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Mammography of the large breast: A comparison study of image quality and radiation dose using two different film formats
European Journal of Radiography (2009) 1, 191e200
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journal homepage: http://ees.elsevier.com/ejradi
Mammography of the large breast: A comparison study of image quality and radiation dose using two different film formats A.D. Oldenborg a,*, J.G. Stalheim a,*, H.S. Aase a, J. Rørvik b,c a
Breast Diagnostic Centre, Department of Radiology, Haukeland University Hospital, 5021 Bergen, Norway Section for Radiology, Department of Surgical Sciences, University of Bergen, Norway c Department of Radiology, Haukeland University Hospital, Bergen, Norway b
Received 25 May 2010; received in revised form 16 September 2010; accepted 20 September 2010
KEYWORDS Mammography; Image quality; Radiation dose; Mosaic technique
Abstract Aim: To compare the differences in image quality and average glandular dose (AGD) of mammograms of the large breast using two different film formats: 18 24 cm and 24 30 cm, and explore factors affecting these differences. Materials and methods: 197 women with breasts too large to be imaged on the standard 18 24 cm film format (f.f.) had one breast imaged in one view on both film formats. An analogue mammography unit was used. A mosaic technique was used in conjunction with the 18 24 cm format. A phantom study was used to compare spatial resolution and contrast of both formats at differing phantom thicknesses. The formats were compared for differences in subjective image quality using the parameters; sharpness, contrast, positioning and compression. The amount of breast tissue exposed more than once using the mosaic technique was calculated. The AGD was calculated for each film format, adjusting for the area exposed more than once during the mosaic technique. Results: Spatial resolution and contrast decreased with increasing phantom thickness on both formats. Spatial resolution was poorer in the nipple-areolar area on the 24 30 cm format than the 18 24 cm format. There was equal or improved image quality on all parameters using the mosaic technique when compared with the 24 30 cm f.f. with the exception of evaluation by the PGMI-system (Perfect, Good, Moderate, Inadequate) of the cranio-caudal images. Women received an average of 0.77 mGy (41.6%) higher AGD when the mosaic technique was used as compared to the 24 30 cm film format. Conclusion: There is a marked improvement in image quality in the use of the 18 24 cm format and mosaic technique. Mosaic technique results in a higher AGD than the 24 30 cm f.f. ª 2010 Euro-med Congress for Radiographers. Published by Elsevier Ltd. All rights reserved.
* Corresponding authors. Tel.: þ47 5597 4000. E-mail addresses: [email protected] (A.D. Oldenborg), [email protected] (J.G. Stalheim). 1756-1175/$36 ª 2010 Euro-med Congress for Radiographers. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ejradi.2010.09.001
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Introduction Mammography is the most sensitive screening examination for breast cancer when used in a population-based screening programme. Two-view mammography screening is offered biennially to all women residents in Norway aged 50e69 years. Since pathological signs are often small and subtle, good image quality is essential if the sensitivity/specificity of the screening examination is to be maintained, and to reduce the incidence of interval breast cancers [1,2]. Due to the small but significant risk of producing a radiation-induced breast cancer [3], the ALARA (As Low As Reasonably Achievable) principle must be applied to the radiation dose given during screening. It is well known that there are difficulties in achieving a good image quality for the large breast. The increased breast thickness can lead to geometric blurring and reduced contrast in the images, and there can be problems positioning the breast. All this can result in difficulties when diagnosing subtle changes [4,5]. Increased breast thickness also results in a higher radiation dose to the glandular tissue [4e6]. Two options are available for imaging the large breast: the mosaic technique using 18 24 cm film format (f.f), or the use of a single 24 30 cm f.f. The former results in an increased radiation dose to the glandular tissue [6], while the latter results in increased secondary radiation and a decreased image quality [7]. The aim of this study is to compare the differences in image quality and radiation dose for these two formats, and explore factors affecting these differences.
Materials and methods Approval for this study was obtained from the local research and ethics committee. 200 women were recruited from the screening programme, and all gave informed and written consent. The following criteria were used:
A.D. Oldenborg et al. The same radiographer took all images, and optimal compression was applied. Mosaic images were labelled 1, 2 & 3 (Fig. 1). The single 24 30 cm image of the whole breast was designated 4, and used as reference and comparison. Compression force applied, compressed breast thickness achieved and mAs used were recorded for each image. The anode/filter combination was set at molybdenum/molybdenum, and the voltage was set at 30 kVp. Objective (technical) image quality was assessed using phantom images. Subjective (clinical) image quality was assessed by means of the patient study. Radiation dose was calculated for mammography images only.
Image quality Phantom study (objective image quality) To assess the effect of the divergence of the X-ray beam and secondary radiation on image resolution and contrast, both film formats were compared using images of tissueequivalent mammography phantoms in connection with line-pair (lp) grids placed in the reference position [8] and around the periphery of the radiation field, and then with a CIRS phantom (Computerised Imaging Reference System, INC) positioned as would a breast. These were imaged at increasing phantom thicknesses, from 4.5 cm to 7 cm, at half centimetre intervals. Patient study (subjective image quality) The original analogue images for each woman were viewed side by side under optimal viewing conditions as stated in the Quality Control Manuel for mammography screening in Norway [8]. Image quality was evaluated by two radiographers and two radiologists, each having a minimum 5 years mammography experience and all working independently. Radiographer analysis The two radiographers graded the images according to the PGMI-system (Perfect, Good, Moderate, Inadequate) of positioning evaluation as described in the Quality Control Manual [8]. This requires that 75% of mammograms score either P or G, and that less than 2% score I [8]. Compression
Inclusion the woman had breasts of a size requiring mosaic technique/18 24 cm f.f. Exclusion the woman had palpated changes in her breasts since last examination; the woman had prosthetic breast implants; the woman had previously undergone breast surgery. Three women were later excluded due to problems with exposure and/or processing of the images, leaving images of 197 women eligible for analysis. An analogue mammography unit (Mammomat 3000, Siemens, Germany) and film/screen combination MinRE (Kodak, Norway) were used for the examination. After the completion of the standard screening examination with mosaic technique/18 24 cm f.f, each woman had one breast imaged in one view using the 24 30 cm f.f.
Figure 1 The order of image acquisition during the mosaic technique for both the mlo-and the cc-view.
