Oxygen‐dependent hyperpolarized 129Xe brain MR

Hyperpolarized (HP) ¹²⁹Xe MR offers unique advantages for brain functional imaging (fMRI) because of its extremely high sensitivity to different chemical environments and the total absence of background noise in biological tissues. However, its advancement and applications are currently plagued by issues of signal strength. Generally, xenon atoms found in the brain after inhalation are transferred from the lung via the bloodstream. The longitudinal relaxation time (T₁) of HP ¹²⁹Xe is inversely proportional to the pulmonary oxygen concentration in the lung because oxygen molecules are paramagnetic. However, the T₁ of ¹²⁹Xe is proportional to the pulmonary oxygen concentration in the blood, because the higher pulmonary oxygen concentration will result in a higher concentration of diamagnetic oxyhemoglobin. Accordingly, there should be an optimal pulmonary oxygen concentration for a given quantity of HP ¹²⁹Xe in the brain. In this study, the relationship between pulmonary oxygen concentration and HP ¹²⁹Xe signal in the brain was analyzed using a theoretical model and measured through in vivo experiments. The results from the theoretical model and experiments in rats are found to be in good agreement with each other. The optimal pulmonary oxygen concentration predicted by the theoretical model was 21%, and the in vivo experiments confirmed the presence of such an optimal ratio by reporting measurements between 25% and 35%. These findings are helpful for improving the ¹²⁹Xe signal in the brain and make the most of the limited spin polarization available for brain experiments. Copyright © 2016 John Wiley & Sons, Ltd.     

Development of a fast method for quantitative measurement of hyperpolarized 129Xe dynamics in mouse brain

A fast method has been established for the precise measurement and quantification of the dynamics of hyperpolarized (HP) xenon‐129 (¹²⁹Xe) in the mouse brain. The key technique is based on repeatedly applying radio frequency (RF) pulses and measuring the decrease of HP ¹²⁹Xe magnetization after the brain Xe concentration has reached a steady state due to continuous HP ¹²⁹Xe ventilation. The signal decrease of the ¹²⁹Xe nuclear magnetic resonance (NMR) signal was well described by a simple theoretical model. The technique made it possible to rapidly evaluate the rate constant α, which is composed of cerebral blood flow (CBF), the partition coefficient of Xe between the tissue and blood (λ_i), and the longitudinal relaxation time (T_1i) of HP ¹²⁹Xe in the brain tissue, without any effect of depolarization by RF pulses and the dynamics in the lung. The technique enabled the precise determination of α as 0.103 ± 0.018 s^‐1 (± SD, n = 5) on healthy mice. To investigate the potential of this method for detecting physiological changes in the brain of a kainic acid (KA) ‐induced mouse model of epilepsy, an attempt was made to follow the time course of α after KA injection. It was found that the α value changes characteristically with time, reflecting the change in the physiological state of the brain induced by KA injection. By measuring CBF using ¹H MRI and ¹²⁹Xe dynamics simultaneously and comparing these results, it was suggested that the reduction of T_1i, in addition to the increase of CBF due to KA‐induced epilepsy, are possible causes of the change in ¹²⁹Xe dynamics. Thus, the present method would be useful to detect a pathophysiological state in the brain and provide a novel tool for future brain study. Copyright © 2011 John Wiley & Sons, Ltd.     

Reduced xenon diffusion for quantitative lung study--the role of SF(6)

The large diffusion coefficients of gases result in significant spin motion during the application of gradient pulses that typically last a few milliseconds in most NMR experiments. In restricted environments, such as the lung, this rapid gas diffusion can lead to violations of the narrow pulse approximation, a basic assumption of the standard Stejskal-Tanner NMR method of diffusion measurement. We therefore investigated the effect of a common, biologically inert buffer gas, sulfur hexafluoride (SF(6)), on (129)Xe NMR and diffusion. We found that the contribution of SF(6) to (129)Xe T(1) relaxation in a 1:1 xenon/oxygen mixture is negligible up to 2 bar of SF(6) at standard temperature. We also measured the contribution of SF(6) gas to (129)Xe T(2) relaxation, and found it to scale inversely with pressure, with this contribution approximately equal to 1 s for 1 bar SF(6) pressure and standard temperature. Finally, we found the coefficient of (129)Xe diffusion through SF(6) to be approximately 4.6 x 10(-6) m(2)s(-1) for 1 bar pressure of SF(6) and standard temperature, which is only 1.2 times smaller than the (129)Xe self diffusion coefficient for 1 bar (129)Xe pressure and standard temperature. From these measurements we conclude that SF(6) will not sufficiently reduce (129)Xe diffusion to allow accurate surface-area/volume ratio measurements in human alveoli using time-dependent gas diffusion NMR.     

