MR gradient echo imaging of intravascular blood oxygenation: T2* determination in the presence of flow

The T₂* relaxation time of blood varies with its oxygen saturation. To evaluate the feasibility of imaging intravascular blood oxygenation in humans using a conventional 1.5T MR system, we have implemented a method to measure T₂* of blood despite the presence of pulsatile flow. The method was tested in a) stationary and flow phantoms, b) blood samples at different levels of oxygen saturation, and c) a human hypoxia model. Our results demonstrate the ability of cardiac-triggered, flow compensated gradient echo imaging to obtain reproducible T₂* measurements of flowing blood in vivo.     

Validation of quantitative estimation of tissue oxygen extraction fraction and deoxygenated blood volume fraction in phantom and in vivo experiments by using MRI

The blood oxygenation level dependent signal of cerebral tissue can be theoretically derived using a network model formed by randomly oriented infinitely long cylinders. The validation of this model by phantom and in vivo experiments is still an object of research. A network phantom was constructed of solid polypropylene strings immersed in silicone oil, which essentially eliminated the effect of spin diffusion. The volume fraction and magnetic property of the string network was predetermined by independent methods. Ten healthy volunteers were measured for in vivo demonstration. The gradient echo sampled spin echo signal was evaluated with the cylinder network model. We found a strong interdependency between the two network characterizing parameters deoxygenated blood volume and oxygen extraction fraction. Here, different sets of deoxygenated blood volume/oxygen extraction fraction values were able to describe the measured signal equally well. However, by setting one parameter constant to a predetermined value, reasonable estimates of the other parameter were obtained. The same behavior was found for the in vivo demonstration. The signal theory of the cylinder network was validated by a well‐characterized phantom. However, the found interdependency that was found between deoxygenated blood volume and oxygen extraction fraction requires an independent estimation of one variable to determine reliable values of the other parameter. Magn Reson Med 63:910–921, 2010. © 2010 Wiley‐Liss, Inc.     

Water proton MR properties of human blood at 1.5 Tesla: magnetic susceptibility, T(1), T(2), T*(2), and non-Lorentzian signal behavior.

Accurate knowledge of the magnetic properties of human blood is required for the precise modeling of functional and vascular flow-related MRI. Herein are reported determinations of the relaxation parameters of blood, employing in vitro samples that are well representative of human blood in situ. The envelope of the blood (1)H(2)O free-induction decay signal magnitude during the first 100 msec following a spin echo at time TE is well- described empirically by an expression of the form, S(t) = S(o). exp[-R(*)(2). (t - TE) - AR*. (t - TE)(2)]. The relaxation parameters AR* and R(*)(2) increase as a function of the square of the susceptibility difference between red blood cell and plasma and depend on the spin-echo time. The Gaussian component, AR*, should be recognized in accurate modeling of MRI phenomena that depend upon the magnetic state of blood. The magnetic susceptibility difference between fully deoxygenated and fully oxygenated red blood cells at 37 degrees C is 0.27 ppm, as determined independently by MR and superconducting quantum interference device (SQUID) measurements. This value agrees well with the 1936 report of Pauling and Coryell (Proc Natl Acad Sci USA 1936;22:210-216), but is substantially larger than that frequently used in MRI literature. Magn Reson Med 45:533-542, 2001.     

Assessment of regional myocardial oxygenation changes in the presence of coronary artery stenosis with balanced SSFP imaging at 3.0 T: theory and experimental evaluation in canines

PURPOSE: To examine the dependence of steady-state free-precession (SSFP) -based myocardial blood-oxygen-level-dependent (BOLD) contrast on field strength using theoretical and experimental models. MATERIALS AND METHODS: Numerical simulations using a two-pool exchange model and a surgically prepared dog model were used to assess the SSFP-based myocardial BOLD signal changes at 1.5T and 3.0T. Experimental studies were performed in eight canines with pharmacological vasodilation under various levels of left circumflex coronary artery stenosis. Experimentally obtained BOLD signal changes were correlated against microsphere-based true flow changes. RESULTS: Theoretical results showed that, at 3.0T, relative to 1.5T, a threefold increase in oxygen sensitivity can be expected. Experimental studies in canines showed near similar results-a 2.5 +/- 0.2-fold increase in BOLD sensitivity at 3.0T relative to 1.5T (P < 0.05). Based on the scatter gram of BOLD data and microsphere data, it was found that the minimum regional flow difference that can be detected with SSFP-based myocardial BOLD imaging at 1.5T and 3.0T were 2.9 and 1.6, respectively (P < 0.05). CONCLUSION: This study demonstrated that SSFP-based myocardial BOLD sensitivity is substantially greater at 3.0T compared with 1.5T. The findings here suggest that SSFP-based myocardial BOLD imaging at 3.0T may have the necessary sensitivity to detect the clinically required minimum flow difference of 2.0.     

Complex relationship between changes in oxygenation status and changes in R*2: the case of insulin and NS-398, two inhibitors of oxygen consumption.

Insulin and NS-398 have been reported to inhibit oxygen consumption in experimental tumor models, thereby increasing oxygenation and radiosensitization. The aim of this work was to use MRI to study changes in murine FSaII tumor hemodynamics after administration of those oxygen consumption inhibitors. A multiple-echo gradient-echo (GRE) MRI sequence (4.7 T) was used to map changes in three factors: the GRE signal (at TE=20 ms), the parameter S0 (theoretical signal at TE=0 ms), and the relaxation rate R*2. Perfusion maps were obtained by dynamic contrast-enhanced (DCE) MRI. Insulin caused a significant decrease in the tumor blood oxygen level-dependent (BOLD) signal over time. factor This was likely the result of decreased blood flow, since both S0 and the percentage of perfused tumor decreased as well. Tumor R*2 did not change significantly in response to the treatments, which is surprising considering that other non-MRI techniques (electron paramagnetic resonance (EPR) oximetry and fiber-optic probes) have shown that tumor oxygenation increases after treatment. This suggests that metabolic changes associated with vasoactive challenges may have an unpredictable influence on blood saturation and R*2. In conclusion, this study further emphasizes the fact that changes in BOLD signal and R*2 in tumors do not depend uniquely on changes in oxygenation status.     

