Hanna Lehto, Jenni Korhonen, Anneli Holmström ja Anja Henner, Oulu University of Applied Sciences (Oamk)
Mammograms have only been digitised in the 21st century and the technology is constantly evolving. Full-field digital mammography equipment (FFDM) have rapidly become more commonplace, but analog CR (Computed Radiography) systems are still used both in Finland and elsewhere in the world. Many X-ray departments are considering replacing CR equipment with digital ones. Does it make sense to completely replace a functioning machine, or should an old device be upgraded to a digital one? This article compares the image quality of the magnification of a mammography using a mammography unit updated with the FeniX digitalisation kit in compared to a corresponding CR unit.
Breast cancer is the most common cancer in women in Finland. Finnish women aged 50–69 are offered the opportunity to participate in breast cancer screening every two years. The purpose of the screening is to detect breast cancer as early as possible. Almost 90% of the women invited are screened. For 2% to 3% of those who participate in the screening, the exam results in a finding that requires further investigation. Magnification of the object found is one of the most common additional examinations of a mammogram. 
Magnification View in Mammography
Magnification is used in mammography as a complementary examination to obtain a more precise image of small changes and microcalcifications in the structures of the breast. The better visibility of small areas is due to an increase in the contrast ratio, which increases according to the degree of magnification used in the mammogram. A magnification view is made by raising the breast closer to the X-ray source with the help of a magnification table, thereby reducing the distance between the breast and the focus of the X-ray tube. The distance between the focus and the image receptor remains standard according to normal mammography .
FeniX is a product developed by Innomentarium Oy, which can be used to upgrade Alpha, Perfoma and Diamond analogue mammography equipment to digital and to better meet modern requirements for image quality. The direct digital detector of the FeniX update package has been clinically researched and found to be of high quality. It’s an easy-to-use and cost-effective way to extend the age of an old unit for years to come. 
Differences in the ability of flat-panel detectors and CR imaging plates to reveal changes in breasts have been studied extensively [4,5,6]. The effect of an anti-scatter grid and focus size on image quality and the mean glandular dose received by the patient has also been studied extensively in various studies [7,8]. This work (thesis) examined the image quality of the magnification view of mammography with images taken by a GE Healthcare’s Performa mammography unit updated with the FeniX digitalisation kit. A comparison was made with magnification views taken with an CR device of the same manufacturer. The comparison looked at the impact of a change in the amount of electricity, focus size and an anti-scatter grid on image quality and radiation dose.
The study was conducted as a phantom study. The data were collected using two GE Healthcare’s Perfoma mammography units, one of which had been upgraded to direct digital with Innomentarium Oy’s FeniX update package, and the other had CR imaging plates in place. In the imaging, the phantom was scanned on top of the magnifying table, and the values obtained by auto exposure were taken into the actual imaging as manual exposure values. The image quality of the magnification view was also studied by increasing and decreasing the amount of electricity by one step.
Two separate detector holders were used when imaging with the direct digitalised device, one of which had grid and the other did not. A 1.8X magnification table was attached to the top of the detector holder. Magnification mammographies were done with a small (0.1 mm) and large (0.3 mm) focus. Imaging done with the CR imaging plate unit was performed only without a grid and with a small focus, using a 1.8X magnification table. The results produced by the mammography devices were compared with each using only images taken without a grid and with a small focus.
The data were collected by scanning a line pair raster and a Gammex 156 mammography phantom or ACR phantom. The line pair raster is used to study the resolution of the image, and the ACR phantom simulates a 4.2 cm-thick breast, of which half is adipose tissue and half is breast tissue. The phantom contains 16 items of different sizes for image quality analysis, but our thesis explored only the low contrast microcalcifications; the phantom contained five of these . Images of the direct digitalised device were also analysed using ImageJ image analysis software.
Better image quality with a direct digitalised device at a lower dose
Images taken with a
direct digitalised device showed all 5/5 microcalcifications (Table 1) at all
exposure parameters except when reducing
the amount of electricity. In that case, only 4/5 microcalcifications were
seen. With the CR device, 4/5 of the microcalcifications were detected
regardless of changes in the amount of electricity. The automation of the CR device
gave a higher amount of electricity and a lower tube voltage than a direct
digitalised device. The exposure time was also longer with the CR device. When
comparing the glandular tissue doses, the maximum dose of the direct digitised
device was 2.39 mGy at exposure parameters of 31 kV 25 mAs. With the CR device,
exposure parameters of 25 kV 50 mAs gave the highest glandular tissue dose,
which was 2.80 mGy. The direct digitalised device produced an image with image
quality that is equal to that of the CR device with a lower glandular tissue
dose, which was 1.53 mGy. Equal quality of image meant that both images showed
the same number of microcalcifications.
TABLE 1. Values of direct digitalised and CR device values when scanning the ACR phantom without a grid and with a small focus.
|Device||Grid||Focus size||kV||mAs||No. of microcalcifications||MGD (mGy)||Exposure (ms)|
* Closest to the values given by auto-exposure
Line pairs of 5.0 lp/mm, the minimum, were seen with both devices. With the CR device, the magnification tool was used in viewing the image to see all of the smallest line pairs Without the magnification tool, line pairs between 3.4 and 4.0 lp/mm were seen variably.
The direct digitalised mammography device provided better magnification image quality with a lower glandular tissue dose than with the CR device. The results were in line with previous studies. Digitalisation does not require the purchase of a new device for better image quality; in light of the study, upgrading an existing device with a digitalisation kit can be recommended. Updating an old device instead of buying a new one is a good decision ecologically and financially.
In many places in Finland, imaging is done with CR devices. According to previous studies, digital devices produce better image quality at lower patient doses. Innomentarium Oy has developed the FeniX digitalisation package for Alpha, Perfoma and Diamond units. The product has been clinically tested. To further develop the product, the image quality of the magnification view of the FeniX digitisation kit was studied, comparing it to the image quality produced by an CR unit. The results of the study supported the results of previous studies. The digitalised device had better image quality at a lower patient dose than in the CR device for mammogram magnification imaging. Upgrading an analogue mammography device to digital rather than buying a new one saves the environment and money.
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