Short Repetition Research Articles

Rapid fat‐suppressed isotropic steady‐state free precession imaging using true 3D multiple‐half‐echo projection reconstruction

Three-dimensional projection reconstruction (3D PR)-based techniques are advantageous for steady-state free precession (SSFP) imaging for several reasons, including the capability to achieve short repetition times (TRs). In this paper, a multi-half-echo technique is presented that dramatically improves the data-sampling efficiency of 3D PR sequences while it retains this short-TR capability. The k-space trajectory deviations are measured quickly and corrected on a per-sample point basis. A two-pass RF cycling technique is then applied to the dual-half-echo implementation to generate fat/water-separated images. The resultant improvement in the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) was demonstrated in volunteer studies. Volumetric images with excellent spatial resolution, coverage, and contrast were obtained with high speed. The non-contrast-enhanced SSFP studies show that this technique has promising potential for MR angiography (MRA).

Variable-rate selective excitation for rapid MRI sequences.

Balanced steady-state free precession (SSFP) imaging sequences require short repetition times (TRs) to avoid off-resonance artifacts. The use of slab-selective excitations is common, as this can improve imaging speed by limiting the field of view (FOV). However, the necessarily short-duration excitations have poor slab profiles. This results in unusable slices at the slab edge due to significant flip-angle variations or aliasing in the slab direction. Variable-rate selective excitation (VERSE) is a technique by which a time-varying gradient waveform is combined with a modified RF waveform to provide the same excitation profile with different RF power and duration characteristics. With the use of VERSE, it is possible to design short-duration pulses with dramatically improved slab profiles. These pulses achieve high flip angles with only minor off-resonance sensitivity, while meeting SAR limits at 1.5 T. The improved slab profiles will enable more rapid 3D imaging of limited volumes, with more consistent image contrast across the excited slab.

「無症候性脳梗塞をどう扱うか」ＭＲＩ診断の立場から―撮像法の正しい選択と解釈

Basic MRI techniques for diagnosis of silent brain infarcts are discussed. Advantages and disadvantages of various basic MR techniques including T1WI, T2WI, FLAIR and PDWI are also evaluated. The use of FLAIR and PDWI is especially suited to differentiate ischemic lesions from CSF-containing spaces such as ventricular system and cerebral sulci. However, surprisingly little is known about the disadvantages of FLAIR. Precise differentiation between complete and incomplete infarct are sometimes difficult on FLAIR especially at the short repetition time. The use of PDWI is suited to this occasion. In conclusion, most useful basic MR imagings for evaluation of silent brain infarcts and white matter lesions are T1WI, T2WI and PDWI. Silent lacunar hemorrhages can be well appreciated on T2 -weighted sequences like gradient-echo.

On the calculation and interpretation of signal intensity in echo-shifted sequences.

Echo-shifted sequences have been shown to be useful in applications where strong T*2-weighting and short repetition times are wanted, such as BOLD-contrast fMRI, MR thermometry, and perfusion studies. However, a full understanding of signal formation with such methods, which is mandatory to optimize sequence parameters for particular applications, has still not been achieved. Here, two methods are proposed to calculate the steady-state signal intensity in coherent TR-periodic and TR-shifted gradient-echo sequences. The integration method, which consists of averaging the steady-state magnetization over all isochromats in a voxel, is shown to be a particularly efficient way of obtaining the analytical expression of the measurable signal. The partition method, based on a physical decomposition of the steady-state magnetization into a sum of contributions from past excitation pulses, reveals that the net transverse magnetization results from a destructive interference between the wanted component and a series of stimulated echoes. The analysis includes off-resonance effects and is illustrated by phantom measurements. Relationships with previous publications on this subject are discussed.

Magnetization recovery for signal enhancement: a fast imaging DEFT-based technique

This paper describes the development and application of a new fast MRI technique based on the DEFT principle. The sequence named MAgnetization RecoverY for Signal Enhancement (MARYSE) is composed of two completely symmetric gradient echoes separated by a 180° refocusing pulse. The RF pulse scheme, 90° x –180° y –90° − x enables restoration of the transverse magnetization along the longitudinal axis, and consequently artificially increases R 1 relaxation rate. In this sequence, the period between the excitation pulse and the restoring pulse (Tem: transverse magnetization evolution time) is very short (<10 ms). This makes possible a significant increase in signal-to-noise ratio, even with a relatively short repetition time (20 ms). Simulations were performed for different values of Tem and TR at definite T 1 and T 2 and for different values of T 1 and T 2 at constant Tem and TR. Relevant signal enhancement for species with long relaxation time constants as compared to classical gradient echo and fast spin-echo imaging was expected. In vitro studies on a fat/water phantom confirmed this simulation. Application of MARYSE to mouse brain imaging permitted to visualize almost completely cerebrospinal fluid of the ventricles, a signal usually partially saturated in fast gradient echo imaging.

