Theoretical investigation of measuring cerebral blood flow in the adult human head using bolus Indocyanine Green injection and near-infrared spectroscopy

Indocyanine green for targeted imaging of the gall bladder and fluorescence navigation

Researchers nowadays have devoted extra attention to the different biomedical applications of indocyanine green (ICG), a US Food and Drug Administration-approved fluorescent compound in the fields such as drug delivery, medical imaging and disease diagnosis. In addition, hepatic function evaluation could be conducted by using ICG before surgical procedures and angiographic assessment of blood. Therefore, ICG will be expected to be excellent imaging and targeting agent in various preclinical and clinical model systems. However, whether ICG possesses the potential for the gall bladder's intraoperative imaging guidance needs to be further explored in vivo animal experiments. Herein, near-infrared fluorophores ICG can display the specific uptake by the gall bladder cells and tissues. The dynamic process of biodistribution and the clearance of ICG in vivo in mice are clearly shown in real-time live-body imaging. Furthermore, ICG was rapidly excreted into the bile and lately biodistributed to the stomach after treatment in mice. Meanwhile, the signal-to-background ratio of the gall bladder demonstrated a tremendously higher level compared to other organs (stomach, heart, liver, lung, pancreas, spleen, intestine and duodenum). In conclusion, fluorescence navigation using ICG fluorescence imaging will provide good visualization and detection of the target lesions (gall bladder) in clinics such as diagnostic medical imaging and intraoperative navigation.

Automated in vivo Assessment of Vascular Response to Radiation using a Hybrid Theranostic X-ray Irradiator/Fluorescence Molecular Imaging System

Hypofractionated stereotactic body radiotherapy treatments (SBRT) have demonstrated impressive results for the treatment of a variety of solid tumors. The role of tumor supporting vasculature damage in treatment outcome for SBRT has been intensely debated and studied. Fast, non-invasive, longitudinal assessments of tumor vasculature would allow for thorough investigations of vascular changes correlated with SBRT treatment response. In this paper, we present a novel theranostic system which incorporates a fluorescence molecular imager into a commercial, preclinical, microCT-guided, irradiator and was designed to quantify tumor vascular response (TVR) to targeted radiotherapy. This system overcomes the limitations of single-timepoint imaging modalities by longitudinally assessing spatiotemporal differences in intravenously-injected ICG kinetics in tumors before and after high-dose radiation. Changes in ICG kinetics were rapidly quantified by principle component (PC) analysis before and two days after 10 Gy targeted tumor irradiation. A classifier algorithm based on PC data clustering identified pixels with TVR. Results show that two days after treatment, a significant delay in ICG clearance as measured by exponential decay (40.5±16.1% P=0.0405 Paired t-test n=4) was observed. Changes in the mean normalized first and second PC feature pixel values (PC1 & PC2) were found (P=0.0559, 0.0432 paired t-test), suggesting PC based analysis accurately detects changes in ICG kinetics. The PC based classification algorithm yielded spatially-resolved TVR maps. Our first-of-its-kind theranostic system, allowing automated assessment of TVR to SBRT, will be used to better understand the role of tumor perfusion in metastasis and local control.

Lawrence K, Amendolia O, Quattrone F, Balu R, Kofke WA, Yodh AG. Noninvasive continuous optical monitoring of absolute cerebral blood flow in critically ill adults

We investigate a scheme for noninvasive continuous monitoring of absolute cerebral blood flow (CBF) in adult human patients based on a combination of time-resolved dynamic contrast-enhanced near-infrared spectroscopy (DCE-NIRS) and diffuse correlation spectroscopy (DCS) with semi-infinite head model of photon propogation. Continuous CBF is obtained via calibration of the DCS blood flow index (BFI) with absolute CBF obtained by intermittent intravenous injections of the optical contrast agent indocyanine green. A calibration coefficient ( γ ) for the CBF is thus determined, permitting conversion of DCS BFI to absolute blood flow units at all other times. A study of patients with acute brain injury ( N = 7 ) is carried out to ascertain the stability of γ . The patient-averaged DCS calibration coefficient across multiple monitoring days and multiple patients was determined, and good agreement between the two calibration coefficients measured at different times during single monitoring days was found. The patient-averaged calibration coefficient of 1.24 × 10 9    ( mL / 100    g / min ) / ( cm 2 / s ) was applied to previously measured DCS BFI from similar brain-injured patients; in this case, absolute CBF was underestimated compared with XeCT, an effect we show is primarily due to use of semi-infinite homogeneous models of the head.

