Author: Cristel Baiu, Laura Castaneda Martinez, Ivan M. Rosado-Mendez ๐จโ๐ฌ
Affiliation: University of Wisconsin, Department of Medical Physics, University of Wisconsin-Madison, Departments of Medical Physics and Radiology, University of Wisconsin-Madison ๐
Purpose: 2D matrix array transducers (2DMAT) are advancing volumetric ultrasound due to their ability to provide more isotropic 3D resolution, fast volumetric imaging, and highly customizable focusing due to the large element count. Knowledge of the exact position of the transducer elements is key for accurate image reconstruction, particularly in novel super-resolution and quantitative imaging methods. This study proposes a phantom-based calibration framework to characterize and compensate for element misalignment in 2DMAT image reconstruction.
Methods: An 8-MHz 2DMAT consisting of 1024 micrometric piezoelectric elements arranged in four 32x8 element sub-apertures was operated with a Verasonics Vantage 256 system. To characterize element misalignment, a phantom with fibers with sub-resolution thickness and parallel to the transducer aperture were immersed in a tissue-mimicking fluid (sound speed 1539.1ยฑ0.6 m/s). Several fiber compositions were compared in terms of their spatial response (measured by the Full-Width-at-Half Maximum (FWHM) of the echo envelope), reflectivity (measured by the time-average power of the echo), and ringing (measured by the echo length). The 2DMAT, mounted on a micrometric motion stage, transmitted a 7.8-MHz, one-cycle plane wave, with RF echo signals sampled at 32 MHz, acquired from individual elements of each aperture. Misalignment was determined by differences in measured time-of-arrivals (TOAs) of echoes from fibers with theoretical estimates.
Results: Stainless steel (SS) was selected after being evaluated against tungsten and nylon. SS showed a FWHM of 930.0ยฑ11.1 ฮผm, reflectivity of 165,622.2ยฑ24,949.4 (mV2ยทs), and pulse length of 0.27ยฑ0.002 ฮผs. Sub-aperture 3 exhibited minimal misalignment with a TOA deviation of 0.004ยฑ0.002 ฮผs, whereas sub-aperture 4 showed the largest at 0.3ยฑ0.02 ฮผs.
Conclusion: A calibration framework was developed and effectively identified element misalignment, with notable offsets in sub-aperture 4. Integrating position corrections into beamforming algorithms aims to improve 3D quantitative ultrasound of microstructural tissue anisotropy.