Thorough and careful review of the breast MRI is essential prior to performing an MRI-directed ultrasound. If a technologist performs the ultrasound study, it is also important to review the MRI study with the technologist. When reviewing the breast MRI study, utilization of 3D reconstructions can help make it easier to understand the location of the lesion in all 3 planes (see Fig. 11.2) and its relationship to surrounding structures [24]. The location and morphology of the MRI-detected lesion are important information to know to determine the expected location and appearance of the lesion at ultrasound.

Lesion location information to note includes the quadrant and o’clock position, distance from nipple, skin, and chest wall, anatomic relationship to surrounding tissue, and its relationship to other landmarks. It is important to keep in mind that the positioning of the breast is different between MRI and ultrasound [19, 24]. Breast US is performed with the patient in the supine or supine oblique position with the arm raised while breast MRI is performed with the patient in the prone position. In the supine position with the arm raised, the breast tissue is flattened and widened which makes the breast tissue, including breast lesions, appear more compact. The distance between the chest wall and the glandular tissue is decreased on US relative to MRI (see Fig. 11.3). With MRI, the breast in the prone position is pendant with little to no compression; this results in the tissue appearing more stretched in the anterior to posterior dimension (see Fig. 11.3). The distance between the chest wall and the glandular tissue is increased and lesions can appear more anterior on MRI than on US images [19, 24]. Carbonaro et al. showed lesion displacement of about 3–6 cm along the three orthogonal directions on prone versus supine MRI [4]. The o’clock position of the lesion in ultrasound can also vary by one or two hours compared to the MRI [17]. Since lesion displacement can vary between ultrasound and MRI, the anatomic relationship of the lesion to surround tissue (subcutaneous fat, glandular tissue, or retroglandular fat) (see Fig. 11.3) can be used to help in identifying a correlate with more confidence [19]. The relationship of the lesion to surrounding tissue is maintained between the two modalities.

More reliable location information to note is the distance to the skin and nipple (see Fig. 11.4) as suggested by Carbonaro et al. [4]. The median lesion-to-skin and lesion-to-nipple displacements were less than 1 cm and that the lesion-to-nipple distance may be the most reliable measure to be used for MRI-directed US [4]. In addition to using the skin and nipple as fixed markers, the relationship of the lesion to co-existing lesions such as cysts, scars, implants, clips, known cancer (see Fig. 11.5), or known fibroadenomas may be helpful. Knowledge of co-existing lesions is also important to prevent erroneous correlation, particularly in patients with multiple lesions within a similar region of the breast [19].

MRI lesion morphology with respect to shape, size and contours can also be useful in finding a lesion on MRI-directed ultrasound (see Fig. 11.6). Perfect morphologic agreement of lesions between the two modalities must not necessarily be expected [10]. Lesions at US tend to look smaller than at MRI as they are compressed in a vertical direction by the ultrasound probe. In addition, round lesions at MRI often appear oval or elliptical at ultrasound [19].

If no sonographic correlate is identified or confident correlation is difficult, MRI-guided biopsy must be performed on all lesions classified as BI-RADS category 4 or 5 at MRI [5, 10, 13, 19, 25].