Chap

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8Sara Chen Gavenonis, MD, andMark Rosen, MD, PhD

Breast MrI

BREAST MRI TECHNIQUE

1. How is breast magnetic resonance imaging (MRI) performed?The patient is required to lie on her stomach in the prone position. A breast-specific coil is used, with the breasts placed into openings or wells that house the receiver coils (devices that detect emitted MRI signal from the body). Prone positioning helps minimize the motion of the breast during respiration. Dependent positioning of the breast also helps to extend the breast away from the chest wall and allows for mild compression of the breast, further minimizing unwanted motion during the MRI examination.

2. What pulse sequences are required for breast MRI?The pulse sequences used to create the different MRI images in a breast MRI examination depend on several factors, including the capabilities and performance of the MRI system used and the type of information (e.g., implant evaluation or cancer identification) sought by the referring physician. In addition, the imaging techniques used by a particular MRI center reflect the experience and preferences of the local radiologists at that center. At our institution, breast MRI is performed in the sagittal plane and includes T1-weighted spin-echo, fat-saturated T2-weighted fast spin-echo (FSE), and fat-saturated three-dimensional T1-weighted spoiled gradient-recalled-echo. Dynamic postcontrast images are obtained resulting in three postcontrast series. Delayed axial T1-weighted fat-saturated images are also obtained for multiplanar imaging correlation and improved visualization of the axillary regions.

Computer-generated subtraction images using the precontrast series as a mask are also routinely used to assess for areas of enhancement. It is important to assess for artifacts from possible motion between the precontrast and postcontrast series to avoid pitfalls when interpreting subtraction images.

3. Is 3-Tesla (T) imaging superior to 1.5-T imaging for breast cancer detection?Compared with imaging at 1.5-T, imaging at 3-T improves the signal-to-noise ratio (S/N) by approximately a factor of 2. This added S/N can be translated into higher spatial resolution and faster imaging (with parallel imaging techniques) without sacrificing image quality. Preliminary results suggest that higher resolution breast MRI at 3-T is feasible. In addition, tissue T1 relaxation rates are longer at 3-T, theoretically improving the image contrast and sensitivity of breast MRI to gadolinium enhancement. The quality of magnetic resonance spectroscopy (MRS) can also be improved at 3-T through improved spectral resolution and the potential for spectroscopy of smaller lesions. No studies have yet shown, however, that 3-T imaging results in superior diagnostic accuracy than imaging at 1.5-T.

4. Can MRS complement MRI for identifying malignant breast lesions?MRS provides information on the relative concentrations of small molecular metabolites in tissue. In clinical breast MRS, the detection of elevated choline levels is associated with malignancy. Preliminary studies have shown that MRS can independently predict malignancy in solid breast lesions. Currently, MRS can be used only in lesions approximately 1 cm or larger in diameter, however, limiting the potential usefulness in routine clinical practice. Nevertheless, several investigators have shown the feasibility of incorporating MRS into breast MRI protocols as an adjunctive method for identifying malignant lesions.

NORMAL ANATOMY ON BREAST MRI

5. What are the components of the female breast?The mature female breast is divided into the following components: glandular elements (ducts and lobules), fibrous supporting structure (Cooper ligaments), and surrounding adipose tissue. Blood vessels and, occasionally, lymph nodes are also found within the breast. Anatomically, the breast is bounded anteriorly, inferiorly, and medially by skin. Posteriorly, it is bounded by the chest wall (pectoralis major and minor muscles, ribs, and intercostal muscles). Superolaterally, the glandular breast tissue variably extends into the axilla, where adipose tissue, major blood vessels, and lymph nodes reside.

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Figure 8-1. A, Sagittal T1-weighted image of normal breast. The fat within the breast is bright, and the fibroglandular elements are intermediate to dark. P = pectoralis major, p = pectoralis minor, L = lung. B, Sagittal FSE T2-weighted image with fat saturation, at same location as A. The suppressed fat is now dark on this image, and the fibroglandular elements are variably intermediate to bright. Higher signal intensity is seen in small cysts (arrows) and in peripheral veins (arrowhead ).

The breast is divided into 8 to 20 segments, each of which is supplied by a major lactiferous duct. The ducts extend from the nipple, each communicating with a separate nipple orifice. The lactiferous duct subdivides into numerous ductules, each of which ramifies into several lobules. A terminal ductule and its associated lobules are classified together as a terminal ductule-lobular unit (TDLU). Most breast cancers develop within the TDLU.

