In vivo proton nuclear magnetic resonance imaging and spectroscopy studies of halocarbon-induced liver damage

Magnetic Resonance Imaging and Spectroscopy: New Noninvasive In Vivo Approaches in Toxicology Research

Magnetic Resonance Imaging (MRI) and Spectroscopy (MRS) give anatomical and biochemical information about a human patient or animal in a non-invasive manner. This unique quality permits the study of toxicological responses of an organ within an intact animal and in a manner in which many fewer animals are needed than by conventional methods of investigation. The use of MRI and MRS in the study of hepatotoxicants, particularly bromobenzene and ethanol, is reviewed. Bromobenzene causes localised hepatic oedema and bioenergetic deterioration; these changes were followed with time by 1H MRI and 31P MRS, respectively. Phosphocholine levels in the liver were found to increase dramatically during bromobenzene-induced damage, possibly related to an intracellular control mechanism in response to tissue damage. The ability of the bromobenzene-challenged liver to metabolise a fructose load was followed by dynamic 31P MRS. Chronic ethanol administration damages the liver. This toxicological process results in the accumulation of fat in the liver, which was followed by fat-selective 1H MRI. When ethanol is no longer administered to the subject, the fatty infiltration subsides, and this process was followed over 16 days in the same animal using fat-selective 1H MRI. Chronic ethanol renders the liver in situ more susceptible to hypoxic injury and less likely to recover afterwards, as shown by 31P MRS.

Prospective respiratory-gated micro-CT of free breathing rodents

Microcomputed tomography (Micro-CT) has the potential to noninvasively image the structure of organs in rodent models with high spatial resolution and relatively short image acquisition times. However, motion artifacts associated with the normal respiratory motion of the animal may arise when imaging the abdomen or thorax. To reduce these artifacts and the accompanying loss of spatial resolution, we propose a prospective respiratory gating technique for use with anaesthetized, free-breathing rodents. A custom-made bed with an embedded pressure chamber was connected to a pressure transducer. Anaesthetized animals were placed in the prone position on the bed with their abdomens located over the chamber. During inspiration, the motion of the diaphragm caused an increase in the chamber pressure, which was converted into a voltage signal by the transducer. An output voltage was used to trigger image acquisition at any desired time point in the respiratory cycle. Digital radiographic images were acquired of anaesthetized, free-breathing rats with a digital radiographic system to correlate the respiratory wave form with respiration-induced organ motion. The respiratory wave form was monitored and recorded simultaneously with the x-ray radiation pulses, and an imaging window was defined, beginning at end expiration. Phantom experiments were performed to verify that the respiratory gating apparatus was triggering the micro-CT system. Attached to the distensible phantom were 100 microm diameter copper wires and the measured full width at half maximum was used to assess differences in image quality between respiratory-gated and ungated imaging protocols. This experiment allowed us to quantify the improvement in the spatial resolution, and the reduction of motion artifacts caused by moving structures, in the images resulting from respiratory-gated image acquisitions. The measured wire diameters were 0.135 mm for the stationary phantom image, 0.137 mm for the image gated at end deflation, 0.213 mm for the image gated at peak inflation, and 0.406 mm for the ungated image. Micro-CT images of anaesthetized, free-breathing rats were acquired with a General Electric Healthcare eXplore RS in vivo micro-CT system. Images of the thorax were acquired using the respiratory cycle-based trigger for the respiratory-gated mode. Respiratory gated-images were acquired at inspiration and end expiration, during a period of minimal respiration-induced organ motion. Gated images were acquired with a nominal isotropic voxel spacing of 44 microm in 20-25 min (80 kVp, 113 mAs, 300 ms imaging window per projection). The equivalent ungated acquisitions were 11 min in length. We observed improved definition of the diaphragm boundary and increased conspicuity of small structures within the lungs in the gated images, when compared to the ungated acquisitions. In this work, we have characterized the externally monitored respiratory wave form of free-breathing, anaesthetized rats and correlated the respiration-induced organ motion to the respiratory cycle. We have shown that the respiratory pressure wave form is an excellent surrogate for the radiographic organ motion. This information facilitates the definition of an imaging window at any phase of the breathing cycle. This approach for prospectively gated micro-CT can provide high quality images of anaesthetized free-breathing rodents.

In vivo assessment of nodularin-induced hepatotoxicity in the rat using magnetic resonance techniques (MRI, MRS and EPR oximetry)

Acute nodularin-induced hepatotoxicity was assessed in vivo, in rats using magnetic resonance (MR) techniques, including MR imaging (MRI), MR spectroscopy (MRS), and electron paramagnetic resonance (EPR) oximetry. Nodularin is a cyclic hepatotoxin isolated from the cyanobacterium Nodularia spumigena. Three hours following the intraperitoneal (i.p.) administration of nodularin (LD50), a region of 'damage', characterized by an increase in signal intensity, was observed proximal to the porta hepatis (PH) region in T2-weighted MR images of rat liver. Image analysis of these regions of apparent 'damage' indicated a statistically significant increase in signal intensity around the PH region following nodularin administration, in comparison with controls and regions peripheral to the PH region. An increase in signal intensity was also observed proximal to the PH region in water chemical shift selective images (CSSI) of nodularin-treated rat livers, indicating that the increased signal observed by MRI is an oedematous response to the toxin. Microscopic assessment (histology and electron microscopy) and serum liver enzyme function tests (aminotransferase (ALT) and aspartate ALT (AST)) confirmed the nodularin-induced tissue injury observed by MRI. In vivo and in vitro MRS was used to detect alterations in metabolites, such as lipids, Glu+Gln, and choline, during the hepatotoxic response (2-3 h post-exposure). Biochemical assessment of perchloric acid extracts of nodularin-treated rat livers were used to confirm the MRS results. In vivo EPR oximetry was used to monitor decreasing hepatic pO2 (approximately 2-fold from controls) 2-3 h following nodularin exposure. In vivo MR techniques (MRI, MRS and EPR oximetry) are able to highlight effects that may not have been evident in single end point studies, and are ideal methods to follow tissue injury progression in longitudinally, increasing the power of a study through repeated measures, and decreasing the number of animals to perform a similar study using histological or biochemical techniques.

