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PubMed2000

Optimal control of molecular motion: design, implementation, and inversion.

This paper reviews recent theoretical and experimental developments aimed at controlling molecular motion using tailored laser fields. Emphasis is given to seeking optimal designs for the laser controls and optimal implementation of the controls in the laboratory. Optimization on both counts provides a rigorous, flexible, and physically attractive means for obtaining the best possible control over molecular motion under any specified conditions. The theoretical design and laboratory implementation of control are best effected by a closed-loop process that draws on observations of the evolving molecular sample to steer it toward the desired target. Going beyond control, similar closed-loop laboratory learning concepts may lead to automated molecular monitors for inversion to systematically identify details of molecular Hamiltonians.

Chemistry, Physical↗
PubMed2006

An inverse method to design RF coil arrays optimized for SENSE imaging.

A new method to design MRI RF coils that are optimized for SENSE (sensitivity encoding) imaging is introduced. In this approach, the inverse problem was solved where the surface current density distribution on a coil former was calculated to maximize the SNR(sense) within a volume of interest (VOI). For that purpose, an analytic relationship was formulated between the SNR(sense) and surface current density on the coil former. The SNR at pixel rho in a SENSE-MR image, SNR(sense,rho), is inversely proportional to the g-factor: therefore, the g-factor was formulated in terms of the B1 distribution of the coils. Then, by specifying the geometry of the desired coil former and using a finite element mesh (FEM), the surface current distribution was calculated to maximize the SNR(sense), by minimizing (1/SNR(sense)) in the VOI using a least squares procedure. A simple two-coil array was designed and built to test the method and phantom images were collected. The results show that the new coil design method yielded better uniformity and SNR in SENSE images compared to those of standard coils.

Algorithms↗
PubMed2002

A time-harmonic inverse methodology for the design of RF coils in MRI.

An inverse methodology is described to assist in the design of radio-frequency (RF) coils for magnetic resonance imaging (MRI) applications. The time-harmonic electromagnetic Green's functions are used to calculate current on the coil and shield cylinders that will generate a specified internal magnetic field. Stream function techniques and the method of moments are then used to implement this theoretical current density into an RF coil. A novel asymmetric coil operating for a 4.5 T MRI machine was designed and constructed using this methodology and the results are presented.

Electromagnetic Fields↗
PubMed2004

Design of an inversion center between two helical segments.

A new strategy is proposed to control the relative orientation of two folded helical oligomers in such a way that they diverge from an aromatic linker and have opposite helical handedness. Mutual steric exclusion between the two helices results from the fact that they cannot be at the same time folded and on the same side of the linker. The concept is validated using the helical conformations of oligoamides of 8-amino-2-quinolinecarboxylic acid, but it should be applicable to many families of oligomers and leads to the first designed meso-helices.

Biomimetic Materials↗
PubMed2002

[Design of an inverse planning system for radiotherapy using linear optimization].

Our approach shows that inverse planning for intensity-modulated beams in radiotherapy can be solved efficiently by the mathematical method of linear optimization. The completeness property of this method guarantees that calculated treatment plans fulfill the dose constraints given by the oncologist. Techniques developed by our group can also avoid the possible infeasibility caused by a physically impossible dose distribution. In contrast to other optimization methods, the simplex algorithm used for linear optimization allows a very high optimization speed, i.e., very short planning cycles. These preconditions were integrated in the development of the software system MIPART ("Munich Inverse Planning And Radiotherapy Treatment"). The object-oriented software architecture of MIPART achieves a maximum of extensibility and flexibility. Our clinical tests show that MIPART can be easily integrated in the clinical routine in spite of the complexity of data in the field of intensity-modulated radiotherapy. Moreover, qualitatively better treatment plans can be generated than in conventional treatment planning, especially in complicated cases.

Humans↗
PubMed1993

RF pulse design using the inverse scattering transform.

