Rotating frame relaxation during adiabatic pulses vs. conventional spin lock: simulations and experimental results at 4 T

Abstract

Spin relaxation taking place during radiofrequency (RF) irradiation can be assessed by measuring the longitudinal and transverse rotating frame relaxation rate constants (R 1ρ and R 2ρ ). These relaxation parameters can be altered by utilizing different settings of the RF irradiation, thus providing a useful tool to generate contrast in MRI. In this work we investigate the dependencies of R 1ρ and R 2ρ due to dipolar interactions and anisochronous exchange (i.e., exchange between spins with different chemical shift δω≠0) on the properties of conventional spin-lock and adiabatic pulses, with particular emphasis on the latter ones which were not fully described previously. The results of simulations based on relaxation theory provide a foundation for formulating practical considerations for in vivo applications of rotating frame relaxation methods. Rotating frame relaxation measurements obtained from phantoms and from the human brain at 4T are presented to confirm the theoretical predictions. Keywordsrotating frame relaxations; spin-lock; adiabatic pulses; dipolar interactions; anisochronous exchange; MR contrast A. IntroductionRotating frame relaxation rate constants, R 1ρ and R 2ρ , characterize relaxation during radiofrequency (RF) irradiation when the magnetization vector is aligned with or perpendicular to the direction of the effective magnetic field ( ), respectively. R 1ρ and R 2ρ reflect the features of the spin dynamic processes, and depend on the properties of the RF irradiation [1][2][3][4]. The latter feature creates the possibility to "manipulate" the measured R 1ρ and R 2ρ by choosing different settings of the RF irradiation, thus leading to the generation of MR contrast [5,6]. Whereas the spin-lattice relaxation rate constant R 1 is sensitive to dynamic processes close to the Larmor frequency (ω 0 /(2π)), which is in the order of MHz for standard in vivo applications, in the majority of cases the rotating frame relaxations are additionally Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. sensitive to fluctuations close to the effective frequency (ω eff /(2π), where ω eff = γB eff and γ is the gyromagnetic ratio), which is in the order of kHz. The enhanced sensitivity of R 1ρ and R 2ρ to molecular dynamics in the kHz range makes rotating frame relaxations a practical tool for gaining information about water spin dynamics and interactions with endogenous macromolecules [7]. Application of these methods holds great potential for addressing several biological questions, especially at high magnetic fields. NIH Public AccessA typical method to measure R 1ρ ...

MRI rotating frame relaxation measurements for articular cartilage assessment

“…Notably, adiabatic T 1ρ can be modified to provide different contrasts by varying the modulation functions of the AFP pulses. Specifically, in agreement with the observations in other tissues [23,24], the adiabatic T 1ρ relaxation time is longer by ~40–50% when using HS1 instead of HS4 pulses (table 1). The fact that most relaxation times differ also in the ND group at 0 and 48 hours is an indication of the apparent sensitivity of the MRI techniques to detect alterations in tissue composition.…”

“…These methods utilize adiabatic full passage (AFP) pulses [21,22]. The parameters which characterize the AFP pulses used for the measurements are critical in determining the values of adiabatic T 1ρ and T 2ρ [23]. Relaxation dispersions of adiabatic T 1ρ and T 2ρ are indeed observed when changing the pulse modulation functions, when using different peak power of the AFP pulses, i.e.…”

“…ω 1 max , or when implementing different amplitudes of the frequency sweep of the pulses [24]. The generation of relaxation dispersion via the parameters of the pulses is realized through the dependency of the instantaneous contribution to the relaxation rate constants on the time-dependent effective frequency, ω eff ( t ), and on the change of its orientation in the first rotating frame with the angular velocity, dα ( t ) /dt [23]. The creation of a broad range of effective frequencies (ω eff (t) ) during the adiabatic pulse constitutes a marked difference of adiabatic rotating frame relaxation methods compared to cw rotating frame relaxation methods, where ω eff is instead constant.…”

“…To minimize SAR, image acquisition using different pulse modulation functions has been suggested as a way to create contrast with adiabatic T 1ρ and T 2ρ methods [23]…”

Teorias adiabáticas e aplicações em Ressonância Magnética Nuclear

“…Vamos a considerar como exemplo numérico, que o pulso aplicadoé um pulso adiabático (7,8,47,48) da seguinte forma…”