Alan Koretsky

Continuous arterial spin labeling using a separate neck labeling coil is an attractive approach for imaging perfusion at 7T. This approach is hindered by specialize hardware including a labeling coil and a separate amplifier located outside the magnet room. On-coil amplifiers have demonstrated higher RF power efficiency with minimal cable loss and patient-coil interaction. This work shows that a separate labeling coil using on-coil amplification can be used for flow driven adiabatic inversion in a flow phantom suggesting that it may provide a suitable approach to simplify the hardware package required as well as to improve CASL studies at 7T.

Meander-line, or zig-zag, MRI surface coils theoretically promise spatially uniform fields with optimal field localization close to the coil. In reality, they suffer poorer than expected field localizations and acquired images are often highly inhomogeneous, plagued by repeating stripe-like signal-loss artifacts. We show that both these detrimental effects arise from coil design based on the same invalid approximation in the underlying theory. Here, the conventional approximation is corrected, yielding a modified coil design that validates the new theory by rectifying the above problems. Specifically, an easily implementable coil correction, which amounts to the addition of a single extra turn of wire, is introduced and shown to increase signal uniformity by an order of magnitude, eliminate image artifacts, and reduce unwanted signal interference from deeper within the sample by tightening the coil field localization to close to the coil, as intended for zig-zag designs. With independent optimization of coil size and imaging depth possible, such corrected meander-lines surface coils may be well suited for large area, near-surface imaging and spectroscopy applications.

PurposeDemonstrating multi-field and inverse-contrast switching of magnetocaloric high-contrast-ratio MRI labels that either have increasing or decreasing moment vs. temperature slopes depending on the material at physiological temperatures and different MRI magnetic field strengths.MethodsTwo Iron-Rhodium samples of different purity (99% and 99.9%) and Lanthanum-Iron-Silicon sample were obtained from commercial vendors. Temperature and magnetic field dependent magnetic moment measurements of the samples were performed on a vibrating sample magnetometer. Temperature-dependent MRI of different Fe-Rh and La-Fe-Si samples were performed on three different MRI scanners at 1T, 4.7T, and 7T to demonstrate both multi-field and inverse-contrast switching.ResultsSharp, first-order magnetic phase transition of each Fe-Rh sample at a physiologically relevant temperature (~37°C) but at different MRI magnetic fields (1T, 4.7T, and 7T, depending on the sample) showed clear image contrast changes in temperature-dependent MRI. Fe-Rh and La-Fe-Si samples with sharp, first-order magnetic phase transitions at the same MRI field of 1T and physiological temperature of 37°C, but with positive and negative slope of magnetization vs. temperature, respectively, showed clear inverse-contrast image changes. We also performed temperature dependent MRI on individual micro-particle samples of La-Fe-Si that also showed sharp image contrast changes.ConclusionMagnetocaloric materials of different purity and composition were demonstrated to act as diverse high-contrast-ratio switchable MRI contrast agents due to their sharp, first order magnetic phase transitions. Thus, we show that a range of magnetocaloric materials can be optimized for unique image contrast response under MRI-appropriate conditions at physiological temperatures and be controllably switched in-situ.