Mammography of the large breast
193 thickness, and the radiologists’ assessment of the glandular composition of the breast. The following formula was used [9]: D Z Kgcs
Figure 2 The radiologists’ ratings, as percentage of the total cases, of image quality parameters sharpness, contrast and positioning. Score 4 shows both formats to be equally good, less than 4 shows better quality in the 18 24 cm format, more than 4 shows poorer quality in the 18 24 cm format.
within the mammogram i.e., the evaluation of separation of structures, exposure and artefacts, was evaluated separately and categorised as either Good (G) or Inadequate (I). Radiologist analysis The radiologists evaluated the image quality according to four of the criteria described in Taplin et al. [2].: sharpness, contrast, compression and positioning. The 24 30 cm image was used as reference, and the mosaic images were graded on a scale of 1e7, as follows: Grade 1: much better image quality on the mosaic images Grade 2: better quality on the mosaic images Grade 3: slightly better quality on the mosaic images Grade 4: equal image quality on both formats Grade 5: slightly poorer quality on the mosaic images Grade 6: poorer quality on the mosaic images Grade 7: much poorer quality on the mosaic images
where: D Z average glandular dose (mGy), K Z skin surface dose, without secondary radiation, g Z conversion factor for HVL and compressed breast thickness, c Z conversion factor for glandular proportion and breast thickness, s Z conversion factor for radiation quality. Where necessary, the conversion factors were interpolated to give values for the HVL used, the glandular proportion and the compressed breast thickness. In order to calculate the actual dose given to the breast when using the mosaic technique the images were first digitalised, and the area of breast exposed in each image was measured. Image 4, containing the whole breast, was used as the reference for calculating the percentage of breast imaged in each of the mosaic images. Calculation of the dose to the whole breast from each image was achieved by multiplying the percentage of breast tissue in each by the dose for that image. These adjusted doses were then summated to give the actual dose to the whole breast by all of the mosaic images together. The following formula was used: ðA D1Þ þ ðB D2Þ þ ðC D3Þ Z actualdose where: A Z proportion of breast imaged in image 1 (%), B Z proportion of breast imaged in image 2 (%), C Z proportion of breast imaged in image 3 (%), D1 Z AGD for image 1, D2 Z AGD for image 2, D3 Z AGD for image 3 The radiation dose was also calculated according to European Protocol [10] (which summates all the doses from each mosaic image without correction for breast area exposed) to show the size of the overestimation of the AGD when using this total dose method. The cases were split into two groups to show the effect on the AGD as a result of the number of images acquired. Results were calculated for each group, for each view and for the study as a whole.
Statistical analysis Radiation dose The average glandular dose (AGD) given for each image was calculated from: exposure factors, the half-value layer (HVL) of the mammography unit, compressed breast
Paired t-tests were used to establish significant differences. P-values less than 0.05 were considered statistically significant. Confidence intervals (CI) are given where appropriate.
Table 1 Mean actual dose (mGy) to subjects for both 18 24 cm and 24 30 cm film formats, and the dose increase (%) received with the 18 24 cm format compared to the 24 30 cm format. Results shown for cc- and mlo-projections specifically and collectively for both groups. cc-projection
Group 1 Group 2
mlo-projection
18 24 (mGy)
24 30 (mGy)
Inc (%)
2.25 2.74
1.63 1.53
38.3 80.0
2
Collective evaluation
18 24 (mGy)
24 30 (mGy)
Inc (%)
18 24 (mGy)
24 30 (mGy)
Inc2 (%)
2.88 3.77
2.14 2.57
36.1 51.3
2.66 3.07
1.96 1.86
36.89 70.78
2
Inc2: Radiation dose increase (%) using mosaic technique compared to 24 30 cm film format.
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Table 2 Mean increase (%) in actual radiation dose correlated to increase in breast area exposed more than once during mosaic technique, compared to dose given using the 24 30 cm film format, and the number (n) of subjects in each group. cc-projection
group 1
group 2
mlo-projection 2
collective evaluation
Area1 (%)
2
n
Inc (%)
n
Inc (%)
n
Inc2 (%)
20e30> 30e40> 40e50> 50e60> 60e70> 70e80> 80e90> 90e100> 70e80> 80e90> 90e100> 100e110> 110e120> 120e130>
5 5 10 6 10 10 11 2
8.28 33.3 21.2 24.9 46.1 52.8 55.4 45.4
3 4 8 23 46 23 3
4.8 17.0 20.8 25.4 40.5 47.6 70.3
4 3 7 4 1
42.7 83.1 93.4 82.1 69.1
8 9 18 29 56 33 14 2 1 7 6 8 5 1
3.4 26.1 21.0 25.3 41.5 49.2 58.6 45.4 9.49 44.0 72.1 90.4 78.1 69.1
1 3 3 1 1
9.49 45.8 61.1 69.3 61.8
Area1: Increase in breast area exposed (%) more than once during the mosaic technique compared to the area exposed using the 24 30 cm film format. Inc2: Increase in actual radiation dose (%) using mosaic technique compared to radiation dose using the 24 30 cm film format.
Results All 197 women in our study, had one 24 30 cm image taken in addition to the standard mosaic images. Seventyeight women were imaged in the cranio-caudal (cc) projection and 119 in the mediolateral-oblique (mlo) projection. The women were divided into 2 groups according to the number of mosaic images required. Group 1, with 2 mosaic images, consisted of 169 women, and group 2, with 3 mosaic images, consisted of 28 women.
Image quality Phantom study (objective image quality) Results of the lp grid measurements showed that both film formats performed within the desired quality control parameters at the reference position on the 4.5 cm phantom. There was little difference between the formats at all other measurements and phantom thicknesses, except where the lp grid represented the area of retroareolar positioning, where the 18 24 cm f.f. showed a greater resolution on all thicknesses (p < 0.05). Both formats showed decreased resolution with increased phantom thickness at all lp positions.
