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Chapter 2: Fundamentals of Achieving Functional and Anatomical Imaging with Breast MRI: Physical and Clinical Concepts
Breast magnetic resonance imaging (MRI) has shown great potential as a diagnostic tool and is rapidly becoming part of the standard of care in breast cancer evaluation. Three major technological advancements have been responsible for the increased utilization that has effectively mainstreamed breast MRI into practice. The first advancement is in recent parallel imaging techniques with coil sensitivity encoding that now permit detailed spatial resolution with rapid temporal acquisition. Temporal resolution gained by parallel imaging is critical in determining dynamic contrast enhancement and washout curves that can aid in distinguishing benign from malignant neoplasms. Of critical importance is the application of bilateral breast imaging that permits contralateral comparison and symmetry evaluation between breasts. Higher field strength magnets (greater than or equal to 1.5 T) have made it possible to attain greater signal-to-noise ratios needed for the application of âparallel imaging.â Second, new shimming and fat saturation methods permit alignment of radio-frequency-pulse sequences "off resonance" (spatially and chemically) to the spatial offset position (imaging "axis") of the breast with improved fat saturation. Distinguishing fat from enhanced lesions is a critical challenge in breast MRI. While subtraction techniques without fat saturation have been used in the past as a method to gain time, the technique requires a nontrivial level of patient compliance because a small positional shift may degrade the readability in the "subtraction" images. A misalignment in position between precontrast and postcontrast images that involves nonrigid spatial deformations can drastically limit diagnostic quality. Such deformations are not correctable even with the use of image processing methods due to nonseparable signal overlap. The third and final advancement has been the commercial availability of MRI biopsy devices that allow tissue-based confirmation of suspicious lesions seen only on MRI, particularly as MRI is known for its high degree of sensitivity to neoplastic lesion conspicuity. As a cross-sectional imaging modality, MRI also offers multiplanar capabilities and the ability to perform maximum intensity projections (MIP) that permit the radiologist to manipulate the images for precise tumor localization.
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