Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Overview of Breast DWI: Diagnosis of Suspicious Lesions Using DWI in Combination With Standard MRI
The Need for Reducing Unnecessary Biopsies in Breast MRI
Dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) of the breast—referred to as standard breast MRI throughout this chapter—is considered the most sensitive test for diagnosis of malignant breast lesions. Standard breast MRI relies on highlighting tumor vasculature by means of intravenously applied gadolinium (Gd)-based contrast media. Because (biologically active) cancers require nutrients, a complex cascade of cytokines induces the growth of new vessels, a process known as neoangiogenesis. This ensures that practically all breast cancers show contrast uptake on DCE MRI images. A lack of enhancement in a technically adequate breast MRI examination excludes biologically significant breast cancer with high certainty. Nonenhancing breast cancers are uncommon, and the negative predictive value of a negative MRI scan for invasive breast cancer has been reported as 99% in two meta-analyses. The high sensitivity is the reason why—besides classical indications, such as high-risk screening and staging and therapy monitoring of breast cancer—breast MRI is gaining increasing acceptance for applications of problem-solving and intermediate risk screening. On the other hand, enhancement itself is not specific: benign ductal proliferation and ductal carcinoma in situ (DCIS) can show similar patterns of angiogenesis as invasive breast cancer. Breast MRI reporting according to the Breast Imaging Reporting and Data System (BI-RADS) lexicon is standardized, but BI-RADS does not include a clinical decision rule such as PI-RADS for the prostate. The lack of formal guidance makes breast MRI interpretation a subjective, experience-dependent task. In addition, BI-RADS semantic features contain redundant information and show significant overlap between benign and malignant lesions. Insecurity of the reporting physician will lead to biopsy and follow-up recommendations that are ultimately unnecessary. False-positive findings pose a challenge to the broad implementation of breast MRI, as the availability of MRI-guided biopsy facilities is limited, and cost-effectiveness of supplemental breast MRI screening in women with dense breasts critically depends on MRI specificity.
Diffusion-weighted imaging (DWI) can help with this clinical problem: it provides additional quantitative information that is fast to acquire. Malignant neoplastic tissue changes cause a characteristic decrease in water diffusivity as measured by the apparent diffusion coefficient (ADC) in standard DWI. This can be put to clinical use, as the probability of cancer decreases with increasing ADC values, so that breast cancer can be excluded practically beyond a certain ADC threshold.
DWI and the ADC in Clinical Practice
CHOICE OF b VALUES
The general physical principle of DWI is covered in Chapter 1 in this book. From a clinical perspective, DWI is a technique sensitive to the molecular (Brownian) motion of water and, in some circumstances, the “pseudoincoherent motion” caused by flowing blood in small vessels (perfusion). To assess water diffusion in a quantitative manner, at least two sets of images must be acquired. The first set of images is a non- (or very low) diffusion-weighted image with recommended b values of 0 (or at least lower than 50) s/mm 2 . As the clinical relevance of using nonzero low b values for suppressing signal from larger vessels in the breast is less evident than in the liver, for example, there is no clear benefit of using b 50 s/mm 2 instead of b 0 s/mm 2 images. The second set of images is diffusion weighted, and a b value of 800 s/mm 2 is currently recommended. This b value is determinant for the DWI contrast level and the calculated ADC value. The simple diffusion model used for DWI assumes a monoexponential signal loss, independent of the choice of b value, assuming free (not hindered) water diffusion. However, in tissues, water diffusion is highly hindered, which results in a decrease of the amount of signal loss when the b value increases. This hindrance is what makes DWI so sensitive to changes in tissue microstructure, as seen in tumors, but it also implies that choice of b values is a major issue for DWI acquisition standardization. Other less important issues for standardization are the pulse gradient setup and timing, which might also affect the ADC to some degree. It is recommended to perform the acquisition of DWI before contrast medium injection, as the local background magnetic gradient created by Gd-filled vessels might result in small ADC underestimation. However, this effect has not been shown to be clinically significant in measuring breast lesion ADC, so that the acquisition of DWI after DCE MRI might be acceptable when clinically necessary. Calculation of the ADC is usually automatically performed by scanner software; the main adjustment that can be performed is a low b value filter that ignores voxels with a base signal below the chosen arbitrary value. This reduces noisy voxels, including those covering suppressed fat signal, but it may make anatomical orientation and correlation with anatomical images difficult. The ADC map itself is a parametric map, the signal intensity of each voxel corresponds to an ADC value that can be considered a spatially resolved quantitative imaging biomarker.
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here
