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. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

On the dual contrast enhancement mechanism in frequency‐selective inversion‐recovery magnetic resonance angiography (IRON‐MRA)

The susceptibility of blood changes after administration of a paramagnetic contrast agent that shortens T₁. Concomitantly, the resonance frequency of the blood vessels shifts in a geometry‐dependent way. This frequency change may be exploited for incremental contrast generation by applying a frequency‐selective saturation prepulse prior to the imaging sequence. The dual origin of vascular enhancement depending first on off‐resonance and second on T₁ lowering was investigated in vitro, together with the geometry dependence of the signal at 3T. First results obtained in an in vivo rabbit model are presented. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

High temporal resolution in vivo blood oximetry via projection‐based T2 measurement

Measuring venous oxygen saturation (HbO₂) in large blood vessels can provide important information about oxygen delivery and its consumption in vital organs. Quantification of blood's T₂ value via MR can be utilized to determine HbO₂ noninvasively. We propose a fast method for in vivo blood T₂ quantification via computing the complex difference of velocity‐encoded projections. As blood flows continuously, its signal can be robustly isolated from the surrounding tissue by computing the complex difference of two central k‐space lines with different velocity encodings. This resultant signal can then be measured as a function of echo time for rapidly quantifying T₂ of blood. We applied the method to quantify HbO₂ in three cerebral veins at rest and in one of the veins in response to hypercapnia. Average HbO₂ measurements in superior sagittal sinus (SSS), straight sinus and internal jugular vein in the group were 63 ± 3%, 68 ± 4% and 65 ± 4%, respectively. Average HbO₂ values in SSS during baseline, hypercapnia, and recovery were 63 ± 2%, 79 ± 5%, and 61 ± 3%, respectively. When compared with standard T₂ quantification techniques, the proposed method is fast, reliable, and robust against partial volume effects. Magn Reson Med 70:785–790, 2013. © 2012 Wiley Periodicals, Inc.     

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.     

Field‐cycling NMR relaxometry with spatial selection

Fast field‐cycling MRI offers access to sources of endogenous information not available from conventional fixed‐field imagers. One example is the T₁ dispersion curve: a plot of T₁ versus field strength. We present a pulse sequence that combines saturation‐recovery/inversion‐recovery T₁ determination with field cycling and point‐resolved spectroscopy localization, enabling the measurement of dispersion curves from volumes selected from a pilot image. Compared with a nonselective sequence, our method of volume selection does not influence measurement accuracy, even for relatively long echo times and in the presence of radiofrequency field nonuniformity. The measured voxel profile, while not ideal, corresponds with that expected from the image slice profile. On a whole‐body fast field‐cycling scanner with 59‐mT detection, the sensitivity of the experiment is sufficient to reveal distinctive “quadrupole dips” in dispersion curves of protein‐rich human tissue in vivo. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

In vivo tumor lactate relaxation measurements by selective multiple‐quantum‐coherence (Sel‐MQC) transfer

The frequency‐selective multiple‐quantum‐coherence (Sel‐MQC) lactate (Lac) filter offers complete lipid and water suppression in a single scan for robust in vivo detection of tumor Lac, even in the presence of abundant lipids. Conversion of the detected signal into accurate tissue concentrations of Lac requires knowledge of in vivo Lac T₁ and T₂ relaxation times. This work reports modifications to the Sel‐MQC pulse sequence, T₁‐ and T₂‐Sel‐MQC, that facilitate relaxation measurements of Lac. The T₁‐Sel‐MQC sequence combines an inversion prepulse with the Sel‐MQC filter. The T₂‐Sel‐MQC sequence incorporates a CH₃‐selective 180° pulse during the MQ preparation period to overcome the J‐modulation effects and allow the insertion of variable echo delays. The performance of these sequences was evaluated with the use of phantoms and subcutaneous murine tumor models in vivo. The present approach will allow investigators to correct for the relaxation‐induced Lac signal loss in Sel‐MQC experiments for the quantitative mapping of in vivo tumor Lac distribution. Magn Reson Med 52:902–906, 2004. © 2004 Wiley‐Liss, Inc.     

