091 Module 2 - Ultrasound Transducers and the Soundbeam

Name the three main transducer groups.

• Linear array

• Curved linear array

• Phased sector array

List the different names for a phased sector array.

• sector probe

• cardiac probe

Explain the meaning of a 1D phased array. Draw an example of a 1D vs a 2D.

1D: single row of crystals

Phased array: crystals are stimulated electronically

Describe the display format, frequency range, and clinical usage of sector transducers.

• pulses originate from the same starting point → very small nearfield

• small footprint for rib access

• pie shaped displays

• prioritize temporal resolution

• BW = 1-5 MHz (adult) and 6-12 MHz (pediatric)

• cardiac application

Describe the display format, frequency range, and clinical usage of linear transducers.

• pulses originate from different starting points and move in the same direction (scan lines are parallel)

• various sizes of footprints (larger than sector)

• rectangular image format (nearfield size = farfield size)

• BW = 12-3 MHz, 12-5 MHz, 18-7 MHz

• superficial structure application

  • breast 

  • tendons and muscle

  • intraoperative

• detail resolution more important than temporal resolution

Nearfield is also called the ______ zone.

Fresnel

Farfield is also called the ______ zone.

Fraunhofer

Describe the display format, frequency range, and clinical usage of curved (convex) linear transducers.

• pulses originate from different starting points and travel in different directions

• wide near and farfield

• larger footprint of various sizes

• BW = 5-1 MHz, 6-2 MHz, 9-4 MHz

• deep structure applications

  • abdominal

  • neonatal

  • endocavity

• detail resolution more important than temporal

List some abilities of 3D matrix transducers.

• uniform slice thickness

  • no focal point in elevational plane

• volumetric imaging (3D imaging)

• scan in multiple planes at the same time

Name the most simple transducer and its applications.

Pedof transducer

• continuous wave

• single crystal design

• Doppler applications only

  • used for cardiac and peripheral vasculature

• only graph, no image

Define and explain Huygen’s principle.

Each individual crystal creates its own wavelet which interact to become a wavefront.

Define wavelet.

Sound produced from 1 transducer crystal

Define wavefront.

Multiple wavelets interacting.

Define sound beam.

Multiple wavefronts interacting.

Name the dimensions of the sound beam. Which dimensions create the imaging plane?

• axial - length

• lateral - width

• elevational - depth

Axial and lateral create the imaging plane.

Label this diagram.

1. focal point

2. elevational plane

3. imaging plane

4. lateral resolution

5. axial resolution

Axial resolution, lateral resolution, and elevational resolution can be grouped together as…

Detail resolution

SPL is proportional to _____, which is inversely proportional to _____.

wavelength, frequency

What determines the area of best elevational resolution?

The focal point of the elevational plane is determined by the type of the transducer.

Label the diagram. Provide 3 different terms for #2 and 2 different terms for #3.

1. focal zone

2. Fresnel zone / near zone length (NZL) / nearfield

3. Fraunhofer zone / farfield

What separates the nearfield and farfield?

Focal point

Where along the sound beam should the structure of interest be?

In the nearfield or focal zone.

Write the equation for NZL.

NZL = [D2 (mm) x f (MHz)] / 6

When aperture increases, the focal point moves _____ to the transducer.

closer

Where along the sound beam is the best lateral resolution? Why?

Focal point, ↓area → ↑intensity

Structures within the focal zone have increased ______ and _______ resolution.

lateral, contrast

Define resolution.

Processing or distinguishing somethin into its consituent parts.

List the three types of ultrasound imaging resolution.

• detail resolution

• contrast resolution

• temporal resolution

Give another term for detail resolution.

Spatial resolution

List some factors determining temporal resolution.

• FR

• FT

• PRP/PRF

• depth

• FOV

• #scan lines

• scan line density

Improving the ______ will improve contrast resolution.

SNR

Define noise in terms of contrast resolution. Where is it mostly seen?

Low level echoes returning from artifact interactions.

Mostly seen in anechoic spaces

Give an example of why noise in anechoic spaces are detrimental to diagnoses.

Noise artifact can present as pathology.

i.e. noise and thrombus look very similar in anechoic spaces

What are the units of measurement for temporal resolution?

