I. Introduction
During mitosis cells undergo the complex process of cell division. The new cell has to pass 4 checkpoints in the cell cycle. In cases where the proper function cannot be guaranteed, the cell undergoes cell death. This self-induced destruction is called apoptosis and plays an important role in tissue regulation and homeostasis. It was originally defined by Kerr et al. 1972 [2]. However sometimes cells mutate and loose the ability to die, when not passing the checkpoints. The cells are undergoing uncontrolled cell division and are likely to proliferate to cancerous tissue or tumors respectively. Most chemotherapeutic agents target the fast and uncontrolled cell division in order to force cells to induce apoptosis by destruction of important cell compartments like the nucleus or die by necrosis. In order to ascertain the success of anticancer treatments, a method for detecting apoptotic regions in cancerous tissue is desirable. Monitoring anticancer treatment responses would allow a fast, individual adaptation of the therapy. Different biochemical techniques have been developed to determine whether cells are undergoing apoptosis. However, these methods are invasive and time consuming. The most promising non-invasive technique uses SPECT (Single Photon Emission Computed Tomography) or PET (positron emission tomography) imaging, combined with specific radioligands. However, these methods need radioactive substances to be introduced into the body and therefore scans cannot be performed repetitively over a longer period of time (Lahorte et al. 2004). The use of bioluminescence markers combined with optical imaging methods for identifying apoptosis work non-invasively as well, but lack penetration depth. Kolios et al. 2002 and Czarnota et al. 1999 used high frequency ultrasound in the range of 20MHz to 60MHz for imaging apoptosis. Since ultrasound intensities used in diagnostics does not harm the human body like PET or X-ray, scanning procedures can be performed repetitively. An imaging method based on ultrasound measurements would offer the ability of monitoring treatment response over an expanded time period.