spin-echo (SE) measurements were used to estimate the apparent transverse relaxation time constant (T 2 † ) of water and metabolites in human brain at 4T and 7T. A significant reduction in the T 2 † values of proton resonances (water, N-acetylaspartate, and creatine/phosphocreatine) was observed with increasing magnetic field strength and was attributed mainly to increased dynamic dephasing due to increased local susceptibility gradients. At high field, signal loss resulting from T 2 † decay can be substantially reduced using a Carr-Purcell-type SE sequence.Magn Theoretical and experimental studies have shown at least a linear increase in sensitivity with magnetic field strength (1,2). On the other hand, the transverse relaxation rate is known to increase with magnetic field strength (3,4), which can result in reduced sensitivity in spin-echo (SE) experiments. The apparent transverse relaxation time (T 2 † ) is related to the intrinsic transverse relaxation time (T 2 ) through the following equation:The first term on the right side of Eq.[1] is the inverse of the intrinsic T 2 and is governed by a number of possible mechanisms, including 1) homonuclear dipole-dipole interaction between protons, which is strongly dependent on rotational correlation time c ; 2) hyperfine (contact) interaction, namely, the change of transverse relaxation time due to interaction with a paramagnetic center; and 3) cross-relaxation, which can be significant in dipole-coupled systems. The second and third terms, T 2,Diffusion and T 2,Exchange , are the transverse relaxation times related to diffusion and exchange of spins between regions with different magnetic field strengths, respectively. These contributions describe the dynamic dephasing regime, whereby the net magnetization is reduced by diffusion and exchange between regions with different magnetic field strengths, which causes the phases of the different spin packets to average out. The opposite situation is defined as the static dephasing regime. NMR signal loss due to static dephasing can be refocused by SE sequences and is therefore not considered here.It is important to investigate: 1) how the increase of field strength causes T 2 † shortening, and 2) how the signal loss from T 2 † decay can be compensated for. Key experiments for answering these questions involve measuring T 2 † at different field strengths and attempting to estimate T 2 . The theory of NMR signal formation in the presence of local magnetic field inhomogeneity was first derived by Carr and Purcell (5), and later generalized by Torrey (6), who incorporated the diffusion effects into the Bloch equations to take into account the actual field distribution. The CarrPurcell (CP) method is the most valuable technique for determining transverse relaxation times. CP experiments are performed by applying a /2 pulse followed by a series of pulses spaced with time interval cp . The value of T 2 † determined with a CP technique can vary with cp because dynamic dephasing and homonuclear spin-spin coupling can cause signi...

A new method to measure rotating frame relaxation and to create contrast for MRI is introduced. The technique exploits relaxation along a fictitious field (RAFF) generated by amplitude- and frequency-modulated irradiation in a sub-adiabatic condition. Here, RAFF is demonstrated using a radiofrequency pulse based on sine and cosine amplitude and frequency modulations of equal amplitudes, which gives rise to a stationary fictitious magnetic field in a doubly rotating frame. According to dipolar relaxation theory, the RAFF relaxation time constant (TRAFF) was found to differ from laboratory frame relaxation times (T1 and T2) and rotating frame relaxation times (T1ρ and T2ρ). This prediction was supported by experimental results obtained from human brain in vivo and three different solutions. Results from relaxation mapping in human brain demonstrated the ability to create MRI contrast based on RAFF. The value of TRAFF was found to be insensitive to the initial orientation of the magnetization vector. Finally, as compared with adiabatic pulse trains of equal durations, RAFF required less radiofrequency power and therefore can be more readily used for rotating frame relaxation studies in humans.

A high-resolution spin-echo imaging method is presented (called CP-LASER) which exploits the spin refocusing capability of an adiabatic Carr-Purcell (CP) pulse sequence to measure apparent 1 H 2 O transverse relaxation (T 2 † ) and generate contrast based on microscopic tissue susceptibility. High-resolution CP-LASER images of the human occipital lobe were acquired at four different echo times from six subjects at 4T and eight subjects at 7T to investigate the effect of magnetic field strength (B 0 ) and the CP interpulse time ( cp ) on T 2 † . Susceptibility contrast was identified and T 2 † was quantified for long cp (>10 ms) and short cp (7 ms at 4T and 6 ms at 7T) in gray matter, white matter, and cerebral spinal fluid.

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Postmortem demonstration of increased iron in the substantia nigra (SN) is a well-appreciated finding in Parkinson's disease (PD). Iron facilitates generation of free radicals, which are thought to play a role in dopamine neuronal loss. To date, however, magnetic resonance imaging (MRI) has failed to show significant in vivo differences in SN iron levels in subjects with PD versus control subjects. This finding may be due to the limitations in tissue contrasts achievable with conventional T(1)- and T(2)-weighted MRI sequences that have been used. With the recent development of novel rotating frame transverse (T(2rho)) and longitudinal (T(1rho)) relaxation MRI methods that appear to be sensitive to iron and neuronal loss, respectively, we embarked on a study of 8 individuals with PD (Hoehn & Yahr, Stage II) and 8 age-matched control subjects. Using these techniques with a 4T MRI magnet, we assessed iron deposits and neuronal integrity in the SN. First, T(2rho) MRI, which is reflective of iron-related dynamic dephasing mechanisms (e.g., chemical exchange and diffusion in the locally different magnetic susceptibilities), demonstrated a statistically significant difference between the PD and control group, while routine T(2) MRI did not. Second, T(1rho) measurements, which appear to reflect upon neuronal count, indicated neuronal loss in the SN in PD. We show here that sub-millimeter resolution T(1rho) and T(2rho) MRI relaxation methods can provide a noninvasive measure of iron content as well as evidence of neuronal loss in the midbrain of patients with PD.

Calculations and experiments were used to examine the B 1 field behavior and signal intensity distribution in a 16-cm diameter spherical phantom excited by a 10-cm diameter surface coil at 300 MHz. In this simple system at this high frequency very complex RF field behavior exists, resulting in different excitation and reception distributions. Included in this work is a straightforward demonstration that coil receptivity is proportional to the magnitude of the circularly polarized component of the B 1 field that rotates in the direction opposite to that of nuclear precession. It is clearly apparent that even in very simple systems in head-sized samples at this frequency it is important to consider the separate excitation and reception distributions in order to understand the signal intensity distribution. Magn Reson Med 47:1026 -1028, 2002.

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 ...