Blood Velocity Measurements Research Articles

Merging computational fluid dynamics and 4D Flow MRI using proper orthogonal decomposition and ridge regression

Time resolved phase-contrast magnetic resonance imaging 4D-PCMR (also called 4D Flow MRI) data while capable of non-invasively measuring blood velocities, can be affected by acquisition noise, flow artifacts, and resolution limits. In this paper, we present a novel method for merging 4D Flow MRI with computational fluid dynamics (CFD) to address these limitations and to reconstruct de-noised, divergence-free high-resolution flow-fields. Proper orthogonal decomposition (POD) is used to construct the orthonormal basis of the local sampling of the space of all possible solutions to the flow equations both at the low-resolution level of the 4D Flow MRI grid and the high-level resolution of the CFD mesh. Low-resolution, de-noised flow is obtained by projecting in vivo 4D Flow MRI data onto the low-resolution basis vectors. Ridge regression is then used to reconstruct high-resolution de-noised divergence-free solution. The effects of 4D Flow MRI grid resolution, and noise levels on the resulting velocity fields are further investigated. A numerical phantom of the flow through a cerebral aneurysm was used to compare the results obtained using the POD method with those obtained with the state-of-the-art de-noising methods. At the 4D Flow MRI grid resolution, the POD method was shown to preserve the small flow structures better than the other methods, while eliminating noise. Furthermore, the method was shown to successfully reconstruct details at the CFD mesh resolution not discernible at the 4D Flow MRI grid resolution. This method will improve the accuracy of the clinically relevant flow-derived parameters, such as pressure gradients and wall shear stresses, computed from in vivo 4D Flow MRI data.

Doppler Measurement of Parasternal Aortic Blood Flow Velocity

Echocardiographic Doppler is capable of continuously measuring blood velocity, and stroke volume at rest and during exercise. Doppler measurements require a near parallel angle between the flow of blood and the ultrasound signal necessitating apical cardiac views to obtain accurate absolute values. However, apical cardiac views are technically challenging during long serial recordings. Parasternal views are less challenging, but do not provide a parallel angle to the aorta. If a relationship exists between the aortic Doppler measurements from these two cardiac windows, a correction calculation can be established to estimate stroke volume from the parasternal view. PURPOSE: To determine if a relationship exists between Doppler measurements made at the aorta in the parasternal long axis and apical three chamber echocardiographic views. METHODS: Healthy college age participants underwent Doppler ultrasound measurements at the aorta in the parasternal long axis and apical three chamber view. Linear cross sectional analysis was performed to assess aortic annulus area and Teichholz stroke volume estimates in the parasternal long axis. Only participants with complete data sets are included in this analysis. Bivariate correlations and t-test were used as appropriate to analyze the data. RESULTS: Of the 18 participants studied, only 12 (66.7%; 6 males, 6 females, age 25.5±1.1yrs, height 156±14.2cm, weight 69.8±4.5kg, body fat 20.6±2%) had adequate Doppler signal strength in the parasternal long axis view for software automated signal tracing. No relationship was found between the time averaged mean blood flow velocity measurements obtained in the parasternal long axis and apical three chamber views (r=-0.216, p=0.498). Parasternal long axis Doppler based stroke volume (25.9±4.9mL/beat) measurements were lower than parasternal long axis Teichholz estimated stroke volume (57.8±4.9mL/beat) measurements (p<0.001), with no correlation between the two (r=0.402, p=0.190). CONCLUSIONS: While this data is preliminary, it suggests that aortic Doppler measurements in the parasternal long axis view suffer from low Doppler signal strength, and are inaccurate when compared to more conventional measurements.

