Phase-sensitive cardiac tagging--REALTAG.

Fully inverting spins, instead of merely saturating them, provides superior contrast for tagging procedures. The resulting improvement in tag contrast-to-noise ratio (CNR) yields higher-precision tag detection. Also, thinner slices and hence reduced tag separations can be employed, providing displacement and strain measurements with better spatial resolution. Alternatively, the improved tag contrast can be used to obtain cine images covering a greater portion of the cardiac cycle. The use of standard magnitude reconstruction for images of these inversion tags causes rectification of the negative-valued signals from the tags, confounding the image interpretation. Therefore, a phase-sensitive reconstruction scheme of the inverted tags must be employed. Here we demonstrate the implementation of inverted tags with phase-sensitive reconstruction in a ramped-flip-angle, steady-state free precession (SSFP) sequence.     

Three-dimensional magnetic resonance myocardial motion tracking from a single image plane.

Three-dimensional imaging for the quantification of myocardial motion is a key step in the evaluation of cardiac disease. A tagged magnetic resonance imaging method that automatically tracks myocardial displacement in three dimensions is presented. Unlike other techniques, this method tracks both in-plane and through-plane motion from a single image plane without affecting the duration of image acquisition. A small z-encoding gradient is subsequently added to the refocusing lobe of the slice-selection gradient pulse in a slice following CSPAMM acquisition. An opposite polarity z-encoding gradient is added to the orthogonal tag direction. The additional z-gradients encode the instantaneous through plane position of the slice. The vertical and horizontal tags are used to resolve in-plane motion, while the added z-gradients is used to resolve through-plane motion. Postprocessing automatically decodes the acquired data and tracks the three-dimensional displacement of every material point within the image plane for each cine frame. Experiments include both a phantom and in vivo human validation. These studies demonstrate that the simultaneous extraction of both in-plane and through-plane displacements and pathlines from tagged images is achievable. This capability should open up new avenues for the automatic quantification of cardiac motion and strain for scientific and clinical purposes.     

High-Density SNP genotyping defines 17 distinct haplotypes of the TNF block in the Caucasian population: implications for haplotype tagging

The region spanning the tumor necrosis factor (TNF) cluster in the human major histocompatibility complex (MHC) has been implicated in susceptibility to numerous immunopathological diseases, including type 1 diabetes mellitus and rheumatoid arthritis. However, strong linkage disequilibrium across the MHC has hampered the identification of the precise genes involved. In addition, the observation of "blocks" of DNA in the MHC within which recombination is very rare, limits the resolution that may be obtained by genotyping individual SNPs. Hence a greater understanding of the haplotypes of the block spanning the TNF cluster is necessary. To this end, we genotyped 32 human leukocyte antigen (HLA)-homozygous workshop cell lines and 300 healthy control samples for 19 coding and promoter region SNPs spanning 45 kb in the central MHC near the TNF genes. The workshop cell lines defined 11 SNP haplotypes that account for approximately 80% of the haplotypes observed in the 300 control individuals. Using the control individuals, we defined a further six haplotypes that account for an additional 10% of donors. We show that the 17 haplotypes of the "TNF block" can be identified using 15 SNPs.     

Pain response measured with arterial spin labeling

The majority of functional MRI studies of pain processing in the brain use the blood oxygenation level‐dependent (BOLD) imaging approach. However, the BOLD signal is complex as it depends on simultaneous changes in blood flow, vascular volume and oxygen metabolism. Arterial spin labeling (ASL) perfusion imaging is another imaging approach in which the magnetically labeled arterial water is used as an endogenous tracer that allows for direct measurement of cerebral blood flow. In this study, we assessed the pain response in the brain using a pulsed‐continuous arterial spin labeling (pCASL) approach and a thermal stimulation paradigm. Using pCASL, response to noxious stimulation was detected in somatosensory cortex, anterior cingulate cortex, anterior insula, hippocampus, amygdala, thalamus and precuneus, consistent with the pain response activation patterns detected using the BOLD imaging approach. We suggest that pCASL is a reliable alternative for functional MRI pain studies in conditions in which blood flow, volume or oxygen extraction are altered or compromised. Copyright © 2013 John Wiley & Sons, Ltd.     