Hyperpolarized (129)Xe T (1) in oxygenated and deoxygenated blood

The viability of the new technique of hyperpolarized (129)Xe MRI (HypX-MRI) for imaging organs other than the lungs depends on whether the spin-lattice relaxation time, T(1), of (129)Xe is sufficiently long in the blood. In previous experiments by the authors, the T(1) was found to be strongly dependent upon the oxygenation of the blood, with T(1) increasing from about 3 s in deoxygenated samples to about 10 s in oxygenated samples. Contrarily, Tseng et al. (J. Magn. Reson. 1997; 126: 79-86) reported extremely long T(1) values deduced from an indirect experiment in which hyperpolarized (129)Xe was used to create a 'blood-foam'. They found that oxygenation decreased T(1). Pivotal to their experiment is the continual and rapid exchange of hyperpolarized (129)Xe between the gas phase (within blood-foam bubbles) and the dissolved phase (in the skin of the bubbles); this necessitated a complicated analysis to extract the T(1) of (129)Xe in blood. In the present study, the experimental design minimizes gas exchange after the initial bolus of hyperpolarized (129)Xe has been bubbled through the sample. This study confirms that oxygenation increases the T(1) of (129)Xe in blood, from about 4 s in freshly drawn venous blood, to about 13 s in blood oxygenated to arterial levels, and also shifts the red blood cell resonance to higher frequency.     

On the oxygenation-dependent (129)Xe T (1) in blood

The spin-lattice relaxation time, T(1), of hyperpolarized (129)Xe in blood is sensitive to blood oxygenation. In particular, it has been shown that (129)Xe T(1) is shorter in venous blood than in arterial blood. We have studied the T(1) of hyperpolarized (129)Xe dissolved in human blood as a function of blood oxygenation level, sO(2), in the physiological oxygenation range. We show that the (129)Xe relaxation rate, T(1)(-1), varies in a nonlinear fashion as a function of sO(2). This finding suggests that direct interaction of xenon with the paramagnetic heme group of deoxyhemoglobin is not the dominant oxygenation-dependent relaxation mechanism for (129)Xe in blood. These results corroborate the idea that the oxygenation-dependence of (129)Xe T(1) is determined by conformational changes of hemoglobin induced by oxygen binding.     

小孔径正交相控阵小动物实验线圈磁共振成像质量研究

目的 比较小孔径正交相控阵小动物实验线圈与C3表面线圈在大鼠头部磁共振成像(MRI)检查的成像质量.方法 将10只雄性SD大鼠分别应用2种线圈进行头部MRI检查.测量大脑皮层、小脑、脑干、肌肉及眼球内的房水等部位的信号强度及背景噪声值,计算信噪比(SNR);由2位放射科医生对图像质量进行评分,将测量数据进行统计分析.结果 所有图像均得到了大脑皮层、小脑、脑干、肌肉及眼球内的房水等部位的SNR值.两种线罔成像质量比较,大脑皮层T1WI横断位(SNRsense-body=41.3±23.5;SNR C3=10.3±0.5;t=-3.21)、T2WI矢状位(SNR sense-body=63.8±16.6;SNR C3=37.9±4.4:t=-4.00)以及T2WI冠状位(SNR sense.body=91.6±23.8;SNR C3=38.5±11.8;t=-5.69)图像SNR差异均有统计学意义(P＜0.01);且在小脑、脑干以及眼球房水测量的结果亦相仿;肌肉组织仅在T1WI横断位(SNR sense-body=39.9±21.1;SNR C3=8.3±1.7;t=-3.64)和T2WI冠状位(SNR sense-body=23.5±9.8:SNR C3=11.9±4.1;t=-3.10)的SNR差异有统计学意义(P＜0.01).采用小孔径正交相控阵线圈检查获得的图像中T2WI冠状面得分最高,而2种线圈检查获得的弥散加权成像(DWI)图像质量评分均最低.结论 在大鼠头部成像MRI检查中,小孔径正交相控阵小动物实验线圈的信噪比和图像质量优于现在经常应用的C3表面线圈.     

