The present invention relates to methods for differentiating a blood vessel from a nonvascular tissue during a minimally invasive surgery using a device. A device may be any penetration sensor device, such as a dilator sensor device, a hollow needle sensor device, or a trocar sensor device, which penetrates the skin and subcutaneous layers to reach a target location inside the body. The penetration sensor device includes a sensor probe which can make optical measurements to determine various parameters of the tissue at the tip of the sensor probe. These parameters may include an optical signal level returned from tissue contacting the tip of the sensor probe, an oxygen saturation level of the tissue, a total hemoglobin concentration, a blood flow, and a pulse. Based on these parameters, the presence or absence of a blood vessel at the tip of the penetration sensor device can be determined while the device travels towards the target location for surgery.
