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A comparative sensitometric x-ray film study
A comparative sensitometric x-ray film study R. D. Fleming, UNIVERSITY
OF
D.D.X., IOWA
MS.,”
COLLEGE
Iowa OF
City, Iowa
DENTISTRY
Eight films were subjected to an analysis of speed, latitude, and average gradient, with x-rays as the exposure medium. Separate sensitometric curves were created for each of the films under different combinations of related factors. The values thus derived indicated the effect of film type, exposure conditions, radiation quality, and all their combined interactions on each of the criteria.
T
his report will present the results of an experiment concerning the et&et of x-rays on photographic and radiographic film emulsions. In order to predict the response of any particular emulsion to electromagnetic radiations, it is necessary to examine the inherent characteristics of that emulsion. Since the time of Becquerel (X396), it has been known that film emulsions are sensitive to electromagnetic radiations in the x-ray region.16 Subsequent to the discovery of photographic emulsions, but quite some time before the discovery of the x-ray in 1895, numerous investigators had attempted to quantitate various emulsion characteristics. However, it was not until 1890 that the first scientifically founded method of emulsion investigation was devised by two investigators named Hurter and Driffield .I6 This type of film evaluation was called sensitometry, a word derived from two Latin words meaning “sensitivity” and “measurement.“’ In their original work, Hurter and Driffield devised an “actinograph,” which is an exposure calculator used to establish the relationship between the amount of silver deposited on a negative and the light transmitted by it.3 They also found that the logarithm of the light transmitted, which they termed density, is proportional to the amount of silver per unit area of the emulsion. They made graduated exposures on emulsion plates through a sector wheel in which successively increasing angular apertures were cut. The density of the developed plate was measured and plotted as a function of the This investigation was supported by United States Public Health Service Training Grant 5 TO1 DE00096, from the National Institute of Dental Research, National Institutes of Health, Bethesda, Md. “Head of Dental Radiology Division. 701
702 FZeming
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logarithm of the exposures. The result was a curve for that particular emulsion under specific physical conditions of exposure and development. This curve has subsequently been given many different names: H and D curve, D log E curve, sensitometric curve, and characteristic curve.*-lo~l6 In this article it will be referred to as a characteristic curve. The general shape of such a curve is shown in Fig. 1, I. Because of its general shape, the characteristic curve may be divided into three distinct regions. 1. Toe-The initial period of the curve from A to B wherein the density increases only slightly with increasing exposure. 2. Straight-line-A period of the curve from B to C wherein the density increases as a direct or linear function of the exposure. 3. Shoulder-That period of the curve above C, the region of overexposure, wherein the curve tends to flatten with further increases in exposure. The curve finally becomes horizontal and then reverses itself. In this region of overexposure, less density is produced for increasing amounts of exposure. This basic curve shape represents the type that evolved from Hurter and Driffield’s original work with photographic emulsions exposed to visible light. The curve shape representative of an x-ray exposure, however, is slightly different and is shown in Fig. 1, II. It begins in the low density region as the visible light exposure curve does, with a gradually increasing toe portion. It simulates the straight-line portion of the visible light exposure curve but, instead of demonstrating a shoulder portion, it continues as a straight line upward. Many investigators in the field believe that there is a definite shoulder portion on an x-ray curve, which will be shown when new densitometers that have the ability to read higher densities are developed. The highest readable density with present equipment is in the range of 6 to 8. With these basic concepts and procedures in mind, a definitive x-ray sensitometric study of eight different emulsions was made. Traditionally, it has been the practice of radiologists to accept, without qualification, the film emulsion types produced by manufacturers. This experiment was conducted to determine whether or not other commercially available film emulsions might be adaptable to diagnostic work in dental radiography and to further determine, in terms of a long-range project, whether film characteristics other than those inherent in intraoral film might serve our diagnostic purpuses more effectively. METHODS AND MATERIALS
The eight different emulsions evaluated in this study are listed in Table I. Royal-X pan and Verichrome were selected because they represented two photographic films with widely separated A.S.A. speed ranges. It was considered useful to determine whether these two light-sensitive films demonstrated the same relative speed differences to x-radiation. (The A.S.A. speed of Verichrome is 40, that of Royal-X pan is 1250.) Infrared and Kodachrome were selected to test the x-radiation response of two films differing greatly in their photographic spectral sensitivities.
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Log Exposure
Fig. 1. Curve I represents a typical curve shape for a film exposed to visible light. Curve 11 represents a typical curve shape for a film exposed to x-rays.
Table I. Film products
CodeNo.
1
Emulsion No. Brand me* Ultra-Speed 02207549 Eodachrome II 29 384 AM N Verichrome 1968 K G 4532-207-42-54 Type M % BB 4128-001-01-10 Royal-X pan 6 Type KK L 4531-llV2-11 H.S. Infrared 5218-92-2J i R 5522-913-2-4 Royal Blue *All fdm products manufactured by Eastman Kodak Company. 1 2 3
I
Expiration date December, 1966 December, 1966 March, 1968 November, 1966 October, 1966 p$&d6;MT November, 1966
Type KK and Type M industrial films were chosen because of their wide relative speed range differences and to evaluate their capabilities for use in the medical/dental field. Royal Blue medical/dental film and dental Ultra-Speed film were included primarily for comparison purposes. A standard 5 by 7 inch cardboard cassette was modified to accept a film strip ‘7 inches long and 35 mm. wide. The outside of the cassette was marked off in twelve vertical l/e inch divisions along the ‘I-inch dimension. A cassette holder exposure device was constructed of Plexiglas and lead. This device was so constructed that the cassette was completely covered with lead except for a l/-inch slit through which the exposures were made (Fig, 2). The cassette was advanced behind the slit in the lead holder, l/s inch at a time, while each of the successive exposures was made. The various x-radiation intensities needed to produce the varying densities for sensitometric curve production can be accomplished by either of two techniques. One is a time-scde exposure technique, in which the cassette holder is maintained at a fixed distance from the x-ray source and the x-ray intensity
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PLAN
ELEVATION Fig.
