Breast DWI Techniques and Processing: The Philips Perspective

Essential Elements of Calibration (B ₀ And B ₁ )

Most breast diffusion scans are performed with fat suppression and an echo-planar imaging (EPI) readout. Both these elements have optimal performance with uniform B ₀ (magnet) and B ₁ (radiofrequency) fields. Homogeneity of these fields heavily depends on the patient body habitus and positioning of the breasts with respect to the magnet and the radio frequency (RF) transmit coil, especially at 3 T. At this field strength, patient specific RF shimming realizes signal uniformity over both breasts as well as uniform fat suppression. Before the actual scan, a B ₁ map is generated to optimize the RF transmission settings to realize a uniform transmit field. A similar strategy is used for image-based B ₀ shimming for which a B ₀ map is acquired before the diffusion-weighted imaging (DWI) scan. The acquisition of the B ₀ map is integrated in the SmartSurvey scan that is the basis for automated, operator-independent planning of the scan geometry. In the B ₀ map, segmentation is performed such that shimming is optimized over the breasts and axillae, the clinically relevant areas. This image-based B ₀ shimming allows for optimal and reproducible fat suppression in bilateral breast magnetic resonance MRI. In case there is focus on a single breast, a shim volume can be planned to optimize the shimming over the indicated location. The three features of patient body habitus, adaptive B ₀ and B ₁ shimming, and automated planning of scan geometries are bundled in the SmartExam Breast option, which allows for robust and reproducible B ₀ shimming for optimal fat suppression and minimal EPI distortion, optimal B ₁ shimming for signal uniformity, and reproducible planning.

Another important aspect for system calibration is the compensation for eddy currents, which arise due to switching gradients and lead to signal loss and distortion. Especially with strong diffusion encoding gradients, these effects can be significant. Eddy current effects are minimized by intrinsic design of the hardware and by preemphasis to compensate upfront for the expected deviations in gradient trajectories. Proper eddy current compensation allows us to use the Stejskal–Tanner type of diffusion encoding without the need for additional sequence compensations, such as twice-refocused diffusion encoding.

Essential elements of acquisition and reconstruction

A DWI sequence consists of the following modules: diffusion encoding, fat suppression, readout, and reconstruction. It is possible to perform diffusion imaging without fat suppression, but the use of fat suppression is general practice and recommended in the guidelines. Readout and reconstruction can be regarded as two separate modules, but as these are very much interdependent, we cover them together.