Diffusion-weighted imaging of the breast-a consensus and mission statement from the EUSOBI International Breast Diffusion-Weighted Imaging working group

Abstract

The European Society of Breast Radiology (EUSOBI) established an International Breast DWI working group. The working group consists of clinical breast MRI experts, MRI physicists, and representatives from large vendors of MRI equipment, invited based upon proven expertise in breast MRI and/or in particular breast DWI, representing 25 sites from 16 countries. The aims of the working group are (a) to promote the use of breast DWI into clinical practice by issuing consensus statements and initiate collaborative research where appropriate; (b) to define necessary standards and provide practical guidance for clinical application of breast DWI; (c) to develop a standardized and translatable multisite multivendor quality assurance protocol, especially for multisite research studies; (d) to find consensus on optimal methods for image processing/analysis, visualization, and interpretation; and (e) to work collaboratively with system vendors to improve breast DWI sequences. First consensus recommendations, presented in this paper, include acquisition parameters for standard breast DWI sequences including specifications of b values, fat saturation, spatial resolution, and repetition and echo times. To describe lesions in an objective way, levels of diffusion restriction/hindrance in the breast have been defined based on the published literature on breast DWI. The use of a small ROI placed on the darkest part of the lesion on the ADC map, avoiding necrotic, noisy or non-enhancing lesion voxels is currently recommended. The working group emphasizes the need for standardization and quality assurance before ADC thresholds are applied. The working group encourages further research in advanced diffusion techniques and tailored DWI strategies for specific indications. Key Points • The working group considers breast DWI an essential part of a multiparametric breast MRI protocol and encourages its use. • Basic requirements for routine clinical application of breast DWI are provided, including recommendations on b values, fat saturation, spatial resolution, and other sequence parameters. • Diffusion levels in breast lesions are defined based on meta-analysis data and methods to obtain a reliable ADC value are detailed.

Keywords: Biomarkers; Breast; Breast neoplasms; Consensus; Diffusion magnetic resonance imaging.

Fig. 1

Diffusion MRI signal decay versus b value. a The diffusion signal attenuation (logarithmic signal attenuation versus b value) follows a straight line when diffusion is free (dotted green line). In tissues, hindrance/restriction of water diffusion by many microscopic obstacles results in a reduced rate of raw signal attenuation and a curvature (cross symbols) which increases with the b value. The ADC value (for instance, calculated from b = 0 and 800 s/mm²) is, thus, lower than the free diffusion coefficient, due to these combined effects. At high b values, the signal may further reach a “noise floor” and no more diffusion information can be extracted. Conversely, at very low b values, the signal attenuation rate can be elevated, because blood circulation in the random capillary network mimics diffusion (pseudo-diffusion, which is referred to as intravoxel incoherent motion (IVIM)). b The conceptual diagram illustrates the need for standardization of the applied b values. As the ADC value is calculated assuming a linear signal decay while the actual signal attenuation is curved, the ADC value decreases when using higher b values, as more restriction/hindrance effects are integrated into the ADC value; this illustrates the importance of using common b values for standardization

Fig. 2

An example of the effect of the noise floor on the observed ADC values. For three hypothetical tissues with free diffusion (no hindrance/restriction effect for simplicity), in the absence of noise (left), the slope gives the ADC values of 1.0, 1.5, and 2.0 10^-3 mm²/s, respectively, for tissues A, B, and C. However, in the presence of noise (right), the noise floor leads to curvature of the signal decay. As a result, the ADC values change. In this example, we measured 0.6, 1.22, and 1.08 10^-3 mm²/s, respectively, for tissues A, B, and C. It should be noted that the observed ADC value in tissue B is now higher than that in tissue C. This order no longer reflects diffusion, but the amount of signal at b = 0 s/mm², which depends solely on tissue magnetization properties (A has the lowest signal level and B the highest). Consequently, a low SNR may result in misclassification of the diffusion level of a lesion. S0 indicates the signal intensity at b = 0 s/mm², and D = diffusion coefficient

Fig. 3

Observed signal decay in benign and malignant lesions depending on baseline T2 signal. Tissues and lesions with a very low water content (e.g., fibrotic parenchyma, scars, low cellular cancers with extensive desmoplastic stromal fibrosis) may not be visible on high b value images and present with an artefactual low signal decay leading to low ADC values. S0 indicates signal intensity at b = 0 s/mm²

Fig. 4

ADC thresholds and value ranges for malignant, benign, and normal tissue. In this graph, the lower horizontal arrows show the range of reported mean ADC values for normal breast tissue, benign, and malignant lesions. The top arrow shows the range of suggested thresholds to differentiate between benign and malignant lesions. Note that this graph simply lists ranges as taken from the original tables and no data pooling was performed. The color bars correspond to the diffusion levels that were defined and agreed upon by the working group in order to standardize the description of the diffusion values

Fig. 5

Clinical examples illustrating the diffusion levels presented in Fig. 4 and Table 2