Creatine kinase (CK) has been implicated in affecting cell growth, and the CK substrates creatine (Cr) and cyclocreatine (CyCr) have been shown to have anti-tumor activity. The influence of Cr and CyCr on liver regeneration following major hepatectomy was evaluated in normal and transgenic mice expressing the human ubiquitous mitochondrial isoform of CK (CK-mit) or the brain isoform of CK (CK-B) or livers expressing both CK-mit and CK-B (CK-comb). Expression of CK isoenzymes had little effect on liver regeneration in the absence of dietary supplementation with Cr or CyCr as assayed by the increase in liver mass. Dietary supplementation with Cr and CyCr significantly reduced liver growth in normal mice. Liver regeneration was almost completely inhibited in mice expressing CK-mit in the presence of Cr. Livers expressing CK-mit regenerated better than normal livers in the presence of CyCr. In mice expressing CK-B, Cr and CyCr had opposite effects from those found in CK-mit mice. In the presence of CyCr, regeneration was inhibited in livers expressing CK-B, and, in the presence of Cr, CK-B-expressing livers regenerated better than normal livers. The amount of DNA synthesized 2 days after hepatectomy confirmed the results obtained from measurements of liver mass for all groups. Growth and DNA synthesis were completely abolished by Cr in CK-mit mice, whereas CyCr mainly affected growth 2 days after hepatectomy in CK-B-expressing mice. Coexpression of the CK isoforms in CK-comb mice ameliorated the effects detected with either isoform alone. Inhibition of growth by Cr and CyCr was not correlated to water accumulation. These results clearly demonstrate isoenzyme and substrate-specific effects of CK on cell growth.

Manganese enhanced MRI (MEMRI) has been used to generate layer-specific contrast in preclinical neuroimaging studies, especially in the cerebellum. However, the cell types that contribute most to MEMRI signal have not yet been identified. In this study, we registered high resolution MEMRI to immunohistochemistry of the cerebellum to verify layer localization of signal. We found that the Purkinje cell layer (PCL) was the source of hyperintensity. Next, we manipulated cell types of the PCL using genetically engineered mouse models and quantified MEMRI signal changes. These results strongly suggest Purkinje cells are the primary cellular source of hyperintensity in cerebellar MEMRI.

Introduction Despite a large body of work mapping the human auditory pathway with functional MRI (fMRI), there has been very little work mapping the rodent auditory system with MRI. Recently, manganese-enhanced MRI (MEMRI) has been used to map the tonotopic reorganization in the auditory midbrain of juvenile mice [1]. This method indicated that auditory midbrain plasticity could have major contributions to the adapting auditory system during development [2]. Drawbacks of this approach were that it was not able to map the auditory cortex and required long-term sound stimulation, which could induce cortical habituation. In the present study, BOLD-fMRI was used to map the sound evoked activity in both the auditory midbrain and the auditory cortex in anesthetized rats. BOLD signal changes were detected at high resolution using an 11.7T MRI. Activation maps could be made from the inferior colliculus (IC) and auditory cortex (AC). This work demonstrates that it should be possible to use BOLD-fMRI to map changes in neural representation in both auditory midbrain and auditory cortex. Methods BOLD-fMRI was performed in 5 rats anesthetized with propofol. Detail procedures of imaging setup and animal preparation for fMRI were similar to those previously described [3]. Briefly, all images were acquired with an 11.7T/31cm horizontal bore magnet (Magnex, Abingdon, UK), interfaced to an AVANCE III console (Bruker, Billerica, MA) and equipped with a 9 cm gradient set. A 3D gradient-echo, EPI sequence was used for the fMRI studies. MRI was run with the following parameters: effective echo time (TE) 16ms, repetition time (TR) 1.5s, bandwidth 150 kHz. This sequence gave isotropic resolution of 300 microns with a 64 x 64 x 32 matrix at FOV 19.2 x 19.2 x 9.6 mm and 200 microns with a 96 x 96 x 32 matrix at FOV 19.2 x 19.2 x 6.4 mm. For acoustic stimulation, electrostatic speakers (STAX) were used to deliver acoustic signal in the magnet during MRI. A 200ms short pulse of broadband noise with 10ms ramp at the edges was repeated with 50ms inter-stimulus duration. To reduce habituation during acoustic stimulation, a randomized sound profile was designed by concatenating the 200ms short pulse of broadband noises with different bandwidths (1-5 kHz, 1-20 kHz, and 1-50 kHz). During imaging, rat ears were covered with customized sound attenuation barrier to reduce the effects of scanner noise. A subskin electrical stimulation with 2.5 mA, 300μs pulses repeated at 3Hz was delivered to the mystacial pads to stimulate whisker cortex. A Block design paradigm was applied for fMRI studies with 2 epochs of 36s on and 24s off for acoustic stimulation and 5 epochs of 30s on and 30s off for mystacial pad stimulation. AFNI software (NIH, Bethesda) and Matlab was used for Image analysis. Results The activity pattern in the auditory cortex and midbrain could be mapped with BOLD-fMRI. To characterize the location of the auditory cortex, the barrel cortex was also mapped It is located rostromedial to the auditory cortex (Fig 1, inset). In Fig 1A and B, a color-coded t-map was superimposed on three 2D coronal slices across the barrel cortex (BC), auditory cortex (AC), and inferior colliculus (IC). The barrel cortex was activated in both hemispheres following electrical stimulation on both sides of the face. Acoustic stimulation was delivered to the left ear, and the AC and IC in the right hemisphere were activated. The BOLD signal change was detected each area, showing a good correlation with the stimulation paradigm. The sound evoked percentage BOLD signal change was around 1% in IC and AC. Electrical stimulation of mystacial pads induced 3-4% BOLD signal enhancement. In addition, 200 micron isotropic 3D EPI images were acquired in the IC with acoustic stimulation of both ears. In fig 1D, we showed the t-map of three consecutive coronal IC slices. A 3D fMRI contour is shown in a segmented brain slab (anatomical IC, green, active IC, red). Conclusions This work successfully detected fMRI activation in the auditory cortex and brainstem with 3D BOLD-fMRI. Future work will characterize auditory plasticity and analyze the interaction between the auditory midbrain and cortex under conditions where plasticity occurs. Reference [1] Yu X et al., PNAS. 104: 12193-8. (2007) [2] Knudsen EI and Brainard MS, Science 253: 85-7 (1991) [3] Silva AC and Koretsky AP, PNAS, 99: 15182-7 (2002) i Somatosensory BC AC IC i ii iii Sound

