Computer-Aided Detection for Breast MRI

Principles of Breast Magnetic Resonance Imaging Interpretation

The sensitivity of breast MRI for detecting breast cancer is high, within the range of 93% to 100%. The primary limitation of dynamic contrast enhanced MRI is a relatively low specificity, reported to be between 37% and 97%. Almost all malignant lesions will enhance after gadolinium administration, and comparing the precontrast and postcontrast sequences to detect areas of enhancement is essential in detecting malignancies. In subtraction images, the precontrast images are subtracted from the postcontrast images, making areas of enhancement more apparent. However, because both benign and malignant breast lesions can enhance, identification of enhancement alone is not sufficient.

Breast MRI specificity relies on analysis of a lesion’s morphological features to distinguish a benign from malignant lesion. Benign lesions tend to be oval with smooth margins and homogeneous internal enhancement, whereas malignant lesions tend to have a round or irregular shape with irregular margins and heterogeneous internal enhancement.

Besides using morphology, breast MRI specificity may be further improved by analyzing lesion kinetics. Some malignant lesions produce angiogenic factors, such as vascular endothelial growth factor (VEGF), that induce the growth of existing capillaries and the formation of vessels to allow the tumor to grow and invade the tissue. The vessels formed by tumor angiogenesis are abnormal and are more porous or leaky than native vessels. Increased enhancement of some cancers relative to the normal, surrounding parenchyma may result from both an increased supply of gadolinium through this proliferative network of vessels and their increased permeability. Increased vascular permeability also results in earlier wash-out of contrast from malignant lesions than from normal breast parenchyma. Therefore lesions on MRI that demonstrate rapid early enhancement followed by a wash-out pattern raise suspicion for associated angiogenesis. For detecting enhancing lesions and assessing their enhancement patterns over time, dynamic contrast-enhanced MRI (DCE-MRI) requires a precontrast and multiple postcontrast sequences (usually three to five).

Computer-Aided Detection for Registration Correction

Many radiologists prefer to interpret subtracted images (postcontrast minus precontrast) because only areas of enhancement in the breast are displayed, reducing distractions and facilitating detection of enhancing lesions ( Figure 3.1 ). However, the quality of subtracted images is significantly affected by patient motion. Most MRI CAD systems, which were developed to allow automated kinetic analysis, provide image registration correction to improve alignment of sequences affected by slight patient motion and reduce subtraction artifact ( Figure 3.2 ). MRI CAD systems usually can correct only for two-dimensional patient motion (the patient rolling left-right or in-out of the coil), not for three-dimensional patient motion (the patient sliding up or down in the magnet). Registration correction is particularly important for breast MRI studies in which fat suppression is not performed and subtraction images are heavily relied upon. Additionally, image registration correction allows for improved three-dimensional image reformation and maximal intensity projection (MIP) generation. However, significant patient motion can lead to erroneous findings on the subtraction images that cannot be overcome by MRI CAD registration correction algorithms. In those instances, interpretation should be based on the source images rather than the subtracted images. Generally, subtraction images should be used only to help detect enhancing lesions. Once detected, the unsubtracted (source) images should be used for analysis of lesion morphology.