In vivo magnetic resonance (MR) study of fatty liver: importance of intracellular ultrastructural alteration for MR tissue parameters change

Fatty liver is thought to have a shorter T1 relaxation time than normal liver tissue, due to the accumulation of triglyceride. Previous studies regarding T1 and T2 relaxation times, however, show widely different results. In order to elucidate the mechanism responsible for the changes and diversity of relaxation times in fatty liver, we created two animal models in 14 rabbits, one acute form (N = 6) and the other chronic form (N = 8). Four rabbits were taken as a control group. Tissue relaxation times and the magnetization transfer (MT) effect of the liver tissue in these two models were measured. The results were correlated with biochemical analysis of water and fat content and histological examination, including findings in light microscopy and electron microscopy. Although the fatty ratio in both forms of fatty liver was similar, their tissue relaxation rate and MT effect were significantly different. The acute form showed prolongation of both T1 and T2 relaxation times (512 +/- 51 msec vs. 710 +/- 95 msec and 39 +/- 1.8 msec vs. 48 +/- 3.7 msec, respectively) and a decrease of the MT effect (50 +/- 5.1% vs. 38 +/- 6.3%), compared to those of the control group and preinduction liver. The chronic form showed shorter T1 and T2 values (526 +/- 36 msec vs. 406 +/- 56 msec and 36 +/- 1.6 msec vs. 33 +/- 2.3 msec, respectively) and a stronger MT effect (21 +/- 0.9% vs. 26 +/- 2.3%). In acute form fatty liver, electron microscopic examination revealed dramatic subcellular changes, such as vesicular transformation, a markedly increased amount of smooth endoplasmic reticulum (SER), and disruption of the crista. These changes were not found in the chronic form fatty liver. From this study, we concluded that the ultrastructural alteration in the subcellular organelles of hepatocyte might play a crucial role for the chameleonic presentation of MR tissue parameters in fatty liver.

The lever-coil: a simple, inexpensive sensor for respiratory and cardiac motion in MRI experiments

Thoracic and abdominal magnetic resonance imaging studies generally require some type of compensation for respiratory and cardiac motions in order to yield artifact-free images with good signal-to-noise ratio. Most techniques for respiratory compensation require the use of a non-NMR sensing device to monitor the subject's chest motion, while cardiac motion compensation generally requires the use of ECG leads within the magnet. An inductive pickup coil placed on the subject's chest is perhaps the simplest and least expensive means of monitoring respiration in a MR scanner. However, due to inductive coupling between the pickup coil, radio frequency resonator and gradient set, this arrangement often results in both NMR and respiratory signal artifacts and can also present a burn hazard to the subject depending on the placement and orientation of the pickup coil. Moreover, the presence of a pickup coil on the chest can degrade local magnetic field homogeneity and thus degrade image quality. Similar problems arise when ECG leads must be connected to the subject for cardiac monitoring and gating. To preserve the benefits of the simple pickup coil while circumventing these limitations, a "lever-coil" sensor is presented in which a pickup coil is mechanically coupled to the subject but is not located within the resonator or gradient coil. This results in much lower mutual inductance between the pickup coil and the resonator or gradients. The optimization of the geometry of the apparatus is discussed and lever-coil signal traces are shown which demonstrate the sensor's ability to simultaneously detect both respiratory and cardiac motion in mice. Finally, respiratory-gated and cardiac-triggered spin echo images of the rat abdomen and mouse heart are presented to demonstrate the utility of the lever-coil sensor.

Non-invasive in vivo magnetic resonance imaging assessment of acute aflatoxin B1 hepatotoxicity in rats

Acute aflatoxin B1 (AFB1)-induced hepatotoxicity was assessed in vivo in male Sprague-Dawley rats (150-300 g) using magnetic resonance imaging (MRI). MRI results were compared to serum enzyme levels, histology and electron microscopy. Twenty-four hours following intraperitoneal delivery of AFB1 (3 mg/kg body weight in a saline/dimethyl sulfoxide (DMSO; 0.03 ml/kg body weight) solution), regions of damage, characterised by increased proton signal intensities in T2-weighted images, were observed in the vicinity of the hepatic portal vein (HPV) and in the right medial regions of the liver. Image analysis of regions of apparent damage around the HPV and right medial regions, following 24 h of AFB1 exposure, indicated statistically significant (P<0.05) increases in proton image signal intensities, when compared to saline/DMSO-treated rats. No significant difference in proton image signal intensities were observed 1-2 h following AFB1 exposure. Twenty-four hours following AFB1 exposure, histopathological assessment was characterised by portal/central vein/artery congestion, sinusoid congestion, nuclear pyknosis and karyolysis, and hepatocyte vacuolation; electron microscopy (EM) examination indicated nuclear debris, swollen cytoplasmic compartments, vacuolation, and the disappearance of the smooth endoplasmic reticulum, and elevated levels of serum aspartate aminotransferase and alanine aminotransferase were found to be significantly different (P<0.01) than controls.

In vivo assessment of microcystin-LR-induced hepatotoxicity in the rat using proton nuclear magnetic resonance (1H-NMR) imaging

Microcystin-LR (MCLR)-induced hepatotoxicity was assessed in vivo in male Sprague-Dawley rats (150-350 g) using magnetic resonance imaging (MRI). Following the intraperitoneal administration of MCLR (LD(50)), a region of damage, characterised by increased signal intensity on T(2)-weighted images, was seen proximal to the hepatic portal vein in the liver. Similarly, increased signal intensity was seen in the chemical-shift selective images (CSSI) of water frequency, proximal to the hepatic portal vein in the liver. This indicates that the increased signal intensity observed in the T(2)-weighted images was due to an increased amount of magnetic resonance (MR) visible protons in the tissue which represents an oedematous response. Image analysis of regions of apparent damage around the hepatic portal vein indicated a statistically significant increase in signal intensity in this region. Mitochondrial swelling and lipid inclusions were observed by transmission electron microscopy (TEM) in samples obtained from the oedematous regions of the liver using spatial coordinates from the magnetic resonance (MR) images. Massive haemorrhagic necrosis and nuclear swelling were observed by light microscopy in the centrilobular regions of the lobules.