8. Diagram
of device
constructed
to hold film
(loaded)
cassette
during
exposure.
is varied by changing the exposure time. The second is an in$ensity-scale technique, in which the exposure time is held constant and the x-ray intensity is varied by changing the distance of the cassette holder device relative to the x-ray source.8tl6 The choice of exposure procedure depends on whether the film is to be exposed directly to x-rays or to x-rays plus blue light from intensifying screens. In this experiment, when direct x-ray exposures of the films were made, the time-scale procedure was used. When intensifying screen exposures were used, the intensity-scale technique was used in order to prevent erroneous results due to reciprocity law failure. 1p3*5,*slopI13I8 All visible light and x-ray intensifying screen exposures (the latter being approximately 98 per cent visible light exposures) are susceptible to reciprocity law failure. Medical and dental x-ray film sensitometry is based on a range of useful densities; this range is from a low of 0.25, which is almost totally devoid of emulsion density, to a high of 2.00, which is almost totally black under usual viewing conditi0ns.l Because the eight selected films had different sensitivities to x-radiation, it was necessary to conduct a pilot study to determine the correct exposure times for the direct-exposure films and the proper distances for the screen-exposure films, so that the twelve densities produced on the films would be within the desired density range. It was also necessary during the pilot study phase to determine accurately the number of roentgens required for each of the many varieties of exposure conditions. This was accomplished with a Nuclear-Chicago “Cutie-Pie” survey meter. For each separate exposure, three meter readings were taken and averaged. Mackl* states that there are five basic photographic properties of an emulsion : (1) speed, (2) emulsion latitude, (3) contrast, (4) grain size of the silver halide crystals, and (5) color sensitivity of the emulsion. The first three
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properties in this list are obtainable directly from the characteristic curve and are the ones that were evaluated in this study. In order to describe how these three properties are derived from the characteristic curve, it will be helpful to consider three hypothetic curves (Figs. 3,4, and 5). Fig. 3 shows graphically how the speed values were derived from the characteristic curves for this study. According to the American Standard Method for the Sensitometry of Medical X-ray Films,’ the speed of a film is defined as the reciprocal of the exposure necessary to produce a density of 1 on the film. The speed values for the experimental films were derived from the characteristic curves according to this principle. In this study, dental Ultra-Speed film was used as a comparison standard, In medical/dental diagnostic roentgenology the speed of the emulsion is an important factor in t,he selection of a film for a specific purpose. It is mandatory to use a film that will give the best diagnostic result with the least amount of radiation to the patient. Since it would be foolhardy to use an extremely fast film that would not reveal the information desired, film selection relative to speed becomes a matter of operator judgment based on the type of information to be gleaned from the final product and the amount of patient exposure necessary to produce this result. The emulsion latitude of a film is defined as that range of exposure corresponding to the straight-line portion of the cha.racteristic curve.13 For the purpose of medical/dental sensitometry, two predetermined density points (0.25 and 2.00)) are located on the characteristic curve, and it is these two points, as they relate to the log exposure axis, that determine the latitude range. Fig. 4 illustrates this point. It can be seen that the flatter the curve slope (film A compared to film B) , the greater the film latitude. Film latitude is an important concept to the radiologist, for if the latitude range of a film is short, the radiologist must be extremely accurate when exposing the film. The range of remnant exposures from the subject (those reaching the film) must be within the correct latitude range of the film in order to produce a maximum film density difference relationship. In this study the numerical difference between the latitude range extremes, on the exposure axis, was used as the latitude value. Contrast is defined as the density difference between two adjacent areas on a film.8* I31I51I6 In film sensitometry, contrast is quantitatively expressed as the gradient of the film and, more specifically, in medical/dental sensitometry it is expressed as the average gradient. Average gradient is defined as the slope of the curve between two predetermined density points (0.25 and 2.00) .a~12117 The average gradient value was derived in the following manner: Referring to the hypothetic curve shown in Fig. 5, a triangle A, B, C can be constructed, with points A and B used to represent the projected density limits of 0.25 and 2.00 on the curve. The average gradient is expressed as a number representing the ratio of side a to side b (0 = a/b). ‘1 8sl6 It can be seen from this expression that as the curve slope becomes flatter the average gradient ratio decreases and vice versa. The rationale of how inherent film contrast is related to average gradient can be described by referring to the hypothetic curve represented in Fig. 6.
Oral slug. May, 1971
706 Fleming
I I.08 Fast
Exposure Speed
-Slow
Fig. 3. Three identically shaped hypothetic characteristic curves. The speed of a film is a function of its proximity to ordinate or the density axis at a horizontal level of one density unit. Note that film A would be the fastest and film C would be the slowest.
Fig. 4. Characteristic curves of two hypothetic radiographic films (I?, high-contrast film; A, low-contrast Mm). The range of exposures that can be covered within the range of useful density (0.25 to 2.00), the latitude of the film, depends on the average gradient.
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I B
2
a
0.25
_-.-
.-_-._.
Fig. 5. A hypothetic characteristic sideb @=a/b).
curve. The average gradient is the ratio of side a to
difference/
Log Exposure
Fig. 6. Two hypothetic characteristic curves. Same amount of remnant radiation (between EZ and E#) exposes both films. The density difference of film B is far greater than the density difference of film 8.
