Magnetization transfer of water T 2 relaxation components in human brain: implications for T 2 -based segmentation of spectroscopic volumes

Helms, Gunther; Piringer, Andreas

doi:10.1016/s0730-725x(01)00396-4

Cited by 13 publications

(28 citation statements)

References 17 publications

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“…Only the latter can be explained by decreasing level of the MT-w signals (due to increase in ␦). CSF was experiencing increased MT effects at longer TR, probably through diffusion to the tissue border (18).…”

Section: Experiments 5: Influence Of Trmentioning

confidence: 99%

High‐resolution maps of magnetization transfer with inherent correction for RF inhomogeneity and T₁ relaxation obtained from 3D FLASH MRI

Helms¹,

Dathe²,

Kallenberg

et al. 2008

Magnetic Resonance in Med

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296

461

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An empirical equation for the magnetization transfer (MT)FLASH signal is derived by analogy to dual-excitation FLASH, introducing a novel semiquantitative parameter for MT, the percentage saturation imposed by one MT pulse during TR. This parameter is obtained by a linear transformation of the inverse signal, using two reference experiments of proton density and T 1 weighting. The influence of sequence parameters on the MT saturation was studied. An 8.5-min protocol for brain imaging at 3 T was based on nonselective sagittal 3D-FLASH at 1.25 mm isotropic resolution using partial acquisition techniques (TR/TE/ ␣ ‫؍‬ 25ms/4.9ms/5°or 11ms/4.9ms/15°for the T 1 reference). A 12.8 ms Gaussian MT pulse was applied 2.2 kHz off-resonance with 540°flip angle. The MT saturation maps showed an excellent contrast in the brain due to clearly separated distributions for white and gray matter and cerebrospinal fluid. Magnetization transfer (MT) is a contrast mechanism in tissue that is based on cross-relaxation or chemical exchange between protons in bulk water and rotationally immobilized macromolecules (1,2). In clinical MRI, MT contrast is invoked by application of a T 2 -selective RF pulse applied prior to slice excitation. The "MT pulse" saturates specifically the macromolecular magnetization, and MT is then observed as a reduction in image intensity. Its normalized value, the MT ratio (MTR), is commonly taken as a measure for the strength of the MT effect, and hence interpreted as a surrogate parameter for macromolecular content or myelination. However, the MTR is not an absolute measure, but depends on the sequence parameters and is influenced by T 1 relaxation and flip angle inhomogeneities.This limitation can be overcome by quantitative Z-spectroscopy measurements to obtain parameter estimates for the binary spin-bath (BSB) model and suitable absorption line shapes (3,4). On clinical MR systems, pulsed MT is combined with spoiled gradient echo sequences (FLASH, fast low angle shot) and suitable adaptations of Henkelman's continuous wave model (5-7). Alternatively, pulsed MT experiments can be approximated by instantaneous events of saturation separating intervals free of irradiation (8). Since brain tissue is characterized by conditions of fast-exchange, this "free" evolution can be described by two exponential time courses, the common T 1 relaxation and the MT. The latter is observed as a reduction of the longitudinal magnetization of free water subsequent to the MT pulse (9). This MT-related saturation increases in time until the exchange equilibrium in the BSB model is restored.The concept of separating T 1 relaxation and MT has been previously applied to progressive partial saturation by repetitive MT pulses (10). Here, it is transferred to the MT-w(eighted) FLASH sequence. A phenomenological signal equation is derived by analogy from a FLASH experiment with two interleaved excitations and recovery times. This equation represents the effects of excitation and relaxation during TR, while any additional saturation du...

