Synopsis

The application of MRE to the breast as an adjunct to classical MR-Mammography is feasible in the clinical workflow. It adds valuable diagnostic information and allows to differentiate different grades of aggressivity via the ratio of viscosity to elasticity. Differentiation between benign and malignant is mainly possible via the viscosity with significant overlap in elasticity. Care must be taken in terms of sequence design as well as hardware to ensure optimal data quality, which is paramount here as lesions tend to be small.

Highlights

• MRE of the breast should be considered as an adjunct to standard MR-Mammography
• Biomechanical parameters do add diagnostic value for lesion classification
• There appears a correlation between aggressiveness of the lesion and increased viscosity
• MRE has significant potential to stage radiation-induced fibrosis in breast therapy
  

Target Audience

• Clinicians and physicists who are searching for new physical parameters that have clinical value for lesion characterization and fibrosis staging

Outcome/Objectives

Upon completion of this course, participants should be able to:

• understand the physical concept of MRE;
• appreciate that for the application of MRE to the breast no simplifications in terms of data acquisition or reconstruction of biomechanical parameters can be made due to the complexity of the organ;
• understand the difference between shear stiffness and shear viscosity, or equivalently shear speed and shear absorption;
• appreciate the added diagnostic value of biomechanical parameters for lesion characterization;
• appreciate the correlation between enhance viscosity of the lesion and aggressiveness; and
  
• understand the potential of MRE for staging fibrosis.
  

Purpose

The aims of this module on breast elastography are to:

• show the various approaches to breast MRE hardware;
• demonstrate current state-of-the-art hardware solutions providing the highest level of fidelity;
• show that simplifications done in data recording or extraction of viscoelastic parameters lead to significant changes in parameters;
  
• outline the current status of clinical evidence supporting the value of MRE for lesion characterization; and
• show the status-quo of fibrosis staging in liver MRE as a motivation to consider the staging of radiation-therapy induced fibrosis in the breast.
  

Required Hardware and Pulse-Sequences to Perform MRE in the Breast

Clinical MRE requires the emission of mechanical shear waves in the frequency range of 10-100Hz into the breast. A simultaneous sensing and recording of the wave propagation is possible using a motion sensitive MRI sequence synchronized to the mechanical excitation. Mechanical shear waves should preferentially illuminate both breasts and also reach upwards towards the axilla to allow for simultaneous lymph-node characterization.

Since the MRI acquisition is intrinsically slow compared to the wave propagation and the MRE sequences are broadband wrt frequency selectivity, hardware must operate at the highest possible fidelity to ensure monochromaticity. Breast parenchyma has short T2 and T2* values imposing short echo times. This enforces fractional motion encoding which leads to the broadband sensitivity of the sequence. The complex intricate geometrical mixture of fatty tissue with glandular tissue requires furthermore either in-phase echo-times to minimize geometrical distortions, or excellent fat suppression if EPI-based readouts are used. Fat suppression is generally reducing the SNR as part of the medium that actually supports the wave propagation is suppressed. Additionally, any geometrical distortion will lead to uncontrollable biases in the viscoelastic parameter estimation.

Clinical Results Regarding Viscoelastic Breast Lesion Characterization

Experts in mammography know that benign and malignant lesions are not uniquely differentiable via their stiffness: malignant can be soft and benign can be hard [1]. This is reflected in the results of the clinical studies performed so far [2-9] and hence does not come with a surprise. The additional diagnostic value appears to originate from the loss modulus (viscosity) which is a physical parameter that is not accessible to manual palpation. It expresses the ability of the material to absorb the energy of the shear wave and hence something present only when vibrating at a finite frequency: slow quasi-static deformation do not implicate viscosity. It appears that malignant lesions tend to exhibit higher values of viscosity and that the ratio of viscosity to elasticity is higher for higher grade lesions. This finding is corroborated by in-vitro work that shows that cells can physically invade through nanoporous tissues in a protease-independent manner if the tissues are more viscous [10].

MRE has demonstrated in numerous studies its ability to stage liver fibrosis [11,12]. Our own recent findings (Rheological Determinants for Simultaneous Staging of Hepatic Fibrosis and Inflammation in Patients with Chronic Liver Disease, under submission) show the ability of multi-frequency MRE to simultaneously estimate liver fibrosis and liver inflammation. Those encouraging preliminary results should be translatable to breast tissue and provide valuable diagnostic information in the context of radiation induced fibrosis.

Conclusions

The application of MRE to the breast as an adjunct to classical MR-Mammography is feasible in the clinical workflow. It adds valuable diagnostic information and allows to differentiate different grades of aggressivity via the ratio of viscosity to elasticity. How these information translate into survival require new studies. Care must be taken in terms of sequence design as well as hardware to ensure optimal data quality, which is paramount here as lesions tend to be small.

Acknowledgements

References

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