1. Epileptic spike 2. Reconstructing brain/cardiac electrical activity from electrical potential measured away from brain 3. Determines the location of sources of current in body from measurements of voltages

Basic principle of Image Formation: ADC convolution Flashcards by J Abow

What is suitable for digital processing?

An image function must be digitised both spatially and in amplitude

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What is a digitiser for?

Sample and quantize analogue video signal

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What does the sampling rate determine?

Spatial resolution of digitized image

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What does quantization level determine?

Number of gray levels in digitized image

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What is the objective of biomedical imaging system?

Make an object or phenomenon within patient’s body visible to observer

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What are the parameters that are required to be minimized?

Artefact
Contrast
Blur
Noise
Distortion

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How can practical system achieve its goal?

Practical imaging system delivers visual information not distorted beyond limit which allows system to achieve its goal

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What are measured from physical principle to an image?

Ultrasound scanner
Gamma camera
X-radiograph
Thermograph
MR scanner
ESI

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What is ultrasound scanner?

A distribution of acoustic scattering centres

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What is Gamma camera?

Uptake of radiopharmaceutical by counting gamma-ray photons

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What is X-radiograph?

Linear attenuation of x-ray photons which are getting through object space

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What is thermograph?

A distribution of temperature

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What is MR scanner?

Magnitude and phase of precessing photons

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What is ESI?

Electrical source imaging

Distribution of electric potential or within brain tissue (based on EEG)

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What is ESI?

Epileptic spike
Reconstructing brain/cardiac electrical activity from electrical potential measured away from brain
Determines the location of sources of current in body from measurements of voltages

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What is EEG inverse problem?

Make mathematical model of head and electrical source
Find location of signal that is responsible for measured EEG data
Invert EEG back to brain space

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What are examples of Electrical source imaging?

Distributed source - electric current encoded by pixel/voxel colour
Single dipole sources

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What happens in early CT imaging devices?

A narrow x-ray beam is scanned across a patient in synchrony with a radiation detector on opposite side of patient

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If the beam is monoenergetic or nearly so, what equation is given for the transmission of x-rays through the patient

I = I0e−µx

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What does attenuation depend on?

Properties of tissues

What constitutes a CT image

gray-scale display of

attenuation coefficients

How can gray-scale image be produced that represents various structures in parient?

By assigning gray levels to different ranges of attenuation

coefficients,

How are multiple x-ray transmission measurements obtained?

By scanning a pencil-like beam of x-rays and an NaI detector in a straight line on opposite side of patiemt

How are 160 x 180 = 28,000 x-ray transmission measurements accumulated?

the angular orientation of the scanning device is incremented
1 degree, and a second translational scan of 160 transmission measurements is performed. This process of translational scanning separated by 1-degree increments is
repeated through an arc of 180 degrees

How can you obtain one projection of an image?

Source and detector move in parallel Rotate and obtain another projection Multiple acquisition with different angulation (arc 180 degrees)

What is analog signal?

Continuous curve

What are the 2 operations that convert analog into digital representation?

1. Sampling | 2. Quantization

Where does sampling occur?

Time domain (spatial)

What does sampling do?

Convert a time-varying voltage signal into discrete-time signal, a sequence of real numbers

What does quantization replace?

Real numbers with approximation from a finite set of discrete values

What is quantization?

Digitization in the signal value (value) = amplitude signal converted into digital

How does ADC work?

Converts the quantities of real world phenomenon into digital language which is used in control systems, data computing, data transmission and information processing

What specifies the sampling frequency?

clock

What are the steps of ADC?

1. Sampling of a sine wave using a 3 bit digitizer | 2. Obtain sampled signal --> Quantization

How is continuous curve encoded?

using a 16 bit ADC

Step-like function

3 bit ADC

What does 3 bit ADC allow for?

Maximum 111 | 8 samples to signal all sigals

What is assigned to quantization?

Numerical value | In form of a binary number

What is resolution of ADC?

is the number of discrete values it can produce over the range of analog values. Measured in bits 8 bit – max 256 levels (28) 16 bit – max 65536 levels (216) 24 bit – max 16777216 levels (224)

What is dynamic range of ADC?

the ratio between the minimum and maximum amplitudes a data acquisition system can capture; typically expressed in decibel (dB). 20*log10(Vmin/Vmax)

What does the dyanamic range describe?

the range of the input signal levels that can be reliably measured simultaneously, in particular the ability to accurately measure small signals in the presence of the large signals

What is Aperture Error?

Difference between actual value of input signal and flat-topped sample value

What does an error in clock translate into?

Error in values

When does the Quantisation error appear due to?

Finite resolution of ADC present in all ADC varies from 0 to ½ of the least significant bit

What is least significant bit?

LSB is the smallest value which can be resolved by an ADC

What is the consequence of passing an analog signal through Quantisation Error?

8 bit digitizer adds noise of up to 1/512 of the full scale value; 12 bit conversion adds a noise of up to 1/8192 of the full scale; 16 bit conversion adds 1/131072

What is the equation of Quantization error?

Sampled Analog signal - Digitized signal

What is 1D sampling thereom?

A continuous-time signal x (t), whose spectral content is limited to frequencies smaller than Fb (i.e., it is band-limited to Fb) can be recovered from its sampled version x (n) if the sampling rate is larger than twice the bandwidth (i.e., if Fs > 2Fb)

What is 2D sampling thereom?

A continuous-space signal s (x; y), whose spectral content is limited to spatial frequencies belonging to the rectangle having semi-edges Fbx and Fby (i.e., band limited to Fbx and Fby) can be recovered from its sampled version s (m; n) if the spatial sampling rates are larger than twice the respective bandwidths (i.e., if Fx > 2Fbx and Fy > 2Fby)