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If two theoretic remnant exposures El and E2 (Fig, 6) from two adjacent areas of a subject expose film A and film B simultaneously, the density difference produced by exposures El and E2 on film A is considerably less than the density difference produced by the same exposures on film B. In fact, film B produces an exaggerated density difference for the same amount of exposure difference. If, on the other hand, the objects to be radiographed have a high inherent differential in structural density, a film with a lower average gradient and a broader latitude could be used satisfactorily. If the average gradient of the characteristic curve is greater than 1.0, the contrast on the film due to the remnant exposure from the patient is increased. The higher the gradient, the greater the increase in contrast. Thus, at densities for which the average gradient is greater than 1.0, the film acts as a “contrast amplifier.“6l 8 Similarly, if the average gradient is less than 1.0, subject contrast is diminished. Many small differences in anatomic structure undergoing study would not be visible if it were not for the ability of the film to amplify the subject contrast. Each of the experimental films was subjected to three different kilovoltages of x-radiation: 50 KVP, 70 KVP, and 90 KVP. In conjunction with the kilovoltage variation, the filtration of the three x-radiation beams was varied: 50 KVP with 1 mm. aluminum total filtration, 70 KVP with 2 mm. aluminum total filtration, and 90 KVP with 3 mm. aluminum total filtration. Screen and no-screen exposures were made on the test films, since both of these exposure types are commonly used in medical/dental diagnostic radiology. The processing of the exposed films for this experiment was thoroughly standardized. Because of the extreme light-sensitive nature of the photographic emulsions, aZZ processing procedures were carried out in total darkness. The six exposed film strips, of each different film type, were placed on three standard 5 by 7 inch developing racks, two to a rack. All films were processed in standard Eastman Kodak dental x-ray liquid developer and fixer. Development time and temperature were accurately maintained at 41/ minutes and 68’ F., respectively. Every 30 seconds during the developing cycle, a burst of nitrogen gas was delivered to the developer solutions to maintain even and constant development. New solutions were used for each replicate or for each separate experimental set of forty-eight films. A series of ninety-six film strips were produced from the experimental procedure. Each film strip contained twelve different densities which, according to the information gained during the pilot phase, would include the predetermined density range of 0.25 and 2.00. Three separate density readings were taken and averaged for each of the twelve film-strip densities and for each of the ninety-six film strips, giving a grand total of 3,456 density readings. The experimental design and subsequent statistical analysis of the experiment were organized on the basis of how each of the three major criteria (speed, latitude, and average gradient) was separatezy affected by the experimental variables studied, that is, experimental replication, the eight film types, three radiation qualities, two different exposure conditions, and all of their many combinations. The type of statistical design used for this experiment is called a “split-split-plot design.” In order to follow the split-split-plot design and
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attempt to eliminate any form of experimental bias, it was necessary to randomize all of the variables that were to be evaluated. First, the eight films were randomly selected for each of the two experimental procedures. Next, the exposure condition, screen or no screen, was randomly selected for each of the eight film types in the two experimental replicates. Finally, the three radiation qualities were randomly selected for each of the eight films with its specific exposure condition for each of the replicates. RESULTS
Ninety-six sensitometric curves were created on Keuffel and Esser graph paper No. 46 1521. Six separate sensitometric curves were developed for each of the eight different film types. The individual curves were transferred from the graph paper to Sl/, by 11 inch sheets of clear plastic. This was accomplished by placing the plastic sheets over the graphs and tracing the position and shape of the curve onto the plastic with l/ss inch plastic colored tape. This tape was color coded to the three different radiation qualities: Red was assigned to represent 50 KVP with 1 mm. aluminum total filtration, I&%X&to represent ‘70 KVP with 2 mm. aluminum total filtration, and Green to represent 90 KVP with 3 mm. aluminum total filtration, This procedure permitted the overlaying of six transfer graphs, one upon another, for each of the separate film types for comparison purposes. For the purpose of this article, it was necessary to distinguish the three levels of x-radiation by using different line configurations in place of color coding as described above. A dot-dash line was used to represent 50 KVP with 1 mm. aluminum t.otal filtration, a dotted line to represent 70 KVP with 2 mm. aluminum total filtration, and a solid line to indicate 90 KVP with 3 mm. aluminum total filtration. The graphs that Fig. 7 comprises will give the reader a visual evaluation of the emulsion characteristics of these eight films. Speed, latitude, and average gradient values were derived from each of the original curves according to the previously described procedure. The data were subjected to an analysis of variance analyzation. This method of statistical analyzation for this experiment was used because it afforded the opportunity to evaluate how each of the three major criteria-speed, latitude, and average gradient-taken separately, was affected by the experimental variables studied, that is, experimental replication, the eight film types, three different radiation qualities, two different exposure conditions, and all their many combinations. When the experimental data for the major criteria-film speed, latitude, and average gradient-were subjected to an analysis of variance, it was found that there was a significant difference at the P = 0.01 level in three of the experimental variables. As might be expected, a significant difference was found when the experimental variable, film type, was analyzed for speed, latitude, and average gradient. A significant difference was demonstrated when the exposure condition, that is, screen exposure or no-screen exposure, was analyzed. And when data from the various combinations of film type and exposure conditions were andyzed, there was an understandably significant difference. It was interesting to note from the analysis of variance tables that the
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Fig.
7. A composite
of the ninety-six
characteristic
curves.
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Fig.
sensitometric
7 cowVc2. (For legend, see page 710.)
x-r&y
film
stztdy
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Oral surg. May, 1971
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Fig. 7
cont’d.
(For legend, see page 710.)
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Fig. 7 cont’d.
(For legend, see page 710.)
quality of the radiation did not show a significant difference when latitude and average gradient were the criteria analyzed but did show a diEerence at the P = 0.01 level when speed was the criterion being evaluated. Another interesting deduction from the analysis of variance tables for the three major criteria was that there was no significant difference between the replicates of the experiment. This would indicate that the experimental design was structurally sound and that experimental replication could be accomplished with no appreciable experimental variation. In our effort to relate this experimental information into a more practical and usable form, a table (Table II) comparing the experimental results of the commonly used dental Ultra-Speed film and the seven other test films was constructed. It can be concluded from Table II that when the latitude of the seven test
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0
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40
Fig. 7 cont’d. (For legend, see page 710.)
Table II. Ultra-Speed film comparisons Film type Royal-X pan Infrared Kodachrome Verichrome Royal Blue KK M
Latitude Greater Greater Greater Greater Approximately equal Approximately equal LeSS
Speed Approximately equal Approximately equal Less LeSS Greater Greater Less
Average gradient Less Approximately Less Less Greater Greater Greater
equal
films is compared to that of Ultra-Speed dental film, all but one (Industrial Type M) shows a latitude either equal to or greater than that of the dental Ultra-Speed standard. When film speed was the consideration criterion, four of the seven films (Royal-X pan, High-Speed Infrared, Royal Blue, and Industrial Type KK) demonstrated speed either equal to or greater than the
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standard film. This was similarly true when average gradient was the single criterion. The four films that had either an equal or greater average gradient were High-Speed Infrared, Royal Blue, Industrial Type KK, and Industrial Type M. A statement of qualification is necessary when a particular film is considered for evaluation on the basis of latitude and average gradient. These two film characteristics are reciprocal quantities; that is, as one increases the other must decrease. Thus, for the same range of useful densities, it is impossible for a film of high contrast to have a wide latitude, or vice versa. However, for the purposes of this experiment, these film characteristics were evaluated separately for each specific film type. A variety of questions concerning this experiment are visually answerable when a careful study of Fig. 7 (the composite of the ninety-six transfer graphs arranged according to film type) is made. These include such questions as the following : A. Does the change in kilovoltage alter the basic curve shape? It can be seen by the uniformity of the three curve shapes for each film type and exposure condition that the kilovoltage change has no appreciable effect on the basic curve shape. B. Does the amount of filtration or the quality of the x-radiation vary the curve shape? Again, it can be seen that there is no relationship between the curve shape and the quality of the radiation. C. Does varying the kilovoltage and filtration alter the sensitivity of the emulsion to x-radiation? As in the previous two statements, it can be seen that there is no apparent correlation between the quality of the x-radiation and the sensitivity of the emulsion. D. Does changing the exposure condition from screen to no-screen change the sensitivity of the emulsion? The wide separation of groupings of screen and no-screen curves indicates that when a screening procedure is used there is a definite increase in film sensitivity. This effect is readily apparent in all the film types, with the exception of the dental Ultrafilm ; it is especially Speed film. This film is Kodak’s new “Morlite” manufactured to be more insensitive to wave lengths in the blue light portion of the spectrum. All of the curves, screen and no-screen, for this film are grouped together within a short exposure or sensitivity range. E. Do the photographic emulsions fall within a usable sensitivity range? It can be seen that the majority of the photographic film curves are too far to the right of the Ultra-Speed curves, indicating that they are too insensitive or that they would require an excessive amount of x-radiation to produce a satisfactory clinical radiograph, This fact, however, does not rule out their usefulness for certain experimental research procedures. F. Is it possible to obtain an x-ray-imposed density on a photographic film by processing it in standard x-ray solutions 1 The experiment demonstrated that it wa;s possible to achieve satisfactory results using only the x-ray solutions for all selected films.
