Phase-contrast Magnetic Resonance Imaging Technique Research Articles

The application of integrated MRI CSF flowmetry in the diagnosis and treatment of CSF dynamic changes in hydrocephalus patients: a systematic review

Background: CSF has contributed to the growth of brain development during the time of evolution and later protects against external trauma. Idiopathic normal pressure hydrocephalus (iNPH) is known as an aberration of intracranial hydrodynamics resulting in the accumulation of CSF endo-ventricular. The use of phase-contrast (PC) MRI is proven effective in assessing the communication of arachnoid cyst and subarachnoid CSF spaces. The CSF flow MRI has been found to show efficacy in differentiation between the communicating and non-communicating hydrocephalus for localizing obstruction levels in obstructive hydrocephalus and later providing important information concerning the preoperative evaluation of NPH, along with the differential diagnosis and the prediction of the related advantages from surgery and post-operative follow-up. Hence, the present study aimed for analyzing the role of MRI CSF flowmetry in the evaluation of patients with suspected hydrocephalus. Method: The role of MRI CSF flowmetry in the evaluation of patients with suspected hydrocephalus was undertaken by conducting a systematic literature review. The current review was reported as required reporting features for systematic reviews and meta-analysis statements (PRISMA). The systematic literature review included a total of 22 studies that analyzed the various applications of MRI CSF flowmetry in the medical field, mainly focusing on the use of MRI CSF flowmetry in the evaluation of patients with suspected hydrocephalus. Results: The result mainly included the identification of the CSF flow by integrating the PC MRI technique and the aqueduct-CSF flow rate was detected accurately showing as statistically significant in diagnosis of NPH. Evaluating the peak velocities and aqueductal stroke volume (ASV) showed a significant increase among hydrocephalus patients. Conclusion: The condition of hydrocephalus makes the ventricular size increase and eventually puts pressure on the brain due to excess accumulation of fluid while blocking the CSF flow after it exits ventricles. Currently, many imaging techniques are available for the detection of hydrocephalus that study the CSF flow dynamics and its related parameters. Among them, the MRI technique is the most reliable, rapid, and most importantly, a non-invasive method for the quantitative measurement of CSF flow rate and intracranial pulsations. Keywords: Intracranial pressure, Magnetic resonance imaging (MRI), MRI CSF Flowmetry, Neurological disorder, CSF dynamics

Comparison of intracranial aneurysm flow quantification techniques: standard PIV vs stereoscopic PIV vs tomographic PIV vs phase-contrast MRI vs CFD

Image-based hemodynamic simulations to assess the rupture risk or improve the treatment planning of intracranial aneurysms have become popular recently. However, due to strong modeling assumptions and limitations, the acceptance...

DPD enables mesoscopic MRI simulation of slow flow

We present a novel method to simulate magnetic resonance imaging (MRI) for the assessment of slow flow at Reynolds number $$Re \approx 0.02$$ . We couple Bloch equations with dissipative particle dynamics (DPD) to study the effect of flow dynamics at the mesoscopic level on acquired MR images. The Bloch equations are used to propagate the evolution of the magnetization of particles while their trajectories are being computed simultaneously based on DPD interaction forces. The magnetic resonance assessment of fluid velocities is performed using a phase-contrast MRI technique, implemented by a spin echo single-sided bipolar gradient sequence. The computational cost for simulating the fluid flow is successfully reduced by an efficient implementation of a vectorized isochromat algorithm. We demonstrate successful simulation of laminar flow, flow with diffusion effects, and flow around an obstacle. The method can be used to simulate convective and diffusive flow MRI experiments at the mesoscopic level.

Quantitative Flow Imaging in Human Umbilical Vessels In Utero Using Nongated 2D Phase Contrast MRI.

