A scout or three-plane localizer is required on all systems to localize the breasts. This allows the technologist to select the appropriate FOV for the patient’s anatomy and scan acquisition plane (FOV considerations are discussed in more detail above).

A fluid-sensitive, typically T2-weighted, sequence is important for improved characterization of lesions and benign findings in the breast. For example, simple cysts, lymph nodes, and some fibroadenomas have high signal on the T2-weighted images. There are multiple acceptable sequence types for fluid-sensitive imaging. The most common are spin echo (SE), fast spin echo (FSE), and short tau inversion recovery (STIR) with an inversion time selected to null fat. These sequences are typically acquired as multi-slice 2D acquisitions because of the long repetition times required for T2-weighting and resulting longer acquisition times, which are more prohibitive for three dimensional (3D) imaging [10]. Thus, the T2-weighted images are typically unable to achieve spatial resolution equivalent to the T1-weighted sequences in a reasonable scan time with adequate SNR. Most protocols utilizing SE or FSE technique for T2-weighted imaging also perform fat suppression in order to readily differentiate bright fluid signal from fat. However, others choose to perform T2-weighted images without fat suppression because it can allow for acquisition of higher spatial resolution images and/or decreased scan times.

If active fat suppression is used for the DCE sequences (discussed below), it is recommended to perform an additional T1-weighted sequence without fat suppression prior to the multi-phase T1-weighted sequences. The sequence is fast, provides an overview of breast anatomy, can aid in assessing the amount of fibroglandular tissue in the breast, and is helpful in distinguishing fat from water-based tissues (such as fibroglandular tissue, breast lesions, etc.). Additionally, this sequence can aid in the identification of fat containing lesions, which is important because lesions containing fat (e.g. fat necrosis) are typically benign. This sequence should be performed with similar parameters and spatial resolution as the multi-phase T1-weighted sequences (as described below), but without active fat suppression, which allows for comparison of lesion characteristics across all the T1-weighted sequences.

The pre-and post-contrast multi-phase T1-weighted MRI images sequences are the most important images for identifying and characterizing lesions. It is imperative that identical scan parameters be used for the multi-phase T1-weighted images so that image registration can be performed, the pre-contrast images can be subtracted from the post-contrast images, and signal differences between sequences can be directly compared. Subtraction images are particularly useful for identifying signal from gadolinium contrast agents and are mandatory if active fat suppression is not utilized so that contrast-enhancement can be readily differentiated from the bright signal of fat (described further below).

The multi-phase T1-weighted images are used for lesion detection, assessment of lesion morphology, and evaluation of lesion contrast enhancement over time. Characterizing lesion morphology, such as shape, margin, and internal enhancement pattern, requires high spatial resolution images with in-plane resolution of ≤1 mm to depict fine features, such as lesion margins. Through-plane slice thickness should be ≤3 mm; however, thinner slices that approach in plane resolution size (i.e. closer to isotropic) decrease volume averaging in the through-plane direction, which can increase the contrast of small lesions compared with background tissue. Additionally, thin slices facilitate higher quality image reformats, eliminating the need to acquire additional images in different planes (discussed further below). Conversely, voxel size should not be so small that SNR suffers.

A 3D GRE pulse sequence is preferred for multi-phase T1-weighted imaging with a short TR. The GRE pulse sequence should be spoiled to avoid any confounding T2 contrast [11]. There are no consensus guidelines for the number of post-contrast acquisitions or total acquisition time for the multi-phase T1-weighted sequences, although at least two post-contrast sequences should be performed in order to allow for the most basic assessment of contrast enhancement kinetic features. Invasive cancers typically enhance early, peaking in enhancement approximately one to two minutes after contrast injection. Although breast cancers more frequently exhibit initial fast enhancement (increase in signal from pre-contrast to first post-contrast series of >100 %) and delayed washout (decrease in signal from first post-contrast to final post-contrast series of >10 %) than benign findings, there remains substantial overlap in the kinetics of malignant and non-malignant lesions of the breast [12]. The multi-phase T1-weighted MRI protocol should be constructed so that one of the early-phase post-contrast sequences will sample the high frequency data at the center of k-space (which defines image contrast) between one and two minutes. This is important to potentially capture the peak enhancement of invasive cancers, but also to differentiate lesions from benign BPE, which typically increases over time (Fig. 1.2). For the majority of Cartesian sequences with rectilinear k-space sampling, the center of the sequence captures the high frequency data. However, Cartesian sequences with elliptical centric k-space sampling and other k-space sampling trajectories, such as radial, may acquire the center of k-space near the beginning of the sequence. Knowledge of the sampling pattern is thus important to properly time the post-contrast sequences.