Hz = frames/sec

When an ultrasound image has choppy sequences, the ______ resolution is low.

temporal

Calculate FR if PRP=3ms and #lines=300. Is this scan diagnostic?

PRP = 3ms

#lines = 300

FT = PRP x #lines

FT = 3ms x 300

FT = 900ms

FR = 1/FT

FR = 1/900ms

FR = 0.0011kHz

FR = 1.1Hz

This is very low temporal resolution as there is only 1.1 frames per second. FR<15Hz so this scan is not diagnostic.

Label the settings listed in this image.

1. transducer BW in MHz

2. FR

3. depth

List some frame time variables that can be changed by the sonographer.

• Maximum depth

• # of scan lines

  • transducer type

  • FOV size

  • scan line density

  • multi-focus

  • THI

  • CD/PD

At the same depth, which transducer has the highest frame rate? Why?

Sector probe - fewest # scan lines and smallest FOV

Decreasing from 10cm depth to 5cm depth, what happens to FR?

↑FR

If scan line density is increased, what will happen to the FR? What kind of resolution(s) does this change?

↓FR → ↓temporal resolution

↑ lateral resolution

What is a reasonable FR before the human eye is capable of detecting “flicker” or “motion blur” or slow motion effect?

15Hz

<15Hz is not diagnostic because the temporal resolution is too slow.

Define scanned modes of imaging and give some examples.

• 2D B-mode

• 3D B-mode

• Colour Doppler

• Power Doppler

Imaging mode that requires multiple scan lines across a FOV.

Define non-scanned modes of imaging and give some examples.

• M-mode

• PW and CW Spectral Doppler

Imaging mode that uses one scan line to produce a graph.

Describe the purpose of nuchal translucency measures. What kind of resolution is required to be high?

Measure of the fetus to predict genetic abnormalities. Requires high detail/spatial resolution to keep the measurement accurate.

Axial resolution =

SPL/2

Axial resolution distinguishes to structures on the ____ axis as the sound beam.

same

If SPL=1mm, what is the required distance between structures to be distinguished.

Axial resolution = SPL/2

AR = 1mm/2

AR = 0.5mm

Any structures equal or greater than 0.55mm is distinguished.

The SPL is 3mm, are reflectors 1mm apart distinguished? What about 2mm?

AR = SPL/2

AR = 3mm/2

AR = 1.5mm

1mm<1.5mm → 1mm apart is distinguished

2mm>1.5mm → 2mm apart is not distinguished

Name 2 influences on axial resolution.

• transducer damping

  • change the number of cycles in the pulse → change PD → change SPL

• frequency

  • change wavelength → SPL

Lateral resolution distinguishes to structures on the ____ axis as the sound beam

perpendicular

Lateral resolution = 

lateral beam width

Lateral resolution is also called…

• transverse resolution

• Azimuthal resolution

Sound beams are not uniform in beam width due to _____, which happens in the nearfield, and _____, which happens in the farfield.

convergence, divergence

Lateral resolution is affected by amount of penetration. True/False.

False - axial resolution is impacted by degree of penetration through frequency.

Where is the worst lateral resolution. Why?

Farfield

Sound beams diverge → ↑area → ↓intensity and ↑beam width

What is the smallest distance between laterally adjacent structures that are distinguished?

2mm

Slice thickness refers to which plane?

Elevational plane.

How is the elevational focal point determined in a 1D array vs 2D array?

1D array - focal point is fixed and determined by lenses or curved crystals.

2D array - focal point is not fixed and can be changed to reduce artifact

Reflectors within the elevational plane can cause ______ ______ ______ in the imaging plane.

partial volume artifact

Where is PVA mostly seen in the image?

Anechoic spaces

What is another name for partial volume artifact?

Slice thickness artifact

Explain why a PVA may appear when a vessel is in long axis but not in short axis.

In long axis, the curved radius of the vessel is in the elevational plane, causing the echo to reflect at an oblique angle that enters the imaging plane. The vessel should be anechoic, hence PVA is seen in the lumen.

In short axis, there is no curved reflector in the elevational plane, hence echoes in the elevational plane reflect at a perpendicular angle and do not enter the imaging plane.