downfalls of a linear array).
Very popular transducer for general sonography

Question 31

What are the advantages and disadvantages of convex array transducers

Answer 31

Advantage
- Smaller footprint
- Wide far FOV
- Relatively wide near FOV

Disadvantage
- Footprint sometimes still too large for very small acoustic windows

Question 32

Describe a radial transducer

Answer 32

Principle used in intravascular catheters. The beam source is made to rotate 360 deg emitting radial beams.
Type 1 is mechanical
Type 2 has a phased array design

The elements form a circle in a radial pattern, allowing for a free internal lumen for a guide wire etc.

Question 33

What are three forms of B-Mode steering

Answer 33

1. Beam steering
2. Sector steering
3. Linear array steering

Question 34

What is beam steering?

Answer 34

This steering forms the field of view - steering from right to left to build the image FOV

Question 35

What is sector steering?

Answer 35

In sector convex transducers, if the entire FOV is not used, the active sector segment may be steered across the available sector area. This enables a reduced sector FOV to be adjusted without moving the probe

Question 36

What is linear array steering?

Answer 36

This useful feature allows the beams to be directed at 90 degrees to an angles specular reflector for better visualisation (e.g. a biopsy needle). It is useful for interfaces that are not perpendicular to the transducer.