Image Acquisition

Image acquisition was carried out at the Lozano Blesa University Hospital (Zaragoza, Spain). The echographic probe was positioned onto the arm of the patient lying supine on a bed. A silicon gel was used as impedance adapter for better ultrasound wave transmission. The probe, once the correct orientation angle was found, was fixed with a probe holder to the table where the patient's arm lies (Fig. 5.1). Telediastolic images were captured and hold, coincident with the peak of the R wave of the electrocardiogram. A SONOS 4500 (Agilent Technologies, Andover, MA, USA) ultrasound system was used in frequency fusion mode and employing a 5.5-7.5 MHz trapezoidal multifrequency probe. Images were transferred to a frame grabber via a video Y/C link and images were digitized at a resolution of 768 x 576 pixels.

During the examination, unavoidable movements take place thus changing the relative position between the transducer and the artery. Therefore, expert intervention is sometimes required to control the image quality by readjusting the orientation of the transducer to keep visible borders as sharp as possible. Both, motion artifacts and successive readjustments may induce changes in image quality as well as changes on extraluminal structures along the sequence. All these factors have to be handled appropriately in the postprocessing stage if the computerized analysis has to be used on a routine basis.

Figure 5.1: Experimental setup.

Each sequence has about 1200 frames and a duration of around 20 min, acquiring each second the last telediastolic frame previously hold. This provides afixed sampling rate irrespective of heart rate, which means a substantial benefit for clinical interpretation, as different stimulus are applied on a time basis along the clinical test. As the dynamics of endothelial vasodilation is much slower than changes happening between cardiac cycles, with this sampling rate, missing information from one heartbeat or using one heartbeat twice does not affect, in practice, the results.

FMD is the vasodilation response to hyperemia after a transitory distal ischemia induced in the forearm using a pneumatic cuff distal to the probe (Fig. 5.1). The dilation mediated by a chemical vasodilator, the nitroglycerin, or nitroglycerin-mediated dilation (NMD), is also registered. Accordingly, five phases of the medical test can be distinguished in each sequence (see Fig. 5.2).

• Rest baseline (B1). Initial rest state preceding distal ischemia. Presents the best image quality in the whole sequence and lasts for about 1 min.

• Distal ischemia (DI). The cuff is inflated and, therefore, the image quality is usually the worst in the sequence. It takes approximately 5 min. This phase ends when the cuff pressure is released.

Figure 5.2: A whole typical examination can be divided into several segments along the sequence: rest baseline (B1), distal ischemia (DI), flow-mediated dilation (FMD), post-FMD baseline (B2), and nitroglycerin-mediated dilation (NMD).

• Flow-mediated dilation (FMD). Response to reactive hyperemia. The maximum %FMD of healthy subjects has a mean value around 5%, and it takes place about 60 sec after the cuff is released [5]. At the beginning of this phase, and coincident with the release of the cuff, it is common to have important motion artifacts. The duration of this phase depends on each patient.

• Post-FMD baseline (B2). The artery diameter returns to a steady state. This state does not necessary have to coincide with that of B1.

• Nitroglycerin-mediated dilation (NMD). Response to the sub lingual supply of a fixed dose of this chemical vasodilator, which is made 8 min later than the cuff release.

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