NMR Measurement of Perfusion Using Arterial Spin Labeling Without Saturation of Macromolecular Spins

“…ASL was accomplished by a separate transmitter coil placed on the subject's neck, as described in Ref. 17, while imaging was carried out using the standard GE birdcage coil. The labeling coil was a custom figure-8 coil (6-cm-diameter loops), which was built such that the two loops were at a 130°angle relative to each other.…”

Section: Human Subjectsmentioning

confidence: 99%

Quantification of perfusion fMRI using a numerical model of arterial spin labeling that accounts for dynamic transit time effects

Hernandez‐García

Lee

Vazquez

et al. 2005

Magnetic Resonance in Med

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We recently presented a new two-coil approach to continuous arterial spin labeling (CASL), (dubbed Turbo-CASL) (1), that significantly enhances the temporal resolution of ASL measurements while the SNR of the measurement is maintained. An additional benefit of the technique is that its sensitivity to transit time changes can be leveraged to increase the sensitivity to brain responses. In brief, Turbo-CASL consists of a continuous labeling experiment in which the control image is collected immediately after the tagging pulse, but before the tag reaches the plane of interest. The tagged image is collected when the tissue concentration of the label reaches its maximum. With Turbo-CASL one can collect samples at a much faster rate compared to standard CASL techniques, and still allow for a long labeling time. In our previous publication (1) we showed that the Turbo-CASL method also had an advantage in terms of SNR per unit of time, even though the amount of accumulated tag is not allowed to reach its steady-state maximum. The method is very sensitive to transit time changes. This makes quantification of perfusion more difficult, but it can also serve to exaggerate the perfusion increases that occur during activation and hence improve detection power, as shown in our previous work. In this follow-up paper we address the quantification issues that arise as a result of these transit time changes.Quantification of blood flow from ASL measurements is a difficult problem that requires the measurement of many parameters. A number of models have been presented to quantify perfusion, for a number of ASL acquisition schemes. These models are largely based on the first-order kinetics of the wash-in and wash-out of the inversion label into the tissue, taking into account its T 1 decay and exchange properties. The current models aim to quantify the microvascular perfusion, and consequently the larger arterial signal is routinely suppressed by the use of crusher gradients or postinversion delays. These delays have the added benefit of desensitizing the signal to the arterial transit time (ATT) (2-4). These models were developed for the cases of steady-state flow and transit time (5-10). Under such circumstances, the uptake of the arterial tag can be modeled as a linear system.However, in addition to the flow increase, neuronal activation is also accompanied by a reduction in the ATT of approximately 10 -20%, as observed by Gonzalez-At et al. (10) and Yang et al. (11). In the case of Turbo-CASL, we have calculated (1,12) that the signal can potentially change approximately 15% by a transit time change alone using the existing models.As a result, the ASL signal observed in our Turbo-CASL approach has nonlinear characteristics that are not easily explained by existing models, and likely distort the underlying flow response. In this study we extended the general framework of the kinetics of the arterial label to the case in which flow and transit times change dynamically in a given paradigm. We then developed and implemented a method...

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“…Assuming arterial blood flow rate is ∼1 cm/s [34], the flow-induced phase encoding error is negligible for a 2 mm slice with 6.8ms TE. In the T 1ρ -weighted images, the non-selective off-resonance pulse irradiates the spins in a much larger volume than that in the imaging slice.…”

Section: Rat Cerebral Vasculaturementioning

confidence: 99%

Water diffusion-exchange effect on the paramagnetic relaxation enhancement in off-resonance rotating frame

Zhang

Xie

2007

Journal of Magnetic Resonance

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The off-resonance rotating frame technique based on the spin relaxation properties of off-resonance T 1ρ can significantly increase the sensitivity of detecting paramagnetic labeling at high magnetic fields. However, the in vivo detectable dimension for labeled cell clusters/tissues in T 1ρ -weighted images is limited by the water diffusion-exchange between mesoscopic scale compartments. An experimental investigation of the effect of water diffusion-exchange between compartments on the paramagnetic relaxation enhancement of paramagnetic agent compartment is presented for in vitro/ in vivo models. In these models, the size of paramagnetic agent compartment is comparable to the mean diffusion displacement of water molecules during the long RF pulses that are used to generate the off-resonance rotating frame. The three main objectives of this study were: (1) to qualitatively correlate the effect of water diffusion-exchange with the RF parameters of the long pulse and the rates of water diffusion, (2) to explore the effect of water diffusion-exchange on the paramagnetic relaxation enhancement in vitro, and (3) to demonstrate the paramagnetic relaxation enhancement in vivo. The in vitro models include the water permeable dialysis tubes or water permeable hollow fibers embedded in cross-linked proteins gels. The MWCO of the dialysis tubes was chosen from 0.1 to 15 kDa to control the water diffusion rate. Thin hollow fibers were chosen to provide submillimeter scale compartments for the paramagnetic agents. The in vivo model utilized the rat cerebral vasculatures as a paramagnetic agent compartment, and intravascular agents (Gd-DTPA) 30 -BSA were administrated into the compartment via bolus injections. Both in vitro and in vivo results demonstrate that the paramagnetic relaxation enhancement is predominant in the T 1ρ -weighted imaging in the presence of water diffusion exchange. The T 1ρ contrast has substantially higher sensitivity than the conventional T 1 contrast in detecting paramagnetic agents, especially at low paramagnetic agent volumetric fractions, low paramagnetic agent concentrations, and low RF amplitudes. Short pulse duration, short pulse recycle delay and efficient paramagnetic relaxation can reduce the influence of water diffusion-exchange on the paramagnetic enhancement. This study paves the way for the design of off-resonance rotating experiments to detect labeled cell clusters/tissue compartments in vivo at a sub-millimeter scale.

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NMR Measurement of Perfusion Using Arterial Spin Labeling Without Saturation of Macromolecular Spins

Cited by 148 publications

References 14 publications

Murine orthostatic response during prolonged vertical studies: Effect on cerebral blood flow measured by arterial spin‐labeled MRI

Murine orthostatic response during prolonged vertical studies: Effect on cerebral blood flow measured by arterial spin‐labeled MRI

Quantification of perfusion fMRI using a numerical model of arterial spin labeling that accounts for dynamic transit time effects

Water diffusion-exchange effect on the paramagnetic relaxation enhancement in off-resonance rotating frame

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