The inverse scattering transform (IST) is a mathematical transformation that can be used to derive RF pulses from functions called continuous spectra describing the final state of the spin system. This paper reviews three seemingly unrelated numerical algorithms that have appeared in the literature, and shows that they are all derivable from the IST. When the continuous spectra are rational, the finite rank kernel method is used to convert the IST to a matrix equation that is easily solved. Another algorithm, equivalent to the so-called "layer stripping" algorithm used in seismology, is derived by assuming that the spectra are Fourier series. Finally, the Shinnar-Le Roux (SLR) algorithm is derived by assuming that the spectra are ratios of Fourier series. With proper interconversion between the rational, series, and ratio of series forms of the continuous spectra, these algorithms generate RF pulses with identical or nearly identical shapes and performance properties, and can be regarded as equivalent.

Algorithms↗
PubMed2002

Inverse-problem approach to designing photonic crystals for cavity QED experiments.

Photonic band gap (PBG) materials are attractive for cavity QED experiments because they provide extremely small mode volumes and are monolithic, integratable structures. As such, PBG cavities are a promising alternative to Fabry-Perot resonators. However, the cavity requirements imposed by QED experiments, such as the need for high Q (low cavity damping) and small mode volumes, present significant design challenges for photonic band gap materials. Here, we pose the PBG design problem as a mathematical inversion and provide an analytical solution for a two-dimensional (2D) crystal. We then address a planar (2D crystal with finite thickness) structure using numerical techniques.

Journal Article↗
PubMed2006

Cloning of full-length cDNA of teleost corticotropin-releasing hormone precursor by improved inverse PCR.

We designed a new inverse PCR protocol combined with switching mechanism at 5' end of RNA transcript (SMART) technology, and applied it to the cloning of teleost corticotropin-releasing hormone precursor cDNA. Due to the advantages of both techniques, this method can efficiently amplify the complete 5'- and 3'-ends of cDNA in a single reaction, and might prove to be an alternative to the conventional rapid amplification of cDNA ends (RACE) approaches.

Amino Acid Sequence↗
PubMed2000

A hybrid, inverse approach to the design of magnetic resonance imaging magnets.

This paper describes a hybrid numerical method of an inverse approach to the design of compact magnetic resonance imaging magnets. The problem is formulated as a field synthesis and the desired current density on the surface of a cylinder is first calculated by solving a Fredholm equation of the first kind. Nonlinear optimization methods are then invoked to fit practical magnet coils to the desired current density. The field calculations are performed using a semi-analytical method. The emphasis of this work is on the optimal design of short MRI magnets. Details of the hybrid numerical model are presented, and the model is used to investigate compact, symmetric MRI magnets as well as asymmetric magnets. The results highlight that the method can be used to obtain a compact MRI magnet structure and a very homogeneous magnetic field over the central imaging volume in clinical systems of approximately 1 m in length, significantly shorter than current designs. Viable asymmetric magnet designs, in which the edge of the homogeneous region is very close to one end of the magnet system are also presented. Unshielded designs are the focus of this work. This method is flexible and may be applied to magnets of other geometries.

Algorithms↗
PubMed2006

Design and modeling of inversion layer ultrasonic transducers using LiNbO3 single crystal.

Using inversion domain engineering controlled by heating temperature, the LiNbO(3) (LNO) piezoelectric plate with both odd and even-order thickness-extensional modes can be excited simultaneously. Therefore, the inversion layer ultrasound transducer is expected to be capable of operating over a wider frequency range. In this paper, the electrical impedance and the acoustic characteristics of LiNbO(3) (LNO) inversion layer transducer have been studied by finite element modeling (FEM). The transducer designed for this study uses a 36 degrees rotated Y-cut LiNbO(3) thin plate with an active element thickness of approximately 100 microm. First the electrical and elastic properties of the 36 degrees rotated Y-cut LNO were obtained by transforming a basic piezoelectric matrix for Z-cut LNO. In order to validate the FEM using the transformed properties several pieces of pure and 50% inversion layer LNO were tested on the electrical impedance analyzer. The modeled impedance characteristics were consistent with the measured data. Next the model was used to design 50-60 MHz transducers using pure and 30% inversion LNO. Two lambda/4 matching layers and a Tungsten loaded epoxy backing were used in these designs. The modeled results show that an over 90% bandwidth transducer can be made with proper matching and 30% inversion layer.

Computer Simulation↗
PubMed1999

A hybrid inverse approach applied to the design of lumped-element RF coils.

