Abstract In NMR imaging experiments, due to the inherent imperfection of the slice profile, not a... more Abstract In NMR imaging experiments, due to the inherent imperfection of the slice profile, not all spins are rotated by the nominal pulse angles. We show how this leads to two artifacts in multiecho NMR images: superimposed mirror-reversed images and a wavelike intensity ...
Steady-state free precession (SSFP) pulse sequences can produce magnetic resonance (MR) images ra... more 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.
PurposeMagnetic resonance guided ultrasonic therapy is a promising minimally invasive technology ... more PurposeMagnetic resonance guided ultrasonic therapy is a promising minimally invasive technology for constantly growing variety of clinical applications. Delivery of focused ultrasound (FUS) energy to the targeted point with optimal intensity is highly desired; however, due to tissue aberrations, optimal focal intensity is not always achieved. Especially in transcranial applications, the acoustic waves are shifted and distorted mainly by the skull. In order to verify that magnetic resonance acoustic radiation force imaging (MR‐ARFI) can be used as a focusing tool in transcranial treatments, such an imaging was appliedin vivo on a porcine brain via ex vivo human skull. Then, by the use of MR‐ARFI technique, an improved ultrasound focusing algorithm is proposed and demonstrated for both transcranial and none brain applications.MethodsMR‐ARFI images were acquired on a GE 1.5 T scanner equipped with InSightec FUS systems ExAblate 2000 and ExAblate 4000. Imaging was performed with MR‐ARF...
Adiabatic RF pulses play an important role in spin inversion where G is the adiabatic parameter d... more Adiabatic RF pulses play an important role in spin inversion where G is the adiabatic parameter due to their robust behavior in presence of inhomogeneous RF fields. These pulses are characterized by the trajectory swept by the tip of the B eff vector and the rate of motion upon it. In this G(v 0 , t) Å v eff (v 0 , t) Éug (v 0 , t)É , [2] paper, a method is described for optimizing adiabatic inversion pulses to achieve a frequency-selective magnetization inversion over a given bandwidth in a shorter time and to improve slice and tan u Å Dv/v 1. Inversion is obtained when the effective profile. An efficient adiabatic pulse is used as an initial condition. field moves the longitudinal magnetization M z from the /z This pulse allows for flexibility in choosing its parameters; in parto the 0z axis over a wide band of Larmor frequencies. ticular, the transition sharpness may be traded off against the In the frame of reference of the slice center, i.e., for v 0 inverted bandwidth. The considerations for selecting the parame-Å v c , we may plot the route traced by the tip of the v eff ters of the pulse according to the requirements of the design are vector. This graph of v 1 (t) vs Dv(v c , t) is called the trajecdiscussed. The optimization process then improves the slice profile tory of the adiabatic pulse. An adiabatic pulse is characterby optimizing the rate of motion along the trajectory of the pulse while preserving the trajectory itself. The adiabatic behavior of ized by its trajectory and the rate of motion of v eff upon the optimized pulses is fully preserved over a twofold range of it. Three classic examples (expressed here as amplitude/ variation in the RF amplitude which is sufficient for imaging applifrequency modulation functions) include the sech/tanh (4), cations in commercial high-field MRI machines. Design examples sin/cos (5), and const/tan (3). Several methods have been demonstrate the superiority of the optimized pulses over the conproposed for the optimization of the modulation functions ventional sech/tanh pulse.
Adiabatic RF pulses play an important role in spin inversion v e (v 0 , t) Å £v 1 (t)xP / Dv(v 0 ... more Adiabatic RF pulses play an important role in spin inversion v e (v 0 , t) Å £v 1 (t)xP / Dv(v 0 , t)zP , [1] due to their robust behavior in the presence of inhomogeneous RF fields. These pulses are characterized by the trajectory swept by where Dv(v 0 , t) Å v(t) 0 v 0 is the resonance offset and the tip of the B eff vector and the rate of motion along it. In this v 0 the Larmor frequency of the spin we are inspecting. £ is paper, we describe a method by which optimized modulation functhe RF field inhomogeneity factor with a nominal value of tions can be constructed to render insensitivity to B 1 inhomogene-1. The adiabatic theorem (2) asserts that the magnetization ity over a predetermined B 1 range and over a wide band of frequenvector m remains spin-locked to v e provided that the rate cies. This is accomplished by requiring that the optimized pulse of precession of m about v e is much faster than the angular fulfill the adiabatic condition over this range of B 1 inhomogeneity and over the desired frequency band for the complete duration of velocity of the motion of v e. Mathematically this is exthe pulse. A trajectory similar to the well-known sech/tanh adiapressed by the adiabatic condition (3), batic pulse, i.e., a half-ellipse, is used. The optimization process improves the slice profile by optimizing the rate of motion along G(v 0 , t) ӷ 1, [2] this trajectory. The optimized pulse can be tailored to the specific design requirements; in particular, the transition sharpness may be where G is the adiabatic parameter, 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 G(v 0 , t) Å v e (v 0 , t) Éug (v 0 , t)É Å (£ 2 v 2 1 / Dv 2) 3/2 £ÉDvv h 1 0 v 1 Dv h É , [3] 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. ᭧ 1997 Academic Press and tan u Å Dv/v 1. Inversion is obtained when the effective field moves the longitudinal magnetization M z from the /z to the 0z axis over a wide band of Larmor frequencies.