Volumetric assessment of afferent blood flow rate provides a measure of global organ perfusion. Phase-contrast magnetic resonance imaging (PCMRI) is a reliable tool for volumetric flow quantification, but given the challenges with motion and lack of physiologic gating signal, such studies, in vivo on the human placenta, are scant. To evaluate and apply a nongated (ng) PCMRI technique for quantifying blood flow rates in utero in umbilical vessels. Prospective study design. Twenty-four pregnant women with median gestational age (GA) 30 4/7 weeks and interquartile range (IQR) 8 1/7 weeks. All scans were performed on a 3.0T Siemens Verio system using the ng-PCMRI technique. The GA-dependent increase in umbilical vein (UV) and arterial (UA) flow was compared to previously published values. Systematic error to be expected from ng-PCMRI, in the context of pulsatile UA flow and partial voluming, was studied through Monte-Carlo simulations, as a function of resolution and number of averages. Correlation between the UA and UV was evaluated using a generalized linear model. Simulations showed that ng-PCMRI measurement variance reduced by increasing the number of averages. For vessels on the order of 2 voxels in radius, partial voluming led to 10% underestimation in the flow. In fetuses, the average flow rates in UAs and UV were measured to be 203 ± 80 ml/min and 232 ± 92 ml/min and the normalized average flow rates were 140 ± 59 ml/min/kg and 155 ± 57 ml/min/kg, respectively. Excellent correlation was found between the total arterial flow vs. corresponding venous flow, with a slope of 1.08 (P = 0.036). Ng-PCMRI can provide accurate volumetric flow measurements in utero in the human umbilical vessels. Care needs to be taken to ensure sufficiently high-resolution data are acquired to minimize partial voluming-related errors. 2 Technical Efficacy Stage 1 J. Magn. Reson. Imaging 2017.

Cardiovascular magnetic resonance elastography: A review.

Cardiovascular diseases are the leading cause of death worldwide. These cardiovascular diseases are associated with mechanical changes in the myocardium and aorta. It is known that stiffness is altered in many diseases, including the spectrum of ischemia, diastolic dysfunction, hypertension and hypertrophic cardiomyopathy. In addition, the stiffness of the aortic wall is altered in multiple diseases, including hypertension, coronary artery disease and aortic aneurysm formation. For example, in diastolic dysfunction in which the ejection fraction is preserved, stiffness can potentially be an important biomarker. Similarly, in aortic aneurysms, stiffness can provide valuable information with regard to rupture potential. A number of studies have addressed invasive and non-invasive approaches to test and measure the mechanical properties of the myocardium and aorta. One of the non-invasive approaches is magnetic resonance elastography (MRE). MRE is a phase-contrast magnetic resonance imaging technique that measures tissue stiffness non-invasively. This review article highlights the technical details and application of MRE in the quantification of myocardial and aortic stiffness in different disease states.

Measurements of wall shear stress and aortic pulse wave velocity in swine with familial hypercholesterolemia

To assess measurements of pulse wave velocity (PWV) and wall shear stress (WSS) in a swine model of atherosclerosis. Nine familial hypercholesterolemic (FH) swine with angioplasty balloon catheter-induced atherosclerotic lesions to the abdominal aorta (injured group) and 10 uninjured FH swine were evaluated with a 4D phase contrast (PC) magnetic resonance imaging (MRI) acquisition, as well as with radial and Cartesian 2D PC acquisitions, on a 3T MR scanner. PWV values were computed from the 2D and 4D PC techniques, compared between the injured and uninjured swine, and validated against reference standard pressure probe-based PWV measurements. WSS values were also computed from the 4D PC MRI technique and compared between injured and uninjured groups. PWV values were significantly greater in the injured than in the uninjured groups with the 4D PC MRI technique (P = 0.03) and pressure probes (P = 0.02). No significant differences were found in PWV between groups using the 2D PC techniques (P = 0.75-0.83). No significant differences were found for WSS values between the injured and uninjured groups. The 4D PC MRI technique provides a promising means of evaluating PWV and WSS in a swine model of atherosclerosis, providing a potential platform for developing the technique for the early detection of atherosclerosis.