Approaching an ecologically valid functional anatomy of spontaneous “willed” action

We used functional magnetic resonance imaging of healthy subjects to investigate the neural basis for spontaneous “willed” action. We hypothesised that such action involves prefrontal cortex (PFC) and supplementary motor area (SMA), in addition to primary motor cortex. Furthermore, we predicted that PFC and SMA would demonstrate similar temporal response dynamics, distinct from primary motor cortex. Specifically, we predicted earlier activation in PFC and SMA, manifest as shorter response latencies compared with primary motor cortex. Six right-handed males participated in an event-related design and were required to generate spontaneous motor acts inside the scanner. By deciding “which” of two buttons to press, and “when” to press them, subjects generated sequences of action that were of high information content (“novelty” or “randomness”). Utilising a short repetition time (1 s), we acquired functional images that covered most of the frontal and parietal cortices. The onset of action was associated with significant activation in bilateral PFC, left primary motor cortex, and, close to the midline, SMA. Following action, mean time to half-maximum blood oxygenation level-dependent response was significantly earlier in left PFC and SMA than primary motor cortex. Our findings suggest that neural correlates of spontaneous willed action are distributed in executive and motor centres, and that temporal response dynamics differentiate “higher” regions from subordinate motor areas.

Task switches under Go/NoGo conditions and the decomposition of switch costs

Alternating switches between two simple S-R tasks are combined with Go/NoGo tasks. Non-switches after Go trials are assumed to selectively profit from stimulus driven repetition benefits, whereas switches after NoGo trials are assumed to be selectively delayed by stimulus driven negative priming. Intentionally driven reconfiguration costs are assessed by RT differences between switches after Go trials (no negative priming) and non-switches after NoGo trials (no repetition benefits). Experiment 1 indicates that with short preparation time repetition benefits, negative priming costs, and intentional components contribute approximately additively to switch costs. Experiment 2 confirms that the delay of switches after NoGo trials is indeed due to negative priming. Experiments 3 and 4 show that repetition benefits and intentional components of switch costs are properly assessed only if the settings assure that participants reconfigure the required task set in NoGo as well as Go trials.

Biological significance of a human enterovirus B-specific RNA element in the 3' nontranslated region.

The secondary structures predicted for the enteroviral 3' nontranslated region (3'NTR) all seem to indicate a conformation consisting of two (X and Y) hairpin structures. The higher-order RNA structure of the 3'NTR appears to exist as an intramolecular kissing interaction between the loops of these two hairpin structures. The enterovirus B-like subgroup possesses an additional stem-loop structure, domain Z, which is not present in the poliovirus-like enteroviruses. It has been suggested that the Z domain originated from a burst of short sequence repetitions (E. V. Pilipenko, S. V. Maslova, A. N. Sinyakov, and V. I. Agol, Nucleic Acids Res. 20:1739-1745, 1992). However, no functional features have yet been ascribed to this enterovirus B-like-specific RNA element in the 3'NTR. In this study, we tested the functional characteristics and biological significance of domain Z. A mutant of the cardiovirulent coxsackievirus group B3 strain Nancy which completely lacked the Z domain and which therefore acquired enterovirus C-like secondary structures exhibited a wild-type growth phenotype, as determined by single-cycle growth analysis with BGM cells. This result proves that the Z domain is virtually dispensable for viral growth in tissue cultures. Partial distortion of the Z domain structure resulted in a disabled virus with reduced growth kinetics, probably due to alternative conformations of the overall structure of the domain. Infection of mice showed that the recombinant coxsackievirus group B3 mutant which completely lacked the Z domain was less virulent. Pancreatic tissues from mice infected with wild-type virus and recombinant virus were equally affected. However, the heart tissue from mice infected with the recombinant virus showed only slight signs of myocarditis. These results suggest that the enterovirus B-like-specific Z domain plays a role in coxsackievirus-induced pathogenesis.

