Separate Radio Frequency Research Articles

Recent MRI advances in experimental stroke.

Recent MRI advances in experimental stroke.

Tandem Ion Trap Design with Enhanced Mass Analysis Capabilities for Large Populations of Ions

A new arrangement consisting of two separate radio frequency (rf) quadrupole ion traps is used to analyze large populations of ions over a wide mass-to-charge (m/z) range. The setup consists of an "accumulation" trap that is maintained at a higher pressure than the second high-performance "analyzer" trap. The two traps are scanned simultaneously, with a mass difference between that determines the residence time and mass range of ions in the analytical trap. Initially, all ions are trapped in the accumulation trap and then mass-selectively ejected into the analyzer trap. As ions arrive in the analyzer trap, they cool through collisions with the buffer gas and then are mass selectively ejected toward the detector. This concurrent linked mass scanning reduces the total number of ions present in the analyzer trap during mass analysis, thereby reducing space charge effects and leading to improved resolution and mass accuracy of analytical spectra.

Sensing Optimization for an Receiver Structure in Cognitive Radio Systems

This paper describes the optimization of spectrum sensing in terms of the throughput of a cognitive radio (CR) system. Dealing with the optimization problem of spectrum sensing, this paper evaluates the throughput of a CR system by considering such situations as the penalty time of a channel search and incumbent user (IU) detection delay caused by a missed detection of an incumbent signal. Also, this paper suggests a serial channel search scheme as the search method for a vacant channel, and derives its mean channel search time by considering the penalty time due to the false alarm of a vacant channel search. The numerical results suggest the optimum sensing time of the channel search process using the derived mean channel search time of a serial channel search in the case of a sensing hardware structure with single radio frequency (RF) path. It also demonstrates that the average throughput is improved by two separate RF paths in spite of the hardware complexity of an RF receiver.

Radio Frequency Noise From the Modulator of a Linac

A novel approach to image-guided radiotherapy being undertaken by a few groups involves the integration of a linac with a magnetic resonance imager (MRI). In order to successfully combine a linac with a MRI, it is important to understand the characteristics and major sources of radio frequency (RF) noise from the pulse power modulator of a linac since these may interfere with the operation of the MRI. The RF noise power spectral density from the modulator of a linac, loaded separately with a magnetron and a resistive load, is measured. The RF fields emitted by the pulse-forming network (PFN) were determined by simulation and compared with measurements. Saturable reactors were introduced in the trigger circuits of the thyratron to reduce the injected voltage spikes into the trigger circuit to assess the impact of these spikes on the RF noise measurements. The results illustrate that the major source of RF noise from the modulator of a linac is the operation of a magnetron. It also eliminates the PFN coil and the grid voltage spikes of thyratron as possible major sources of RF noise. For linac-MRI systems the modulator of a linac should be housed in a separate RF cage from the MRI.

Trap design for insertable second‐nuclei radiofrequency coils for magnetic resonance imaging and spectroscopy

AbstractThe application of nuclear magnetic resonance spectroscopy in metabolic studies usually includes concurrent operation at two different frequencies to enable excitation and detection of two different nuclei. When using separate radiofrequency coils for the multinuclear excitation, coupling between the two coils has been shown to be a problem, resulting in resonance frequency shifting and reduced sensitivity of the 1H coil. Among the methods to reduce this coupling is the placement of an 1H trap on the second‐nuclei coil. This article presents the benefits of this method, specifically in this case for 31P imaging/spectroscopy, through modeling results, bench measurements, imaging, and spectra acquisition. The use of a resonant trap in conjunction with a modular design permitted the insertion of the phosphorus coil as an “add‐on” into multiple existing proton coils, suggesting particular applicability in the high‐field arena, where spectroscopy holds great promise, but where proton coil design complexity exists that is not easily re‐engineered for dual‐nuclei use. © 2009 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 35B: 121–132, 2009

Development of an Electrophysiology (EP)-Enabled Intracardiac Ultrasound Catheter Integrated With NavX 3-Dimensional Electrofield Mapping for Guiding Cardiac EP Interventions

