26 - Breast Lesions: Computerized Analysis of Magnetic Resonance Imaging

Abstract

This chapter presents the role of computerized analysis of magnetic resonance imaging (MRI) in the diagnosis of breast lesions. It is important to review the basic mechanisms of contrast uptake in the breast and the limitations of contrast-enhanced (CE) MRI to visualize this uptake in order to fully appreciate the rationale for computerized analysis of CE MRI. Magnetic resonance imaging acquisition protocols are defined by the sequences at which the radiofrequency field in the coil is turned on and off. Typical techniques are Tl-weighted spin-echo and gradient-echo imaging. The patient is examined in prone orientation using a dedicated bilateral breast coil. A series of precontrast images is made, followed by intravenous application of the contrast agent at a well-defined rate by means of a power injector. The presence of malignant disease is correlated with several temporal and morphological characteristics of the contrast enhancement at CE MRI. To visualize the enhancement, subtraction images are typically computed. The precontrast series subtracted from the postcontrast series (wash-in series) emphasize the early kinetics caused by transfer of the contrast agent from the neovascular system to the leakage space. The first postcontrast series subtracted from the last postcontrast series (washout series) visualize early departure of the contrast agent from the leakage space back to the neovascular system. The methods used to visualize the contrast enhancement are multiplanar reformatting (MPR) and maximum-intensity projection (MIP). In the first method, cross sections through the CE MRI volume are calculated at various depths and directions, and in the second method the wash-in volume is projected in one or multiple directions. The resulting MIP views allow superior assessment of multifocal enhancement patterns that extend into multiple cross sections.