Image quality control in breast ultrasound

Abstract

Sonography is well suited for breast studies. Adequate equipment is needed to acquire high quality images because several technical factors influence ultrasound images. Thus, the use of high frequency dynamic scanning probes, the ultrasound beam focusing corrected for the near field, the adjustment of the gain and image contrast may all interfere with ultrasound beam reflection and scattering, determined by the heterogeneity of the gland parenchyma. In the last few years, a line of ultrasound equipment dedicated to this kind of application has been developed with ‘small parts’ transducers and frequencies ranging 10–13 MHz. These units can improve the evaluation of superficial structures and provide diagnostic results that conventional equipment cannot achieve. The higher the quality, the more a sonographic image corresponds to real anatomy. This capability depends on the different kinds of system resolution. Axial spatial resolution is the capability to resolve discrete structures along the beam axis. Pulse length is inversely proportional to frequency and thus, the higher the transducer frequency, the better the axial resolution. However, the increase in frequency reduces the depth of penetration of the ultrasound beam. The spectrum of frequencies emitted by the crystal has been recently modified in order to obtain a good trade-off between the beam resolution and its penetration. Indeed, the development of the multifrequency technology allowed to improve the near field resolution while retaining a good penetration into the distant field. Furthermore, the use of compound ceramics with a broad bandwidth helps Doppler analysis because flow studies are optimized by low frequencies, whereas two-dimensional morphologic imaging is optimized by high frequencies. Lateral spatial resolution is the capability to resolve discrete structures perpendicular or lateral to the beam axis. This parameter strictly depends on the size of the ultrasound beam section and it is optimal only in the focal area. Therefore, it improves with narrow beams. Several transducers are available in breast sonography, but the most adequate one is currently the annular transducer. The equipment should be able to detect even slight differences in acoustic impedance between the several breast tissues. This may be obtained by optimizing the dynamic range and the pre- and postprocessing setting. Apart from equipment, two other technical factors should be optimized to obtain high quality images, namely beam intensity and gain curve. A new Doppler technique has been recently introduced: power Doppler, which allows the demonstration of breast nodule vascularization with higher sensitivity than color Doppler. Finally, a rigorous examination technique is required to obtain high quality images. In the last few years, several quality assurance programs have been introduced. Dedicated phantoms are generally used. Recently, computer systems have been also developed.