Contrast-enhanced magnetic resonance (MR) imaging is an imaging technique that characterizes morphological and functional aspects of breast lesions. The principle underlying breast MR imaging is neoangiogenesis in tumors. Invasive breast cancers have high metabolic demand for oxygen and nutrients often resulting in hypoxia within the tumor cells which in turn releases vascular endothelial growth factors that induce formation of new vessels. The angiogenic activity of cancers results in a complex network of newly formed vessels that lack normal anatomical structure resulting in capillary leakage, arteriovenous shunting, and increased perfusion. Malignant lesions with tumor angiogenesis would therefore show rapid and intense enhancement, while benign lesions generally have slower and less intense enhancement. Factors that determine enhancement include vessel density and vessel permeability, rate of contrast leak within the tumor tissue, T1 relaxivity of glandular parenchyma and tumor cells, and volume and relaxivity of contrast injected.

The technique assesses tissue perfusion and cellularity. In this chapter we discuss optimal pulse sequences and protocols for breast imaging and image analysis to diagnose various breast pathologies.

Breast MR imaging depends on optimum levels of spatial and temporal resolution both of which have to be of the highest level. Current breast MR imaging techniques provide high-spatial-resolution tissue characteristics on multiplanar imaging, functional information on perfusion cellularity, and capillary leakage. Tissue characterization is further enabled by knowing T1 and T2 relaxation times, the use of fat and fluid suppression, and diffusion imaging.

Breast MR imaging provides information on physical and physiologic tissue characteristics. The protocols for breast MR should be designed in such a way that these characteristics are obtained. Protocols can be modified for specific conditions like patient having implants though most studies are designed to include the basic mandatory sequences.

Magnetic field strengths of 1.5–3 T should be used to increase signal-to-noise ratio and diagnostic accuracy. Higher field strength further improves spatial and temporal resolution. Dedicated multi-element breast surface coils are used to provide higher signal-to-noise ratio and offer the advantage of the ability to use parallel imaging techniques. Currently available breast coils have a multicoil phased-array configuration that is optimized for bilateral simultaneous imaging of breasts, chest wall, and axillae. Bilateral breast MR imaging is currently performed with 4-channel, 8-channel, or 16-channel coils, with better signal-to-noise ratios from higher number of channels. Bilateral imaging has the advantage of being quicker and more convenient for the patient. It also allows direct comparison between the two breasts in one field of view, thereby facilitating the assessment of symmetry during screening. Most coils are open at the side to allow lateral access for intervention and are equipped with a compression plate. The advantages of breast compression are minimization of patient movement, decreased thickness of the breast, and shorter imaging time. However, excessive compression may compromise the enhancement of breast tumors, leading to a false-negative result. Therefore, only minimal or no compression should be applied to immobilize the breast during routine breast MR examinations.

Current MR mammogram protocols include T1 and T2 TSE, fat-suppressed T2 or STIR (short tau inversion recovery), and diffusion and dynamic contrast sequences. All dynamic breast MR protocols use three-dimensional T1-weighted gradient sequences. Three-dimensional imaging indicates that phase encoding, frequency encoding, and section encoding are all achieved by applying suitable gradients during image acquisition. 3D imaging has the inherent advantage of better T1 contrast and higher signal-to-noise ratio. The GRE pulse sequence is spoiled to avoid any confounding T2 contrast. All pulse sequences should use the shortest possible repetition time, a large flip angle, and a short echo time, which should be set in phase (4.6 ms at 1.5 T) for non-fat-saturated subtracted protocols.

T1 TSE sequences identified fat within some lesions like fibroadenomas which provide a clue to the diagnosis. The inherent contrast provided by the fat within breast tissue helps identify tumors which usually appear diffusely hypointense.

T2-weighted images help identify cysts and fluid-containing ducts that appear bright. Diffusion imaging performed at different b values identifies tumor tissue based on the principle of restricted diffusion in malignant tissues. However, there are quite a few pitfalls as protein-rich fluid-containing lesions and inflammation may have restricted diffusion.

Contrast acquisition in the sagittal plane is most preferred. The technical advantage of the sagittal plane is that a relatively small FOV is sufficient which improves the spatial resolution at a given acquisition matrix, acquisition speed with no penalty in spatial resolution. Fat suppression is more homogeneous. The disadvantage of the sagittal plane is that far too many sections would be needed to cover both breasts. This disadvantage can be overcome by the use of bilateral dynamic imaging and subtraction artifacts that make images very difficult to read. The transverse plane is preferable if bilateral imaging is desired or required.

Bilateral dynamic imaging is available in most recent scanners. Bilateral imaging is important in all women undergoing screening and in all who undergo staging for known breast cancer.

All breast imaging protocols need some sort of fat suppression or subtraction to highlight enhancing lesions. Active fat suppression, or fat saturation, means that the signal from fatty tissue is specifically eliminated by additional radiofrequency pulses or by choosing selective water excitation. In addition, active fat suppression requires a very homogeneous magnetic field across the entire FOV, which is difficult to achieve with bilateral imaging.

Subtraction does not require extra acquisition time and is preferred for dynamic bilateral imaging. The main disadvantage of subtraction is that it may suffer from patient motion. Subtraction errors occur if the pre-contrast mask and the post-contrast images are not entirely congruent due to even subtle patient motion.

Breast MR imaging studies require injection of gadolinium chelates. The contrast agent is injected intravenously in the antecubital vein at a dose of 0.1 mmol/kg. Contrast is injected with a power injector at a rate of 3 mL/s, followed by a 20-mL saline flush. The first post-contrast acquisition is started after the entire volume of contrast has been injected.

Normal fibroglandular tissue produces background enhancement on dynamic contrast images. Background enhancement on MR imaging is analogous to fibroglandular tissue density on mammography and must be mentioned while reporting dynamic contrast-enhanced MR mammography. This enhancement is a mimicker and confounder of breast malignancies. Background enhancement is usually multifocal or diffuse and is classified as severe, moderate, and mild. Severe enhancement appears in the early phases and can be diffuse or multifocal. Mild and moderate enhancement appears in the late phases and is usually multifocal. At times, no background enhancement may be observed in which case the fibroglandular tissue appears homogeneously dark across the different phases. The amount of background enhancement does not necessarily correlate with the amount of parenchymal tissue. Mammographically dense breasts may have no or minimal background enhancement, whereas breasts with scattered fibroglandular tissue may display strong enhancement. The degree of background enhancement also varies in different phases of the menstrual cycle. Estrogen induces hyperemia, vasodilatation, and capillary leakiness which results in marked enhancement of the glandular tissue during the first and last weeks of the menstrual cycle (when estrogen levels peak). Benign parenchymal enhancement usually shows persistent or plateau curves. To avoid excessive background enhancement, breast MR imaging should be performed during the second and third weeks of the menstrual cycle when possible. Background enhancement is also increased during lactation and in patients on hormonal replacement therapy. Tamoxifen therapy decreases enhancement. Radiation treatment causes parenchymal edema resulting in prominent enhancement even after completion of therapy.

The enhancement kinetics are analyzed by visually assessing dynamic images and using ROIs to obtain time-signal intensity curves. ROIs are placed over the region which has most intense enhancement on first or second dynamic images. ROIs should include not exceeding 3–4 pixels, and care should be taken that the ROI does not extend beyond the lesion.