Theory Flashcards Preview

Medical Imaging > Theory > Flashcards

Flashcards in Theory Deck (167)
Loading flashcards...

Ultrasound definition

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


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.


Inverse piezoelectric effect

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



Device that converts electric energy into mechanical and vice versa


Damping (US)

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


Acoustic impedance

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



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


Pulse echo method

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


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


Use of US

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


Source of US

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


Ultrasound pulse...

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



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


Detection of US

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


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



Pixels - proportional to amplitude of reflected US signal


Doppler shift

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


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


Digital image

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


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)


Color histogram

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


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(?)



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


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.


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


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



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


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


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


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