[Ultrasound Physics] COMPONENTS OF ULTRASOUND

[Contents]

1. Pulser

2. Beam former

3. Transducer

4. Receiver

5. Memory (Scan converter)

6. Display

* In order: Pulser → Beam former → Transducer → Receiver → Memory (scan converter) → Display

 

* Analog signal

- does not have discrete steps

- values may vary continuously between minimum and maximum point

               (continuous variation of the signal is possible)

 

* Digital signal

- have discrete values that have fixed steps between values

- bits determine levels in digital system

 

1. Pulser (= Voltage Generator, Transmitter) [★]

1) originates action

 

2) Sends an Electric voltage pulse (EVP) to transducer through the beam former

- EVP (electric) = analog part

- Starting piezoelectric effect

 

3) Output power control

- adjust to increase or decrease the intensity of transmitted pulse

- output power: most closely affects patient exposure

 

4) PRF (Pulse repetition frequency) of Pulser = PRF of Transducer

2. Beam Former [★★]

1) Sends EVP to Transducer.

- EVP (electric) = analog part

 

2) Responsible for

- Aperture control

- Beam steering

- Focusing (shaping of beam)

- Apodization: reducing side lobes, grading lobes, gets rid of additional beams

(improves lateral resolution)

 

3. Transducer [★★★]

1) Converts one form of energy to another

 

2) Change EVP to MVP, vice versa (w/ applied pressure) = The Piezoelectric Effect

- The Piezoelectric Effect

: Mechanical deformation results when an electric voltage is applied to certain crystal materials

: Varying electrical signal is produced when the crystal structure is mechanically deformed

- When electric signal is applied to a piezoelectric element

               : element expands and contracts to produce mechanical vibrations (sound waves)

- EVP and MVP: neither analog nor digital

: Mechanical Voltage pulse (MVP) = sound wave

- Sends long MVP into body. Receives returning MVP

- Converts returning MVP to EVP, sends EVP to receiver

 

4) Matching layer: between piezoelectric element and tissue

- places on face of element

- reduce acoustic impedance mismatch between the element and tissue

               : improve sound transmission, reduces reflection

- multiple matching layers

: increase transducer bandwidth (short duration) → improve axial resolution

- optimal thickness = ¼ wavelength

- not) used for focusing

 

5) Backing material

- Advantage

: dampen US pulse and reduce spatial pulse length → improves axial resolution

: control ringing of piezoelectric element

- Disadvantage

: reduces sensitivity

: decrease overall intensity of sound beam

 

6) Radiofrequency shield

- used around the crystal and backing material

- Purpose

               : reduce noise from electromagnetic interference

               : enhance sensitivity to weak signals

 

7) commonly used material in modern transducer element

- lead zirconate titanate

 

8) Strain relief

- portion of transducer that protects the insertion of the cable into the transducer housing

- area prone to wear and tear with repeated use and bending of the cable

- strain relief area should be regularly inspected for cracks and exposed wiring

- use of a transducer should be discontinued if a crack appears in any area

 

9) Damage to the lens or transducer crystals

- result

               : degradation of image quality

               : underestimation of maximum flow velocity

4. Receiver/Signal Processor

 

1) Receives EVP signals from Transducer

- EVP (electric) = analog part

 

2) Alters the signal to make suitable for processing in memory

- Improves image

- Any pre-processing function = Receiver

- Refines signal through 5 functions of Pre-processing (ACDCR)

 

* Gain

- brightness of entire image changes.

- system control that determines the amount of amplification that occurs in the receiver

 

3) Between Receiver and Memory: Analog to Digital Converter (ADC)

5. Memory/Image Processor/Scan Converter (if digital) [★★]

 

1) Storage for the signals returning

- Digital

: stores signals as numbers

: Only Digital or Scan Converter are currently in use

: Allows more signal storage per location

 

2) Storage components

- Matrix board (checker board) = 1 Bit

: pixel (individual square) = picture element

- each pixel stores a signal (signal location)

- binary number (0-1) stored in pixel

- single bit can represent 2 levels of information

: 8bits (Matrix boards) = 1 Byte

- Matrix boards can be stacked

: Allows multiple signals stored per single location → Better Image

: Usually 6-8 matrix boards

- spatial resolution of scan converter is determined by # of pixels in the matrix

- Between Memory (scan converter) and Display: Digital to Analog Converter (DAC)

6. Display/Cathode Ray Tube (CRT)

 

1) Viewing Tube

- Phosphor covered tube

- the image is made for viewing

- not what we see. inside machine

 

2) Sends EVP to the monitor

- EVP (electrical) = analog

 

3) Three colors used on color monitor to produce range of available colors

- RGB (red, green, blue)

 

7. Five Functions of the Receiver (ACCDR) [★★★]

1) Amplification (= receiver gain) [dB]

- Returning echoes vary in strength

- Each returning signal is amplified uniformly

- purpose of preamplification of incoming signal

               : to increase echo voltages before noise is induced through the cable

- Operator adjustable

 

2) Compensation [=Time Gain Compensation (TGC); Depth Gain Compensation (DGC)] [dB] [★★★★★]

- Compensates for attenuation at different depths

- Produces an image with uniform brightness regardless of the depth of the reflector.

: optimize the image producing very bright echoes on display

: equalize differences in echo amplitudes received from similar structures at different depths

- Operator adjustable

 

3) Compression (=Log Compression; Dynamic Range) [dB]

- Dynamic Range/Shades of Gray

               : ratio of the largest to the smallest signal that a system can handle

: Determines the extent a signal can vary and maintain accuracy

: Narrow dynamic range = fewer shades of gray (High contrast)

: Wide dynamic range = many shades of gray (Low contrast)

- display cannot accommodate the wide dynamic range of the incoming signals

- Decreases dynamic range of the processed signal (Equalizes difference between signals)

               : Increases smaller signals, reduces larger signals

: Keeps gray scale images within 20 distinguishable shades of gray

- Does not alter the incoming signals; i.e. larger signals are still large and smaller signals remain small.  

- Partially operator adjustable (gray scale adjustments)

 

4) Demodulation (=Detection)

- Changes the form of echo voltages into an appropriate form for the display monitor.

1. Rectification: Turns negative portion of radiofrequency (RF) signal to positive

2. Smoothing (enveloping): even out the rough edges.

- Not operator adjustable (the only function that is not operator adjustable)

 

5) Rejection/ filter (=Threshold; Suppression)

- eliminates non-diagnostically relevant low-level signals/noise → cleaner image.

               : increased rejection (threshold) level → decreased low-level echoes

- Operator adjustable

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