Toward individualized SAR models and in vivo validation.

The specific absorption rate (SAR) is a limiting constraint in sequence design for high-field MRI. SAR estimation is typically performed by numerical simulations using generic human body models. This entails an intrinsic uncertainty in present SAR prediction. This study first investigates the required detail of human body models in terms of spatial resolution and the number of soft tissue classes required, based on finite-differences time-domain simulations of a 3 T body coil. The numerical results indicate that a resolution of 5 mm is sufficient for local SAR estimation. Moreover, a differentiation between fatty tissues, water-rich tissues, and the lungs was found to be essential to represent eddy current paths inside the human body. This study then proposes a novel approach for generating individualized body models from whole-body water-fat-separated MR data and applies it to volunteers. The SAR hotspots consistently occurred in the arms due to proximity to the body coil as well as in narrow regions of the muscles. An initial in vivo validation of the simulated fields in comparison with measured B(1)-field maps showed good qualitative and quantitative agreement.

REF2 encodes an RNA-binding protein directly involved in yeast mRNA 3'-end formation.

The Saccharomyces cerevisiae mutant ref2-1 (REF = RNA end formation) was originally identified by a genetic strategy predicted to detect decreases in the use of a CYC1 poly(A) site interposed within the intron of an ACT1-HIS4 fusion reporter gene. Direct RNA analysis now proves this effect and also demonstrates the trans action of the REF2 gene product on cryptic poly(A) sites located within the coding region of a plasmid-borne ACT1-lacZ gene. Despite impaired growth of ref2 strains, possibly because of a general defect in the efficiency of mRNA 3'-end processing, the steady-state characteristics of a variety of normal cellular mRNAs remain unaffected. Sequencing of the complementing gene predicts the Ref2p product to be a novel, basic protein of 429 amino acids (M(r), 48,000) with a high-level lysine/serine content and some unusual features. Analysis in vitro, with a number of defined RNA substrates, confirms that efficient use of weak poly(A) sites requires Ref2p: endonucleolytic cleavage is carried out accurately but at significantly lower rates in extracts prepared from delta ref2 cells. The addition of purified, epitope-tagged Ref2p (Ref2pF) reestablishes wild-type levels of activity in these extracts, demonstrating direct involvement of this protein in the cleavage step of 3' mRNA processing. Together with the RNA-binding characteristics of Ref2pF in vitro, our results support an important contributing role for the REF2 locus in 3'-end processing. As the first gene genetically identified to participate in mRNA 3'-end maturation prior to the final polyadenylation step, REF2 provides an ideal starting point for identifying related genes in this event.

A quaternary transcription termination complex. Reciprocal stabilization by Rho factor and NusG protein.

The Escherichia coli protein NusG is known to modulate Rho-dependent transcription termination in vivo. We have shown that it can also alter the pattern of Rho-dependent RNA endpoints in vitro, at lower NusG concentrations than can be explained by reported interactions between NusG and Rho or RNA polymerase. Three observations in vitro now suggest a model to account for these effects of NusG on Rho-dependent termination. First, the presence of NusG circumvents the interference with Rho function caused by adding DNA oligonucleotides complementary to particular segments of the Rho binding site. Second, when NusG is added to stalled elongation complexes, the off-rate of Rho from nascent RNA is slowed. Third, NusG associates stably with the elongation complex only when Rho is also present and bound to the nascent RNA. Our observations are consistent with a model in which NusG and Rho participate in an interdependent association with the transcribing RNA polymerase and the nascent RNA to facilitate the recognition and use of termination signals. Common structural and functional features shared with complexes that carry out processive antitermination are discussed.

[3D motion adapted gating: a new navigator technique to shorten the acquisition time for coronary MRA]. / 3D Motion Adapted Gating: Eine neue Navigatortechnik zur Verkürzung der Messzeit bei der MR-Koronarangiographie.

