Characterising the performance of a high resolution ultrasound scanner for pre-clinical ultrasound imaging

Research using small animals continues to play a key role in biological, biomedical and veterinary science. In particular both mouse and rat models have become increasingly popular as research tools due to the fact that 90% of the genomic sequences in these rodents are identical to those found in humans. The versatility of employing a high resolution ultrasound scanner to perform small animal microimaging in vivo has recently been demonstrated [1]. However, in common with medical imaging systems, the technical performance of high resolution scanners is difficult to quantify. We have employed a novel measurement technique previously developed for medical imaging (the Resolution Integral) to study the grey-scale imaging performance of a high resolution scanner (Vevo 770, Visualsonics) with probe centre frequencies of 25-55 MHz. We designed and manufactured a high resolution test object containing 30 wall-less anechoic pipe structures in blocks of agar based tissue mimic. The pipe diameters ranged from 45 mum to 1.5 mm. Each probe was scanned over the surface of the test object and a series of images of each pipe was captured. The axial depth range over which each pipe could be visualised (L) was determined, and plotted as a function of plusmn, where plusmn is proportional to the reciprocal of pipe diameter. The Resolution Integral (R) was calculated for each probe by measuring the area under the curve. Characteristic Resolution and Depth of Field were also determined using the same set of measurements. Results were obtained using with probe models RMV710, RMV707B, RMV704, RMV708 and RMV711 (centre-frequencies 25, 30, 40, 55 and 55 MHz respectively). The measured values of R were 18, 23, 22, 21, 24 respectively, and the corresponding Depths of Field/Characteristic Resolutions were 5.4 mm/289 m; 5.3 mm/225 m; 3.0 mm/137 m; 2.8 mm/131m and 3.6 mm /145 m. We have successfully extended the range of application of Resolution Integral measurement to a high resolution microimaging system.