Optimized visualization of vessels in contrast enhanced intracranial MR angiography

In this study, the problem of small vessel visualization in magnetic resonance angiography is addressed. The loss of vessel contrast due to slow flow-related signal saturation can be compensated by the T₁ reduction obtained from the use of an MR contrast agent, such as Gd-DTPA. The vesselfbackground signal-difference-to-noise ratio (SDNR) is shown to strongly depend on the imaging parameters, as well as on the time course of the blood T₁ values obtained from the contrast injection. Specifically, it was found that vessel SDNR increases almost linearly with TR, if the sampling bandwidth is reduced proportionately.     

Noninvasive measurement of renal hemodynamic functions using gadolinium enhanced magnetic resonance imaging

A technique for the assessment of single kidney hemodynamic functions utilizing a novel MR pulse sequence in conjuinction with MR contrast material administration is described. Renal extraction fraction (EF) is derived by measuring the concentration of the incoming contrast agent in the renal artery and the outgoing concentration in the renal vein. The glomerular filtration rate (GFR) can then be determined by the product of EF and renal plasma flow. A modified iniversion recovery MR pulse sequence is used to measure the T₁ of moving blood. This pulse sequence uses a spatially nonselective inversion pulse. A series of small flip angle detection pulses are then used to monitor the recovery of longitudinal spin magnetization in an image plane intersecting the renal vessels. The recovery rate is measured in each vessel and the T₁ of blood determined. These T₁ measurements are then used to determine the ratio of contrast concentration in the renal arteries and veins. Blood flow measurements can be obtained simultaneously with T₁ measurements by inserting flow-encoding magnetic field gradients into the pulse seqiuence. Preliminary results in human volunteers suggest the feasibility of noninvasively determining hemodynamic functions with magnetic resonance.     

In vivo differentiation of two vessel wall layers in lower extremity peripheral vein bypass grafts: Application of high‐resolution inner‐volume black blood 3D FSE

Lower extremity peripheral vein bypass grafts (LE‐PVBG) imaged with high‐resolution black blood three‐dimensional (3D) inner‐volume (IV) fast spin echo (FSE) MRI at 1.5 Tesla possess a two‐layer appearance in T1W images while only the inner layer appears visible in the corresponding T2W images. This study quantifies this difference in six patients imaged 6 months after implantation, and attributes the difference to the T₂ relaxation rates of vessel wall tissues measured ex vivo in two specimens with histologic correlation. The visual observation of two LE‐PVBG vessel wall components imaged in vivo is confirmed to be significant (P < 0.0001), with a mean vessel wall area difference of 6.8 ± 2.7 mm² between contrasts, and a ratio of T1W to T2W vessel wall area of 1.67 ± 0.28. The difference is attributed to a significantly (P < 0.0001) shorter T₂ relaxation in the adventitia (T₂ = 52.6 ± 3.5 ms) compared with the neointima/media (T₂ = 174.7 ± 12.1 ms). Notably, adventitial tissue exhibits biexponential T₂ signal decay (P < 0.0001 vs monoexponential). Our results suggest that high‐resolution black blood 3D IV‐FSE can be useful for studying the biology of bypass graft wall maturation and pathophysiology in vivo, by enabling independent visualization of the relative remodeling of the neointima/media and adventitia. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Quantitative determination of magnetic force on a coronary stent in MRI

Purpose:

To introduce an analytical method for a quantitative determination of magnetic force on a coronary stent in the magnetic resonance imaging (MRI) magnet that is generally applicable to metallic implants. Magnetic forces on metallic implants in the MRI magnets are traditionally determined empirically by measuring deflection from the vertical plane at the central axis of the magnet and at the point of the largest force along the longitudinal axis of the magnet.

