High temporal resolution OCT using image-based retrospective gating

Abstract

High temporal resolution OCT imaging is very advantageous for analyzing cardiac mechanics in the developing embryonic heart of small animals. An image-based retrospective gating technique is presented to increase the effective temporal resolution of an OCT system and to allow visualization of systolic dynamics in 3D. The gating technique employs image similarity measures for rearranging asynchronously acquired input data consisting of a time series of 2D images at each z position along the heart volume, to produce a time sequence of 3D volumes of the beating heart. The study includes a novel robust validation technique, which quantitatively evaluates the accuracy of the gating technique, in addition to visual evaluations by 2D multiplanar reformatting (MPR) and 3D volume rendering. The retrospective gating and validation is demonstrated on a stage 14 embryonic quail heart data set. Using the validation scheme, it is shown that the gating is accurate within a standard deviation of 4.7 ms, which is an order of magnitude shorter than the time interval during which systolic contraction (approximately 50 ms) occurs in the developing embryo. This gating method has allowed, for the first time, clear visualization of systolic dynamics of the looping embryonic heart in 3D.

Fig. 1

A schematic of our image-based gating algorithm. As shown, the major steps are: (i) decimate images, (ii) determine cardiac period at each slice position, (iii) reassemble or wraparound data to produce one correctly ordered, unevenly spaced set of images from a cardiac cycle at each slice, (iv) interpolate linearly to create even temporal spacing, (v) determine relative shift from one slice to the next (vi) determine absolute time shift at each slice, and (vii) reorder image data to create volumes over the cardiac cycle.

Fig. 2

(A) Schematic showing proposed validation scheme. The real-time volume is captured in 103 ms, while image-based retrospective gating data represents a 4.9 ms time interval. The left part of Fig. 2(a) shows an en face real-time OCT image with the B-scan direction superimposed on the image. The right part of Fig. 2(a) shows various volumes with yellow arrows pointing to the B-scan that represents the identical phase in the cardiac cycle. (B, C) slice 31 from a real-time volume (volume scan A) and an image-based retrospective gated volume (volume 24). (D, E) slice 45 from a real-time volume (volume scan A) and an image-based retrospective gated volume (volume 24). In slice 31 the real-time volume and gated volume are not synchronized. The images are taken at different phases in the cardiac cycle as clearly seen within the yellow oval. Slice 45 in volume 24 is synchronized with slice 45 in the real-time volume as can be seen by the similarity in the 2 images.

Fig. 3

(3.16 MB) A (2D + time) sagittal (en face) movie created from 4D retrospective gated OCT data of the stage 14 embryonic quail heart. The slice was chosen 313 μm deep into the tissue. With retrospective gating, cardiac systolic dynamics can be clearly visualized with a high temporal resolution in planes different from the OCT imaging plane.

Fig. 4

4D Visualization of retrospective gated OCT data from stage 14 embryonic quail heart. Movies of 4D gated data (90 volumes in a single cardiac cycle, or ∼270 volumes/s) are shown in sagittal or en face (a) (2.7 MB), transverse (b) (2.9 MB), and coronal (c) (3.2 MB) views. In each case, a cutaway view of the beating heart with a moving orthogonal 2D slicer in the plane of interest is shown.

Fig. 5

(3.4 MB) Digitally reconstructed radiograph (DRR) volume rendering movie created using gated data from stage 14 embryonic quail heart. A summing of voxel intensities along rays cast through the volume perpendicular to the viewing plane has been used to create a 3D effect with the rotating and beating heart volume.

Fig. 6

Validation of retrospective gating algorithm using two different reference volume scans A (left) and B (right). A linear fit is obtained through matching volume numbers in gated data as per the method described in Section 2.3. The slope of the fit obtained on gated data is 1.364 ms/slice for scan A and 1.120 ms/slice for scan B. The slope of reference scan is 1.275 ms/slice.

Fig. 7

Validation of image-based retrospective gating using input data with different integer decimation factors. In each case, the B-scan frame was decimated by an integer number n in both x and y directions. Following decimation, the gating algorithm (Section 2.2) and the proposed validation scheme (Section 2.3) were applied. We observe that, up to a decimation factor of 8, the slope of the validation plot (slice number versus matching volume number in gated data) closely matches the ideal slope from the reference volume.