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The design and subsequent analysis of this experiment not only demonstrated the resolution of these questions visually but also provided statistical evidence to support these observations. The stability of the experimental design and the statistical balance of this experiment are easily appreciated from the repeatability of the curve shape and position from one replicate to another. SUMMARY
A group of eight films, four photographic (Royal-X pan, High-Speed Infrared, Kodachrome II, Verichrome) , two industrial x-ray (KK, M) , and two commonly used dental and medical x-ray films (Ultra-Speed, Royal Blue), were subjected to an analysis of three of their inherent characteristics-speed, latitude, and average gradient. X-rays were used as the exposure medium. To accomplish the analysis, it was necessary to create separate sensitometric curves for each of the films under different combinations of related factors (ninety-six curves in all). From these curves, it was possible to derive certain values that represented the three criteria-speed, latitude, and average gradient. These values indicated the effect of film type, exposure conditions (screen or no-screen), radiation quality, and all their combined interactions on each of the criteria. Analysis of variance was the method used for the statistical evaluation of the experimental data. Many interesting results were deduced from this experiment : 1. For all practical purposes, the kilovoltage and filtration of the radiation beam had little or no effect on such conditions as curve shape, curve position, or emulsion sensitivity. 2. There was, however, a definite statistical difference in emulsion sensitivity when the exposure condition was changed from screen to no-screen in all the test emulsions with the exception of dental UltraSpeed film. This emulsion has been manufactured so that it is less sensitive to the blue light portion of the spectrum. 3. Some of the test emulsions not presently used in medical/dental diagnostic radiography demonstrated very acceptable speed, latitude, and average gradient characteristics. 4. X-radiation could produce usable densities on photographic emulsions. 5. Even photographic emulsions could be processed in x-ray processing solutions with satisfactory results. 6. The A.S.A. speed ratings of the photographic emulsions correlated very well with their x-ray sensitivities. 7. There was no apparent color effect of x-radiation on the specially color-sensitized emulsion of Kodachrome II. 8. Verichrome and Kodachrome II demonstrated an emulsion sensitivity to x-radiation that was too slow for practical clinical usage. In view of the varied and interesting results of this experiment it is my intention to continue this study with a wider variety of emulsions and subso quently to submit those emulsions of promise to an aluminum step wedge and patient evaluation.
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REFERENCES 1. American
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. ii: 14. 15. 16. 17.
Standards Association: American Standard Method for the Sensitometry of Medical X-ray films, A.S.A. PH2. 9, 1956. Arkin, H., and Colton, R. R.: Tables for Statisticians, ed. 2, New York, 1964, Barnes & Noble; In&. Bell, G. E.: The Photographic Action of X-rays, Brit. J. Radiol. 9: 578-605, 1936. an Introductory Text, New York, 1964, The Macmillan Goldstein, A. : Biostatistics, Company. Corney, G. M.: Gamma-Ray Sensitivities of Several Black and White Negative Photographic Films, Photo. Sci. Tech. (Series II), 2: 146-148,1955. Comey, G. M. : Notes from Eastman Kodak Company course, June, 1965. Eastman Kodak Company: Practical Densitometry, Kodak Sales Service Pamphlet No. E-59, Rochester, N. Y., June, 1963. Eastman Kodak Company: Sensitometric Properties of X-ray Films, Rochester, N. Y., December, 1963. Eastman Kodak Company: Kodak Personal Monitoring Films, Special Sensitized Products Sales Division, Pamphlet No. P-31, Rochester, N. Y., March, 1964. Eastman Kodak Company (X-ray Division): Radiography in Modern Industry, ed. 2, Rochester. N. Y.. 1957. Glasser, O., Q&by, E. H., Taylor, L. S., Weatherwax, J. L., and Morgan, R. H.: Physical Foundations of Radioloav.Y” I ed. 3, New York. 1962. Haruer & Row. Henrikson, C. 0.: Speed and Contrast of Dental Film< Acta Radiol. 1: 66-80, 1963. Johns, H. E.: The Physics of Radiology, ed. 2 (revised), Springfield, Ill., 1964, Charles C Thomas, Publisher, pp. 575-590. Maek, J. E.: The Photographie Process, New York, 1939, McGraw-Hill Book Company, Inc. Mat&on, 0. : Practical Photographic Problems in Radiography, Acta Radiol., Supp. 120, pp. l-206, 1955. Mees, C. E. Ii.: The Theory of the Photographic Process, Ed. revised ed., New York, 1954, The Macmillan Company, pp. 101-124, 147-166, 199-216, 298-330, 503-584, 657-670, 773-777. Trout, E. D., Kelley, J. P., and Cathey, G. A.: The Use of Filters to Control Radiation Exposure to the Patient in Diagnostic Radiology, Amer. J. Roentgen. 67: 946963, 1952.