Table 3
Patient study (subjective image quality) All mammograms except the mlo-view on the 24 30 cm f.f. fulfilled the goals of the PGMI evaluation [10]. Seventy-four percent of cc-projection mammograms in the 18 24 cm format, and 90% in the 24 30 cm f.f. were classified as Perfect (P) or Good (G), (p < 0.05). In the mlo-projection, the figures were 78% and 68% for the 18 24 cm f.f. and the 24 30 cm f.f., respectively (p > 0.05). Radiographers evaluated compression within the mammograms to be good in 80.8% of mammograms on the 24 30 cm f.f, and in 94.2% on the 18 24 cm f.f. The radiologists, however, evaluated compression as equally good on both formats in 78% of cc-mammograms, but in 79% of mlo-mammograms there was better compression with the 18 24 cm f.f. The radiologists rated sharpness, contrast and positioning to be either equally good with both formats or best
Mean breast area (%) exposed more than once when using the mosaic technique. cc-projection
Group 1 Group 2
Results from the CIRS phantom measurements showed the 24 30 cm f.f. to be slightly superior in the high contrast areas (p < 0.01), while the 18 24 cm f.f. was superior in the middle contrast areas (p < 0.05). Both formats performed alike in the low contrast regions. Contrast results were poorer with increased phantom thickness in both formats.
mlo-projection 1
Collective evaluation 1
n
area (95%CI)
n
area (95%CI)
n
area1 (95%CI)
59 19
60.5 (55.4 / 65.7) 103.2 (97.5 / 108.8)
110 9
62.2 (59.9 / 64.4) 92.5 (82.1 / 102.8)
169 28
61.6 (59.3 / 63.9) 99.7 (94.7 / 104.8)
area1: Breast area exposed more than once during the mosaic technique given as a percent of the total breast area measured on the 24 30 cm format.
Mammography of the large breast
195 Breasts only marginally too large for the standard 18 24 cm format showed the greatest area of tissue exposed more than once with the mosaic technique (Fig. 3). There was no significant difference between the compression force applied or the compressed breast thickness achieved between the film formats (p > 0.05).
Secondary findings In this study, 49.7% of the women had less than 30% glandular tissue. Only 2.5% had over 70% glandular tissue. In two cases, important diagnostic information was either not seen or not considered suspicious on the large format (Fig. 4a and b).
Discussion
Figure 3 The correlation between breast size and percentage of breast exposed more than once during mosaic technique.
with the 18 24 cm format, with only a few exceptions (Fig. 2). The poorest quality was present when the combination 24 30 cm f.f. and mlo-view were used.
Radiation dose In all but 6 of the women, the dose given with the mosaic technique was greater than the dose given using the 24 30 cm format. A calculation of radiation doses from all mammograms in the study showed that the women in group 1 received an average 36.89% greater radiation dose, and group 2 received an average 70.78% greater radiation dose when the mosaic technique was used, than that received when the 24 30 cm film format was used (Table 1). The average for all the women was 41.6% (0.77 mGy). Statistical significance between film formats: p < 0.01 for all results.
Total dose and actual dose A collective calculation of the mean total dose showed an overestimation of 25% against the mean actual dose. For group 1 the dose was overestimated by 21% (0.56 mGy), for group 2 by 44.6% (1.37 mGy). The mean total dose was higher than the mean actual dose for all groups and projections; the difference, however, was greatest for the mlo-projection.
Factors affecting the radiation dose Increasing the area exposed more than once increased the difference in radiation dose between formats (Table 2). The proportion of the breast exposed more than once during the mosaic technique for each group and each projection are shown in Table 3.
We have been unable to find previous studies showing the effect using different film format on the image quality in mammography of large breasts, although it is accepted that the use of a large format reduces the radiation dose to these women [6]. This was also shown in this study. The radiation dose in our study was on the average 41.6% greater when using the mosaic technique/18 24 cm f.f, than when using the 24 30 cm f.f.
Image quality Phantom study (objective image quality) The phantom study confirms previous findings of decreasing spatial resolution and contrast with increasing object thickness [4,5]. The difference in spatial resolution between formats was greatest at the nipple-areolar and subareolar region, an area already known to be difficult to evaluate on analogue mammography images [11]. Patient study (subjective image quality) Guest et al. [4] has shown that it is more difficult to comply with image criteria for the mlo-view than the cc-view on overweight women. The mlo-view on the 24 30 cm format was the only image to fail the PGMI requirements of positioning in our study. Radiologists also rated positioning to be poorer on the 24 30 cm format in more than 40% of cases. Failures in positioning may give an increase in the incidence of interval invasive cancers [2]. In the study by Guest et al. the radiographers reported extremes of weight to be the most difficult to position [4]. The large breast is often heavy and difficult for the radiographer to lift, hold and smooth out with one hand. In our study this resulted in skin folds and a poorly outstretched submammary angle. Evaluation of compression in the mlo-view also favoured the 18 24 cm format. The apparent difference in opinion between radiographers and radiologists regarding the evaluation of compression is due to the differing methods of evaluation. Radiographers have assessed each image against the requirements of the Quality Control Manual. Radiologists have compared the two formats against each other. Although many of the 24 30 cm f.f. images passed the quality control requirements, they were still considered inferior to the 18 24 cm f.f. from a clinical point of view. Problem areas on the 24 30 cm format were insufficient
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Figure 4 (a) Microcalcifications only visible on the 18 24 cm format. (b) The star-shaped lesion only suspicious on the 18 24 cm format.
Mammography of the large breast
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Figure 5 The difference in penetration of the pectoralis muscle and diffuseness of the mammilla area of the breast on the 24 30 cm format film compared with the 18 24 cm format mosaic images.
penetration of the pectoralis muscle leading to weak projection of lymph nodes (Fig. 5), an inability to image the muscle to the level of the nipple, under exposure of the retro-areolar area, and poorly separated structures. The mosaic technique eliminates problems with positioning and compression. The first mosaic image includes the axilla and pectoralis muscle, without the need to lift the whole breast. The compression force applied and mAs chosen by the ionisation chamber are optimal for the muscle area, producing good penetration. Mosaic image 2 concentrates on the glandular area of the breast, and the radiographer is able to use both hands to position this area, ensuring the submammary angle is well positioned, and the
Figure 6 The missing triangle of breast on the 18 24 cm format due to changes in the positioning of the woman between image 1 (bar line) and image 2 (dotted line).
