Relaxometry of Tissue

Koenig, Seymour H.; Brown, Rodney D.

doi:10.1002/9780470034590.emrstm0466

Cited by 12 publications

(33 citation statements)

References 39 publications

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“…Whereas the phenomenology of water-1 H relaxation in tissue is well documented (1)(2)(3), the underlying molecular mechanism remains controversial (4 -9). The task of elucidating the relaxation mechanism is complicated by incomplete knowledge about the chemical composition and supramolecular structure of most tissues.…”

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confidence: 99%

Molecular basis of water proton relaxation in gels and tissue

Chávez

Halle

2006

Magnetic Resonance in Med

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An extensive set of water-1 H magnetic relaxation dispersion (MRD) data are presented for aqueous agarose and gelatin gels. It is demonstrated that the EMOR model, which was developed in a companion paper to this study (see Halle, this issue), accounts for the dependence of the water-1 H spin-lattice relaxation rate on resonance frequency over more than four decades and on pH. The parameter values deduced from analysis of the 1 H MRD data are consistent with values derived from 2 H MRD profiles from the same gels and with small-molecule reference data. This agreement indicates that the water-1 H relaxation dispersion in aqueous biopolymer gels is produced directly by exchange-mediated orientational randomization of internal water molecules or labile biopolymer protons, with little or no role played by collective biopolymer vibrations or coherent spin diffusion. This ubiquitous mechanism is proposed to be the principal source of water-1 H spin-lattice relaxation at low magnetic fields in all aqueous systems with rotationally immobile biopolymers, including biological tissue. The same mechanism also contributes to transverse and rotating-frame relaxation and magnetization transfer at high fields. Intrinsic contrast in magnetic resonance images of soft tissue is generated largely by spatial variations in spin relaxation rates. Whereas the phenomenology of water-1 H relaxation in tissue is well documented (1-3), the underlying molecular mechanism remains controversial (4 -9). The task of elucidating the relaxation mechanism is complicated by incomplete knowledge about the chemical composition and supramolecular structure of most tissues. In addition, it is usually not possible to vary biopolymer composition, pH, or temperature in a controlled way while maintaining the integrity of the tissue. For these reasons, most mechanistic studies have been carried out on model systems, such as aqueous biopolymer gels, with relaxation characteristics similar to those of tissue. Here we report an extensive set of water-1 H relaxation data from two widely used tissue models: aqueous gels of agarose and gelatin.Detailed information about relaxation mechanisms in complex systems can be obtained from the magnetic relaxation dispersion (MRD) profile, that is, the field/frequency dependence of the spin-lattice relaxation rate, R 1 ϭ 1/T 1 . Ideally, the dispersion profile should be recorded from typical MRI frequencies of order 100 MHz down to the kHz range. The water-1 H MRD profiles reported here cover more than four frequency decades. The strong frequency dependence of R 1 at low fields, which is the focus of the present study, can be used to enhance image contrast in prepolarized MRI experiments (10). The low-field R 1 dispersion also yields information about the zero-frequency dipolar contributions to transverse relaxation and steadystate magnetization transfer and to the low-frequency dipolar contribution to rotating-frame spin-lattice relaxation.Agarose is a linear polysaccharide with a disaccharide repeat (agarobiose) composed...

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mentioning

confidence: 99%

Molecular basis of water proton relaxation in gels and tissue

Chávez

Halle

2006

Magnetic Resonance in Med

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“…7, but broadened on the high-frequency side due to I-protons with shorter residence times. This is indeed what is observed in most tissues (1,2). Typically the dispersion is only measured down to 10 kHz, where the approach to the low-frequency R 1 plateau is just barely apparent (see the I ϭ 10 -5 s curve in Fig.…”

Section: From Model System To Tissuementioning

confidence: 86%

“…This has led to the mistaken belief that the motional correlation time (deduced from the dispersion midpoint) is invariant with respect to system properties and temperature (2,4,5). In fact, the apparent correlation time obtained in this way is not related to molecular motions.…”

