This dynamic spectral-domain OCE technique was first applied on the 3-layer tissue phantoms. In the tissue phantom illustrated here, the top layer had an elastic modulus of ~100 kPa and a thickness of ~200 µm. The middle layer had an elastic modulus of ~10 kPa and a thickness of ~400 µm, and the bottom layer had an elastic modulus of ~100 kPa and a thickness of ~2400 µm, which is shown in the structural OCT image in
. The OCE images under driving frequencies of 20 Hz and 100 Hz are shown in and (green channel), with the structural OCT image as the background (red channel). Under 20 Hz excitation, the OCE signal is shown predominantly in the middle layer. When the excitation frequency is increased to 100 Hz, the OCE signal is shown predominantly in the top and middle layers.
Fig. 4 OCT and OCE images of a three-layer tissue phantom. (a) Structural OCT image. Young's moduli are labeled for each corresponding layer. The red axis denotes the A-scan direction and the dotted lines denote the boundaries of the layers. (b) OCE image under (more ...)
Strain rates as calculated in Eq. (1)
and shown in the OCE images are used to differentiate mechanical properties under different driving frequencies. If the mechanical properties of samples result in resonance at a given driving frequency, the vibration strain rates will increase. This feature of OCE imaging may be used to mechanically characterize specific regions within heterogeneous samples. For OCE using 20 Hz excitation, the strain rate of the soft middle layer is relatively high, suggesting it is near resonance. The stiff top layer is not effectively compressed, suggesting it is far from resonance. The OCE image therefore has a low strain rate in this top layer. The low strain rate in the bottom layer suggests that it is also off resonance, but is nevertheless transmitting vibrations to the middle layer. Thus in the OCE image, appreciable strain rate values are observed. However, the OCT signal for the bottom layer also has lower SNR compared to the other two layers, and the resulting higher phase noise causes attenuation of the OCE signal in this layer, at both excitation frequencies [16
]. For OCE at 100 Hz excitation, larger strain rate values with bulk motion in the top and bottom layers suggest that the top and bottom layers are close to a resonance.
The OCE results from rat tumor tissue are shown in
. and are the OCE images of the tissue under 45 Hz and 313 Hz excitations, respectively. The corresponding OCT structural image and histological image are shown in and , respectively. From , one can observe that under 45 Hz excitation, the OCE image highlights predominantly the adipose tissue region (left side of image), while under 313 Hz, the OCE image highlights predominantly the tumor tissue region (right side of image). The 45 Hz and 313 Hz frequencies are near the previously measured mechanical resonances for rat adipose and tumor tissue [13
], respectively. Therefore in the heterogeneous rat tumor tissue, different tissue types were selectively highlighted under different driving frequencies according to their different resonances. Furthermore, in , the OCE image includes two regions from the right side (orange arrows) which corresponds to the highly scattering region from the OCT signal in . From the histological image in , it is shown that this region corresponds to local areas of adipose cells and connective tissues (orange arrows) within the tumor, which is poorly differentiated from the OCT scattering image. On the other hand, in , the OCE image includes some regions from the left side (blue arrow) which corresponds to the low scattering region from the OCT signal in . According to the histological image in , those regions correspond to invading tumor tissues in the adipose side (blue arrow).
Fig. 5 OCE results on ex vivo rat tumor tissue. (a) OCE image under 45 Hz mechanical excitation. (b) OCT structural image of the tissue. (c) OCE image under 313 Hz mechanical excitation. (d) Corresponding histological image. Scale bar applies to all the images. (more ...)
The image acquisition and processing speeds of this OCE technique have been dramatically improved compared with the previous studies [5
]. The acquisition speed is 4 s per frame for a 1 kHz axial scan rate and 0.8 s per frame for a 10 kHz axial scan rate. The processing speed is approximately 1 s per frame using Matlab on a PC with a dual core 2.0 GHz AMD AthlonTM
CPU and 2 Gigabytes of memory. With these higher acquisition and processing rates, this dynamic OCE technique has the potential for volumetric biomechanical imaging, and with state-of-the-art OCT hand held probes, there is also the potential for non-destructive in vivo
and clinical applications using a ring actuator design [7