6. What is the normal appearance of the breast on MRI?On MRI, one can distinguish between the fatty adipose tissue and the fibroglandular elements, or parenchyma, on the basis of signal intensity. On T1-weighted imaging, the fat is bright, whereas the parenchyma is intermediate to dark (Fig. 8-1A). The appearance of the breast on T2-weighted imaging depends on whether fat suppression is used. Most radiologists use fat suppression on T2-weighted imaging. On fat-suppressed T2-weighted imaging, breast tissue is intermediate to bright (Fig. 8-1B). This fat suppression eliminates or reduces the high signal intensity of fat to highlight the bright signal of fluid in ducts or cysts or the bright signal intensity of certain masses, such as lymph nodes or fibroadenomas.

INDICATIONS FOR BREAST MRI

7. What are the indications for diagnostic breast MRI?Two general categories of use of a breast imaging modality are screening and diagnostic. The use of breast MRI as a diagnostic study has been well established. In patients with a known breast cancer, breast MRI is useful in defining the extent of disease in the ipsilateral breast for surgical planning purposes and useful in screening for a synchronous cancer in the contralateral breast. Breast MRI can also be used for monitoring local tumor response to neoadjuvant chemotherapy. Breast MRI can be used to look for an index breast lesion when there is metastatic axillary lymphadenopathy with no suspicious mammographic findings in the ipsilateral breast. In addition, breast MRI can sometimes be helpful in the work-up of indeterminate mammographic and ultrasound (US) findings, or in cases of an area of clinical concern with no suspicious mammographic or US findings. Breast MRI can also be performed for evaluation of the integrity of breast prostheses (implants) (see subsequent section on implant evaluation).

8. What are the indications for screening breast MRI?In March 2007, the American Cancer Society published guidelines for the use of breast MRI as a screening modality. Based on evidence, annual screening breast MRI is now recommended for patients with a deleterious BRCA mutation, for untested first-degree relatives of BRCA carriers, and for patients with a lifetime risk approximately 20% to 25% or greater, as defined by BRCAPRO or other models that are largely dependent on family history. Also, annual screening breast MRI is recommended for specific subgroups, such as patients who received radiation to the chest between age 10 and 30 years and patients (and their first-degree relatives) with syndromes known to predispose to malignancy (Li-Fraumeni, Cowden, and Bannayan-Riley-Ruvalcaba syndromes).

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Beyond these specific groups of patients, annual screening breast MRI is not currently recommended, especially given the relatively low specificity of breast MRI and the propensity for false-positive findings. Investigation continues into the use of breast MRI as a screening modality in patients with a prior personal history of breast cancer, patients with heterogeneously dense or extremely dense breasts on mammography, and in patients with a prior history of a breast biopsy with pathology results of atypia or lobular carcinoma in situ.

9. What is the appearance of (invasive) breast cancer on MRI? Do all breast cancers appear as focal enhancing masses?Breast cancer can appear as an enhancing mass or an enhancing region of the breast after contrast agent administration (Fig. 8-2). Certain malignancies, such as invasive lobular cancer or ductal carcinoma in situ (DCIS), may manifest as focal or segmental areas or regions of enhancement (also termed nonmass enhancement) (Fig. 8-3).

10. Do all enhancing abnormalities in the breast represent cancer?No. Although MRI is very sensitive for detecting breast cancer, it is less specific because many other breast entities can be enhancing. Many benign masses, such as fibroadenomas and intramammary lymph nodes, also enhance after gadolinium administration. Proliferative breast changes, such as fibrocystic disease or ductal hyperplasia, may also enhance after gadolinium administration. Even normal breast parenchyma may enhance after contrast administration, especially in premenopausal women or women receiving hormone replacement therapy.

11. Does an irregular or spiculated margin in an enhancing lesion always represent breast cancer?Generally, an enhancing mass with irregular or spiculated margins is more suspicious for malignancy than a mass with smooth or lobulated margins. Some enhancing masses with irregular margins are ultimately benign (e.g., radial scars), however, and some cancers may manifest with smooth margins. Internal morphology is also an important indicator of malignancy on MRI. Masses that are centrally necrotic (often seen in larger cancers) may show heterogeneous internal enhancement or irregular peripheral rim enhancement (Fig. 8-4), or both.

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Figure 8-2. A, Breast cancer on T1-weighted spoiled gradient-recalled-echo image (TR = 7.7 ms, TE = 1.8 ms). The cancer (arrow) is isointense to the background glandular tissue. B, After gadolinium administration, the cancer is seen as an avidly enhancing mass. The contrast-enhanced images also help depict the irregular and spiculated borders of the mass.

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Figure 8-4. Invasive ductal carcinoma on MRI (T1-weighted spoiled gradient-recalled-echo image, first postcontrast series). Note the index mass has smooth margins compared with Figure 8-2. There is also heterogeneous enhancement of the lesion, with irregular peripheral enhancement.