retro-areolar area is smoothed out. Optimal compression force and exposure factors, due to the homogenic nature of the breast tissue included in each of these images, produces sharper images. The combination of mlo-view and 24 30 cm format did poorer on the evaluation of contrast and sharpness in comparison with the 18 24 cm format mosaic images, so that it was more difficult to diagnose small and subtle changes in the breast tissue. A failure in sharpness is also associated with the incidence of interval cancer in some cases [2]. PGMI evaluation of positioning in the cc-view showed both formats to be of a good standard. However, the mosaic cc-images scored poorer than the 24 30 cm format images. The larger format showed under exposure of, poorly separated structures in, and skin folds in the retroareolar area. There was also difficulty imaging the nipple in profile. Problems areas for the cc-view mosaic images were skin folds against the chest wall and inadequate visualisation of the lateral tail. A triangular area close to the chest wall was also discovered to be missing in the 18 24 cm format when compared to the 24 30 cm format, and was the main reason for this poorer PGMI score. This was due to the positioning technique used (Fig. 6), with rotation of the woman between mosaic image 1 and 2. This is a problem easily solved by the radiographer by ensuring the whole breast is drawn out evenly from the chest wall without rotation in relation to the film. The large format cc-view scored better than the large format mlo-view on both sharpness and contrast, with 80% of images being equally as good as the mosaic images. However, 20% of images were still regarded best on the 18 24 cm format. Obese women are more likely to have a false-positive screening examination than normal weight women, which
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Figure 7 (a & b) The effect of positioning of the breast in relation to the ionisation chamber. Both subjects have almost identical proportions of breast exposed more than once. The subject in Fig. 7a receives only a 3% higher dose with mosaic technique compared to the larger format, due to fatty tissue covering the ionisation chamber in image 2. The glandular rich breast of the subject in Fig. 7b receives 34.5% higher dose when using the mosaic technique compared to the larger format.
may be another result of poor image quality due to increased scattered radiation leading to poorer image contrast [12]. This study shows that image quality can be improved for women with large breasts by using an 18 24 cm format film and the mosaic technique. Our study was composed of only supposedly healthy women in a screening situation. In those few cases where important diagnostic information was seen, it appeared on
both formats, except in 2 cases where this information was either visible exclusively in the mosaic images, or only regarded as suspicious in the mosaic images (Fig. 4a and b). Both were mlo-views, shown earlier to be of much poorer quality in the 24 30 cm format, and the information was in the retro-areolar area which is already known to be more difficult to evaluate on anaolgue mammography images [11]. This area also showed poorer spatial resolution on the
Mammography of the large breast 24 30 cm format in our phantom study. In these two cases a screening examination based only on the 24 30 cm format would have given a false-negative result.
Radiation dose Calculation of the actual dose received with mosaic technique was a major part of this study. This was a timeconsuming exercise, and is not practical for use in regular dose control procedures. We were unable to accurately calculate the dose received for women in group 2 as it was not possible to calculate the proportion of breast tissue irradiated more than twice. The total dose method proposed in the European Protocol on Dosimetry [10] is far more practical, but overestimates the actual dose received by an average of 25%. The overestimation is greatest for the mlo-view. The positioning of the breast in relation to the ionisation chamber had an effect on the difference in dose received between formats. When similar breast tissue covers the ionisation chamber on all images the difference in dose between formats is less than when one of the images has tissue of a much different density covering the ionisation chamber. Fig. 7 demonstrates this. The amount of breast exposed more than once is almost identical in both women. However, dose increase when using the mosaic technique contra the large format was 3% in Fig. 7a and 34.5% in Fig. 7b. Six of the study women received a lower dose with the mosaic technique than the 24 30 cm format, and this was entirely due to the positioning of the breast structures in relation to the ionisation chamber. Calculation of the actual dose, taking into account the proportion of breast exposed in each image, resulted in a lower dose with the mosaic technique than with the 24 30 cm format. In accordance with the findings of Young et al. [6], we found that the amount of breast tissue exposed more than once in the mosaic technique had an effect on the radiation dose received. The greater the amount of tissue exposed more than once, the greater was the dose received. The area exposed more than once was greatest for those women with breasts only marginally too large for the 18 24 cm format. Usual practice at the department has been to exploit as much of the film area as possible at each exposure. Smaller breasts would therefore be only slightly repositioned between exposures. Larger breasts would need to be totally repositioned between exposures to ensure that all the breast tissue was exposed at least once. A portion of the breast must always be included on both images in order to ensure total coverage, however, the amount of breast tissue exposed more than once can be kept to a minimum if we allow part of the film to be unused. Results for some of the women in group 2 appear to indicate that more than 100% of the breast was exposed more than once, thus suggesting that the whole breast would have fit on one view. This discrepancy is due to there being 3 exposures required to cover the whole breast, resulting in some breast tissue being irradiated 3 times. We were unable to calculate the size of this area, thus resulting in an overestimation of the area of breast tissue exposed more than once in the group 2 women, and a subsequent overestimation of the actual radiation dose for this group. However, as this is
199 a very small group (only 14% of the study women and 2% of the screening population) this discrepancy will have little effect on the average radiation dose increase shown in this study when using the mosaic technique. Large breasts are usually associated with a lower percentage of glandular tissue compared with the normal sized breast [12], and it is the glandular tissue which is radiation sensitive. Almost half of the women in this study had less than 30% glandular tissue. Only 2.5% of the women had more than 70% glandular tissue, and these women would benefit most from the dose-saving technique of breast repositioning. The majority of these results are likely to be applicable for both analogue and digital mammography systems. Positioning techniques, compression applied and geometric differences, due to detector size differences, apply to both units. The nipple-areolar area and microcalcifications are more easily visible on a digital system, and even though image postprocessing on a digital unit can compensate a little for insufficient breast compression it does not change the fact that good compression is a must for optimal images. Radiation doses received with digital units are lower than with analogue units [13]; however, the difference in doses between formats would likely be of similar proportions to this study.
Conclusion Image quality was noticeably better in the mosaic images than in the 24 30 cm format images, especially in the mlo-view. There is a higher radiation dose to the glandular tissue when the mosaic technique is used than when the 24 30 cm f.f. is used. Limiting the amount of breast tissue exposed more than once in the mosaic technique can minimise this difference. We believe that the 18 24 cm f.f/mosaic technique is the better choice when examining women with large breasts, as the improved image quality increases the diagnostic performance of mammography. Curbing the radiation dose must not come at the cost of a good diagnostic image.