To meet the challenge of providing adequate coverage at high field strength, a combination of two... more To meet the challenge of providing adequate coverage at high field strength, a combination of two complementary methods is proposed: reshaped waveforms with the VERSE technique and a modulated angle refocusing train. It was hypothesized that pulses can be reshaped and flip angles can be modulated such that power is cut in half and thus coverage doubled. Volunteer experiments at 3T using an SSFSE sequence in the abdomen demonstrate a power reduction of 50% with unchanged contrast and SNR and improved resolution.
Where 0 ≤ t ≤ TR, is off-resonance, is a phase generated by the electronics and is the phas... more Where 0 ≤ t ≤ TR, is off-resonance, is a phase generated by the electronics and is the phase accrual at t = TR. If TR < T2 magnetization from previous TR’s affects the magnetization M in the current TR. Figure 1 shows a GRE sequence with RF pulses P1 to PN with flip angle , and a phase between RF pulses. The initial magnetization immediately after P1 at t = 0 is M(1) and the magnetization immediately after Pn is M(n).
The NMR excitation of the double quantum (DQ) transition of a spin system with I=1 in a solid by ... more The NMR excitation of the double quantum (DQ) transition of a spin system with I=1 in a solid by double-frequency irradiation fields is studied. The efficiency of the DQ excitation by the two irradiation fields applied simultaneously at the two sides of the single quantum frequency spectrum of this system is calculated. The theory of Shirley for periodical time-dependent Hamiltonians is extended to describe these NMR experiments. The two-photon character of the DQ excitation is discussed and the two-photon resonance conditions are derived. The effective rf irradiation intensity on the DQ transition is calculated to be ω1ω2{(ωQ−ωT)−1 +(ωQ+ωT)−1}, where ω1 and ω2 are the intensities of the two rf irradiation fields, 2ωT the frequency distance between these rf fields and ωQ the quadrupole frequency of the spin system. This result is valid for all ωQ values as long as ω1, ω2< ‖ ωQ−ωT ‖. Accurate computer calculations are performed to examine the exact time evolution of the density ma...
Abstract In NMR imaging experiments, due to the inherent imperfection of the slice profile, not a... more Abstract In NMR imaging experiments, due to the inherent imperfection of the slice profile, not all spins are rotated by the nominal pulse angles. We show how this leads to two artifacts in multiecho NMR images: superimposed mirror-reversed images and a wavelike intensity ...
Steady-state free precession (SSFP) pulse sequences can produce magnetic resonance (MR) images ra... more 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.
PurposeMagnetic resonance guided ultrasonic therapy is a promising minimally invasive technology ... more PurposeMagnetic resonance guided ultrasonic therapy is a promising minimally invasive technology for constantly growing variety of clinical applications. Delivery of focused ultrasound (FUS) energy to the targeted point with optimal intensity is highly desired; however, due to tissue aberrations, optimal focal intensity is not always achieved. Especially in transcranial applications, the acoustic waves are shifted and distorted mainly by the skull. In order to verify that magnetic resonance acoustic radiation force imaging (MR‐ARFI) can be used as a focusing tool in transcranial treatments, such an imaging was appliedin vivo on a porcine brain via ex vivo human skull. Then, by the use of MR‐ARFI technique, an improved ultrasound focusing algorithm is proposed and demonstrated for both transcranial and none brain applications.MethodsMR‐ARFI images were acquired on a GE 1.5 T scanner equipped with InSightec FUS systems ExAblate 2000 and ExAblate 4000. Imaging was performed with MR‐ARF...
Adiabatic RF pulses play an important role in spin inversion where G is the adiabatic parameter d... more Adiabatic RF pulses play an important role in spin inversion where G is the adiabatic parameter due to their robust behavior in presence of inhomogeneous RF fields. These pulses are characterized by the trajectory swept by the tip of the B eff vector and the rate of motion upon it. In this G(v 0 , t) Å v eff (v 0 , t) Éug (v 0 , t)É , [2] paper, a method is described for optimizing adiabatic inversion pulses to achieve a frequency-selective magnetization inversion over a given bandwidth in a shorter time and to improve slice and tan u Å Dv/v 1. Inversion is obtained when the effective profile. An efficient adiabatic pulse is used as an initial condition. field moves the longitudinal magnetization M z from the /z This pulse allows for flexibility in choosing its parameters; in parto the 0z axis over a wide band of Larmor frequencies. ticular, the transition sharpness may be traded off against the In the frame of reference of the slice center, i.e., for v 0 inverted bandwidth. The considerations for selecting the parame-Å v c , we may plot the route traced by the tip of the v eff ters of the pulse according to the requirements of the design are vector. This graph of v 1 (t) vs Dv(v c , t) is called the trajecdiscussed. The optimization process then improves the slice profile tory of the adiabatic pulse. An adiabatic pulse is characterby optimizing the rate of motion along the trajectory of the pulse while preserving the trajectory itself. The adiabatic behavior of ized by its trajectory and the rate of motion of v eff upon the optimized pulses is fully preserved over a twofold range of it. Three classic examples (expressed here as amplitude/ variation in the RF amplitude which is sufficient for imaging applifrequency modulation functions) include the sech/tanh (4), cations in commercial high-field MRI machines. Design examples sin/cos (5), and const/tan (3). Several methods have been demonstrate the superiority of the optimized pulses over the conproposed for the optimization of the modulation functions ventional sech/tanh pulse.