Blood Flow Simulation in Patient-Specific Segmented Hepatic Arterial Tree

PurposeSelective Internal Radiation Therapy is an emerging, minimally invasive therapy of liver cancer. Millions of microspheres are injected through a catheter into tumor vascular supply. Microspheres distribution in liver is strongly dependent on patient's arteries geometry and catheter tip positioning, which is currently chosen by the physician based on qualitative image interpretation. A patient-specific numerical simulation of blood flow would have a crucial importance in therapy optimization, allowing in-silico research of microspheres trajectories. Material and methodsWe developed a procedure to segment patient's hepatic arterial vasculature with a Hessian based approach on 3D angiography, and to perform blood flow numerical simulation in the extracted geometry with ANSYS Fluent. In vivo blood velocity measurements were performed using phase contrast MRI to constrain these simulations. ResultsFlow results in the larger vessels are coherent with literature, but very little is known for smaller arteries: simulation results are compared to velocity measures on hepatic arteries down to 2.2 mm of diameter on phase contrast MRI, which gave encouraging results for validation. ConclusionA viable procedure for arteries segmentation and patient-specific blood flow simulation is proposed, phase contrast MRI allowing for tuning and validation of velocity values. The proposed personalized blood flow simulation is compulsory for the simulation of microspheres trajectories in the aim of tumor targeting.

Ultrasound imaging velocimetry with interleaved images for improved pulsatile arterial flow measurements: a new correction method, experimental and in vivo validation.

Blood velocity measurements are important in physiological science and clinical diagnosis. Doppler ultrasound is the most commonly used method but can only measure one velocity component. Ultrasound imaging velocimetry (UIV) is a promising technique capable of measuring two velocity components; however, there is a limit on the maximum velocity that can be measured with conventional hardware which results from the way images are acquired by sweeping the ultrasound beam across the field of view. Interleaved UIV is an extension of UIV in which two image frames are acquired concurrently, allowing the effective interframe separation time to be reduced and therefore increasing the maximum velocity that can be measured. The sweeping of the ultrasound beam across the image results in a systematic error which must be corrected: in this work, we derived and implemented a new velocity correction method which accounts for acceleration of the scatterers. We then, for the first time, assessed the performance of interleaved UIV for measuring pulsatile arterial velocities by measuring flows in phantoms and in vivo and comparing the results with spectral Doppler ultrasound and transit-time flow probe data. The velocity and flow rate in the phantom agreed within 5-10% of peak velocity, and 2-9% of peak flow, respectively, and in vivo the velocity difference was 9% of peak velocity. The maximum velocity measured was 1.8 m s-1, the highest velocity reported with UIV. This will allow flows in diseased arteries to be investigated and so has the potential to increase diagnostic accuracy and enable new vascular research.

Doppler ultrasonography of the pectinis oculi artery in harpy eagles (Harpia harpyja).

Twenty harpy eagles (Harpia harpyja) without systemic or ocular diseases were examined to measure blood velocity parameters of the pectinis oculi artery using Doppler ultrasonography. Pectinate artery resistive index (RI) and pulsatility index (PI) were investigated using ocular Doppler ultrasonography. The mean RI and PI values across all eyes were 0.44±0.10 and 0.62±0.20 respectively. Low RI and PI values found in the harpy eagle´s pectinis oculi artery compared with the American pekin ducks one and other tissue suggest indeed a high metabolic activity in pecten oculi and corroborates the hypothesis of a nutritional function and/or intraocular pressure regulation.

Computer-aided identification of novel ophthalmic artery waveform parameters in healthy subjects and glaucoma patients

Purpose: Arterial waveform parameters (WPs) are commonly used to monitor and diagnose systemic diseases. Color Doppler Imaging (CDI) is a consolidated technique to measure blood velocity profile in some of the major ocular vessels. This study proposes a computer-aided manipulation process of ophthalmic artery (OA) CDI images to classify and quantify WPs that might be significant in the assessment of glaucoma.Methods: Fifty CDI images acquired by four different operators on nine healthy individuals and 38 CDI images of 38 open-angle glaucoma (OAG) patients were considered. An ad-hoc semi-automated image processing code was implemented to detect the digitalized OA velocity waveform and to extract the WPs. Concordance correlation coefficient (CCC), two-sample t-test and Pearson’s correlation coefficient were used to test for similarities, differences and associations among variables.Results: The OA-CDI images manipulation proposed showed a higher concordance between measured peak systolic velocity (PSV) data and extracted PSV data (0.80≤CCC≤0.98) than on end diastolic velocity (EDV) (0.45≤CCC≤0.63) and resistive index (RI) (0.30≤CCC≤0.58) data. In OAG patients, EDV, RI, subendocardial viability ratio (SEVR), period (T), area ratio (f) and normalized distance between ascending and descending limb (DAD/T) were found statistically correlated to at least one of the following factors: gender, age, ocular medications and year of diagnosis. When compared to healthy individuals, OAG patients OA-CDI profiles showed statistically higher values of f (p &lt; 0.001) and DAD/T (p = 0.002) (p-values corrected by age and gender).Conclusion: The proposed computer-aided manipulation of OA-CDI images allowed to identify DAD/T as a novel WP that vary significantly among healthy individuals and OAG patients, and among female and male OAG patients. Future studies on longitudinal OAG data are suggested to investigate the potential of DAD/T to predict severity and progression of the disease.