Acknowledgements Statistical analysis assistance: Ragnhild Sørum, Cancer Registry of Norway The authors have no relevant financial interest in this article.
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Mammography of the large breast: A comparison study of image quality and radiation dose using two different film formats A.D. Oldenborg a,*, J.G. Stalheim a,*, H.S. Aase a, J. Rørvik b,c a
Breast Diagnostic Centre, Department of Radiology, Haukeland University Hospital, 5021 Bergen, Norway Section for Radiology, Department of Surgical Sciences, University of Bergen, Norway c Department of Radiology, Haukeland University Hospital, Bergen, Norway b
Received 25 May 2010; received in revised form 16 September 2010; accepted 20 September 2010
KEYWORDS Mammography; Image quality; Radiation dose; Mosaic technique
Abstract Aim: To compare the differences in image quality and average glandular dose (AGD) of mammograms of the large breast using two different film formats: 18 24 cm and 24 30 cm, and explore factors affecting these differences. Materials and methods: 197 women with breasts too large to be imaged on the standard 18 24 cm film format (f.f.) had one breast imaged in one view on both film formats. An analogue mammography unit was used. A mosaic technique was used in conjunction with the 18 24 cm format. A phantom study was used to compare spatial resolution and contrast of both formats at differing phantom thicknesses. The formats were compared for differences in subjective image quality using the parameters; sharpness, contrast, positioning and compression. The amount of breast tissue exposed more than once using the mosaic technique was calculated. The AGD was calculated for each film format, adjusting for the area exposed more than once during the mosaic technique. Results: Spatial resolution and contrast decreased with increasing phantom thickness on both formats. Spatial resolution was poorer in the nipple-areolar area on the 24 30 cm format than the 18 24 cm format. There was equal or improved image quality on all parameters using the mosaic technique when compared with the 24 30 cm f.f. with the exception of evaluation by the PGMI-system (Perfect, Good, Moderate, Inadequate) of the cranio-caudal images. Women received an average of 0.77 mGy (41.6%) higher AGD when the mosaic technique was used as compared to the 24 30 cm film format. Conclusion: There is a marked improvement in image quality in the use of the 18 24 cm format and mosaic technique. Mosaic technique results in a higher AGD than the 24 30 cm f.f. ª 2010 Euro-med Congress for Radiographers. Published by Elsevier Ltd. All rights reserved.
* Corresponding authors. Tel.: þ47 5597 4000. E-mail addresses: [email protected] (A.D. Oldenborg), [email protected] (J.G. Stalheim). 1756-1175/$36 ª 2010 Euro-med Congress for Radiographers. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ejradi.2010.09.001
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Introduction Mammography is the most sensitive screening examination for breast cancer when used in a population-based screening programme. Two-view mammography screening is offered biennially to all women residents in Norway aged 50e69 years. Since pathological signs are often small and subtle, good image quality is essential if the sensitivity/specificity of the screening examination is to be maintained, and to reduce the incidence of interval breast cancers [1,2]. Due to the small but significant risk of producing a radiation-induced breast cancer [3], the ALARA (As Low As Reasonably Achievable) principle must be applied to the radiation dose given during screening. It is well known that there are difficulties in achieving a good image quality for the large breast. The increased breast thickness can lead to geometric blurring and reduced contrast in the images, and there can be problems positioning the breast. All this can result in difficulties when diagnosing subtle changes [4,5]. Increased breast thickness also results in a higher radiation dose to the glandular tissue [4e6]. Two options are available for imaging the large breast: the mosaic technique using 18 24 cm film format (f.f), or the use of a single 24 30 cm f.f. The former results in an increased radiation dose to the glandular tissue [6], while the latter results in increased secondary radiation and a decreased image quality [7]. The aim of this study is to compare the differences in image quality and radiation dose for these two formats, and explore factors affecting these differences.
Materials and methods Approval for this study was obtained from the local research and ethics committee. 200 women were recruited from the screening programme, and all gave informed and written consent. The following criteria were used:
A.D. Oldenborg et al. The same radiographer took all images, and optimal compression was applied. Mosaic images were labelled 1, 2 & 3 (Fig. 1). The single 24 30 cm image of the whole breast was designated 4, and used as reference and comparison. Compression force applied, compressed breast thickness achieved and mAs used were recorded for each image. The anode/filter combination was set at molybdenum/molybdenum, and the voltage was set at 30 kVp. Objective (technical) image quality was assessed using phantom images. Subjective (clinical) image quality was assessed by means of the patient study. Radiation dose was calculated for mammography images only.
Image quality Phantom study (objective image quality) To assess the effect of the divergence of the X-ray beam and secondary radiation on image resolution and contrast, both film formats were compared using images of tissueequivalent mammography phantoms in connection with line-pair (lp) grids placed in the reference position [8] and around the periphery of the radiation field, and then with a CIRS phantom (Computerised Imaging Reference System, INC) positioned as would a breast. These were imaged at increasing phantom thicknesses, from 4.5 cm to 7 cm, at half centimetre intervals. Patient study (subjective image quality) The original analogue images for each woman were viewed side by side under optimal viewing conditions as stated in the Quality Control Manuel for mammography screening in Norway [8]. Image quality was evaluated by two radiographers and two radiologists, each having a minimum 5 years mammography experience and all working independently. Radiographer analysis The two radiographers graded the images according to the PGMI-system (Perfect, Good, Moderate, Inadequate) of positioning evaluation as described in the Quality Control Manual [8]. This requires that 75% of mammograms score either P or G, and that less than 2% score I [8]. Compression
Inclusion the woman had breasts of a size requiring mosaic technique/18 24 cm f.f. Exclusion the woman had palpated changes in her breasts since last examination; the woman had prosthetic breast implants; the woman had previously undergone breast surgery. Three women were later excluded due to problems with exposure and/or processing of the images, leaving images of 197 women eligible for analysis. An analogue mammography unit (Mammomat 3000, Siemens, Germany) and film/screen combination MinRE (Kodak, Norway) were used for the examination. After the completion of the standard screening examination with mosaic technique/18 24 cm f.f, each woman had one breast imaged in one view using the 24 30 cm f.f.
Figure 1 The order of image acquisition during the mosaic technique for both the mlo-and the cc-view.