MR imaging of hepatic injury in the LEC rat under a high magnetic field (7.05 T)

Visualization of copper-induced hepatitis (CuH) in LEC rats was performed by using an MRI apparatus equipped with a magnet producing a high magnetic field of 7.05 T. When three groups of LEC rats (6-16 [pre-hepatitis], 15-26 [acute hepatitis] and 40-77 [chronic hepatitis] weeks old) were examined by MRI under T2-weighted imaging conditions which are suitable for the diagnosis of human hepatitis, hypointense MR images of the livers were, as a whole, obtained in all groups, suggesting that these conditions were not adequate for imaging of CuH of LEC rats. The shortening of the T1 and T2 relaxation times of livers due to an excess amount of paramagnetic irons under the high magnetic field was responsible for the lowering of MR signal intensities of the livers, especially those of 15 to 26-week old rats showing acute hepatitis. However, theoretical calculation of the MR signal intensities using the T1 and T2 relaxation times of the livers indicated that their imaging might be possible under proton density-weighted conditions even with a high magnetic field. Experimental results showed that hepatic injury was visualized as hyperintense regions in the MR image of the liver in the acute-phase rat.

A compact respiratory-triggering device for routine microimaging of laboratory mice

A partial-body plethysmograph was developed for measuring the respiratory flow of anesthetized mice during routine microimaging experiments performed in the close confines of an 89-mm-diameter, vertical-bore magnet. Respiratory flow patterns were used for synchronizing conventional T2-weighted spin-echo imaging with the respiratory cycle, thereby, significantly reducing motion-induced artifacts and increasing observed liver lesion contrast.

Nuclear magnetic resonance imaging as a tool to estimate the mass of the Pectoralis muscle of chickens in vivo

1. Nuclear magnetic resonance imaging (MRI) was used to assess the advantages of the technique in determining the size (volume) and shape of the Pectoralis muscle (Pectoralis major and minor) in broiler chickens, non-invasively and in vivo. 2. The imaging was performed using a Spectroscopy Imaging System 2.0 Tesla/31 cm bore imaging spectrometer. Three-dimensional reconstruction of transverse images was used to estimate the size of the Pectoralis muscle of chickens ranging in body weight from 362 to 1643 g. 3. Regression analysis resulted in R2 values of 0.92 and 0.99 for the relationship between Pectoralis muscle weight, body weight and muscle volume, respectively. 4. It is concluded that MRI and 3-dimensional image reconstruction may be used to estimate the Pectoralis muscle size and shape. This may be readily extended to monitor the influence of various factors on the growth and development of specific organs and tissues in the body.

In vivo 1H-NMR microimaging with respiratory triggering for monitoring adoptive immunotherapy of metastatic mouse lymphoma

The metastatic ESb-MP murine lymphoma in DBA/2 mice has been used as a model for investigating metastatic disease and its cure by adoptive immunotherapy (ADI) as monitored by in vivo multislice spin-echo 1H NMR microimaging at 7 T. isoflurane inhalation anesthesia facilitated long measurement sessions, and respiratory gating with a fiber-optic sensor greatly reduced motional artifacts. With T2 weighting (TR = 2 s, TE = 30 ms) mean signal-to-noise ratios of 30 and 15 for kidney and liver, respectively, were achieved in 20 min (100-micron pixels, 1-mm slices, 25-mm field of view). Without the use of contrast agents, metastases with diameters > or = 0.3 mm in the imaged plane could be detected as hyperintense lesions in kidney (contrast ratio ca. 1.4) and liver (contrast ratio ca. 2) with a confidence level of > 98%. For the first time the complete eradication of late-stage macroscopic metastases by ADI could be demonstrated noninvasively by MRI.

In vivo study of halothane hepatotoxicity in the rat using magnetic resonance imaging and 31P spectroscopy

Using magnetic resonance imaging (MRI) and spectroscopy (MRS), in vivo halothane hepatotoxicity was assessed in male Wistar rats. With 1.5% halothane in 100 or 20% O2, an edematous region, characterized by increased intensity on T2 weighted images and an increase in regional tissue water content (rho water), was seen proximal to the hepatic portal vein in the liver. Both spin-lattice relaxation (T1) and spin-spin relaxation (T2) increased in this region, relative to distal regions of the liver. Similarly, a high signal intensity on proton density weighted images was observed in this area. As halothane anaesthesia progressed, a decrease in the adenosine triphosphate-inorganic phosphate ratio (ATP/Pi) and an increase in the phosphomonoester-phosphodiester (PME/PDE) ratio was detected in the liver. In addition, intracellular pH decreased and intracellular free magnesium concentration [Mg2+] increased with time of exposure. Excessive vacuolation, ribosomal disappearance from rough endoplasmic reticulum, mitochondrial swelling and fragmentation of smooth endoplasmic reticulum were observed by transmission electron microscopy (TEM) in samples from the edematous region of the liver.

Comparison of Zn and vitamin E for protection against hyperoxia-induced lung damage

The objective of the present study was to demonstrate an antioxidant function for Zn in vivo by comparing the efficacy of Zn or Vitamin E without additional energy intake for protection of Zn-deficient (ZnDF) or energy-restricted (ER) rats from hyperoxia-induced lung damage. Zn (200 mumol ZnCl2/kg b.wt.) or Vitamin E (100 mg alpha-tocopherol/kg b.wt.) was injected IP before exposure to 85% oxygen or air for 5 d. During the exposure period, all injected ZnDF or ER rats were restricted to 5 g Zn-deficient or Zn-adequate diet/day, respectively, the amount of diet consumed by the untreated ZnDF or ER rats. We clearly demonstrated that injection of Zn without additional energy intake can protect ZnDF and ER rats from hyperoxia-induced lung damage assessed by the histopathological scoring system and magnetic resonance imaging (MRI). Vitamin E was not as effective as Zn in either ZnDF or ER rats for preventing hyperoxia-induced lung damage. Zn injection did not exert its antioxidant effect through increased lung CuZn-superoxide dismutase activity or metallothionein. This difference in the effectiveness of Vitamin E and Zn for hyperoxic protection in lung injury may be due to the specificity of antioxidant function, i.e., vitamin E inhibits oxidation of membrane lipids and Zn protects sulfhydryl groups of proteins.

Inhibitory effect of phenyl N-terf-butyl nitrone on Kupffer cell phagocytosis

Light microscopy studies of rat liver were conducted after injection of India ink. The data indicated that Kupffer cell phagocytosis was inhibited by C-phenyi-N-tert-butyl nitrone (PBN), as well as by the Kupffer cell antagonist gadolinium chloride.