Silicone, like water, has a longer T2 relaxation time than fat. Thus, on a standard T2-weighted sequence without fat suppression, water will be brighter than silicone, which is brighter than fat [13]. For evaluation of silicone implant rupture, it is often ideal to have a sequence that suppresses both the water and fat, leaving silicone as the only material remaining bright on imaging. This is possible using a T2-weighted FSE pulse sequence with water suppression and an inversion pulse with an inversion time to null fat [13].

High spatial resolution and high temporal resolution are typically competing demands of MR acquisition. High spatial resolution imaging results in longer scan times, decreasing the temporal resolution. High spatial resolution is critical for accurate assessment of lesion morphologic features, but high temporal resolution is necessary for accurate depiction of lesion enhancement curves over time. Studies have shown that when forced to compromise, spatial resolution and accurate depiction of lesion morphology is more important to diagnostic accuracy than characterization of the signal enhancement curve. Thus, when creating a breast MRI protocol, spatial resolution is often prioritized over temporal resolution [14].

In general, the compromises between spatial and temporal resolution have decreased with state-of-the art MR systems and breast coils. The ACR Breast MRI Accreditation Program [9] requires that the early-phase post-contrast sequence be completed by four minutes after contrast injection; however, three minutes or less is most desirable [14, 15] and should be achievable without difficulty on modern scanners. Parallel imaging is now standard on modern MRI systems and also helps to shorten scan times [16–19]. There has been much research in recent years to develop novel accelerated MRI acquisition techniques to provide simultaneously high spatial and high temporal resolution scans. These include techniques such as novel k-space sampling schemes and reconstruction using high spatial frequency k-space data from adjacent time frames (view-sharing) [20–23]. Some of these methods are being used to obtain hybrid high spatiotemporal resolution imaging protocols [24, 25], although their exact benefit in the routine clinical setting is not known.

Subtraction of post-contrast images from the pre-contrast T1-weighted images is required to remove bright signal from fat (if active fat suppression is not used) so that contrast-enhancing lesions can more easily be seen. However, regardless of the use of active fat suppression for the DCE MR images, the use of image subtraction can be helpful since it removes non-enhancing fibroglandular tissue and other non-enhancing anatomy (besides adipose tissue), enabling easier visualization of potential suspicious areas of contrast enhancement. Subtraction images are most valuable for accentuating enhancing lesions that are evident on the early-phase (for the same reasons described above) DCE images, but can be performed for any of the post-contrast sequences by simply subtracting the pre-contrast images’ signal from the desired post-contrast images. One of the most commonly used subtraction reformats is a 3D subtraction maximum intensity projection (MIP) that is created from an individual subtraction series (most frequently the first post-contrast DCE series). The MIP is valuable clinically as an “overview” image that allows for quick assessment of symmetry, BPE, and the presence of suspicious findings.

If the acquired images have sufficient spatial resolution and thin slices, multi-planar reformats (MPRs) can be performed (Fig. 1.3). MPRs allow suspicious findings to be evaluated in multiple planes, aiding detection and characterization, as some lesion types such as non-mass enhancement can be at times easier to detect in a second plane. Also, creation of MPRs eliminates the need to acquire additional sequences in perpendicular planes, saving overall scan time.

There are inherent challenges to performing breast MRI that must be addressed to obtain consistent high quality breast MR examinations. First and foremost, a quality breast MRI program includes highly trained technologists who regularly perform breast MRI and are comfortable with appropriate patient positioning and communication. Protocol sequences and sequence timings should be as consistent as possible from patient to patient, regardless of breast size or body habitus. However, the FOV can be adjusted for body habitus for image optimization and reduction of artifact as needed. Finally, there are technical and physical challenges to obtaining high quality breast MRI, some of which are accentuated at higher magnetic field strength, which require attention and are discussed in greater detail below.

Imaging artifacts can occur in breast MRI scans from a variety of sources. Such artifacts are important to recognize as they can cause misinterpretation by obscuring and/or mimicking pathology. Several of the most common artifacts affecting clinical breast MRI are summarized here.