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Section: Experiments 5: Influence Of Trmentioning

confidence: 99%

High‐resolution maps of magnetization transfer with inherent correction for RF inhomogeneity and T₁ relaxation obtained from 3D FLASH MRI

Helms¹,

Dathe²,

Kallenberg

et al. 2008

Magnetic Resonance in Med

Self Cite

296

461

View full text Add to dashboard Cite

show abstract

“…In the hypothetical absence of direct saturation, the steady state approaches CW saturation (Eq. [5]) for PR 3 0. Already for ␦ f ϭ 4% direct saturation dominates the PR-dependence, which thus loses the typical features seen in Figure 3. resents a different relation between fast transfer and slow relaxation.…”

Section: Three Domains Of Pr-dependencementioning

confidence: 96%

“…The idealized case without direct saturation would approach the steady state of the ideal continuous wave saturation (Eq. [5]) for PR 3 0 (20). This is about 76% for both WM and GM.…”

Section: Direct Saturation Of Bulk Watermentioning

confidence: 99%

“…An alternative approach is to combine trains of equidistant MT-pulses with a "single-shot" read-out of the signal. This has been used for quantitative echo-planar imaging (3), MT-spectroscopy with single volume localization (4), and to identify contributions from tissue to the apparent component of cerebro-spinal fluid (CSF) in T 2 -based segmentation (5). A complete separation of the MTpreparation of longitudinal magnetization from the read-out sequence allows for faster repetition and/or for utilizing MT-pulses of higher power without violating specific-absorption-rate limits.…”

Section: Introductionmentioning

confidence: 99%

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Pulsed saturation of the standard two‐pool model for magnetization transfer. Part I: The steady state

Helms

Hagberg

2004

Concepts Magnetic Resonance

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ABSTRACT:A general framework for magnetization transfer (MT) of a two-pool system with linear exchange for arbitrary saturation by periodic radio-frequency pulses was derived. It is based on a novel parameterization adapted to the time evolution of saturation recovery. The conditions in tissue permit a description in analogy to partial saturation of a homogeneous liquid. In this approximation, the direct saturation of bulk water is amplified by MT. Rapid transfer equilibrates the saturation of the pools in the early phase of free evolution. This kinetic "pre-equilibrium" relaxes slowly with a common relaxation rate.

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“…[7] Thus, A 2 is expressed as linear combination of A and I. Because A is a 2 ϫ 2 matrix, trace and determinant of A (see Eqs.…”

Section: Solving the Recursionmentioning

confidence: 99%

Pulsed saturation of the standard two‐pool model for magnetization transfer. Part II: The transition to steady state

Helms

Dathe

Hagberg

2004

Concepts Magnetic Resonance

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ABSTRACT:The transition of the tissue signal to steady state under periodic selective saturation of macromolecular magnetization can be observed by single-shot echo-planar imaging. The general solution for a two-pool system with linear exchange kinetics contains two transient components. The rapid minor transient causes an initial delay of the transition for fast pulse repetition (PR) and weak saturation. The slow major transient combines progressive direct saturation and transferred saturation. Its PR-dependence provides similar information as the steady state, but is less sensitive to direct saturation and fitting errors. Sampling at different PR allows to quantify all system parameters.

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Magnetization transfer of water T 2 relaxation components in human brain: implications for T 2 -based segmentation of spectroscopic volumes

Cited by 13 publications

References 17 publications

High‐resolution maps of magnetization transfer with inherent correction for RF inhomogeneity and T₁ relaxation obtained from 3D FLASH MRI

High‐resolution maps of magnetization transfer with inherent correction for RF inhomogeneity and T₁ relaxation obtained from 3D FLASH MRI

Pulsed saturation of the standard two‐pool model for magnetization transfer. Part I: The steady state

Pulsed saturation of the standard two‐pool model for magnetization transfer. Part II: The transition to steady state

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Magnetization transfer of water T 2 relaxation components in human brain: implications for T 2 -based segmentation of spectroscopic volumes

Cited by 13 publications

References 17 publications

High‐resolution maps of magnetization transfer with inherent correction for RF inhomogeneity and T1 relaxation obtained from 3D FLASH MRI

High‐resolution maps of magnetization transfer with inherent correction for RF inhomogeneity and T1 relaxation obtained from 3D FLASH MRI

Pulsed saturation of the standard two‐pool model for magnetization transfer. Part I: The steady state

Pulsed saturation of the standard two‐pool model for magnetization transfer. Part II: The transition to steady state

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High‐resolution maps of magnetization transfer with inherent correction for RF inhomogeneity and T₁ relaxation obtained from 3D FLASH MRI

High‐resolution maps of magnetization transfer with inherent correction for RF inhomogeneity and T₁ relaxation obtained from 3D FLASH MRI