Optical clearing of flowing blood using dextrans with spectral domain optical coherence tomography

Comparison of the effects of contrast medium and low-molecular-weight dextran on coronary optical coherence tomographic imaging in relatively complex coronary lesions

Background

Optical coherence tomography (OCT) has gained increasing popularity in coronary artery intervention due to its high resolution and excellent tissue correlation as a novel intravascular imaging modality. However, the current use of OCT requires contrast agent injection for imaging, and excessive use of contrast agents may adversely affect renal function, exacerbate cardiac burden, and even lead to contrast agent-induced nephropathy and heart failure. In recent years, several researchers have proposed the use of low molecular weight dextran (LMWD) as a substitute for contrast agents in OCT imaging because of its low toxicity, low cost, and wide availability. However, the inclusion of lesions in these studies is relatively simple, and the image quality criteria remain to be optimized.

Methods

This study included 26 patients with coronary artery disease who were scheduled for OCT imaging in a real-world clinical practice involving various complex lesions. All patients underwent two OCT examinations at the same vascular site, one each using contrast agent and LMWD. Both contrast media and LMWDs were infused by an autoinjector. The primary endpoint of the study was the average image quality score. Secondary endpoints included clear image length, clear image segments, minimum lumen area, average lumen area, and contrast-induced nephropathy, among others.

Results

In terms of image clarity, the average image quality score was similar when comparing contrast media with LMWD (3.912 ± 0.175 vs. 3.769 ± 0.392, P = 0.071). The lengths of the clear images and the segments of the clear images were also similar between the two groups (50.97 ± 16.25 mm vs. 49.12 ± 18.15 mm, P = 0.110; 255.5 ± 81.29 vs. 250.5 ± 89.83, P = 0.095). Additionally, strong correlations were noted between the two flushing solutions regarding the minimum lumen area and mean lumen area. During their hospital stay, none of the patient exhibited deterioration in renal function, and no patient experienced any major adverse cardiovascular events.

Conclusions

The quality of coronary artery OCT imaging using LMWD may be comparable to that achieved with traditional contrast agents, even in real-world clinical practice involving various complex lesions. For high-risk patients, LMWD may serve as an excellent substitute for contrast agents in OCT examinations.

Characterization of radiofrequency ablated myocardium with optical coherence tomography

Certain types of cardiac arrhythmias are best treated with radiofrequency (RF) ablation, in which an electrode is inserted into the targeted area of the myocardium and then RF electrical current is applied to heat and destroy surrounding tissue. The resulting ablation lesion usually consists of a coagulative necrotic core surrounded by a rim region of mixed viable and non-viable cells. The characterization of the RF ablated lesion is of potential clinical importance. Here we aim to elaborate optical coherence tomography (OCT) imaging for the characterization of RF-ablated myocardial tissue. In particular, the underlying principles of OCT and its polarization-sensitive counterpart (PS-OCT) are presented, followed by the knowledge needed to interpret their optical images. Studies focused on real-time monitoring of RF lesion formation in the myocardium using OCT systems are summarized. The design and development of various hybrid probes incorporating both OCT guidance and RF ablation catheters are also discussed. Finally, the challenges related to the transmission of OCT imaging systems to cardiac clinics for real-time monitoring of RF lesions are outlined.

Phase transitions and mechanisms of cryoprotection of serum-/xeno-free media based on dextran and dimethyl sulfoxide

The development of serum-/xeno-free media may help avoid the drawbacks of using serum and its components, such as probable contamination, instability of composition, or difficulty in sterilization. The objectives of this research were to investigate the use of combinations of a permeating cryoprotective agent (Me₂SO) and non-permeating (polyvinyl alcohol, polyvinylpyrrolidone, polyethylene glycol, hydroxyethyl starch, dextran) polymers for cryopreservation of interstitial cells (ICs) of rat testis, and to propose the mechanism of cryoprotection of such compositions. In the course of this study, the best combination was 100 mg/ml dextran (M.m. 40 kDa) (Dex40) with 0.7 M Me₂SO in Ham's F12. The ICs were additionally cooled and warmed to different end temperatures (-30, -50, -50 and -196 °C) to determine which temperature intervals contributed most to the IC loss. Then, the cryoprotective action of this serum-/xeno-free medium was investigated in comparison with serum or albumin-containing media by differential scanning calorimetry (DSC) and thermomechanical analysis (TMA). The results showed that the medium based on Dex40 did not decrease the amount of ice formed. However, it could undergo other phase separation and phase transformation to form glassy states. Potential cell-damaging physical processes such as eutectic crystallization/melting, recrystallization of NaCl and/or Me₂SO derivatives, found in serum-containing media and taking place in specific temperature intervals, were not observed in the Dex40 based media. This was in good correlation with indicators of cell survival. Additionally, the application of Dex40 allowed using Me₂SO in lower concentrations (0.7 M) than required for serum-containing media (1.4 M), which may decrease the toxicity of serum-/xeno-free media.