An in vivo 19F NMR study of isoflurane elimination as a function of age in rat brain.

In vivo 19F NMR at 4.7 T has shown that the biphasic elimination of the vapor anesthetic isoflurane from rat brain is ca 15% slower in old (23-24 months) animals compared with young (5-6 months) animals. The fast kinetic component has a t1/2 of ca 7-9 min and the slow event, 100-115 min. Gas chromatographic measurement of arterial blood elimination displays age attenuation to the same extent, although a monophasic kinetic process (6-7 min). The slow wash-out from brain is thought to involve elimination from intracranial fatty tissue as postulated by others in rabbit brain. Longitudinal relaxation time measurements show monoexponential recovery and essentially identical values for young (1.09 + 0.11 s) and old (1.04 +/- 0.09 s) animals. For dipalmitoylphosphatidylcholine vesicles the monoexponential recovery also suggests rapidly exchanging averaged homogeneous lipid environments for the anesthetic, but the longer T1s (2.75 +/- 0.25 s) imply less restricted mobility compared with brain. Single T2 values were obtained in vivo, indicating either a single compartment or rapid exchange between multiple environments. These measurements were inconsistent, undoubtedly as a result of B1 inhomogeneity. The age-attenuated elimination kinetics for isoflurane are consistent with poorer cardiopulmonary function, whereas the T1 data suggest similar environments for the anesthetic in young and old brain tissue.     

Regional fractional ventilation mapping in spontaneously breathing mice using hyperpolarized 129Xe MRI

The feasibility of ventilation imaging with hyperpolarized (HP) ¹²⁹Xe MRI has been investigated for quantitative and regional assessment of ventilation in spontaneously breathing mice. The multiple breath ventilation imaging technique was modified to the protocol of spontaneous inhalation of HP ¹²⁹Xe delivered continuously from a ¹²⁹Xe polarizer. A series of ¹²⁹Xe ventilation images was obtained by varying the number of breaths before the ¹²⁹Xe lung imaging. The fractional ventilation, r, was successfully evaluated for spontaneously breathing mice. An attempt was made to detect ventilation dysfunction in the emphysematous mouse lung induced by intratracheal administration of porcine pancreatic elastase (PPE). As a result, the distribution of fractional ventilation could be visualized by the r map. Significant dysfunction of ventilation was quantitatively identified in the PPE‐treated group. The whole‐lung r value of 0.34 ± 0.01 for control mice (N = 4) was significantly reduced, to 0.25 ± 0.07, in PPE‐treated mice (N = 4) (p = 0.038). This study is the first application of multiple breath ventilation imaging to spontaneously breathing mice, and shows that this methodology is sensitive to differences in the pulmonary ventilation. This methodology is expected to improve simplicity as well as noninvasiveness when assessing regional ventilation in small rodents. Copyright © 2014 John Wiley & Sons, Ltd.     

Hyperpolarized 129Xe MRI using isobutene as a new quenching gas

The use of a quenching gas, isobutene, with a low vapor pressure was investigated to enhance the utility of hyperpolarized ¹²⁹Xe (HP Xe) MRI. Xenon mixed with isobutene was hyperpolarized using a home‐built apparatus for continuously producing HP Xe. The isobutene was then readily liquefied and separated almost totally by continuous condensation at about 173 K, because the vapor pressure of isobutene (0.247 kPa) is much lower than that of Xe (157 kPa). Finally, the neat Xe gas was continuously delivered to mice by spontaneous inhalation. The HP Xe MRI was enhanced twofold in polarization level and threefold in signal intensity when isobutene was adopted as the quenching gas instead of N₂. The usefulness of the HP Xe MRI was verified by application to pulmonary functional imaging of spontaneously breathing mice, where the parameters of fractional ventilation (r_a) and gas exchange (f_D) were evaluated, aiming at future extension to preclinical studies. This is the first application of isobutene as a quenching gas for HP Xe MRI.     