Reprint requests to: Dr. R. D. Fleming Dental Radiology Division University of Iowa College of Dentistry Dental Building Iowa City, Iowa 52240
OF
D.D.X., IOWA
MS.,”
COLLEGE
Iowa OF
City, Iowa
DENTISTRY
Eight films were subjected to an analysis of speed, latitude, and average gradient, with x-rays as the exposure medium. Separate sensitometric curves were created for each of the films under different combinations of related factors. The values thus derived indicated the effect of film type, exposure conditions, radiation quality, and all their combined interactions on each of the criteria.
T
his report will present the results of an experiment concerning the et&et of x-rays on photographic and radiographic film emulsions. In order to predict the response of any particular emulsion to electromagnetic radiations, it is necessary to examine the inherent characteristics of that emulsion. Since the time of Becquerel (X396), it has been known that film emulsions are sensitive to electromagnetic radiations in the x-ray region.16 Subsequent to the discovery of photographic emulsions, but quite some time before the discovery of the x-ray in 1895, numerous investigators had attempted to quantitate various emulsion characteristics. However, it was not until 1890 that the first scientifically founded method of emulsion investigation was devised by two investigators named Hurter and Driffield .I6 This type of film evaluation was called sensitometry, a word derived from two Latin words meaning “sensitivity” and “measurement.“’ In their original work, Hurter and Driffield devised an “actinograph,” which is an exposure calculator used to establish the relationship between the amount of silver deposited on a negative and the light transmitted by it.3 They also found that the logarithm of the light transmitted, which they termed density, is proportional to the amount of silver per unit area of the emulsion. They made graduated exposures on emulsion plates through a sector wheel in which successively increasing angular apertures were cut. The density of the developed plate was measured and plotted as a function of the This investigation was supported by United States Public Health Service Training Grant 5 TO1 DE00096, from the National Institute of Dental Research, National Institutes of Health, Bethesda, Md. “Head of Dental Radiology Division. 701
702 FZeming
OraI Burg. May, 1971
logarithm of the exposures. The result was a curve for that particular emulsion under specific physical conditions of exposure and development. This curve has subsequently been given many different names: H and D curve, D log E curve, sensitometric curve, and characteristic curve.*-lo~l6 In this article it will be referred to as a characteristic curve. The general shape of such a curve is shown in Fig. 1, I. Because of its general shape, the characteristic curve may be divided into three distinct regions. 1. Toe-The initial period of the curve from A to B wherein the density increases only slightly with increasing exposure. 2. Straight-line-A period of the curve from B to C wherein the density increases as a direct or linear function of the exposure. 3. Shoulder-That period of the curve above C, the region of overexposure, wherein the curve tends to flatten with further increases in exposure. The curve finally becomes horizontal and then reverses itself. In this region of overexposure, less density is produced for increasing amounts of exposure. This basic curve shape represents the type that evolved from Hurter and Driffield’s original work with photographic emulsions exposed to visible light. The curve shape representative of an x-ray exposure, however, is slightly different and is shown in Fig. 1, II. It begins in the low density region as the visible light exposure curve does, with a gradually increasing toe portion. It simulates the straight-line portion of the visible light exposure curve but, instead of demonstrating a shoulder portion, it continues as a straight line upward. Many investigators in the field believe that there is a definite shoulder portion on an x-ray curve, which will be shown when new densitometers that have the ability to read higher densities are developed. The highest readable density with present equipment is in the range of 6 to 8. With these basic concepts and procedures in mind, a definitive x-ray sensitometric study of eight different emulsions was made. Traditionally, it has been the practice of radiologists to accept, without qualification, the film emulsion types produced by manufacturers. This experiment was conducted to determine whether or not other commercially available film emulsions might be adaptable to diagnostic work in dental radiography and to further determine, in terms of a long-range project, whether film characteristics other than those inherent in intraoral film might serve our diagnostic purpuses more effectively. METHODS AND MATERIALS
The eight different emulsions evaluated in this study are listed in Table I. Royal-X pan and Verichrome were selected because they represented two photographic films with widely separated A.S.A. speed ranges. It was considered useful to determine whether these two light-sensitive films demonstrated the same relative speed differences to x-radiation. (The A.S.A. speed of Verichrome is 40, that of Royal-X pan is 1250.) Infrared and Kodachrome were selected to test the x-radiation response of two films differing greatly in their photographic spectral sensitivities.
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Log Exposure
Fig. 1. Curve I represents a typical curve shape for a film exposed to visible light. Curve 11 represents a typical curve shape for a film exposed to x-rays.
Table I. Film products
CodeNo.
1
Emulsion No. Brand me* Ultra-Speed 02207549 Eodachrome II 29 384 AM N Verichrome 1968 K G 4532-207-42-54 Type M % BB 4128-001-01-10 Royal-X pan 6 Type KK L 4531-llV2-11 H.S. Infrared 5218-92-2J i R 5522-913-2-4 Royal Blue *All fdm products manufactured by Eastman Kodak Company. 1 2 3
I
Expiration date December, 1966 December, 1966 March, 1968 November, 1966 October, 1966 p$&d6;MT November, 1966
Type KK and Type M industrial films were chosen because of their wide relative speed range differences and to evaluate their capabilities for use in the medical/dental field. Royal Blue medical/dental film and dental Ultra-Speed film were included primarily for comparison purposes. A standard 5 by 7 inch cardboard cassette was modified to accept a film strip ‘7 inches long and 35 mm. wide. The outside of the cassette was marked off in twelve vertical l/e inch divisions along the ‘I-inch dimension. A cassette holder exposure device was constructed of Plexiglas and lead. This device was so constructed that the cassette was completely covered with lead except for a l/-inch slit through which the exposures were made (Fig, 2). The cassette was advanced behind the slit in the lead holder, l/s inch at a time, while each of the successive exposures was made. The various x-radiation intensities needed to produce the varying densities for sensitometric curve production can be accomplished by either of two techniques. One is a time-scde exposure technique, in which the cassette holder is maintained at a fixed distance from the x-ray source and the x-ray intensity
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PLAN
ELEVATION Fig.
