Recently, a T2-Relaxation-Under-Spin-Tagging (TRUST) MRI technique was developed to quantitatively estimate blood oxygen saturation fraction (Y) via the measurement of pure blood T2. This technique has shown promise for normalization of fMRI signals, for the assessment of oxygen metabolism, and in studies of cognitive aging and multiple sclerosis. However, a human validation study has not been conducted. In addition, the calibration curve used to convert blood T2 to Y has not accounted for the effects of hematocrit (Hct). In the present study, we first conducted experiments on blood samples under physiologic conditions, and the Carr-Purcell-Meiboom-Gill (CPMG) T2 was determined for a range of Y and Hct values. The data were fitted to a two-compartment exchange model to allow the characterization of a three-dimensional plot that can serve to calibrate the in vivo data. Next, in a validation study in humans, we showed that arterial Y estimated using TRUST MRI was 0.837±0.036 (N=7) during the inhalation of 14% O2, which was in excellent agreement with the gold-standard Y values of 0.840±0.036 based on Pulse-Oximetry. These data suggest that the availability of this calibration plot should enhance the applicability of TRUST MRI for non-invasive assessment of cerebral blood oxygenation.
The oxygen extraction fraction of the brain reports on the balance between oxygen delivery and consumption and can be used to assess deviations in physiological homeostasis. This is relevant clinically as well as for calibrating blood oxygen level-dependent functional MRI responses. Oxygen extraction fraction is reflected in the arteriovenous difference in oxygen saturation fraction (Y v 2 Y a ), which can be determined from venous T 2 values when arterial oxygenation is known. A pulse sequence is presented that allows rapid measurement (<1 min) of blood T 2 s in the internal jugular vein. The technique combines slice-saturation and blood inflow to attain high signal-to-noise ratio in blood and minimal contamination from tissue. The sequence is sensitized to T 2 using a nonselective Carr-Purcell-Meiboom-Gill T 2 preparation directly after slice saturation. Fast scanning (pulse repetition time of about 2 sec) is possible by using a nonselective saturation directly after acquisition to rapidly achieve steady-state longitudinal magnetization. The venous T 2 (for 10 msec Carr-Purcell-Meiboom-Gill interecho time) for normal volunteers was 62.4 6 6.1 msec (n 5 20). A calibration curve relating T 2 to blood oxygenation was established using a blood perfusion phantom. The oxygen extraction fraction (OEF) of the brain reports on the balance between oxygen delivery and consumption (1). Slight changes in OEF may reflect physiological perturbation, and a method to rapidly and noninvasively assess this parameter should be useful for clinical assessment of brain homeostasis. In addition, the blood oxygen level-dependent (BOLD) functional MRI effect reflects focal changes in OEF during activation (2-4) and changes in baseline OEF will affect the size of the BOLD effect. Measurement of whole-brain OEF would therefore be useful to calibrate the BOLD effect for baseline blood oxygenation in a manner similar to Lu et al. (5). The OEF is defined as (1,6):in which CMRO 2 is the cerebral metabolic rate for oxygen and C a the oxygen content, the product of total hemoglobin concentration [Hb tot ] in mM and Y a arterial oxygen saturation fraction. CBF is the cerebral blood flow.The venous deoxygenation fraction (1 À Y v ) is directly related to OEF via the modified Kety equation accounting for hypoxia (3)The blood deoxygenation fraction (1 À Y) in any type of vessel is directly related to 1/T 2 in that vessel (7-12) and calibration curves relating these two parameters can be used to determine Y-values from absolute T 2 (13-17). Determination of the venous relaxation times T 2,v , would allow determination of the oxygenation fractions of blood in these vessels, and, if Y a is known (or assumed) and a calibration curve available, the subsequent calculation of OEF.Measurement of blood T 2 in vivo is challenging because of the rapid flow of blood in larger vessels and the small size of microvascular blood vessels. To avoid the wash-in/wash-out effect encountered in conventional spin-echo sequences for large blood vessels, magnetization-pre...
Purpose Demonstrate the applicability of natural D-glucose as a T2 MRI contrast agent. Methods D-glucose solutions were prepared at multiple concentrations and variable pH. The relaxation rate (R2 = 1/T2) was measured at 3, 7, and 11.7T. Additional experiments were performed on blood at 11.7 T. Also, a mouse was infused with D-glucose (3.0 mmol/kg) and dynamic T2 weighted images of the abdomen acquired. Results The transverse relaxation rate depended strongly on glucose concentration and solution pH. A maximum change in R2 was observed around physiological pH (pH 6.8-7.8). The transverse relaxivities at 22°C (pH 7.3) were 0.021, 0.060, and 0.077 s-1mM-1 at 3.0, 7.0, and 11.7T, respectively. These values showed good agreement with expected values from the Swift-Connick equation. There was no significant dependence on glucose concentration or pH for T1 and the diffusion coefficient for these solutions. The transverse relaxivity in blood at 11.7 T was 0.09 s-1mM-1. The dynamic in vivo experiment showed a 10% drop in signal intensity after glucose infusion followed by recovery of the signal intensity after about 50-100 s. Conclusion Glucose can be used as a T2 contrast agent for MRI at concentrations that are already approved for human use.
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