Evelyn E. Babcock

Measurement of intracellular triglyceride stores by 1H spectroscopy: validation in vivo

We validate the use of 1H magnetic resonance spectroscopy (MRS) to quantitatively differentiate between adipocyte and intracellular triglyceride (TG) stores by monitoring the TG methylene proton signals at 1.6 and 1.4 ppm, respectively. In two animal models of intracellular TG accumulation, intrahepatic and intramyocellular TG accumulation was confirmed histologically. Consistent with the histological changes, the methylene signal intensity at 1.4 ppm increased in both liver and muscle, whereas the signal at 1.6 ppm was unchanged. In response to induced fat accumulation, the TG concentration in liver derived from 1H MRS increased from 0 to 44.9 ± 13.2 μmol/g, and this was matched by increases measured biochemically (2.1 ± 1.1 to 46.1 ± 10.9 μmol/g). Supportive evidence that the methylene signal at 1.6 ppm in muscle is derived from investing interfascial adipose tissue was the finding that, in four subjects with generalized lipodystrophy, a disease characterized by absence of interfacial fat, no signal was detected at 1.6 ppm; however, a strong signal was seen at 1.4 ppm. An identical methylene chemical shift at 1.4 ppm was obtained in human subjects with fatty liver where the fat is located exclusively within hepatocytes. In experimental animals, there was a close correlation between hepatic TG content measured in vivo by 1H MRS and chemically by liver biopsy [ R = 0.934; P < .0001; slope 0.98, confidence interval (CI) 0.70-1.17; y-intercept 0.26, CI -0.28 to 0.70]. When applied to human calf muscle, the coefficient of variation of the technique in measuring intramyocellular TG content was 11.8% in nonobese subjects and 7.9% in obese subjects and of extramyocellular (adipocyte) fat was 22.6 and 52.5%, respectively. This study demonstrates for the first time that noninvasive in vivo 1H MRS measurement of intracellular TG, including that within myocytes, is feasible at 1.5-T field strengths and is comparable in accuracy to biochemical measurement. In addition, in mixed tissue such as muscle, the method is clearly advantageous in differentiating between TG from contaminating adipose tissue compared with intramyocellular lipids.

Hippocampal volume, spectroscopy, cognition, and mood in patients receiving corticosteroid therapy

BACKGROUND Hippocampal volume reduction, declarative memory deficits, and cortisol elevations are reported in persons with major depressive disorder; however, data linking cortisol elevations with hippocampal atrophy are lacking. Prescription corticosteroid-treated patients offer an opportunity to examine corticosteroid effects on hippocampal volume and biochemistry and memory in humans. METHODS Seventeen patients on long-term prescription corticosteroid therapy and 15 controls of similar age, gender, ethnicity, education, height, and medical history were assessed with magnetic resonance imaging and proton magnetic resonance spectroscopy, the Rey Auditory Verbal Learning Test, Stroop Color Word Test and other neurocognitive measures, the Hamilton Rating Scale for Depression, Young Mania Rating Scale, and Brief Psychiatric Rating Scale. RESULTS Compared with controls, corticosteroid-treated patients had smaller hippocampal volumes and lower N-acetyl aspartate ratios, lower scores on the Rey Auditory Verbal Learning Test and Stroop Color Word Test, and higher Hamilton Rating Scale for Depression and Brief Psychiatric Rating Scale scores. CONCLUSIONS Patients receiving chronic corticosteroid therapy have smaller hippocampal volumes, lower N-acetyl aspartate ratios, and declarative memory deficits compared with controls. These findings support the idea that corticosteroid exposure appears to be associated with changes in hippocampal volume and functioning in humans.

Diffusion tensor tractography of traumatic diffuse axonal injury.