Cerebral blood flow and autoregulation: current measurement techniques and prospects for noninvasive optical methods

Cerebral blood flow (CBF) and cerebral autoregulation (CA) are critically important to maintain proper brain perfusion and supply the brain with the necessary oxygen and energy substrates. Adequate brain perfusion is required to support normal brain function, to achieve successful aging, and to navigate acute and chronic medical conditions. We review the general principles of CBF measurements and the current techniques to measure CBF based on direct intravascular measurements, nuclear medicine, X-ray imaging, magnetic resonance imaging, ultrasound techniques, thermal diffusion, and optical methods. We also review techniques for arterial blood pressure measurements as well as theoretical and experimental methods for the assessment of CA, including recent approaches based on optical techniques. The assessment of cerebral perfusion in the clinical practice is also presented. The comprehensive description of principles, methods, and clinical requirements of CBF and CA measurements highlights the potentially important role that noninvasive optical methods can play in the assessment of neurovascular health. In fact, optical techniques have the ability to provide a noninvasive, quantitative, and continuous monitor of CBF and autoregulation.

Optimization of the method for assessment of brain perfusion in humans using contrast-enhanced reflectometry: multidistance time-resolved measurements

The aim of the study was to determine optimal measurement conditions for assessment of brain perfusion with the use of optical contrast agent and time-resolved diffuse reflectometry in the near-infrared wavelength range. The source-detector separation at which the distribution of time of flights (DTOF) of photons provided useful information on the inflow of the contrast agent to the intracerebral brain tissue compartments was determined. Series of Monte Carlo simulations was performed in which the inflow and washout of the dye in extra- and intracerebral tissue compartments was modeled and the DTOFs were obtained at different source-detector separations. Furthermore, tests on diffuse phantoms were carried out using a time-resolved setup allowing the measurement of DTOFs at 16 source-detector separations. Finally, the setup was applied in experiments carried out on the heads of adult volunteers during intravenous injection of indocyanine green. Analysis of statistical moments of the measured DTOFs showed that the source-detector separation of 6 cm is recommended for monitoring of inflow of optical contrast to the intracerebral brain tissue compartments with the use of continuous wave reflectometry, whereas the separation of 4 cm is enough when the higher-order moments of DTOFs are available.

Sensitivity of near-infrared spectroscopy and diffuse correlation spectroscopy to brain hemodynamics: simulations and experimental findings during hypercapnia

Near-infrared spectroscopy (NIRS) and diffuse correlation spectroscopy (DCS) are two diffuse optical technologies for brain imaging that are sensitive to changes in hemoglobin concentrations and blood flow, respectively. Measurements for both modalities are acquired on the scalp, and therefore hemodynamic processes in the extracerebral vasculature confound the interpretation of cortical hemodynamic signals. The sensitivity of NIRS to the brain versus the extracerebral tissue and the contrast-to-noise ratio (CNR) of NIRS to cerebral hemodynamic responses have been well characterized, but the same has not been evaluated for DCS. This is important to assess in order to understand their relative capabilities in measuring cerebral physiological changes. We present Monte Carlo simulations on a head model that demonstrate that the relative brain-to-scalp sensitivity is about three times higher for DCS (0.3 at 3 cm) than for NIRS (0.1 at 3 cm). However, because DCS has higher levels of noise due to photon-counting detection, the CNR is similar for both modalities in response to a physiologically realistic simulation of brain activation. Even so, we also observed higher CNR of the hemodynamic response during graded hypercapnia in adult subjects with DCS than with NIRS.

Intraoperative assessment of human spinal cord perfusion using near infrared spectroscopy with indocyanine green tracer technique

BACKGROUND CONTEXT

Despite the significant interest in the assessment of human cerebral perfusion, investigations into human spinal cord perfusion (SCP) are scarce. Current intraoperative monitoring of spinal cord relies on the assessment of neural conduction as a surrogate for SCP. However, there are various inherent limitations associated with the use of these techniques. Near infrared spectroscopy (NIRS) has been successfully used for monitoring and assessment of human cerebral perfusion and has shown promising results in intraoperative assessment of SCP in animal models.