8. Diagram
of device
constructed
to hold film
(loaded)
cassette
during
exposure.
is varied by changing the exposure time. The second is an in$ensity-scale technique, in which the exposure time is held constant and the x-ray intensity is varied by changing the distance of the cassette holder device relative to the x-ray source.8tl6 The choice of exposure procedure depends on whether the film is to be exposed directly to x-rays or to x-rays plus blue light from intensifying screens. In this experiment, when direct x-ray exposures of the films were made, the time-scale procedure was used. When intensifying screen exposures were used, the intensity-scale technique was used in order to prevent erroneous results due to reciprocity law failure. 1p3*5,*slopI13I8 All visible light and x-ray intensifying screen exposures (the latter being approximately 98 per cent visible light exposures) are susceptible to reciprocity law failure. Medical and dental x-ray film sensitometry is based on a range of useful densities; this range is from a low of 0.25, which is almost totally devoid of emulsion density, to a high of 2.00, which is almost totally black under usual viewing conditi0ns.l Because the eight selected films had different sensitivities to x-radiation, it was necessary to conduct a pilot study to determine the correct exposure times for the direct-exposure films and the proper distances for the screen-exposure films, so that the twelve densities produced on the films would be within the desired density range. It was also necessary during the pilot study phase to determine accurately the number of roentgens required for each of the many varieties of exposure conditions. This was accomplished with a Nuclear-Chicago “Cutie-Pie” survey meter. For each separate exposure, three meter readings were taken and averaged. Mackl* states that there are five basic photographic properties of an emulsion : (1) speed, (2) emulsion latitude, (3) contrast, (4) grain size of the silver halide crystals, and (5) color sensitivity of the emulsion. The first three
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properties in this list are obtainable directly from the characteristic curve and are the ones that were evaluated in this study. In order to describe how these three properties are derived from the characteristic curve, it will be helpful to consider three hypothetic curves (Figs. 3,4, and 5). Fig. 3 shows graphically how the speed values were derived from the characteristic curves for this study. According to the American Standard Method for the Sensitometry of Medical X-ray Films,’ the speed of a film is defined as the reciprocal of the exposure necessary to produce a density of 1 on the film. The speed values for the experimental films were derived from the characteristic curves according to this principle. In this study, dental Ultra-Speed film was used as a comparison standard, In medical/dental diagnostic roentgenology the speed of the emulsion is an important factor in t,he selection of a film for a specific purpose. It is mandatory to use a film that will give the best diagnostic result with the least amount of radiation to the patient. Since it would be foolhardy to use an extremely fast film that would not reveal the information desired, film selection relative to speed becomes a matter of operator judgment based on the type of information to be gleaned from the final product and the amount of patient exposure necessary to produce this result. The emulsion latitude of a film is defined as that range of exposure corresponding to the straight-line portion of the cha.racteristic curve.13 For the purpose of medical/dental sensitometry, two predetermined density points (0.25 and 2.00)) are located on the characteristic curve, and it is these two points, as they relate to the log exposure axis, that determine the latitude range. Fig. 4 illustrates this point. It can be seen that the flatter the curve slope (film A compared to film B) , the greater the film latitude. Film latitude is an important concept to the radiologist, for if the latitude range of a film is short, the radiologist must be extremely accurate when exposing the film. The range of remnant exposures from the subject (those reaching the film) must be within the correct latitude range of the film in order to produce a maximum film density difference relationship. In this study the numerical difference between the latitude range extremes, on the exposure axis, was used as the latitude value. Contrast is defined as the density difference between two adjacent areas on a film.8* I31I51I6 In film sensitometry, contrast is quantitatively expressed as the gradient of the film and, more specifically, in medical/dental sensitometry it is expressed as the average gradient. Average gradient is defined as the slope of the curve between two predetermined density points (0.25 and 2.00) .a~12117 The average gradient value was derived in the following manner: Referring to the hypothetic curve shown in Fig. 5, a triangle A, B, C can be constructed, with points A and B used to represent the projected density limits of 0.25 and 2.00 on the curve. The average gradient is expressed as a number representing the ratio of side a to side b (0 = a/b). ‘1 8sl6 It can be seen from this expression that as the curve slope becomes flatter the average gradient ratio decreases and vice versa. The rationale of how inherent film contrast is related to average gradient can be described by referring to the hypothetic curve represented in Fig. 6.
Oral slug. May, 1971
706 Fleming
I I.08 Fast
Exposure Speed
-Slow
Fig. 3. Three identically shaped hypothetic characteristic curves. The speed of a film is a function of its proximity to ordinate or the density axis at a horizontal level of one density unit. Note that film A would be the fastest and film C would be the slowest.
Fig. 4. Characteristic curves of two hypothetic radiographic films (I?, high-contrast film; A, low-contrast Mm). The range of exposures that can be covered within the range of useful density (0.25 to 2.00), the latitude of the film, depends on the average gradient.
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I B
2
a
0.25
_-.-
.-_-._.
Fig. 5. A hypothetic characteristic sideb @=a/b).
curve. The average gradient is the ratio of side a to
difference/
Log Exposure
Fig. 6. Two hypothetic characteristic curves. Same amount of remnant radiation (between EZ and E#) exposes both films. The density difference of film B is far greater than the density difference of film 8.