Hippocampal molecular mechanisms involved in the enhancement of fear extinction caused by exposure to novelty

Exposure to a novel environment enhances the extinction of contextual fear. This has been explained by tagging of the hippocampal synapses used in extinction, followed by capture of proteins from the synapses that process novelty. The effect is blocked by the inhibition of hippocampal protein synthesis following the novelty or the extinction. Here, we show that it can also be blocked by the postextinction or postnovelty intrahippocampal infusion of the NMDA receptor antagonist 2-amino-5-phosphono pentanoic acid; the inhibitor of calcium/calmodulin-dependent protein kinase II (CaMKII), autocamtide-2–related inhibitory peptide; or the blocker of L-voltage–dependent calcium channels (L-VDCCs), nifedipine. Inhibition of proteasomal protein degradation by β-lactacystin has no effect of its own on extinction or on the influence of novelty thereon but blocks the inhibitory effects of all the other substances except that of rapamycin on extinction, suggesting that their action depends on concomitant synaptic protein turnover. Thus, the tagging-and-capture mechanism through which novelty enhances fear extinction involves more molecular processes than hitherto thought: NMDA receptors, L-VDCCs, CaMKII, and synaptic protein turnover.Frey and Morris (1, 2) and their collaborators (3–7) proposed a mechanism whereby relatively “weak” hippocampal long-term potentiation (LTP) or long-term depression (LTD) lasting only a few minutes can nevertheless “tag” the synapses involved with proteins synthesized ad hoc, so that other plasticity-related proteins (PRPs) produced at other sets of synapses by other LTPs or LTDs can be captured by the tagged synapses and strengthen their activity to “long” LTPs or LTDs lasting hours or days (8). LTDs and LTPs can “cross”-tag each other; that is, LTDs can enhance both LTDs and LTPs, and vice versa (6, 8). Because many learned behaviors rely on hippocampal LTP or LTD (7–9), among them the processing of novelty (9, 10) and the making of extinction (11–13), interactions between consecutive learnings can also be explained by the “tagging-and-capture” hypothesis (9, 10, 13), whose application to behavior became known as “behavioral tagging and capture” (5, 7, 9, 13). Typically, exposure to a novel environment [e.g., a nonanxiogenic 50 × 50 × 40-cm open field (OF) (5, 7, 9, 10, 14)] is interpolated before testing for another task, which becomes enhanced (4–10, 13). The usual reaction to novelty is orienting and exploration (14), followed by habituation of this response (14–16). Habituation is perhaps the simplest form of learning, and it consists of inhibition of the orienting/exploratory response (14, 16).We recently showed that the brief exposure of rats to a novel environment (the OF) within a limited time window enhances the extinction of contextual fear conditioning (CFC) through a mechanism of synaptic tagging and capture (13), which is a previously unidentified example of behavioral tagging of inhibitory learning. Fear extinction is most probably due to LTD in the hippocampus (11, 12), although the possibility that it may also involve LTP is not discarded (13). The enhancement of extinction by novelty probably relies on the habituation to the novel environment, which is also probably due to LTD (15, 16). The enhancement of extinction by the exposure to novelty depends on hippocampal gene expression and ribosomal protein synthesis following extinction training and on both ribosomal and nonribosomal protein synthesis caused by the novel experience (13). Nonribosomal protein synthesis that can be blocked by rapamycin is believed to be dendritic (13, 17), so it would be strategically located for tagging-and-capture processes, but it has not been studied in synaptic tagging to date (3–8) or in other forms of behavioral tagging (7–10). As occurs with the interactions between LTPs and/or LTDs (4), the enhancement of extinction by novelty relies on hippocampal but not amygdalar processes (13).Recent findings indicate that several hippocampal processes related to learning and memory, such as the reconsolidation of spatial learning, are highly dependent on NMDA glutamate receptors, calcium/calmodulin protein kinase II (CaMKII), and long-term voltage channel blockers (L-VDCCs), which, in turn, rely on the proteasomal degradation of proteins (18). Here, we study the effects of an NMDA blocker, 2-amino-5-phosphono pentanoic acid (AP5); the L-VDCC blocker nifedipine (Nife); a CaMKII inhibitor, the autocamtide-2–related inhibitory peptide (AIP); and the irreversible proteasome blocker β-lactacystin (12, 13) on the interaction between novelty and extinction (11). As will be seen, we found that both the setting up of tags by extinction and the presumable production of PRPs by the processing of novelty are dependent on NMDA receptors, CaMKII, and L-VDCCs. This endorses and expands the hypothesis that the novelty–extinction interaction relies on synaptic tagging and capture (13).     