An infrared device for monitoring the respiration of small rodents during magnetic resonance imaging

A device that uses infrared reflectometry for monitoring the respiratory waveform of small rodents during MRI was developed. This system uses a photoplethysmograph coupled to the animal by a light pipe to detect movements as small as 1.0 mm. This device can also be used for monitoring larger animals or for controlling respiratory-compensated or respiratory-triggered MRI.

Detection of bromobenzene-induced hepatocellular necrosis using magnetic resonance microscopy

The authors used magnetic resonance (MR) microscopy to assess hepatic tissue damage induced by bromobenzene both in living rats and in fixed rat liver tissues. Experiments were conducted at 7 Tesla on three groups of Fisher rats treated with bromobenzene at a single dose of 68, 135, and 269 mg/kg, respectively. Optical microscopy of hematoxylin and eosin stained sections showed liver damage only at the highest dose, whereas with MR microscopy, tissue alterations were detected at all three doses both in vivo and ex vivo. The contrast mechanism of the superior sensitivity of MR microscopy is believed to be related to the changes in local diffusion coefficients that accompany cellular degeneration and death, although other contrast mechanisms may also be involved. The superior sensitivity of MR microscopy, as demonstrated in this study, has many implications for potential use of MR techniques to perform in vivo histology.

31P NMR study of acute toxic effects of cadmium chloride on rat liver

In this study, acute effects of cadmium ions (Cd2+) on energy metabolism in rat livers were analyzed in vivo after intravenous administration using 31P NMR. Both inorganic phosphate (Pi) and nucleotide triphosphate (NTP) peaks of in vivo Cd-treated livers gradually decreased over a 6-h period. In the extract, NTP peaks in Cd-treated livers were lower, as in the in vivo experiments, but the Pi peak was significantly higher than the control. The apparent decrease in Pi in in vivo liver treated with Cd could be caused by the reduced visibility of Pi because of its uptake into mitochondria from cytoplasm, accompanied by the uncoupling of oxidative phosphorylation by Cd2+. These results indicated that total Pi in the hepatocytes increases after Cd administration. However, only 10% of Pi was visible in Cd-treated livers in vivo, whereas 34% of Pi was visible in controls. Significant increases in phosphocholine and glycerophosphocholine were also observed in extracts of Cd-treated livers.

Diagnosis of persistent intestinal ischemia in the rabbit using proton magnetic resonance imaging

Noninvasive diagnosis of persistent intestinal ischemia remains an elusive goal. Magnetic resonance imaging (MRI) recognizes changes in tissue water content, and several authors have demonstrated increased intensity within 6 hours of intestinal ischemia. To simulate the clinical situation more closely, we studied the efficacy of MRI in differentiating ischemic from viable segments of bowel 24 hours after injury in a rabbit model. A segment of distal ileum was rendered ischemic by vascular isolation and ligation. Controls underwent sham operation without vascular ligation. After 24 hours, multislice transverse scans were done using both T1 and T2 weighting. Image intensity was calculated from the isolated loop (absolute intensity), and paraspinal muscle intensity was used as an internal standard to calculate relative intensity (isolated bowel/paraspinal muscle). Animals were killed and bowel necrosis was confirmed histologically. Both absolute and relative intensity were significantly higher in animals undergoing persistent intestinal ischemia. This was true using both T1 and T2 weighting. In a further group of rabbits using the same model, intensity was calculated both before and after intravenous gadolinium. No significant difference was seen between sham and ischemic animals. Our data show that (1) MRI can differentiate ischemic from viable bowel 24 hours after ischemic injury, and (2) the use of intravenous contrast does not improve accuracy. We conclude that MRI may represent a useful noninvasive technique for the diagnosis of persistent intestinal ischemia and that clinical studies should be initiated.

Influences of dietary deoxycholic acid on progression of hepatocellular neoplasms and expression of glutathione S-transferases in rats

Serial magnetic resonance imaging (MRI) was used to evaluate the influences of dietary deoxycholic acid (DCA) on the rate of progression of chemically induced hepatocellular neoplasms in rats. Male Fischer-344 rats with established persistent hepatocellular nodules generated by the Solt-Farber protocol were exposed to dietary DCA (0.3%) between 6 and 12 mo of age. Growth of nodules and carcinomas in vivo was measured by morphometric quantification of tumor images obtained every 6 wk. The final stages of neoplastic progression were determined by terminal histopathological examination and by expression and functional evaluation of glutathione S-transferase (GST) isoenzyme phenotypes. Dietary DCA increased the number of hepatocellular neoplasms per rat, accelerated the rate of growth of persistent nodules, and increased the histological progression of liver tumors. Expression of immunoreactive GST subunits Yf, Ya, and Yb1 was induced in early persistent nodules, a pattern that was maintained throughout the study in both basal diet and DCA-fed groups. However, 5% of early nodules and about 75% of advanced neoplasms were partially or completely deficient in GST Yb2 expression in both groups. DCA did not alter the cytosolic activity for the GST substrates 1-chloro-2,4-dinitrobenzene (CDNB) or trans-4-phenyl-3-buten-2-one (tPBO) in tumors or surrounding liver. However, in both groups, CDNB activity was increased in the tumors relative to the surrounding nonneoplastic tissue, whereas activity for tPBO, a substrate more specific for the Yb2 subunit, was reduced in the tumors. All advanced neoplasms were similarly more resistant than surrounding liver to DNA-binding metabolites of aflatoxin B1 or benzo[a]pyrene. These data demonstrate that DCA can increase the progression of established hepatocellular nodules to larger, more advanced neoplasms but does not preferentially select for a specific GST phenotype. Preferential loss of constitutively expressed GST Yb2 in both basal diet and DCA-fed groups may be an important aspect of progression from resistant nodules to advanced cancers in this model. These studies also demonstrate that serial MRI is a useful tool for measuring the rates of enlargement and patterns of growth in established hepatocellular neoplasms.