Volumetric flow rate of ophthalmic artery in healthy individuals using a new magnetic resonance imaging technique

AbstractPurpose To measure the volume flow rate of the ophthalmic artery (OA). A phase contrast magnetic resonance imaging (PCMRI) protocol was developed to assess volume flow rate of OA in healthy young (HY) and elderly (HE).Methods The study was performed with a 3‐Tesla scanner on HY (n=51,28females,age=64‐80ys) and HE (n=51,28females,age=21‐30ys). The OA were localized by a 3D time of flight (3DTOF) sequence. To measure the volume flow rate, a 2D PCMRI sequence was optimized to reach a spatial resolution of 0.35mm/pixel. Based on 3DTOF data, the flow measurement was perpendicular to OA. Mean volume flow rate (Qmean), resistive index (RI), arterial volume change (ΔV) and symmetry index (SI) were computed from the volume flow rate curve. Accuracy of MRI measures was studied using a vessel‐phantom (Ø=1.4mm). Six constant flow rates (5.54‐34.41mL/min) were used as reference.Results Phantom investigation showed good agreement between the reference and MRI measurements with a percentage error <7%. The mean and SD of RI (HE:0.68±0.08; HY:0.63±0.09), ΔV (HE:26.35±14.03uL; HY:18.51±7.23uL) and SI (HE:0.58±0.03; HY:0.63±0.03) of OAs volume flow rate curve were significantly higher in the elderly group compared to the young group (RI,p<0.05; ΔV,p<0.001; SI:p<0.001). No statistical difference was found in Qmean between HE(10.91±5.44mL/min) and HY (10.27±4.46mL/min) individuals (p>0.05).Conclusion Volume flow rate measurement of OA vessels using PCMRI is possible. Contrary to color Doppler imaging, PCMRI technique does not alter IOP during the measurements. This method may provide a precise description of the dynamics of the volume flow rate of OA during the cardiac cycle.

Quantitative assessment of cardiac output and left ventricular function by noninvasive phase‐contrast and cine MRI: Validation study with invasive pressure‐volume loop analysis in a swine model

To validate noninvasive cardiac output measurements of phase-contrast magnetic resonance imaging (PC-MRI) and cine MRI using an invasive pressure-volume (PV) loop technique on a swine model. We compared three methods for evaluating cardiac function at rest and under pharmaceutical low-dose inotropic infusion conditions: 1) phase-contrast MRI, 2) cine MRI, and 3) PV loop relationship. These measurements were made in 14 domestic pigs under rest conditions. Identical MRI acquisitions and PV loop analysis were performed on six pigs from the same group that received an infusion of dobutamine 2.5 μg/kg/min. Cardiac outputs from all measurements were analyzed and compared using linear regression and Bland-Altman analysis. Noninvasive PC-MRI and cine MRI did not show any significant differences compared to an invasive PV loop technique for measurement of cardiac output under both rest (PC-MRI, cine MRI, and PV loop, 3.17 ± 0.45, 3.18 ± 0.61, 3.45 ± 0.41 L/min, respectively) and pharmaceutical low-dose inotropic infusion conditions (PC-MRI, cine MRI, and PV loop, 4.78 ± 0.53, 4.7 ± 0.6, 4.96 ± 0.48 L/min, respectively). Statistical analysis showed good agreement of cardiac output measurements at rest (R(2) = 0.83) and under low-dose inotropic infusion conditions (R(2) = 0.74) using PC-MRI and PV loop techniques. Cardiac output measurement using cine MRI and PV loop techniques also showed good agreement at rest (R(2) = 0.85) and under low-dose inotropic infusion conditions (R(2) = 0.76). Furthermore, cardiac outputs determined with the three modalities showed good agreement over a wide range of heart rates (90-180 bpm). MRI can provide a reliable, noninvasive measurement of cardiac output that can be carried out without the complications that are inherent with current invasive procedures.

