Purchase this article with an account.
D.J. Coleman, R.H. Silverman, M.J. Rondeau, H.O. Lloyd; Ultra–High Resolution Harmonic Ultrasound Imaging of the Choroid . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3506.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
Imaging of the choroid is of increasing importance with the advent of improved therapeutics for a wide range of macular and retina diseases. Conventional ultrasound has excellent penetration of the choroid but without sufficient resolution to document early changes in the choroid. Newer 20 MHz ultrasound has improved resolution (on the order of 75 microns axially) but some changes in choroidal micro–architecture remain unresolved at this level. Resolution of choroidal structures can be further improved using non–linear imaging techniques where the RF signal returned from the harmonic of the transducer’s fundamental frequency is used to improve resolution. In this study we describe the application of various harmonic ultrasound techniques to choroidal imaging to improve resolution and signal to noise ratio (SNR).
10 and 20 MHz transducers were excited under conditions resulting on non–linear propagation with resultant formation of harmonics at double the fundamental. System configurations were tested to obtain optimal clinical imaging in terms of bandwidth and harmonic peak frequency. Bandpass filtering and pulse inversion methods were used to separate the harmonic from the fundamental emitted frequency data. Spectral parameter imaging was evaluated as a means of additional improvement in SNR and speckle suppression.
Sufficient SNR was available for harmonic imaging of the retina and choroid with 10 and 20 MHz fundamental frequency transducers using both the filtering and pulse inversion method. 12 bit digitization significantly improved dynamic range of the images and mid–band fit parameter images improved structural segmentation of the choroid with enhanced speckle suppression.
Significantly improved resolution, SNR, and speckle suppression of ultrasound images of the choroid are obtained with harmonic imaging techniques. Improved resolution allows for improved blood flow measurements in smaller vessels and improved tissue microstructure estimation using wavelet techniques. The higher resolution and depth of penetration provided by this technique allows improved evaluation of the entire retina, choroid and sclera as well as the juxtascleral orbital fat pad and the optic nerve.
This PDF is available to Subscribers Only