Figure 3b. Color (inset box) and spectral Doppler US images of the superficial palmar branch of the radial artery (red area in inset box) in a healthy volunteer demonstrate blood flow toward the transducer before radial artery compression (left of arrow). (a) In one hand, flow reverses after radial artery compression (right of arrow), indicating a continuous palmar arch. (b) In the contralateral hand, flow does not reverse after radial artery compression (right of arrow), indicating an incomplete arch.