WE‐E‐210A‐01: Functional Cardiovascular MRI: Assessment, Visualization and Quantification of 3D Blood Flow Characteristics

Magnetic Resonance Imaging (MRI) techniques provide a non‐invasive method for the highly accurate anatomic depiction of the heart and vessels. Most MR‐sequences demonstrate more or less significant sensitivity to flow and motion, which can lead to artifacts in many applications. The intrinsic motion sensitivity of MRI can, however, also be used to image vessels like in phase contrast (PC) MR‐angiography or to quantify blood flow and motion of tissue. Such techniques offer the unique possibility to acquire spatially registered functional information simultaneously with the morphological data within a single experiment. Characterizations of the dynamic components of blood flow and cardiovascular function provide insight into normal and pathological physiology and have made considerable progress in recent yearsTo synchronize flow or motion sensitive measurements with periodic tissue motion or pulsatile flow, data acquisition is typically gated to the cardiac cycle and time resolved (CINE) anatomical images are collected to depict the dynamics of tissue motion and blood flow during the cardiac cycle. Visualization and quantification of blood flow and tissue motion using PC MRI has been widely used in a number of applications. In addition to analyzing tissue motion such as left ventricular function, time‐resolved 2D PC MRI techniques have proven to be useful tools for the assessment of blood flow within the cardiovascular system.Moreover, 3D spatial encoding offers the possibility of isotropic high spatial resolution and thus the ability to measure and visualize the temporal evolution of complex flow and motion patterns in a 3D‐volume. In this context, ECG synchronized and respiration controlled flow sensitive 3D MRI using 3‐directional velocity encoding (also termed ‘flow sensitive 4D MRI’) can be employed to detect and visualize global and local blood flow characteristics in targeted vascular regions (aorta, cranial arteries, carotid arteries, etc.). For the analysis and visualization of complex, three‐directional blood flow within a 3D volume, various visualization tools, including 2D vector‐fields, 3D streamlines and particle traces, have been reported. In addition more advanced data quantification strategies of directly measured (e.g. flow rates) or derived parameters (e.g. pressure difference maps, wall sheer stress, pulse wave velocity, etc.) are promising as new clinical markers for the characterization of cardiovascular disease.This lecture will provide an overview of the MR imaging principles and advanced acquisition methods, data processing, flow visualization and quantification strategies, and clinical applications of flow sensitive MRI imaging.Learning Objectives:1. Understand the basic and advanced methods for flow measurements using MRI2. Understand techniques for 3D flow visualization and quantification of blood flow and derived parameters3. Understand the issues related to clinical applications of flow‐sensitive MRI

Changing Concepts of Cerebrospinal Fluid Hydrodynamics: Role of Phase-Contrast Magnetic Resonance Imaging and Implications for Cerebral Microvascular Disease

Phase-contrast magnetic resonance imaging (PC-MRI) or flow-sensitive MRI can be used to noninvasively measure intracranial vascular and CSF flow. Monro-Kellie homeostasis is the complex compensatory mechanism for the increase in intracranial blood volume during systole. Through PC-MRI techniques, our understanding of Monro-Kellie homeostasis and the associated intracranial hydrodynamics has greatly improved. Failure of this homeostatic mechanism has been implicated in a wide range of cerebral disorders, including vascular and Alzheimer's dementia, late-onset depression, benign and secondary intracranial hypertension, communicating and normal pressure hydrocephalus, and age-related white matter changes. The most common mode of homeostatic failure is due to vascular disease with decreased cerebral arterial compliance. This has wide-reaching implications in the investigation of patients with cerebral vascular disease. Here we discuss the role of PC-MRI in the study of cerebral hydrodynamics and the current understanding of Monro-Kellie homeostasis in both healthy and disease states. Quantitative assessment of the changes in this homeostatic mechanism using PC-MRI has important implications in the development of biomarkers of vascular involvement in disease with application in diagnosis, treatment planning, phenotype identification, and outcome assessment in clinical trials.