Purpose:The sensitivity of pseudo-continuous arterial spin labeling (PCASL) to off-resonance effects (ΔB0) is a major limitation at ultra-high field (≥7T). The aim of this study was to assess the effectiveness of different PCASL ΔB0 compensation methods at 7T and measure the labeling efficiency with off-resonance correction.Theory and Methods:Phase offset errors induced by ΔB0 at the feeding arteries can be compensated by adding an extra RF phase increment and transverse gradient blips into the PCASL RF pulse train. The effectiveness of an average field correction (AVGcor), a vessel-specific field-map-based correction (FMcor) and a vessel-specific prescan-based correction (PScor) were compared at 7T. After correction, the PCASL labeling efficiency was directly measured in feeding arteries downstream from the labeling location.Results:The perfusion signal was more uniform throughout the brain after off-resonance correction. Whole-brain average perfusion signal increased by a factor of 2.4, 2.5 and 2.1 respectively with AVGcor, FMcor and PScor compared to acquisitions without correction. With off-resonance correction, the maximum labeling efficiency was ~0.68 at mean B1 (B1mean) of 0.70μT when using a mean gradient (Gmean) of 0.25mT/m.Conclusion:Either a prescan or a field map can be used to correct for off-resonance effects and retrieve a good brain perfusion signal at 7T. Although the three methods performed well in this study, FMcor may be better suited for patient studies because it accounted for vessel-specific ΔB0 variations. Further improvements in image quality will be possible by optimizing the labeling efficiency with advanced hardware and software while satisfying SAR-constraints.

Transgenic mice overexpressing beta-tropomyosin have increased myofilament Ca(2+) sensitivity that we hypothesized would result in altered relationships among pressure and heart rates, intracellular Ca(2+), and myocardial O(2) consumption. In perfused hearts from transgenic mice there was a marked negative force-frequency response between 6 and 10 Hz with a 30 +/- 3% reduction in peak-positive first derivative of pressure development over time (dP/dt) compared with 14 +/- 2% in wild-type mice (P < 0.001). At 8 Hz systolic pressures were normal, though peak systolic intracellular Ca(2+) was significantly reduced in transgenic mice versus wild type (726 +/- 61 vs. 936 +/- 67 nM, P < 0.05) indicating an alteration in the pressure-Ca(2+) relationship. Over a wide range of positive and negative inotropic interventions there were normal developed pressures, though marked elevations in myocardial O(2) consumption (15-54%). Because pressures are normal and intracellular Ca(2+) decreased and myocardial O(2) consumption increased, this suggests that these abnormalities are at least in part compensatory mechanisms to the altered myofilament function.