Studies on bromobenzene-induced hepatotoxicity using in vivo MR microscopy with surgically implanted RF coils

Using surgically implanted RF coils at 300 MHz, three-dimensional microscopic MR images of rat liver were obtained in vivo to follow the development of pathology induced by bromobenzene exposure. Formalin fixed specimens of liver from these animals were also imaged using in vitro MR microscopy, followed by conventional optical microscopy. All MR images were acquired using a spin-warp pulse sequence with TR = 950 ms and TE = 23 ms. The in vivo images were reconstructed as 256(2) x 32 arrays with a voxel size of (50 microns)2 x 219 microns, while the in vitro images were reconstructed as 256(2) x 128 arrays, giving an isotropic resolution at (39 microns)3. Based on results from six animals, we have found in all animals exposed to bromobenzene, image intensity decreased in specific hepatic tissue regions. These regions were well correlated to low signal intensity areas observed in in vitro MR images at higher resolution. Conventional optical microscopy indicated that the low signal intensity regions corresponded to areas of necrosis. The decrease in signal intensity is consistent with increased local diffusion coefficients as a result of necrosis. This study demonstrates that MR microscopy with implanted RF coils can be successfully used to follow tissue pathological changes in living tissues.

In vivo and in vitro 31P magnetic resonance spectroscopic studies of the hepatic response of healthy rats and rats with acute hepatic damage to fructose loading

The hepatic response to a fructose challenge for control rats, and rats subjected to an acute sublethal dose of carbon tetrachloride (CCl4) or bromobenzene (BB), was compared using dynamic in vivo 31P MRS. Fructose loading conditions were used in which control rats showed only a modest increase in hepatic phosphomonoester (PME), and a small decrease in ATP, Pi, and intracellular pH after fructose administration. Both CCl4 and BB-treated rats showed a much greater fructose-induced accumulation of PME than did controls. Trolox C, a free radical scavenger, prevented most of this PME increase. BB-treated rats, given sufficient time to recover from the hepatotoxic insult, responded to the fructose load similarly to controls. Liver aldolase activities of control, toxicant-treated rats, and toxicant plus Trolox C-treated rats correlated inversely with PME accumulation after fructose loading (correlation coefficient: -0.834, P < 0.05). Perchloric acid extracts of rat livers studied by in vitro 31P MRS confirmed that the PME accumulation after fructose loading is mainly due to an increase in fructose 1-phosphate. These studies are consistent with the aldolase-catalyzed cleavage of fructose 1-phosphate being rate-limiting in hepatic fructose metabolism, and that the CCl4 and BB treatment modify and inactivate the aldolase enzyme.

Use of nitroxides as MRI contrast agents to study in vivo carbon tetrachloride induced hepatotoxicity in rats

CCl4 and related compounds, such as halothane, are metabolized by the liver to form free radical intermediates, which are thought to be implicated in the hepatotoxic response. Two to three hours following CCl4 exposure (i.p.) there is a localized edematous region surrounding the portal vein which is observable by proton MRI in vivo. Enhancement of the CCl4-induced edematous region was possible using Gd-DTPA, a paramagnetic contrast agent. However, with the use of a nitroxide contrast agent (3-PCA) there was no enhancement, but rather a significant diminution of the CCl4-induced edematous response. These results suggest that the nitroxide contrast agents, which are themselves free radicals, act as free radical scavengers and therefore reduce the formation of the CCl4-induced hepatic 'damage' observed in proton MR images.

MRI study of the inhibitory effect of new spin traps on in vivo CCl4-induced hepatotoxicity in rats

Acute CCl4 hepatotoxicity is thought to occur as a result of free radicals generated from the metabolism of CCl4 in the liver. With the use of MRI it is possible to detect in vivo a CCl4-induced localized edematous region surrounding the major branch of the hepatic portal vein in the right lobe. Inhibition of the CCl4-induced response has been obtained by pretreatment with the spin trap, PBN, 30 min prior to CCl4 exposure. The inhibitory effect of two new spin traps, M3PO or methyl-DMPO, and PhM2PO or phenyl-DMPO, on in vivo CCl4-induced acute hepatotoxicity was investigated. Both PhM2PO and M3PO were found to inhibit the CCl4-induced response at lower concentrations (0.35 M/kg body weight) than PBN (0.70 M/kg body weight). However, both M3PO and PhM2PO were also found to induce an edematous response at the same concentrations used for the PBN studies (0.70 M/kg body weight). PhM2PO, at a concentration of 0.35 M/kg body weight, was 93% as efficient as PBN, at a concentration of 0.70 M/kg body weight; whereas M3PO, at a concentration of 0.35 M/kg, was 89% as efficient as PBN at 0.70 M/kg body weight. Electron micrographs were obtained from small liver sections taken in proximity to the major branch of the hepatic portal veins of all treatment groups. The electron microscopy investigations support the MRI findings.

Diagnosis of intestinal ischemia in the rat using magnetic resonance imaging

Noninvasive diagnosis of persistent ischemia after intestinal revascularization has remained an elusive goal. Because magnetic resonance imaging (MRI) can detect changes in tissue water, we studied its efficacy in differentiating ischemic from perfused intestine in an animal model. Six-week-old rats were subjected to (1) 30-min superior mesenteric artery (SMA) occlusion and reperfusion, (2) permanent SMA ligation, or (3) sham operation, and were then imaged for 90 min using a small-animal MRI scanner with T1 weighting (TR = 1000 msec, TE = 25 msec). In an additional group of rats, the experiment was repeated using a new contrast technique consisting of oral ferrite to decrease luminal signal and intravenous gadolinium to increase bowel wall signal. Mean abdominal intensity over the scanning period was calculated for each animal (n = 5 rats per experimental group). Definition of individual bowel loops was subjectively improved in animals scanned with intravenous and oral contrast. Mean abdominal intensity was significantly lower in ligated vs sham rats (43.90 +/- 8 vs 59.63 +/- 6 and 46.19 +/- 6 vs 54.26 +/- 6, with and without contrast, respectively). There was no significant difference in intensity between reperfused and sham animals. MRI differentiated persistently ischemic bowel from viable bowel in this model, both with and without the use of contrast. These data suggest that MRI may have a potential role in the noninvasive diagnosis of persistent intestinal ischemia.

Respiratory triggered imaging with an optical displacement sensor

Motion of abdominal organs with respiration is a major problem in NMR spectroscopy and imaging thereof. Triggering each phase-encoding step with respiration or gating a number of phase-encoding steps is one approach to the problem. The design of a sensor for small animal experiments has not been as simple. An optical device, implemented with polymer optical fibres is described, along with associated hardware and electronics which can act as a trigger for small animal NMR experiments. A brief description of a similar device for human application is also given. 2DFT spin-echo and B0 susceptibility images, both triggered and untriggered, are presented to validate the technique.