Coronary hemodynamics during hemopump left-intraventricular assistance.

Hemopump left intraventricular pumping (HP) can permit percutaneous transluminal angioplasty (PTCA) in high-risk patients. Benefits may be related to left ventricular unloading or myocardial perfusion improvement, or both. Direct ultrasonic measurements of coronary blood flow were made in the dilated vessel after a successful PTCA in five patients. A 3 Fr intracoronary Doppler catheter was placed in the coronary artery to measure flow velocities (maximal or diastolic velocity; minimum or systolic velocity and mean velocity). A SwanGanz catheter was used to measure the cardiac index and pulmonary capillary wedge pressure. Mean aortic pressures were monitored through an 8 Fr guiding catheter. Measurements were made after a 5-min period of minimal speed (T0) of the HP to avoid retrograde regurgitation through the turbine; during the increase from minimum to maximal speed (T1); after a 5-min period of maximal HP flow (3l/min) (T2) and after HP was pulled back (T3). From T0 to T2, cardiac index rose from 1.93 +/- 0.38 to 3.26 +/- 0.35 l/min/m2 and capillary wedge pressure decreased from 18 +/- 6 to 13 +/- 5 mmHg (p less than 0.05); from T2 to T3, cardiac index decreased to 2.4 +/- 0.4 while capillary wedge pressure increased to 17 +/- 5 (p less than 0.05). Mean arterial pressure and heart rate did not change significantly throughout the study. When the hemopump flow was raised to high speed, coronary blood flow increased immediately but returned shortly to baseline values.(ABSTRACT TRUNCATED AT 250 WORDS)     

Whole-brain blood flow and oxygen metabolism in the rat during nitrous oxide anesthesia.

The Kety-Schmidt washout technique has been modified to measure whole-brain blood flow and metabolism in the rat. During nitrous oxide anesthesia, 14 rats exhaled (133)Xe, and continuous and simultaneous arterial and cerebral venous samples were drawn from a femoral artery and the transverse sinus of the brain. Extracerebral contamination of the venous sample was minimal, and equilibration of (133)Xe in brain tissue and blood was obtained after 10-24 min of inhalation. Cerebral blood flow was calculated from the total activity of the mechanically integrated arterial and venous samples according to the principle of Scheinberg and Stead. At a mean Paco2 of 40 mmHg, CBF averaged 98 +/- 6 (SEM) ml/100 g-min and CMRO2 averaged 5.4 +/- 0.7 (SEM) ml/100 g-min. CBF changed 2.4% with each millimeter Hg change of Paco2 while CMRO2 changed only insignificantly. The values obtained for CBF are higher than reported for man and large laboratory animals bur reflect the proportionately greater amount of gray matter in the rat brain.     

Effect of signal-to-noise ratio and spectral linewidth on metabolite quantification at 4 T

The accuracy and precision of measurements of metabolite concentrations from short echo-time spectra has previously been characterized at l.5 T as a function of signal-to-noise ratio (SNR) and peak linewidth. The purpose of this study was to characterize the systematic error in quantification of metabolite concentrations associated with linewidth and SNR for the major metabolites of interest in the short echo-time 1H-MR spectrum at 4 T. Simulated 4 T LASER localized spectra (TE = 46 ms) were generated with full width at half maximum (FWHM) over the range 4-14 Hz, and SNR over the range 5-500 by adding 100 Gaussian-distributed noise realizations at each combination of SNR and linewidth. Linewidth and SNR were treated as independent parameters, and therefore an increase in linewidth at a constant SNR resulted in increased metabolite areas. All spectra were fitted in the time domain using identical prior-knowledge and relative parameter starting values. Six metabolites (N-acetylaspartate, glutamate, creatine, myo-inositol, glycerophosphocholine, phosphocholine) were quantified with >90% accuracy and <10% standard deviation at SNR = 10 for linewidths ranging from 8 to 14 Hz FWHM. These simulations did not consider additional sources of variation, including eddy current artifacts, incomplete macromolecule baseline removal, and incomplete water suppression. Regardless, the results show that metabolite quantification from 4 T short echo-time 1H-MRS is sensitive to SNR and linewidth.     