To change the levels of expression and isoenzyme distribution of creatine kinase (CK) in muscle, transgenic technology was used to express the B subunit of CK in mouse muscle. Normally, mammalian skeletal muscle contains the MM dimer of CK. The BB dimer and MB heterodimer of CK can be found in brain and heart, respectively. Heterologous genes consisting of skeletal and cardiac muscle-specific actin promoters fused to the genomic coding region of the B form of CK were used to create transgenic mice. Lines were established from the three highest expressing founders. Analysis of skeletal muscle extracts revealed that all three lines had an increase in total CK activity measured under maximal velocity conditions. The highest expressing line, 7001, had a CK activity 150% that of control muscle. Nuclear magnetic resonance saturation transfer was used to measure the in vivo rate of the CK reaction. In 7001 hindlimb muscles, the CK catalyzed reaction was 200% that of control muscle. The elevation in CK activity in transgenic muscle was accompanied by significant changes in the composition of the cytosolic isoenzyme ratio of CK. In control, 100% of CK was MM, whereas 7001 had 60 +/- 18% MM, 32 +/- 10% MB, and 8 +/- 2% BB. There were no changes in ATP, phosphocreatine, Pi, or creatine levels in transgenic muscle compared with control. Immunofluorescence of myofibrils isolated from control and transgenic muscle revealed specific association of CK to the M line. Small amounts of MB CK were detected on myofibrils from transgenic mice. Transgenic mice expressing the B subunit of CK in muscle represent a first step toward altering CK isoforms so as to elucidate the specific roles of these isoforms in energy metabolism.

The effects of an intraperitoneal dose of fructose on hepatic metabolism in transgenic mice expressing creatine kinase in liver were investigated using phosphorus-31 nuclear magnetic resonance (31P-NMR). Transgenic mice were fed diets containing varying amounts of creatine (Cr; 0-12%). It has previously been shown that 31P-NMR spectra of transgenic mice have a peak due to phosphocreatine (PCr), the intensity of which was proportional to the amount of Cr in the diet. No PCr peak was detected in control mice or transgenic mice not fed Cr. In the present study NMR spectra were collected before and for a 1-h recovery period after infusion of 0.15 mmol/10 g body wt fructose. In all mice infusion of fructose resulted in a two- to threefold elevation of phosphomonoesters. In control and non-Cr-fed transgenic mice this was accompanied by a 60% reduction of the inorganic phosphate (Pi) and a 50% fall in ATP. In transgenic mice fed Cr, the extent of reduction of Pi was dependent on the level of PCr and was markedly reduced compared with controls. Falls in Pi of 46, 24, and 6% were detected 12.5 min after fructose infusion in low, intermediate, and high PCr-containing livers, respectively. The presence of PCr also protected hepatic ATP levels from a fructose load. Transgenic mice fed on high or intermediate Cr diets showed no significant loss of ATP. However, livers with low levels of PCr lost ATP during a fructose challenge. From the equilibrium established by creatine kinase, free ADP levels were calculated throughout the fructose dose. Fructose caused a 2.5-fold increase in free ADP. This rise in ADP was independent of the total Cr or whether Pi and ATP were reduced by fructose infusion. These results indicate that an increase in ADP is not sufficient to cause depletion of ATP during a fructose challenge.

