Spatial phase encoding exploiting the Bloch–Siegert shift effect

Kartäusch, Ralf; Drießle, Toni; Kampf, Thomas; Basse-Lüsebrink, Thomas C.; Hoelscher, Uvo Christoph; Jakob, Peter M.; Fidler, Florian; Helluy, Xavier

doi:10.1007/s10334-013-0417-0

Cited by 12 publications

(18 citation statements)

References 22 publications

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“…Although a much weaker B + 1 gradient of approximately 2 mT/m was used here experimentally, this stronger gradient is not difficult to achieve in practice. 5,7 In general, 𝜔 off should be minimized to maximize the Bloch-Siegert frequency gradient. However, excessive reduction of the frequency offset can produce out-of band excitation (Figure 3B, at 𝜔 off = 2.5 kHz) or move multiphoton resonances closer in B + 1 to the single-photon resonance (Figure 6) and potentially into the FOV.…”

Section: Discussionmentioning

confidence: 99%

“…For example, to create a frequency gradient of 1 MHz/m (produced by a

{B}_0

gradient of approximately 23.5 mT/m) using the Bloch–Siegert shift, using the

{\omega}_{\mathrm{off}}

= 7.5 kHz value used throughout this work, a

{B}_1^{+}

gradient of approximately the same strength, 23.5 mT/m, would be required. Although a much weaker

{B}_1^{+}

gradient of approximately 2 mT/m was used here experimentally, this stronger gradient is not difficult to achieve in practice 5,7 . In general,

{\omega}_{\mathrm{off}}

should be minimized to maximize the Bloch–Siegert frequency gradient.…”

Section: Discussionmentioning

confidence: 99%

“…This property has allowed the Bloch–Siegert shift to be used for mapping

{B}_1^{+}

fields in MRI 18 and calibrating decoupling field strength in NMR 19,20 . More recently, the phenomenon has been exploited as an encoding mechanism for

{B}_0

gradient‐free imaging 5,21,22 . In this work, the Bloch–Siegert shift is used to localize excitation.…”

Section: Introductionmentioning

confidence: 99%

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Selective excitation localized by the Bloch–Siegert shift and a B1+ gradient

Martin

Srinivas

Vaughn

et al. 2022

Magnetic Resonance in Med

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Purpose To perform B1+$$ {B}_1^{+} $$‐selective excitation using the Bloch–Siegert shift for spatial localization. Theory and Methods A B1+$$ {B}_1^{+} $$‐selective excitation is produced by an radiofrequency (RF) pulse consisting of two summed component pulses: an off‐resonant pulse that induces a B1+$$ {B}_1^{+} $$‐dependent Bloch–Siegert frequency shift and a frequency‐selective excitation pulse. The passband of the pulse can be tailored by adjusting the frequency content of the frequency‐selective pulse, as in conventional B0$$ {B}_0 $$ gradient‐localized excitation. Fine magnetization profile control is achieved by using the Shinnar–Le Roux algorithm to design the frequency‐selective excitation pulse. Simulations analyzed the pulses' robustness to off‐resonance, their suitability for multi‐echo spin echo pulse sequences, and how their performance compares to that of rotating‐frame selective excitation pulses. The pulses were evaluated experimentally on a 47.5 mT MRI scanner using an RF gradient transmit coil. Multiphoton resonances produced by the pulses were characterized and their distribution across B1+$$ {B}_1^{+} $$ predicted. Results With correction for varying B1+$$ {B}_1^{+} $$ across the desired profile, the proposed pulses produced selective excitation with the specified profile characteristics. The pulses were robust against off‐resonance and RF amplifier distortion, and suitable for multi‐echo pulse sequences. Experimental profiles closely matched simulated patterns. Conclusion The Bloch–Siegert shift can be used to perform B0$$ {B}_0 $$‐gradient‐free selective excitation, enabling the excitation of slices or slabs in RF gradient‐encoded MRI.

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Section: Discussionmentioning

confidence: 99%

“…For example, to create a frequency gradient of 1 MHz/m (produced by a

{B}_0

gradient of approximately 23.5 mT/m) using the Bloch–Siegert shift, using the

{\omega}_{\mathrm{off}}

= 7.5 kHz value used throughout this work, a

{B}_1^{+}

gradient of approximately the same strength, 23.5 mT/m, would be required. Although a much weaker

{B}_1^{+}

gradient of approximately 2 mT/m was used here experimentally, this stronger gradient is not difficult to achieve in practice 5,7 . In general,

{\omega}_{\mathrm{off}}

should be minimized to maximize the Bloch–Siegert frequency gradient.…”

Section: Discussionmentioning

confidence: 99%

“…This property has allowed the Bloch–Siegert shift to be used for mapping

{B}_1^{+}

fields in MRI 18 and calibrating decoupling field strength in NMR 19,20 . More recently, the phenomenon has been exploited as an encoding mechanism for

{B}_0

gradient‐free imaging 5,21,22 . In this work, the Bloch–Siegert shift is used to localize excitation.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Selective excitation localized by the Bloch–Siegert shift and a B1+ gradient

Martin

Srinivas

Vaughn

et al. 2022

Magnetic Resonance in Med

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“…The gr-MRI images were free of distortions, and had similar overall geometry and quality to the commercial spectrometer’s images. The ability to generate frequency-swept pulses using gr-MRI was also validated, which is needed for adiabatic excitations [20] and may be useful for RF encoding using the Bloch-Siegert shift [21, 22]. …”