A comparison of cardiac (31)P MRS at 1.5 and 3 T

(31)P MRS was evaluated on normal volunteers at 1.5 and 3 T, and the signal-to-noise ratio (SNR) of the two field strengths was calculated. The in vivo spin-lattice, T(1), relaxation times for PCr and gamma-ATP, which are essential for correcting for the effects of radiofrequency saturation on the PCr/ATP ratio, were determined at 3 T. The T(1) values for six volunteers were 3.8 +/- 0.7 s for PCr (mean +/- SD) and 2.4 +/- 1.1 s for gamma-ATP, which are similar to reported values at 1.5 T, allowing us to use protocols developed at 1.5 T at the new clinical field strength of 3 T. Direct comparison between 1.5 T and 3 T in the same 10 subjects, using coils of identical geometry and identical pulse sequences gave a mean SNR for PCr at 3 T which was 206 +/- 94% of that at 1.5 T. The linewidth for PCr increased from 13 +/- 6 Hz at 1.5 T to 22 +/- 12 Hz at 3 T. The coefficient of variation in the measurement of PCr/ATP, based on the Cramer-Rao lower bounds, was reduced from 32 +/- 25% at 1.5 T to 18 +/- 13% at 3 T. Thus, (31)P MRS at 3 T is greatly improved by the increase in SNR compared with acquisitions at 1.5 T because of the higher field strength. Copyright (c) 2008 John Wiley & Sons, Ltd.     

Focal brain ischemia in rat: acute changes in brain tissue T1 reflect acute increase in brain tissue water content

Several recent studies have reported changes of brain tissue T(1) in ischemic models during the first minutes after occlusion of the middle cerebral artery (MCA). In order to assess whether these tissue T(1) changes are related to an increase in tissue water content, we performed T(1) (7 T) and tissue water content measurements in a rat model (n = 10, Sprague-Dawley) of focal cerebral ischemia (intraluminal occlusion model). The tissue water content was determined using a gravimetric technique. The animals were divided into two groups: an ischemic group, with an effective MCA occlusion (n = 6) and a control group, with animals having undergone sham surgery but no MCA occlusion (n = 4). In the ipsilateral cortex, the tissue water content was 81.1 +/- 0.7% at 2 h 15 min following ischemic insult (contralateral value: 79.3 +/- 0.5%). Concomitantly, the tissue T(1) in the ipsilateral cortex was 2062 +/- 60 ms at ischemia onset + 1 h (contralateral 1811 +/- 28 ms) and 2100 +/- 38 ms at ischemia onset + 2 h (contralateral 1807 +/- 18 ms). The tissue T(1) and tissue water content values measured in the contralateral area do not differ from the values obtained in the control group. A significant T(1) increase is observed at ischemia onset + 1 h (+ 14%) and ischemia onset (+ 2 h) + 16%, together with a significant increase in tissue water content (+ 2.3%). This suggests that there is an increase in tissue water content concomitant with cell swelling during the first hours of ischemia.     

An account of the discrepancy between MRI and PET cerebral blood flow measures. A high-field MRI investigation

There is controversy concerning the discrepancy between absolute cerebral blood flow (CBF) values measured using positron emission tomography (PET) and magnetic resonance imaging (MRI). To gain insight into this problem, the increased signal-to-noise ratio (SNR) and extended T(1) relaxation times of blood and tissue at 3.0 T were exploited to perform pulsed arterial spin labeling (PASL) MRI measurements as a function of spatial resolution and post-labeling delay. The results indicate that, when using post-labeling delays shorter than 1500 ms, MRI gray matter flow values may become as high as several times the correct CBF values owing to tissue signal contamination by remaining arterial blood water label. For delays above 1500 ms, regional PASL-based CBF values (n = 5; frontal gray matter: 48.8 +/- 3.3(SD) ml/100 g/min; occipital gray matter: 49.3 +/- 4.5 ml/100 g/min) comparable with PET-based measurements can be obtained by using spatial resolutions comparable with PET (5-7.5 mm in-plane). At very high resolution (2.5 x 2.5 x 3 mm(3)), gray matter CBF values were found to increase by 10-20%, a consequence attributed to reduction in partial volume effects with cerebrospinal fluid and white matter. The recent availability of MRI field strengths of 3.0 T and higher will facilitate the use of MRI-based CBF measurements in the clinic.     