Section: Adiabatic Regimementioning

confidence: 99%

“…Previous water-1 H and 2 H MRD studies of chemically cross-linked protein gels revealed a remarkable invariance of the dispersion frequency with respect to temperature and biopolymer (2,4,5). The authors interpreted that finding in terms of the two-pool model by postulating the existence, in all investigated proteins, of a class of water molecules with a long (1 s), but temperature-independent, residence time (2,4,5). Such unphysical assumptions are not required by the EMOR model, which predicts an invariant dispersion frequency that is equal to the residual dipole frequency ⍀ D , in the adiabatic regime.…”

Section: Other Modelsmentioning

confidence: 99%

“…[1] is variously referred to as a strong-collision process or a continuous Kubo-Anderson process (22). With a propagator of this form, the time correlation functions relevant to dipolar relaxation (13) decay exponentially with the mean residence time as the correlation time: [2] where the subscript denotes "exchange." The spectral density function associated with exchange-mediated orientational randomization of the dipole-dipole vector between an I-spin and a particular P-spin (labeled k) thus has the Lorentzian form (19)…”

Section: Emor Modelmentioning

confidence: 99%

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Molecular theory of field‐dependent proton spin‐lattice relaxation in tissue

Halle

2006

Magnetic Resonance in Med

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A molecular theory is presented for the field-dependent spinlattice relaxation time of water in tissue. The theory attributes the large relaxation enhancement observed at low frequencies to intermediary protons in labile groups or internal water molecules that act as relaxation sinks for the bulk water protons. Exchange of intermediary protons not only transfers magnetization to bulk water protons, it also drives relaxation by a mechanism of exchange-mediated orientational randomization (EMOR). An analytical expression for T 1 is derived that remains valid outside the motional-narrowing regime. Cross-relaxation between intermediary protons and polymer protons plays an important role, whereas spin diffusion among polymer protons can be neglected. For sufficiently slow exchange, the dispersion midpoint is determined by the local dipolar field rather than by molecular motions, which makes the dispersion frequency insensitive to temperature and system composition. The EMOR model differs fundamentally from previous models that identify collective polymer vibrations or hydration water dynamics as the molecular motion responsible for spin relaxation. Unlike previous models, the EMOR model accounts quantitatively The spin-lattice relaxation time, T 1 , of tissue water is one of the principal contrast modalities in clinical MRI. Although the phenomenology of water-1 H relaxation in soft tissue is well documented (1-3), the molecular determinants of natural and pathological T 1 variations have not been established. Because of the molecular-level complexity of biological tissue, studies aimed at unraveling the relaxation mechanism have often employed aqueous gels as tissue models. The magnetic relaxation dispersion (MRD), that is, the variation of R 1 ϭ 1/T 1 with magnetic field strength (or Larmor frequency) is particularly informative about the molecular motions that induce spin relaxation. However, even though numerous MRD studies of tissue models have been conducted, we still cannot predict the MRD profile from known system properties. There is even disagreement about the nature of the molecular motions that are responsible for the observed relaxation dispersion. For example, some authors invoke slow water dynamics in the hydration layer at the biopolymer surface (4 -6), while others focus on collective biopolymer vibrations (7-10).We recently reported an extensive set of water-2 H MRD data from polysaccharide and polypeptide gels (11) (Vaca Chávez et al., submitted). Since cross-relaxation and spin diffusion can be neglected for the quadrupolar 2 H nuclide, the analysis of 2 H MRD data is relatively straightforward. The 2 H MRD profiles can be accounted for quantitatively by a model that attributes the relaxation dispersion to a small deuteron population in intimate and long-lived association with the biopolymer. These intermediary deuterons either reside in labile groups (such as hydroxyl or amino groups) or belong to water molecules that are temporarily trapped within the biopolymer. According to this model, spin re...

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Top 5 Common Misconceptions About the Medical Inpatient: Approaching Cardiovascular Prevention for the Hospitalist

Levy

2011

Clinical Cardiology

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Relaxometry of Tissue

Cited by 12 publications

References 39 publications

Molecular basis of water proton relaxation in gels and tissue

Molecular basis of water proton relaxation in gels and tissue

Molecular theory of field‐dependent proton spin‐lattice relaxation in tissue

Top 5 Common Misconceptions About the Medical Inpatient: Approaching Cardiovascular Prevention for the Hospitalist

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