## 1 Introduction  
Cryosurgery/cryotherapy is the medical application of controlled localized freezing to renew, remove or destroy pathological tissues [1]. It is achieved by local rapid cooling to extremely low temperatures, typically by liquid nitrogen injection or a cold \"finger\", followed by a relatively slower thawing process. Tissue injury from a freeze-thaw procedure can broadly be attributed to three sequential, though overlapping, mechanisms: (a) an initial physical phase, where cell necrosis results directly from the exposure of the tissue to low temperature; the resulting intracellular micro-crystals and micro-bubbles as well as osmotic processes during thawing destroy the cell structures [2]; (b) a vascular phase, where tissue damage occurs from blood supply alterations [3]; and (c) an immunologic-inflammatory phase, where inflammatory processes are triggered by the cryo-injury and persist for a couple of weeks. The latter mechanism is suggested to be particularly important in tumor therapy [4].  
After a cryosurgery session, the treated tissue will alter progressively over time due to the aforementioned mechanisms. It is therefore crucial to monitor, in real-time, the expansion and depth of the \"ice-ball\", ideally by measuring physical state changes and/or dynamic temperature gradients (isotherms) of the tissue. To date, there is no easy real-time monitoring of the cooling process neither can the precise depth of the isotherms be routinely determined in vivo. Thus, clinical cryosurgery relies on the \"expert knowledge\" of the physicians. So far, monitoring techniques that are only used in experimental or in clinical trials, include direct temperature measurements utilizing infrared thermography [5] and single or multiple needle-mounted fiber Bragg gratings or thermocouples, inserted at appropriate tissue sites to measure local temperatures [6,7]. Ultrasound shadowing has also been employed [8]. Finally, imaging techniques such as optical coherent tomography (OCT), [9] vibroacoustography, [10] electrical impedance tomography [11], and magnetic resonance imaging (MRI) [8,12,13] have also been reported. In particular, OCT [9] is a well establish interferometric method that has been used experimentally in similar studies. However, in a recent clinical study [PERSON] et al. [14] reported that in normal ex vivo skin and in skin lesions in vivo, the opaque ice ball induced by cryo-therapy treatment does not allow the determination of tissue freezing depth by OCT. Other remote optical imaging techniques such as camera or high-resolution multispectral imaging [15,16] may also be considered but could be handicapped by ambient light variations and the formation of frozen due during and immediately after cryogen applications, which obscures the treated area, thus impairing data collection in the initial stages of the treatment. In general, there are several disadvantages when using the above techniques: (a) in vivo skin remains frozen for a very short time making difficult to collect and process relevant data; (b) the contact instruments can't be used due to thermal interference in cryo-therapy; (c) some methods rely on high cost instrumentation, which renders them impractical for routine cryosurgery. Here, we demonstrate a technique based on fiber optic-array geometry that exploits the differential optical backscattering and diffusion properties of frozen-unfrozen ex vivo porcine tissues to monitor the topology and depth of the frozen region in real time.