BACKGROUND Diffuse axonal injury is a common consequence of traumatic brain injury that frequently involves the parasagittal white matter, corpus callosum, and brainstem. OBJECTIVE To examine the potential of diffusion tensor tractography in detecting diffuse axonal injury at the acute stage of injury and predicting long-term functional outcome. DESIGN Tract-derived fiber variables were analyzed to distinguish patients from control subjects and to determine their relationship to outcome. SETTING Inpatient traumatic brain injury unit. PATIENTS From 2005 to 2006, magnetic resonance images were acquired in 12 patients approximately 7 days after injury and in 12 age- and sex-matched controls. MAIN OUTCOME MEASURES Six fiber variables of the corpus callosum, fornix, and peduncular projections were obtained. Glasgow Outcome Scale-Extended scores were assessed approximately 9 months after injury in 11 of the 12 patients. RESULTS At least 1 fiber variable of each region showed diffuse axonal injury-associated alterations. At least 1 fiber variable of the anterior body and splenium of the corpus callosum correlated significantly with the Glasgow Outcome Scale-Extended scores. The predicted outcome scores correlated significantly with actual scores in a mixed-effects model. CONCLUSION Diffusion tensor tractography-based quantitative analysis at the acute stage of injury has the potential to serve as a valuable biomarker of diffuse axonal injury and predict long-term outcome.

Non-invasive determination of tumor oxygen tension and local variation with growth

PURPOSE The objective was to develop and demonstrate a novel noninvasive technique of measuring regional pO2 in tumors. The method is based on measuring 19F nuclear magnetic resonance spin-lattice relaxation rate (R1 = 1/T1) of perfluorocarbon (PFC) emulsion discretely sequestered in a tumor. METHODS AND MATERIALS We have examined pO2 in the Dunning prostate tumor R3327-AT1 implanted in a Copenhagen rat. Oxypherol blood substitute emulsion was administered intravenously and became sequestered in tissue. Proton magnetic resonance imaging (MRI) showed tumor anatomy and correlated 19F MRI indicated the distribution of perfluorocarbon. Fluorine-19 spectroscopic relaxometry was used to measure pO2 in the tumor and repeated measurements over a period of 3 weeks showed the variation in local pO2 during tumor growth. RESULTS Perfluorocarbon initially resided in the vascularized peripheral region of the tumor: 19F nuclear magnetic resonance R1 indicated pO2 approximately 75 torr in a small tumor (approximately 1 cm) in an anesthetized rat. As the tumor grew, the sequestered PFC retained its original distribution. When the tumor had doubled in size the residual PFC was predominantly in the core of the tumor and the pO2 of this region was approximately 1 torr indicating central tumor hypoxia. CONCLUSION We have demonstrated a novel noninvasive approach to monitoring regional tumor pO2. Given the critical role of oxygen tension in tumor response to therapy this may provide new insight into tumor physiology, the efficacy of various therapeutic approaches, and ultimately provide a clinical technique for assessing individual tumor oxygenation.

Perfluorocarbon imaging in vivo: A 19F MRI study in tumor-bearing mice

Multiresonance perfluorocarbon emulsions (Oxypherol and Fluosol-DA) were imaged in tumor-bearing mice using 19F spin-echo magnetic resonance imaging in vivo. Multiple thin-slice fluorine images free of chemical shift artifacts were obtained in 13 minutes and these were correlated with proton images obtained during the same experiment to delineate the anatomic distribution of perfluorocarbons. Sequential images were used to determine the time course of the distribution and the retention of the compounds in tumors and organs. 19F MR spectroscopy was used ex vivo to determine with high sensitivity the relative concentration of perfluorocarbons in different tissues and organs and to confirm the results obtained from imaging experiments. The fluorine images visually demonstrated the preferential localization of the perfluorocarbons in the liver and spleen; shortly after injection, the images also revealed the highly vascularized tumor-chest wall interface. Imaging and spectroscopy together showed that the perfluorocarbons were removed from the blood pool within hours and remained sequestered in tissues at later times; the highest concentrations were found in the spleen and liver, where the agents were retained without spectral changes for the duration of these studies. The perfluorocarbons accumulated within tumors at dose-dependent concentrations, one to two orders of magnitude smaller than those observed in the spleen and liver.

Edge Artifacts in MR Images: Chemical Shift Effect

The boundaries of some organs as seen in clinical magnetic resonance images appear to be asymmetric. This effect is caused by chemical shift differences between the resonant frequencies of the hydrogen nuclei of water and fat. The zeugmatographic technique maps resonant frequencies to unique spatial locations. These differences in resonant frequencies can appear as artifactual misplacement of information as this one-to-one correspondence is lost. Various phantoms are used to demonstrate that the boundary artifact appears only in the direction of the read (frequency-encoding) gradient when media of two different chemical shifts are separated by an interface. When the relative shift is less than the width of the interface, the boundary appears to be asymmetric; when the relative shift is greater than the width of the interface, a bright band is seen along one edge with a dark band along the other. This artifact is more pronounced in low resolution images than in high resolution images, and these effects are seen even when the relative chemical shift is smaller than a pixel bandwidth. These effects are explained both conceptually and analytically. The clinician should be aware of the potential presence of this artifact at boundary interfaces that bear diagnostic significance.