A simple device for respiratory gating for the MRI of laboratory animals

Respiratory motion must be overcome if MRI of the abdomen, even at the lowest resolution, is to be performed satisfactorily. A simple and reliable respiratory gating device, based on the interruption of an infrared (IR) optical beam is described. This device has the advantage that gating is based on the position of the chest as opposed to its velocity, and that it can be used without degrading the radiofrequency isolation of a Faraday cage. Its use in animal MRI is illustrated by high resolution (200 microns) images of in vivo rat liver and kidney.

A piezoelectric respiratory monitor for in vivo NMR

This report describes a simple electronic device employing a piezoelectric element which serves as a sensitive detector of motion. The device is useful as a monitor of respiratory motion for nuclear magnetic resonance animal experiments in vivo. It can also provide a trigger pulse for respiratory gating experiments.

Use of 1H/23Na and 1H/31P double frequency tuned birdcage coils to study in vivo carbon tetrachloride-induced hepatotoxicity in rats

In vivo 1H and 23Na magnetic resonance imaging (MRI) and 31P magnetic resonance spectroscopy (MRS) techniques were used to study CCl4-induced acute hepatotoxicity in rats in situ. One or two hours following exposure to CCl4, a localized edematous region was detected in the liver by 1H MRI. The CCl4-induced edema was localized in a region surrounding the hepatic portal vein. With the use of a 23Na/1H double frequency tuned bird-cage imaging coil an increase in Na+ ion flux was also observed in the same region as the edematous region detected by 1H-MRI. Pretreatment with alpha-phenyl-tert-butyl nitrone (PBN), a free radical spin trap, 30 min prior to CCl4 exposure, was found to reduce the CCl4-induced edematous response in the liver observed in either 1H or 23Na-NMR images. Inhibition of the CCl4-induced edematous response in rat liver by PBN demonstrates that free radical intermediates, arising from the metabolism of CCl4, are possibly the key causal agents in the initiation of the edematous response. In addition, with the use of a 31P/1H double frequency tuned bird-cage imaging/spectroscopy coil, localized 31P spectra (ISIS) were obtained from the regions of CCl4-induced "tissue damage" observed in the 1H-MRI images. The most notable changes observed from the 31P spectra were an increase in inorganic phosphate (Pi) and a decrease in hepatocytosolic pH in the CCl4-treated rat livers in comparison to saline-treated control livers.

Proton magnetic resonance imaging and phosphorus-31 magnetic resonance spectroscopy studies of bromobenzene-induced liver damage in the rat

Respiratory-gated proton magnetic resonance imaging was used to study the response of the rat liver in situ to bromobenzene, a classic hepatotoxicant. A localized region of high proton signal intensity in the perihilar region of the liver was seen 24-48 hr after an intraperitoneal injection of bromobenzene. Localized proton magnetic resonance spectra from within this region indicated that the increased proton signal intensity was not due to accumulation of fat in the liver, but primarily due to a longer T2 for the proton resonance of water. This is consistent with acute edema in this localized region. In vivo 31P magnetic resonance spectroscopy studies of the same rat livers in situ were performed. Spectroscopic conditions were determined whereby localized, quantitative 31P spectra could be obtained. Using these methods, 10 mmol/kg bromobenzene was found after 24 hr to cause a number of statistically significant (p less than 0.05) effects: a decrease in adenosine 5'-triphosphate levels from 4.1 +/- 0.5 to 3.0 +/- 0.5 mM, a decrease in phosphodiester levels from 11.3 +/- 0.9 to 9.3 +/- 0.7 mM and an increase in the phosphomonoesters from 3.0 +/- 0.4 to 5.5 +/- 1.2 mM (mean +/- standard deviation). High resolution in vitro 31P spectra of perchloric acid extracts of these rat livers showed that the increased phosphomonoester resonance was due to a selective 4.3-fold increase in phosphocholine. Thus, our in vivo and in vitro 31P magnetic resonance spectra are consistent with the hypothesis that a phosphatidylcholine-specific phospholipase C (generating phosphocholine and diacylglycerol) is activated during tissue damage. Both the imaging and spectroscopy results obtained with bromobenzene closely resemble CCl4-induced liver changes previously reported, and may reflect a generalized response of the liver to any acutely acting toxic chemical.

Spin trapping of free radical metabolites of carbon tetrachloride in vitro and in vivo: effect of acute ethanol administration

A single dose of ethanol, when administered 18 hr prior to CCl4, potentiates the hepatotoxicity of the halocarbon. In these studies, spin trapping and electron spin resonance (ESR) spectroscopy methods were utilized to determine whether a single ethanol dose increased the metabolism of CCl4 to free radical intermediates. When hepatic microsomes from ethanol-treated or control rats were incubated with CCl4 and the spin trapping agent alpha-phenyl-N-tert-butylnitrone (PBN), the ESR signal of the trichloromethyl radical adduct of PBN was of similar intensity in both groups. The ethanol dose also failed to induce p-nitrophenol hydroxylase activity. When PBN and CCl4 were administered to rats, liver extracts contained ESR signals resulting primarily from the trichloromethyl radical adduct of PBN, and the signals were of similar intensity in both experimental groups. Higher concentrations of the carbon dioxide anion radical adduct of PBN were detected in plasma samples from ethanol-treated rats. However, when hepatocytes from ethanol-treated and control rats were incubated with PBN and CCl4, ESR signals of the carbon dioxide adduct were of similar intensity. These data suggest that the higher concentrations of the carbon dioxide adduct in the blood of ethanol-treated rats may be explained by early CCl4-induced damage to liver cell membranes, rather than increased rates of formation. The data in this report fail to support the hypothesis that a single dose of ethanol stimulates the hepatic metabolism of CCl4 to the trichloromethyl radical. Alternatively, ethanol may potentiate CCl4 toxicity by affecting some critical metabolic step subsequent to trichloromethyl radical formation.