MRI myocardial motion and fiber tracking: a confirmation of knowledge from different imaging modalities

A helical configuration underlies the anatomy of cardiac structure, and a structure/function relationship is needed to determine if the ventricular myocardial band model defines this spatial relationship. This report explores how studies of velocity-encoded phase contrast magnetic resonance imaging (MRI) for myocardial motion and fiber tracking algorithms that imply fiber orientations can (a) quantify regional myocardial wall motion of the entire heart, (b) determine if these motion of implied fiber orientation link with the helical heart model, and (c) reveal if this new knowledge correlates with imaging information from other different imaging modalities. Accumulated left ventricular motion patterns that accurately differentiate radial (i.e. contraction and expansion), rotational (i.e. twisting and untwisting), and longitudinal (i.e. lengthening and shortening) motion components are correlated with structure/function data achieved by sonomicrometer crystals, echocardiography, corrosion casts, and MUGA recordings. Acceleration fiber tracking to determine fiber orientation and cardiac motion during the ejection and rapid filling phases of the cardiac cycle corresponded to maximal force displayed by ultrasonic crystals placed into the angulation of the presumed functional units of the descending and ascending segments of the apical loop of the helical ventricular myocardial band, and motion by echocardiographic recordings. These integrated findings imply a favourable interaction of MRI with the myocyte orientation of the helical ventricular myocardial band. These composite findings indicate that phase contrast MRI techniques for high temporal resolution velocity mapping during cardiac motion and myocardial fiber tracking confirm other technologies, and centralize the capacity of MRI to link other imaging methods together relative to a single helical structural model. The close agreement amongst a spectrum of imaging studies provide a very powerful integration that transcends a single look; the same thing is observed by each component of global technology, thereby implying that the helical ventricular band is the structural basis for these functional changes.

Investigating myocardial motion by MRI using tissue phase mapping

Velocity-encoded phase contrast magnetic resonance imaging (MRI) provides a tool to quantify regional myocardial wall motion of the entire heart. It allows the acquisition of three-directional velocity vector fields with high spatial resolution that reflect the temporal evolution of myocardial velocities over the cardiac cycle. In contrast to other imaging modalities such as echocardiography left ventricular performance can be assessed without limited anatomical or functional coverage. Compared to other techniques that quantify local myocardial contractility (e.g. implanted ultrasonic crystals) by means of regional displacement, phase contrast MRI provides information about local and global left ventricular velocities (i.e. motion) by utilizing the intrinsic motion sensitivity of MRI. The resultant motion components of contraction, expansion, rotation, lengthening, and shortening of the left ventricle are described in high spatial and temporal detail. Phase contrast measurements were performed in 12 healthy volunteers with a respiratory-gated technique in order to achieve a high temporal resolution of 13.8ms to demonstrate the detailed assessment of global and regional myocardial motion. Data revealed details in left ventricular motion patterns that were previously not seen in phase contrast measurements and are only known from echocardiography. For all volunteers, characteristic myocardial motion patterns and locally different radial (i.e. contraction and expansion), rotational (i.e. twisting and untwisting) and longitudinal (i.e. lengthening and shortening) motion components could be detected with high accuracy. The phase contrast MRI technique for high temporal resolution velocity mapping is therefore very promising for the investigation and better understanding of the myocardial motion in normal subjects and patients with disturbed left ventricular performance and may validate further testing of different models of cardiac structure.

