MR breast imaging guides interventional procedures

Rapid enhancement and early washout are generally observed in breast carcinomas, but there are exceptions to this pattern. Certain carcinomas, particularly lobular cancer and carcinoma in situ, may enhance slowly or not at all. On the other hand, some enhancement can be seen in benign lesions such as fibroadenomas. Because of its high sensitivity, MRI reveals small lesions that may not be visible on mammography and/or ultrasound. MRI of the breast thus produces a diagnostic dilemma, as these lesions are often nonpalpable, precluding accurate and safe excision.3 If diagnostic MRI of the breast identifies a suspicious lesion, every effort should be made to reidentify the lesion on conventional modalities such as mammography or ultrasound. If the lesion can again be clearly identified, localization or biopsy should be performed using a conventional modality. MR-guided localization or biopsy should be limited to BI-RADS IV and V lesions that are visible only on MRI. Short-term follow-up is recommended for BI-RADS III lesions.4,5 Most MR-guided interventions are performed in closed magnets. As a result, only the identification of the lesion and verification of correct needle position are performed under MR guidance. All other steps of the procedure (wire placement, biopsy, and treatment) are performed outside the magnet. Some groups have reported on MR-guided interventions in open magnets,6 which have low magnetic field strengths (0.2T to 0.5T) and provide direct vertical or horizontal access to the patient. Open systems appear to have advantages over closed systems in that they provide direct access to the breast during the entire intervention and allow real-time monitoring of the needle insertion and placement. In addition, they allow interventions in the direction of the magnetic field, which minimizes susceptibility artifacts. But their low field strength makes open magnets unsuitable for diagnostic imaging, and they are not as prevalent as closed magnets. MR-guided preoperative localization or biopsy procedures are usually performed with stereotactic devices that immobilize the breast and allow a more precise needle placement, resulting in high accuracy.7-14 Various systems have been described in the literature with the patient in the supine, prone, or prone decubitus position. Most stereotactic devices allow simultaneous localization of two or more lesions within one or both breasts (Figure 1). Interventional procedures using a freehand technique have almost completely lost their importance.15 LOCALIZATION AND BIOPSY MR can be used for both biopsy and localization. Preoperative localization. Fully MR-compatible materials should be used for MR-guided preoperative localization (needle-localized open breast biopsy). Several manufacturers offer MR-compatible needles, hookwires, and coaxial needles composed of titanium or nickel-chromium alloys. Artifacts caused by these materials usually pose no problems. All preoperative localization and biopsy procedures are performed under local anesthesia in sterile conditions. The needle is placed under MR guidance, the correct needle position is verified, and the hookwire is then released (Figure 2). Because lesions referred for MR-guided localization are usually small, their accurate localization is critically important. A wire deviation of 10 mm is regarded as acceptable, however.4 An alternative two-step procedure has been described in which clip placement via a coaxial needle is done under MR guidance, and subsequent localization of the clip with a hookwire is accomplished under mammographic guidance.4 Percutaneous biopsy. Reports on fine-needle aspiration biopsy (FNAB) under MR guidance are