Materials and Methods:

Magnetic and chemical characterization of the stents was performed by a commercial magnetometer and energy‐dispersive X‐ray spectroscopy. Magnetic force on the stents fabricated of paramagnetic alloys (surgical stainless steel and cobalt–chromium) was determined by measuring the stent's magnetic dipole moment and employing the on‐axis magnetic field profile of an MRI magnet.

Results:

The maximum force on the stainless steel stent was found to be F_S,max = 0.18 mN, whereas on the cobalt–chromium stent it was F_C,max = 0.06 mN.

Conclusion:

The magnetic force on the investigated paramagnetic stents is even smaller than the gravitational force acting on the stents in the Earth's gravity field, so that it has no physiological impact on the stented vessels. J. Magn. Reson. Imaging 2013;37:391–397. © 2012 Wiley Periodicals, Inc.     

Measurement of regional blood oxygenation and cerebral hemodynamics

An echo planar linewidth mapping technique, Shufflebutt, has allowed temporal measurements of changes in linewidth caused by static inhomogeneities (ΔLWSI) and transverse relaxation rate (ΔR2) in models of hypoxia and hypercapnia. We demonstrate these changes are due to intravascular susceptibility differences(Δ_X) between the blood and tissue. Contrast agent injections at a /Δ_X equivalent to that of deoxygenatetd blood showed a twofold difference between the contrast agent and physiological anoxia values. Hypercapnia decreased both ΔLWSI and ΔR2 consistent with an increase in blood oxygenation. We attribute these findings to constant oxygen extraction during an increase in blood flow, resulting in less deoxygenated venous blood and thus reduced Δ_X. For in vivoperturbations we found that ΔR/ΔR2′ ≈ 0.33, a ratio much different from that measured in whole blood phantoms (ΔR/ΔR2′ ≈ 2). This demonstrates that signal changes in these studies are produced predominantly by dephasing of extravascular protons due to field inhomogeneities produced by intravascular deoxygenated hemoglobin (deoxyHb).     

Investigation of relationships between transverse relaxation rate,diffusion coefficient,and labeled cell concentration in ischemic rat brain using MRI

MRI has been used to evaluate labeled cell migration and distribution. However, quantitative determination of labeled cell concentration using MRI has not been systematically investigated. In the current study, we investigated the relationships between labeled cell concentration and MRI parameters of transverse relaxation rate, R₂, and apparent diffusion coefficient (ADC), in vitro in phantoms and in vivo in rats after stroke. Significant correlations were detected between iron concentration or labeled cell concentration and MRI measurements of R₂, ADC, and ADC×R₂ in vitro. In contrast, in vivo labeled cell concentration did not significantly correlate with R₂, ADC, and ADC×R₂. A major factor for the absence of a significant correlation between labeled cell concentration and MRI measurements in vivo may be attributed to background effects of ischemic tissue. By correcting the background effects caused by ischemic damage, ΔR₂ (difference in R₂ values in the ischemic tissue with and without labeled cells) exhibited a significant correlation to labeled cell concentration. Our study suggests that MRI parameters have the potential to quantitatively determine labeled cell concentration in vivo. Magn Reson Med, 2009. © 2008 Wiley‐Liss, Inc.     

Susceptibility weighted imaging at ultra high magnetic field strengths: Theoretical considerations and experimental results

We present numerical simulations and experimental results for susceptibility weighted imaging (SWI) at 7 T. Magnitude, phase, and SWI contrast were simulated for different voxel geometries and imaging parameters, resulting in an echo time of 14 msec for optimum contrast between veins and surrounding tissue. Slice thickness of twice the in‐plane voxel size or more resulted in optimum vessel visibility. Phantom and in vivo data are in very good agreement with the simulations and the delineation of vessels at 7 T was superior compared to lower field strengths. The phase of the complex data reveals anatomical details that are complementary to the corresponding magnitude images. Susceptibility weighted imaging at very high field strengths is a promising technique because of its high sensitivity to tissue susceptibility, its low specific absorption rate, and the phase's negligible sensitivity to B₁ inhomogeneities. Magn Reson Med 60:1155–1168, 2008. © Wiley‐Liss, Inc.     

