Revisiting the Role of MRI Breast in Detection, Treatment Planning and Follow-up of Breast Pathologies

Introduction

Breast cancer has been ranked as the number one cancer among Indian females, with an age-adjusted rate as high as 25.8 per 100,000 women and a mortality 12.7 per 100,000 women in the years 2017-2018. For several years imaging has been playing a key role in screening as well as diagnosis and post treatment follow up of breast malignancies. Mammography is considered the single most effective screening tool. In conjunction with mammography, ultrasound is almost essential when working up a suspicious lesion.

Magnetic resonance imaging (MRI) is a relatively newer addition to our breast imaging armamentarium. MRI has several unique advantages over mammography and ultrasound. It is being performed more frequently to improve primary and recurrent tumour detection and to assess the patient's response to therapy. Sensitivity approaches 90%, while the specificity ranges from 37% to 100%.¹

MRI is particularly of importance when screening younger women (under 40 years), women with dense breasts, and women at high risk. It has several advantages, the first being lack of ionising radiation. Others include higher spatial resolution, the ability to detect occult, multifocal/multicentric disease, a better ability to assess the true size of a cancer and the ability to image the chest wall.²

Indications for breast MRI^3,4:

1. As per the American College of Radiology (ACR) appropriateness criteria, MRI is indicated as a screening modality for high-risk women. This includes women with a breast cancer gene (BRCA) gene mutation and their untested first-degree relatives, women with a history of chest irradiation between the ages of 10 and 30 years, women with a 20% or greater lifetime risk of breast cancer.
2. Staging evaluation to define the extent of cancer or presence of multifocal or multicentric cancer in the ipsilateral breast.
3. Screening for contralateral breast cancer at the time of initial diagnosis.
4. Evaluation before and after neoadjuvant therapy to define extent of disease, response to treatment, and potential for Case Studies The Images Students’ Corner the most commonly used post-processing tool. Placing a region of interest (ROI) on the most intensely enhancing lesion creates breast conserving therapy.
5. Detection of additional disease in women with mammographically dense breasts.
6. Identification of primary cancer in women with axillary nodal adenocarcinoma or Paget's disease of the nipple, where the breast primary has not been identified with other techniques.
7. Positive surgical margins post breast conservation surgery.
8. Assessment of silicone breast implant integrity.
9. Evaluation of equivocal mammographic findings.
10. Differentiation between postoperative recurrence and fat necrosis.
11. Differentiation between postoperative recurrence and fat necrosis.
12. MRI-guided biopsies and interventions.

Breast MRI protocol

Ideal time for performing breast MRI: The 2nd week of the menstrual cycle is ideal for performing a breast MRI. As per a recent literature review, there is no major difference in the MRI appearance of breast parenchyma over the menstrual cycle phases.⁵

Dedicated breast coils should be used to perform breast MRI. The patient is placed in a prone position. Multiplanar pre- and post-contrast MRI scans ideally include axial T2, Short Tau Inversion Recovery (STIR) (preferably in sagittal plane), Diffusion Weighted Imaging (DWI) (with b value of 800 mm2/sec) and dynamic gradient echo post-contrast 3D T1 sequences. A native T1-weighted acquisition should be obtained prior to contrast material administration. A single dose of 0.1 mmol/kg body weight is recommended. There is no strong data available to support an advantage for diagnostic accuracy from double dosing. T1-weighted imaging may be performed with or without fat suppression. Images are usually acquired in the axial plane, which is faster than sagittal acquisition and provides a better overview of both breasts. After contrast material administration, the T1-weighted acquisition is repeated to depict enhancing abnormalities. It is essential to obtain an post-contrast image approximately 60–90 seconds after contrast administration, as most malignancies will show peak enhancement at that time. For images obtained without fat suppression, creating subtraction images is essential. These are also important for acquisitions with fat suppression as they help differentiate truly enhancing lesions from lesions with native high T1 signal intensity.⁶

Post Processing: Maximum intensity projection (MIP) provides an excellent overview of image data and can direct the radiologist to suspicious lesions in the breast. However, motion artefacts, chemical shift artefacts, and poor fat suppression may obscure small lesions on MIP images. Multiplanar reformation (MPR) helps pinpoint the location of enhancing lesions in 3D. It allows the clinician to examine lesions’ internal structure and margins from different perspectives. Creating signal-intensity time curves is

the most commonly used post-processing tool. Placing a region of interest (ROI) on the most intensely enhancing lesion creates a time-intensity curve.

