One of the most actual tasks in medical diagnostics is pathology zone extraction in medical images acquired with Magnetic Resonance Imaging, Computer Tomography, etc. As a result, a problem of medical image 3D visualization develops. In this work the use of bi-cubic Coons surfaces with the following transformation to bi-cubic Bezier surfaces is proposed. Such approach allows using standard OpenGL graphic library for medical image visualization and pathology zone extraction. This allows estimating the volume of such zones, which improves medical diagnostics quality