Adiabatic RF pulses play an important role in spin inversion v e (v 0 , t) Å £v 1 (t)xP / Dv(v 0 ... more Adiabatic RF pulses play an important role in spin inversion v e (v 0 , t) Å £v 1 (t)xP / Dv(v 0 , t)zP , [1] due to their robust behavior in the presence of inhomogeneous RF fields. These pulses are characterized by the trajectory swept by where Dv(v 0 , t) Å v(t) 0 v 0 is the resonance offset and the tip of the B eff vector and the rate of motion along it. In this v 0 the Larmor frequency of the spin we are inspecting. £ is paper, we describe a method by which optimized modulation functhe RF field inhomogeneity factor with a nominal value of tions can be constructed to render insensitivity to B 1 inhomogene-1. The adiabatic theorem (2) asserts that the magnetization ity over a predetermined B 1 range and over a wide band of frequenvector m remains spin-locked to v e provided that the rate cies. This is accomplished by requiring that the optimized pulse of precession of m about v e is much faster than the angular fulfill the adiabatic condition over this range of B 1 inhomogeneity and over the desired frequency band for the complete duration of velocity of the motion of v e. Mathematically this is exthe pulse. A trajectory similar to the well-known sech/tanh adiapressed by the adiabatic condition (3), batic pulse, i.e., a half-ellipse, is used. The optimization process improves the slice profile by optimizing the rate of motion along G(v 0 , t) ӷ 1, [2] this trajectory. The optimized pulse can be tailored to the specific design requirements; in particular, the transition sharpness may be where G is the adiabatic parameter, 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 G(v 0 , t) Å v e (v 0 , t) Éug (v 0 , t)É Å (£ 2 v 2 1 / Dv 2) 3/2 £ÉDvv h 1 0 v 1 Dv h É , [3] 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. ᭧ 1997 Academic Press and tan u Å Dv/v 1. Inversion is obtained when the effective field moves the longitudinal magnetization M z from the /z to the 0z axis over a wide band of Larmor frequencies.
To meet the challenge of providing adequate coverage at high field strength, a combination of two... more To meet the challenge of providing adequate coverage at high field strength, a combination of two complementary methods is proposed: reshaped waveforms with the VERSE technique and a modulated angle refocusing train. It was hypothesized that pulses can be reshaped and flip angles can be modulated such that power is cut in half and thus coverage doubled. Volunteer experiments at 3T using an SSFSE sequence in the abdomen demonstrate a power reduction of 50% with unchanged contrast and SNR and improved resolution.
Where 0 ≤ t ≤ TR, is off-resonance, is a phase generated by the electronics and is the phas... more Where 0 ≤ t ≤ TR, is off-resonance, is a phase generated by the electronics and is the phase accrual at t = TR. If TR < T2 magnetization from previous TR’s affects the magnetization M in the current TR. Figure 1 shows a GRE sequence with RF pulses P1 to PN with flip angle , and a phase between RF pulses. The initial magnetization immediately after P1 at t = 0 is M(1) and the magnetization immediately after Pn is M(n).
The NMR excitation of the double quantum (DQ) transition of a spin system with I=1 in a solid by ... more The NMR excitation of the double quantum (DQ) transition of a spin system with I=1 in a solid by double-frequency irradiation fields is studied. The efficiency of the DQ excitation by the two irradiation fields applied simultaneously at the two sides of the single quantum frequency spectrum of this system is calculated. The theory of Shirley for periodical time-dependent Hamiltonians is extended to describe these NMR experiments. The two-photon character of the DQ excitation is discussed and the two-photon resonance conditions are derived. The effective rf irradiation intensity on the DQ transition is calculated to be ω1ω2{(ωQ−ωT)−1 +(ωQ+ωT)−1}, where ω1 and ω2 are the intensities of the two rf irradiation fields, 2ωT the frequency distance between these rf fields and ωQ the quadrupole frequency of the spin system. This result is valid for all ωQ values as long as ω1, ω2< ‖ ωQ−ωT ‖. Accurate computer calculations are performed to examine the exact time evolution of the density ma...
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