Mechanisms of Contrast in NMR Imaging

Wehrli, Félix W.; MacFall, James R.; Shutts, Deborah; Breger, R K; Herfkens, Robert J.

doi:10.1097/00004728-198406000-00001

Cited by 115 publications

(45 citation statements)

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“…On the other hand, contrast of the NMR images is strongly dependent on imaging pulse sequence parameters, i.e., TR, TI, and TE (Crooks et aL, 1982;Wehrli et aL, 1984). As a consequence, calcu lation of relaxation ti me is commonly employed as an index of tissue injury.…”

Section: Figmentioning

confidence: 99%

Characterization of Experimental Ischemic Brain Edema Utilizing Proton Nuclear Magnetic Resonance Imaging

Kato

Kogure

Ohtomo

et al. 1986

J Cereb Blood Flow Metab

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Summary:Correlations between Tl and T2 relaxation times and water and electrolyte content in the normal and ischemic rat and gerbil brains were studied by means of both nuclear magnetic resonance (NMR) spectroscopic and imaging methods. In the spectroscopic experiment on excised rat brains, Tl was linearly dependent on tissue water content and T2 was prolonged in edematous tissue to a greater extent than expected by an increase in water content, showing that T2 possesses a greater sensi tivity for edema identification and localization. Changes in Na + and K + content of the tissue mattered little in the prolongation of relaxation times. Serial NMR imaging of gerbil brains insulted with permanent hemispheric isch- Proton nuclear magnetic resonance (NMR)imaging enables sensitive detection of cerebral ischemia within several hours after the onset of dis ease (Buonanno et aI. , 1983; Mano et aI. , 1983; Spetzler et aI. , 1983). T1 (spin-lattice) and T2 (spin-spin) relaxation times are prolonged in isch emic tissue, and the degree of damage and extent of injury can be pictorialized by NMR imaging. Pro longation of relaxation times is considered to be a result of the development of brain edema (Go and Edzes, 1975; Naruse et aI. , 1982). However, the contrast of the NMR image is arbitrarily controlled by the pulse sequence used. As a consequence, the Abbreviations used: IR, inversion recovery; NMR, nuclear magnetic resonance; PO, proton density; SE, spin echo; SR, sat uration recovery; TE, time to echo; n, time of inversion; TR , time of repetition. 212emia offered early lesion detection in TJ-and especially T2-weighted images (detection as soon as 30 min after insult). The progressive nature of lesions was also imaged. Calculated TJ and T2 relaxation times in regions of interest correlated excellently with tissue water con tent (r = 0.892 and 0.744 for T 1 and T2, respectively). As a result, detection of cerebral ischemia utilizing NMR imaging was strongly dependent on a change in tissue water content. The different nature of Tl and T2 relax ation times was also observed.

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Section: Figmentioning

confidence: 99%

Characterization of Experimental Ischemic Brain Edema Utilizing Proton Nuclear Magnetic Resonance Imaging

Kato

Kogure

Ohtomo

et al. 1986

J Cereb Blood Flow Metab

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“…The reemitted signal can be localized in space to create a digital image by application of magnetic gradients. For the partial saturation or the spin echo sequence (12), the signal S at any point in the image is given by Eq. 1 as S = NO41 -2e(-TR-TE/2)/T1 + e-TRIT1ie-TE/T2 [1] where TR is the repetition time between pulse sequences, TE is the time between the 900 radio frequency pulse and the formation of the spin echo signal, NH is the proton density, and T1 and T2 are the spin lattice and spin-spin relaxation times of the sample.…”

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confidence: 99%

Magnetic resonance microscopy of changes in water content in stems of transpiring plants.

Johnson¹,

Brown²,

Kramer³

1987

Proc. Natl. Acad. Sci. U.S.A.

