1. A method comprising: capturing a first intra-operative 2-D ultrasound image depicting at least a portion of a first flexible surgical needle instrument disposed at a first position with respect to a target tissue of a patient and not a second flexible surgical needle instrument disposed at a second position with respect to the target tissue of the patient, wherein the second position is different from the first position and the first and second flexible surgical needle instruments are concurrently present respectively at the first and second positions when the first intra-operative 2-D ultrasound image is captured; capturing a second intra-operative 2-D ultrasound image depicting at least a portion of the second flexible surgical needle instrument disposed at the second position with respect to the target tissue of the patient and not the first flexible surgical needle instrument disposed at the first position with respect to the target tissue of the patient, wherein the first and second surgical instruments are concurrently present respectively at the first and second positions when the second intra-operative 2-D ultrasound image is captured, wherein the first and second flexible surgical needle instruments are sequentially manually positioned at the first and second positions; transforming each of the first and second intra-operative 2-D ultrasound images to a three-dimensional model such that (1) the first position of the portion of the first flexible surgical needle instrument is rendered with the three-dimensional model, and (2) the second position of the portion of the second flexible surgical needle instrument is rendered with the three dimensional model by registering the first intra-operative 2-D ultrasound image to the 3-D model and registering the second intra-operative 2-D ultrasound image to the 3-D model, wherein the 3-D model is generated based on a pre-operative image of the target tissue of the patient acquired before the first flexible surgical needle instrument is disposed at the first position with respect to the target tissue of the patient; calculating a three-dimensional distance between the portion of the first flexible surgical needle instrument and the portion of the second flexible surgical needle instrument based on the three-dimensional model; and determining a size of an irreversible electroporation zone from the three-dimensional distance between the portion of the first flexible surgical needle instrument and the portion of the second flexible surgical needle instrument.