PURPOSE: A major problem of free breathing coronary MR angiography (MRA) with respiratory navigator gating is low navigator efficiency and prolonged scan time due to irregular breathing patterns. 3D motion adapted gating (MAG) is a new adaptive navigator technique, which adapts in real time to changes of the end-expiratory position of diaphragm. This study evaluates the influence of 3D MAG on coronary MRA. METHODS AND MATERIALS: In 3D MAG, two additional gating windows are grouped around the conventional window. Additionally, each gating window is divided into three bands assigned to different portions of the k-space. The scan is terminated when three consecutive bands are filled and one complete image data set is collected. Free breathing navigator-gated coronary MRA was performed on 48 patients with suspected coronary artery disease. In random order, each patient underwent an ECG-gated, a 3D segmented k-space gradient echo sequence using 3D MAG and a conventional navigator technique. The coronary MRA was evaluated and compared using the following parameters: 1. navigator efficiency and scan time; 2. visualized coronary artery length; 3. qualitative assessment of image quality; and 4. detection of stenoses > 50 % in comparison with catheter angiography. RESULTS: Coronary MRA with 3D MAG had a significant increase in the average navigator efficiency (46 % +/- 12 % vs. 38 % +/- 12 %, p < 0.05), resulting in a significantly shorter scan time (mean: 18 % +/- 4 %, p < 0.05) for coronary MRA with 3D MAG compared to conventional navigator technique. Scans with and without 3D MAG had no significant differences in the continuously visualized vessel lengths, in the assessed image quality and in the sensitivity and specificity (83 % and 89 % vs. 83 % and 88 %, p > 0.05) of detecting coronary artery stenoses > 50 %. CONCLUSION: The 3D MAG technique improves the navigator efficiency and significantly (p < 0.05) shortens the scan time of navigator gated coronary MRA while maintaining image quality and diagnostic accuracy in the detection of coronary artery stenoses.

NusG alters rho-dependent termination of transcription in vitro independent of kinetic coupling.

To complement the recent discovery that rho-dependent termination in E. coli requires nusG protein in vivo, we have tested the effect of purified nusG protein on rho-dependent termination in vitro. With the well-characterized trp t' terminator of E. coli, and no other proteins than E. coli RNA polymerase and rho factor, nusG causes a proximal shift in the terminated RNA endpoints, compared to the endpoints generated by rho alone. The presence of nusG also enhances rho-mediated termination on partially defective mutant trp t' templates. We rule out explanations such as a change in the kinetic coupling between rho and RNA polymerase or a nusG-mediated increase in the affinity of rho for RNA. We also detect no difference in the helicase rate of rho in the presence of nusG. Even assays with completely stalled and isolated ternary complexes indicate that rho is able to effect the release of RNA with the assistance of nusG at points preceding the most proximal release sites observed in the absence of nusG. Our observations support a model in which nusG acts as a component of the transcription complex, possibly interacting with both rho and RNA polymerase as it governs accessibility to the nascent transcript.

On the performance and accuracy of 2D navigator pulses.

The purpose of this study was to investigate and to optimize the performance of two-dimensional spatially selective excitation pulses used for navigator applications on a clinical scanner. The influence of gradient imperfections, off-resonance effects, and incomplete k-space covering on the pencil beam-shaped spatial excitation profile of the 2D RF pulse was studied. The studies involved experiments performed on phantoms and in vivo. In addition, simulations were carried out by numerical integration of the Bloch equations. The accuracy of positioning of the pencil beam was increased by a factor of three by employing a simple correction scheme for the compensation of gradient distortions. The spatial selectivity of the 2D RF pulse was improved by taking sampling density corrections into account. The 2D RF pulse performance was found to be sufficient to monitor the diaphragm motion even at moderate gradient strength. For applications, where a high spatial resolution is required or a less characteristic contrast is present a strong gradient system is recommended.

Intestinal Ca2+ wave dynamics in freely moving C. elegans coordinate execution of a rhythmic motor program.

Defecation in the nematode worm Caenorhabditis elegans is a highly rhythmic behavior that is regulated by a Ca(2+) wave generated in the 20 epithelial cells of the intestine, in part through activation of the inositol 1,4,5-trisphosphate receptor. Execution of the defecation motor program (DMP) can be modified by external cues such as nutrient availability or mechanical stimulation. To address the likelihood that environmental regulation of the DMP requires integrating distinct cellular and organismal processes, we have developed a method for studying coordinate Ca(2+) oscillations and defecation behavior in intact, freely behaving animals. We tested this technique by examining how mutations in genes known to alter Ca(2+) handling [including egl-8/phospholipase C (PLC)-beta, kqt-3/KCNQ1, sca-1/sarco(endo)plasmic reticulum Ca(2+) ATPase, and unc-43/Ca(2+)-CaMKII] contribute to shaping the Ca(2+) wave and asked how Ca(2+) wave dynamics in the mutant backgrounds altered execution of the DMP. Notably, we find that Ca(2+) waves in the absence of PLCbeta initiate ectopically, often traveling in reverse, and fail to trigger a complete DMP. These results suggest that the normal supremacy of the posterior intestinal cells is not obligatory for Ca(2+) wave occurrence but instead helps to coordinate the DMP. Furthermore, we present evidence suggesting that an underlying pacemaker appears to oscillate at a faster frequency than the defecation cycle and that arrhythmia may result from uncoupling the pacemaker from the DMP rather than from disrupting the pacemaker itself. We also show that chronic elevations in Ca(2+) have limited influence on the defecation period but instead alter the interval between successive steps of the DMP. Finally, our results demonstrate that it is possible to assess Ca(2+) dynamics and muscular contractions in a completely unrestrained model organism.

