Principles Of Exposure 1 Chapter 6 PDF

PRINCIPLES OF EXPOSURE 1 Chapter 6 Photographic Characteristics of x-ray film LECTURER HANEEN ABUANZEH  Photographic Density u It’s the Film’s response to incident radiation and its depend on the tissue absorption u Tissue Absorption Dependencies 1. Patient 1...

PRINCIPLES OF EXPOSURE 1 Chapter 6 Photographic Characteristics of x-ray film LECTURER HANEEN ABUANZEH  Photographic Density u It’s the Film’s response to incident radiation and its depend on the tissue absorption u Tissue Absorption Dependencies 1. Patient 1. composition 2. thickness 2. Beam u Energy spectrum 1. kVp 2. phase 3. filtration PHOTOGRAHIC / OPTICAL DENSITY (OD ) u Measure of film blackness or opacity u The density :The photographic effect producing different degree of blackness on the film measured by light absorption through it. Why Logarithms? 1. Easily represent large dynamic ranges factors of 10 2. Represent physiologic response of eye to differences in light intensity PHOTOGRAHIC / OPTICAL DENSITY (OD ) u Opacity u ability of film to block light = Io/It u Transmittance u ability of film to transmit light=It/Io — Higher density value means: — darker film — less light transmitted * Opacity is doubled by an increase in density of 0.3 * Useful densities range: 0.3 - 2.0 * 50% down to 1% of light transmitted Base + Fog u Unexposed film has optical density > 0 (min = 0.12) v Base The plastic material absorbs small amount of light blue dye OD ~ 0.07 v Fog Development of unexposed silver halide grains OD ~ 0.05 PHOTOGRAHIC / OPTICAL DENSITY (OD ) The Plot derived by giving a film a series of exposures, developing the film, and plotting the resulting density against the known exposure. Sensitometric Curve /Characteristic Curve / H & D Curve u Sensitometry is the study of the relationship between the intensity of exposure of the film and the blackness after the film is processed. u Base + fog: The optical density of a film due to its base density plus any action of the developer on the unexposed silver halide crystals usually 0.15 -0.30. u Sensitivity (speed): The reciprocal of the exposure value needed to achieve a film net optical density of 1.00 u Gamma (contrast): The average gradient of the characteristic curve u Latitude: The range of exposures that can be recorded and visualized on the film. Characteristic Curve — Linear portion in mid densities v Flatter portions at bottom & top v change in exposure results in little density change — Shoulder v flat portion of curve near top v high exposure & density\ v Very high and very low variations of exposure make very small changes in density. — Toe v flat portion of curve near bottom v low exposure & density v Very high and very low variations of exposure make very small changes in density.  Radiographic Contrast u Density difference between image areas Density u Depends upon Difference 1. Subject contrast Film 2. Film contrast Contrast Subject Contrast  Radiographic Contrast Subject Contrast Dependency Film Contrast Dependency 1. Thickness 1. Characteristic curve of the film 2. Density 2. Film density 3. Atomic differences 3. Screen or direct x-ray exposure 4. Energy spectrum (kvp) 4. Film processing 5. Contrast material 6. Scatter radiation Characteristic curve of a radiographic film Optical Saturation Density (OD) OD2 range visually evaluable f densities g = (OD2 - OD1) / (log E2 - log E1) g The g of a film: the gradient of the «straight OD1 line» portion of the Normal range characteristic curve of exposures Base + fog E1 E2 Log Exposure (mR) 11 Characteristic curve of a radiographic film u Film Gamma q maximum slope of characteristic curve D2 - D1 Gamma = -------------------- log E2 - log E1 q Slope shows change in film density for given change in exposure q Ranges from 2.0 – 3.5 1. Film Gamma u Average gradient q slope between points with densities at ends of useful range (straight line portion )of characteristic curve q Usually between 0.25 and 2.0 v >1: exaggerates subject contrast v (typical for x-ray film) v =1: no change in subject contrast v