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Image quality and breast dose of 24 screen–film combinations for mammography

¹ Medical Physics Department, Medical School, University of Athens, 75 Mikras Asias, 115 27, Athens , ² Medical Physics Unit, Konstantopoulio-Agia Olga Hospital, 3-5 Agias Olgas, Nea Ionia, 142 33, Athens, Greece

	Abstract

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In this study the effect of different mammographic screen–film combinations on image quality and breast dose, and the correlation between the various image quality parameters, breast dose and the sensitometric parameters of a film were investigated. Three Agfa (MR5-II, HDR, HT), two Kodak (Min-R M, Min-R 2000), one Fuji (AD-M), one Konica (CM-H) and one Ferrania (HM plus) single emulsion mammographic films were combined with three intensifying screens (Agfa HDS, Kodak Min-R 2190 and Fuji AD-MA). The film characteristics were determined by sensitometry, while the image quality and the dose to the breast of the resulting 24 screen–film combinations were assessed using a mammography quality control phantom. For each combination, three images of the phantom were acquired with optical density within three different ranges. Two observers assessed the quality of the 72 phantom images obtained, while the breast dose was calculated from the exposure data required for each image. Large differences among screen–film combinations in terms of image quality and breast dose were identified however, that, could not be correlated with the film's sensitometric characteristics. All films presented the best resolution when combined with the HDS screen at the expense of speed, and the largest speed when combined with the AD-MA screen, without degradation of the overall image quality. However, an ideal screen–film combination presenting the best image quality with the least dose was not identified. It is also worth mentioning that the best performance for a film was not necessarily obtained when this was combined with the screen provided by the same manufacturer. The results of this study clearly demonstrate that comparison of films based on their sensitometric characteristics are of limited value for clinical practice, as their performance is strongly affected by the screens with which they are combined.

	Introduction

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The main concern in mammography screening is the detection of features characteristic of breast disease. These features often have sizes of the order of 1 mm and differ from the normal tissue only slightly in composition, thus setting high requirements for the resolution and contrast that an imaging system must offer in order to be appropriate for mammography [1]. On the other hand, given the high radiosensitivity of the breast and the large number of women examined many times during their life, it is evident that the doses during mammography should be kept as low as possible.

While digital mammography may look promising, the vast majority of mammography examinations are still carried out with screen–film systems. In recent years, most film manufacturers have presented new films and intensifying screens for mammography that reduce the dose to the breast and produce the image quality required to maintain the diagnostic sensitivity and specificity of mammography at high levels. However, while the design is the major factor in determining the performance of a film, this may be affected by the processing conditions, such as the chemicals used, their temperature and the processing time [2]. Inappropriate chemicals or a developing temperature lower than recommended may result in unacceptable mammograms and this is why some films have been modified to be less dependent on processing conditions [3, 4].

Film characteristics can be determined and monitored for changes due to processing by sensitometry. However, film performance will be dependent on the screen with which it is combined and thus for clinical practice the characteristics of the screen–film combination rather than those of film or screen separately are of interest [5]. The screen–film characteristics can be determined and monitored using an appropriate quality control (QC) phantom, with which changes in image quality due to processing or other reasons can be identified.

One of the parameters routinely monitored with the QC phantom is the background or reference optical density (OD) of the mammographic images. Apart from the personal preferences of radiologists, it has been shown that for a given screen–film combination, subtle details and small contrast differences are best accentuated when the film OD is within a certain range [6–8]. For this reason it has been recommended that each institution should determine the optimum OD for the screen–film used and the processing conditions specific to it [7].

In this study, eight films were combined with three intensifying screens and the resulting 24 screen–film combinations were compared in terms of image quality and breast dose. Film characteristics were determined by sensitometry, whereas the image quality and speed of the screen–film combinations were assessed using a QC phantom to obtain images within three different OD ranges. Our main objective was to investigate the effect of different screens on a certain film and search for any correlation between the image quality, breast dose and the sensitometric parameters of a film.

	Materials and methods

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The eight single emulsion mammographic films tested in this study were the MR5-II, HDR, HT (Agfa-Gevaert N.V., Mortsel, Belgium), Min-R M, Min-R 2000 (Eastman Kodak Company, New York, NY), AD-M (Fuji Photo Film Co. Ltd, Tokyo, Japan), CM-H (Konica Corporation, Tokyo, Japan) and HM plus (Ferrania Sp A, Ferrania (SV), Italy). One box from each film type was used to avoid little differences that may exist among different film batches or films that may have been stored for different times and under different storage conditions [9]. The three intensifying screens used in this study were the HDS (Agfa), the Min-R 2190 (Kodak) and the AD-MA (Fuji).

