Null Subtraction Beamforming for Improved Vessel Resolution in Volumetric Contrast-Enhanced Ultrasound

Contrast-enhanced ultrasound (CEUS) is a vascular imaging technique that has several clinical and preclinical applications. It is generally performed in 2-dimensional (2-D) space, which is not ideal for imaging complicated blood vessel networks. Alternatively, volumetric data can be acquired at a high frame rate with 2-D matrix arrays, although the image quality is reduced. Alternatives to conventional delay-and-sum (DAS) beamforming can be used to compensate for the poor resolution and high side lobe levels. The purpose of this research was to determine if the recently developed null subtraction beamforming technique can improve vessel resolution during volumetric CEUS imaging. A tissue-mimicking phantom with a 2.5 mm vessel lumen was perfused with a microbubble contrast agent and imaged with a programmable US scanner (Vantage 256, Verasonics Inc) equipped with a 1024-element matrix array transducer (7.5 MHz, Vermon). Volumetric data was acquired and beamformed with both null subtraction and conventional DAS. The resulting temporal maximum intensity projections were compared. Qualitatively, the vessel lumen was clearer and side lobe noise was greatly reduced by null subtraction beamforming. The signal-to-noise ratio (SNR) of the vessel was also improved from 9.8 dB for DAS to 13.3 dB for null subtraction. Null subtraction beamforming is a simple technique that improves the quality of volumetric CEUS images acquired using a 2-D matrix array transducer.