PURPOSE

The aim of this study was to investigate whether it is possible to monitor physiological changes in human SCP intraoperatively using NIRS with indocyanine green (ICG) tracer technique. We used this technique to calculate the human spinal cord carbon dioxide (CO₂) reactivity index. In addition, we investigated whether the lamina causes significant attenuation of NIRS signals.

STUDY DESIGN/SETTING

Intraoperative human experimental study.

PATIENT SAMPLE

Eighteen patients undergoing elective posterior cervical spine surgery.

OUTCOME MEASURES

Carbon dioxide reactivity of human SCP.

METHODS

Nine patients underwent transdural assessment of SCP, with an additional nine patients undergoing translaminar measurements. Patients' SCP was continuously monitored using an NIRO-500 NIRS monitor via a set of purpose built optodes. Their arterial ICG concentration was simultaneously assessed using a pulse dye densitometer. Patients' end-tidal CO₂ was gradually increased by 7.5 mm Hg and then returned back to baseline. Three sets of measurements were taken: baseline, hypercapnic, and return to baseline.

RESULTS

After hypercapnia, SCP increased by a mean of 57.2 ± 23.3% in the transdural group and 46.6 ± 36.3% in the translaminar group. Carbon dioxide reactivity index was 7.6 ± 3.2%ΔSCP/mm Hg in the transdural group and 6.4 ± 5.3 %ΔSCP/mm Hg in the translaminar group. There was no significant difference in the increase in SCP (p=.475) or the CO₂ reactivity index (p=.581) observed between the transdural and the translaminar groups.

CONCLUSIONS

Intraoperative NIRS with ICG tracer technique can identify an increase in the SCP in response to hypercapnia. It is possible to use this technique for monitoring SCP over the dura and the lamina. This technique could potentially be used to provide insight in to the pathophysiology and autoregulation of commonly acquired spinal cord conditions. Further research assessing the use of NIRS for monitoring of SCP is required.

An algorithm for assessment of inflow and washout of optical contrast agent to the brain by analysis of time-resolved diffuse reflectance and fluorescence signals

In optical measurements of the brain oxygenation and perfusion the problem of contamination of the signals with the components related to the extracerebral tissues remains an obstacle limiting clinical applicability of the technique. In this paper we present an algorithm allowing for derivation of signals related to the changes in absorption in the intracerebral tissues based on analysis of time-resolved diffuse reflectance and fluorescence. The proposed method was validated in series of Monte Carlo simulations in which inflow and washout of an optical contrast agent into the two-layered human head model was considered. It was shown that the decomposed intracerebral component of the signal can be derived with uncertainty of about 5%. This result suggests that the method proposed can be applied in improved estimation of brain perfusion parameters based on the bolus-tracking technique.

A review of indocyanine green fluorescent imaging in surgery

The purpose of this paper is to give an overview of the recent surgical intraoperational applications of indocyanine green fluorescence imaging methods, the basics of the technology, and instrumentation used. Well over 200 papers describing this technique in clinical setting are reviewed. In addition to the surgical applications, other recent medical applications of ICG are briefly examined.

Quantitative investigation of the effect of the extra-cerebral vasculature in diffuse optical imaging: a simulation study

Diffuse optical imaging (DOI) is a non invasive technique allowing the recovery of hemodynamic changes in the brain. Due to the diffusive nature of photon propagation in turbid media and the fact that cerebral tissues are located around 1.5 cm under the adult human scalp, DOI measurements are subject to partial volume errors. DOI measurements are also sensitive to large pial vessels because oxygenated and deoxygenated hemoglobin are the dominant chromophores in the near infrared window. In this study, the effect of the extra-cerebral vasculature in proximity of the sagittal sinus was investigated for its impact on DOI measurements simulated over the human adult visual cortex. Numerical Monte Carlo simulations were performed on two specific models of the human head derived from magnetic resonance imaging (MRI) scans. The first model included the extra-cerebral vasculature in which constant hemoglobin concentrations were assumed while the second did not. The screening effect of the vasculature was quantified by comparing recovered hemoglobin changes from each model for different optical arrays and regions of activation. A correction factor accounting for the difference between the recovered and the simulated hemoglobin changes was computed in each case. The results show that changes in hemoglobin concentration are better estimated when the extra-cerebral vasculature is modeled and the correction factors obtained in this case were at least 1.4-fold lower. The effect of the vasculature was also examined in a high-density diffuse optical tomography configuration. In this case, the difference between changes in hemoglobin concentration recovered with each model was reduced down to 10%.