Introduction to the Special Issue on Flow Measurement and Imaging

The investigation of flow by ultrasound (frequently referred to as Doppler ultrasound) has fascinated several generations of researchers. Thanks to the contributions of pioneers, such as P. N. T. Wells, J. M. Reid, D. W. Baker, P. Peronneau, L. Pourcelot, B. A. J. Angelsen, and others, the feasibility of blood velocity measurements based on the estimate of the phase shift in the received echo-signals has been well established since the 1970s. A first peak of interest in this topic was achieved during the 1980s, when real-time spectral Doppler and color flow imaging could be implemented in most ultrasound machines and rapidly became standard tools in clinical practice. By the end of the century, most aspects of Doppler ultrasound had been definitively clarified by the work of researchers, such as D. H. Evans, R. S. C. Cobbold, A. P. G. Hoeks, and many others.

Contrast enhanced computerized tomography measurement of vascular blood flow

In this work, we study the measurement of blood velocity with contrast enhanced computed tomography. The transport equation is used as a constraint to obtain stable solutions. The inverse problem is formulated as an optimal control problem. The density of the contrast agent is reconstructed together with the flow field. The inversion scheme is tested on a simple phantom. The reconstruction of the velocity is improved but the convergence of the method is slow.

Label free measurement of retinal blood cell flux, velocity, hematocrit and capillary width in the living mouse eye.

Measuring blood cell dynamics within the capillaries of the living eye provides crucial information regarding the health of the microvascular network. To date, the study of single blood cell movement in this network has been obscured by optical aberrations, hindered by weak optical contrast, and often required injection of exogenous fluorescent dyes to perform measurements. Here we present a new strategy to non-invasively image single blood cells in the living mouse eye without contrast agents. Eye aberrations were corrected with an adaptive optics camera coupled with a fast, 15 kHz scanned beam orthogonal to a capillary of interest. Blood cells were imaged as they flowed past a near infrared imaging beam to which the eye is relatively insensitive. Optical contrast of cells was optimized using differential scatter of blood cells in the split-detector imaging configuration. Combined, these strategies provide label-free, non-invasive imaging of blood cells in the retina as they travel in single file in capillaries, enabling determination of cell flux, morphology, class, velocity, and rheology at the single cell level.

Phase-contrast MRI with hybrid one and two-sided flow-encoding and velocity spectrum separation.

To develop and evaluate a phase-contrast MRI (PC-MRI) technique with hybrid one and two-sided flow-encoding and velocity spectrum separation (HOTSPA) for accelerated blood flow and velocity measurement. In the HOTSPA technique, the two-sided flow encoding (FE) is used for two FE directions and one-sided is used for the remaining FE direction. Such a temporal modulation of the FE strategy allows for separations of the Fourier velocity spectrum into components for the flow-compensated and the three-directional velocity waveforms, accelerating PC-MRI by encoding three-directional velocities using only two repetition times (TRs) instead of four TRs as in standard PC-MRI. The HOTSPA was evaluated and compared with standard PC-MRI in the common carotid arteries of six healthy volunteers. Total volumetric flow and peak velocity measurements based on HOTSPA and the conventional PC-MRI were in good agreement with a bias of -0.005 mL (-0.1% relative bias error) for total volumetric flow and 1.21 cm/s (1.1% relative bias error) for peak velocity, although the total acquisition time was 50% of the conventional PC-MRI. The proposed HOTSPA technique achieved nearly two-fold acceleration of PC-MRI while maintaining accuracy for total volumetric flow and peak velocity quantification by separating the paired acquisitions in the Fourier velocity spectrum domain. Magn Reson Med 78:182-192, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Evaluation of Retinal Circulation Using Segmental-Scanning Doppler Optical Coherence Tomography in Anesthetized Cats.