Detrimental effects of BOLD signal in arterial spin labeling fMRI at high field strength.

Arterial spin labeling (ASL) MRI is a useful technique for noninvasive measurement of cerebral blood flow (CBF) in humans. High field strength provides a unique advantage for ASL because of longer blood T(1) relaxation times, making this technique a promising quantitative approach for functional brain mapping. However, higher magnetic field also introduces new challenges. Here it is shown that the CBF response determined using ASL functional MRI (fMRI) at 3.0 T contains significant contamination from blood-oxygenation-level-dependent (BOLD) effects. Due to interleaved acquisitions of label and control images, difference in blood oxygenation status between these two scans can cause incomplete cancellation of the static signal upon image subtraction, resulting in a BOLD-related artifact in the estimated CBF hemodynamics. If not accounted for, such an effect can complicate the interpretation of the ASL results, e.g., causing a delayed onset and offset of the response, or inducing an artifactual poststimulus undershoot. The BOLD contribution also decreases the sensitivity of ASL-based fMRI. Correction methods are proposed to reduce the artifact, giving increased number of activated voxels (18+/-5%, P=0.006) and more accurate estimation of CBF temporal characteristics.     

Acute cerebral ischemia in rats studied by Carr-Purcell spin-echo magnetic resonance imaging: assessment of blood oxygenation level-dependent and tissue effects on the transverse relaxation.

Acute cerebral ischemia has been shown to be associated with an enhanced transverse relaxation rate in rat brain parenchyma, chiefly due to the blood oxygenation level-dependent (BOLD) effect. In this study, Carr-Purcell R(2) (CP R(2)), acquired both with short and long time intervals between centers of adiabatic pi-pulses (tau(CP)), was used to assess the contributions of BOLD and tissue effects to the transverse relaxation in two brain ischemia models of rat at 4.7 T. R(1rho) and diffusion MR images were also acquired in the same animals. During the first minutes of global ischemia, the long tau(CP) R(2) in brain parenchyma increased, whereas the short tau(CP) R(2) was unchanged. Based on the simulations, and using constraints of intravascular BOLD effect on parenchymal R(2), the former observation was ascribed to be due to susceptibility changes arising in the extravascular compartment. R(1rho) declined almost immediately after the onset of focal cerebral ischemia, and further declined during the evolution of ischemic damage. Interestingly, short tau(CP) CP R(2) started to decline after some 20 min of focal ischemia and declined over a time course similar to that of R(1rho), indicating that it may be an MRI marker for irreversible tissue changes in cerebral ischemia. The present results show that CP R(2) MRI can reveal both tissue- and blood-derived contrast changes in acute cerebral ischemia.     

紫外线照射充氧自血回输对兔急性梭曼中毒血清一氧化氮及一氧化氮合酶水平的影响

目的:探讨紫外线照射充氧自血回输(UB IO)治疗梭曼急性中毒的机理。方法:建立兔急性梭曼中毒模型。100只家兔随机分为正常对照组、中毒组、常规治疗组、UB IO治疗组及UB IO加常规治疗(复合治疗)组。观察14 d,检测兔血清一氧化氮(NO)及一氧化氮合酶(NOS)水平的变化。结果:正常对照组中兔血清NO及NOS水平分别为(8.38±3.40)μmol/L及(19.25±2.11)U/m l,中毒组中兔血清NO及NOS水平分别为(6.81±1.57)μmol/L及(17.46±1.12)U/m l,两组间有显著性差异(P<0.05)。经UB IO和复合治疗后,NO浓度分别为(7.88±2.05)及(8.03±2.46)μmol/L,NOS水平分别为(19.23±2.67)及(18.73±2.51)U/m l,与中毒组比较有显著差异(P<0.05)。结论:梭曼急性中毒后可使血清NO及NOS水平发生改变,此改变可能参与梭曼中毒的损伤,UB IO治疗对NO、NOS有极为有益的调节作用,能有效地治疗梭曼急性中毒。     

Quantification of myocardial blood flow and blood flow reserve in the presence of arterial dispersion: a simulation study.

Myocardial blood flow (MBF) can be quantified using dynamic T1-weighted MRI of diffusible tracers and a mathematical model of underlying vasculature. Quantification of MBF by means of T1- weighted MRI requires knowledge of the arterial input function (AIF). The AIF can be estimated from the left ventricular (LV) cavity. However, dispersion may occur between the LV and the tissue of interest because of the laminar blood flow profiles, branching of venules, and because of stenosis. To evaluate the influence of dispersion on the results of MBF quantification, a simulation study was performed. The dispersion was described as a convolution of the AIF with an exponential residue function. Synthetic tissue and AIF curves were analyzed and the derived parameters fit to the simulated parameters. The results show that an unaccounted dispersion may result in a systematic underestimation of MBF up to approximately 50%. Underestimation increases with increasing dispersion and with increasing MBF. Assuming equal dispersion at rest and during hyperemia, myocardial perfusion reserve (MPR) estimates are also susceptible to underestimation of approximately 20%. An unaccounted dispersion therefore can lead to systematic underestimation of both blood flow and perfusion reserve.