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Biomedical subjects

Y Zur

Publications and source records attributed to Y Zur.

15 recordsLinked to original sources
PubMed2000

Design of improved spectral-spatial pulses for routine clinical use.

Spectral-spatial pulses (spsp pulses) selectively excite spins at spatial location z and spectral frequency (due to chemical shift and/or field inhomogeneity) v. In this work we discuss the design of improved spsp pulses for fat signal suppression. Optimal pulses are designed as optimal constant ripple FIR filters using the inverse SLR transform. Spsp pulses with thin slices are obtained by modifying the phases between subpulses, thereby eliminating unwanted magnetization lobes. Robust spsp pulses at off-center slices are obtained with a prescan calibration. These pulses are used either for selective fat saturation or for selective water excitation. It is shown that spsp pulses suppress fat signal better than conventional fat saturation pulses. Using the techniques presented in this article, we replaced all the fat saturation pulses on our systems with spsp pulses and obtained a significant improvement in image quality.

Adipose Tissue↗
PubMed1997

A new diffusion SSFP imaging technique.

In this paper a new diffusion sensitive steady-state free precession (SSFP) pulse sequence with a reduced sensitivity to physiological brain motion is presented. The signal attenuation due to diffusion in this SSFP sequence is derived theoretically and confirmed experimentally with a phantom. It is shown that for brain tissue this signal attenuation is approximately independent of T1 and T2, but depends only on the pulse sequence used, i.e., the timing and the size of the RF and the gradient pulses. On this basis the diffusion constant can be calculated for any region in the image. Diffusion sensitive images of the brain obtained with our pulse sequence are presented and shown to be superior over an image obtained with a "conventional" diffusion sensitive SSFP sequence.

Brain↗
PubMed1997

Design of adiabatic pulses for fat-suppression using analytic solutions of the Bloch equation.

Discrimination between signals produced by fat and by water is an important issue in MRI. One efficient approach is to perform fat-suppression by selective inversion. This technique exploits the transition region of a selective RF pulse to invert the longitudinal lipid magnetization while leaving the magnetization of the water protons untouched. The damaging effects of RF field inhomogeneity may be overcome by using pulses based on the adiabatic fast passage principle (AFP). In particular, the well-known sech/tanh adiabatic pulse is a robust and efficient pulse that is obtained as an analytic solution of the Bloch equation. In this paper, a wider class of analytic solutions of the Bloch equation is presented of which the sech/tanh driving function is merely a particular case. The new pulse exhibits an asymmetric distribution of magnetization with one transition sharper than the other. The sharper transition can be used to perform the required selective discrimination between signals. The resulting pulse features excellent adiabatic behavior. Moreover, the transition width of the new pulse can be reduced by a factor of about 2/3 with respect to an equal-duration sech/tanh pulse. The performance of the new pulse is compared with a similar sech/tanh pulse with the aid of a practical design example.

Adipose Tissue↗
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↗
PubMed1996

An algorithm for eddy currents symmetrization and compensation. off.

Eddy currents, which are induced in the magnet cryostat by pulsed magnetic field gradients in MRI, generate undesired eddy fields within the imaging volume. In this work, an automated and computerized algorithm to compensate these eddy currents is presented. The compensation is done in two steps: (i) Eddy fields are symmetrized electronically with an R-C filter. (ii) The symmetric eddy fields are compensated by another R-C filter. The compensation algorithm is iterative; therefore, errors that remain from one iteration are eliminated in the next iteration. Hence, the compensation process is very robust and accurate. It is shown that all the even harmonics of the eddy fields are eliminated by the symmetrization process, but the odd field harmonics remain. The amplitude of these odd harmonics can be significantly reduced if the gradient coils are designed so that the field they generate is spatially similar to the eddy fields.

Algorithms↗
PubMed1996

A new adiabatic inversion pulse.