To study retinal blood flow (RBF) measurement reproducibility using segmental-scanning Doppler optical coherence tomography (DOCT) in vitro in glass capillaries and in vivo in anesthetized cats. As a preliminary study, the flow rates of human blood through glass capillaries were changed by using an infusion pump and measured at 13 preset velocities by DOCT. For in vivo measurement, the cats were anesthetized using sevoflurane. The flow in the parent vessel was compared with the sum of the flow values in the two daughter vessels. The RBF was measured using two different instruments: bidirectional laser Doppler velocimetry (LDV) and DOCT. The reproducibility of the measurements was assessed by calculating the coefficients of variation (CVs) for repeated measurements of RBF at the superior retinal arterioles and venules. In vitro, the flow velocities measured by DOCT agreed well with the preset velocities. In vivo, the flow in the parent vessel agreed with the sum of the flow values in the two daughter vessels. In addition, there were no significant differences in the mean averaged CVs of the RBF in both the arterioles and venules between LDV and DOCT. The newly developed segmental-scanning DOCT revealed the accuracy of the measurement in in vitro glass capillaries and reproducibility of the measurements of blood velocity in both the retinal arterioles and venules in anesthetized cats.

A non-contact approach for PWV detection: application in a clinical setting

A need for screening methods for arteriosclerosis led to the development of several approaches to measure pulse wave velocity (PWV) being indicative of arterial stiffness. Carotid-femoral PWV (cfPWV) can be measured between common carotid artery (CCA) and femoral artery (FA) displaying the physiologically important stiffness of the conduit arteries. However, this measurement approach has several disadvantages, and a local PWV-measurement of CCA-stiffness has been proposed as an alternative in the past.In the presented pilot study, laser Doppler vibrometry (LDV) is used to measure PWV locally in the CCA (PWVLDV) in 48 patients aged between 48 and 70, with known atherosclerotic arterial disease: stabilized coronary artery disease (CAD), cerebro-vascular disease (CVD) or peripheral artery disease (PAD). Additionally, cfPWV, CCA distensibility coefficient (DC), CCA intima-media thickness (IMT), blood pressure (BP) and age were evaluated.LDV is a valid method for local PWV-measurement. The method is potentially easy to use, and causes no discomfort to the patient. PWVLDV correlates with age (R = 0.432; p = 0.002) as reported in related studies using other techniques, and measured values lay between 2.5 and 5.8 m s−1, which is well in line with literature measures of local PWV in the CCA.In conclusion, PWVLDV potentially is a marker for arterial health, but more research in a larger and more homogeneous patient population is mandatory. In future studies, blood velocity measurements should be incorporated, as well as a reference method such as pulse wave imaging (PWI) or magnetic resonance imaging (MRI).

Neurovascular 4DFlow MRI (Phase Contrast MRA): emerging clinical applications

Recent advances in 4DFlow MRI (Phase Contrast MRA) acquisition and reconstruction enable high resolution exams to be obtained in practical imaging times. 4DFlow MRI provides images of vascular morphology and quantitative measurements of blood velocity throughout a 3D imaging volume. Hemodynamic parameters such as flow volume, relative wall shear stress, streamlines, vorticity and pressure gradients can be derived from the velocity data. The combination of anatomic vessel wall imaging, lumen visualization and physiologic data derived from accelerated 4DFlow MRI augments the characterization of intracranial arterial stenosis, aneurysms, vascular malformations and dural sinus pathology. This review provides an update for clinicians interested in 4DFlow MRI of the brain.

Original Research: Sickle cell anemia and pediatric strokes: Computational fluid dynamics analysis in the middle cerebral artery.

