Reply to 'Letter to the editor: ‘Assessment of flow-mediated dilation in humans: a methodological and physiological guideline''

reply: We would like to thank Mc Loughlin and Mc Loughlin (4a) for their response and for bringing attention to the appropriate selection of transducer operating frequency for flow-mediated dilation investigations. We agree that employing a transducer with a higher central frequency (i.e., 12 MHz) will yield a greater detail resolution in the axial plane when interrogating vessels such as the brachial artery, and this would provide a greater sensitivity in detecting small changes in flow-evoked vasodilatation. However, in our review (8), we discussed not only the brachial artery but also the common femoral (9), profunda femoris (12), superficial femoral (1, 2, 10, 12), popliteal (5, 6, 11), and posterior tibial (3, 7) arteries. When these lower-limb vessels are insonated at an optimal region in terms of flow conditions, for example, at a distance where entrance effects are minimized, the artery depth is often greater than that expected for a typical brachial artery. Given that the average attenuation coefficient is 0.5 dB·cm−1·MHz−1 (4), there is considerable degradation of image quality when employing a high-frequency probe while imaging a deeper lying vessel. This can render the image unusable when combined with automated analysis software. The subsequent trade-off between detail resolution and depth penetration is a further consideration for researchers when performing flow-mediated dilation investigations; however, the minimum transducer recommended is 7.5 MHz for all peripheral arteries so as to maintain adequate detail resolution. Certainly, we advocate the use of higher transducer frequencies (i.e., 12 MHz) when investigating more superficial vessels such as the brachial and radial arteries.