Mammography of the large breast
193 thickness, and the radiologists’ assessment of the glandular composition of the breast. The following formula was used [9]: D Z Kgcs
Figure 2 The radiologists’ ratings, as percentage of the total cases, of image quality parameters sharpness, contrast and positioning. Score 4 shows both formats to be equally good, less than 4 shows better quality in the 18 24 cm format, more than 4 shows poorer quality in the 18 24 cm format.
within the mammogram i.e., the evaluation of separation of structures, exposure and artefacts, was evaluated separately and categorised as either Good (G) or Inadequate (I). Radiologist analysis The radiologists evaluated the image quality according to four of the criteria described in Taplin et al. [2].: sharpness, contrast, compression and positioning. The 24 30 cm image was used as reference, and the mosaic images were graded on a scale of 1e7, as follows: Grade 1: much better image quality on the mosaic images Grade 2: better quality on the mosaic images Grade 3: slightly better quality on the mosaic images Grade 4: equal image quality on both formats Grade 5: slightly poorer quality on the mosaic images Grade 6: poorer quality on the mosaic images Grade 7: much poorer quality on the mosaic images
where: D Z average glandular dose (mGy), K Z skin surface dose, without secondary radiation, g Z conversion factor for HVL and compressed breast thickness, c Z conversion factor for glandular proportion and breast thickness, s Z conversion factor for radiation quality. Where necessary, the conversion factors were interpolated to give values for the HVL used, the glandular proportion and the compressed breast thickness. In order to calculate the actual dose given to the breast when using the mosaic technique the images were first digitalised, and the area of breast exposed in each image was measured. Image 4, containing the whole breast, was used as the reference for calculating the percentage of breast imaged in each of the mosaic images. Calculation of the dose to the whole breast from each image was achieved by multiplying the percentage of breast tissue in each by the dose for that image. These adjusted doses were then summated to give the actual dose to the whole breast by all of the mosaic images together. The following formula was used: ðA D1Þ þ ðB D2Þ þ ðC D3Þ Z actualdose where: A Z proportion of breast imaged in image 1 (%), B Z proportion of breast imaged in image 2 (%), C Z proportion of breast imaged in image 3 (%), D1 Z AGD for image 1, D2 Z AGD for image 2, D3 Z AGD for image 3 The radiation dose was also calculated according to European Protocol [10] (which summates all the doses from each mosaic image without correction for breast area exposed) to show the size of the overestimation of the AGD when using this total dose method. The cases were split into two groups to show the effect on the AGD as a result of the number of images acquired. Results were calculated for each group, for each view and for the study as a whole.
Statistical analysis Radiation dose The average glandular dose (AGD) given for each image was calculated from: exposure factors, the half-value layer (HVL) of the mammography unit, compressed breast
Paired t-tests were used to establish significant differences. P-values less than 0.05 were considered statistically significant. Confidence intervals (CI) are given where appropriate.
Table 1 Mean actual dose (mGy) to subjects for both 18 24 cm and 24 30 cm film formats, and the dose increase (%) received with the 18 24 cm format compared to the 24 30 cm format. Results shown for cc- and mlo-projections specifically and collectively for both groups. cc-projection
Group 1 Group 2
mlo-projection
18 24 (mGy)
24 30 (mGy)
Inc (%)
2.25 2.74
1.63 1.53
38.3 80.0
2
Collective evaluation
18 24 (mGy)
24 30 (mGy)
Inc (%)
18 24 (mGy)
24 30 (mGy)
Inc2 (%)
2.88 3.77
2.14 2.57
36.1 51.3
2.66 3.07
1.96 1.86
36.89 70.78
2
Inc2: Radiation dose increase (%) using mosaic technique compared to 24 30 cm film format.
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Table 2 Mean increase (%) in actual radiation dose correlated to increase in breast area exposed more than once during mosaic technique, compared to dose given using the 24 30 cm film format, and the number (n) of subjects in each group. cc-projection
group 1
group 2
mlo-projection 2
collective evaluation
Area1 (%)
2
n
Inc (%)
n
Inc (%)
n
Inc2 (%)
20e30> 30e40> 40e50> 50e60> 60e70> 70e80> 80e90> 90e100> 70e80> 80e90> 90e100> 100e110> 110e120> 120e130>
5 5 10 6 10 10 11 2
8.28 33.3 21.2 24.9 46.1 52.8 55.4 45.4
3 4 8 23 46 23 3
4.8 17.0 20.8 25.4 40.5 47.6 70.3
4 3 7 4 1
42.7 83.1 93.4 82.1 69.1
8 9 18 29 56 33 14 2 1 7 6 8 5 1
3.4 26.1 21.0 25.3 41.5 49.2 58.6 45.4 9.49 44.0 72.1 90.4 78.1 69.1
1 3 3 1 1
9.49 45.8 61.1 69.3 61.8
Area1: Increase in breast area exposed (%) more than once during the mosaic technique compared to the area exposed using the 24 30 cm film format. Inc2: Increase in actual radiation dose (%) using mosaic technique compared to radiation dose using the 24 30 cm film format.
Results All 197 women in our study, had one 24 30 cm image taken in addition to the standard mosaic images. Seventyeight women were imaged in the cranio-caudal (cc) projection and 119 in the mediolateral-oblique (mlo) projection. The women were divided into 2 groups according to the number of mosaic images required. Group 1, with 2 mosaic images, consisted of 169 women, and group 2, with 3 mosaic images, consisted of 28 women.
Image quality Phantom study (objective image quality) Results of the lp grid measurements showed that both film formats performed within the desired quality control parameters at the reference position on the 4.5 cm phantom. There was little difference between the formats at all other measurements and phantom thicknesses, except where the lp grid represented the area of retroareolar positioning, where the 18 24 cm f.f. showed a greater resolution on all thicknesses (p < 0.05). Both formats showed decreased resolution with increased phantom thickness at all lp positions.