Children with sickle cell anemia (SCA) have a high incidence of strokes, and transcranial Doppler (TCD) identifies at-risk patients by measuring blood velocities in large intracerebral arteries; time-averaged mean velocities greater than 200 cm/s confer high stroke risk and warrant therapeutic intervention with blood transfusions. Our objective was to use computational fluid dynamics to alter fluid and artery wall properties, to simulate scenarios causative of significantly elevated arterial blood velocities. Two-dimensional simulations were created and increasing percent stenoses were created in silico, with their locations varied among middle cerebral artery (MCA), internal carotid artery (ICA), and anterior cerebral artery (ACA). Stenoses placed in the MCA, ICA, or ACA generated local increases in velocity, but not sufficient to reach magnitudes > 200 cm/s, even up to 75% stenosis. Three-dimensional reconstructions of the MCA, ICA, and ACA from children with SCA were generated from magnetic resonance angiograms. Using finite element method, blood flow was simulated with realistic velocity waveforms to the ICA inlet. Three-dimensional reconstructions revealed an uneven, internal arterial wall surface in children with SCA and higher mean velocities in the MCA up to 145 cm/s compared to non-SCA reconstructions. There were also greater areas of flow recirculation and larger regions of low wall shear stress. Taken together, these bumps on the internal wall of the cerebral arteries could create local flow disturbances that, in aggregate, could elevate blood velocities in SCA. Identifying cellular causes of these microstructures as adhered blood cells or luminal narrowing due to endothelial hyperplasia induced by disturbed flow would provide new targets to treat children with SCA. The preliminary qualitative results provided here point out the critical role of 3D reconstruction of patient-specific vascular geometries and provide qualitative insight to complex interplay between vascular geometry and rheological properties possibly altered by SCA.

Ultrasound diagnosis and management of acquired uterine enhanced myometrial vascularity/arteriovenous malformations

Arteriovenous malformation is a short circuit between an organ's arterial and venous circulation. Arteriovenous malformations are classified as congenital and acquired. In the uterus, they may appear after curettage, cesarean delivery, and myomectomy among others. Their clinical feature is usually vaginal bleeding, which may be severe, if curettage is performed in unrecognized cases. Sonographically on 2-dimensional grayscale ultrasound scanning, the pathologic evidence appears as irregular, anechoic, tortuous, tubular structures that show evidence of increased vascularity when color Doppler is applied. Most of the time they resolve spontaneously; however, if left untreated, they mayrequire involved treatments such as uterine artery embolization or hysterectomy. In the past, uterine artery angiography was the gold standard for the diagnosis; however, ultrasound scanning has diagnosed successfully and helped in the clinical management. Recently, arteriovenous malformations have been referred to as enhanced myometrial vascularities. The purpose of this study was to evaluate the role of transvaginal ultrasound scanning in the diagnosis and treatment of acquired enhanced myometrial vascularity/arteriovenous malformations to outline the natural history of conservatively followed vs treated lesions. This was a retrospective study to assess the presentation, treatment, and clinical pictures of patients with uterine Enhanced myometrial vascularity/arteriovenous malformations that were diagnosed with transvaginal ultrasound scanning. We reviewed both (1) ultrasound data (images, measured dimensions, and Doppler blood flow that were defined by its peak systolic velocity and (2) clinical data (age, reproductive status, clinical presentation, inciting event or procedure, surgical history, clinical course, time intervals that included detection to resolution or detection to treatment, and treatment rendered). The diagnostic criteria were "subjective" with a rich vascular network in the myometrium with the use of color Doppler images and "objective" with a high peak systolic velocity of ≥20 cm/sec in the vascular web. Statistical analysis was performed and coded with statistical software where necessary. Twenty-seven patients met the diagnostic criteria of uterine enhanced myometrial vascularity/arteriovenous malformation. Mean age was 31.8 years (range, 18-42 years). Clinical diagnoses of the patients included 10 incomplete abortions, 6 missed abortions, 5 spontaneous complete abortions, 5 cesarean scar pregnancies, and 1 molar pregnancy. Eighty-nine percent of patients had bleeding (n= 24/27), although 1patient was febrile, and 2 patients were asymptomatic. Recent surgical procedures were performed in 55.5% patients (15/27) that included curettage (n= 10), cesarean deliveries (n= 5), or both (n= 1); 4 patients had a remote history of uterine surgery that included myomectomy. Treatment was varied and included expectant treatment alone in 48% of the patients with serial ultrasound scans and serum human chorionic gonadotropin until resolution (n= 13/27 patients), uterine artery embolization (29.6%; 8/27 patients), methotrexate administration (22.2%; 6/27 patients), hysterectomy (7.4%; 2/27 patients), and curettage (3.7%; 1/27 patients). Three patients required a blood transfusion. Of the 9 patients whose condition required embolization, the conditions of 7 patients resolved after the procedure although 1 patient's condition required operative hysteroscopy and 1 patient's condition required hysterectomy for intractable bleeding. Average peak systolic velocity after embolization in the 9 patients was 85.2 cm/sec (range, 35-170 cm/sec); the average peak systolic velocity of the 16 patients with spontaneous resolution was 58.5 cm/sec (range, 23-90 cm/sec). Acquired enhanced myometrial vascularity/arteriovenous malformations occurred after unsuccessful pregnancies or treatment procedures that included uterine curettage, cesarean delivery, or cesarean scar pregnancy. Triage of patients for expectant treatment vs intervention with uterine artery embolization based on their clinical status, which was supplemented by objective measurements of blood velocity measurement in the arteriovenous malformation, appears to be a good predictor of outcome. Ultrasound evaluation of patients with early pregnancy failure and persistent bleeding should be considered for evaluation of a possible enhanced myometrial vascularity/arteriovenous malformation.

