Breast cancer is the most common cancer among women. The established screening method to detect breast
cancer is X-ray mammography. However, X-ray frequently provides poor contrast of tumors located within
glandular tissue. In this case, additional modalities like MRI are used for diagnosis in clinical routine. A new
imaging approach is Ultrasound Computer Tomography, generating three-dimensional speed of sound images.
High speed of sound values are expected to be an indicator of cancerous structures. Therefore, the combination of
speed of sound images and X-ray mammograms may benefit early breast cancer diagnosis. In previous work, we
proposed a method based on Finite Elements to automatically register speed of sound images with the according
mammograms. The FEM simulation overcomes the challenge that X-ray mammograms show two-dimensional
projections of a deformed breast whereas speed of sound images render a three-dimensional undeformed breast in
prone position. In this work, 15 datasets from a clinical study were used for further evaluation of the registration
quality. The quality of the registration was measured by the displacement of the center of a lesion marked in both
modalities. We found a mean displacement of 7.1 mm. For visualization, an overlay technique was developed,
which displays speed of sound information directly on the mammogram. Hence, the methodology provides a
good basis for multimodal diagnosis using mammograms and speed of sound images. It proposes a guidance tool
for radiologists who may benefit from the combined information.