Abstract
Purpose:
Acoustic radiation force (ARF) exerts a compressive force, or stress, on tissue upon absorption. The effect of such force can be monitored with the same transducer used to generate the ‘push’, making the push and detection of stress-induced displacements (strain) inherently co-aligned. Our aim was to assess the effect of ARF on the posterior coats and orbital tissues of the in vivo rabbit eye as a means to probe tissue stiffness.
Methods:
We used an 18 MHz single-element transducer with a 31 mm focal length. Eyes were oriented to allow the ultrasound beam to enter the eye anterior to the equator so as to avoid absorption and refraction by the lens. After focusing on the retina, the transducer emitted a series of ten 18 MHz tonebursts at 1 msec intervals with a 25% duty cycle. Radiofrequency pulse/echo data were digitized at a pulse repetition rate of 1 kHz before, during and after ARF. Echo data (32 µm long kernel) during and for 15 msec post-push were cross-correlated with pre-push data over an 80 µm long window to determine ARF-induced displacements during the push and during relaxation. Displacement values were used to generate color-coded displacement images superimposable upon conventional grey-scale images representing echo amplitude.
Results:
Color-coded displacements superimposed upon the B-mode image allowed identification of displacement magnitude and direction with anatomy. Artifactual large positive and negative displacements were seen at the choroid as a result of confusion of the tracking algorithm by perfusion-associated speckle motion. Scleral displacement was minimal, but large strains of up to 16 µm were seen in extraocular muscle and orbital fat, with rapid recovery, usually in 1 or 2 msec, with overshoot to negative displacement common.
Conclusions:
ARF provides a non-invasive means for assessment of relative tissue stiffness that can be applied to the posterior coats as well as orbital tissues. Using power levels within FDA safety guidelines, this technique offers a means to probe changes in tissue stiffness in the posterior coats and orbital tissues in vivo.
Keywords: 552 imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) •
452 choroid •
708 sclera