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If two theoretic remnant exposures El and E2 (Fig, 6) from two adjacent areas of a subject expose film A and film B simultaneously, the density difference produced by exposures El and E2 on film A is considerably less than the density difference produced by the same exposures on film B. In fact, film B produces an exaggerated density difference for the same amount of exposure difference. If, on the other hand, the objects to be radiographed have a high inherent differential in structural density, a film with a lower average gradient and a broader latitude could be used satisfactorily. If the average gradient of the characteristic curve is greater than 1.0, the contrast on the film due to the remnant exposure from the patient is increased. The higher the gradient, the greater the increase in contrast. Thus, at densities for which the average gradient is greater than 1.0, the film acts as a “contrast amplifier.“6l 8 Similarly, if the average gradient is less than 1.0, subject contrast is diminished. Many small differences in anatomic structure undergoing study would not be visible if it were not for the ability of the film to amplify the subject contrast. Each of the experimental films was subjected to three different kilovoltages of x-radiation: 50 KVP, 70 KVP, and 90 KVP. In conjunction with the kilovoltage variation, the filtration of the three x-radiation beams was varied: 50 KVP with 1 mm. aluminum total filtration, 70 KVP with 2 mm. aluminum total filtration, and 90 KVP with 3 mm. aluminum total filtration. Screen and no-screen exposures were made on the test films, since both of these exposure types are commonly used in medical/dental diagnostic radiology. The processing of the exposed films for this experiment was thoroughly standardized. Because of the extreme light-sensitive nature of the photographic emulsions, aZZ processing procedures were carried out in total darkness. The six exposed film strips, of each different film type, were placed on three standard 5 by 7 inch developing racks, two to a rack. All films were processed in standard Eastman Kodak dental x-ray liquid developer and fixer. Development time and temperature were accurately maintained at 41/ minutes and 68’ F., respectively. Every 30 seconds during the developing cycle, a burst of nitrogen gas was delivered to the developer solutions to maintain even and constant development. New solutions were used for each replicate or for each separate experimental set of forty-eight films. A series of ninety-six film strips were produced from the experimental procedure. Each film strip contained twelve different densities which, according to the information gained during the pilot phase, would include the predetermined density range of 0.25 and 2.00. Three separate density readings were taken and averaged for each of the twelve film-strip densities and for each of the ninety-six film strips, giving a grand total of 3,456 density readings. The experimental design and subsequent statistical analysis of the experiment were organized on the basis of how each of the three major criteria (speed, latitude, and average gradient) was separatezy affected by the experimental variables studied, that is, experimental replication, the eight film types, three radiation qualities, two different exposure conditions, and all of their many combinations. The type of statistical design used for this experiment is called a “split-split-plot design.” In order to follow the split-split-plot design and
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attempt to eliminate any form of experimental bias, it was necessary to randomize all of the variables that were to be evaluated. First, the eight films were randomly selected for each of the two experimental procedures. Next, the exposure condition, screen or no screen, was randomly selected for each of the eight film types in the two experimental replicates. Finally, the three radiation qualities were randomly selected for each of the eight films with its specific exposure condition for each of the replicates. RESULTS
Ninety-six sensitometric curves were created on Keuffel and Esser graph paper No. 46 1521. Six separate sensitometric curves were developed for each of the eight different film types. The individual curves were transferred from the graph paper to Sl/, by 11 inch sheets of clear plastic. This was accomplished by placing the plastic sheets over the graphs and tracing the position and shape of the curve onto the plastic with l/ss inch plastic colored tape. This tape was color coded to the three different radiation qualities: Red was assigned to represent 50 KVP with 1 mm. aluminum total filtration, I&%X&to represent ‘70 KVP with 2 mm. aluminum total filtration, and Green to represent 90 KVP with 3 mm. aluminum total filtration, This procedure permitted the overlaying of six transfer graphs, one upon another, for each of the separate film types for comparison purposes. For the purpose of this article, it was necessary to distinguish the three levels of x-radiation by using different line configurations in place of color coding as described above. A dot-dash line was used to represent 50 KVP with 1 mm. aluminum t.otal filtration, a dotted line to represent 70 KVP with 2 mm. aluminum total filtration, and a solid line to indicate 90 KVP with 3 mm. aluminum total filtration. The graphs that Fig. 7 comprises will give the reader a visual evaluation of the emulsion characteristics of these eight films. Speed, latitude, and average gradient values were derived from each of the original curves according to the previously described procedure. The data were subjected to an analysis of variance analyzation. This method of statistical analyzation for this experiment was used because it afforded the opportunity to evaluate how each of the three major criteria-speed, latitude, and average gradient-taken separately, was affected by the experimental variables studied, that is, experimental replication, the eight film types, three different radiation qualities, two different exposure conditions, and all their many combinations. When the experimental data for the major criteria-film speed, latitude, and average gradient-were subjected to an analysis of variance, it was found that there was a significant difference at the P = 0.01 level in three of the experimental variables. As might be expected, a significant difference was found when the experimental variable, film type, was analyzed for speed, latitude, and average gradient. A significant difference was demonstrated when the exposure condition, that is, screen exposure or no-screen exposure, was analyzed. And when data from the various combinations of film type and exposure conditions were andyzed, there was an understandably significant difference. It was interesting to note from the analysis of variance tables that the
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Fig.
7. A composite
of the ninety-six
characteristic
curves.
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7 cowVc2. (For legend, see page 710.)
x-r&y
film
stztdy
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Oral surg. May, 1971
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Fig. 7
cont’d.
(For legend, see page 710.)
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Fig. 7 cont’d.
(For legend, see page 710.)
quality of the radiation did not show a significant difference when latitude and average gradient were the criteria analyzed but did show a diEerence at the P = 0.01 level when speed was the criterion being evaluated. Another interesting deduction from the analysis of variance tables for the three major criteria was that there was no significant difference between the replicates of the experiment. This would indicate that the experimental design was structurally sound and that experimental replication could be accomplished with no appreciable experimental variation. In our effort to relate this experimental information into a more practical and usable form, a table (Table II) comparing the experimental results of the commonly used dental Ultra-Speed film and the seven other test films was constructed. It can be concluded from Table II that when the latitude of the seven test
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40
Fig. 7 cont’d. (For legend, see page 710.)
Table II. Ultra-Speed film comparisons Film type Royal-X pan Infrared Kodachrome Verichrome Royal Blue KK M
Latitude Greater Greater Greater Greater Approximately equal Approximately equal LeSS
Speed Approximately equal Approximately equal Less LeSS Greater Greater Less
Average gradient Less Approximately Less Less Greater Greater Greater
equal
films is compared to that of Ultra-Speed dental film, all but one (Industrial Type M) shows a latitude either equal to or greater than that of the dental Ultra-Speed standard. When film speed was the consideration criterion, four of the seven films (Royal-X pan, High-Speed Infrared, Royal Blue, and Industrial Type KK) demonstrated speed either equal to or greater than the
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standard film. This was similarly true when average gradient was the single criterion. The four films that had either an equal or greater average gradient were High-Speed Infrared, Royal Blue, Industrial Type KK, and Industrial Type M. A statement of qualification is necessary when a particular film is considered for evaluation on the basis of latitude and average gradient. These two film characteristics are reciprocal quantities; that is, as one increases the other must decrease. Thus, for the same range of useful densities, it is impossible for a film of high contrast to have a wide latitude, or vice versa. However, for the purposes of this experiment, these film characteristics were evaluated separately for each specific film type. A variety of questions concerning this experiment are visually answerable when a careful study of Fig. 7 (the composite of the ninety-six transfer graphs arranged according to film type) is made. These include such questions as the following : A. Does the change in kilovoltage alter the basic curve shape? It can be seen by the uniformity of the three curve shapes for each film type and exposure condition that the kilovoltage change has no appreciable effect on the basic curve shape. B. Does the amount of filtration or the quality of the x-radiation vary the curve shape? Again, it can be seen that there is no relationship between the curve shape and the quality of the radiation. C. Does varying the kilovoltage and filtration alter the sensitivity of the emulsion to x-radiation? As in the previous two statements, it can be seen that there is no apparent correlation between the quality of the x-radiation and the sensitivity of the emulsion. D. Does changing the exposure condition from screen to no-screen change the sensitivity of the emulsion? The wide separation of groupings of screen and no-screen curves indicates that when a screening procedure is used there is a definite increase in film sensitivity. This effect is readily apparent in all the film types, with the exception of the dental Ultrafilm ; it is especially Speed film. This film is Kodak’s new “Morlite” manufactured to be more insensitive to wave lengths in the blue light portion of the spectrum. All of the curves, screen and no-screen, for this film are grouped together within a short exposure or sensitivity range. E. Do the photographic emulsions fall within a usable sensitivity range? It can be seen that the majority of the photographic film curves are too far to the right of the Ultra-Speed curves, indicating that they are too insensitive or that they would require an excessive amount of x-radiation to produce a satisfactory clinical radiograph, This fact, however, does not rule out their usefulness for certain experimental research procedures. F. Is it possible to obtain an x-ray-imposed density on a photographic film by processing it in standard x-ray solutions 1 The experiment demonstrated that it wa;s possible to achieve satisfactory results using only the x-ray solutions for all selected films.
