# Wavelia Breast Imaging: The Optical Breast Contour Detection Subsystem
## 2 Materials and Methods  
### Wavelia Breast Examination: OBCD Subsystem Architecture  
As already mentioned in the introduction, the MBI and OBCD subsystems of the Wavelia investigational medical device are housed in two separate examination tables. This is a temporary technical limitation of the Wavelia prototype #1 system, which is planned to be resolved in the future with integration of the two subsystems in the same examination table. Such an integration is not trivial though, mainly due to two well-identified technical challenges:  
* The circular array of microwave sensors moves along an opaque cylindrical container filled with a creamy opaque transition liquid. These opaque materials have dielectric properties appropriately selected to favor the penetration of the electromagnetic waves in the breast, while being biocompatible and acceptable in the clinical setting. Their replacement by semi-transparent counterparts with the desired properties is feasible, but not evident or straightforward;* An appropriate motorization system needs to be designed, such that the MBI and OBCD scans can be performed sequentially, while the sensors (optical and microwave) are not in close vicinity to each other during the respective scans. Given the temporary technical constraint of housing the MBI and OBCD subsystems in two separate examination tables and in order for the 3D data generated by the OBCD subsystem to be useful a priori information for the MBI subsystem, the two examination tables have been manufactured to be identical. In addition, they both use the same 3D reference system (Wavelia reference system), located on each examination table plane and centered on the circular opening, as seen in Figure 1.  
The examination procedure is divided into two steps. In the first step the patient is lying in prone position on the examination table of the OBCD subsystem. The dedicated circular opening on the examination table permits a 3D optical scan of the breast and then a priori information for the MBI subsystem is computed. The opening is fitted with a plastic ring to protect the patient's breast. In the second step the patient is moved to the MBI subsystem and asked to lie in prone position on the examination table. The breast is introduced in the dedicated circular opening and immersed in a tube containing a specific liquid, which will serve as coupling (transition) medium between the imaging system and the breast. A horizontal circular probe array located outside the tube, slides vertically along the tube to reach a set of predefined vertical scan positions. It is important that the patient is lying in the same prone position during both examinations. The objective is to facilitate the registration between the 3D data generated by the OBCD subsystem and the 3D data generated by the MBI subsystem. For the examination on the OBCD subsystem, the patient is lying on the examination table, with her breast under examination inserted in the circular opening of the examination table. For this examination, the breast is hanging below the examination table; no coupling liquid is used. An infrared 3D stereoscopic camera is placed below the examination table, at a distance of several tens of centimeters below the breast. A motorization system enables azimuthal motion of the camera in one single horizontal plane with an angular step of 30\({}^{\circ}\) (see Figure 2). The azimuthal scan of the 3D camera permits to acquire twelve 3D point clouds and then to reconstruct the external envelope of the breast with sub-millimetric precision. The working environment inside the OBCD subsystem where the 3D breast scan is performed is small, closed and with no light. The small closed environment is essentially a multi-reflection environment, for which stereoscopic cameras are meant to be efficient. In addition, the obscurity of the environment justifies the selection of an infrared camera.