Quantitative Measurements on the Human Ascending Aortic Flow Using 2D Cine Phase-Contrast Magnetic Resonance Imaging

The flow in the human ascending aorta was quantified using two-dimensional (2D) cine phase-contrast magnetic resonance imaging (MRI). The quality and reliability of the method were demonstrated with a specially designed phantom model; the flow rate determined with the MRI agreed well with that obtained with a measuring cylinder. The method was then used to measure the aortic blood flow of three healthy human volunteers. The velocity profiles at the supra-aortic valvular plane and ascending aortic plane (approximately 2 and 5cm distal to the aortic valve, respectively) were significantly different. At the peak of systole, the profile was almost axisymmetric at the supra-aortic valvular plane, while it was skewed towards the anterior side of the vessel at the ascending aorta. The Reynolds number, volume flow rate, and stroke volume were all within the normal physiological range. This study demonstrated that the 2D cine phase-contrast MRI technique can be used to provide detailed information on the flow velocity and configuration of a blood vessel, making it a promising tool for analyzing complex hemodynamics in the aorta.

Flow patterns in the aortic root and the aorta studied with time-resolved, 3-dimensional, phase-contrast magnetic resonance imaging: implications for aortic valve–sparing surgery

Objective Sparing the aortic valve has become a surgical option for patients who require repair of aortic root ectasia and have normal valve leaflets. Surgical approaches to valve sparing differ with regard to preservation of the native sinuses of Valsalva. The role of the sinuses and the importance of maintaining them remain controversial. Methods By using a time-resolved, 3-dimensional, phase-contrast magnetic resonance imaging technique, aortic root and aortic blood velocity data were acquired from 2 patients with Marfan syndrome 6 months after aortic valve–sparing surgery with straight Dacron grafts and contrasted with data from 6 normal volunteers. Results In normal aortas vortical blood flow became apparent in the individual sinuses after peak systole. The vortices filled the available space behind the valve leaflets and persisted until diastole, expanding and moving inward during aortic valve closure. In contrast, no vortices were observed in the postoperative patients with Marfan syndrome with negligible sinuses. Conclusions Changes in supravalvular flow accompany loss of sinus architecture. Whether the presence, size, and velocity of supravalvular vortices affects the function or durability of the preserved aortic valve remains to be studied.

Basal cerebral blood flow is dependent on the nitric oxide pathway in elderly but not in young healthy men

Objective. Brain perfusion is tightly regulated over a wide range of blood pressures by local regulation of cerebral blood flow (CBF). Ageing is associated with impaired CBF and impaired nitric oxide mediated vasodilator responses. The role of nitric oxide in the regulation of basal CBF in young and older subjects was investigated, using the nitric oxide synthase inhibitor l-NMMA as pharmacological tool. Methods. We used a gradient echo phase-contrast magnetic resonance imaging technique to investigate the role of nitric oxide in the regulation of cerebral blood flow in young (25±7.1 years; n=8) and old (78±6.6 years; n=7) volunteers. The study was performed in a double-blinded fashion and consisted of two study days. On one day the effects of the intravenously infused L-NMMA on CBF and blood pressure was measured and on the other day the effects of a matching placebo. Results. Basal CBF was significantly lower in old compared to young subjects (590±20 vs 704±20 ml/min), while the cerebral vascular resistance (CVR) levels were significantly higher (0.15±0.01 (arbitrary units) vs 0.12±0.01, respectively). Infusion of l-NMMA significantly increased mean arterial pressure in both groups (2.8±1.2 mmHg; p=0.02 in the young and in the old subjects 5.6±1.1 mmHg; p<0.001). Infusion of l-NMMA significantly decreased CBF (49±12 ml/min; p<0.001) and increased CVR (0.02±0.004; p<0.001) in the old subjects but did not significantly influence cerebral circulation in the young subjects. Conclusion. We conclude that compared to young subjects, in old people CBF is impaired, and dependent on the intactness of the nitric oxide pathway.

Three-directional myocardial motion assessed using 3D phase contrast MRI.