To develop a technique for measurement of regional renal perfusion with magnetic resonance (MR) imaging. Quantitative renal perfusion images in rats were obtained by measurement of the reduction in kidney MR image signal intensity after steady state magnetic labeling of arterial blood in the suprarenal aorta. Labeling was achieved with adiabatic fast passage inversion of arterial water. Cortical renal blood flow was 4.9 mL/g/min +/- 0.15 (12 rats), which correlated well with previous measurements obtained with other techniques. Serial perfusion images obtained every 5 minutes during intravenous infusion of either acetylcholine or angiotensin II showed that one increased and the other decreased renal blood flow, respectively, also correlating with previous measurements. Quantitative measurement of cortical renal blood flow can be obtained with proton MR imaging techniques, with use of endogenous arterial water as a tracer. This technique should be readily applicable to measurement of renal perfusion in humans.

Renal intracellular pH (pHi) was measured in vivo from the chemical shift (sigma) of inorganic phosphate (Pi), obtained by 31P-nuclear magnetic resonance spectroscopy (NMR). pH was calculated from the difference between sigma Pi and sigma alpha-ATP. Changes of sigma Pi closely correlated with changes of sigma monophosphoesters; this supports the hypothesis that the pH determined from sigma Pi represents pHi. Renal pH in control rats was 7.39 +/- 0.04 (n = 8). This is higher than pHi of muscle and brain in vivo, suggesting that renal Na-H antiporter activity raises renal pHi. To examine the relationship between renal pH and ammoniagenesis, rats were subjected to acute (less than 24 h) and chronic (4-7 days) metabolic acidosis, acute (20 min) and chronic (6-8 days) respiratory acidosis, and dietary potassium depletion (7-21 days). Acute metabolic and respiratory acidosis produced acidification of renal pHi. Chronic metabolic acidosis (arterial blood pH, 7.26 +/- 0.02) lowered renal pHi to 7.30 +/- 0.02, but chronic respiratory acidosis (arterial blood pH, 7.30 +/- 0.05) was not associated with renal acidosis (pH, 7.40 +/- 0.04). At a similar level of blood pH, pHi was higher in chronic metabolic acidosis than in acute metabolic acidosis, suggesting an adaptive process that raises pHi. Potassium depletion (arterial blood pH, 7.44 +/- 0.05) was associated with a marked renal acidosis (renal pH, 7.17 +/- 0.02). There was a direct relationship between renal pH and cardiac K+. Rapid partial repletion with KCl (1 mmol) significantly increased renal pHi from 7.14 +/- 0.03 to 7.31 +/- 0.01.(ABSTRACT TRUNCATED AT 250 WORDS)

Magnetic resonance imaging (MRI) has become established as an important imaging modality for the clinical management of disease. This is primarily due to the great tissue contrast inherent in magnetic resonance images of normal and diseased organs. Due to the wide availability of high field magnets and the ability to generate large and rapidly switched magnetic field gradients there is growing interest in applying high resolution MRI to obtain microscopic information. This symposium on MRI microscopy highlights new developments that are leading to increased resolution. The application of high resolution MRI to significant problems in developmental biology and cancer biology will illustrate the potential of these techniques.In combination with a growing interest in obtaining high resolution MRI there is also a growing interest in obtaining functional information from MRI. The great success of MRI in clinical applications is due to the inherent contrast obtained from different tissues leading to anatomical information.