Adiabatic pulses play an important role in magnetization inversion in the presence of RF field inhomogeneity. In this work the authors present an efficient adiabatic inversion pulse that is able to selectively invert magnetization over a large frequency bandwidth in a short time. The pulse is constructed in two steps: (i) the optimal trajectory is determined and (ii) the optimal rate of motion along that trajectory is determined. The resulting pulse enables separately controlling and trading off the pulse duration against the transition width. The superiority of this pulse over the well known sech/tanh adiabatic pulse is demonstrated in a scenario where a large bandwidth should be inverted at a short time using limited B1 amplitude.

Algorithms↗
PubMed1996

Design of adiabatic selective pulses using optimal control theory.

Optimal control theory has been applied in the past for the design of RF pulses for selective excitation. This was the outcome of having established the controllability of the MR spin system for the selective excitation problem. "Minimum distance" was the main formulation used for the solution. Because of their robust behavior in the presence of inhomogeneous RF fields, adiabatic pulses play an important role in spin inversion and excitation. In this study, we present a method for incorporating adiabaticity into the optimal control problem by enhancing the cost functional with an appropriate term. Two different types of adiabatic terms are proposed. Furthermore, two methods are used to solve the optimal control problem, namely the Hamiltonian approach and the solution by mathematical programming. Design examples include both a frequency selective pulse for performing fat suppression by inversion and a regular inversion pulse. It is shown that, in the course of optimization, the pulse designer can trade-off slice resolution against pulse adiabaticity.

Humans↗
PubMed1991

MR angiography without subtraction.

A new NMR method for producing angiograms with strong suppression of the static spin signal and without the need for subtraction is described and demonstrated. A velocity-selective pulse sequence was implemented whereby the magnetization of all stationary spins is driven to the -z axis and is not detected, while maximizing the signal intensity of the moving spins. A theory of the method is presented and gives good agreement with experimental results obtained on a flow phantom. It is shown theoretically and experimentally that high-quality velocity-independent angiograms of the head and neck can be obtained with strong suppression of static spin signal when TR approximately T1. The method can be extended to produce three-dimensional angiograms.

Angiography↗
PubMed1991

Multiecho, spin-echo sequence to eliminate unwanted echoes.

Two types of artifacts--a mirror-reversed image about the phase-encode direction and a wave-like intensity variation across the image--may appear when using a conventional multiecho spin-echo sequence. We describe and demonstrate a new method, which eliminates such artifacts, using only one excitation per phase-encoding step and without the need for spoiler gradients.

Electromagnetic Phenomena↗
PubMed1991

Spoiling of transverse magnetization in steady-state sequences.

A detailed analysis is presented of a method to eliminate transverse magnetization prior to each rf excitation in pulse sequences with TR less than T2. It is shown that artifact-free images with high T1 contrast can be obtained only if a phase shift that is incremented during each TR interval is applied to the transverse magnetization. Computer simulations are used to show that when this phase increment is 117 degrees, the steady-state transverse magnetization prior to each rf pulse is nulled over a wide range of T1, T2, and rf tip angles, resulting in optimal T1 contrast. Such nulling of steady-state transverse magnetization cannot be obtained by using large gradient pulses, or gradients of random or linearly incremented amplitude. Images of phantoms and human subjects confirm the theoretical predictions.

Artifacts↗
PubMed1990

Motion-insensitive, steady-state free precession imaging.

Steady-state free precession (SSFP) pulse sequences employing gradient reversal echoes and short repetition time (TR) between successive rf excitation pulses offer high signal-to-noise ratio per unit time. However, SSFP sequences are very sensitive to motion. A new SSFP method is presented which avoids the image artifacts and loss of signal intensity due to motion. The pulse sequence is designed so that the time integral of each of the three gradients is zero over each TR time interval. The signal then consists of numerous echoes which are superimposed. These echoes are isolated by combining the data from N different scans. In each scan a specific phase shift is added during every TR interval. Each of these N isolated echoes produces a motion-insensitive, artifact-free image. Because all the echoes are sampled simultaneously, the signal-to-noise ratio per unit time in this SSFP method is higher than in existing SSFP techniques which sample only one echo at a time. The new method was implemented and used to produce both two- and three-dimensional images of the head and cervical spin of a human patient. In these images the high signal intensity of cerebrospinal fluid is preserved regardless of its motion. Further work is required to evaluate the imaging parameters (TR, TE, rf tip angle) so as to give optimal tissue contrast for the various echoes.

