Film Sensitometry NATOMY AND PHYSIOLOGY 1 LYMPHATIC SYSTEM AND IMMUNITY LYMPHATIC PATHWAYS  Lymphatic pathways begins as lymphatic capillaries that merge to form lymphatic vessels. These in tu PDF

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NATOMY AND PHYSIOLOGY 1 LYMPHATIC SYSTEM AND IMMUNITY LYMPHATIC PATHWAYS  Lymphatic pathways begins as lymphatic capillaries that merge to form lymphatic vessels. These in turn lead to larger vessels, trunks, and ducts that unite with the veins in the thorax  Lymphatic vessels tran...

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C H A P T E R

21

Film Sensitometry

KEY TERMS average gradient base plus fog calibrate Text not available due to copyright restrictions

characteristic curve contrast D log E curve definition densitometer detail Dmax gamma gradient point Hurter and Driffield (H&D) curve

OBJECTIVES Upon completion of this chapter, the student should be able to: ◾

Define sensitometry as it is applied to radiography.

◾

Describe the production of a step wedge on radiographic film through the use of a penetrometer and a sensitometer.

penetrometer

◾

Explain the function of a densitometer.

resolution

◾

Estimate the percentage of light transmitted by a radiograph according to optical density logarithms.

reversal

◾

Construct a D log E curve from sensitometric data.

sensitometer

◾

State acceptable base-plus-fog and diagnostic-range optical density numbers.

◾

Describe the effect of automatic-processor reducing agents on the shape of a D log E curve, especially with relation to speed and contrast.

speed exposure point

◾

Define resolution, speed, contrast, and latitude.

speed point

◾

Explain the effect of silver halide crystal size on image resolution.

latitude opacity optical density numbers

resolving power

sensitometric curve sensitometry sharpness solarization

step wedge straight-line portion 294

CHAPTER 21

OBJECTIVES (continued)

Film Sensitometry

◾

Calculate gamma, gradient point, average gradient, and latitude from D log E curves.

◾

Discuss the physical and processing factors that affect film speed.

◾

Analyze D log E curves to determine speed, contrast, and latitude relationships.

◾

Calculate speed points, speed exposure points, and relative speeds from D log E curves.

◾

Discuss the relationships between speed, contrast, and latitude.

Sensitometry is the measurement of the characteristic responses of film to exposure and processing and it is accomplished by exposing and processing a film and then measuring and evaluating the resulting densities. Sensitometric methods are useful to evaluate technical factor exposure systems, films, intensifying screens, and processing equipment, and to maintain technical exposure factor charts.

295

to as a step wedge because of its shape. It is used to produce a step wedge on radiographic film by exposure to x-rays (Figure21-2). Because of the vast number of variables in x-ray-generating equipment, the use of a penetrometer is not recommended for quality control monitoring of film processors. However, it is an excellent method for monitoring both x-ray equipment and film/intensifying-screen combinations because it reproduces the variables associated with a clinical situation.

SENSITOMETRIC EQUIPMENT Sensitometer Either a penetrometer or a sensitometer is required to produce a uniform range of densities on a film, and a densitometer is required to provide an accurate reading of the amount of light transmitted through the film.

Penetrometer A penetrometer is a series of increasingly thick, uniform absorbers. They are usually made of aluminum steps, although tissue-equivalent plastic is sometimes used (Figure21-1). A penetrometer is referred

A sensitometer is designed to expose a reproducible, uniform, optical step wedge onto a film (Figure21-3). It contains a controlled-intensity light source (a pulsed stroboscopic light is best) and a piece of film with a standardized optical step wedge image (a step tablet). The controlled light source reproduces the same amount of light each time it is triggered. Voltage fluctuations and other factors that might cause the intensity to vary are controlled by circuits that supply an exact quantity of power to a capacitor that discharges to the stroboscopic light

FIGURE21-1. Two basic types of penetrometers or step wedges: (A) aluminum and (B) tissue equivalent plastic. (Courtesy of Fluke Biomedical.)