We have developed an integrated high-resolution intracardiac echocardiography (ICE) catheter for electrophysiology (EP) testing, which can be coregistered in 3-dimensional space with EP testing and ablation catheters using electrofield sensing. Twelve open-chest pigs (34-55 kg) and 3 closed-chest pigs were studied. After introduction from the jugular or femoral venous locations, the 9F side-looking, highly steerable (0 degrees -180 degrees), 64-element array catheters could be manipulated easily throughout the right side of the heart. Multisite cardiac pacing was performed for assessing left ventricular (LV) synchrony using tissue Doppler methods. Also, in the open-chest pigs, right atrial (RA) and right ventricular (RV) ablations were performed with a separate radio frequency catheter under fluoroscopic guidance and visualized with ICE to characterize the changes. In the 3 closed-chest pigs, electrofield NavX 3-dimensional coregistration (St Jude Medical Corp, Minneapolis, MN) allowed us to test whether this additional feature could shorten the time necessary to perform 4 targeted ablations in each animal while imaging the ablation catheter and the adjacent region by ICE. Intracardiac anatomy, tricuspid, aortic, pulmonary, and mitral valve function, and pulmonary vein flow were all imaged reproducibly from scanning locations in the RA or RV in all animals, along with assessment of cardiac motion and the effects of multisite pacing. Three-dimensional electrofield displays detailed the spatial relationship between the ICE catheter and ablation catheters such that the time to visualize and ablate 4 sites in each of the 3 closed-chest animals was reduced. This new technology is a first step in the integration of ICE with EP procedures.

Measurement of cerebral perfusion territories using arterial spin labelling

The ability to assess the perfusion territories of major cerebral arteries can be a valuable asset to the diagnosis of a number of cerebrovascular diseases. Recently, several arterial spin labeling (ASL) techniques have been proposed for determining the cerebral perfusion territories of individual arteries by three different approaches: (1) using a dedicated labeling radio frequency (RF) coil; (2) applying selective inversion of spatially confined areas; (3) employing multidimensional RF pulses. Methods that use a separate labeling RF coil have high signal-to-noise ratio (SNR), low RF power deposition, and unrestricted three-dimensional coverage, but are mostly limited to separation of the left and right circulation, and do require extra hardware, which may limit their implementation in clinical systems. Alternatively, methods that utilize selective inversion have higher flexibility of implementation and higher arterial selectivity, but suffer from imaging artifacts resulting from interference between the labeling slab and the volume of interest. The goal of this review is to provide the reader with a critical survey of the different ASL approaches proposed to date for determining cerebral perfusion territories, by discussing the relative advantages and disadvantages of each technique, so as to serve as a guide for future refinement of this promising methodology.

Multiple‐region gradient arrays for extended field of view, increased performance, and reduced nerve stimulation in magnetic resonance imaging

This article presents a novel design for magnetic resonance imaging (MRI) gradient systems. This design may allow the development of MRI scanners that are capable of imaging large regions with high performance while minimizing the potential for nerve stimulation. The general concept of the gradient system is that spatial oscillation is incorporated such that each gradient coil creates multiple, approximately linear gradient regions that oscillate in gradient polarity. Separate radiofrequency (RF) coil arrays are designed to be sensitive to the signals within each linear region and thus allow signal measurements to be obtained separately from each region. Enabling image acquisition in the transition region that separates each pair of adjacent linear regions requires a second gradient system with imaging regions that overlap and coincide with the transition regions of the first gradient system. Imaging the extended field of view (FOV) is accomplished by interleaved operation of the two gradient systems. Simulated annealing is used to create designs for both longitudinal and transverse gradient systems with two imaging regions.

Fluctuation in a helicon plasma with additional immersed antenna

Fluctuations in the floating potential are measured in a cylindrical magnetised plasma, as the phase between a primary Helicon antenna and a secondary immersed antenna is varied. The two antennas are powered by separate radio frequency (RF) generators of 13.56 MHz controlled by a single oscillator and phase shifter. The Helicon antenna is separated from the plasma by a glass tube, while the immersed copper antenna is in direct contact with the plasma. The image presented here shows floating potential fluctuations of /spl sim/100 Hz where the amplitude of the fluctuations vary dramatically with the phase between the two RF generators.

Remotely detected high-field MRI of porous samples

Remote detection of NMR is a novel technique in which an NMR-active sensor surveys an environment of interest and retains memory of that environment to be recovered at a later time in a different location. The NMR or MRI information about the sensor nucleus is encoded and stored as spin polarization at the first location and subsequently moved to a different physical location for optimized detection. A dedicated probe incorporating two separate radio frequency (RF)—circuits was built for this purpose. The encoding solenoid coil was large enough to fit around the bulky sample matrix, while the smaller detection solenoid coil had not only a higher quality factor, but also an enhanced filling factor since the coil volume comprised purely the sensor nuclei. We obtained two-dimensional (2D) void space images of two model porous samples with resolution less than 1.4 mm 2. The remotely reconstructed images demonstrate the ability to determine fine structure with image quality superior to their directly detected counterparts and show the great potential of NMR remote detection for imaging applications that suffer from low sensitivity due to low concentrations and filling factor.

