Mammography of the large breast: A comparison study of image quality and radiation dose using two different film formats

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.