UNIT III

Creating the Image

% Light transmitted

OD #

X-ray tube

A.

4

0.01

3

0.1

2

1

1

10

B.

0.3 0.9 1.5 2.1 2.7 3.3 0.6 1.2 1.8 2.4 3.0 Log relative exposure

© Cengage Learning 2013

296

FIGURE21-2. A density curve produced from graphing the exposures of a penetrometer exposure. (A) The exposure. (B) The graph.

Cover with foam to ensure good film-wedge contact

Pulsed stroboscopic stroboscopic light light

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Optical step wedge

FIGURE21-3. A sensitometer.

when triggered. The optical step wedge absorbs a calibrated amount of this light, leaving a uniform and reproducible “light penetrometer” to expose any film placed in the sensitometer over the optical

step wedge. The optical step wedge should not be touched because hands leave a film of oil that interferes with the light intensity. Optical step wedges (step tablets) are available in 11- and 21-step versions. The 11-step wedges usually increase density 100percent (by a factor of two) per step. The 21-step wedges usually increase density 41percent (by a factor of 1.41times [which is 2 ]) per step. Because the rigid control of the densities produced on the film eliminates other variables, sensitometer-produced step wedges are perfect for processor quality control monitoring. Very slight density differences can be detected by sensitometric equipment. When a film is processed, there is a tendency for exhausted reducing agents and bromine ions to be carried backward on the emulsion as it is driven through the rollers of an automatic processor. For this reason, sensitometric film strips should be fed into automatic processors with either the long axis of the step wedge parallel to the entrance rollers or with the light edge entering the processor first.

CHAPTER 21 Sensitometric strips are also produced electronically by most laser and dry imaging systems (when images are transferred to film) as well as by most Picture Archiving and Communications Systems PACS systems for use in calibrating the sensitometric response of flat-panel monitors.

Densitometer A densitometer is an instrument that provides a readout of the amount of blackening (density) on a film. A densitometer consists of a calibrated uniform light source, a stage for placing the film to be measured, a light aperture to control the amount of light from the source, a sensor arm with an optical sensor, a readout display, and a calibration control (Figure 21-4). Density readings are accomplished by comparing the amount of light emitted by the light source with the amount of light transmitted through the film. To do this, the densitometer must be calibrated before each reading by recording the amount of light the light source is emitting. This is done by pushing the sensor arm so that the sensor is in contact with the light source (this eliminates the inverse square law factor) and by using the calibration control to set the readout display at zero. This calibrates or zeroes the densitometer and prepares it for a reading. When a film is placed on

Calibration control

IO CALIBRAT

N

1.64

FIGURE21-4. A densitometer.

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Optical sensor Calibrated uniform light source

OD

log10

Io It

where: OD = optical density number Io = intensity of the incident light It = intensity of the transmitted light This formula can be stated as the log of the intensity of the incident light divided by the intensity of the light transmitted through the film. Radiographic film densities range from OD 0.0to 4.0. The ability of a film to stop light is termed opacity. Opacity is calculated using the following formula: Io It

where: Io = intensity of the incident light It = intensity of the transmitted light

Sensor arm Stage

Aperture

297

the stage and the sensor arm is pushed down into contact with the film, the densitometer can calculate the difference between the calibration intensity and the intensity of light the film is transmitting. Because films are sensitive to a wide range of exposures, their densities are best visualized if the range is compressed into a logarithmic scale. When using a logarithmic scale with a base of 10, an increment of 0.3represents a doubling of exposure. This is because the log of 2 is 0.3. The numbers that are displayed by a densitometer are known as optical density numbers. They can be expressed with the term OD in front of the number (e.g., OD 1.5). They are calculated using the following formula:

opacity

Readout display

Film Sensitometry

Note that density is the log10 of opacity (density = log10 of opacity). Table21-1 shows both the opacity and optical density numbers for various percentages of light transmitted within the radiographic film density range of 0.0 to 4.0. For example, if a region of a radiograph has an OD of 1.0, this means only 10percent or 1/10 of the incident light is transmitted through the radiograph in this region. The opacity of the region would be 10. If the OD number is increased to 1.3, the opacity is doubled (to 20) and the percentage of light transmitted through the film is halved (to 5percent or 1/20). Increments of 0.3 changes in OD numbers represent a doubling or halving of opacity.