Relation of retinal blood flow and retinal oxygen extraction during stimulation with diffuse luminance flicker.

Cerebral and retinal blood flow are dependent on local neuronal activity. Several studies quantified the increase in cerebral blood flow and oxygen consumption during activity. In the present study we investigated the relation between changes in retinal blood flow and oxygen extraction during stimulation with diffuse luminance flicker and the influence of breathing gas mixtures with different fractions of O2 (FiO2; 100% 15% and 12%). Twenty-four healthy subjects were included. Retinal blood flow was studied by combining measurement of vessel diameters using the Dynamic Vessel Analyser with measurements of blood velocity using laser Doppler velocimetry. Oxygen saturation was measured using spectroscopic reflectometry and oxygen extraction was calculated. Flicker stimulation increased retinal blood flow (57.7 ± 17.8%) and oxygen extraction (34.6 ± 24.1%; p < 0.001 each). During 100% oxygen breathing the response of retinal blood flow and oxygen extraction was increased (p < 0.01 each). By contrast, breathing gas mixtures with 12% and 15% FiO2 did not alter flicker–induced retinal haemodynamic changes. The present study indicates that at a comparable increase in blood flow the increase in oxygen extraction in the retina is larger than in the brain. During systemic hyperoxia the blood flow and oxygen extraction responses to neural stimulation are augmented. The underlying mechanism is unknown.

Utility of TEE in Anesthesia for Non Cardiac Surgery: A Practical Review

Two dimensional echocardiography of the heart gives the accurate dimensions of cardiac anatomical structures. In addition, Doppler effect by virtue of measuring blood velocity gives critical information regarding functioning of the heart (cardiac output and flow across cardiac valves). Trans-esophageal echocardiography (TEE) has revolutionized surgical and anesthetic management in patients undergoing cardiac surgery ever since its introduction in 1980s, but its intraoperative utility in non-cardiac surgery is a new emergent dimension. We review here, the nascent and lesser studied use of TEE in patients undergoing non-cardiac surgery, concentrating on five basic views which can be rapidly viewed even by lesser trained anesthesiologists in the learning curve to provide valuable information requisite for non cardiac surgery.

Hemodynamics of the pectinis oculi artery in American pekin ducks (Anas platyrhynchos domestica).

To measure blood velocity parameters of the main detectable branch of the pectinis oculi artery and compare with values found in other arteries in other species to form a hypothesis of the function of the pecten oculi in birds. Eleven American pekin ducks (Anas platyrhynchos domestica) without ocular diseases were examined with ocular Doppler ultrasonography. The pectinate artery resistive index (RI) and pulsatility index (PI) were calculated. The mean of RI and PI values resulted, respectively, in: left eye (0.43 ± 0.07; 0.58 ± 0.13), right eye (0.37 ± 0.09; 0.47 ± 0.14), and both eyes (0.40 ± 0.08; 0.53 ± 0.14). The low RI and PI values found in the main branch of the pectinis oculi artery compared with other arteries in other tissues suggest a high metabolic activity in the pecten oculi, and it could indicate a nutritional function and/or intraocular pressure regulation.

