There is a pressing need for noninvasive methods for continuous monitoring of total hemoglobin concentration ([THb]) in blood. We proposed to use an optoacoustic technique for noninvasive [THb] measurement by probing of arteries or veins. In our previous work we demonstrated that our optoacoustic system is capable of detecting signals from the radial artery with high resolution, contrast, and signal-to-noise ratio. In clinical studies, we confirmed the ability of our system to monitor [THb] changes continuously, noninvasively, and in real time. However, absolute measurements of [THb] with high accuracy must account for blood vessel diameter. We designed and built a new broadband optoacoustic transducer that detected both the anterior and posterior (back) walls of blood vessels with high resolution. The optoacoustic signals from our radial artery phantom (silicon tube with a diameter of 1.6 mm filled with arterial sheep blood and immersed in 0.625% Intralipid solution) contained a distinct peak from the back wall of the tube. The characteristic parameters derived from the signals (the amplitude and the derivative of the normalized signal near the back wall) proved to be linearly dependent on [THb] in the physiological range. We also tested our modified system in vivo in radial arteries of healthy volunteers. The posterior walls of the arteries were well resolved, permitting accurate measurement of vessel diameter. The characteristic parameters of the signals were compared to those of signals from blood obtained for close tube diameter and various THb concentrations. The [THb] values derived from the in vivo measured optoacoustic signals were close to invasively measured ones.