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Flashcards in Theory Deck (167)
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1

Ultrasound definition

- Mechanical oscillation
- Higher freq (pitch) than 20000 Hz (upper limit human ear)
- Longitudinal in liquid and gas
- Both (longit.& transv.) in solids

2

Direct piezoelectric effect

US -> electric energy
- Detection of ultrasound
- Piezoelectric insulators accumulate electric charge and gets polarized when mechanical strain (pressure, tension) is applied on them.
This results in measurable potential difference between electrodes on opposite sides of crystals.

3

Inverse piezoelectric effect

Electric energy -> US
- In the transducer
- Electric voltage applied on electrodes of piezoelectric material -> becomes deformed

4

Transducer

Device that converts electric energy into mechanical and vice versa

5

Damping (US)

Parameter that characterises attenuation of US
- J0: initial intensity
- J: intensity after passing theough layer
*Energy lost as heat

6

Acoustic impedance

Characterizes acoustic properties of medium
- Z = density*velocity
- Boundary surface/interface: acoustic impedance changes
(Higher difference=higher reflection)

7

Reflexivity

Ratio of reflected:incident intensity
R=0: no reflection
R=1: total reflection

8

Pulse echo method

d=(c*t)/2
Calculate distance between transducer and reflecting boundary surface
c: sound velocity (known for medium)
t: elapsed time from emission to arrival of sound

9

Doppler effect

Frequency (and wavelength change as a result of relative motion and source of the observer
- Towards you: shorter wavelength - higher pitch
- Away: longer wavelength - lower pitch

10

Use of US

1) Diagnostic (ultrasonography, medical sonography)
- Non-invadive
2) Therapy
- Rheumatology
- Musculoskeletal disorders (arthrosis)
- Removal of dental calculus

11

Source of US

Sine wave oscillator conduct electric pulse (MHz) to trancducer containing the piezoelectric crystals (converts electric->US "Inverse piezoelectric effect")

12

Ultrasound pulse...

Should be short - only a couple of time periods (sine waves)

13

Eigen-frequency

Frequency of largest amplitude vibration of a solid material (own resonance frequency)
*For best US - electric signal should match Eigen-frequency of the piezoelectric material

14

Detection of US

- US -> transducer (polarizes) -> electric signal (direct piezoelectric effect) -> conducted through cable to electronic amplifier

15

Diagnostic US imaging methods

1) One dimentional A-image (amplitude modulation)
2) One dimentional B-image (brightness modulation)
3) Two-dimentional B-image (2D, brightness modulated)
- Series of one dimentional B-images at diff. angles
4) M image (motion)
- Info: position of the given surface as function of time
5) Reconstructed 3D image (tomography)
6) Reconstructed 4D image
- Time is 4th dimention - US movie created
7) Doppler methods

16

Brightness

Pixels - proportional to amplitude of reflected US signal

17

Doppler shift

= f - f0
Proportional to the relative velocity (v/c) and to the incident frequency

18

Doppler methods

1) Doppler time-velocity image:
- Doppler-frequency-shift plotted as a function of time - corresponds to velocity of observed surface

2) Color-coded Doppler image:
- Color coded velocity information

3) Doppler flow meter
- Can measure velocity of blood flow in larger blood vessels

19

Digital image

Information displayed at different discrete spatial points in the form of color
- 2 or 3 dimentional array or matrix of picture elements

20

Characteristics of the digital image

1) Picture element (pixel)
2) Information associated with the pixel
- XY location: coordinates related to spatial resolution
- Color depth: intensities related to color/gray-scale resolution
3) Spatial resolution
- Number of resolved pixels in the X and Y directions
4) Grayscale/color depth
- Number of resolved colors/grayscale intensities (bit)

21

Color histogram

- Resolved intensities may be displayed as a function
- Relative frequency of colors or grayscale intensities in the image

22

Image enhancement techniques

1) Contrast manipulation
- Color transfer function: assigns color to pixel densities (expressed in numerical values)
2) Convolution
- Kernel operation - se eget kort
- Blurry vs skarpt
3) Rank operations
- The pixel is exchanged for another from its ranked neighborhood (e.g noise removal - prikker)
- Min, max, median, mean(?)

23

Convolution

Image enhancement technique
- Kernel operation -> smoothing, sharpening, edge detect.
- Convolution is the process of adding each element of the image to its local neighbors, weighted by the kernel

24

Fourier transformation + principle

Fourier principle: any function may be generated as the sum of sine function and its harmonics

Fourier transform: decomposes a function of time (a signal) into the frequencies that make it up (FFT, inverse FFT, masked FFT)

Smoothie analogy:
- What does the Fourier Transform do? Given a smoothie, it finds the recipe.
- How? Run the smoothie through filters to extract each ingredient.
- Why? Recipes are easier to analyze, compare, and modify than the smoothie itself.
- How do we get the smoothie back? Blend the ingredients.

25

Binary transformation

The image is partitioned according to certain parameters
- Tresholding, segmentation
- Excecution:
1: Select a certain grayscale range of the image
2: The selected pixels form the "foreground"
3: The rest of the pixels form the "background"
- Bildet blir svart og hvitt i stedet for gråskala

26

Binary operations

1) Boolean functions (of image a and b):
- a OR b (union)
- a AND b (intersection)
- a NOR b (union+intersection?)
- NOT a (complement of a)
*Boolean operation: separation of touching objects

2) Erosion, Dilatation, Opening, Closing
- Moving pixels from the foreground to the background and vice versa
- Erosion: svarte prikkene blir mindre og mindre
- Dilatation: svarte prikkene blir større og større

27

DICOM

Digital imaging and communications in medicine
- En standard for håndtering, lagring, utskrift og overføring av medisinske digitale bilder og informasjon relatert til disse bildene

28

X-rays (characteristic, wavel, freq, energy)

- Electromagnetic wave
- Wavelength: 10 - 0.01 nm (10nm: soft x-r, 0,01: hard)
- Frequency: 30 * 10^15 - 30 * 10^18 Hz (petaHz-exaHz)
- Energy: 120 eV - 120keV

29

Generation of X-ray - rotating anode tube

- X-rays are emitted when a high-speed electron hits a metal target - is decelerated (1 % of electron beam converted to protons, rest to heat)
- Rotating anode to deal with heating problem
- Hot-wire cathode (heated by DC circuit)
- Anode at 45 degrees angle to direct toward window

30

2 mechanisms of X-ray generation + spectrum

1) Bremsstrahlung
- Breaking/deceleration radiation
- Continous energy spectrum

2) Characteristic radiation
- X-ray fluorescence
- Electron collides w/K-shell electron -> K-shell electron ejected -> higher shell electron jumps down -> emission of a single X-ray photon
- Linear energy spectrum