BME5002 Image Processing Lecture 1 John G. Harris 1/26/01

Image Processing Goals Improve image quality Image enhancement Feature extraction Computer-aided diagnosis Required for image formation for some modalities (CT, MRI, etc.)

Aspects of Image Quality Contrast vs. dynamic range Blur Noise

Contrast vs. Dynamic Range

Characteristic Film Curve OPTICAL DENSITY LOG(EXPOSURE) toe shoulder base + fog

4 Major Causes of Blur BODY SCREEN FILM FOCAL SPOT

Diffusion of Light in Screen

Film Characteristics BLUR (mm) FILM SPEED LOG( EXPOSURE TIME)

Methods to Reduce Blur Smaller focal spot Thinner Screens Place film farther away Slower film

Range of Blur Values 0.5mm for normal radiographs 0.15mm for mammograms BLUR

Noise Quantum noise in body, screen, film Nonuniformity of film Noise reduces visibility of low contrast features

Quantum Noise Standard deviation of noise is proportional to the square root of the number of photons Therefore the effect of quantum noise can be reduced by increasing the exposure (intensity x time).

Blur vs. Noise Tradeoff image film screen X-rays sensitive film MORE NOISEMORE BLUR

Can Reduce Quantum Noise More intensity More exposure time Thicker screen More efficiency in screen All of the above require less sensitive film