Ethanol-induced fatty liver in the rat examined by in vivo 1H chemical shift selective magnetic resonance imaging and localized spectroscopic methods

In vivo 1H magnetic resonance imaging (MRI), chemical shift selective imaging (CSI), and localized (VOSY) 1H magnetic resonance spectroscopy (MRS) were used to study fatty infiltration in the livers of rats chronically fed an ethanol-containing all-liquid DeCarli-Lieber diet. Conventional total proton MRI showed a somewhat hyperintense liver for ethanol-fed rats, compared with pair-fed controls. CSI showed a dramatic increase in the fat signal intensity for ethanol-treated rats that was fairly homogeneous throughout the liver. However, CSI also showed a substantial decrease in the water signal intensity for the ethanol-treated rats compared to pair-fed control rats. 1H VOSY MR spectra also showed a 5.5-fold increase in the methylene resonance (1.3 ppm) of fat and a 50-70% decrease in the water resonance (4.8 ppm). Relative in vivo proton T1 and T2 relaxation times for the water resonance separate from the fat resonance, determined from modified VOSY experiments, were found to tend to increase and decrease, respectively, for ethanol-treated rat livers compared with controls. The decrease in hepatic water signal intensity could be accounted for by the decrease in T2 and decrease in water density due to the presence of accumulated hepatic fat (approximately 25 mg/g wet weight of liver). When ethanol was withdrawn from the chronically treated rats, fatty infiltration was observed by both CSI and VOSY spectra to revert toward control values with a half-life of 2-4 days. By day 16, however, the signal intensity for hepatic fat was still significantly higher than control levels. In vitro 1H MRS studies of chloroform-methanol extracts confirmed the 5.5-fold increase in total hepatic fat induced by the chronic ethanol treatment, and showed further that triacylglycerols were increased 7.7-fold, cholesterol was increased fourfold, and phospholipids were increased 3.3-fold, compared with liver extracts from pair-fed control rats.

The response of the rat liver in situ to bromobenzene--in vivo proton magnetic resonance imaging and 31P magnetic resonance spectroscopy studies

Proton magnetic resonance imaging (MRI) and 31P magnetic resonance spectroscopy (MRS) have been used to study the response of the rat liver in situ to bromobenzene, a classic hepatotoxicant. A localized region of high proton signal intensity was seen in the perihilar region of the liver 24 hr after injection of a sublethal dose of bromobenzene. The signal intensity of the entire liver was increased at 48 hr with a gradual return approaching control values by 120 hr. These results are consistent with acute hepatic edema followed by repair of the damaged tissue. In vivo 31P MRS studies of the same rat livers were performed under conditions whereby localized, quantitative spectra could be obtained without surgical intervention. Initial concentrations of the major endogenous phosphorus-containing metabolites within the livers of control rats were 2.97 +/- 0.43 mM for the phosphomonoesters (PME), 2.92 +/- 0.56 mM for inorganic phosphate, 11.3 +/- 1.0 mM for phosphodiesters (PDE), 4.09 +/- 0.54 mM for ATP, and 0.56 +/- 0.50 mM for ADP and the intracellular pH was 7.39 +/- 0.14 (mean +/- SD, n = 10). Bromobenzene was found to cause statistically significant (p less than 0.05) changes in several of these metabolites: a decrease in hepatic ATP levels (20% at 24 hr; 27% at 48 hr), a decrease in PDE levels (15% at 24 hr; 18% at 48 hr), and an increase in the PME (63% at 24 hr; 84% at 48 hr). Both the proton MRI and the 31P MRS changes have an onset of 15-20 hr and maximum effect at 25-60 hr, but the MRS changes returned to normal well before the MRI changes. The decreased ATP levels indicate deleterious effects of bromobenzene on the bioenergetic status of the liver in situ, while the increase in PME, due to a selective increase in phosphocholine, suggests the activation of a phosphatidylcholine-specific phospholipase C in response to tissue damage. Trolox C, a potent inhibitor of lipid peroxidation, prevented the bromobenzene-induced hepatic edema (i.e., the increase in proton MRI signal intensity) and the bioenergetic deterioration (i.e., the decrease in ATP levels). However, the bromobenzene-induced increase in PME levels was not prevented by Trolox C. These results indicate that the process of lipid peroxidation plays a significant role in the hepatotoxicity of bromobenzene within the intact animal.

In vivo NMR, biochemical, and histologic evaluation of alcohol-induced fatty liver in rat and a comparison with CCl4 hepatotoxicity

Magnetic resonance imaging (MRI) and spectroscopy (MRS) were used to follow the time course of ethanol-induced fatty liver in a group of 10 rats fed a diet containing 12% alcohol (ethanol) over a 5-week period. The MR data consisted of T1-weighted images, in vivo 1H spectra, and in vivo T1 relaxation measurements. Changes in short TR images as a result of fatty accumulation were noted only as a slight increase in liver intensity relative to surrounding muscle. A poorly correlated (r = 0.54) increase in water T1 with time was observed. No statistically significant changes in lipid T1 were found. MRS derived lipid content was compared with biochemically derived total lipids and histology. MRS determined liver lipids were found to increase linearly with time (r = 0.91). Biochemically derived lipid content also increased with prolonged exposure to ethanol (r = 0.96). The averages of MRS derived lipid content agreed well with the average changes in biochemically determined total lipid concentration. Histologic examination revealed slight to moderate changes in fatty accumulation with significant variation in the group at the end of the study. On an individual basis the MRS and histologic evaluation were highly correlated (r = 0.94).

Enhancement of carbon tetrachloride-induced liver injury by a single dose of ethanol: proton magnetic resonance imaging (MRI) studies in vivo

Magnetic resonance imaging (MRI) and localized magnetic resonance spectroscopy (MRS) were used to study the effects of a single dose of ethanol, given 18 h prior to experiments, on CC14-induced acute hepatotoxicity in rats in situ. Localized edema in the centrilobular region of the liver, following exposure to ethanol and CCl4, was detected by 1H-MRI techniques. The edema was characterized by a volume selective spectroscopy (VOSY) method, which measured an increase in water concentration from ethanol and CCl4-treated rat livers, in comparison to control livers. Electron microscopy (EM) of the high intensity regions of the ethanol/CCl4 treated liver sections revealed dramatic subcellular changes such as fragmentation of the granular endoplasmic reticulum (ER), formation of large vacuoles and lipid droplets in the cytoplasmic matrix and extensive swelling of the mitochondria as well as disruption of the cristae. Pretreatment with alpha-phenyl tert-butyl nitrone (PBN), a free radical spin trap, prior to halocarbon exposure, was found to reduce the CC14-mediated high intensity region in the liver images. Electron microscopy of the PBN pretreated CCl4 exposed rat liver sections revealed only minor observable differences in subcellular organization, such as some swelling of the mitochondria, when compared to controls. In addition, these data suggest that ethanol may potentiate CCl4 hepatotoxicity by increased formation of free radical intermediates. Inhibition of the CCl4-induced edematous response in rat liver by PBN demonstrates that free radical intermediates, arising from the metabolism of CCl4, are possibly the causal factor in the initiation of the edema.

