I. Introduction
Blood flow is one of the most important physiological parameters and also one of the most difficult to measure accurately. Instruments for measuring the flow through blood vessels within the body have to meet certain stringent specifications; eg. Sensitivity and stability requirements depend upon the magnitude of flow, location and the diameter of the vessels. Blood flow not just ensures transportation of nutrients, hormones and metabolic wastes but also helps in early diagnosis of certain diseases and most importantly talks volumes about the cardiac activity. With the changing lifestyle and food habits the risk of cardiac related disorders is also increasing, as a result of which deaths due to heart stroke is very much common these days. Due to this, it becomes very important to monitor the functionality of heart and measuring the blood flow serves the purpose efficiently. There are many widely used techniques for measuring the blood flow. They can be categorized into two types, the invasive (surgical) techniques and non-invasive (through the skin) techniques [1].