Breast Lesions Evaluation
Breast Lesions Evaluation
Abstract & Commentary
Synopsis T2*-weighted first pass MR imaging, a sequence applied while the patient is still undergoing MR imaging for tumor evaluation of the breast and following more standard T1-weighted perfusion studies, can help differentiate benign from malignant lesions.
Source: Kvistad KA, et al. Breast Lesions: Evaluation with dynamic contrast-enhanced T1-weighted MR imaging and with T2*-weighted first-pass perfusion MR imaging. Radiology 2000;216:545-553.
Mri of the breast is directed, using very different protocol sequences, to evaluating silicone—especially important in determining implant integrity—and in detection and diagnosis of breast cancer. The former study requires no intravenous contrast and the latter is predicated upon contrast-enhanced properties of breast lesions.
Results have invariably reported improved sensitivity for breast cancer using contrast-enhanced MRI when compared to mammography, but the efficacy of this imaging modality has been limited by low specificity. Most investigators have used criteria for diagnosing malignancy based on lesion morphology and contrast enhancement characteristics. The latter is performed with an emphasis on either spatial or temporal resolution, with most practices scanning the entire breast. Certain studies have demonstrated increased accuracy in performing temporal images on one detected lesion (1 plane of section) only, instead of the whole breast.
Lesion morphology is evaluated using parameters similar to mammography; namely, circumscribed masses are more likely benign, and ones with irregular margins more often malignant. Incorporated into the analysis are contrast enhancing characteristics using T1-weighted perfusion characteristics. Classically, malignant tumors demonstrate rapid contrast enhancement—sometimes showing rim enhancement first—and rapid "washout." Conversely, benign lesions demonstrate slower, gradual enhancement and demonstrate either a plateau of the contrast intensity signal over the period of the examination, or slow washout.
The low specificity associated with MRI of breast lesions is secondary in large part to such classic descriptions. Many benign lesions enhance quickly, and many malignancies enhance more slowly. This may be related to tumor vessel density and permeability. In addition, washout characteristics vary. The most common differential dilemma occurs for rapidly enhancing focal areas of the breast.
This study included 131 patients with breast lesions, many of which were large and palpable, in an attempt to study a novel method for helping to differentiate benign from malignant breast lesions, given the problems outlined above. Commonly used spoiled GRASS techniques were used for initial T1-weighted perfusion imaging, followed by the application of T2*-weighted perfusion imaging. The latter protocol analyzed signal loss with a threshold established as 20% from baseline, for discriminating benign (< 20%) from malignant (> 20%) lesions. Fifty-seven of 72 carcinomas, but only four of 58 benign lesions, had a signal intensity loss of 20% or more during T2* first-pass imaging, resulting in a sensitivity of 79% and specificity of 93%. Indeed, the four benign lesions that violated this threshold determination included three patients who were premenopausal or receiving exogenous hormonal therapy. There was no attempt to image patients at specific times in the menstrual cycle.
Combined with a sensitivity and specificity of 80% and 67%, respectively, for T1-weighted perfusion imaging where signal intensity was greater than 90% over baseline on the first obtained image (early enhancement), the use of both T1-weighted and T2*-weighted first-pass imaging affords the radiologist the opportunity to maintain high sensitivity while significantly increased specificity of diagnosis, for MRI defined abnormalities.
Increased vascularity and capillary diameter increase the fractional volume of the intravascular space in carcinomas compared with other tissues, likely accounting for the high susceptibility effects observed at T2*-weighted imaging. The observations regarding the signal loss at T2*-weighted imaging for the first pass were made despite the estimated 50% of injected contrast agent entering the extracellular space after the first pass through the capillary bed.
Comment by r. James Brenner, MD, JD
It has almost become cliché to acknowledge that MRI of the breast for tumor imaging is more sensitive than mammography, but the lack of specificity limits its application. Although some researchers have claimed high specificity, such results need further validation. Part of the reason for this dilemma relates to both the nature of and imaging limitations when using intravenous contrast agents such as gadolinium compounds.
To optimize the contrast-enhanced image, most researchers have used either a fat-suppression technique or the use of computer-assisted subtraction of pre- and post-contrast enhanced images. The former technique does not usually lend itself to the acquisition of early images. Even the latter technique—where images are acquired immediately following injection—may be "too delayed" if certain models are correct. Such considerations have led researchers to evaluate very fast image acquisition techniques such as echo-planar imaging. Current applications, however, produce images that limit the second component of image analysis, namely, morphology.
Three years ago, a single study reported on a novel approach to differentiating benign from malignant lesions detected on T1-weighted perfusion imaging where rapid early enhancement was observed by using the technique described in this report. Two patterns of signal loss were described at that time, with a second report (both cited in this paper) corroborating the conclusions that signal loss during T2*-weighted first-pass perfusion can significantly assist in differentiation of benign and malignant lesions.
In many cases, the identification of a lesion by T1-weighted perfusion may prompt sonographic corroboration and image-guided tissue sampling to resolve the issue. But many of these lesions may not be imaged by conventional means—especially if MR is applied with wider latitude than currently used. The ability to differentiate cancer from benign conditions for MRI-detected lesions that cannot be evaluated by more conventional techniques (e.g., mammography, ultrasound) would alleviate the need for complicated MR-guided excision or tissue sampling. The latter effort has prompted multiple attempts to provide guidance systems, all of which need to be evaluated in terms of resource allocation and expected yield, notwithstanding accuracy.
If MRI is to be considered as a screening tool for high-risk populations (a term which is subject to innumerable definitions), then MR-guided tissue sampling will be necessary. However, the volume of lesions detected by MR that require biopsy needs to be dramatically reduced to allow this approach to be feasible. Differentiating techniques such as described here may assist in such a solution, and need to be further studied.
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