For each film a 21-step sensitometric strip was produced, using an X-Rite 334 sensitometer (X-Rite, Grandville, MI) operated in the green spectrum. All films were processed in a daylight processor (Curix Capacity, Agfa) with nominal processing time 90 s (22 s developing time) and with the developer temperature set to 36°C. The developer type was the Eos Dev (Agfa) and the fixer type was the G334i (Agfa). All films were processed sequentially, immediately after exposure and on the same day, to avoid day-to-day variations in processing conditions caused by the ageing of the chemicals that may have variable effects on the characteristics of each film [9, 10].

The OD of the 21 steps of the sensitometric strips was measured using a calibrated optical densitometer (RMI 331, X-Rite). For each film the Hurter-Driffield (H&D) curve was plotted and the following sensitometric parameters were derived: OD of base plus fog (OD_b+f), maximum OD (OD_max), average gradient (AG), film gamma () and film speed. The AG and are the slopes of the H&D curve for ODs from 0.25+ODb+f to 2.0+OD_b+f and from 1.0+OD_b+f to 2.0+OD_b+f, respectively. AG and are both used as indices of film contrast, however, only can be used to reproduce the linear part of the H&D curve. The film speed was defined as the reciprocal of the relative light exposure required to obtain an OD of 1+OD_b+f. Using this definition, the higher the film speed the less exposure is needed for a given OD. In order to illustrate the expected increase in breast dose – according to sensitometry – when a film other than the fastest one is used, the sensitometric relative dose index (SRDI) was defined as the reciprocal of the relative speed value. The SDRIs were expressed as percentages of the smallest SRDI value (highest speed) that was taken as 100%. It must be noted that from preliminary sensitometric tests it has been confirmed that for different sheets of the same film type processed within the same day, variations of less than ±0.01 in OD_b+f, ±10% in speed and ±0.1 in OD_max, AG and should be expected.

To evaluate the characteristics of screen–film combinations, a mammography QC phantom was employed (breast phantom, Model 18-222; Nuclear Associates, Division of Victoreen Inc., NY). This phantom is realistically shaped and equivalent to an average firm breast of 4.5 cm compressed thickness, consisting of 50% adipose and 50% glandular tissue. It includes 12 groups of calcium carbonate specks (simulating microcalcifications), 7 hemispheric masses composed of 75% glandular and 25% adipose equivalent tissue (simulating tumours) and a wax insert with 5 embedded nylon fibres (simulating glandular tissue fibrils). The phantom also contains a five-step stepwedge, simulating breast areas with compositions 100% adipose, 70% adipose–30% glandular, 50% adipose–50% glandular, 30% adipose–70% glandular and 100% glandular tissue. Finally, two line-pair test targets (5–20 lp mm^–1 each), one parallel and one perpendicular to the anode–cathode axis and a central area where the background OD is measured, are included. A similar phantom (without the nylon fibres and with only one line-pair test target) was used by Nassivera and Nardin [11].

The phantom was exposed using a Senographe 500T mammography unit (CGR, Buc, France). All exposures were made with the Mo/Mo target filter combination, constant tube potential (28 kV_p), large focal spot (0.3 mm nominal size) and without the breast compression paddle. Using manual mAs selection technique, images of the phantom were acquired until for each screen–film combination three films with OD as close as possible to the central OD of three different optical density ranges (0.70–1.10, 1.11–1.50, 1.51–2.00) were produced to account for the wide range of ODs that can be encountered within actual mammographs. Phantom images were processed in the same processor and on the same day as the sensitometric strips, so both film characteristics and screen–film performance were determined under the same processing conditions. It must be noted that as the Kodak and Fuji cassettes were not compatible with the Agfa daylight processor, the films exposed with these cassettes had to be transferred manually to the Agfa cassette in order to be fed into the processor.

For each one of the resulting 72 phantom images, the OD of the central area was measured with the densitometer, as well as the OD of the areas simulating 100% adipose and 100% glandular tissue. The OD difference of these two areas can be used as an index of screen–film contrast (CI) and, according to the phantom manufacturer, it should be 0.28.

All phantom images were examined using a viewing box especially designed for mammography, featuring adjustable brightness, masking shutters and a magnifying glass. The shutters were closed down to the phantom image size and the brightness was adjusted as necessary to obtain the best possible conditions for viewing each type of simulated lesion, while for speck groups the magnifying glass was also used. A magnifying glass supplied with the QC phantom was used to inspect the line pair object. The above details are mentioned, as viewing conditions are very important for interpreting mammograms or scoring phantom images [12].