High resolution general purpose D-layer experiment for EISCAT incoherent scatter radars using selected set of random codes

Abstract. The ionospheric D-layer is a narrow bandwidth radar target often with a very small scattering cross section. The target autocorrelation function can be obtained by transmitting a series of relatively short coded pulses and computing the correlation between data obtained from different pulses. The spatial resolution should be as high as possible and the spatial side lobes of the codes used should be as small as possible. However, due to the short pulse repetition period (in the order of milliseconds) at any instant, the radar receives detectable scattered signals not only from the pulse illuminating the D-region but also from 3–5 ambiguous-range pulses, which makes it difficult to produce a reliable estimate near zero lag of the autocorrelation function. A new experimental solution to this measurement problem, using a selected set of 40-bit random codes with 4 µs elements giving 600 m spatial resolution is presented. The zero lag is approximated by dividing the pulse into two 20-bit codes and computing the correlation between those two pulses. The lowest altitudes of the E-layer are measured by dividing the pulse into 5 pieces of 8 bits, which allows for computation of 4 lags. In addition, coherent integration of data from four pulses is used for obtaining separately the autocorrelation function estimate for the lowest altitudes and in cases when the target contains structures with a long coherence time. Design details and responses of the experiment are given, and analysed test data are shown.Key words. Radio science (signal processing); Ionosphere (plasma temperature and density; instruments and techniques)

C 13 nuclear Overhauser polarization nuclear magnetic resonance in rotating solids: Replacement of cross polarization in uniformly C13 labeled molecules with methyl groups

A new C13 polarization technique in solids is presented on the basis of a recently proposed C13–C13 recoupling sequence [13C–1H dipolar-assisted rotational resonance (DARR), K. Takegoshi, S. Nakamura, and T. Terao, Chem. Phys. Lett. 344, 631 (2001)] operative under fast magic angle spinning (MAS), in which a rf field is applied to H1H with a rotary resonance condition but none to C13. The H1 irradiation in DARR saturates H1 signals, leading to the C13 signal enhancement due to the nuclear Overhauser effect for fast rotating methyl groups, if any. If we use a uniformly C13 labeled sample, C13–C13 polarization transfer enhanced by DARR successively distributes the enhanced methyl carbon polarization to the other C13 spins, leading to uniform enhancement for all C13 spins even under very fast MAS. In uniformly C13 labeled rotating samples, the enhancement factor in cross polarization (CP) is about 2.4, while in the present nuclear Overhauser polarization (NOP), it is 3.0 in the fast rotation limit of the methyl groups. While the CP enhancement becomes smaller for molecules with short T1ρ of H1 or C13, NOP would work well for such mobile molecules, and also NOP enables us to acquire a signal with a short repetition time even if H1 T1 is long. Further, NOP has the advantage of quantitativeness, and is very easy to carry out, being insensitive to the adjustment of rf field intensity and requiring only very low rf power. These features are demonstrated for uniformly C13, N15-labeled L-threonine and uniformly C13, N15-labeled glycylisoleucine. NOP-MAS is also applied for a naturally abundant C13 sample.

RF inhomogeneity compensation in structural brain imaging.

Three-dimensional T(1)-weighted magnetization-prepared rapid gradient-echo (MP-RAGE) sequences with centric phase encoding (PE) in the inner loop provide structural brain images with a high spatial resolution and high tissue contrast. A disadvantage of this sequence type is the susceptibility to inhomogeneities of the radiofrequency (RF) coil, which may result in poor image contrast in some peripheral regions. A special excitation pulse is presented which compensates for these effects in both the head/foot and anterior/posterior directions. This pulse has a duration of only 1.3 ms and is thus compatible with the short repetition times (TRs) required for MP-RAGE imaging. It is shown experimentally that images acquired with the compensation pulse may be segmented without using intensity correction algorithms during data postprocessing.

Effect of Respiratory CO2 Changes on the Temporal Dynamics of the Hemodynamic Response in Functional MR Imaging

Increasing end-expiratory CO2 levels (PETCO2) increases the dispersion and the time of maximum of the hemodynamic response curve in human primary visual cortex. This was demonstrated using event-related multislice functional magnetic resonance imaging (fMRI) with short repetition time and 3-s flicker light stimulation. Measurements were performed at 5 different PETCO2 levels between 20 and 60 mmHg using hyperventilation or by adding CO2 to the inspired air. Between 30 and 60 mmHg the full-width-at-half-maximum of the hemodynamic response curve induced by visual stimulation increased nearly linearly at 130 ms per mmHg PETCO2. Consistent with previous studies a concomitant decrease of the signal amplitude was observed at PETCO2 values below 40 mmHg and above 50 mmHg. The relevance of these findings for the temporal resolution of fMRI and especially of event-related methods is discussed.