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Differences in water content and degree of binding in the various stem tissues of Pelargonium hortorum were observed by magnetic resonance imaging. 'H images were obtained with a resolution of 100 ,um in the transverse plane and a slice thickness of 1250 pum. It was possible to distinguish the principal tissues of the stem by differences in their proton density or apparent water content and spin lattice relaxation time (T,) or degree of water binding. Measurements were made while the plant was slowly and actively transpiring. In the slowly transpiring plant, T, of various tissues ranged from an average of 659 to 865 ms with a proton density variation offrom 72 to 100%. In the actively transpiring plant, T, ranged from an average of 511 to 736 ms, and the proton density was reduced, ranging between 62 and 88% of the peak value found in the slowly transpiring plant. The fibrous sheath surrounding the vascular tissue and the epidermal region was found to have the highest spin density and T1. This paper describes an investigation of the use of magnetic resonance imaging (MRI) for nondestructive research on the content and binding of water in plant tissues. Since the initial demonstration of spatially localized magnetic resonance by Lauterbur (1), it has been clear that MRI has potential as a valuable tool in the study ofplants. Several early researchers demonstrated images of fruits and nuts (2-4), and MRI has been applied to studies of intact plants (5, 6). Studies have been limited by picture volume elements (voxels) with sampling volumes of >10 mm3. But the resolution has been extended into the microscopic domain with voxels as small as 0.012 mm3 with a sufficiently high signal to noise ratio to distinguish subtle differences in signal intensity (7-11). This permits production of images that distinguish the various tissues in a plant stem.A magnetic resonance image is generated by placing a sample in a strong magnetic field that causes the protons (primarily in water) to precess synchronously about the field at the Larmor frequency. Externally applied radio frequency pulses at the Larmor frequency can be used to manipulate the precessing protons away from their equilibrium position (precessing about the field). In the process the protons absorb energy that is later reemitted. The reemitted signal can be localized in space to create a digital image by application of magnetic gradients. For the partial saturation or the spin echo sequence (12), the signal S at any point in the image is given by Eq. 1 as S = NO41 -2e(-TR-TE/2)/T1 + e-TRIT1ie-TE/T2[1]where TR is the repetition time between pulse sequences, TE is the time between the 900 radio frequency pulse and the formation of the spin echo signal, NH is the proton density, and T1 and T2 are the spin lattice and spin-spin relaxation times of the sample. For a more complete description of the technology see ref. 13. One application of MRI in plant science would be to monitor water movement through plant tissue, using change in NH as a gauge of water content...

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“…Pour cette séquence qui nécessite deux impulsions radiofré-quences, le temps de répétition joue un rôle important vis-à-vis du retour à l'équilibre des aimantations. Si cet intervalle de temps est court, la pondération du signal est alors gouvernée par la relaxation spin-milieu (ou relaxation longitudinale) : les régions à temps de relaxation spin-milieu (T.,) courts apparaissent comme les plus intenses sur l'image [15]. Lorsque le temps de répétition est suffisamment long pour permettre à la relaxation longitudinale de s'effectuer totalement, la pondération du signal dépend des temps de relaxation spin-spin : les régions les plus intenses sont celles qui présentent les temps de relaxation spin-spin (T2) les plus longs.…”

Section: Séquences D'imagerieunclassified

“…Les techniques d'imagerie par réso-nance magnétique s'avèrent être des outils adaptés au diagnostic pré-coce des lésions radioinduites [6]. Les lésions superficielles et profondes induites localement [8,10] se traduisent habituellement par des modifications des temps de relaxation magnétique nucléaire qui jouent le rôle de facteur de contraste des images [15]. Après irradiation, l'apparition d'un hypersignal en profondeur, observé expérimentalement dans les muscles squelettiques de porc et de lapin a déjà pu être corrélée à la dose de rayonnement reçu [5,11].…”

Section: Introductionunclassified

Mise en évidence des effets d'une irradiation aiguë localisée par imagerie de résonance magnétique du tissu cutané

et al. 1993

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Over-exposures with gamma rays of a 192 Ir source performed on pig skin, have been followed by magnetic resonance imaging. A group of 3 minipigs was given single doses of 96, 32 and 64 Gy applied to the back skin (between 17 and L4) from bottom to neck respectively. NMR images were taken from a Siemens Magnetom 63 SP system operating at 1.5 tesla. T 2 -weighted spin echo images showed that signal intensities in the irradiated areas were significantly decreased during the three weeks following irradiation. The signal evolution was similar in skeletal striated muscle, subcutaneous adipose tissue and skin but with noticeably different amplitudes. The larger variation was observed on skin. This observation, which can be done in the early days after the exposure, suggests that the method could be an important aid to diagnose or to predict tissue necrosis and fibrosis.

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Mechanisms of Contrast in NMR Imaging

Cited by 115 publications

References 0 publications

Characterization of Experimental Ischemic Brain Edema Utilizing Proton Nuclear Magnetic Resonance Imaging

Characterization of Experimental Ischemic Brain Edema Utilizing Proton Nuclear Magnetic Resonance Imaging

Magnetic resonance microscopy of changes in water content in stems of transpiring plants.

Mise en évidence des effets d'une irradiation aiguë localisée par imagerie de résonance magnétique du tissu cutané

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