0885–3010/$25.00 © 2011 IEEE 316 IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, . 58, . 2, FEBRUARY 2011 Abstract—The frequency response of ultrasonic detectors is commonly calibrated by finding their sensitivity to incident plane waves at discrete frequencies. For certain applications, such as the emerging field of optoacoustic tomography, it is the response to point sources emitting broadband spectra that needs to be found instead. Although these two distinct sensi- tivity characteristics are interchangeable in the case of a flat detector and a point source at infinity, it is not the case for detectors with size considerably larger than the acoustic wave- length of interest or those having a focused aperture. Such geometries, which are common in optoacoustics, require direct calibration of the acoustic detector using a point source placed in the relevant position. In this paper, we report on novel cross-validating optoacoustic methods for measuring the fre- quency response of wideband acoustic sensors. The approach developed does not require pre-calibrated hydrophones and therefore can be readily adopted in any existing optoacoustic measurement configuration. The methods are successfully con- firmed experimentally by measuring the frequency response of a common piezoelectric detector having a cylindrically focused shape. I. I M  popular piezoelectric ultrasonic detectors at- tain relatively limited bandwidth and non-uniform frequency characteristics, leading to distortion of the de- tected acoustic signals. In imaging applications, signal distortions are translated into image artifacts and loss of accuracy. Such artifacts can be reduced if the detector characteristics which are relevant to the imaging modal- ity are known and corrected for. Thus, the calibration of ultrasonic detectors is not only important for assessing image fidelity, but may also be used to improve the overall quantification abilities. Several calibration methods have been developed using acoustic plane waves [1]–[6]. One of the early methods employed was the reciprocity method, in which the same transducer or transducers is used both as the source and as the detector [1]–[3]. Using this technique, the efficiency of the transducer in both modes is embodied in the mea- sured acoustic signal. The known relation between these two efficiencies, manifested in the reciprocity parameter, is then utilized to extract both efficiencies from the measure- ment. One of the advantages of the reciprocity method is that it does not require using a primary standard, i.e., an acoustic detector whose response is known. Another ap- proach which avoids using a primary standard is to mea- sure the driving voltage fed into a transducer and use a modeling approach to calculate the formed acoustic field [4]. The calibration is performed by placing a hydrophone in front of the transducer and comparing its measured signal to the one predicted by the model. When a primary standard is used, the calibration of acoustic detectors is performed by creating a wide-band acoustic field and mea- suring it with both the primary standard and the sensor that is to be calibrated. By comparing the two detected signals, the frequency response of the detector can be cal- culated. The acoustic fields can be created by wide-band transducers [5], [6], or by nonlinear acoustic propagation effects [7]. The primary standard may be a pre-calibrated hydrophone [7] or a setup based on optical interferometry [6], [7]. In recent years, there has been a growing interest in new imaging modalities based on the thermoacoustic ef- fect, which require calibrated acoustic sensors to perform quantified imaging [8]–[14]. In optoacoustic tomography, for instance, ultra-wideband acoustic fields are generated by thermal expansion of tissue exposed to high-power short laser pulses. Here, the imaged object acts as an acoustic (or optoacoustic) source. Because the detection is acous- tic, optoacoustic tomography is capable of mapping opti- cal contrast while attaining diffraction-limited ultrasonic resolution, unaffected by light diffusion. Furthermore, techniques like multispectral optoacoustic tomography (MSOT), in which the laser operates at multiple wave- lengths, extend this mapping capability to imaging and mapping bio-distribution of spectrally-distinct molecular biomarkers, also attracting a great deal of interest from the biological and medical communities [13], [14]. Optoa- coustic imaging typically operates with much wider-band signals compared with conventional ultrasound imaging; therefore, the detector characterization needs to be per- formed over wider frequency bands to attain accurate im- age quantification. One of the common detection geometries in optoacous- tic tomography is based on transducers cylindrically fo- cused in the detection plane [8]–[12]. Because of their large detection area, the use of focused sensors significantly im- Optoacoustic Methods for Frequency Calibration of Ultrasonic Sensors Amir Rosenthal, Vasilis Ntziachristos, and Daniel Razansky Manuscript received July 15, 2010; accepted November 4, 2010. A. R. acknowledges the financial support of the European Community’s Seventh Framework Programme (FP7/2007-2013 under grant agreement number 235689). D. R. acknowledges support from the German Research Foundation (DFG) Research Grant (RA 1848/1) and the European Re- search Council Starting Grant. V. N. acknowledges financial support from the European Research Council Advanced Investigator Award, and the BMBF’s Innovation in Medicine Award. The authors are with the Institute for Biological and Medical Imaging (IBMI), Technical University of Munich and Helmholtz Center Munich, Neuherberg, Germany (e-mail: eeamir@gmail.com). A. Rosenthal is also with the Cardiovascular Research Center (CVRC) and Cardiology Division, Massachusetts General Hospital and Harvard Medical School, Boston MA. Digital Object Identifier 10.1109/TUFFC.2011.1809