Optical bedside monitoring of cerebral perfusion: technological and methodological advances applied in a study on acute ischemic stroke

We present results of a clinical study on bedside perfusion monitoring of the human brain by optical bolus tracking. We measure the kinetics of the contrast agent indocyanine green using time-domain near-IR spectroscopy (tdNIRS) in 10 patients suffering from acute unilateral ischemic stroke. In all patients, a delay of the bolus over the affected when compared to the unaffected hemisphere is found (mean: 1.5 s, range: 0.2 s to 5.2 s). A portable time-domain near-IR reflectometer is optimized and approved for clinical studies. Data analysis based on statistical moments of time-of-flight distributions of diffusely reflected photons enables high sensitivity to intracerebral changes in bolus kinetics. Since the second centralized moment, variance, is preferentially sensitive to deep absorption changes, it provides a suitable representation of the cerebral signals relevant for perfusion monitoring in stroke. We show that variance-based bolus tracking is also less susceptible to motion artifacts, which often occur in severely affected patients. We present data that clearly manifest the applicability of the tdNIRS approach to assess cerebral perfusion in acute stroke patients at the bedside. This may be of high relevance to its introduction as a monitoring tool on stroke units.

Quantitative measurement of cerebral blood flow in a juvenile porcine model by depth-resolved near-infrared spectroscopy

Nearly half a million children and young adults are affected by traumatic brain injury each year in the United States. Although adequate cerebral blood flow (CBF) is essential to recovery, complications that disrupt blood flow to the brain and exacerbate neurological injury often go undetected because no adequate bedside measure of CBF exists. In this study we validate a depth-resolved, near-infrared spectroscopy (NIRS) technique that provides quantitative CBF measurement despite significant signal contamination from skull and scalp tissue. The respiration rates of eight anesthetized pigs (weight: 16.2+/-0.5 kg, age: 1 to 2 months old) are modulated to achieve a range of CBF levels. Concomitant CBF measurements are performed with NIRS and CT perfusion. A significant correlation between CBF measurements from the two techniques is demonstrated (r(2)=0.714, slope=0.92, p<0.001), and the bias between the two techniques is -2.83 mL min(-1)100 g(-1) (CI(0.95): -19.63 mL min(-1)100 g(-1)-13.9 mL min(-1)100 g(-1)). This study demonstrates that accurate measurements of CBF can be achieved with depth-resolved NIRS despite significant signal contamination from scalp and skull. The ability to measure CBF at the bedside provides a means of detecting, and thereby preventing, secondary ischemia during neurointensive care.

Biodistribution of encapsulated indocyanine green in healthy mice

Indocyanine green (ICG) is a fluorescent probe used in various optically mediated diagnostic and therapeutic applications. However, utility of ICG remains limited by its unstable optical properties and nonspecific localization. We have encapsulated ICG within electrostatically assembled mesocapsules (MCs) to explore its potential for targeted optical imaging and therapy. In this study, we investigate how the surface coating and size of the MCs influences ICG's biodistribution in vivo. ICG was administered intravenously to Swiss Webster mice as a free solution or encapsulated within either 100 nm diameter MCs coated with dextran; 500 nm diameter MCs coated with dextran; or 100 nm diameter MCs coated with 10 nm ferromagnetic iron oxide nanoparticles, themselves coated with polyethylene glycol. ICG was extracted from harvested blood and organs at various times and its amount quantified with fluorescence measurements. MCs containing ICG accumulated in organs of the reticuloendothelial system, namely, the liver and spleen, as well as the lungs. The circulation kinetics of ICG appeared unaffected by encapsulation; however, the deposition within organs other than the liver suggests a different biodistribution mechanism. Results suggest that the capsules' coating influences their biodistribution to a greater extent than their size. The MC encapsulation system allows for delivery of ICG to organs other than the liver, enabling the potential development of new optical imaging and therapeutic strategies.

Changes in cerebral blood flow after acetazolamide: an experimental study comparing near-infrared spectroscopy and SPECT

BACKGROUND AND PURPOSE

It is important to find a reliable and bedside method, which can estimate the cerebral blood flow (CBF) of patients in clinical settings. Estimation of CBF by calculating a blood flow index (BFI) using continuous wave near-infrared spectroscopy (CW-NIRS) and indocyanine green (ICG) as an i.v. tracer has been proposed to be a feasible and promising method. To validate if the BFI method can detect relative changes in CBF we compared data with the established method (133)Xenon single photon emission computer tomography ((133)Xe-SPECT).

