Harris R A, Follett D H, Halliwell M, Wells P N
University Department of Electrical Engineering, Bristol General Hospital, England.
Ultrasound Med Biol. 1991;17(6):547-58. doi: 10.1016/0301-5629(91)90025-r.
According to elementary theory, the resolution of an ultrasonic imaging system increases with the ultrasonic frequency. However, frequency is limited by frequency-dependent attenuation. For imaging at any required depth, resolution improvement beyond the limit imposed by ultrasonic frequency can be obtained by increasing the ultrasonic intensity. This is itself, however, dependent on safety considerations and the effects of nonlinearity. In homogeneous media, image resolution increases with decreasing f-number. Particularly at low f-numbers, however, tissue inhomogeneity leads to a deterioration in image quality. Inhomogeneity may also be considered in terms of phase aberration. It has been found that for a given aperture, image degradation due to phase aberration is worse at higher frequencies. Schemes have been proposed for correction of this problem, but so far model systems do not lend themselves to clinical application. Deconvolution is unsatisfactory, speed correction is impracticable and synthetic aperture scanning and holography are virtually useless in biological tissues. Ultrasound-computed tomography has had only limited success. Speckle reduction can improve target detectability, but at the expense of resolution. Time-frequency control provides a useful partial solution to the problem of resolution reduction resulting from attenuation. It is clear that improved resolution would result in significant clinical benefits. An optimisation system for aperture size and ultrasonic frequency is proposed with signal averaging for resolution enhancement of a defined object area. This would have a compact ultrasonic beam and would allow frame rate to be traded for resolution, by means of signal averaging.
根据基本理论，超声成像系统的分辨率随超声频率的增加而提高。然而，频率受到与频率相关的衰减的限制。对于任何所需深度的成像，通过增加超声强度可以获得超出超声频率所限制的分辨率提升。然而，这本身取决于安全考量以及非线性效应。在均匀介质中，图像分辨率随f数的减小而提高。然而，特别是在低f数时，组织的不均匀性会导致图像质量下降。不均匀性也可以从相位像差的角度来考虑。已经发现，对于给定孔径，由于相位像差导致的图像退化在较高频率时更严重。已经提出了校正这个问题的方案，但到目前为止，模型系统并不适用于临床应用。反卷积并不令人满意，速度校正不切实际，合成孔径扫描和全息术在生物组织中几乎毫无用处。超声计算机断层扫描仅取得了有限的成功。散斑减少可以提高目标可检测性，但以分辨率为代价。时频控制为因衰减导致的分辨率降低问题提供了一个有用的部分解决方案。显然，提高分辨率将带来显著的临床益处。提出了一种孔径大小和超声频率的优化系统，并通过信号平均来提高定义目标区域的分辨率。这将产生紧凑的超声束，并通过信号平均允许以帧率换取分辨率。
