Biomedical Image Processing

Imaging Systems

Human Visual Pathway

Rod (blue to green light): 498 nmL- cones (red or long wavelength): 564 nmM-cones (green or medium wavelength): 533 nmS-cones (blue or short wavelength): 437 nm

Brightness adaptation or accommodation Local comparisons

The human visual response is limited to detecting brightness changes of about 2–3%, so that typically it can distinguish only around 25–30 brightness levels in a scene.

The eye cannot distinguish two objects as separate unless the light from them falls on two different cones

Photographic film

• Incoming signals emitted from, transmitted through or reflected from a three-dimensional (3-D) object gray or color

• Radiographic film thin sheet of inert plastic, the base, coated on both sides with emulsion.

• Film emulsion sub-micron sized microcrystals of silver halide in a gelatin base

• Radiography Latent image• Development microcrystals to a dark silver deposit fixation with a thiosulfate solution washing

Response of film to light

Contrast of the film (γ)

Typically, an x-ray film has a γ value of around 2.0, measured at an optical density of around 1.2–1.25, the center of its approximately linear range. The speed of a film is a measure of the ease with which useful optical density is reached; it is defined as the inverse of the exposure that achieves an optical density value of 1.0 above the fog and base level.

• Point spread function (PSF) image of a point object pinhole radiograph of the x-ray source Source, focusing elements and detectors overall

blurring Two small objects (points) within this distance are

blurred to such an extent that they cannot be resolved as two separate entities

Spatial resolution of the film

Ability of an imaging system to form an accurate image of a sinusoidal pattern spatial frequency, f.

• Modulation transfer function (MTF) loss of image contrast as a function of spatial frequency

MTF= 0.1 fl limiting spatial frequency highest spatial frequency that the imaging system can reasonably record

Other sensors

• Ionization chambers • Scintillation detectors• CCD

Digitizing an Image

• Spatial quantizationThe sampling frequency determines the

distance between samples, and this distance becomes the linear pixel size.

Nyquist frequencyPixelationAliasing

Intensity quantization

The discrete pixels formed around the sampled locations comprise the spatially quantized image, but the values within the pixels are still the sampled values measured from the original analog (i.e. continuous) image. In order to form a digital image, these values need to be assigned to a finite set of discrete values. This is the second step in the process of digitizing an analog image, and is known as intensity (or brightness) quantization.

Quality of an Image• Spatial resolution and pixel size

• Brightness resolution false contouring or posterization

• Noise content Unwanted, random (stochastic) fluctuations in an image

Photon noiseElectronic noise Noise from screen/ filmQuantization noise

• Effect of noise on detectability

Color Images

• Pseudocolor added to improve our visualization of the image, not to attempt to replicate the true colors of the features in the image

• RGB model

• HSV (hue, saturation, value)

Region of interest

Nondestructive Overlay

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