Characterization of the spatial resolution of different high-frequency imaging systems using a novel anechoic-sphere phantom

Abstract

The spatial resolution of high-frequency ultrasound (HFU, >20 MHz) imaging systems is usually determined using wires perpendicular to the beam. Recently, two tissue-mimicking phantoms (TMPs) were developed to estimate the three-dimensional (3D) resolution. Each of the TMPs consist of nine, 1 cm wide slabs of tissue-mimicking material containing randomly distributed anechoic spheres. All anechoic spheres in one slab have the same dimensions, and their diameter is increased from 0.1 mm in the first slab to 1.09 mm in the last. The scattering background for one set of slabs was fabricated using 3.5 µm glass beads, while those of the second set were 6.4 µm. The ability of a HFU system to detect these spheres against a speckle background provides a realistic estimation of its 3D spatial resolution. In the present study, these TMPs were used with HFU systems using single-element transducers, linear arrays and annular arrays. The TMPs were immersed in water and each slab was scanned using a VisualSonics™ Vevo 770 and Vevo 2100, and a custom HFU system based on a 5-element annular array. The annular array had a nominal center frequency of 40 MHz, a focal length of 12 mm, and a total aperture of 6 mm. A syntheticfocusing algorithm was used to form images with an increased depth-of-field. The penetration depth was increased by using a linear-chirp signal spanning 15 to 65 MHz over 4 µs. Results obtained with the custom system were compared to those of the Vevo systems (40 MHz probes RMV-704 and MS-550D) in terms of sphere detection, i.e., 3D spatial resolution, and contrast-tonoise ratio (CNR). Resulting B-mode images indicated that only the linear-array transducer failed to clearly resolve the 0.2 mm spheres, which showed that the 3D spatial resolution of the singleelement and annular-array transducers was superior to that of the linear array. The single-element transducer could only detect these spheres over a narrow 1.5 mm depth-of-field, while the annular array was able to detect them to depths of at least 7 mm. For any size of the anechoic spheres, the annular array excited by a chirp-coded signal provided images of the highest contrast, with a maximum CNR of 1.8 at the focus, compared to 1.3 when using impulse excitation and 1.6 with the single-element transducer and linear array. This imaging configuration also provided CNRs above 1.2 over a wide depth range of 8 mm, while CNRs would quickly drop below 1 outside the focal zone of the other configurations.

Correspondence - Characterization of the effective performance of a high-frequency annular-array-based imaging system using anechoic-pipe phantoms

“…2(c5)]. This showed that despite the better contrast and SNR provided by the linear array, the annular array had a higher effective resolution and enabled a better detection of the pipes smaller than 200 μ m. A similar result was found in a previous study [6] using phantoms with anechoic spheres 200 μ m in diameter.…”

“…Tissue-mimicking (TM) phantoms can be used to provide a more realistic medium for device characterization because they simulate conditions more relevant to scanning within tissue. In a previous study [6], we characterized the detection capabilities of a custom HF imaging system based on an annular-array transducer, and two HF commercial scanners from VisualSonics (VisualSonics Inc., Toronto, ON, Canada): the Vevo 770 (single-element transducer) and Vevo 2100 (linear-array transducer). The characterization method was based on a TM phantom containing anechoic spheres with sizes ranging from 1090 to 100 μ m. The ability of the scanners to detect those spheres was dependent on their spatial resolution in all directions simultaneously (3D resolution).…”

High-frequency annular array with coaxial illumination for dual-modality ultrasonic and photoacoustic imaging

“…In a previous study, we used a 40 MHz, 5-element annular array, and a synthetic-focusing (SF) algorithm to achieve a sixfold increase in DOF compared to an equivalent single-element transducer. 7,8 Using a similar array, we demonstrated combined high-frequency (HF) ultrasound (HFU) and PAI with broad, uniform illumination from a bifurcated optical-fiber assembly. 9 One application that is gaining attention is vascular imaging in small animals, such as mice, for studying human disease 10,11 and mammalian development.…”

A flexible annular array imaging platform for micro-ultrasound

“…The size of the anechoic spheres was in the range of 280 to 350 μm, controlled by two dedicated sieves (Fisherbrand sieves, Fisher Scientific, Pittsburgh, PA). The contrast-to-noise ratio (CNR) of the anechoic spheres against the background, which was described in detail by Filoux et al [38], was used to evaluate the improvement of depth-of-field by the proposed imaging platform.…”