Measurement of tissue optical properties in the context of tissue optical clearing

Nowadays, dynamically developing optical (photonic) technologies play an ever-increasing role in medicine. Their adequate and effective implementation in diagnostics, surgery, and therapy needs reliable data on optical properties of human tissues, including skin. This paper presents an overview of recent results on the measurements and control of tissue optical properties. The issues reported comprise a brief review of optical properties of biological tissues and efficacy of optical clearing (OC) method in application to monitoring of diabetic complications and visualization of blood vessels and microcirculation using a number of optical imaging technologies, including spectroscopic, optical coherence tomography, and polarization- and speckle-based ones. Molecular modeling of immersion OC of skin and specific technique of OC of adipose tissue by its heating and photodynamic treatment are also discussed.

Dextran or Saline Can Replace Contrast for Intravascular Optical Coherence Tomography in Lower Extremity Arteries

PURPOSE

To examine the hypothesis that alternative flush media could be used for lower extremity optical coherence tomography (OCT) imaging in long lesions that would normally require excessive use of contrast.

METHODS

The OPTical Imaging Measurement of Intravascular Solution Efficacy (OPTIMISE) trial was a single-center, prospective study (ClinicalTrials.gov identifier NCT01743872) that enrolled 23 patients (mean age 68±11 years; 14 men) undergoing endovascular intervention involving the superficial femoral artery. Four flush media (heparinized saline, dextran, carbon dioxide, and contrast) were used in succession in random order for each image pullback. Quality was defined as ≥270° visualization of vessel wall layers from each axial image. Mean proportions (± standard deviation) of image quality for each flush medium were assessed using 1-way analysis of variance and are reported with the 95% confidence intervals (CI).

RESULTS

Four OCT catheters failed, leaving 19 patients who completed the OCT imaging protocol; from this cohort, 51 highest quality runs were selected for analysis. Average vessel diameter was 3.99±1.01 mm. OCT imaging allowed 10- to 15-μm resolution of the lumen border, with diminishing quality as vessel diameter increased. Plaque characterization revealed fibrotic lesions. Mean proportions of image quality were dextran 87.2%±12% (95% CI 0.81 to 0.94), heparinized saline 74.3%±24.8% (95% CI 0.66 to 0.93), contrast 70.1%±30.5% (95% CI 0.52 to 0.88), and carbon dioxide 10.0%±10.4% (95% CI 0.00 to 0.26). Dextran, saline, and contrast provided better quality than carbon dioxide (p<0.001).

CONCLUSION

OCT is feasible in peripheral vessels <5 mm in diameter. Dextran or saline flush media can allow lesion characterization, avoiding iodinated contrast. Carbon dioxide is inadequate for peripheral OCT imaging. Axial imaging may aid in enhancing durability of peripheral endovascular interventions.

Dynamic and quantitative assessment of blood coagulation using optical coherence elastography

Reliable clot diagnostic systems are needed for directing treatment in a broad spectrum of cardiovascular diseases and coagulopathy. Here, we report on non-contact measurement of elastic modulus for dynamic and quantitative assessment of whole blood coagulation using acoustic radiation force orthogonal excitation optical coherence elastography (ARFOE-OCE). In this system, acoustic radiation force (ARF) is produced by a remote ultrasonic transducer, and a shear wave induced by ARF excitation is detected by the optical coherence tomography (OCT) system. During porcine whole blood coagulation, changes in the elastic property of the clots increase the shear modulus of the sample, altering the propagating velocity of the shear wave. Consequently, dynamic blood coagulation status can be measured quantitatively by relating the velocity of the shear wave with clinically relevant coagulation metrics, including reaction time, clot formation kinetics and maximum shear modulus. The results show that the ARFOE-OCE is sensitive to the clot formation kinetics and can differentiate the elastic properties of the recalcified porcine whole blood, blood added with kaolin as an activator, and blood spiked with fibrinogen.