J‐difference lactate editing at 3.0 Tesla in the presence of strong lipids

Purpose

To implement in vivo detection of lactate in the presence of lipids by proton magnetic resonance spectroscopy at a 3 Tesla (T) field strength for potential applications in human tumors outside of the brain.

Materials and Methods

The BASING J‐difference sequence was implemented in the presence of high lipid concentrations in phantoms and in vivo at 3 Tesla.

Results

The effectiveness of the lactate editing scheme is demonstrated in phantoms containing both lactate and lipids and in vivo in ischemic induced human muscle.

Conclusion

The ability of the BASING J‐difference technique to detect lactate in the presence of strong lipid signals outside the brain at 3T is feasible. This robust technique should permit noninvasive lactate measurements in human tumors to investigate its potential as a prognostic indicator. J. Magn. Reson. Imaging 2008;28:1492–1498. © 2008 Wiley‐Liss, Inc.     

Phase‐based regional oxygen metabolism (PROM) using MRI

Venous oxygen saturation (Y_v) in cerebral veins and the cerebral metabolic rate of oxygen (CMRO₂) 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 CMRO₂ using noninvasive imaging. This article proposes a technique to quantify CMRO₂ 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 CMRO₂ 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 CMRO₂ (mean ± SD, n = 12), which is consistent with values previously reported by positron emission tomography and MRI. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Real‐time RF pulse adjustment for B0 drift correction

Although the magnetic field of an MR scanner is very stable under little or no load, it can become less stable under heavy‐duty cycle conditions, such as in diffusion tensor imaging (DTI). Uncorrected, such field drifts lead to an apparent image shift along the phase‐encoding direction and decreasing effectiveness of fat saturation pulses. A method is presented to adjust the center frequency of all RF pulses and the receiver in real time during the acquisition. No data postprocessing or changes to the sequence timing are necessary. In vivo acquisitions were performed to assess the prolonged effectiveness of fat saturation. Field drifts of approximately 2.5 Hz/min were measured and corrected during DTI acquisitions at b‐values of up to 3000 s/mm². The effectiveness of fat saturation diminished over the duration of an 18‐min acquisition when the drift was left uncorrected. The proposed method corrects for apparent image shift and ensures continuously effective fat saturation over the duration of an acquisition. Magn Reson Med, 2006. © 2006 Wiley‐Liss, Inc.     

Chemical shift‐induced phase errors in phase‐contrast MRI

Phase‐contrast magnetic resonance imaging is subject to numerous sources of error, which decrease clinical confidence in the reported measures. This work outlines how stationary perivascular fat can impart a significant chemical shift induced phase‐contrast magnetic resonance imaging measurement error using computational simulations, in vitro, and in vivo experiments. This chemical shift error does not subtract in phase difference processing, but can be minimized with proper parameter selection. The chemical shift induced phase errors largely depend on both the receiver bandwidth and the TE. Both theory and an in vivo comparison of the maximum difference in net forward flow between vessels with and without perivascular fat indicated that the effects of chemically shifted perivascular fat are minimized by the use of high bandwidth (814 Hz/px) and an in‐phase TE (high BW‐TE_IN). In healthy volunteers (N = 10) high BW‐TE_IN significantly improves intrapatient net forward flow agreement compared with low bandwidth (401 Hz/px) and a mid‐phase TE as indicated by significantly decreased measurement biases and limits of agreement for the ascending aorta (1.8 ± 0.5 mL vs. 6.4 ± 2.8 mL, P = 0.01), main pulmonary artery (2.0 ± 0.9 mL vs. 11.9 ± 5.8 mL, P = 0.04), the left pulmonary artery (1.3 ± 0.9 mL vs. 5.4 ± 2.5 mL, P = 0.003), and all vessels (1.7 ± 0.8 mL vs. 7.2 ± 4.4 mL, P = 0.001). Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