Analysis of residual coronary artery motion for breath hold and navigator approaches using real-time coronary MRI.

Coronary artery MRI methods utilize breath holds, or diaphragmatic navigators, to compensate for respiratory motion. To increase image quality and navigator (NAV) gating efficiency, slice tracking is used, with more sophisticated affine motion models recently introduced. This study assesses the extent of remaining coronary artery motion in free breathing NAV and single and multi breath hold coronary artery MRI. Additionally, the effect of the NAV gating window size was examined. To visualize and measure the respiratory induced motion, an image containing a coronary artery cross section was acquired at each heartbeat. The amount of residual coronary artery displacement was used as a direct measure for the performance of the respiratory motion correction method. Free breathing studies with motion compensation (slice tracking with 5 mm gating window) had a similar amount of residual motion (0.76+/-0.17 mm) as a single breath hold (0.52+/-0.20 mm) and were superior to multiple breath holds (1.22+/-0.60 mm). Affine NAV methods allowed for larger gating windows ( approximately 10 mm windows) with similar residual motion (0.74+/-0.17 mm). In this healthy adult cohort (N=10), free-breathing NAV methods offered respiratory motion suppression similar to a single breath hold.

Free breathing 3D balanced FFE coronary magnetic resonance angiography with prolonged cardiac acquisition windows and intra-RR motion correction.

A shortcoming of today's coronary magnetic resonance angiography (MRA) is its low total scan efficiency (<5%), as only small well-defined fractions of the respiratory (50%) and cardiac (10%) cycle are used for data acquisition. These precautions are necessary to prevent blurring and artifacts related to respiratory and cardiac motion. Hence, scan times range from 4 to 9 min, which may not be tolerated by patients. To overcome this drawback, an ECG-triggered, navigator-gated free breathing radial 3D balanced FFE sequence with intra-RR motion correction is investigated in this study. Scan efficiency is increased by using a long cardiac acquisition window during the RR interval. This allows the acquisition of a number of independent k-space segments during each cardiac cycle. The intersegment motion is corrected using a self-guided epicardial fat tracking procedure in a postprocessing step. Finally, combining the motion-corrected segments forms a high-resolution image. Experiments on healthy volunteers are presented to show the basic feasibility of this approach.

Efficient foldover suppression using SENSE.

Parallel imaging techniques, which in principle represent procedures of unfolding a reduced dataset, are well known and well established in MR imaging. This paper presents a further application of one particular reconstruction method, the SENSE algorithm, considered from a different point of view to remove potential foldover in conventional images acquired with multiple receive coils. Based on the coil sensitivity information, a body coverage map in the excited plane is calculated. This is used together with the measured raw data in a SENSE-type reconstruction to optimize the signal-to-noise ratio (SNR) as well as to remove foldover reliably by unfolding the image to a larger field of view. The reconstruction is performed automatically, without any user interaction, and does not affect data acquisition. Based on phantom and in vivo studies, which retain high image quality after the removal, the potential and limits of this approach are discussed, also taking into account future scanner hardware that will support a large number of parallel receiver channels.

Free-breathing whole-heart coronary MRA with 3D radial SSFP and self-navigated image reconstruction.

Respiratory motion is a major source of artifacts in cardiac magnetic resonance imaging (MRI). Free-breathing techniques with pencil-beam navigators efficiently suppress respiratory motion and minimize the need for patient cooperation. However, the correlation between the measured navigator position and the actual position of the heart may be adversely affected by hysteretic effects, navigator position, and temporal delays between the navigators and the image acquisition. In addition, irregular breathing patterns during navigator-gated scanning may result in low scan efficiency and prolonged scan time. The purpose of this study was to develop and implement a self-navigated, free-breathing, whole-heart 3D coronary MRI technique that would overcome these shortcomings and improve the ease-of-use of coronary MRI. A signal synchronous with respiration was extracted directly from the echoes acquired for imaging, and the motion information was used for retrospective, rigid-body, through-plane motion correction. The images obtained from the self-navigated reconstruction were compared with the results from conventional, prospective, pencil-beam navigator tracking. Image quality was improved in phantom studies using self-navigation, while equivalent results were obtained with both techniques in preliminary in vivo studies.

