Dosimetry in Digital Breast Tomosynthesis

Open Access
Author:
Bradley, Lena Renee
Area of Honors:
Physics
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Mark Strikman, Thesis Supervisor
  • Dr Andrew D A Maidment, Thesis Supervisor
  • Richard Wallace Robinett, Honors Advisor
Keywords:
  • dosimetry
  • digital breast tomosynthesis
  • DBT
  • dose
  • mammography
  • dosimeter
  • ionization chamber
Abstract:
This thesis presents a study of dosimetry for the emerging imaging modality of Digital Breast Tomosynthesis (DBT). In the last decade, Digital Mammography (DM) has become the gold standard of diagnostic breast imaging, especially for regular breast cancer screening. This modality, however, is known to have significant limitations due to the fact that a mammogram displays a two-dimensional x-ray projection view through a three-dimensional breast. Because the third spatial dimension is compressed during imaging, a mammogram often shows the superposition of nonadjacent tissues, a phenomenon which can both obscure small cancers and cause the appearance of non-existent structures. DBT is an emerging imaging modality that seeks to avoid this problem by acquiring multiple projection views through the breast over a range of angles. These images can then be mathematically combined and reconstructed to form a 3D image of the breast. As DBT becomes more widespread, a standard method of dosimetry and quality assurance is necessary to ensure safe, consistent and controlled results. Although DM dosimetry is standardized, there is currently no such procedure for DBT. Therefore, the application of DM dosimeters in DBT geometries was investigated, with special focus toward their dependence on the DBT projection angle and x-ray energy. Both experimental results and numeric models show that mammographic dosimeters have varying degrees of dependence upon projection angle and x-ray energy. These can introduce significant errors in DBT measurements if left uncorrected. It is possible to compute factors that correct for these errors, but they are highly dependent on the specifics of the dosimeter, imaging system and x-ray beam. Instead, it is recommended that an angle-independent dosimeter be used. To this end, cylindrical, open-air ionization chambers, which would be symmetric with respect to projection angle, were constructed and tested. Chambers constructed with varying wall materials were also studied. For some constructed chambers, experimental results confirmed almost no angular dependence and energy dependence comparable to that of mammographic dosimeters applied in DM. Although the chamber construction would benefit from the refinements of commercial development, this proof-of-concept study shows that such a design is well suited for DBT dosimetry, especially as compared to mammographic dosimeters.