Effect of fibrinogen on blood coagulation detected by optical coherence tomography

Our previous work demonstrated that an optical coherence tomography (OCT) technique and the parameter 1/e light penetration depth (d1/e) were able to characterize the whole blood coagulation process in contrast to existing optical tests that are performed on plasma samples. To evaluate the feasibility of the technique for quantifying the effect of fibrinogen (Fbg) on blood coagulation, a dynamic study of d1/e of blood in various Fbg concentrations was performed in static state. Two groups of blood samples of hematocrit (HCT) in 35, 45, and 55% were reconstituted of red blood cells with: 1) treated plasma with its intrinsic Fbg removed and commercial Fbg added (0-8 g L(-1)); and 2) native plasma with commercial Fbg added (0-8 g L(-1)). The results revealed a typical behavior due to coagulation induced by calcium ions and the clotting time is Fbg concentration-dependent. The clotting time was decreased by the increasing amount of Fbg in both groups. Besides, the blood of lower HCT with various levels of Fbg took shorter time to coagulate than that of higher HCT. Consequently, the OCT method is a useful and promising tool for the detection of blood-coagulation processes induced with different Fbg levels.

Optimal flushing agents for integrated optical and acoustic imaging systems

An increasing number of integrated optical and acoustic intravascular imaging systems have been developed and hold great promise for accurately diagnosing vulnerable plaques and guiding atherosclerosis treatment. However, in any intravascular environment, the vascular lumen is filled with blood, a high-scattering source for optical and high-frequency ultrasound signals. Blood must be flushed away to provide clearer images. To our knowledge, no research has been performed to find the ideal flushing agent for combined optical and acoustic imaging techniques. We selected three solutions as potential flushing agents for their image-enhancing effects: mannitol, dextran, and iohexol. Testing of these flushing agents was performed in a closed-loop circulation model and in vivo on rabbits. We found that a high concentration of dextran was the most useful for simultaneous intravascular ultrasound and optical coherence tomography imaging.

In vitro assessment of effects of hyperglycemia on the optical properties of blood during coagulation using optical coherence tomography

No published reports have demonstrated the capability of the optical coherence tomography technique for quantifying the optical coherence tomography signal slope, 1/e light penetration depth, and attenuation coefficient of hyperglycemic blood by an in vitro assessment. The purpose of this study was to investigate the effects of hyperglycemia on optical properties during in vitro blood coagulation by optical coherence tomography. Normal whole blood acted as the control group. After 1-h coagulation, the average optical coherence tomography signal slope decreased approximately 23.3 and 16.7%, and the 1/e light penetration depths increased approximately 21.5 and 19.2% for the control and hyperglycemic groups, respectively. It could be seen from the 1/e light penetration depth evolution curves that the blood coagulation time was about (425 ± 19) s for normal whole blood and (367 ± 15) s for the hyperglycemic blood. The coagulation time decreased 13.6% for the hyperglycemic blood compared with that for normal whole blood. There was statistically significant difference in blood coagulation time between the hyperglycemic and normal whole blood (p < 0.05). The results suggested that hyperglycemia has a procoagulant effect. Our experiment was the first reported study of monitoring hyperglycemic blood coagulation using OCT. We conclude that OCT is potential technique to quantify and follow the liquid-gel transition of hyperglycemic blood coagulation.

Optical clearing at cellular level

Strong light scattering in tissues and blood reduces the usability of many optical techniques. By reducing scattering, optical clearing enables deeper light penetration and improves resolution in several optical imaging applications. We demonstrate the usage of optical tweezers and elastic light scattering to study optical clearing [one of the major mechanisms-matching of refractive indices (RIs)] at the single particle and cell level. We used polystyrene spheres and human red blood cells (RBCs) as samples and glycerol or glucose water solutions as clearing agents. Optical tweezers kept single microspheres and RBCs in place during the measurement of light scattering patterns. The results show that optical clearing reduces the scattering cross section and increases g. Glucose also decreased light scattering from a RBC. Optical clearing affected the anisotropy factor g of 23.25-μm polystyrene spheres, increasing it by 0.5% for an RI change of 2.2% (20% glycerol) and 0.3% for an RI change of 1.1% (13% glucose).