Two observers scored the images independently and any disagreements were resolved by consensus. Five scores were recorded for each film: one for the speck-groups, one for masses, one for fibres and two for the two line-pair test targets. For ambiguous decisions concerning not clearly visualized structures, a 0.5 mark was assigned. In order to have a single index characterizing the screen–film performance, a total score (TS) was calculated using the following weighting coefficients: 0.4 for specks, 0.35 for masses and 0.25 for fibres. These coefficients were selected after consulting with five radiologists about the clinical importance of each simulated structure for diagnosis. Since the two scores for the line-pair test targets have no straightforward clinical relevance, their mean value was calculated for reference only (resolution score).

For screen–film combination comparisons in terms of breast dose, the entrance surface air-kerma (ESAK) at the phantom surface was calculated from the mAs selected for each exposure. For the range of mAs selections utilized in this study, the output at 28 kVp defined at the phantom entrance surface was 98±2 µGy mAs^–1. Furthermore, the ESAK required to achieve a net OD of 1 was calculated by interpolation from the ODs and the ESAKs of the three films acquired for each combination. The resulting ESAKs for a net OD = 1 were used to derive the relative dose index (RDI), expressed as a percentage of the smallest observed value, that was considered as 100%. Using this definition, the larger the RDI, the larger the dose to the breast and the smaller the speed of a given screen–film combination.

To investigate the correlation between the various image quality parameters, sensitometric parameters, OD and dose indices linear regression analysis was used. A correlation coefficient (r) larger than 0.7 was taken as an indication of good correlation. Specifically, the correlations of all the image quality scores (TS, specks, masses, fibres and resolution) with AG, , SRDI, CI, OD and ESAK were investigated. Furthermore, the correlation of TS with resolution, the correlations of CI with AG, , SDRI, OD and ESAK and the correlations of SDRI with RDI and ESAK were also investigated.

	Results

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The H&D curves for all the films studied are plotted in Figure 1, while their sensitometric parameters are given in Table 1. In Figure 1, the large differences among the H&D curve shapes, the high speed of Min-R 2000, the low speed of AD-M and the non-typical but similar H&D curve shapes of HDR and HM plus should be noted. From Table 1, it can be seen that the Min-R 2000 presents the highest speed and , while the CM-H has the highest AG.

View larger version (27K): [in this window] [in a new window]   Figure 1. The Hurter-Driffield(H&D) curves of the eight films included in this study are given. The optical densities (ODs) of the 21 steps of the sensitometric strips correspond to log relative exposure values (LogE) that range from 0 to 3, in steps of 0.15 each. In these figures, only the ODs for LogEge;0.9 (steps 7 to 21) are presented in order to enhance the visibility of the differences in the linear part and the shoulder of the H&D curves.

View this table: [in this window] [in a new window]   Table 3. The best screen for a given film in terms of total score(TS), index of screen–film contrast (CI), line pairs per millimetre (lp mm–1) and relative dose index (RDI). Letters in parentheses give reference to Tables 2a, 2b or 2c where each value can be found. The largest values overall for each parameter are given in bold

View this table: [in this window] [in a new window]   Table 4. The best film for a given screen in terms of total score(TS), index of screen–film contrast (CI), line pairs per millimetre (lp mm–1) and relative dose index (RDI). Letters in parentheses give reference to Tables 2a, 2b or 2c where each value can be found. The largest values overall for each parameter are given in bold

Finally, concerning the correlation between image quality parameters, sensitometric parameters and dose, no correlation coefficient larger than 0.7 was calculated in any of the correlations tested. The largest correlation coefficient calculated was that between CI and OD, which was 0.66 but increased to 0.87 when the MR5-II combinations were excluded, demonstrating that for modern high contrast films the screen–film contrast increases with OD. It must be clarified, however, that this correlation has been assessed for ODs up to 2.0 and it is expected that for higher ODs the CI will start to decrease again as the films become saturated.

The poor correlations of image quality parameters (TS, specks, masses, fibres, resolution and CI) with film contrast (, AG) and the poor correlation of RDI with SRDI, confirmed that film performance is strongly affected by the screen. Concerning the lack of correlation of TS with CI, resolution and dose, the following remarks should be made. While combinations with high TS generally had high CI, there were many cases with high CI and low TS. There were also many combinations with high TS that, however, exhibited low resolution score and vice versa. Finally, slow combinations did not always give high TS, as expected according to the general principle that the higher the dose the lower the quantum mottle.

	Discussion

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The major conclusion of this study was that film characteristics are modified by intensifying screens in such a significant and variable way, that comparisons among films based on the manufacturer's specifications or sensitometry are of limited value. Indeed, a film with given technical specifications or sensitometric characteristics, when combined with different screens may exhibit improved or degraded performance.