MR coronary artery imaging with 3D motion adapted gating (MAG) in comparison to a standard prospective navigator technique.

Magnetic resonance coronary angiography (MRCA) has been proven to be feasible for imaging of the proximal and medial portions of the three main coronary arteries. Free breathing techniques allow for high resolution imaging but prolong scan time. This could potentially be shortened by improving the efficiency, robustness and accuracy of the navigator gating algorithm. Aim of this study was to determine the feasibility, efficiency, and image quality of a new motion compensation algorithm (3D-MAG) for coronary artery imaging with navigator techniques. In 21 patients the coronaries were imaged in plane with a 3D k-space segmented gradient echo sequence. A T2 preparation prepulse was used for suppression of myocardial signal, during free breathing and a navigator technique with using real time slice following and a gating window of 5 mm was applied to suppress breathing motion artefacts. Imaging was performed with standard gating and compared to 3D-MAG. Image quality was visually compared, contrast-to-noise and signal-to-noise ratio were calculated, the length of visualized coronary arteries was measured and scan duration and scan efficiency were calculated. Standard navigator imaging was feasible in 19 of 21 (90.5%) patients 3D-MAG in 21/21 (100%). Scan efficiency and duration was significantly improved with 3D-MAG (p < .05) without change in image quality. 3D-MAG is superior to conventional navigator correction algorithms. It improves feasibility and scan efficiency without reduction of image quality. This approach should be routinely used for MR coronary artery imaging with navigator techniques.

3D motion adapted gating (3D MAG): a new navigator technique for accelerated acquisition of free breathing navigator gated 3D coronary MR-angiography.

This study aimed to evaluate the influence of a new navigator technique (3D MAG) on navigator efficiency, total acquisition time, image quality and diagnostic accuracy. Fifty-six patients with suspected coronary artery disease underwent free breathing navigator gated coronary MRA (Intera, Philips Medical Systems, 1.5 T, spatial resolution 0.9x0.9x3 mm3) with and without 3D MAG. Evaluation of both sequences included: 1) navigator scan efficiency, 2) total acquisition time, 3) assessment of image quality and 4) detection of stenoses >50%. Average navigator efficiencies of the LCA and RCA were 43+/-12% and 42+/-12% with and 36+/-16% and 35+/-16% without 3D MAG (P<0.01). Scan time was reduced from 12 min 7 s without to 8 min 55 s with 3D MAG for the LCA and from 12 min 19 s to 9 min 7 s with 3D MAG for the RCA (P<0.01). The average scores of image quality of the coronary MRAs with and without 3D MAG were 3.5+/-0.79 and 3.46+/-0.84 (P>0.05). There was no significant difference in the sensitivity and specificity in the detection of coronary artery stenoses between coronary MRAs with and without 3D MAG (P>0.05). 3D MAG provides accelerated acquisition of navigator gated coronary MRA by about 19% while maintaining image quality and diagnostic accuracy.

Biosynthesis of a low-molecular-mass rat submandibular gland mucin glycoprotein in COS7 cells.

We have examined the biosynthesis of a low-molecular-mass mucin from rat submandibular gland (RSMG) expressed recombinantly in COS7 tissue culture cells, focusing primarily on the addition of carbohydrate to the protein core of the mucin. We find evidence for N-linked glycosylation, but this modification is not required for secretion of the mucin. Similarly, although the recombinant RSMG mucin, like its native counterpart, contains large amounts of O-linked carbohydrate, chain extension beyond the initial O-linked GalNAc moiety is not required for secretion. We have identified partially glycosylated mucin by a combination of metabolic pulse-chase and lectin precipitations of the biosynthetic intermediates. Our results suggest that the addition of GalNAc to threonine and serine in the RSMG mucin does not occur simultaneously, as has been described for other O-glycosylated proteins.

Improved 3D spiral imaging for coronary MR angiography.

Thin-slab 3D spiral imaging has been used for MR angiography to image selected coronary arteries. Improved scan efficiency was achieved using a train of multiple spiral interleaves within each single R-R interval acquired in the late diastole. Data acquisition was performed during free breathing, using navigator gating. Additionally, prospective slice tracking was applied to further reduce the sensitivity to motion. The application of a T(2)-preparation pulse and fat suppression increased the contrast between blood and myocardium. Experiments performed on healthy volunteers are presented to show the feasibility of this approach, which allows coronary artery imaging of selected vessels within a few minutes. Magn Reson Med 45:172-175, 2001.