Fractional flow reserve (FFR): Fractional flow reserve (FFR) is measured using an ultrsound Doppler wire. It measures the pressure before and after a narrowing in the coronary artery. The ratio between the two is known as FFR. It will indicate the significance of the narrowing noted on coronary angiography. If the ratio is low, that means the obstruction is severe. This is useful in assessing the significance of borderline narrowing noted on coronary angiography. There is hardly any need for this test if the narrowing is obviously severe or mild.