Non-canonical amino acid labeling in vivo to visualize and affinity purify newly synthesized proteins in larval zebrafish

Protein expression in the nervous system undergoes regulated changes in response to changes in behavioral states, in particular long-term memory formation. Recently, methods have been developed (BONCAT and FUNCAT), which introduce non-canonical amino acids bearing small bio-orthogonal functional groups into proteins using the cells' own translational machinery. Using the selective 'click reaction', this allows for the identification and visualization of newly synthesized proteins in vitro. Here we demonstrate that non-canonical amino acid labeling can be achieved in vivo in an intact organism capable of simple learning behavior, the larval zebrafish. We show that azidohomoalanine is metabolically incorporated into newly synthesized proteins, in a time- and concentration-dependent manner, but has no apparent toxic effect and does not influence simple behaviors such as spontaneous swimming and escape responses. This enables fluorescent labeling of newly synthesized proteins in whole mount larval zebrafish. Furthermore, stimulation with a GABA antagonist that elicits seizures in the larval zebrafish causes an increase in protein synthesis throughout the proteome, which can also be visualized in intact larvae.     

Continuous arterial spin labeling using a train of adiabatic inversion pulses

PURPOSE: To develop a simple and robust magnetic resonance imaging (MRI) pulse sequence for the quantitative measurement of blood flow in the brain and cerebral tumors that has practical implementation advantages over currently used continuous arterial spin labeling (CASL) schemes. MATERIALS AND METHODS: Presented here is a single-coil protocol that uses a train of hyperbolic secant inversion pulses to produce continuous arterial spin inversion for perfusion weighting of fast spin echo images. Flow maps of normal rat brains and those containing a 9L gliosarcoma orthotopic tumor model conditions were acquired with and without carbogen. RESULTS: The perfusion-weighted images have reduced magnetization transfer signal degradation as compared to the traditional single-coil CASL while avoiding the use of a more complex two-coil CASL technique. Blood flow measurements in tumor and normal brain tissue were consistent with those previously reported by other CASL techniques. Contralateral and normal brain showed increased blood flow with carbogen breathing, while tumor tissue lacked the same CO(2) reactivity. CONCLUSION: This variation of the CASL technique is a quantitative, robust, and practical single-coil method for measuring blood flow. This CASL method does not require specialized radiofrequency coils or amplifiers that are not routinely used for anatomic imaging of the brain, therefore allowing these flow measurements to be easily incorporated into traditional rodent neuroimaging protocols.     

Combined tag tracking and strain reconstruction from tagged cardiac MR images without user-defined myocardial contours

PURPOSE: To develop an unsupervised method for measuring quantitative three-dimensional regional strain in the left ventricular wall from tagged cardiac MR images. MATERIALS AND METHODS: A total of 10 normal human volunteers and eight patients with myocardial infarction were imaged using a parallel tagged imaging protocol. Each study was analyzed using the combined tag tracking and strain reconstruction (COTTER) algorithm. In contrast to existing techniques, which first track tag lines independently in each slice, then reconstruct myocardial deformation, the COTTER algorithm fits a three-dimensional cardiac deformation model directly to the image data. This approach ensures that tag line positions identified in the image data are consistent from slice to slice. A total of 10 imaging studies (six normals, four patients) were used to optimize parameters of the COTTER algorithm. RESULTS: In the remaining eight imaging studies, the root-mean-square difference between tags tracked by COTTER and user-supervised analysis was 0.66 pixels at end-systole. The correlation coefficient between circumferential shortening strains at end-systole computed by COTTER and user-supervised analysis was 0.84 (P < 0.005) at the midwall. CONCLUSION: The COTTER algorithm can compute accurate measurements of three-dimensional regional strain without user supervision.     

Peak-combination HARP: a method to correct for phase errors in HARP

PURPOSE: To introduce a method to correct phase errors (e.g., from B0 inhomogeneity) in tagging images, which may affect harmonic phase (HARP) evaluation. MATERIALS AND METHODS: The phase images corresponding to the negative and positive harmonic peaks in k-space are combined before HARP evaluation to eliminate any spurious phase. To validate in vivo, two complementary spatial modulation of magnetization (CSPAMM) data sets were collected for each volunteer and evaluated with conventional HARP, using either the positive or the negative harmonic peak, and with peak-combination HARP. RESULTS: Elimination of phase distortion by peak combination was observed in vitro and in vivo. Improved reproducibility of motion parameters was found with peak-combination HARP. CONCLUSION: With peak-combination HARP, reproducibility of contractile parameters is improved, and consequently, the number of subjects needed to detect statistically significant changes in contractile function can be reduced to one third compared to conventional HARP evaluation.