In vivo NMR imaging and spectroscopic investigation of renal pathology in lean and obese rat kidneys

Diabetic nephropathy is a major cause of end-stage renal failure. While our understanding of the pathogenesis of nephropathy is incomplete, progressive glomerular injury appears to play a significant role in the decline of renal function. Proton NMR spectroscopy and imaging techniques were used to address changes in renal pathology associated with glomerular mesangial expansion in vivo in kidneys from spontaneously obese and lean (control) littermate Zucker rats. Fully functioning rat kidneys were surgically exposed and externalized for direct NMR signal detection via a coil placed around the organ. High-resolution (78 μm in plane) proton images were obtained at 4.7 T magnetic field strength revealing fine structure within the well-defined cortical and medullary regions. The obese rat kidney images were distinct in appearance from the lean kidney images and exhibited marked cortical expansion as well as increased overall kidney size. Enlargement of mean glomerular diameter was verified histologically in the obese kidneys as compared with the lean kidneys. Proton T₁ and T₂ relaxation times were determined from the entire kidney using standard spectroscopic techniques, and from specific regions within the kidney from multiple T₁ and T₂-weighted images. Additionally, image contrast enhancement resulting from saturation transfer between protons in restricted-mobility environments and mobile water protons within the kidney was investigated in the lean and obese rat kidneys using magnetization-transfer imaging techniques. At the early stage of renal injury examined in this study, diseased and healthy kidneys could not be differentiated on the basis of relaxation times alone. The magnitude of saturation transfer obtained in cortical tissue in the lean and obese kidneys was also not statistically significantly different. However, the magnitude of saturation transfer achieved in the medullary tissue of obese kidneys was statistically significantly less than that achieved in lean kidneys.     

Real‐time dynamic frequency and shim correction for single‐voxel magnetic resonance spectroscopy

Subject motion during brain magnetic resonance spectroscopy acquisitions generally reduces the magnetic field (B₀) homogeneity across the volume of interest or voxel. This is the case even if prospective motion correction ensures that the voxel follows the head. We introduce a novel method for rapidly mapping linear variations in B₀ across a small volume using two‐dimensional excitations. The new field mapping technique was integrated into a prospectively motion‐corrected single‐voxel ¹H magnetic resonance spectroscopy sequence. Interference with the magnetic resonance spectroscopy measurement was negligible, and there was no penalty in scan time. Frequency shifts were also measured continuously, and both frequency and first‐order shim corrections were applied in real time. Phantom experiments and in vivo studies demonstrated that the resulting motion‐ and shim‐corrected sequence is able to mitigate line broadening and maintain spectral quality even in the presence of large‐amplitude subject motion. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Application of a 3D volume 19FMR imaging protocol for mapping oxygen tension (pO2) in perfluorocarbons at low field

A limited flip angle gradient-echo 3D volume acquisition imaging protocol for mapping partial pressure of oxygen (pO₂) in perfluorocarbon compounds (PFCs) at low field (0.14 T) is presented. The pO₂ measurement method is based on the paramagnetic effect of dissolved molecular oxygen (O₂) which reduces the PFC ¹⁹F T_1? Specific objectives related to imaging of PFCs through use of the protocol include improved image signal-to-noise characteristics and elimination of ¹⁹F chemical shift artifacts. A parametric Wiener deconvolution filtering algorithm is used for suppression of ¹⁹F chemical shift artifacts. Application of the protocol is illustrated in a series of calculated pO₂ maps of a gas equilibrated, multi-chamber phantom containing perfluorotributylamine (FC-43). The utility of the protocol is demonstrated in vivo through images of a commercially available perfluorocarbon based blood substitute emulsion containing FC-43 sequestered in the liver and spleen of a rat.