In Vivo Short Echo Time 1H-Magnetic Resonance Spectroscopic Imaging (MRSI) of the Temporal Lobes

Two different methodologies for obtaining PRESS-localized magnetic resonance spectroscopic imaging (MRSI) data from the mesial and lateral temporal lobes were investigated. The study used short echo times (30 ms) and long repetition times (3000 ms) to minimize relaxation effects. Inhomogeneity and spectral distortions from the proximity of the temporal bones precluded the attainment of consistently good-quality data from both temporal lobes at once. Even when the right and left temporal lobes were studied separately, distortions often disturbed spectra from the anterior lateral temporal lobe. Quantitative analysis using LCModel was therefore performed only on the posterior lateral temporal lobe, and the posterior, middle, and anterior mesial temporal lobe. No significant left-right differences in metabolite content were found in a series of 10 controls. Significantly higher concentrations of myoinositol and choline were found in the anterior mesial temporal lobe, even when grey matter content was included as a covariate. The concentration of N-acetyl aspartate plus N-acetyl aspartyl glutamate (NAc) was not found to vary significantly along the length of the hippocampus. The previously observed lower anterior ratios of NAA to creatine plus choline (NAA/(Cr + Cho) may instead have been due to higher anterior choline. Large differences in metabolite concentrations were seen between posterior lateral temporal lobe (predominantly subcortical white matter) and the posterior mesial temporal lobe, most notably lower creatine, glutamate/glutamine, and myo-inositol, and higher NAA/(Cr + Cho) in the lateral than mesial temporal lobe. This pattern was similar to that previously seen for grey/white matter differences in the frontal, parietal and occipital regions.

Pulsed electric field breakage of cellular tissues: visualisation of percolative properties

Results of numerical and experimental investigation of the electric breakage of a cellular material in pulsed electric fields (PEF) are presented. The numerical model simulates the conductive properties of a cellular material by a two-dimensional array of biological cells. The application of an external field in the form of an idealised square pulse sequence with a pulse duration t i , and a pulse repetition time Δ t is assumed. The simulation model includes the known mechanisms of temporal and spatial evolution of the conductive properties of different microstructural elements in a tissue. The kinetics of breakage at different values of electric field strength E, t i and Δ t was studied in the experimental investigation. A 5×55-mm cylindrical slab of apple was taken as a sample. The results of the experimental and numerical studies were compared. A hypothesis for the nature of tissue properties evolution after PEF treatment was proposed. This phenomenon was considered as a correlated percolation governed by two key processes: resealing of cells and moisture transfer processes inside the cellular structure. The breakage kinetics was shown to be very sensitive to the repetition times Δ t of the PEF treatment. Correlated percolation patterns were observed in a case when Δ t exceeded the characteristic time of the moisture transfer processes while random percolation patterns were observed in other cases. The long-term mode of the pulse repetition times in PEF treatment allows the experimental visualisation of macroscopic percolation channels in the sample. Considerable differences were observed between the damage kinetics at long and short repetition times both for experimental and simulation data.

Magnetization preparation during the steady state: fat-saturated 3D TrueFISP.

A novel fat saturation scheme is proposed which combines spectral fat saturation with a steady-state 3D true Fast Imaging with Steady Precesson (TrueFISP) sequence. Fat saturation consisted of a conventional frequency-selective excitation pulse surrounded by spoiler gradients. This saturation block was periodically repeated within a continuously running TrueFISP sequence. Except for the fat signals, the steady-state signal formation and the resulting image contrast of TrueFISP was not modified by the periodically inserted fat saturation block. This was achieved by a alpha/2 flip-back pulse before the fat saturation block, which stores the established steady-state transverse magnetization as pure longitudinal magnetization. After fat saturation this longitudinal magnetization was excited by a alpha/2 preparation pulse to continue the TrueFISP acquisition. The resulting images show contrast identical to conventional TrueFISP images, but without the usually very bright fat signals. The short repetition time allows acquisition of a 3D data set of the abdomen within a single breath-hold.

Intravascular MR imaging-guided balloon angioplasty with an MR imaging guide wire: feasibility study in rabbits.

To develop a technique for intravascular magnetic resonance (MR)-guided balloon angioplasty with use of an MR imaging guide wire. An MR imaging guide wire (0.6-mm loopless antenna) that could be placed within a balloon catheter was manufactured. The guide wire was expected to function as either an MR receiver probe in real-time MR imaging or a guide wire for use with interventional devices. Laparotomy was performed in eight rabbits, and a dilatable stenosis was created at the upper abdominal aorta. Balloon angioplasty, validated at pre- and postoperative MR aortography with renal contrast enhancement was performed by using a 1.5-T MR unit with a fast spoiled gradient-echo pulse sequence, short repetition and echo times, and a rate of three frames per second. During MR tracking, the entire length of the MR imaging guide wire was always visible as a band of high signal intensity. In all cases, the MR imaging guide wires were passed through the aortic stenoses dilated by means of balloon inflation. Before balloon angioplasty, flow in the aorta distal to the stenosis was decreased, which caused mild contrast enhancement in each kidney. After balloon angioplasty, distal flow was restored, resulting in substantial renal enhancement. The MR imaging guide wire is a potential tool for use in endovascular interventional MR imaging.