Section: Discussionmentioning

confidence: 99%

gr-MRI: A software package for magnetic resonance imaging using software defined radios

Hasselwander

Cao

Grissom

2016

Journal of Magnetic Resonance

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The goal of this work is to develop software that enables the rapid implementation of custom MRI spectrometers using commercially-available software defined radios (SDRs). The developed gr-MRI software package comprises a set of Python scripts, flowgraphs, and signal generation and recording blocks for GNU Radio, an open-source SDR software package that is widely used in communications research. gr-MRI implements basic event sequencing functionality, and tools for system calibrations, multi-radio synchronization, and MR signal processing and image reconstruction. It includes four pulse sequences: a single-pulse sequence to record free induction signals, a gradient-recalled echo imaging sequence, a spin echo imaging sequence, and an inversion recovery spin echo imaging sequence. The sequences were used to perform phantom imaging scans with a 0.5 Tesla tabletop MRI scanner and two commercially-available SDRs. One SDR was used for RF excitation and reception, and the other for gradient pulse generation. The total SDR hardware cost was approximately $2000. The frequency of radio desynchronization events and the frequency with which the software recovered from those events was also measured, and the SDR’s ability to generate frequency-swept RF waveforms was validated and compared to the scanner’s commercial spectrometer. The spin echo images geometrically matched those acquired using the commercial spectrometer, with no unexpected distortions. Desynchronization events were more likely to occur at the very beginning of an imaging scan, but were nearly eliminated if the user invoked the sequence for a short period before beginning data recording. The SDR produced a 500 kHz bandwidth frequency-swept pulse with high fidelity, while the commercial spectrometer produced a waveform with large frequency spike errors. In conclusion, the developed gr-MRI software can be used to develop high-fidelity, low-cost custom MRI spectrometers using commercially-available SDRs.

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“…The Bloch–Siegert shift imparted by off-resonant RF pulses has been recently exploited for frequency [17] and phase [18] encoding using pure transmit RF field (

B_{1}^{+}

) spatial encoding. In this approach, an RF coil with a spatially varying

B_{1}^{+}

amplitude along the encoding direction is used (typically a linear profile).…”

Section: Introductionmentioning

confidence: 99%

Transmit Array Spatial Encoding (TRASE) using broadband WURST pulses for RF spatial encoding in inhomogeneous B0 fields

Stockmann

Cooley

Guérin

et al. 2016

Journal of Magnetic Resonance

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Transmit Array Spatial Encoding (TRASE) is a promising new MR encoding method that uses transmit RF (B1+) phase gradients over the field-of-view to perform Fourier spatial encoding. Acquisitions use a spin echo train in which the transmit coil phase ramp is modulated to jump from one k-space point to the next. This work extends the capability of TRASE by using swept radiofrequency (RF) pulses and a quadratic phase removal method to enable TRASE where it is arguably most needed: portable imaging systems with inhomogeneous B0 fields. The approach is particularly well-suited for portable MR scanners where (a) inhomogeneous B0 fields are a byproduct of lightweight magnet design, (b) heavy, high power-consumption gradient coil systems are a limitation to siting the system in non-conventional locations and (c) synergy with the use of spin echo trains is required to overcome intra-voxel dephasing (short T2∗) in the inhomogeneous field. TRASE does not use a modulation of the B0 field to encode, but it does suffer from secondary effects of the inhomogeneous field. Severe artifacts arise in TRASE images due to off-resonance effects when the RF pulse does not cover the full bandwidth of spin resonances in the imaging FOV. Thus, for highly inhomogeneous B0 fields, the peak RF power needed for high-bandwidth refocusing hard pulses becomes very expensive, in addition to requiring RF coils that can withstand thousands of volts. In this work, we use swept WURST RF pulse echo trains to achieve TRASE imaging in a highly inhomogeneous magnetic field (ΔB0/B0 ~ 0.33% over the sample). By accurately exciting and refocusing the full bandwidth of spins, the WURST pulses eliminate artifacts caused by the limited bandwidth of the hard pulses used in previous realizations of TRASE imaging. We introduce a correction scheme to remove the unwanted quadratic phase modulation caused by the swept pulses. Also, a phase alternation scheme is employed to mitigate artifacts caused by mixture of the even and odd-echo coherence pathways due to defects in the refocusing pulse. In this paper, we describe this needed methodology and demonstrate the ability of TRASE to Fourier encode in an inhomogeneous field (ΔB0/B0 ~ 1% over the full FOV).

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Spatial phase encoding exploiting the Bloch–Siegert shift effect

Abstract: BS-gradients were demonstrated as a feasible option for spatial phase encoding. Furthermore, undistorted BS-SET images could be obtained using the proposed reconstruction method.

Cited by 12 publications

References 22 publications

Selective excitation localized by the Bloch–Siegert shift and a B1+ gradient

Selective excitation localized by the Bloch–Siegert shift and a B1+ gradient

gr-MRI: A software package for magnetic resonance imaging using software defined radios

Transmit Array Spatial Encoding (TRASE) using broadband WURST pulses for RF spatial encoding in inhomogeneous B0 fields

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