Preclinical hyperpolarized 129Xe MRI: ventilation and T2* mapping in mouse lungs at 7 T using multi‐echo flyback UTE

Fast apparent transverse relaxation (short T₂*) is a common obstacle when attempting to perform quantitative ¹H MRI of the lungs. While T₂* times are longer for pulmonary hyperpolarized (HP) gas functional imaging (in particular for gaseous ¹²⁹Xe), T₂* can still lead to quantitative inaccuracies for sequences requiring longer echo times (such as diffusion weighted images) or longer readout duration (such as spiral sequences). This is especially true in preclinical studies, where high magnetic fields lead to shorter relaxation times than are typically seen in human studies. However, the T₂* of HP ¹²⁹Xe in the most common animal model of human disease (mice) has not been reported. Herein, we present a multi‐echo radial flyback imaging sequence and use it to measure HP ¹²⁹Xe T₂* at 7 T under a variety of respiratory conditions. This sequence mitigates the impact of T₁ relaxation outside the animal by using multiple gradient‐refocused echoes to acquire images at a number of effective echo times for each RF excitation. After validating the sequence using a phantom containing water doped with superparamagnetic iron oxide nanoparticles, we measured the ¹²⁹Xe T₂* in vivo for 10 healthy C57Bl/6 J mice and found T₂* ~ 5 ms in the lung airspaces. Interestingly, T₂* was relatively constant over all experimental conditions, and varied significantly with sex, but not age, mass, or the O₂ content of the inhaled gas mixture. These results are discussed in the context of T₂* relaxation within porous media.     

Sensitive and fast T1 mapping based on two inversion recovery images and a reference image

We developed a fast method to obtain T1 relaxation maps in magnetic resonance imaging (MRI) based on two inversion recovery acquisitions and a reference acquisition, while maintaining high sensitivity by utilizing the full dynamic range of the MRI signal. Optimal inversion times for estimating T1 in the human brain were predicted using standard error propagation theory. In vivo measurements on nine healthy volunteers yielded T1 values of 1094+/-18 ms in gray matter and 746+/-40 ms in white matter, in reasonable agreement with literature values using conventional approaches. The proposed method should be useful for clinical studies because the T1 maps can be obtained within a few seconds.     

In vivo brain (31)P-MRS: measuring the phospholipid resonances at 4 Tesla from small voxels

An optimized phosphorous ((31)P) three-dimensional chemical-shift imaging (3D-CSI) protocol was developed at 4 T to study the phospholipid metabolism from discrete regions in the human brain without the need for (1)H-decoupling or nuclear Overhauser enhancement (NOE). In this study, a spherically bound, weighted average, random point omission 3D-CSI technique was developed and tested, based on methods proposed in the literature. The technique yields a significant (p < 0.001, two-tailed, 5% confidence level) increase in signal-to-noise (SNR) efficiency over conventional 3D-CSI (phantom 32%), without an increase in voxel bleedthrough. An automated time-domain fitting procedure utilizing prior spectral knowledge quantified the individual brain phospholipid metabolites from 15 cm(3) effective (8.0 cm(3) nominal) volumes from the left/right-parieto-occipital cortex and left/right thalamus in 10 normal volunteers. Individual constituents from the phosphomonoester (PME) region; phosphoethanolamine (PEth), phosphocholine (PCh) and the phosphodiester (PDE) region; glycerophosphoethanolamine (GPEth), glycerophosphocholine (GPCh) and membrane phospholipids (MP) were separately quantified to assess the precision of our method at 4 T against previous (1)H-decoupled (31)P-MRS brain studies at lower fields and much larger voxels. Derived concentrations (mM/l tissue) for PEth, PCh, GPEth, GPCh and MP in the left-parieto-occipital cortex were 0.81 +/- 0.21, 0.46 +/- 0.14, 0.74 +/- 0.30, 1.15 +/- 0.43 and 1.54 +/- 0.95 mM, respectively, and 0.94 +/- 0.16, 0.46 +/- 0.17, 0.83 +/- 0.22, 1.14 +/- 0.40 and 1.26 +/- 0.78 mM for the right parieto-occipital cortex. Derived concentrations (mM/l tissue) for PEth, PCh, GPEth, GPCh and MP in the left-thalamus were 0.69 +/- 0.18, 0.42 +/- 0.16, 0.63 +/- 0.20, 1.05 +/- 0.42 and 0.93 +/- 0.56 mM, respectively, and 0.68 +/- 0.24, 0.34 +/- 0.18, 0.60 +/- 0.23, 1.09 +/- 0.36 and 0.74 +/- 0.48 mM for the right-thalamus. This is the first study to our knowledge that has been able to quantify each of these individual phospholipid metabolites from such small voxels in the brain within a clinically reasonable scan time and without (1)H-decoupling or NOE.     

