Chirped CPMG for well-logging NMR applications

Casabianca, Leah B.; Mohr, Daniel; Mandal, Soumyajit; Song, Yi‐Qiao; Frydman, Lucio

doi:10.1016/j.jmr.2014.02.025

Cited by 29 publications

(21 citation statements)

References 28 publications

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“…The larger bandwidth of the chirped pulse is evident, and it should be reflected by a larger number of contributing spins and thus eventually a higher sensitivity. As in our previous studies [14], the differences between the two spectral bandwidths-and hence between the pools of contributing nuclei-are ca. fourfold.…”

Section: Data Processingsupporting

confidence: 69%

“…In the standard CPMG sequence, all echo shapes can be summed up and Fourier transformed to obtain the desired profile. In the case of the chirped CPMG sequence, odd and even echoes are different [14,18]. The 90°excitation pulse is twice as long as the 180°pulses; as a result of this all isochromats contributing to the NMR signal refocus simultaneously, generating an echo that is similar to that obtained from a standard CPMG sequence [18].…”

Section: Cpmg Experimentsmentioning

confidence: 99%

“…Chirped radiofrequency (RF) pulses have long been used in solution NMR for broadband decoupling [10,11]; approaches that excite wideline NMR spectra of spins C1/2 in solids have also been proposed and are nowadays also routinely used [12,13]. In a recent study, we have demonstrated the potential advantages of relying on chirped CPMG sequences in well-logging scenarios [14]. In general, frequency-swept pulses in all these instances have been shown to excite a larger spectral bandwidth as compared to hard pulses for the same RF peak power [15,16]; ca.…”

Section: Introductionmentioning

confidence: 99%

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Single-Sided Stray-Field NMR Profiling Using Chirped Radiofrequency Pulses

et al. 2015

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Single-sided nuclear magnetic resonance (NMR) scanners find increased use in applications where non-destructive measurements are needed. These singlesided scanners are characterized by a weak magnetic field and a large stray magnetic field gradient. These characteristics make these scanners suitable for determining a sample's proton density profile, or for mapping NMR properties such as T 1 , T 2 or diffusivity as a function of distance. The strong stray-field gradient generated by these magnets dictates a need for relatively high transmission/reception bandwidths, even when thin slices are involved. Consequently, scanning a large volume demands multiple separate measurements, associated with long scan times, potential inaccuracies associated with mechanical misplacements and limitations in tackling certain in vivo or dynamic systems. This work explores the consequences of replacing the hard pulses in the usual multi-echo sequence used in this kind of scanner, with frequency-swept (chirped) pulses. It was found that, under identical echo times and number of echoes, peak power-limited cases like the ones usually involved in these setups endow chirped-pulse sequences with a higher sensitivity than their squarepulse counterparts. Furthermore, data can be extracted in this manner faster; it can also be measured from larger slabs following a single excitation, thereby avoiding the need for multiple mechanical motions of the scanner/sample. Still, at least with

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Section: Data Processingsupporting

confidence: 69%

Section: Cpmg Experimentsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Single-Sided Stray-Field NMR Profiling Using Chirped Radiofrequency Pulses

et al. 2015

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“…Methods for creating long spin echo trains in highly inhomogeneous fields have been demonstrated using frequency-swept pulses that excite and refocus all spins over the sample with dramatically lower peak power levels compared to hard pulses [9,10]. In Ref.…”

Section: Introductionmentioning

confidence: 99%

“…In Ref. [9], frequency-swept pulses were played in a CPMG spin echo train to acquire echoes from a sample with a 120 kHz linewidth in an inhomogeneous B 0 field. Compared with hard pulses, the frequency-swept pulses provided a 3-fold or greater signal enhancement and corresponding increase in the spectral width of the acquired data (Fourier transform of the echoes).…”

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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The Fun of Applications – a Perspective

Song

2022

Magnetic Resonance Microscopy

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Chirped CPMG for well-logging NMR applications

Cited by 29 publications

References 28 publications

Single-Sided Stray-Field NMR Profiling Using Chirped Radiofrequency Pulses

Single-Sided Stray-Field NMR Profiling Using Chirped Radiofrequency Pulses

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

The Fun of Applications – a Perspective

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