BME5002Image 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

0

1

2

3

LOG(EXPOSURE)

toe

shoulder

base + fog

4 Major Causes of Blur

BODY

SCREEN

FILM

FOCAL SPOT

Diffusion of Light in Screen

Film Characteristics

0.1

0.2

0.4

0.8

BLUR(mm)

FILM SPEED

100 20050 400 800

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 NOISE MORE BLUR

Can Reduce Quantum Noise

• More intensity

• More exposure time

• Thicker screen

• More efficiency in screen

• All of the above require less sensitive film