Characterizing the imaging environment for supine breast MRI.

Purpose: Supine breast MRI has the potential to improve over standard prone breast magnetic resonance imaging (MRI) in terms of efficiency and image quality, image alignment with diagnostic and treatment procedures, and overall accessibility. This study aims to characterize potential technical challenges of imaging in the supine position: (i) $B_0$  field inhomogeneities, (ii) $B_1^{+}$  variations, (iii) respiratory-induced breast motion, and (iv) supine breast geometry.

Methods: Ten healthy subjects were scanned at 3T in both prone and supine positions to quantify and compare (i) and (ii) between both positions, and to assess (iii) in the supine position. Breast image volumes from a wider population (N = 40, healthy volunteers and patients) were analyzed to obtain breast shape metrics to characterize (iv).

Results: $B_0$  field inhomogeneity increased from prone positioning (2SD: $122\text{Hz}\pm 25\text{Hz}$ ) to supine positioning (2SD: $152\text{Hz}\pm 15\text{Hz}$ ), and $B_1^{+}$  flip angle variations (from prescribed ${30}^{\circ }$ ) were greater in the supine position (2SD ranging $7^{\circ }$ to ${13}^{\circ }$ ) than in the prone position (2SD ranging $6^{\circ }$ to $8^{\circ }$ ). Breast tissue displacement (median [IQR] across all analyzed locations and subjects) was similar along A-P (1.4 [0.5] mm) and R-L (1.9 [1.5] mm) directions. Breast geometry varied greatly, with the outer breast perimeter ranging from 34 to 68 cm, and maximum breast tissue thickness ranging from 2 to 9 cm.

Conclusion: Supine positioning for breast MRI may lead to greater $B_0$  inhomogeneities and greater $B_1^{+}$  variations when compared to prone positioning, and breast motion can be substantial. Breast geometry varies greatly among the female population, and shape metrics can inform supine-dedicated coil development.