Introduction Studies of the rodent barrel cortex have shown that the critical period of thalamocortical plasticity ends in the first week after birth[1]. Barrel cortical plasticity occurred after this critical period is usually considered to occur in corticocortical connections. In this study, plasticity associated with unilateral denervation of the infraorbital (IO) nerve was studied in the protected whisker cortex in juvenile rats. Previously, plasticity after unilateral denervation of forepaw, hindpaw and IO has been shown to lead to detection of increased bilateral cortical fMRI responses to stimulation of the protected side in rats [2,3]. Here, we focused on determining whether there were changes in the contralateral thalamocortical pathway to the good whisker pad that could explain the increased cortical fMRI. First, stronger BOLD response was detected in the contralateral barrel cortex of rats with unilateral infraorbital denervation (IO rats) in comparison to sham rats. The relation between thalamus and cortical fMRI was consistent with a strengthening of the thalamocortical input [2]. To further analyze the underlying circuit changes, Manganese-enhanced MRI (MEMRI) was applied to map the contralateral thalamocortical connection between the ventral posteromedial/posterior nucleus of thalamus (VPM/PO) and barrel cortex. MEMRI can be used to trace the antegrade neuronal projections into layers 4/5 of the barrel cortex [3]. The synaptic strength of thalamocortical projection can be estimated by measuring the amount of Mn transported from VPM/PO to the Layer 4-5 of the barrel cortex. Mnenhanced signal in the Layer 4-5 of IO rats was significantly higher than that of the sham rats. This result indicated a strengthened thalamocortical projection toward the contralateral barrel cortex of the good whisker pad in IO rats. This layer-specific thalamocortical plasticity changes led us to studying the underlying synaptic mechanism with in vitro slice electro-physiology. Synaptic responses of Layer 4 stellate cells, the major cortical neurons receiving thalamocortical inputs, were recorded after stimulation of VPM thalamic projection fibers. The increased amplitude of Sr-induced miniature excitatory postsynaptic current (EPSC) in IO rats indicated a postsynaptic modulation on the strengthened thalamocortical inputs. In summary, by combing multi-modal MRI imaging methods and electrophysiology strengthening of the thalamocortical synapse was demonstrated in the whisker barrel system of juvenile rats. Methods Unilateral infraorbital denervation and sham surgeries were performed on rats at postnatal 4 weeks. MRI imaging and electrophysiological recordings were done at postnatal 6-7 weeks. BOLD-fMRI was performed on 18 rats anesthetized with α-chloralose. MEMRI were done on 20 rats. Detailed procedures for imaging and animal preparation for BOLD-fMRI and MEMRI were similar to those previously described [4, 5]. Briefly, all images were acquired with an 11.7T/31cm horizontal bore magnet (Magnex, Abingdon, UK), interfaced to an AVANCE III console (Bruker, Billerica, MA) and equipped with a 12 cm gradient set. A custom-built, 9 cm diameter transmitter coil was used for transmit and a custom-built surface coil was used for receive employing a transmit/receive decoupling device. A singleshot 3D gradient-echo, EPI sequence was used for the fMRI studies (matrix 64 x 64 x 32, TE 16ms, TR 1.5s, isotropic resolution, 300μm). A sub-skin electrical stimulation with 2.5 mA, 300μs pulses, 3Hz was delivered to forepaw (FP) and whisker pads in a block design (30s on/off). MPRAGE (TI, 1s) was used to examine Mn-tracing (matrix 192x192x16, in plane 100μm, thickness, 500μm). AFNI software was used for fMRI image processing. For group analysis, custom C++ scripts were developed to register MRI images to rat brain atlas to define brain ROIs. Student’s t test (two-tail) was used for statistical analysis. Results Fig1A shows averaged 2D fMRI β-maps overlaid on anatomical MRI images across the barrel cortex (BC) and the forepaw S1 (FP) cortex of IO and Sham rats. Bilateral activation in the BC was observed in IO rats (contra-BC left; ipsi-BC, right). Group analysis of the mean β value in contralateral BC ROI showed a significantly increased β value in the IO rats up to 65% compare to only 6% increase in the contralateral FP ROI. In addition, little difference (less than 5%) was observed in the contralateral VPM/PO (data not shown)[2]. This result indicated a specific plasticity changes in the contralateral BC of IO rats. Next, we locally administrated Mn into the VPM/PO of the contralateral thalamus (50mM, 200nl). Detectable Mn signal in the Layer 4-5 first appears 5 h after Mn injection. In IO rats, Mn-enhanced MRI signal in the Layers 4-5 of the BC was significantly higher than that of sham rats with 37% signal increase, but no difference was detected in the layer 4-5 of FP/HP S1 cortex(less than 2%). This result indicated an increased…