Table 3
Patient study (subjective image quality) All mammograms except the mlo-view on the 24 30 cm f.f. fulfilled the goals of the PGMI evaluation [10]. Seventy-four percent of cc-projection mammograms in the 18 24 cm format, and 90% in the 24 30 cm f.f. were classified as Perfect (P) or Good (G), (p < 0.05). In the mlo-projection, the figures were 78% and 68% for the 18 24 cm f.f. and the 24 30 cm f.f., respectively (p > 0.05). Radiographers evaluated compression within the mammograms to be good in 80.8% of mammograms on the 24 30 cm f.f, and in 94.2% on the 18 24 cm f.f. The radiologists, however, evaluated compression as equally good on both formats in 78% of cc-mammograms, but in 79% of mlo-mammograms there was better compression with the 18 24 cm f.f. The radiologists rated sharpness, contrast and positioning to be either equally good with both formats or best
Mean breast area (%) exposed more than once when using the mosaic technique. cc-projection
Group 1 Group 2
Results from the CIRS phantom measurements showed the 24 30 cm f.f. to be slightly superior in the high contrast areas (p < 0.01), while the 18 24 cm f.f. was superior in the middle contrast areas (p < 0.05). Both formats performed alike in the low contrast regions. Contrast results were poorer with increased phantom thickness in both formats.
mlo-projection 1
Collective evaluation 1
n
area (95%CI)
n
area (95%CI)
n
area1 (95%CI)
59 19
60.5 (55.4 / 65.7) 103.2 (97.5 / 108.8)
110 9
62.2 (59.9 / 64.4) 92.5 (82.1 / 102.8)
169 28
61.6 (59.3 / 63.9) 99.7 (94.7 / 104.8)
area1: Breast area exposed more than once during the mosaic technique given as a percent of the total breast area measured on the 24 30 cm format.
Mammography of the large breast
195 Breasts only marginally too large for the standard 18 24 cm format showed the greatest area of tissue exposed more than once with the mosaic technique (Fig. 3). There was no significant difference between the compression force applied or the compressed breast thickness achieved between the film formats (p > 0.05).
Secondary findings In this study, 49.7% of the women had less than 30% glandular tissue. Only 2.5% had over 70% glandular tissue. In two cases, important diagnostic information was either not seen or not considered suspicious on the large format (Fig. 4a and b).
Discussion
Figure 3 The correlation between breast size and percentage of breast exposed more than once during mosaic technique.
with the 18 24 cm format, with only a few exceptions (Fig. 2). The poorest quality was present when the combination 24 30 cm f.f. and mlo-view were used.
Radiation dose In all but 6 of the women, the dose given with the mosaic technique was greater than the dose given using the 24 30 cm format. A calculation of radiation doses from all mammograms in the study showed that the women in group 1 received an average 36.89% greater radiation dose, and group 2 received an average 70.78% greater radiation dose when the mosaic technique was used, than that received when the 24 30 cm film format was used (Table 1). The average for all the women was 41.6% (0.77 mGy). Statistical significance between film formats: p < 0.01 for all results.
Total dose and actual dose A collective calculation of the mean total dose showed an overestimation of 25% against the mean actual dose. For group 1 the dose was overestimated by 21% (0.56 mGy), for group 2 by 44.6% (1.37 mGy). The mean total dose was higher than the mean actual dose for all groups and projections; the difference, however, was greatest for the mlo-projection.
Factors affecting the radiation dose Increasing the area exposed more than once increased the difference in radiation dose between formats (Table 2). The proportion of the breast exposed more than once during the mosaic technique for each group and each projection are shown in Table 3.
We have been unable to find previous studies showing the effect using different film format on the image quality in mammography of large breasts, although it is accepted that the use of a large format reduces the radiation dose to these women [6]. This was also shown in this study. The radiation dose in our study was on the average 41.6% greater when using the mosaic technique/18 24 cm f.f, than when using the 24 30 cm f.f.
Image quality Phantom study (objective image quality) The phantom study confirms previous findings of decreasing spatial resolution and contrast with increasing object thickness [4,5]. The difference in spatial resolution between formats was greatest at the nipple-areolar and subareolar region, an area already known to be difficult to evaluate on analogue mammography images [11]. Patient study (subjective image quality) Guest et al. [4] has shown that it is more difficult to comply with image criteria for the mlo-view than the cc-view on overweight women. The mlo-view on the 24 30 cm format was the only image to fail the PGMI requirements of positioning in our study. Radiologists also rated positioning to be poorer on the 24 30 cm format in more than 40% of cases. Failures in positioning may give an increase in the incidence of interval invasive cancers [2]. In the study by Guest et al. the radiographers reported extremes of weight to be the most difficult to position [4]. The large breast is often heavy and difficult for the radiographer to lift, hold and smooth out with one hand. In our study this resulted in skin folds and a poorly outstretched submammary angle. Evaluation of compression in the mlo-view also favoured the 18 24 cm format. The apparent difference in opinion between radiographers and radiologists regarding the evaluation of compression is due to the differing methods of evaluation. Radiographers have assessed each image against the requirements of the Quality Control Manual. Radiologists have compared the two formats against each other. Although many of the 24 30 cm f.f. images passed the quality control requirements, they were still considered inferior to the 18 24 cm f.f. from a clinical point of view. Problem areas on the 24 30 cm format were insufficient
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Figure 4 (a) Microcalcifications only visible on the 18 24 cm format. (b) The star-shaped lesion only suspicious on the 18 24 cm format.
Mammography of the large breast
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Figure 5 The difference in penetration of the pectoralis muscle and diffuseness of the mammilla area of the breast on the 24 30 cm format film compared with the 18 24 cm format mosaic images.
penetration of the pectoralis muscle leading to weak projection of lymph nodes (Fig. 5), an inability to image the muscle to the level of the nipple, under exposure of the retro-areolar area, and poorly separated structures. The mosaic technique eliminates problems with positioning and compression. The first mosaic image includes the axilla and pectoralis muscle, without the need to lift the whole breast. The compression force applied and mAs chosen by the ionisation chamber are optimal for the muscle area, producing good penetration. Mosaic image 2 concentrates on the glandular area of the breast, and the radiographer is able to use both hands to position this area, ensuring the submammary angle is well positioned, and the
Figure 6 The missing triangle of breast on the 18 24 cm format due to changes in the positioning of the woman between image 1 (bar line) and image 2 (dotted line).