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Figure 8-3. A and B, DCIS on MRI (sagittal subtraction images, first postcontrast series). In the lower inner left breast, avid segmental enhancement extends from the nipple posteriorly toward the chest wall.

2. Do malignant lesions enhance more strongly and more rapidly than benign lesions?Although breast cancers do tend to enhance more rapidly and more intensely than benign lesions, some cancers may enhance less prominently; invasive lobular cancer can be a subtle area of enhancement on MRI (Fig. 8-5). In such cases, identification of the cancer may require close comparison between the unenhanced and enhanced T1-weighted images or the use of computer-generated subtraction images. Occasionally, noninvasive cancers (e.g., DCIS) enhance minimally or not at all.

Kinetic patterns of enhancement are crucial when evaluating suspicious enhancing lesions. Lesions that enhance minimally during the “first-pass” of a contrast agent (generally within 1 to 2 minutes after the intravenous bolus of gadolinium) but then steadily increase in intensity over time are typically benign, whereas lesions that enhance markedly in the first pass, but then decrease in their intensity are usually malignant. Exceptions exist in both cases, however, and a combination of morphologic and vascular enhancement information should be analyzed in the interpretation of breast MRI.

13. How are breast lesion kinetic patterns categorized?The rate of initial enhancement (between the precontrast and first postcontrast series) can be described as rapid, intermediate, or slow. Generally, more vascular lesions have a more rapid rate of enhancement during this initial time period. Malignant breast lesions tend to be more vascular and often enhance markedly during the first pass of contrast agent. After the initial phase of enhancement, lesions continue to enhance gradually (type I curve), stabilize (plateau) their intensity (type II curve), or decrease (washout) their intensity (type III curve). Type I enhancement is termed persistent; type II enhancement, plateau; and type III, washout. Type III enhancement generally is the most suspicious pattern for malignancy.

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Figure 8-5. A and B, Subtly enhancing cancer (invasive lobular carcinoma) shown on contrast-enhanced T1-weighted spoiled gradient-recalled-echo imaging before (A) and after (B) contrast agent administration. The cancer is shown as an irregular region of glandular-type intensity, with enhancement mildly greater in degree than that of the surrounding glandular tissue. C, Enhancement (arrows) is much more apparent in the computer-generated subtraction (postcontrast minus precontrast) image.

14. Do all breast cancers show early rapid enhancement, followed by washout (type III enhancement)?Although this pattern is not always seen in breast cancer, many invasive cancers show at least some areas of this pattern of enhancement (termed washout pattern). Malignant breast lesions often have higher degrees of angiogenesis, which has been shown to be essential for tumor growth and metastasis. New vessels in this microenvironment are abnormally leaky to gadolinium. As a result, malignant lesions tend to enhance markedly after gadolinium contrast administration, and washout more rapidly as the arterial contrast concentration declines.

15. How can biopsy specimens of suspicious lesions that are identified only on breast MRI be obtained?When a suspicious breast lesion is identified by MRI, it is imperative to correlate the findings with findings on mammography and US. Often, an MRI abnormality has a corresponding mammographic or US finding. If there are no suspicious intermodality correlates for a suspicious breast MRI finding, an MRI-guided tissue sampling procedure is required. MRI-guided needle localization and core biopsy can be performed with specialized MRI-compatible equipment. If MRI-guided core biopsy is performed, MRI-compatible tissue markers are often also placed to mark the site of biopsy.

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Key Points: MRI Breast Cancer Detection

1. Intravenous contrast agent (gadolinium) is required to evaluate the breast parenchyma for malignancy on MRI.

2. MRI is nearly 100% sensitive for invasive breast cancer, but is less sensitive for DCIS. MRI can detect foci of cancer that are occult on mammography and physical examination.

3. Cancer detection requires identification of foci or regions of abnormal enhancement.4. Differentiation between benign and malignant on breast MRI requires evaluation of lesion morphology

(e.g., borders) and enhancement kinetics.5. The use of breast MRI for diagnostic purposes has been established. It is useful in defining extent of disease

in a patient with known cancer, for monitoring local tumor response to neoadjuvant chemotherapy, and for implant evaluation.

6. In 2007, the American Cancer Society published guidelines for use of screening MRI, with annual screening breast MRI recommended in specific patient groups at high risk for breast cancer. Investigation continues into the use of breast MRI as a screening modality in many other patient groups.

IMPLANT EVALUATION BY MRI

16. What are the components of a breast prosthesis?There are two broad categories of breast implants: saline-only implants and silicone-containing implants. All implants (including saline-only implants) have a thin elastomer shell, generally derived from a silicone polymer. Saline implants often contain an access port that allows for injection of additional saline into the lumen because these implants are also used as tissue expanders. Silicone implants may be single-lumen or dual-lumen, usually with a silicone-containing inner lumen surrounded by a saline-containing outer lumen.