A combination of inverse procedures is employed in the design of radio-frequency (RF) coils with specific examples in, but not restricted to, magnetic resonance imaging. The first inverse procedure is the use of functional methods for the optimization of coil characteristics subject to restrictions on the field behavior. Continuous current distributions are derived from analysis of the fields they are required to produce. To make use of these distributions at a desired frequency, the method of moments is applied as a second inverse procedure to a discretized version of the current distribution. The advantage of this hybrid technique is that it provides a computational algorithm for optimization of feeding, tuning, impedance matching and other aspects of RF coil design. A prototype RF coil has been built using the engineering values predicted by the theory. Experimental results including images acquired from the prototype coil are presented.

Algorithms↗
PubMed1992

Inverse protein folding problem: designing polymer sequences.

We consider the question of how to design proteins. How can we find "good" amino acid sequences (i) that fold to a desired "target" structure as a native conformation of lowest accessible free energy and (ii) that will not simultaneously fold to many other conformations of the same free energy? Current protein designs often focus on helix propensities and turns. We focus here on designing the hydrophobicity. For a model of self-avoiding hydrophobic/polar chains on two-dimensional square lattices, geometric proofs and exhaustive enumerations show the following results. (i) The strategy hydrophobic residues inside/polar residues outside is not optimal. Placement of additional hydrophobic residues on the surface is often necessary. (ii) To avoid unwanted conformations, the designed sequence must have neither too many nor too few hydrophobic residues. (iii) The computational complexity of inverse folding appears to be in a different class than folding: unlike the folding problem, the design problem does not scale exponentially with chain length. Some design strategies, described here for the lattice model, produce good sequences and scale only linearly with chain length.

Drug Design↗
PubMed2004

Exploring the limits of broadband excitation and inversion pulses.

The design of broadband excitation and inversion pulses with compensation of B(1)-field inhomogeneity is a long standing goal in high resolution NMR spectroscopy. Most optimization procedures used so far have been restricted to particular pulse families to keep the scale of the problem within manageable limits. This restriction is unnecessary using efficient numerical algorithms based on optimal control theory. A systematic study of rf-limited broadband excitation by optimized pulses and broadband inversion by optimized pulses with respect to bandwidth and B(1)-field is presented. Upper limits on minimum pulse lengths are set for different degrees of pulse performance.

Algorithms↗
PubMed1997

Design of selective adiabatic inversion pulses using the adiabatic condition

Adiabatic RF pulses play an important role in spin inversion due to their robust behavior in the presence of inhomogeneous RF fields. These pulses are characterized by the trajectory swept by the tip of the Beff vector and the rate of motion along it. In this paper, we describe a method by which optimized modulation functions can be constructed to render insensitivity to B1 inhomogeneity over a predetermined B1 range and over a wide band of frequencies. This is accomplished by requiring that the optimized pulse fulfill the adiabatic condition over this range of B1 inhomogeneity and over the desired frequency band for the complete duration of the pulse. A trajectory similar to the well-known sech/tanh adiabatic pulse, i.e., a half-ellipse, is used. The optimization process improves the slice profile by optimizing the rate of motion along this trajectory. The optimized pulse can be tailored to the specific design requirements; in particular, the transition sharpness may be traded off against the inverted bandwidth. Two design examples, including experimental results, demonstrate the superiority of the optimized pulses over the conventional sech/tanh pulse: in the first example, a large frequency band is to be inverted using a weak RF amplitude in a short time. In the second example, a pulse with a very sharp transition is required. Copyright 1997 Academic Press. Copyright 1997Academic Press

Journal Article↗
PubMed1987

The design of a multiple inversion recovery sequence for T1 measurement.

Inversion recovery has the potential to be a powerful method of determining the values of T1 in in vivo studies. However, because of its relative slowness it is not practical to undertake several experiments with different values of the interval between the magnetization inverting 180 degree pulse and the interrogating 90 degree one. The multiple inversion recovery method described here uses a series of sampling pulses, most much less than 90 degrees, to produce a series of images. It is shown that slice shape is relatively reproducible from one sample to another and that, largely as a result, the accuracy of the sequence in measuring T1 is encouraging.

