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Flashcards in Ultra-Sound Imaging Deck (48):

What is Ultra-Sound?

A mechanical wave with frequency in the RF spectrum between 1 & 15MHz


Speed of sound in tissue

~1540 m/s


How are US waves produced?

Transducer with array of up to 512 active sources


Wave Propagation Diagram (2)

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  1. Compressional Pressure pc
  2. Rarefactional pressure pr

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Direction of particles vibration is the same as ...

... the wave propagation, US waves are purely longitudinal


An US wave has a higher velocity if ... (2)

  • More rigid tissue
  • Less dense tissue


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Particle Velocity Equation

Time derivtive of particle displacement


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Pressure Eqn

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Characteristic Acoustic Impedance Equation

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Z determined by ...

... the physical properties of the tissue

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Strongest reflected signal is received if ...

... the angle between incident wave and boundary is 90 degrees


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Backscatter signal detected by transducer is maximised if ...

... Z1 or Z2 is zero


If Z1 and Z2 are equal in value, there is ...

... no backscattered signal and the tissue boundary is undetectable


Scattering by small structure

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  1. Rayleigh scattering of an US beam by a structure which is small (US wavelength comparison), also known as speckle noise
  2. Scattering from close together structure produces waves that add constructively
  3. Scattering structure which are far from each other produces either constructive or destructive waves



US Instrumentation Diagram (4)

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  1. A-D converter
  2. Logarithmic Compression
  3. Time-Gain Compensation
  4. Tx/Rx Switch

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US Generation Method (5)

  • Frequency generator sends i/p signal to transducer
  • Pulse voltage sigals are amplified & fed to Tx/Rx switch
  • Amplified V is converted to mehanical pressure wave
  • Backscattered pressure waves converted to voltages
  • Time-gain compensation is used to reduce the dyamic range of the signals



Transducer Diagram (4)

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  1. Epoxy
  2. Backing Material
  3. PZT-4
  4. Matching Layer

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What is PZT made from?

Lead Zirconate Titanate


Distance of Matching Layer Equation

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Pulse produced at a certain rate are called ...

... Pulse Repetition Rate (PRR)


Increasing the mechanical damping of a transducer ...

... reduces the duration of the pressure wave


Efficient damping of a transducer is also important for achieving ...

... a large frequency b/w


B/W of modern transducer is ...

... extremely large


Term for near-field zone

Fresnel Zone


Term for far-field zone

Fraunhofer Zone


Boundary between near and far-field zone is the ...

... near-field boundary (NFB) and occurs @ dist ZNFB


ZNFB equation

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Lateral Resolution

FWHM = 2.36\alpha


Axial Resolution Eqn

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Diagram of US Wave tissue boundary

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A linear array consists of how many rectangularly shaped PZT elements?

128 - 512


Each element of the linear array is ... (2)

  • Unfocused
  • Mechnically & Electrically isolated from its neighbours



Sequential excitation os a small group of elements produces a series...

... US line which lie parallel to one another, thus an image is built up



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Why are voltage pulses slightly delayed in time w.r.t each other?

To produce an effective focused beam for each line


What sized are phased arrays in comparison to linear arrays?

  • Phased: 1 - 3cm
  • Linear: 10 - 15cm



When are Linear Arrays used?

When a large FoV is required for images close to the surface

e.g. - musculoskeletal applications


When are phased arrays used?

When the acoustic window is small

e.g. - cardiac imaging where the US must pass between the ribs to avoid reflection from the bone


How is Beamforming achieved? (3)

  • Applying voltage pulses to each individual element of the array at different times
  • Produces a composite waveform
  • Applying voltages symmetriclly w.r.t the centre element causes the beam to focus at a point half-way along the array

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How does Beam steering work?

Changing the patern of excitation of all elements

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What is Dynamic Focusing? (2)

  • The resolution of the image
  • Used to focus at specific depths in the tissue


How does dynamic focussing work? (2)

  • A small number of elements are used to acquire signals from very close to transducer surface
  • Subsequent excitations using increasingly higher number of elements for focussing at points deeper within the tissue


Amplitude (A)-mode scanning acquires ..

... a 1-D line image which plots amplitude of the backscattered echo vs time & uses HF (10-20 MHz)

applications: non-invasive measuring of corneal thickness


Moiton (M)-mode data aquisition involves acquiring a ...

... a continouous series of A-mode lines and displaying them as a function of time.

The brightness represents the amplitude of the backscattered echo.

Applications: cardiac imaging & foetal ultrasound


2-D Brightness (B) mode scans ... 

... each line in the image is an A-mode line with the intensity of each echo being represented bu the brightness of the scan.

B-Mode is the most commonly used procedure


Clinical Applications of US (3)

  • Obstetrics and Gynaecology
    • Size of foetal head and extend of developing brain
    • Condition of spine
  • Breast Imaging
    • Differentiate between solid and cystic lessions
  • Echocardiography
    • Ventricular size and function



SNR Characteristics (3)

  • Higher the intensity of transmitted US pulses, the higher the amplitude of detected signals
  • Higher the op. freq of the transducer, the greater the tissue attenuation and lower the signals from deeper parts within the body
  • Stronger the focussing @ a particular point, the higher the signal at that point. outside of this depth, SNR is very low



Spatial Resolution Characteristics (2)

  • Lateral Resolution: Higher the freq, the better the lateral resolution for both single element and phased array transducers
  • Axial Resolution: given by half the length of  the ultrasound pulse. Higher degree of damping (higher op. freq), the shorter the pulse and better axial resolution


What changes in the images from left to right?

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  • Right - no signal penetrated through the object
  • Middle - lots of noise in middle image
  • Left - Better contrast and best resolution


Frequency is the changing paramter, higher freq = highre resolution, BUT less penetrating depth