Evaluation of optical coherence tomography for the measurement of the effects of activators and anticoagulants on the blood coagulation in vitro

Optical properties of human blood during coagulation were studied using optical coherence tomography (OCT) and the parameter of clotting time derived from the 1/e light penetration depth (d(1/e)) versus time was developed in our previous work. In this study, in order to know if a new OCT test can characterize the blood-coagulation process under different treatments in vitro, the effects of two different activators (calcium ions and thrombin) and anticoagulants, i.e., acetylsalicylic acid (ASA, a well-known drug aspirin) and melagatran (a direct thrombin inhibitor), at various concentrations are evaluated. A swept-source OCT system with a 1300 nm center wavelength is used for detecting the blood-coagulation process in vitro under a static condition. A dynamic study of d1/e reveals a typical behavior due to coagulation induced by both calcium ions and thrombin, and the clotting time is concentration-dependent. Dose-dependent ASA and melagatran prolong the clotting times. ASA and melagatran have different effects on blood coagulation. As expected, melagatran is much more effective than ASA in anticoagulation by the OCT measurements. The OCT assay appears to be a simple method for the measurement of blood coagulation to assess the effects of activators and anticoagulants, which can be used for activator and anticoagulant screening.

Characterization of flowing blood optical property under various fibrinogen levels using optical coherence tomography

The feasibility of characterization of human blood fibrinogen levels using optical coherence tomography (OCT) was investigated. Three groups of blood samples were reconstituted of red blood cells: 1) phosphate-buffered saline; 2) plasma with its intrinsic fibrinogen removed and commercial fibrinogen added; and 3) native plasma with various fibrinogen levels (0-12 g/L). OCT signal slope (OCTSS) of blood was extracted from OCT depth-reflectivity profiles. Effects of hematocrit (HCT) and blood flow on OCTSS of the blood under various fibrinogen concentrations were also studied. The results of blood flowing at 5 mm/s showed that OCTSS of all the three groups at HCT of 40% decreases with the increasing fibrinogen concentration up to a certain level, i. e., >8, 6, and 4 g/L for groups 1, 2, and 3, respectively. The blood of group 2 at HCTs of 30%, 40%, and 50% had a rapid decrease in OCTSS in the range of fibrinogen concentration of 0-2, 0-6, and 0-10 g/L, respectively. OCTSS value of blood flowing at 2.5 mm/s was lower than that at 5 mm/s at each fibrinogen concentration. In conclusion, OCTSS has a strong correlation with plasma fibrinogen concentration, and has the potential to identify the abnormal level of human blood fibrinogen.

Optical coherence tomography to investigate optical properties of blood during coagulation

This study investigates the optical properties of human blood during the coagulation process under statics using optical coherence tomography (OCT). OCT signal slope (OCTSS) and 1∕e light penetration depth (d(1∕e)) were obtained from the profiles of reflectance versus depth. Results showed that both OCTSS and d(1∕e) were able to sensitively differentiate various stages of blood properties during coagulating. After 1 h clotting, OCTSS decreased by 47.0%, 15.0%, 13.7%, and 8.5% and d(1∕e) increased by 34.7%, 29.4%, 24.3%, and 22.9% for the blood samples at HCT of 25%, 35%, 45%, and 55%, respectively. The slope of d(1∕e) versus time (S(r), ×10(-4) mm∕s), associated with clot formation rate decreased from 6.0 ± 0.3, 3.7 ± 0.5 to 2.3 ± 0.4 with the increasing of HCT from 35%, 45%, to 55%. The clotting time (t(c)) from the d(1∕e) evolution curves was estimated to be 1969 ± 92 s, 375 ± 12 s, 455 ± 11 s, and 865 ± 47 s for the blood of 25%, 35%, 45%, and 55%. This study demonstrates that the parameters (t(c) and S(r)) from the variations in d(1∕e) had better sensitivity and smaller standard deviation. Furthermore, blood hematocrit affecting backscattering properties of blood during coagulation was capable of being discerned by OCT parameters. It is concluded that OCT is a potential technique to quantify and follow the liquid-gel transition of blood during clotting.