METHODS

Ten healthy subjects were investigated before and after a bolus of acetazolamide. NIRS data were obtained using a multi source detector separation configuration in order to assess a corrected BFI (BFI(corr)) value, which attempts to eliminate contamination of skin blood flow.

RESULTS

Data obtained showed no significant correlation between CBF changes measured by (133)Xe-SPECT and BFI(corr) (0.133, P = 0.732). After acetazolamide, a 49% increase in CBF was detected using the (133)Xe-SPECT method, whereas no changes in any ICG variables were observed after acetazolamide.

CONCLUSION

The study shows that it is not possible to obtain reliable BFI data, which reflect changes in CBF after acetazolamide infusion, using the CW-NIRS and ICG method.

Quantitative analysis of peripheral tissue perfusion using spatiotemporal molecular dynamics

Background

Accurate measurement of peripheral tissue perfusion is challenging but necessary to diagnose peripheral vascular insufficiency. Because near infrared (NIR) radiation can penetrate relatively deep into tissue, significant attention has been given to intravital NIR fluorescence imaging.

Methodology/Principal Findings

We developed a new optical imaging-based strategy for quantitative measurement of peripheral tissue perfusion by time-series analysis of local pharmacokinetics of the NIR fluorophore, indocyanine green (ICG). Time-series NIR fluorescence images were obtained after injecting ICG intravenously in a murine hindlimb ischemia model. Mathematical modeling and computational simulations were used for translating time-series ICG images into quantitative pixel perfusion rates and a perfusion map. We could successfully predict the prognosis of ischemic hindlimbs based on the perfusion profiles obtained immediately after surgery, which were dependent on the preexisting collaterals. This method also reflected increases in perfusion and improvements in prognosis of ischemic hindlimbs induced by treatment with vascular endothelial growth factor and COMP-angiopoietin-1.

Conclusions/Significance

We propose that this novel NIR-imaging-based strategy is a powerful tool for biomedical studies related to the evaluation of therapeutic interventions directed at stimulating angiogenesis.

Monte Carlo algorithm for efficient simulation of time-resolved fluorescence in layered turbid media

We present an efficient Monte Carlo algorithm for simulation of time-resolved fluorescence in a layered turbid medium. It is based on the propagation of excitation and fluorescence photon bundles and the assumption of equal reduced scattering coefficients at the excitation and emission wavelengths. In addition to distributions of times of arrival of fluorescence photons at the detector, 3-D spatial generation probabilities were calculated. The algorithm was validated by comparison with the analytical solution of the diffusion equation for time-resolved fluorescence from a homogeneous semi-infinite turbid medium. It was applied to a two-layered model mimicking intra- and extracerebral compartments of the adult human head.

Dimenhydrinate effect on cerebral oxygen status and salivary chromogranin-A during cognitive tasks

To investigate the effects of dimenhydrinate on cerebral oxygen status (COS; cerebral oxygenated hemoglobin concentration changes) and salivary chromogranin-A (CgA) during a cognitive test battery, a double-blind, placebo-controlled, randomized, crossover protocol was used to examine the effect of 50 mg of dimenhydrinate or placebo in 12 subjects. This test battery includes tests of both reaction time and fundamental cognitive ability and was used in the assessment of pilots. Poor cognitive performance was observed in the subjects taking dimenhydrinate. We used two-channel near-infrared spectroscopy to investigate the effects of dimenhydrinate on the COS. With the one exception of shifting attention task in the left forehead, no significant difference was found between dimenhydrinate and placebo during the tasks of the test battery. Under placebo treatment, on the other hand, CgA levels were significantly elevated during cognitive testing when compared with baseline. However, CgA levels were not significantly elevated above baseline following dimenhydrinate. The present study is one of the first to demonstrate that the first-generation antihistamine drugs altered the responses of salivary CgA during cognitive tasks. The changes in salivary CgA secretion, as a result of dimenhydrinate administration, may serve as a sensitive biomarker of a psychological status such as a drug-induced sedation during the performance of a cognitive test battery. Further studies, however, are required to examine the usefulness of this sensitive biomarker in investigation of psychological agents during cognitive tasks.