Enhancement Pattern: Time-intensity curve analysis

Dynamic analysis investigates the permeability of the vessels that supply a lesion. This is done by obtaining a series of T1 weighted acquisitions between 5 and 7 minutes after contrast material administration. In the case of leaky vessels, the peak contrast material accumulation will have passed, and the contrast material is being removed from the lesion. In lesions with less permeable vessels, the contrast gradient over the vessel wall will still be positive, and therefore the enhancement of the lesion continues to increase. This is reflected in the shape of the time-signal intensity curves; a persistent increase is most commonly seen in benign lesions, whereas a decrease in the late phase is common in malignant lesions. To improve lesion classification, the most suspicious curve observed (washout > plateau > persistent) within a small ROI (typically 3×3×1 voxels) in the lesion is used. Approximately 85% of cancers manifest with a washout curve. Persistent curves are rare in malignancies, although they may be present in ductal carcinoma in situ (DCIS) and more diffuse-growing invasive cancers, particularly lobular breast cancers.⁶

Three patterns of enhancement kinetic curves/Kuhl enhancement curves may be seen. (Figure 1)

• Type I curve: Usually considered benign
• Type II curve: Considered concerning for malignancy
• Type III curve: Considered strongly suggestive of malignancy

Figure 1: source graphic has been self-created. Conceptualised from Ref: American College of Radiology https://www.acr.org/-/ media/ACR/Files/RADS/BI-RADS/BIRADS-Reference-Card.pdf

Reporting template and MRI lexicon: Standardized reporting templates as well as lexicon and final assessment categories must be used in reporting breast MRI. The report should include clinical details with the indication for the study, relevant family history and comparison with previous imaging, if any. The report should end with a final Breast Imaging Reporting and Data System (BIRADS) assessment category. The ACR BIRADS atlas fifth edition may be used for reference. The key concepts from the same are shown in Figures 2 and 3.

Figure 2: Ref American College of Radiology https://www.acr.org/-/media/ACR/Files/RADS/BI-RADS/BIRADS-Reference-Card.pdf

Figure 3: Ref American College of Radiology https://www.acr.org/-/media/ACR/Files/RADS/BI-RADS/BIRADS-Reference-Card.pdf

Figure 4: Ref Source: Nanavati Max Superspeciality Hospital

Anatomy on MRI breast

Approach to MRI may vary and will depend on the clinical context. The following (Figure 5) is one suggested approach:

Figure 5: Approach to MRI breast interpretation

Figure 6: Contrast-enhanced axial bilateral breast maximum-intensity-projection (MIP) MRI shows patterns of background parenchymal enhancement. A. Minimal B. Mild C. Moderate D. Marked background parenchymal enhancement

Non-mass enhancement (NME): Is defined as an enhancing abnormality that is not associated with the three-dimensional volume of a mass, its shape, or outlining, and is separate from the Background Parenchymal Enhancement (BPE). This may be seen with pseudo-angiomatous stromal hyperplasia (PASH), apocrine metaplasia, radiation effects, atypical ductal hyperplasia (ADH), flat epithelial atypia, intraductal papilloma, radial scar or complex sclerosing lesion, with ductal carcinoma in-situ (DCIS), invasive ductal carcinoma (IDC), and invasive lobular carcinoma (ILC).⁷

DCIS and breast MRI: In the clinical setting of DCIS, MRI is useful to rule out underlying invasive carcinoma. This is accomplished due to MRI’s high negative predictive value (NPV) in diagnosing invasive breast cancer. MRI and mammography complement each other in the diagnosis of DCIS. Although MRI has greater overall sensitivity, it does not detect all DCIS cases, especially those that are non-enhancing and are only visible as calcifications on mammography. Mammography is similarly limited in the detection of DCIS, especially in the absence of calcifications, where lesions are only detected as abnormal enhancement on MRI. Research has shown that the overall sensitivity of MRI for high-grade DCIS is higher than that of mammography.3 There are two main types of in situ breast cancer: DCIS and lobular carcinoma in situ (LCIS), also known as lobular neoplasia. Other in situ breast cancers have characteristics of both ductal and lobular carcinomas or have unknown origins.8,9 Figures 7 to 13 illustrate few examples to demonstrate these concepts,

Limitations and pitfalls of breast MRI:

1. MRI may not detect all DCIS cases, especially those which are non-enhancing and only visible as calcifications on mammography.
2. Relatively low specificity and positive predictive value.
3. Long scan times, necessity of contrast agent administration and necessity for dedicated breast coils may limit the utilisation of breast MRI.

Conclusion and future directions

Breast MRI is a potential tool for detecting occult breast malignancy and determining the multi-centricity of disease. Despite some limitations and a degree of lack of standardization of technique, the future holds promise with upcoming abbreviated and ultra-fast breast MRI protocols.10,11 Increased availability of MRI-guided interventions, in particular, will improve the detection rate of malignant lesions, especially those presenting with patterns of NME.