On the cause of increased aliasing in the slice‐select direction in 3D contrast‐enhanced magnetic resonance angiography

The combination of short repetition times and large flip angles typically used in 3D contrast-enhanced magnetic resonance angiography (3D CE MRA) can significantly alter the expected shape of the slab profile for unenhanced tissues, which can cause increased aliasing in the slice select direction. In this work, this increased slice select aliasing is demonstrated and explained from both theoretical and experimental points of view. The effect is due to the Ernst angle of unenhanced background tissue occurring on the falling edges of the flip angle profile that has been set for the significantly reduced T1 of contrast-enhanced blood. The deleterious aliasing effects are magnified substantially when the chosen volume is placed close to surface coil reception with the slice select direction perpendicular to the coil axis. Magn Reson Med 44:336–338, 2000. © 2000 Wiley-Liss, Inc.

Contrast-enhanced MRI of the lung

The lung has long been neglected by MR imaging. This is due to unique intrinsic difficulties: (1) signal loss due to cardiac pulsation and respiration; (2) susceptibility artifacts caused by multiple air-tissue interfaces; (3) low proton density. There are many MR strategies to overcome these problems. They consist of breath-hold imaging, respiratory and cardiac gating procedures, use of short repetition and echo times, increase of the relaxivity of existing spins by administration of intravenous contrast agents, and enrichment of spin density by hyperpolarized noble gases or oxygen. Improvements in scanner performance and frequent use of contrast media have increased the interest in MR imaging and MR angiography of the lung. They can be used on a routine basis for the following indications: characterization of pulmonary nodules, staging of bronchogenic carcinoma, in particular assessment of chest wall invasion; evaluation of inflammatory activity in interstitial lung disease; acute pulmonary embolism, chronic thromboembolic pulmonary hypertension, vascular involvement in malignant disease; vascular abnormalities. Future perspectives include perfusion imaging using extracellular or intravascular (blood pool) contrast agents and ventilation imaging using inhalation of hyperpolarized noble gases, of paramagnetic oxygen or of aerosolized contrast agents. These techniques represent new approaches to functional lung imaging. The combination of visualization of morphology and functional assessment of ventilation and perfusion is unequalled by any other technique.

Three-dimensional cardiac cine magnetic resonance imaging with an ultrasmall superparamagnetic iron oxide blood pool agent (NC100150)

The purpose of this study was to assess image quality of three-dimensional (3D) cardiac cine magnetic resonance (MR) imaging before and after administration of a T1-shortening ultrasmall superparamagnetic iron oxide blood pool agent (NC100150). 3D cardiac cine MR imaging was performed in 13 volunteers using a radiofrequency-spoiled cardiac-gated 3D cine gradient-echo sequence with short repetition and echo times. Compared with precontrast images, postcontrast images showed no enhancement in fat and skeletal muscle, moderate enhancement in myocardium, and significant enhancement in ventricular cavity. After contrast injection, the signal ratio of the ventricular chamber to the myocardium significantly increased, and dramatic improvements were seen in the quality of the cineangiographic images and the depiction of cardiac valves. This quantitative study has shown that 3D cardiac cine MR imaging using a blood pool agent provided MR ventriculography and cineangiography with excellent image quality.

Generation of multiple pulses with extremely short pulse repetition interval

A new type of multipulse generator is proposed for the generation of high power pulses with an extremely short interpulse repetition interval. In this system, an air-core step-up transformer is used with a magnetic switch and a pulse forming line, which is charged in the double-resonance mode. Numerical simulations of this system have shown that 600 kV, 56 ns pulses, matched to a 20 /spl Omega/ impedance can be produced with a minimum pulse separation of 2.7 /spl mu/s. A small system was constructed to demonstrate the production of double pulses experimentally. The system contains two first stage capacitors of 150 nF each, a 1:15 air-core step-up transformer, a magnetic switch using cobalt based material, a second stage capacitor of 1.6 nF, and an 80 /spl Omega/ load. Double-pulses 100 ns wide, peak voltages of 85 kV (first pulse) and 90 kV (second pulse) have been successfully generated with an interval of 3.75 /spl mu/s between pulses, when the first stage capacitors were charged to 14 kV (for the first pulse) and 17 kV (for the second pulse).