Quantifying Doppler angle and mapping flow velocity by a combination of Doppler-shift and Doppler-bandwidth measurements in optical Doppler tomography

New insight into rheology and flow properties of complex fluids with Doppler optical coherence tomography

Flow properties of complex fluids such as colloidal suspensions, polymer solutions, fiber suspensions and blood have a vital function in many technological applications and biological systems. Yet, the basic knowledge on their properties is inadequate for many practical purposes. One important reason for this has been the lack of effective experimental methods that would allow detailed study of the flow behavior of especially opaque multi-phase fluids. Optical Coherence Tomography (OCT) is an emerging technique capable of simultaneous measurement of the internal structure and motion of most opaque materials, with resolution in the micrometer scale and measurement frequency up to 100 kHz. This mini-review will examine the recent results on the use of Doppler-OCT in the context of flows and rheological properties of complex fluids outside biomedical field.

Absolute retinal blood flow measurement with a dual-beam Doppler optical coherence tomography

PURPOSE

To test the capability of a novel dual-beam Doppler optical coherence tomography (OCT) technique for simultaneous in vivo measurement of the Doppler angle and, thus, the absolute retinal blood velocity and the retinal flow rate, without the influence of motion artifacts.

METHODS

A novel dual-beam Doppler spectral domain OCT (SD-OCT) was developed. The two probing beams are separated with a controllable distance along an arbitrary direction, both of which are controlled by two independent 2D optical scanners. Two sets of optical Doppler tomography (ODT) images are acquired simultaneously. The Doppler angle of each blood vessel segment is calculated from the relative coordinates of the centers of the blood vessel in the two corresponding ODT images. The absolute blood flow velocity and the volumetric blood flow rate can then be calculated. To measure the total retinal blood flow, we used a circular scan pattern centered at the optic disc to obtain two sets of concentric OCT/ODT images simultaneously.

RESULTS

We imaged two normal human subjects at ages of 48 and 34 years. The total retinal blood flow rates of the two human subjects were calculated to be 47.01 μL/min (older subject) and 51.37 μL/min (younger subject), respectively. Results showed that the performance of this imaging system is immune to eye movement, since the two sets of ODT images were acquired simultaneously.

CONCLUSIONS

The dual-beam OCT/ODT system is successful in measuring the absolute retinal blood velocity and the volumetric flow rate. The advantage of the technique is that the two sets of ODT images used for the calculation are acquired simultaneously, which eliminates the influence of eye motion and ensures the accuracy of the calculated hemodynamic parameters.

Imaging flow dynamics in murine coronary arteries with spectral domain optical Doppler tomography

Blood flow in murine epicardial and intra-myocardial coronary arteries was measured in vivo with spectral domain optical Doppler tomography (SD-ODT). Videos at frame rates up to 180 fps were collected and processed to extract phase shifts associated with moving erythrocytes in the coronary arteries. Radial averaging centered on the vessel lumen provided spatial smoothing of phase noise in a single cross-sectional frame for instantaneous peak velocity measurement without distortion of the flow profile. Temporal averaging synchronized to the cardiac cycle (i.e., gating) was also performed to reduce phase noise, although resulting in lower flow profiles. The vessel angle with respect to incident imaging beam was measured with three-dimensional raster scans collected from the same region as the high speed cross-sectional scans. The variability in peak phase measurement was 10-15% from cycle to cycle on a single animal but larger for measurements among animals. The inter-subject variability is attributed to factors related to real physiological and anatomical differences, instrumentation variables, and measurement error. The measured peak instantaneous flow velocity in a ~40-µm diameter vessel was 23.5 mm/s (28 kHz Doppler phase shift). In addition to measurement of the flow velocity, we observed several dynamic features of the vessel and surrounding myocardium in the intensity and phase sequences, including asymmetric vessel deformation and rapid flow reversal immediately following maximum flow, in confirmation of known coronary artery flow dynamics. SD-ODT is an optical imaging tool that can provide in vivo measures of structural and functional information on cardiac function in small animals.