Humans↗
PubMed1988

An analysis of fast imaging sequences with steady-state transverse magnetization refocusing.

Recently, several groups have proposed and demonstrated the use of rapid imaging methods, using short pulse repetition times and gradient-reversal echoes. Here, we analyze the behavior of the magnetization and the resulting image contrasts in such sequences for the case where the pulse repetition time TR is of the order of, or shorter than, the transverse relaxation time T2, and the transverse magnetization is not destroyed between phase-encoding cycles. Exact analytical expressions describing the signal evolution between the pulses are derived, taking into account the effects of resonance offsets and flip angles, and examining the influence of constant-phase or alternate-phase RF pulse trains. It is shown that for typical imaging sequences two distinct echo signals will develop between pulses, which may have a detrimental effect on image quality if they partially overlap within the sampling window. It is shown that artifact-free images can be obtained only if the two echo signals overlap precisely, which seems technically close to impossible to achieve, or if they are sufficiently separated in time to allow sampling of only one of the signals.

Image Enhancement↗
PubMedDate not supplied

The effects of simultaneous pulsing in different gradient coils on the nuclear magnetic resonance imaging of oblique slices.

The definition and mapping of oblique planes by magnetic resonance imaging (MRI) requires the simultaneous application of two or three orthogonal gradients to define the desired intermediate direction of the frequency encoding or "readout" gradient. Each of the three main gradient coils produces different patterns of eddy currents. Consequently, the application of dephasing and rephasing lobes of these gradients will produce echoes at slightly different times for each gradient. If two or three gradients are applied simultaneously to create an arbitrary view direction, the resulting echo will therefore be shifted in time and considerably reduced in intensity. In this article, we present an analysis of the behavior of the magnetization in a typical two-dimensional Fourier transform pulse sequence for the imaging of oblique slices. The theoretical displacements in time and reduction in intensity of the echo amplitudes are calculated and compared to the experimental behavior. We show that, in spite of this phenomenon, the final image suffers only marginally in signal-to-noise ratio, provided the slice width is small compared to the field of view. This is due to the fact that there always exists a cycle in the sequence in which the phase-encoding gradient almost completely compensates for the above described effect.

Humans↗
PubMedDate not supplied

Gradient moment nulling for steady-state free precession MR imaging of cerebrospinal fluid.

Steady-state free precession (SSFP) pulse sequences can produce magnetic resonance (MR) images rapidly, in which cerebrospinal fluid (CSF) is several times more intense than the other tissues. However, motion in the presence of magnetic field gradients reduces the intensity of CSF drastically, unless the time integral of the gradient waveform between each radio-frequency (rf) pulse vanishes. The consequences of motion on SSFP are explored here in detail theoretically and experimentally. The principle of gradient moment nulling is applied with the objective of giving CSF in SSFP images uniformly high intensity everywhere, in spite of motion. Theoretical analysis of the phase of the transverse magnetization from a group of isochromats, with a trajectory described by a Taylor series, reveals how motion along each direction disrupts SSFP and also causes ghost artifacts. Images of CSF in the cervical spine are found to have less extensive flow voids and weaker ghosts from pulsation if the first moment calculated from the rf pulse to the center of the gradient echo vanishes for both the frequency encoding and slice selection gradient waveforms. However, first-order moment nulling of the phase encoding gradient waveform is unnecessary for SSFP imaging of CSF.

Cerebrospinal Fluid↗