Retrospective correction for induced magnetic field inhomogeneity in measurements of large-vessel hemoglobin oxygen saturation by MR susceptometry

Simultaneous mapping of temporally-resolved blood flow velocity and oxygenation in femoral artery and vein during reactive hyperemia

BACKGROUND

Post-occlusive hyperemia is often used as a paradigm to evaluate vascular reactivity, for example by measuring post-ischemic flow-mediated dilation, arterial blood flow or temporally resolved venous blood oxygenation (HbO2). Here we demonstrate the feasibility of a simultaneous measurement of blood flow and HbO2 in the femoral circulation as part of a single procedure.

METHODS

A multi-echo GRE pulse sequence was designed and implemented to collect velocity-encoded projections in addition to full-image echoes for field mapping as a means to quantify intravascular magnetic susceptibility. The method's feasibility was evaluated at 3T in a small pilot study involving two groups of healthy subjects (mean ages 26 ± 1.6 and 59 ± 7.3 years, N = 7 and 5, respectively) in terms of six parameters characterizing the time-course of reactive hyperemia and their sensitivity to differentiate age effects. The reproducibility was assessed on two of the seven young healthy subjects with three repeated measurements.

RESULTS

The physiological parameters agree with those obtained with current methods that quantify either velocity or HbO2 alone. Of the six measures of vascular reactivity, one from each group was significantly different in the two subject groups (p < 0.05) even though the study was not powered to detect differences. The mean coefficient of variation (CV) from two subjects undergoing repeat scans were approximately 8% for the oximetric and the arterial velocimetric parameters in the femoral vein and artery, respectively, considerably below intersubject CVs (20 and 35%, for the young and older subject groups, respectively).

CONCLUSION

The proposed method is able quantify multiple parameters that may lead to more detailed assessment of peripheral vascular reactivity in a single cuff paradigm rather than in separate procedures as required previously, thereby improving measurement efficiency and patient comfort.

Accuracy of the cylinder approximation for susceptometric measurement of intravascular oxygen saturation

Susceptometry-based MR oximetry has previously been shown suitable for quantifying hemoglobin oxygen saturation in large vessels for studying vascular reactivity and quantification of global cerebral metabolic rate of oxygen utilization. A key assumption underlying this method is that large vessels can be modeled as long paramagnetic cylinders. However, bifurcations, tapering, noncircular cross-section, and curvature of these vessels produce substantial deviations from cylindrical geometry, which may lead to errors in hemoglobin oxygen saturation quantification. Here, the accuracy of the "long cylinder" approximation is evaluated via numerical computation of the induced magnetic field from 3D segmented renditions of three veins of interest (superior sagittal sinus, femoral and jugular vein). At a typical venous oxygen saturation of 65%, the absolute error in hemoglobin oxygen saturation estimated via a closed-form cylinder approximation was 2.6% hemoglobin oxygen saturation averaged over three locations in the three veins studied and did not exceed 5% for vessel tilt angles <30° at any one location. In conclusion, the simulation results provide a significant level of confidence for the validity of the cylinder approximation underlying MR susceptometry-based oximetry of large vessels.

Phase-based regional oxygen metabolism (PROM) using MRI

Venous oxygen saturation (Y(v) ) in cerebral veins and the cerebral metabolic rate of oxygen (CMRO(2)) are important indicators for brain function and disease. Although MRI has been used for global measurements of these parameters, currently there is no recognized technique to quantify regional Y(v) and CMRO(2) using noninvasive imaging. This article proposes a technique to quantify CMRO(2) from independent MRI estimates of Y(v) and cerebral blood flow. The approach uses standard gradient-echo and arterial spin labeling acquisitions to make these measurements. Using MR susceptometry on gradient-echo phase images, Y(v) was quantified for candidate vein segments in gray matter that approximate a long cylinder parallel to the main magnetic field. Local cerebral blood flow for the identified vessel was determined from a corresponding region in the arterial spin labeling perfusion map. Fick's principle of arteriovenous difference was then used to quantify CMRO(2) locally around each vessel. Application of this method in young, healthy subjects provided gray matter averages of 59.6% ± 2.3% for Y(v), 51.7 ± 6.4 mL/100 g/min for cerebral blood flow, and 158 ± 18 μmol/100 g/min for CMRO(2) (mean ± SD, n = 12), which is consistent with values previously reported by positron emission tomography and MRI.

MRI estimation of global brain oxygen consumption rate

Measuring the global cerebral metabolic rate of oxygen (CMRO(2)) is a valuable tool for assessing brain vitality and function. Measurement of blood oxygen saturation (HbO(2)) and flow in the major cerebral outflow and inflow vessels can provide a global estimate of CMRO(2). We demonstrate a rapid noninvasive method for quantifying CMRO(2) by simultaneously measuring venous oxygen saturation in the superior sagittal sinus with magnetic resonance susceptometry-based oximetry, a technique that exploits the intrinsic susceptibility of deoxygenated hemoglobin, and the average blood inflow rate with phase-contrast magnetic resonance imaging. The average venous HbO(2), cerebral blood flow, and global CMRO(2) values in eight healthy, normal study subjects were 64%+/-4%, 45.2+/-3.2 mL per 100 g per minute, and 127+/-7 micromol per 100 g per minute, respectively. These values are in good agreement with those reported in literature. The technique described is noninvasive, robust, and reproducible for in vivo applications, making it ideal for use in clinical settings for assessing the pathologies associated with dysregulation of cerebral metabolism. In addition, the short acquisition time (approximately 30 seconds) makes the technique suitable for studying the temporal variations in CMRO(2) in response to physiologic challenges.

