Magnetization transfer or spin-lock? An investigation of off-resonance saturation pulse imaging with varying frequency offsets

John L. Ulmer, Vincent Mathews, Craig A. Hamilton, Allen D. Elster, Paul R. Moran

Research output: Contribution to journalArticle

55 Citations (Scopus)

Abstract

PURPOSE: To characterize near-resonance saturation pulse MR imaging on a 1.5-T scanner in order to gain insight into underlying mechanisms that alter tissue contrast and to optimize the technique for neuroimaging. METHODS: Off- resonance saturation pulses were applied to T1-weighted, spin-density- weighted, and T2-weighted sequences at frequency offsets ranging from 50 Hz to 20 000 Hz down field from water resonance. Suppression ratios were determined at each offset for phantom materials (MnCl2 solution, gadopentetate dimeglumine, corn oil, water, and agar), normal brain structures, and a variety of brain lesions. RESULTS: Signal suppression of MnCl2 on T1-weighted images occurred at offsets of less than 2000 Hz even though no macromolecules were present in the solution. Only those phantom materials and tissues with short or intermediate T1 relaxation times and relatively large T1/T2 ratios were sensitive to changing frequency offsets. Suppression of brain increased from approximately 20% at 2000 Hz offset to approximately 45% when the offset was reduced to 300 Hz. In human subjects, the net effect of reducing the frequency offset was to increase T2 contrast on T1-weighted, spin-density-weighted, and T2-weighted images. Distilled water and contrast material did not suppress except at very low offsets (2 indicates that magnetization transfer is not the sole mechanism of contrast in near- resonance saturation MR imaging. Spin-lock excitation can reasonably explain the behavior of the phantom solutions and the increase in T2 contrast of tissues achieved as the frequency offset is decreased from 2000 Hz to 300 Hz. Below 300 Hz, saturation is presumably caused by spin-tip effects. With our pulse design, an offset of 300 Hz is optimal for many routine clinical imaging examinations.

Original languageEnglish (US)
Pages (from-to)805-819
Number of pages15
JournalAmerican Journal of Neuroradiology
Volume17
Issue number5
StatePublished - 1996
Externally publishedYes

Fingerprint

Water
Brain
Gadolinium DTPA
Corn Oil
Neuroimaging
Contrast Media
Agar
manganese chloride

Keywords

  • Magnetic resonance, experimental
  • Magnetic resonance, magnetization transfer

ASJC Scopus subject areas

  • Clinical Neurology
  • Radiology Nuclear Medicine and imaging
  • Radiological and Ultrasound Technology

Cite this

Magnetization transfer or spin-lock? An investigation of off-resonance saturation pulse imaging with varying frequency offsets. / Ulmer, John L.; Mathews, Vincent; Hamilton, Craig A.; Elster, Allen D.; Moran, Paul R.

In: American Journal of Neuroradiology, Vol. 17, No. 5, 1996, p. 805-819.

Research output: Contribution to journalArticle

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N2 - PURPOSE: To characterize near-resonance saturation pulse MR imaging on a 1.5-T scanner in order to gain insight into underlying mechanisms that alter tissue contrast and to optimize the technique for neuroimaging. METHODS: Off- resonance saturation pulses were applied to T1-weighted, spin-density- weighted, and T2-weighted sequences at frequency offsets ranging from 50 Hz to 20 000 Hz down field from water resonance. Suppression ratios were determined at each offset for phantom materials (MnCl2 solution, gadopentetate dimeglumine, corn oil, water, and agar), normal brain structures, and a variety of brain lesions. RESULTS: Signal suppression of MnCl2 on T1-weighted images occurred at offsets of less than 2000 Hz even though no macromolecules were present in the solution. Only those phantom materials and tissues with short or intermediate T1 relaxation times and relatively large T1/T2 ratios were sensitive to changing frequency offsets. Suppression of brain increased from approximately 20% at 2000 Hz offset to approximately 45% when the offset was reduced to 300 Hz. In human subjects, the net effect of reducing the frequency offset was to increase T2 contrast on T1-weighted, spin-density-weighted, and T2-weighted images. Distilled water and contrast material did not suppress except at very low offsets (2 indicates that magnetization transfer is not the sole mechanism of contrast in near- resonance saturation MR imaging. Spin-lock excitation can reasonably explain the behavior of the phantom solutions and the increase in T2 contrast of tissues achieved as the frequency offset is decreased from 2000 Hz to 300 Hz. Below 300 Hz, saturation is presumably caused by spin-tip effects. With our pulse design, an offset of 300 Hz is optimal for many routine clinical imaging examinations.

AB - PURPOSE: To characterize near-resonance saturation pulse MR imaging on a 1.5-T scanner in order to gain insight into underlying mechanisms that alter tissue contrast and to optimize the technique for neuroimaging. METHODS: Off- resonance saturation pulses were applied to T1-weighted, spin-density- weighted, and T2-weighted sequences at frequency offsets ranging from 50 Hz to 20 000 Hz down field from water resonance. Suppression ratios were determined at each offset for phantom materials (MnCl2 solution, gadopentetate dimeglumine, corn oil, water, and agar), normal brain structures, and a variety of brain lesions. RESULTS: Signal suppression of MnCl2 on T1-weighted images occurred at offsets of less than 2000 Hz even though no macromolecules were present in the solution. Only those phantom materials and tissues with short or intermediate T1 relaxation times and relatively large T1/T2 ratios were sensitive to changing frequency offsets. Suppression of brain increased from approximately 20% at 2000 Hz offset to approximately 45% when the offset was reduced to 300 Hz. In human subjects, the net effect of reducing the frequency offset was to increase T2 contrast on T1-weighted, spin-density-weighted, and T2-weighted images. Distilled water and contrast material did not suppress except at very low offsets (2 indicates that magnetization transfer is not the sole mechanism of contrast in near- resonance saturation MR imaging. Spin-lock excitation can reasonably explain the behavior of the phantom solutions and the increase in T2 contrast of tissues achieved as the frequency offset is decreased from 2000 Hz to 300 Hz. Below 300 Hz, saturation is presumably caused by spin-tip effects. With our pulse design, an offset of 300 Hz is optimal for many routine clinical imaging examinations.

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