[Ultrasound Physics] TRANSDUCERS AND ARRAYS

[Contents]

 

1. Types of Displays

2. Terms to Know 

3. Linear array

4. Phased array

5. Convex/Curved Arrays

6. Lobes

1. Types of Displays

1) A-Mode, or Amplitude Modulation [★]

- display of amplitude spikes of different heights

- does not need scan converter

- only 1 line of sight are sampled and displayed

- used for ophthalmology studies to detect finding in the optic nerve

- consists of x-axis (depth/distance) and y-axis (amplitude)

- ex) internal contents of a simple cyst: an area with no spikes

 

2) B-Mode, or Brightness Modulation

- most common form of ultrasound imaging

- display of 2D map of B-mode data

- based on brightness with absence of vertical spikes

- brightness depends upon the amplitude/intensity of echo (image of large/brighter and small dots)

- no y-axis on B-Mode. x-axis (depth) and z-axis (echo intensity or amplitude)

- transducers used for diagnostic ultrasound are damped to improve axial resolution

               : damping – reduce pulse duration and spatial pulse length

               : usually have pulse length of 1-3 cycles

- frequency for doppler is usually lower than for imaging in a given transducer

               : B-mode real-time imaging = 10-50 Hz

- blood vessels usually appear anechoic on B-mode imaging

               : reflection from the RBC is too weak to be displayed

 

3) M-Mode, or Motion Mode (= Time Motion or TM-Mode) [★★]

- display of a one-dimensional image

- used for analyzing moving body parts (commonly in cardiac and fetal cardiac imaging)

               : time, motion pattern, and amplitude

- useful for measuring dimensions of structures

               : produces display more similar to a tracing than actual anatomic picture

- repeatedly measure distance of the object from a single transducer at a given moment

: record the amplitude and rate of motion in real time

- limitations

               : information is obtained along only one line of sight

                              - single sound beam (single line of sight) transmitted

                              - depth of reflectors along a single line of sight vs. time

: displayed as dots of varying intensities

               : motion lateral to the transducer is not displayed

               : 2-D shape of a structure is not shown

2. Terms to Know

 

1) Sector display

- wedge of a circle.  A narrow near field and broader far field.

 - phased array

 

2) Linear display=rectangular

- linear array

 

3) Footprint

- Aperture of the outside of probe. What touches the patient

- small footprint transducers

               : beam diverges very rapidly in the far field → poor lateral resolution

 

 4) Array

- A collection of transducer crystal elements.

 

5) Steering

-  Sending the pulse out in different directions

- Directing the beam

- Electronic steering/Phasing

: steering by using small time delays between excitation pulses to each elements within the array

 

6) Dynamic receive focusing [★]

- rephrase the signals by dynamically introducing electronic delay circuitry upon reception

- curved wavefront cause reflected echoes to be received at varying times at the different elements

- echoes that are received first are held in delay circuitry

: until all of the echoes from the same depth have been received

- time delays before echo signals from array elements are combined

: results in constructive interference of waves

: produces higher amplitude and more focused signal

 

7) Dynamic aperture [★]

- change number of elements in array receiving reflections

: deeper reflections (take longer to return) - more elements used to receive

- minimize the degree to which beam width varies with depth

 

8) f-number

- ratio of focal length to the size of the aperture

               = focal length/aperture

 

* Main advantage of intracavitary probe

- closer to the area of interest: higher frequency transducer can be used

→ superior spatial resolution

 

* Standoff pad

- used to evaluate superficial mass

- Advantage: increases the distance between transducer and the mass

               → mass will be located close to the elevational focus of sound beam

3. Linear array [★★★★★]

1)

2) creates a rectangular image

- same distance between scan lines in both the near and far fields

- best suited to vascular imaging

 

3) beam focusing: electronically

 

4) beam steering: electronically

               : create trapezoidal shape to improve field of view

 

5) slice thickness: determined by the point of mechanical focusing, along the width of array

: with 1D linear array

 

6) Linear Phased Array

- Elements arranged in a line (linear)

- Display=Sector image (fan)

: Greater depth = greater gaps between scan lines (worse Lateral resolution)

- Small footprint

: Compact: good for tight spaces (예: between ribs)

 

7) Linear Sequential Array 

 

- Elements arranged in a line

- Display=Rectangular Image

- Large footprint

- One element damaged → Vertical drop out line directly below damaged element

4. Phased array [★★★★★]

 

1) Sector display

2) beam focusing: electronically

 

3) beam steering: electronically

 

4) mechanically focused along the elevational dimension

 

5) fires all of the elements for each acoustic scan line using small time delays to steer the beam

- compared to non-phased array

               : smaller transducer footprint

               : precise beam focusing and steering

               : higher sensitivity and dynamic range

               : reduce low amplitude echoes, which removes grating and side lobes

               : ideal for cardiac imaging (poor abdominal)

5. Convex/Curved Arrays [★★★]

 

1)

2) Crystals arranged along an arc

 

3) Display= Blunted Sector Image

: Greater depth = greater gaps between scan lines (worse Lateral resolution)

 

4) Large/Long Footprint (Large up to 10cm long)

               : acquire sonogram with the largest possible field of view both in near field an at depth

 

5) Electronic Steering

 

6) Electronic Focusing

* Annular array

 

 

- beam is symmetric about the beam axis

               : lateral resolution = elevational resolution

 

* Advantage of single crystal (pure-wave crystal) transducer

- wide bandwidth

6. Lobes

1) artifact from sound energy transmitted in a direction other than along the beam’s main axis

- Unwanted sound energy

- side lobe: created by a single crystal transducer

- grating lobe: created by array transducers

 

2) produced by all probes

 

3) Side lobe

- far field energy interferes with lateral resolution (degrades lateral resolution)

- <10% of energy of main US beam (lobes are weaker than primary beam)

- minimized in

               : phased array transducers

               : pulsed mode operation, broad bandwidth pulses

- eliminated by

               : apodization

                              - varying the excitation voltage to each element in the group used to form US pulse

- maximizing excitation voltage for elements near the center of the beam

- reducing it toward the periphery

               : use tissue harmonics

 

4) Grating lobes [★]

-

 

- summation of side lobes generated by linear arrays

: cause smearing of ultrasound beam → degrade lateral resolution

- related to the spacing of elements in the array

- if spacing reduced to less than one wavelength: grating lobes are eliminated

               (but elements are always spaced greater than 1 wavelength)

- Subdicing the elements into smaller sub-elements

: decrease effective distance between elements → reduce grating lobes

- relatively small when sound beam is unsteered

: beam steering (trapezoidal display) → grating lobes become prominent

- technique most helpful in reducing grating lobe = tissue harmonic imaging

 

Reference

 

* Davies Ultrasound Physics review

* https://sites.google.com/site/lindadmsportfolio/ultrasound-physics/

* https://sites.google.com/site/nataljasultrasoundphysics/

* https://sites.google.com/site/ektasphysicseportfolio/doppler