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The design and subsequent analysis of this experiment not only demonstrated the resolution of these questions visually but also provided statistical evidence to support these observations. The stability of the experimental design and the statistical balance of this experiment are easily appreciated from the repeatability of the curve shape and position from one replicate to another. SUMMARY
A group of eight films, four photographic (Royal-X pan, High-Speed Infrared, Kodachrome II, Verichrome) , two industrial x-ray (KK, M) , and two commonly used dental and medical x-ray films (Ultra-Speed, Royal Blue), were subjected to an analysis of three of their inherent characteristics-speed, latitude, and average gradient. X-rays were used as the exposure medium. To accomplish the analysis, it was necessary to create separate sensitometric curves for each of the films under different combinations of related factors (ninety-six curves in all). From these curves, it was possible to derive certain values that represented the three criteria-speed, latitude, and average gradient. These values indicated the effect of film type, exposure conditions (screen or no-screen), radiation quality, and all their combined interactions on each of the criteria. Analysis of variance was the method used for the statistical evaluation of the experimental data. Many interesting results were deduced from this experiment : 1. For all practical purposes, the kilovoltage and filtration of the radiation beam had little or no effect on such conditions as curve shape, curve position, or emulsion sensitivity. 2. There was, however, a definite statistical difference in emulsion sensitivity when the exposure condition was changed from screen to no-screen in all the test emulsions with the exception of dental UltraSpeed film. This emulsion has been manufactured so that it is less sensitive to the blue light portion of the spectrum. 3. Some of the test emulsions not presently used in medical/dental diagnostic radiography demonstrated very acceptable speed, latitude, and average gradient characteristics. 4. X-radiation could produce usable densities on photographic emulsions. 5. Even photographic emulsions could be processed in x-ray processing solutions with satisfactory results. 6. The A.S.A. speed ratings of the photographic emulsions correlated very well with their x-ray sensitivities. 7. There was no apparent color effect of x-radiation on the specially color-sensitized emulsion of Kodachrome II. 8. Verichrome and Kodachrome II demonstrated an emulsion sensitivity to x-radiation that was too slow for practical clinical usage. In view of the varied and interesting results of this experiment it is my intention to continue this study with a wider variety of emulsions and subso quently to submit those emulsions of promise to an aluminum step wedge and patient evaluation.
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REFERENCES 1. American
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. ii: 14. 15. 16. 17.
Standards Association: American Standard Method for the Sensitometry of Medical X-ray films, A.S.A. PH2. 9, 1956. Arkin, H., and Colton, R. R.: Tables for Statisticians, ed. 2, New York, 1964, Barnes & Noble; In&. Bell, G. E.: The Photographic Action of X-rays, Brit. J. Radiol. 9: 578-605, 1936. an Introductory Text, New York, 1964, The Macmillan Goldstein, A. : Biostatistics, Company. Corney, G. M.: Gamma-Ray Sensitivities of Several Black and White Negative Photographic Films, Photo. Sci. Tech. (Series II), 2: 146-148,1955. Comey, G. M. : Notes from Eastman Kodak Company course, June, 1965. Eastman Kodak Company: Practical Densitometry, Kodak Sales Service Pamphlet No. E-59, Rochester, N. Y., June, 1963. Eastman Kodak Company: Sensitometric Properties of X-ray Films, Rochester, N. Y., December, 1963. Eastman Kodak Company: Kodak Personal Monitoring Films, Special Sensitized Products Sales Division, Pamphlet No. P-31, Rochester, N. Y., March, 1964. Eastman Kodak Company (X-ray Division): Radiography in Modern Industry, ed. 2, Rochester. N. Y.. 1957. Glasser, O., Q&by, E. H., Taylor, L. S., Weatherwax, J. L., and Morgan, R. H.: Physical Foundations of Radioloav.Y” I ed. 3, New York. 1962. Haruer & Row. Henrikson, C. 0.: Speed and Contrast of Dental Film< Acta Radiol. 1: 66-80, 1963. Johns, H. E.: The Physics of Radiology, ed. 2 (revised), Springfield, Ill., 1964, Charles C Thomas, Publisher, pp. 575-590. Maek, J. E.: The Photographie Process, New York, 1939, McGraw-Hill Book Company, Inc. Mat&on, 0. : Practical Photographic Problems in Radiography, Acta Radiol., Supp. 120, pp. l-206, 1955. Mees, C. E. Ii.: The Theory of the Photographic Process, Ed. revised ed., New York, 1954, The Macmillan Company, pp. 101-124, 147-166, 199-216, 298-330, 503-584, 657-670, 773-777. Trout, E. D., Kelley, J. P., and Cathey, G. A.: The Use of Filters to Control Radiation Exposure to the Patient in Diagnostic Radiology, Amer. J. Roentgen. 67: 946963, 1952.
Reprint requests to: Dr. R. D. Fleming Dental Radiology Division University of Iowa College of Dentistry Dental Building Iowa City, Iowa 52240