Quantitative ATP synthesis in human liver measured by localized 31P spectroscopy using the magnetization transfer experiment

The liver plays a central role in intermediate metabolism. Accumulation of liver fat (steatosis) predisposes to various liver diseases. Steatosis and abnormal muscle energy metabolism are found in insulin-resistant and type-2 diabetic states. To examine hepatic energy metabolism, we measured hepatocellular lipid content, using proton MRS, and rates of hepatic ATP synthesis in vivo, using the 31P magnetization transfer experiment. A suitable localization scheme was developed and applied to the measurements of longitudinal relaxation times (T1) in six healthy volunteers and the ATP-synthesis experiment in nine healthy volunteers. Liver 31P spectra were modelled and quantified successfully using a time domain fit and the AMARES (advanced method for accurate, robust and efficient spectral fitting of MRS data with use of prior knowledge) algorithm describing the essential components of the dataset. The measured T1 relaxation times are comparable to values reported previously at lower field strengths. All nine subjects in whom saturation transfer was measured had low hepatocellular lipid content (1.5 +/- 0.2% MR signal; mean +/- SEM). The exchange rate constant (k) obtained was 0.30 +/- 0.02 s(-1), and the rate of ATP synthesis was 29.5 +/- 1.8 mM/min. The measured rate of ATP synthesis is about three times higher than in human skeletal muscle and human visual cortex, but only about half of that measured in perfused rat liver. In conclusion, 31P MRS at 3 T provides sufficient sensitivity to detect magnetization transfer effects and can therefore be used to assess ATP synthesis in human liver.     

The performance of interventional loopless MRI antennae at higher magnetic field strengths

Interventional, "loopless antenna" MRI detectors are currently limited to 1.5 T. This study investigates whether loopless antennae offer signal-to-noise ratio (SNR) and field-of-view (FOV) advantages at higher fields, and whether device heating can be controlled within safe limits. The absolute SNR performance of loopless antennae from 0.5 to 5 T is investigated both analytically, using electromagnetic (EM) dipole antenna theory, and numerically with the EM method of moments, and found to vary almost quadratically with field strength depending on the medium's electrical properties, the noise being dominated by direct sample conduction losses. The prediction is confirmed by measurements of the absolute SNR of low-loss loopless antennae fabricated for 1.5, 3, and 4.7 T, immersed in physiologically comparable saline. Gains of 3.8 +/- 0.2- and 9.7 +/- 0.3-fold in SNR, and approximately 10- and 50-fold gains in the useful FOV area are observed at 3 and 4.7 T, respectively, compared to 1.5 T. Heat testing of a 3 T biocompatible nitinol-antenna fabricated with a redesigned decoupling circuit shows maximum heating of approximately 1 degrees C for MRI operating at high MRI exposure levels. Experiments in the rabbit aorta confirm the SNR and FOV advantages of the 3 T antenna versus an equivalent commercial 1.5 T device in vivo. This work is the first to study the performance of experimental internal MRI detectors above 1.5 T. The large SNR and FOV gains realized present a major opportunity for high-resolution imaging of vascular pathology and MRI-guided intervention.