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Ultra-Sound Imaging Flashcards

(48 cards)

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1
Q

What is Ultra-Sound?

A

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

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2
Q

Speed of sound in tissue

A

~1540 m/s

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3
Q

How are US waves produced?

A

Transducer with array of up to 512 active sources

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4
Q

Wave Propagation Diagram (2)

A
  1. Compressional Pressure pc
  2. Rarefactional pressure pr
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5
Q

Direction of particles vibration is the same as …

A

… the wave propagation, US waves are purely longitudinal

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6
Q

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

A
  • More rigid tissue
  • Less dense tissue
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7
Q

Particle Velocity Equation

A

Time derivtive of particle displacement

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8
Q

Pressure Eqn

A
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9
Q

Characteristic Acoustic Impedance Equation

A
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10
Q

Z determined by …

A

… the physical properties of the tissue

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11
Q

Strongest reflected signal is received if …

A

… the angle between incident wave and boundary is 90 degrees

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12
Q

Backscatter signal detected by transducer is maximised if …

A

… Z1 or Z2 is zero

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13
Q

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

A

… no backscattered signal and the tissue boundary is undetectable

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14
Q

Scattering by small structure

A
  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
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15
Q

US Instrumentation Diagram (4)

A
  1. A-D converter
  2. Logarithmic Compression
  3. Time-Gain Compensation
  4. Tx/Rx Switch
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16
Q

US Generation Method (5)

A
  • 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
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17
Q

Transducer Diagram (4)

A
  1. Epoxy
  2. Backing Material
  3. PZT-4
  4. Matching Layer
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18
Q

What is PZT made from?

A

Lead Zirconate Titanate

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19
Q

Distance of Matching Layer Equation

A
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20
Q

Pulse produced at a certain rate are called …

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A

… Pulse Repetition Rate (PRR)

21
Q

Increasing the mechanical damping of a transducer …

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A

… reduces the duration of the pressure wave

22
Q

Efficient damping of a transducer is also important for achieving …

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A

… a large frequency b/w

23
Q

B/W of modern transducer is …

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A

… extremely large

24
Q

Term for near-field zone

25
Term for far-field zone
Fraunhofer Zone
26
Boundary between near and far-field zone is the ...
... near-field boundary (NFB) and occurs @ dist ZNFB
27
ZNFB equation
28
Lateral Resolution
FWHM = 2.36\alpha
29
Axial Resolution Eqn
30
Diagram of US Wave tissue boundary
31
A linear array consists of how many **rectangularly shaped PZT elements?**
128 - 512
32
Each element of the linear array is ... (2)
* Unfocused * Mechnically & Electrically isolated from its neighbours
33
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
34
Why are voltage pulses **slightly delayed** in time w.r.t each other?
To produce an **effective focused beam** for each line
35
What sized are phased arrays in comparison to linear arrays?
* Phased: 1 - 3cm * Linear: 10 - 15cm
36
When are Linear Arrays used?
When a **large FoV** is required for images close to the surface e.g. - musculoskeletal applications
37
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
38
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
39
How does Beam steering work?
Changing the **patern of excitation** of all elements
40
What is Dynamic Focusing? (2)
* The resolution of the image * Used to focus at specific depths in the tissue
41
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
42
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
43
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
44
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
45
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
46
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
47
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
48
What changes in the images from left to right?
* 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

Bio-Med (9 decks)

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