17. How does one distinguish between silicone and saline implants on MRI?Because silicone and saline are hypointense on T1-weighted imaging and hyperintense on T2-weighted imaging, differentiation between the two requires more sophisticated MRI. More heavily T2-weighted sequences (with echo times on the order of 200 ms) accentuate slight differences in the T2 relaxation times of these substances (Fig. 8-6A). Frequency-selective water saturation can effectively eliminate the signal intensity of saline, while preserving the bright silicone signal on T2-weighted images (Fig. 8-6B). More advanced imaging methods can be used to create images with signal contribution from only one moiety or the other.

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Figure 8-6. A, Normal appearance of a silicone implant on MRI. Sagittal heavily T2-weighted FSE (TR = 5016 ms, TE = 202 ms) image shows an intact single-lumen silicone implant. Dark lines extending into the center of the implant are normal radial folds of the outer elastomer shell. The silicone is intermediately high in signal intensity, and fluid condensation within the radial folds (arrow) is brighter than silicone. B, Silicone-only imaging on MRI. Sagittal short-tau inversion recovery image (TR = 4600 ms, TE = 105 ms) with frequency-selective water suppression yields an image in which only silicone contributes bright signal intensity. This sequence is useful for showing extracapsular ruptures with extravasation of free silicone into the breast or chest wall.

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18. What is the normal MRI appearance of a breast implant?A normal breast implant is oval or ellipsoid in shape. An outer fibrous capsule forms as part of the normal tissue response to the implant. On T2-weighted MRI, the lumen of the implant should be uniformly bright with no internal debris. Infolding of the outer implant shell is common and manifests as radially oriented protrusions (“radial folds”) into the lumen of the implant. This is a common appearance of the normal implant and is not a sign of implant rupture. Condensation of fluid outside the implant shell but within the fibrous capsule is also a common phenomenon and should not be viewed as a sign of rupture.

19. What are the different types of implant rupture?The different types of implant rupture include gel bleed, contained (intracapsular) rupture, and extracapsular rupture. These categories refer to the degree of disruption of the elastomer shell of the implant and the external fibrous capsule that develops around it.

20. What is gel bleed?Gel bleed refers to transudation of small amounts of silicone gel through tiny perforations, or microtears, of the outer elastomer shell. Gel bleed is diagnosed by visualization of a small amount of silicone gel external to an otherwise intact elastomer shell, but contained within the external fibrous capsule. The term gel bleed is reserved for MRI evaluation of silicone implants. It is impossible on MRI to distinguish between saline extrusion through a microtear of the elastomer shell and normal fluid condensation between the elastomer shell and the fibrous capsule.

Figure 8-7. Axial T2-weighted FSE image of a silicone implant with contained (intracapsular) rupture. The silicone is contained by the outer fibrous capsule that develops within the breast after implant placement. The fragmented strands of the outer elastomer shell of the implant can be seen floating freely within the contained silicone, resembling strands of linguine (arrows).

21. Describe the appearance of a contained implant rupture.A contained rupture can occur in either saline or silicone implants. In a contained rupture, there is complete or near-complete separation of the elastomer shell from the fibrous capsule and fragmentation of the elastomer shell. The contents of the implant lumen (saline or silicone) remain contained, however, within the external fibrous capsule. The hallmark of contained rupture on MRI is the “linguine sign” (Fig. 8-7), in which strands of the remnant elastomer shell float freely in the contents of the implant lumen. These are identified as dark linear strands on T2-weighted images.

22. What are the MRI findings of extracapsular rupture?Extracapsular rupture refers to rupture of the external fibrous capsule. For either silicone or saline implants, an extracapsular rupture can be diagnosed when there is gross deformity or collapse of the fibrous capsule. For silicone-containing implants, an extracapsular rupture is diagnosed whenever free silicone is identified within the extracapsular soft tissues of the breast or within axillary lymph nodes. When one seeks to identify the presence of free silicone within

breast tissue, one must separate not only silicone and saline signals, but also signals of breast parenchyma and fat. Silicone-only imaging sequences are essential for detecting free silicone in the breast or axilla.

Key Points: Implant Evaluation

1. Breast implants can be saline-only, silicone-only, or a combination of saline and silicone (dual-lumen).2. MRI is the most accurate imaging modality for evaluating implant integrity.3. Implant rupture can be intracapsular or extracapsular.4. Depiction of free silicone in the breast tissue is the hallmark of extracapsular rupture.

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