Instructive of the variable effect that a screen may have on a film, it can be seen that while Min-R 2000 was the fastest film and remained the fastest when combined with the Fuji screen, the RDI ratio of CM-H and Min-R 2000 combinations with Fuji screen was 1.04 while for film only the respective ratio of SDRI was 1.32. That means that the Fuji screen spectral emission better matched the spectral sensitivity of CM-H compared with Min-R 2000. The slowest film, according to sensitometry, was the AD-M, which remained the slowest when combined with all screens. The SDRI for AD-M was 167 while its smallest RDI was 191 and it was observed when combined with the Fuji screen.

Examples of the largest variations observed in score and dose when a film was combined with different screens are: the HM plus where the TS was 36% larger with the Fuji than with the Agfa screen (Table 2c), the Min-R 2000 where the CI was 46% and 22% larger with the Agfa than with the Kodak screen (Tables 2a and 2c, respectively), the Min-R 2000 where the resolution was 26% larger with the Agfa than with the Fuji screen (Table 2b) and the HT (Agfa) where 63% more dose is required with the Agfa screen than with the Fuji screen.

As previously mentioned, from Tables 2–4 some conclusions may be drawn concerning the superiority of certain combinations over others in terms of image quality or speed. However, the absolute values of scores and other screen–film characteristics may be quite different on other mammographic facilities, given the strong dependence of film characteristics on processing conditions [13, 14]. This must be emphasised, as the objective of this study was not to recommend or condemn certain films or screens but to investigate the effect of screens on the performance of films. Although most screen–film comparisons in the literature have been carried out using the same processing conditions for all films, it must be noted that the general notion is that a film would perform optimally when it is processed according to the recommendations of the manufacturer. Even so, this does not annul the fact that the breast dose and image quality for a film optimally processed will again vary, depending on the screen with which it is combined, and that some films will be affected by the screen more than others.

Even if it were assumed that the processing conditions were optimal for all films, it would again be difficult to select the best screen–film combination from those studied, as there are no established criteria about what increase in breast dose is justified by a superior image quality. For example the AD-MA/AD-M presented a TS of 7.3 (11 specks, 6 masses, 3 fibres) and an ESAK of 6.2 mGy while the AD-MA/Min-R 2000 a TS of 6.4 (9 specks, 5 masses, 4 fibres) with an ESAK of 2.4 mGy. To conclude which is the best combination, one has to decide if the 14% increase in TS could justify the 158% increase in breast dose. The same question still holds when considering that certain combinations (as the AD-MA/AD-M) exhibited slightly larger TS for larger ODs but with disproportional increase in breast dose.

An important remark should also be made concerning the OD of the films studied. It is obvious that the films included in Table 2a are of too low OD and few of the films included in Table 2c are of too high OD, compared with the target OD range of 1.3 to 1.8 proposed for mammography [1]. Nonetheless, certain combinations exhibited better scores in Table 2a than in Tables 2b and 2c, while most of the films of Table 2c with ODs larger than 1.8 exhibited scores similar to those of Table 2b. In clinical practice, however, given that the wide OD variations within a mammogram are not uncommon, some areas may present similar ODs with those of Table 2a or larger than 1.8 and therefore the performance of a screen–film combination within all OD ranges is of interest. In this context, comparisons based on the TSm may be considered more relevant to the clinical situation than comparisons based on the TS within only one OD range. The variability of TS with OD should always be considered when selecting the central OD setting of the automatic exposure control (AEC) system based on the results of phantom scores.

Some final comments should be made concerning the method used to assess the image quality of screen–film combinations. Phantom scoring does not always represent clinical practice, as in actual mammograms the performance of a given combination will be also dependent on the breast type [15]. Furthermore, phantom scoring may be somewhat biased, as it relies on the detection of structures known to be present at specific positions [16]. Nevertheless, phantoms are considered as the best way for the objective evaluation of image quality and various models with fixed or randomly positioned details are extensively used. Caldwell et al [17] agreed on the usefulness of such phantoms for the objective evaluation of image quality and also reported that a subjective assessment of image quality is better accomplished with an anthropomorphic breast phantom than with actual mammograms, where the variability among radiologists was higher. However, they noted that no significant correlation was found between the various methods used to evaluate image quality and concluded that more work is required to obtain an index of true image quality correlated with the probability of correct diagnosis.

In conclusion, image quality and dose in mammography are more strongly dependent on screen–film combination than on film or screen separately. While sensitometry remains an important tool for determining and monitoring the film characteristics [18], it is of little value when the image quality and breast dose in clinical mammograms are of concern. Therefore, any change of film or screen type in a mammographic facility should be carefully investigated with a phantom, for determining the performance of the selected screen–film combination and for adjusting the AEC system to the optimum OD range for this combination.

Received for publication February 26, 2005. Revision received May 23, 2005. Accepted for publication June 7, 2005.

	References

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