Isolated tumor growth in a surgically formed skin pedicle in the rat: A new tumor model for NMR studies☆

We have developed a new tumor model, the skin flap pedicle, which is ideally suited for study by such techniques as magnetic resonance imaging (MRI) and spectroscopy (MRS) and positron emission tomography (PET) to evaluate changes in metabolism/physiology in tumors before and after treatment. Heretofore, results from such studies were generally obtained from tumors growing in the thigh or flank region and these sites of growth often compromised the quality of the results, because of the contribution of normal tissues such as muscle and bone. In an attempt to circumvent such problems, we developed a surgical technique for creating a tubular skin pedicle and subsequently transplanting tumors in the lumen of such pedicles. We have successfully grown Dunning Prostate tumors (R-3327) as PEDICLE-TUMORS (PED-TUM). This paper describes the surgical procedure for formation of the skin pedicle, implantation of the tumor, data on the growth and histology and discusses the application of the PED-TUM to research problems. We compare the NMR characteristics of the tumor grown in the traditional SC thigh position with that growth in the pedicle and demonstrate the utility and advantages of the new model for studies of tumor physiological function and structure.

Flourine-19 NMR spectroscopy and imaging investigations of myocardial perfusion and cardiac function

Fluorine-19 nuclear magnetic resonance images of rabbit hearts have been obtained with the administration of a fluorinated contrast compound. The use of fluorine-labeled compounds allows differences in tissue perfusion to be demonstrated between hearts with and without coronary artery ligation. In postischemic reperfusion studies, regional hypoperfusion is also demonstrated by 19F imaging. The results of in vivo rabbit studies show that 19F NMR can be used to monitor the rate of blood pool and extravascular space dilution of a single bolus injection of either a water soluble or emulsified fluorine-labeled agent.

NMR indirect detection of glutamate to measure citric acid cycle flux in the isolated perfused mouse heart

13C‐edited proton nuclear magnetic resonance (NMR) spectroscopy was used to follow enrichment of glutamate C3 and C4 with a temporal resolution of ∼20 s in mouse hearts perfused with 13C‐enriched substrates. A fit of the NMR data to a kinetic model of the tricarboxylic acid (TCA) cycle and related exchange reactions yielded TCA cycle (V tca) and exchange (V x) fluxes between α‐ketoglutarate and glutamate. These fluxes were substrate‐dependent and decreased in the order acetate (V tca=14.1 μmol g−1 min−1; V x=26.5 μmol g−1 min−1)>octanoate (V tca=6.0 μmol g−1 min−1; V x=16.1 μmol g−1 min−1)>lactate (V tca=4.2 μmol g−1 min−1; V x=6.3 μmol g−1 min−1).

Direct observation of lactate and alanine by proton double quantum spectroscopy in rat hearts supplied with [3-13C]pyruvate.

The13C‐fractional enrichments in the lactate and alanine methyl carbon positions were determined by1H NMR spectroscopy of extracts of rat hearts perfused with various concentrations of [3‐13C]pyruvate ± unlabeled glucose or acetate. In general, the13C‐fractional enrichment of the alanine methyl carbon pool paralleled the13C‐fractional enrichment of the acetyl‐CoA which entered the TCA cycle (as determined by13C‐isotopomer analysis) while the13C‐fractional enrichment of the lactate methyl carbon was always significantly lower, consistent with a pool of lactate which does not mix with exogeneous [3‐13C]pyruvate. This has also been examined in intact, perfused, KCl‐arrested rat hearts supplied with [3‐13C]pyruvate by proton double quantum motabolite specific spectroscopy (MSS). A comparison of MSS spectra of intact hearts with one pulse spectra of extracts of those same hearts indicates there is a sizeable non‐enriched pool of lactate in the intact hearts which is not visible by NMR spectroscopy.