Creating the Image

TABLE 21-1. Example Opacities, Optical Density Numbers, and Light Transmission Percentages

1

0.0

100

2

0.3

50

4

0.6

25

8

0.9

12.5

10

1.0

10

20

1.3

5

40

1.6

2.5

80

1.9

1.25

100

2.0

1

200

2.3

0.5

400

2.6

0.25

800

2.9

0.125

1,000

3.0

0.1

2,000

3.3

0.05

4,000

3.6

0.025

8,000

3.9

0.0125

10,000

4.0

0.01

D

3

Percentage of Light Transmitted through Film

2 Density

OD Number

E

C

Diagnostic density range

1 B A 0.3 0.9 1.2 1.2 1.5 1.8 1.8 2.1 2.4 2.7 3.0 0.3 0.6 0.9 3.0 Log xposure Log exposure eexposure

FIGURE21-5. A typical D log E (characteristic, sensitometric, or H&D) curve. (A) Base plus fog; (B) toe; (C) straight-line portion; (D) shoulder; (E) Dmax. © Cengage Learning 2013

Opacity

4

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UNIT III

298

THE D LOG E CURVE Sensitometry is normally shown as a graphic relationship between the amount of exposure and the resultant density on the film (Figure21-5). The horizontal exposure axis (x axis) is compressed into a logarithmic scale and the vertical optical density axis (y axis) is shown as a logarithmic scale (OD numbers are logarithmic). Consequently, the curves are known as density log exposure, orD log E curves. They are also called characteristic, sensitometric, and Hurter and Driffield (H&D) curves after the two photographers who first described the relationships in 1890. The important elements of a typical D log E curve are the base plus fog, toe, straight-line portion (gamma), shoulder, and maximum density (Dmax). The base plus fog (b+f) (see Figure21-5A) is the density at no exposure, or the density that is

inherent in the film. It includes the density of the film base, including its tints and dyes, plus any fog the film has experienced. Radiographic film base density ranges around OD 0.05–0.10. Processing the film usually adds about OD 0.05–0.10 in fog density. The total base plus fog is seldom below OD 0.10 but should not exceed OD 0.22. Fog may be caused by heat, chemical fumes, light, and x-radiation. Over time, the natural amounts of these radiations will produce a slight density that is sometimes called age fog. Most of the fog level will be produced by the chemical processing system. This includes the hyperactivity of the developer solution, primarily caused by the high temperature at which automatic processors operate. Phenidone is the reducing agent that controls the subtle gray tones early in the development process. This region of the curve is known as the toe (see Figure21-5B) and is predominantly controlled by phenidone. The straight-line portion of the curve is that portion between the toe and shoulder (see Figure 21-5C). It is usually fairly straight because the film is reacting in a linear fashion to exposure

CHAPTER 21 in the range of its primary sensitivity, which is in this region. The range of diagnostic densities varies from a low of OD 0.25–0.50 to a high of OD 2.0–3.0. The majority of diagnostic-quality information on a radiograph will measure between OD 0.5 and OD 1.25. These densities are within the straight-line portion of the curve. Hydroquinone is the reducing agent that controls the heavy black tone later in the development process. This region of the curve is known as the shoulder (see Figure 21-5D) and is entirely controlled by hydroquinone. Dmax is the maximum density the film is capable of recording (see Figure 21-5E). It is the highest point on the D log E curve. It represents the point where all the silver halides have a full complement of silver atoms and cannot accept more. Additional exposure beyond Dmax will result in less density because silver atoms attached to sensitivity specks will be ionized again, reversing their charge and causing them to be repelled from the speck. This process of reversal, or solarization, reduces the intensity of the latent image and will produce less density. The true D log E curve is bell-shaped (Figure21-6). Duplication film is actually film that is pre-exposed to Dmax so that additional exposure will cause a reversed, duplicated image instead of a negative one.