Compact Laser Doppler Flowmeter (LDF) Fundus Camera for the Assessment of Retinal Blood Perfusion in Small Animals.

PurposeNoninvasive techniques for ocular blood perfusion assessment are of crucial importance for exploring microvascular alterations related to systemic and ocular diseases. However, few techniques adapted to rodents are available and most are invasive or not specifically focused on the optic nerve head (ONH), choroid or retinal circulation. Here we present the results obtained with a new rodent-adapted compact fundus camera based on laser Doppler flowmetry (LDF).MethodsA confocal miniature flowmeter was fixed to a specially designed 3D rotating mechanical arm and adjusted on a rodent stereotaxic table in order to accurately point the laser beam at the retinal region of interest. The linearity of the LDF measurements was assessed using a rotating Teflon wheel and a flow of microspheres in a glass capillary. In vivo reproducibility was assessed in Wistar rats with repeated measurements (inter-session and inter-day) of retinal arteries and ONH blood velocity in six and ten rats, respectively. These parameters were also recorded during an acute intraocular pressure increase to 150 mmHg and after heart arrest (n = 5 rats).ResultsThe perfusion measurements showed perfect linearity between LDF velocity and Teflon wheel or microsphere speed. Intraclass correlation coefficients for retinal arteries and ONH velocity (0.82 and 0.86, respectively) indicated strong inter-session repeatability and stability. Inter-day reproducibility was good (0.79 and 0.7, respectively). Upon ocular blood flow cessation, the retinal artery velocity signal substantially decreased, whereas the ONH signal did not significantly vary, suggesting that it could mostly be attributed to tissue light scattering.ConclusionWe have demonstrated that, while not adapted for ONH blood perfusion assessment, this device allows pertinent, stable and repeatable measurements of retinal blood perfusion in rats.

Local Vascular Hemodynamic Response During and After Passive Stretching

PURPOSE: Only few investigations focused on the peripheral hemodynamic responses to passive limb stretching (PLS), and only in the animal model. Therefore, the aim of this study was to determine in healthy humans the acute effects of PLS on limb blood flow (LBF). Our hypothesis was that peripheral hemodynamic responses would be affected by the PLS, because of the repetitive ischemia/reperfusion cycles induced by the passive muscle tension. METHODS: Twelve healthy individuals (age: 22 ± 3 yrs) underwent PLS of the knee extensors. The PLS procedure (45 s stretch/passive flexion, 15 s release/passive extension) was repeated five times. Measurements of arterial blood velocity and vessel diameter were taken in the passively stretched leg, distal to the inguinal ligament and proximal to the deep superficial femoral bifurcation by ultrasound system. RESULTS: PLS had a significant influence on peripheral hemodynamic. Specifically, LBF responses during the 1st passive flexion of PLS transitory increased with respect to baseline by ∼78% (from 495±110 to 882±121 ml/min; P<0.05). Interestingly, a second hyperemia (to 880±118 ml/min; P<0.05) was recorded during the limb passive extension after the 1st PLS. During the 2nd PLS procedure, the hyperemic response recorded during the passive flexion was markedly reduced with respect to the 1st (608±103 ml/min), while LBF recorded during the passive extension was similar to the 1st PLS (915±131 ml/min). From the 3rd PLS procedure, the hyperemic responses to PLS were negligible, while the peaks of LBF were consistently elevated. CONCLUSION: In agreement with our hypothesis, PLS significantly influenced peripheral circulation in the passively stretched knee extensors. However, the time response of these alterations in the peripheral hemodynamic suggests two different components of this phenomenon. The hyperemic response corresponding to the flexion phase of PLS may be explained by an acute release of nitric oxide derived from the mechanical stretch of the vessels. Afterwards, the response decreased progressively during the 2nd, 3rd, 4th, and 5th PLS procedures. Conversely, the constant hyperemic responses recorded during the passive extensions of PLSs were possibly caused by a reduced ischemia determined by a diminished muscle tension.