Velocity variation assessment of red blood cell aggregation with spectral domain Doppler optical coherence tomography

We propose spectral domain Doppler optical coherence tomography (SD-D-OCT) to qualitatively measure red blood cell aggregation. Variance/standard deviation (SD) of the Doppler frequency spectrum in Doppler variance imaging of flowing blood under shearing conditions was developed as a new aggregation index. In in vitro microchannel-flow experiments, porcine blood at various hematocrits with aggregation characteristics induced by dextran 500 or at the presence of plasma fibrinogen was measured by a SD-D-OCT system with a spectrum centered at 1310 nm. The effects of shear rate, hematocrit, aggregation level on the SD values were investigated. The results demonstrate that Doppler variance imaging using the SD value was capable of differentiating the normal and the aggregated blood with hematocrits of 30-55% flowing at a shear rate of 40-60 s(-1). The SD value was found to be nonsignificant hematocrit-dependent. It is also a sensitive and repeatable aggregation index for comparison between nonaggregated and aggregated blood samples.

Feasibility of Doppler variance imaging for red blood cell aggregation characterization

An abnormal level of red blood cell (RBC) aggregation is a clinical condition associated with many pathologies. Our purpose is to investigate the feasibility of spectral domain phase-resolved optical Doppler tomography incorporated with Doppler variance imaging to characterize RBC aggregation in flowing blood. Variance/standard deviation (SD) of the Doppler frequency spectrum of porcine blood flowing through a glass tube (diameter 300 microm) at 4.7 mms is measured. For nonaggregating RBCs with hematocrits of 20, 40, and 60%, the mean centerline SD values were 34.2+/-1.2, 47.8+/-1.0, and 47.2+/-0.5, respectively. After dextran 500 induction, the SD value decreased significantly to 29.9+/-0.7, 37.9+/-3.0, and 40.1+/-0.6 (P<0.01) in the aggregated blood, respectively. The results demonstrate that Doppler variance imaging using the SD value is capable of differentiating normal and aggregated blood.

Virtual histology of the human heart using optical coherence tomography

Optical coherence tomography (OCT) allows for the visualization of micron-scale structures within nontransparent biological tissues. For the first time, we demonstrate the use of OCT in identifying components of the cardiac conduction system and other structures in the explanted human heart. Reconstructions of cardiac structures up to 2 mm below the tissue surface were achieved and validated with Masson Trichrome histology in atrial, ventricular, sinoatrial nodal, and atrioventricular nodal preparations. The high spatial resolution of OCT provides visualization of cardiac fibers within the myocardium, as well as elements of the cardiac conduction system; however, a limiting factor remains its depth penetration, demonstrated to be approximately 2 mm in cardiac tissues. Despite its currently limited imaging depth, the use of OCT to identify the structural determinants of both normal and abnormal function in the intact human heart is critical in its development as a potential aid to intracardiac arrhythmia diagnosis and therapy.

Retention of polarization signatures in SHG microscopy of scattering tissues through optical clearing

Polarization responses in Second Harmonic Generation (SHG) imaging microscopy are a valuable method to quantify aspects of tissue structure, and may be a means to differentiate normal and diseased tissues. Due to multiple scattering, the polarization data is lost in turbid tissues. Here we investigate if this information can be retained through the use of optical clearing which greatly reduces the scattering coefficient and increases the corresponding mean free path. To this end, we have measured the SHG intensity as a function of laser polarization and the SHG signal anisotropy in murine tendon and striated muscle over a depth range of 200 microns. We find that the laser polarization is highly randomized in the uncleared tissues at depths corresponding to only 2-3 scattering collisions (50- 10 microns). This depolarization of the laser is also reflected in the randomized anisotropy of the SHG signal as it is created over a range of polarization states. In strong contrast, both polarization signatures are significantly retained through ~200 microns of tissue thickness following treatment with 50% glycerol. Moreover, the measured polarization responses for both tendon and striated muscle are consistent with the extent of reduction of the respective scattering coefficients upon clearing. We suggest the method will be applicable to SHG imaging of connective disorders as well as cancer through several hundred microns of extracellular matrix.