Scanning protocols dedicated to smart velocity ranging in spectral OCT

We introduce a new type of scanning protocols, called segmented protocols, which enable extracting multi-range flow velocity information from a single Spectral OCT data set. The protocols are evaluated using a well defined flow in a glass capillary. As an example of in vivo studies, we demonstrate two- and three-dimensional imaging of the retinal vascular system in the eyes of healthy volunteers. The flow velocity detection is performed using a method of Joint Spectral and Time domain OCT. Velocity ranging is demonstrated in imaging of retinal vasculature in the macular region and in the optic disk area characterized by different flow velocity values. Additionally, an enhanced visualization of retinal capillary network is presented in the close proximity to macula.

Dual-beam Fourier domain optical Doppler tomography of zebrafish

We have developed a dual-beam Fourier domain optical Doppler tomography (FD-ODT) system to image zebrafish (Danio rerio) larvae. Two beams incident on the zebrafish with a fixed angular separation allow absolute blood flow velocity measurement to be made regardless of vessel orientation in a sagittal plane along which the heart and most of the major vasculature lie. Two spectrometers simultaneously acquire spectra from two interferometers with a typical (maximum) line rate of 18 (28) kHz. The system was calibrated using diluted milk and microspheres and a 0.5-mm thick flow cell. The average deviation from the set velocity from 1.4 to 34.6 mm/s was 4.1%. Three-dimensional structural raster videos were acquired of an entire fish, and through the head, heart, and upper tail of the fish. Coarse features that were resolved include the telencephalon, retina, both heart chambers (atrium and ventricle), branchial arches, and notochord. Other fine structures within these organs were also resolved. Zebrafish are an important tool for high-throughput screening of new pharmacological agents. The ability to generate high-resolution three-dimensional structural videos and accurately measure absolute flow rates in major vessels with FD-ODT provides researchers with additional metrics by which the efficacy of new drugs can be assessed.

Measurement of absolute flow velocity vector using dual-angle, delay-encoded Doppler optical coherence tomography

Single-beam laser Doppler measurements of flow velocity are only sensitive to the velocity component parallel to the optical axis. We describe a simple modification to a standard Doppler optical coherence tomography (OCT) system using a single sample beam that provides velocity information from multiple angles within the beam. By introducing a glass plate midway into the OCT beam path, the sample beam is divided into several components, each with a different group delay and each providing a separate interferogram with its own effective Doppler angle. By combining the Doppler shift measured in each of these component interferograms, the flow velocity vector is fully determined.

Direct bidirectional angle-insensitive imaging of the flow signal intensity in Doppler optical coherence tomography

We introduce a new method, to our knowledge, for direct detection of flow signal intensity by stationary target rejection. In our system, two delay lines are constructed with identical scanning speed and ranging depth. One delay line is used for depth ranging as well as phase modulation, and the other one acts as a full-range retroreflector (FRRR). The signal from this FRRR carries the overall features of local phase modulation, and it is used as the local oscillator for coherent demodulation. With this setup, stationary targets can be rejected at a 4-kHz high-pass cutoff frequency of the filter that follows the demodulator, compared with 20 kHz for conventional fixed-frequency demodulation. This technique features angle insensitivity and provides flow direction as well by implementing standard in-phase and quadrature detection. Besides the direct directional detection of flow signal intensity, flow speed information can be acquired with postprocessing.

Digital signal processor-based real-time optical Doppler tomography system

We present a real-time data-processing and display unit based on a custom-designed digital signal processor (DSP) module for imaging tissue structure and Doppler blood flow. The DSP module is incorporated into a conventional optical coherence tomography system. We also demonstrate the flexibility of embedding advanced Doppler processing algorithms in the DSP module. Two advanced velocity estimation algorithms previously introduced by us are incorporated in this DSP module. Experiments on Intralipid flow demonstrate that a pulsatile flow of several hundred pulses per minute can be faithfully captured in M-scan mode by this DSP system. In vivo imaging of a rat's abdominal blood flow is also presented.