Evaluation of cuff-induced ischemia in the lower extremity by magnetic resonance oximetry

OBJECTIVES

The aim of this study was to evaluate vascular function in the lower extremities by making direct time-course measurement of oxygen saturation in the femoral/popliteal arteries and veins during cuff-induced reactive hyperemia with magnetic resonance imaging-based oximetry.

BACKGROUND

Magnetic resonance imaging-based oximetry is a new calibration-free technique taking advantage of the paramagnetic nature of blood that depends on the volume fraction of deoxyhemoglobin in red blood cells.

METHODS

We compared post-occlusive blood oxygenation time-course of femoral/popliteal vessels in: 1) young healthy subjects (YH) (n = 10; mean ankle-brachial index [ABI] 1.0 +/- 0.1, mean age 30 +/- 7 years); 2) peripheral arterial disease (PAD) patients (n = 12; mean ABI 0.6 +/- 0.1, mean age 71 +/- 9 years); and 3) age-matched healthy control subjects (AHC) (n = 8; mean ABI 1.1 +/- 0.1, mean age 68 +/- 9 years). Blood oxygenation was quantified at 3.0-T field strength with a field mapping pulse sequence yielding the magnetic susceptibility difference between blood in the vessels and surrounding muscle tissue from which the intravascular blood oxygen saturation is computed as %HbO(2).

RESULTS

Significantly longer washout time (42 +/- 16 s vs. 14 +/- 4 s; p < 0.0001) and lower upslope (0.60 +/- 0.20 %HbO(2)/s vs. 1.32 +/- 0.41 %HbO(2)/s; p = 0.0008) were observed for PAD patients compared with healthy subjects (YH and AHC combined). Furthermore, greater overshoot was observed in YH than in AHC (21 +/- 8 %HbO(2) vs. 10 +/- 5 %HbO(2); p = 0.0116).

CONCLUSIONS

Post-occlusive transient changes in venous blood oxygenation might provide a new measure of vascular competence, which was found to be reduced in subjects with abnormal ABI, manifesting in prolonged recovery during the early phase of hyperemia.

Quantitative susceptibility map reconstruction from MR phase data using bayesian regularization: validation and application to brain imaging

The diagnosis of many neurologic diseases benefits from the ability to quantitatively assess iron in the brain. Paramagnetic iron modifies the magnetic susceptibility causing magnetic field inhomogeneity in MRI. The local field can be mapped using the MR signal phase, which is discarded in a typical image reconstruction. The calculation of the susceptibility from the measured magnetic field is an ill-posed inverse problem. In this work, a bayesian regularization approach that adds spatial priors from the MR magnitude image is formulated for susceptibility imaging. Priors include background regions of known zero susceptibility and edge information from the magnitude image. Simulation and phantom validation experiments demonstrated accurate susceptibility maps free of artifacts. The ability to characterize iron content in brain hemorrhage was demonstrated on patients with cavernous hemangioma. Additionally, multiple structures within the brain can be clearly visualized and characterized. The technique introduces a new quantitative contrast in MRI that is directly linked to iron in the brain.

Accuracy and precision of MR blood oximetry based on the long paramagnetic cylinder approximation of large vessels

An accurate noninvasive method to measure the hemoglobin oxygen saturation (%HbO(2)) of deep-lying vessels without catheterization would have many clinical applications. Quantitative MRI may be the only imaging modality that can address this difficult and important problem. MR susceptometry-based oximetry for measuring blood oxygen saturation in large vessels models the vessel as a long paramagnetic cylinder immersed in an external field. The intravascular magnetic susceptibility relative to surrounding muscle tissue is a function of oxygenated hemoglobin (HbO(2)) and can be quantified with a field-mapping pulse sequence. In this work, the method's accuracy and precision was investigated theoretically on the basis of an analytical expression for the arbitrarily oriented cylinder, as well as experimentally in phantoms and in vivo in the femoral artery and vein at 3T field strength. Errors resulting from vessel tilt, noncircularity of vessel cross-section, and induced magnetic field gradients were evaluated and methods for correction were designed and implemented. Hemoglobin saturation was measured at successive vessel segments, differing in geometry, such as eccentricity and vessel tilt, but constant blood oxygen saturation levels, as a means to evaluate measurement consistency. The average standard error and coefficient of variation of measurements in phantoms were <2% with tilt correction alone, in agreement with theory, suggesting that high accuracy and reproducibility can be achieved while ignoring noncircularity for tilt angles up to about 30 degrees . In vivo, repeated measurements of %HbO(2) in the femoral vessels yielded a coefficient of variation of less than 5%. In conclusion, the data suggest that %HbO(2) can be measured reproducibly in vivo in large vessels of the peripheral circulation on the basis of the paramagnetic cylinder approximation of the incremental field.