4

D max Solarization reversal

3

299

FILM CHARACTERISTICS The primary characteristics of film are classified as resolution, speed, contrast, and latitude. Sensitometry permits analysis of speed, contrast, and latitude within the normal exposure ranges of the film. Extremely long or high-intensity exposure can overload the silver halide crystals and cause a phenomenon known as reciprocity failure. Although films are designed to handle a wide range of exposures, when unusual circumstances require large exposures, films may deviate from their expected performance.

Resolution Resolution is the ability to accurately image an object. It is also called detail, sharpness, definition, and resolving power. Resolution is measured by the ability to see pairs of lines. The unit of resolution is line pairs per millimeter, expressed as lp/mm. Film resolution is determined by the size of the silver halide crystals. Smaller crystals will darken a smaller area of the film, whereas larger ones will darken larger areas. Information that is smaller than an individual silver halide crystal cannot be visualized. An inverse relationship exists between film resolution and crystal size (the smaller the crystals, the higher the resolution; the larger the crystals, the lower the resolution). Silver halide crystals are sometimes called grains, thus the term graininess for poor resolution. Although film graininess can sometimes be seen, radiographic film/screen system resolution is generally controlled by the size of the intensifying-screen phosphors, not the size of the silver halide crystals in the film emulsion.

Speed

1

1.0 2.0 Log exposure

3.0

4.0

FIGURE21-6. Solarization or reversal of duplicating film.

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Density

2

Film Sensitometry

The amount of density (degree of blackening) a film produces for a given amount of exposure is the film speed. It is determined by the film’s sensitivity to exposure. Speed is controlled by the activity of the phenidone because it affects the toe of the D log E curve. The position of the toe determines how soon the straight-line portion will begin, and this indicates the overall speed of the film. Figure21-7

300

UNIT III

Creating the Image

4 Film A Film B

3

Speed point 1

0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 Log relative exposure

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Density

2

FIGURE21-7. The effect of film speed on D log E curves.

illustrates the effect of the toe and shoulder on the overall position of the curve. Film sensitivity is determined primarily by the size of the silver halide crystals. However, the number of sensitivity specks and the thickness of the emulsion layer also have an effect. Larger crystals will receive more photons because of the greater area they cover. Larger crystals will darken a greater area of the film than smaller crystals with the same exposure. Therefore, film speed and crystal size are directly related (the larger the crystals, the faster the film speed; the smaller thecrystals, the slower the film speed). Film speed and the number of sensitivity specks are also directly related for the same reason. A thicker emulsion layer will place more crystals in a given area. Each incoming photon may interact with more than one crystal, so when more crystals are stacked on top of one another in the same area, the same number of photons will produce more film density. Therefore, film speed and thickness of emulsion layer are directly related (the thicker the emulsion, the faster the film speed; the thinner the emulsion, the slower the film speed).

Screen films are capable of responding to (producing visible densities for) exposures as low as 1mR and as high as 1,000mR. In Figure21-7, film A produces all density levels with less exposure than film B requires for the same density. This demonstrates that film A is more sensitive to exposure, or faster. Film B is less sensitive, or slower. The speed point of a film is that point on the D log E curve where a density of OD 1.0 + b+f is achieved. The American National Standards Institute (ANSI) specifies x-ray film speed as the exposure required to reach OD 1.00. However, many users add base plus fog to this standard. The speed exposure point is the log exposure that will produce the speed point for a given film. Film A in Figure21-7 has a speed exposure point of 1.5, and film B has a speed exposure point of 2.0. In clinical radiography it is important to be able to adjust technical factors from one film to another. The radiographer must be able to calculate the difference in exposure that will produce a diagnostic-quality image on a new film when the proper factors are known for a previous one. In Figure21-7, film B would require a log exposure of 2.0 to produce OD 1.0. Film A would require a log exposure of 1.5 to produce the same density. The difference in ...