Validation of a radio frequency identification system for monitoring the feeding patterns of feedlot cattle

The feed bunk attendance patterns of six crossbred beef heifers (averaging 481±22 kg) and six crossbred beef steers (averaging 471±44 kg) being fed an 80% steam-rolled barley grain:15% barley silage and 5% supplement diet were monitored for 52 days using a radio frequency (RF) system (GrowSafe ® Systems Ltd., Airdrie, AB). The system was validated by three methods: (1) comparing the duration and frequency of feeding visits recorded by the RF system against those calculated from video surveillance data; (2) comparing the bunk attendance data provided by two separate RF transponders carried by a single animal; and (3) documenting the relationship between bunk attendance and actual feeding time. When one considered the ability of the RF system to monitor the presence or absence of an animal, total daily error in the RF system was 6% as compared to the videotaped data. Incidence of errors due to RF interference and false antennae readings were 2.4 and 3.6%, respectively. The RF system was concluded to be a highly effective tool for documenting bunk attendance. Feeding durations documented by animals bearing two transponders were highly correlated, averaging 88.4±1.5 and 84.5±1.5 min day −1 ( R 2=0.96). Frequency of bunk visits was also highly correlated ( R 2=0.96). Approximately 84% of total bunk attendance time was spent consuming feed; 16% was spent on non-feeding activity. On average, the cattle made 29±11.8 visits day −1, with 55.8% of visits associated with feeding activity and 44.2% with non-feeding activity. These observations suggest that attendance duration may be a better indicator of an animal’s feed intake than frequency of attendance at the bunk. Future studies with GrowSafe ® or other RF systems that do not automatically record feed intake should incorporate a correction factor to relate attendance to feeding time. Factors were identified that could introduce errors with the GrowSafe ® system. These include non-grounded, metal material (connected in a loop configuration), particularly when associated with the feed bunk area.

5530351 NMR tomography apparatus with combined radio frequency antenna and gradient coil

In a combined radiofrequency antenna and gradient coil in a magnetic resonance imaging apparatus, radiofrequency signals are coupled into conductor loops of the gradient coil so that these loops are operated as elements of a radiofrequency antenna. The installation space for a separate radiofrequency antenna can thus be saved. Given the same patient-receiving volume, the diameter of the cylindrical opening of a magnet or the spacing between pole shoes can be reduced, so that the magnets can be more economically built.

Etching 0.35 μm polysilicon gates on a high-density helicon etcher

This article describes an evaluation of a helicon high-density plasma etch tool for etching 0.35 μm polysilicon gates. The objective of the evaluation was to establish whether it is possible to etch 0.35 μm lines of both doped and undoped polysilicon (simultaneously) while maintaining straight profiles across the wafer and a very high polysilicon-to-oxide selectivity. Several high-density plasma (HDP) polysilicon etch tools for 0.35 μm technology have been tested at Texas Instruments. It is known that HDP tools operating at low pressures can obtain high polysilicon etch rates and anisotropic etch profiles. Most of the HDP sources utilize two separate radio-frequency (rf) power supplies; one to generate the high-density plasma and a second to bias the chuck. This allows relatively independent control of the ion density and the ion energy. It is possible to attain high polysilicon-to-oxide selectivity in the overetch step by using a high flux (high rf power to the plasma) of low energy (low rf power to the chuck) ions. This high selectivity is particularly important when long overetches are required with thin gate oxides.

17O-decoupled 1H detection using a double-tuned coil

17O-decoupled proton MR spectroscopy and imaging with a double-tuned radiofrequency (RF) coil at 2 T was used to detect and quantify H 2 17O in tissue phantoms containing various concentrations of 17O-enriched water in 5% gelatin. The pulse sequence used in these experiments consisted of a conventional proton spinecho sequence with RF irradiation at the 17O resonance frequency applied between the proton 90° pulse and the signal acquisition window. The double-tuned coil provided several advantages over systems using separate RF coils for 17O decoupling and proton excitation/detection, including ensuring that the same (or similar) sample volumes are excited and decoupled and permitting accurate calibration of the 17O decoupling pulse amplitude. The efficiency of 17O decoupling as a function of decoupling RF amplitude, decoupling duration, and decoupling resonance offset was investigated. Finally, the specific absorption rate of the 17O decoupled pulse sequence was investigated and found to lie within federal guidelines at 1.5 T.

Collisional dephasing of the Na13d−13ftransition using the Ramsey method of separated rf fields

The collisional dephasing of the Na 13d-13f transition by Ne and CO has been investigated using the Ramsey method of separated radio-frequency fields. Upper limits for the dephasing cross sections are determined which are far smaller than the geometric size of the excited Na atoms. This leads to the conclusion that the separation of the potential cruves of the Na 13d-X and 13f-X states is nearly constant as a function of internuclear separation.