retro-areolar area is smoothed out. Optimal compression force and exposure factors, due to the homogenic nature of the breast tissue included in each of these images, produces sharper images. The combination of mlo-view and 24 30 cm format did poorer on the evaluation of contrast and sharpness in comparison with the 18 24 cm format mosaic images, so that it was more difficult to diagnose small and subtle changes in the breast tissue. A failure in sharpness is also associated with the incidence of interval cancer in some cases [2]. PGMI evaluation of positioning in the cc-view showed both formats to be of a good standard. However, the mosaic cc-images scored poorer than the 24 30 cm format images. The larger format showed under exposure of, poorly separated structures in, and skin folds in the retroareolar area. There was also difficulty imaging the nipple in profile. Problems areas for the cc-view mosaic images were skin folds against the chest wall and inadequate visualisation of the lateral tail. A triangular area close to the chest wall was also discovered to be missing in the 18 24 cm format when compared to the 24 30 cm format, and was the main reason for this poorer PGMI score. This was due to the positioning technique used (Fig. 6), with rotation of the woman between mosaic image 1 and 2. This is a problem easily solved by the radiographer by ensuring the whole breast is drawn out evenly from the chest wall without rotation in relation to the film. The large format cc-view scored better than the large format mlo-view on both sharpness and contrast, with 80% of images being equally as good as the mosaic images. However, 20% of images were still regarded best on the 18 24 cm format. Obese women are more likely to have a false-positive screening examination than normal weight women, which
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Figure 7 (a & b) The effect of positioning of the breast in relation to the ionisation chamber. Both subjects have almost identical proportions of breast exposed more than once. The subject in Fig. 7a receives only a 3% higher dose with mosaic technique compared to the larger format, due to fatty tissue covering the ionisation chamber in image 2. The glandular rich breast of the subject in Fig. 7b receives 34.5% higher dose when using the mosaic technique compared to the larger format.
may be another result of poor image quality due to increased scattered radiation leading to poorer image contrast [12]. This study shows that image quality can be improved for women with large breasts by using an 18 24 cm format film and the mosaic technique. Our study was composed of only supposedly healthy women in a screening situation. In those few cases where important diagnostic information was seen, it appeared on
both formats, except in 2 cases where this information was either visible exclusively in the mosaic images, or only regarded as suspicious in the mosaic images (Fig. 4a and b). Both were mlo-views, shown earlier to be of much poorer quality in the 24 30 cm format, and the information was in the retro-areolar area which is already known to be more difficult to evaluate on anaolgue mammography images [11]. This area also showed poorer spatial resolution on the
Mammography of the large breast 24 30 cm format in our phantom study. In these two cases a screening examination based only on the 24 30 cm format would have given a false-negative result.
Radiation dose Calculation of the actual dose received with mosaic technique was a major part of this study. This was a timeconsuming exercise, and is not practical for use in regular dose control procedures. We were unable to accurately calculate the dose received for women in group 2 as it was not possible to calculate the proportion of breast tissue irradiated more than twice. The total dose method proposed in the European Protocol on Dosimetry [10] is far more practical, but overestimates the actual dose received by an average of 25%. The overestimation is greatest for the mlo-view. The positioning of the breast in relation to the ionisation chamber had an effect on the difference in dose received between formats. When similar breast tissue covers the ionisation chamber on all images the difference in dose between formats is less than when one of the images has tissue of a much different density covering the ionisation chamber. Fig. 7 demonstrates this. The amount of breast exposed more than once is almost identical in both women. However, dose increase when using the mosaic technique contra the large format was 3% in Fig. 7a and 34.5% in Fig. 7b. Six of the study women received a lower dose with the mosaic technique than the 24 30 cm format, and this was entirely due to the positioning of the breast structures in relation to the ionisation chamber. Calculation of the actual dose, taking into account the proportion of breast exposed in each image, resulted in a lower dose with the mosaic technique than with the 24 30 cm format. In accordance with the findings of Young et al. [6], we found that the amount of breast tissue exposed more than once in the mosaic technique had an effect on the radiation dose received. The greater the amount of tissue exposed more than once, the greater was the dose received. The area exposed more than once was greatest for those women with breasts only marginally too large for the 18 24 cm format. Usual practice at the department has been to exploit as much of the film area as possible at each exposure. Smaller breasts would therefore be only slightly repositioned between exposures. Larger breasts would need to be totally repositioned between exposures to ensure that all the breast tissue was exposed at least once. A portion of the breast must always be included on both images in order to ensure total coverage, however, the amount of breast tissue exposed more than once can be kept to a minimum if we allow part of the film to be unused. Results for some of the women in group 2 appear to indicate that more than 100% of the breast was exposed more than once, thus suggesting that the whole breast would have fit on one view. This discrepancy is due to there being 3 exposures required to cover the whole breast, resulting in some breast tissue being irradiated 3 times. We were unable to calculate the size of this area, thus resulting in an overestimation of the area of breast tissue exposed more than once in the group 2 women, and a subsequent overestimation of the actual radiation dose for this group. However, as this is
199 a very small group (only 14% of the study women and 2% of the screening population) this discrepancy will have little effect on the average radiation dose increase shown in this study when using the mosaic technique. Large breasts are usually associated with a lower percentage of glandular tissue compared with the normal sized breast [12], and it is the glandular tissue which is radiation sensitive. Almost half of the women in this study had less than 30% glandular tissue. Only 2.5% of the women had more than 70% glandular tissue, and these women would benefit most from the dose-saving technique of breast repositioning. The majority of these results are likely to be applicable for both analogue and digital mammography systems. Positioning techniques, compression applied and geometric differences, due to detector size differences, apply to both units. The nipple-areolar area and microcalcifications are more easily visible on a digital system, and even though image postprocessing on a digital unit can compensate a little for insufficient breast compression it does not change the fact that good compression is a must for optimal images. Radiation doses received with digital units are lower than with analogue units [13]; however, the difference in doses between formats would likely be of similar proportions to this study.
Conclusion Image quality was noticeably better in the mosaic images than in the 24 30 cm format images, especially in the mlo-view. There is a higher radiation dose to the glandular tissue when the mosaic technique is used than when the 24 30 cm f.f. is used. Limiting the amount of breast tissue exposed more than once in the mosaic technique can minimise this difference. We believe that the 18 24 cm f.f/mosaic technique is the better choice when examining women with large breasts, as the improved image quality increases the diagnostic performance of mammography. Curbing the radiation dose must not come at the cost of a good diagnostic image.
Acknowledgements Statistical analysis assistance: Ragnhild Sørum, Cancer Registry of Norway The authors have no relevant financial interest in this article.
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