Brain↗
PubMed2006

Designed interaction potentials via inverse methods for self-assembly.

We formulate statistical-mechanical inverse methods in order to determine optimized interparticle interactions that spontaneously produce target many-particle configurations. Motivated by advances that give experimentalists greater and greater control over colloidal interaction potentials, we propose and discuss two computational algorithms that search for optimal potentials for self-assembly of a given target configuration. The first optimizes the potential near the ground state and the second near the melting point. We begin by applying these techniques to assembling open structures in two dimensions (square and honeycomb lattices) using only circularly symmetric pair interaction potentials; we demonstrate that the algorithms do indeed cause self-assembly of the target lattice. Our approach is distinguished from previous work in that we consider (i) lattice sums, (ii) mechanical stability (phonon spectra), and (iii) annealed Monte Carlo simulations. We also devise circularly symmetric potentials that yield chainlike structures as well as systems of clusters.

Journal Article↗
PubMed2006

Simplifying minimally invasive transhiatal esophagectomy with the inversion approach: Lessons learned from the first 20 cases.

HYPOTHESIS: The laparoscopic transhiatal esophagectomy can be simplified and performed safely and effectively by using a novel esophageal inversion technique. DESIGN: Case series describing technique, initial experience, and learning curve with laparoscopic inversion esophagectomy. SETTING: Tertiary care university hospital and veteran's hospital. PATIENTS: Twenty consecutive patients with high-grade dysplasia (n = 16) and esophageal adenocarcinoma (n = 4). INTERVENTION: Laparoscopic inversion esophagectomy, a totally laparoscopic approach to transhiatal esophagectomy that incorporates distal to proximal inversion to improve mediastinal exposure and ease of dissection. MAIN OUTCOME MEASURES: Perioperative end points and complications, compared between the first and second groups of 10 patients. RESULTS: There were 19 men and 1 woman. Median operative time was 448 minutes. Median blood loss was 175 cm3. Median intensive care unit stay was 4 days, and median total hospital stay was 9 days. Overall anastomotic leak rate was 20%. Five patients developed an anastomotic stricture, all successfully managed with endoscopic dilation. There were 2 recurrent laryngeal nerve injuries, which resolved. There was no intraoperative or 30-day mortality. Between the first 10 consecutive cases and last 10 procedures, the incidence of anastomotic leak and stricture formation decreased from 30% to 10% and 40% to 10%, respectively. During this period, the number of lymph nodes harvested increased 9-fold, and duration of intensive care unit stay decreased from 8.00 to 2.50 days. CONCLUSIONS: Laparoscopic inversion esophagectomy is a safe procedure. The learning curve for the inversion approach is approximately 10 operations in the hands of esophageal surgeons with advanced laparoscopic expertise.

Adenocarcinoma↗
PubMed2001

Dose verification for patients undergoing IMRT.

At Emory Clinic intensity-modulated radiation therapy (IMRT) was started by using dynamic multileaf collimators (dMLC) as electronic tissue compensators in August 1998. Our IMRT program evolved with the inclusion of a commercially available inverse treatment planning system in September 1999. While the introduction of electronic tissue compensators into clinical use did not affect the customary radiation oncology practice, inverse treatment planning does alter our basic routines. Basic concepts of radiation therapy port designs for inverse treatment planning are different from conventional or 3D conformal treatments. With inverse treatment planning, clinicians are required to outline a gross tumor volume (GTV), a clinical target volume (CTV), critical normal structures, and to design a planning target volume (PTV). Clinicians do not designate the volume to be shielded. Because each IMRT radiation portal is composed of many beamlets with varying intensities, methods and practice used to verify delivered dose from IMRT portals are also different from conventional treatment portals. Often, the validity of measured data is in doubt. Therefore, checking treatment planning computer output with measurements are confusing and fruitless, at times. Commissioning an IMRT program and routine patient dose verification of IMRT require films and ionization chamber measurements in phantom. Additional specialized physics instrumentation is not required other than those available in a typical radiation oncology facility. At this time, we consider that routine quality assurance prior to patient treatments is necessary.

Humans↗