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Feature Breast imaging


summation of the projection values (figure 3). This allows the physician to obtain sections comparable to those of conventional tomography, but exempt from the critics previously explained. Tomosynthesis, therefore, does not


provide direct projection images, but reconstructed images of any individual layers through several available algorithms. The aim is to remove the ‘structured noise’ from the upper and lower slices from each image. Reconstructed algorithms used in the


first generation of devices (including FBP- Filtered Back Projection algorithm, ideal for 360° CT acquisitions reconstruction but not optimal in DBT reconstruction, where it generates noise and artifacts) were recently abandoned for iterative algorithms, such as the SART - Simultaneous Algebraic Reconstruction Technique, and the MLEM - Maximum Likelihood Expectation Maximization. These can improve imaging quality through the final reduction of streaking artifacts, as well the increasing of contrast-to-noise ratio, thus improving the visibility of microcalcifications and skin edge. DBT allows the detection of a greater


number of expansive lesions and a better morphological analysis of masses and architectural distortions. This is thanks to the contrast of findings greater than the background, given by the more shade structures belonging to the upper and lower layers, and also to the smaller amount of noise (figure 4). It therefore exceeded one of the limits of two- dimensional imaging, which is the masking of lesions caused by superimposition of normal structures.


The possibility of separating different


layers suggests a possible reduction of false negatives and false positives due to overlapping. According to data from early trials,


DBT is designed to offer the conspicuity of a higher percentage of breast cancers than conventional mammography, reducing false negative (FN) percentages at an estimated value of around 15%. More recent studies indicate about 30% increased DBT sensitivity and specificity compared to FFDM with a recalls reduction in screening by approximately 40%. A further advantage of DBT is given


by the lack of need for operator training (the breast is positioned just like a conventional mammography in MLO and/


4


or CC projection) and for the radiologist (as he continues to perform diagnosis from images with mammogram features). It’s still an ongoing study to compare the clinical performance of FFDM in two projections (CC + MLO) and those of DBT in a single projection (MLO) in compliance with dose constraint. Until the clinical ‘non-inferiority’ of DBT compared to FFDM is demonstrated, it is not


reasonable to increase dose in DBT. For this reason, the dose is restrained so as not to exceed the dose of a two- projection FFDM. Features common to every DBT


systems are the execution mode (MLO projection), acquisition time (10-20 sec) and reconstruction time (between 40 and 180 sec), slices thickness (1mm), display mode (single slice, or slab cine loop), chance 


Imaging & Diagnostics Issue 4 2011 17


FIGS 1, 2, 3, 4 1


2


3


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