Regional myocardial function is a complex entity consisting of motion in three dimensions (3D). Besides magnetic resonance imaging (MRI), no other noninvasive technique can give a true 3D description of cardiac motion. Using a time-resolved 3D phase contrast technique, three-dimensional image volumes containing myocardial velocity data in six normal volunteers were acquired. Coordinates and velocity information were extracted from nine points placed in different myocardial segments in the left ventricle (LV), and decomposed into longitudinal (V(L)), radial (V(R)), and circumferential (V(C)) velocity components. Our findings confirm a longitudinal apex-to-base gradient for the LV, with only a small motion of the apex. The mean velocity for V(L) for all the basal segments was higher compared to the midsegments during systole [3.5+/-1.2 vs. 2.5+/-1.7 cm/s (p<0.01)], early filling [-6.9+/-1.8 vs. -4.9+/-1.8 cm/s (p<0.001)], and during atrial contraction [-2.2+/-1.4 vs. -1.6+/-1.3 cm/s (p<0.05)]. A similar pattern was observed when comparing velocities from the midsegments to the apex. Radial velocity was higher during early filling in the midportion of the lateral [-4.9+/-2.7 vs. -3.2+/-1.6 cm/s (p<0.05)] wall compared to the basal segments, no difference was observed for the septal [-2.0+/-1.5 vs. -0.3+/-2.5 cm/s (p=0.15)], anterior [-5.8+/-3.3 vs. -4.0+/-1.7 cm/s (p=0.17)], and posterior [-2.3+/-2.1 vs. -2.5+/-1.0 cm/s (p=0.78)] walls. When observing the myocardial velocity in a single point and visualizing the movement of the main direction of the velocities in this point as vectors in velocity vector plots like planes, it is clear that myocardial movement is by no means one dimensional. In conclusion, our time-resolved 3D, phase contrast MRI technique makes it feasible to extract myocardial velocities from anywhere in the myocardium, including all three velocity components without the need for positioning any slices at the time of acquisition.

Nitric oxide mediates hypoxia-induced cerebral vasodilation in humans.

Nitric oxide (NO) plays a pivotal role in the regulation of peripheral vascular tone. Its role in the regulation of cerebral vascular tone in humans remains to be elucidated. This study investigates the role of NO in hypoxia-induced cerebral vasodilatation in young healthy volunteers. The effect of the NO synthase inhibitor N(G)-monomethyl-L-arginine (L-NMMA) on the cerebral blood flow (CBF) was assessed during normoxia and during hypoxia (peripheral O(2) saturation 97 and 80%, respectively). Subjects were positioned in a magnetic resonance scanner, breathing normal air (normoxia) or a N(2)-O(2) mixture (hypoxia). The CBF was measured before and after administration of L-NMMA (3 mg/kg) by use of phase-contrast magnetic resonance imaging techniques. Administration of L-NMMA during normoxia did not affect CBF. Hypoxia increased CBF from 1,049 +/- 113 to 1,209 +/- 143 ml/min (P < 0.05). After L-NMMA administration, the augmented CBF returned to baseline (1,050 +/- 161 ml/min; P < 0.05). Similarly, cerebral vascular resistance declined during hypoxia and returned to baseline after administration of L-NMMA (P < 0.05 for both). Use of phase-contrast magnetic resonance imaging shows that hypoxia-induced cerebral vasodilatation in humans is mediated by NO.

Normal renal blood flow measurement using phase-contrast cine magnetic resonance imaging.

This study assesses the ability of a cardiac-gated phase-contrast magnetic resonance imaging (MRI) technique to measure renal blood flow (RBF) noninvasively in humans. In nine normal volunteers, total RBF in the renal arteries and in the left renal vein was estimated by MRI and correlated with RBF determined by the clearance of para-aminohippuric acid (CPAH) and the hematocrit level. Correlation of RBF estimated from left renal vein flow, with RBF by CPAH-hematocrit, yielded r = .86 (P less than .003). Repeated measurement of RBF by MRI demonstrated a high degree of reproducibility, with coefficients of variation ranging from 4.8% to 8.9%. However, the MRI measurements of arterial flow did not significantly correlate with the standard measurements. Reproducible noninvasive measurement of normal RBF is possible with the phase-contrast MRI technique used to measure renal venous blood flow.