1 Breast MRI: Overview
Gillian M. Newstead
Abstract
The first chapter provides a general overview of the clinical applications and diagnostic utility of breast MRI and the need for specific individual technical protocols for both screening and diagnostic indications. The shortcomings of mammographic screening, particularly for those women at high risk or with dense breasts at mammography, have led to increased use of an abbreviated MRI sequence for breast screening. MRI standards, protocols and practice guidelines for screening are reviewed and discussed. MRI is a functional imaging technique and there is growing interest in establishing functional breast imaging methods using dynamic contrast-enhanced MRI (DCE-MRI) with kinetic analyses for classification of tumor biology and correlation with genomic markers. MRI is well suited for this type of analysis, identifying features that may reflect underlying malignant potential, providing an imaging biomarker that could play a key role in the development of precision medical therapy and personalized cancer care.
The diagnostic applications of breast MRI require a standard full protocol and may need additional advanced sequences according to the clinical indication for each individual patient. Indications for diagnostic MRI include assessment of disease extent at the time of initial cancer diagnosis, postoperative evaluation of residual disease following cancer excision, identification of recurrent tumor and monitoring of women undergoing cancer treatment with neoadjuvant chemotherapy. Other applications discussed in this chapter include problem-solving for difficult clinical or imaging findings, nipple discharge, implant assessment and use of MR-Guided biopsy.
Keywords: clinical applications breast MRI, MRI screening techniques, clinical utilization of breast MRI screening, practice guidelines for breast MRI, diagnostic MRI applications, known cancer MRI applications, MRI as a problem-solving tool, MR-Guided therapy, MR imaging biomarker
1.1 Introduction
Breast imaging has played an important role in the rapid development and change in breast cancer management over the past decades, not only by the introduction of new technical tools to aid in cancer detection and improve diagnosis, but also by the use of these tools to provide diagnostic information necessary to guide therapy. Over past years, the role of the radiologist has greatly changed; supervision of breast cancer screening programs using multimodal techniques and increased participation in multidisciplinary conferences have resulted in improved care for women diagnosed with breast cancer. When an abnormality is detected at screening, or when a patient is referred with clinical symptoms, the radiologist performs a diagnostic evaluation, discusses the pertinent findings with the patient and her family, and is usually the first physician to decide if a biopsy is necessary based on imaging findings. The whole gamut of interventional procedures, using stereotactic, ultrasound, or magnetic resonance imaging (MRI) guidance, provide accurate and timely diagnoses of nonpalpable image-detected lesions, with the additional advantage of limited patient morbidity when compared to surgical excision. When a malignant or uncertain finding is diagnosed, the radiologist will communicate the biopsy results with the patient and, in many instances, guide her to treating physicians for further management. New imaging techniques have benefitted patients, not only by allowing detection of an increased number of small cancers, but also by providing important diagnostic information essential for treatment, the goal being to provide optimal treatment for each individual cancer patient.
Mammography has been the tried-and-true breast imaging method for many decades, providing over 80% sensitivity for breast cancer detection in postmenopausal women and even greater sensitivity in the symptomatic population, when combined with breast ultrasound.1 It would be logical therefore to ask why another imaging modality such as breast MRI is needed. It is well recognized that the sensitivity of mammography is substantially lower in the dense breast, less than 50% even when full-field digital mammography (FFDM) is used, as shown in the Digital Mammographic Imaging Screening Trial (DMIST).2 Mammography suffers from inherent limitation of image contrast; many suspect lesions are therefore indeterminate and require further imaging evaluation and biopsy. Other important shortcomings of mammographic screening include recognized observer limitations as well as a propensity for detection of lesions with less aggressive histology, which may result in underdiagnosis of biologically aggressive disease. The failure to detect some high-grade breast cancers with mammography is driven largely by the degree of breast tissue density, but also by the very nature of rapidly growing, biologically relevant cancers that exhibit imaging features that are indistinguishable from normal breast tissue. Failure to diagnose these cancers at mammographic screening will result in progression of disease and diagnosis of advanced-stage interval cancers. Breast ultrasound is now commonly used for screening, and although this technique will identify some additional mammographically occult cancers, identification of ductal carcinoma in situ (DCIS) is challenging, and ultrasound suffers from low specificity in the screening setting.3,4 Breast cancer continues to represent a major cause of cancer death in women; therefore, continued search for improved breast cancer screening methods is needed.
Against this background of mammography and ultrasound imaging, breast MR technology has improved steadily over the last three decades in large part due to research collaboration between radiologists, technologists, physicists, and industry scientists. Continued focus on research and educational efforts aimed toward improving diagnosis and patient care have resulted in increased use of breast MRI as a valued clinical resource for women's health care. The main driver for the increasing adoption of breast MRI in clinical practice is its extremely high sensitivity, approaching 100% for cancer detection. Multiple studies have shown that standard dynamic contrast-enhanced MRI (DCE-MRI) for breast imaging achieves the highest sensitivity of any imaging modality, identifying cancer regardless of radiographic breast density, stage (DCIS or invasive), tumor type, or postsurgical changes.5,6,7,8 Criticisms of low specificity in the early years of breast MRI might be expected with such a sensitive modality, but with current techniques and protocols, the specificity and positive predictive value (PPV) for malignancy can exceed that of breast ultrasound and mammography. Recognition of the salient MRI features of benign and malignant disease, and the specific morphologic and kinetic characteristics associated with various malignant subtypes, allows radiologists to provide important diagnostic information that can guide therapy. DCE-MRI protocols at both 1.5 and 3.0 Tesla provide excellent spatial resolution and accurate analysis of lesion morphology. Dedicated software providing semiquantitative analysis of lesion kinetics and display of its internal enhancement characteristics can improve diagnostic specificity. Measures of the initial rise and delayed phase of contrast enhancement within lesions are displayed as standard signal-intensity-versus-time-intensity curves (TICs). These computer-aided, semiquantitative, kinetic analysis tools have the advantage of being relatively simple to implement into routine practice and are widely used in the United States.
1.2 Breast MRI as a Biomarker
Traditionally, breast cancer treatment was determined by two major factors: (1) tumor histology, assessed by classifications based on grade and morphology such as ductal, lobular, mucinous tubular etc., and (2) the TNM staging method, based on cancer size, nodal status, and presence or absence of distant metastases. During the last 10 years or so, molecular subtyping of breast cancer has assumed a more important role in treatment planning and cancer classification has moved beyond the basic histologic assessment of prior years, to encompass treatment stratification based on tumor biology and gene expression profiles. There is a growing interest in establishing functional breast imaging methods, using DCE-MRI and its kinetic and morphologic analyses, for classification of tumor biology and correlation with genomic markers. MRI is a functional imaging technique and is well suited for this type of analysis. If MRI is to fulfill this role, the semiquantitative MRI methods used in current practice that document change in signal intensity over time will not be sufficient, and protocols that provide quantitative measurements of concentration of contrast media in tissues will be needed. Quantitative kinetic analysis depends on intrinsic lesion characteristics, acquisition parameters, and concentration, all of which play a role in signal enhancement. This type of analysis is necessary if we want to move beyond the binary choice, "is the lesion in question benign or malignant?" to ask whether a detected tumor has features that may reflect underlying biology and play a key role in precision medicine and personalized cancer care.
There is thus a need for further development of quantitative MRI biomarkers for prediction and prognosis. The goal of an imaging biomarker is to improve risk stratification, provide the right treatment for the right patient at the right time, reduce the variability in interpretation, and avoid trial and error in treatment. Quantitative imaging methods,...