In vivo proton magnetic resonance imaging and localized spectroscopic analysis of polycystic kidney disease in DBA/2FG-pyc mice

Proton magnetic resonance imaging (MRI) and localized spectroscopy techniques were used to study polycystic kidney disease (PKD) in DBA/2FG-pcy (pcy) mice, which are an animal model for the adult form of human PKD. A volume selective spectroscopy (VOSY) method was used to obtain localized proton spectra as well as apparent T1 and T2 relaxation times of the kidneys of either pcy or DBA/2FG normal (DBA) mice. Localization of the proton spectra was based on spatial coordinates determined from the proton images. Increases in apparent kidney T1 and T2 relaxation times were observed for pcy mice in comparison to those measured in DBA mice. Localized kidney T1 values from pcy mice were found to range from 799 to 1395 msec whereas DBA mice kidney T1 values were 617 to 774 msec. Localized T2 values measured from pcy mice kidneys ranged from 90 to 172 msec in comparison to kidney T2 values of 50 to 72 msec for DBA mice. The onset of cyst formation in the kidneys of pcy mice as a function of age was also studied. As early as 4 to 5 weeks of age it was possible to detect in vivo changes in pcy mice kidney T1 and T2 values.

Chronic carbon tetrachloride and phospholipase D hepatotoxicity in rat: in vivo 1H magnetic resonance, total lipid analysis, and histology

Magnetic resonance (MR) imaging, localized in vivo proton spectroscopy, and T1 relaxation measurements were obtained from the livers of rats treated chronically with carbon tetrachloride and phospholipase D. The MR data correlated well with lipid changes measured biochemically and histologically. MR images appeared generally hyperintense during fatty infiltration, changing to hypointense mottling during cirrhosis. Water T1 relaxation times showed no statistically significant change at any time during the experiments from the control value of 908 ms (SE = 42 ms). Minor changes in lipid T1 values with time were noted. The average lipid T1 curve demonstrated a linear relation with time (r = 0.81), increasing from the control value of 283 ms (+/- 16 ms) to 365 ms (+/- 53 ms) at the end of the third week and decreasing slightly through the end of the experiment. Water-suppressed in vivo spectra showed quantitative changes in liver lipids which correlated well with the biochemical and histologic analysis. From the MR images and spectroscopy results it was possible to distinguish early fatty liver from more advanced cirrhosis.

Sodium-23 and proton nuclear magnetic resonance imaging studies of carbon tetrachloride-induced liver damage in the rat

Magnetic resonance imaging techniques were used to investigate the response of the liver of the rat in situ to a toxicological challenge by carbon tetrachloride. Proton images were taken as transverse slices through the rat before and after intraperitoneal administration of the hepatotoxin. Two to three hours after carbon tetrachloride was given, a region of high proton signal intensity was observed where the portal vein enters the liver. Sodium-23 images were also taken, and a region of high sodium intensity was observed in the same location within the liver as the increased proton intensity. The results suggest that acute administration of carbon tetrachloride induces localized liver damage in the region where the hepatotoxin first enters the liver. This liver damage is manifest as edema with a buildup of sodium ion and water, which can be readily detected by sodium-23 and proton NMR imaging techniques, respectively.

MRI detection of hyperoxia-induced lung edema in Zn-deficient rats

In a pioneering application of proton Magnetic Resonance Imaging (MRI), lung edema has been monitored in vivo in Zn-deficient rats exposed to 85% oxygen. Dietary Zn appears to play a role in protecting against hyperoxia-induced lung damage.

The effect of phenyl tert-butyl nitrone (PBN) on CCl4-induced rat liver injury detected by proton magnetic resonance imaging (MRI) in vivo and electron microscopy (EM)

Acute intoxication by CCl4 induces morphological changes in rat liver which are readily detectable by 1H-NMR imaging techniques in situ. Two to four hours after the administration of CCl4, regions of high proton signal intensity are observed in the centrilobular region of the liver. The regions of high signal intensity are attributed to the formation of local edema as a result of CCl4-induced damage. Electron microscopy of the high intensity regions of CCl4 treated liver sections revealed characteristic subcellular changes which include the disapperance of ribosomes from the rough endoplasmic reticulum (RER), the fragmented appearance of the smooth endoplasmic reticulum (SER), formation of vacuoles in the cisternae and swelling of the mitochondria. Pretreatment of rats with alpha-phenyl-tert-butyl nitrone (PBN), a free radical spin trap, prior to halocarbon exposure, was found to reduce the halocarbon-induced edema in the liver. Electron microscopy of the PBN pretreated CCl4 exposed rat liver sections revealed no observable changes in subcellular organization when compared to controls.

In vivo and in vitro 31P-NMR spectroscopy of rat liver treated with halocarbons

Both in vivo and in vitro 31P-NMR spectroscopy were used to demonstrate metabolic changes in rat liver as a function of time after exposure to either carbon tetrachloride (CCl4) or bromotrichloromethane (BrCCl3). The inorganic phosphate resonance, measured in vivo, moves upfield, which is associated with a decrease in cytosolic pH over a 12 or 20 h period (for BrCCl3 or CCl4, respectively). Intoxication by CCl4 or BrCCl3 causes an intracellular acidosis to pH 7.05 or 6.82 (+/- 0.05), respectively. Also, it has been found that halocarbon exposure increases the amounts of phosphomonoesters (PME) detected. High resolution in vitro 31P-NMR spectroscopy studies of perchloric acid extracts of CCl4-treated rat livers indicated a significant increase in the height of the phosphocholine resonance in the PME region 4-5 h after CCl4 exposure.