MRI imaging methods using a single excitation

Details MRI imaging methods using a single excitation MRI imaging methods using a single excitation

2001-10-15

2003-09-09

Lateef, Marvin M. (Department: 3737)

Surgery

Diagnostic testing

Detecting nuclear, electromagnetic, or ultrasonic radiation

C324S312000, C324S316000, C324S319000, C382S128000, C382S131000

Reexamination Certificate

active

06618607

ABSTRACT:

FIELD OF THE INVENTION
The present invention is related to functional magnetic resonance imaging (fMRI) methods.
BACKGROUND OF THE INVENTION
Functional magnetic resonance imaging (fMRI) has been used widely in brain imaging studies for the past several years. See e.g., J. W. Belliveau, D. N. Kennedy, R. C. McKinstry, B. R. Buchbinder, R. M. Weisskoff, M. S. Cohen, J. M. Vevea, T. J. Brady and B. R. Rosen, “Functional mapping of the human visual cortex by magnetic resonance imaging,”
Science
254, 716-719, 1991; K. K. Kwong, J. W. Belliveau, D. A. Chesler, I. E. Goldberg, R. M. Weisskoff, B. P. Poncelet, D. N. Kennedy, B. E. Hoppel, M. S. Cohen, R. Turner, H.-M. Cheng, T. J. Brady and B. R. Rosen, “Dynamic magnetic resonance imaging of human brain activity during primary sensory stimulation,”
Proc. Natl. Acad. Sci. USA
89, 5675-5679, 1992; P. A. Bandettini, E. C. Wong, R. S. Hinks, R. S. Tikofsky, and J. S. Hyde, “Time course EPI of human brain function during task activation,”
Magn. Reson. Med.
25, 390-397, 1992; S. Ogawa, D. W. Tank, R. Menon, J. M. Ellerman, S.-G. Kim, H. Merkle, and K. Ugurbil, “Intrinsic signal changes accompanying sensory stimulation: functional brain mapping with magnetic resonance imaging,”
Proc. Natl. Acad. Sci. USA
89, 5951-5955, 1992; and R. Menon, S. Ogawa, D. W. Tank, and K. Ugurbil, “4 Tesla gradient recalled echo charateristics of photic stimulation-induced signal changes in the human primary visual cortex,”
Magn. Reson. Med.
30, 380-386, 1993. One of the most often used methods is the gradient-recalled echo-planar imaging (EPI) technique because of its good sensitivity to the blood oxygenation level dependent signal and high speed. See S. Ogawa, R. S. Menon, D. W. Tank, S.-G. Kim, H. Merkle, J. M. Ellerman and K. Ugurbil, “Functional brain mapping by blood oxygenation level-dependent contrast magnetic resonance imaging,”
Biophys. J.
64, 803-812, 1993. However, its usage is limited in areas with severe static inhomogeneity induced by susceptibility effect near air/tissue interfaces.
One potential problem in gradient-recalled EPI using a long echo time is the severe signal losses at areas with large static inhomogeneities. These areas include the ventral frontal, medial temporal and inferior temporal regions that experience inhomogeneities induced by the susceptibility effects near air/tissue interfaces. For fMRI studies that use both the gradient-recalled EPI and high field scanners, these signal losses may prevent investigation of the human cognitive processes such as the memory and attention studies. Methods have been developed to recover the signal losses, however, these methods typically involved multiple excitations, thus, compromising the temporal resolution.
Susceptibility artifacts can be manifested primarily in two ways: signal losses and geometric distortions. In general, a long echo time makes an MRI system more prone to signal losses in the presence of an inhomogeneous field because of the intra-voxel dephasing, and a long readout time typically leads to geometric distortions due to the reduced sampling frequency and reduced readout gradient strength. Typically, pronounced susceptibility-related field variation along the slice-selective direction in combination with a long echo time results in severe signal losses, while the inhomogeneity in-plane combined with a long readout time leads to geometric distortions. Thus, signal losses can be caused by the susceptibility-induced gradient along the slice-selective direction. Because of the long echo time typically used in fMRI experiments to maximize the sensitivity toward the signal changes, the signal losses at areas near air/tissue interfaces may be severe. Refined methods to recover these signals may be needed in order to study brain function at these areas.
Several research groups have addressed these sorts of signal losses using various techniques. One such technique is to use a thinner slice thickness to reduce the field change across the slice. See I. R. Young, I. J. Cox, D. J. Bryant, and G. M. Bydder, “The benefits of increasing spatial resolution as a means of reducing artifacts due to field inhomogeneities,”
Magn. Reson. Imag.
6, 585-590, 1988. This technique may be relatively easy to implement but it may reduce SNR as well as the spatial coverage per unit time.
Frahm et al. originally proposed to use multiple refocusing gradients to effectively compensate the field inhomogeneities. J. Frahm, K. D. Merboldt, W. Hanicke, “Direct FLASH MR imaging of magnetic field inhomogeneities by gradient compensation,”
Magn. Reson. Med.
6, 474-480, 1988. This method was later adopted by several other groups and applied more recently in functional MRI. See e.g., R. J. Ordidge, J. M. Gorell, J. C. Deniau, R. A. Knight, J. A. Helpern, “Assessment of relative brain iron concentrations using T
2
-weighted and T
2
*-weighted MRI at 3 Tesla,”
Magn. Reson. Med.
32, 335-341, 1994; R. T. Constable, “Functional MR imaging using gradient-echo echo-planar imaging in the presence of large static field inhomogeneities,”
J Magn. Reson. Im.
5, 746-752, 1995; Q. X. Yang, B. J. Dardzinski, S. Li, P. J. Eslinger, M. B. Smith, “Multi-gradient echo with susceptibility compensation (MGESIC): demonstration of fMRI in the olfactory cortex at 3T,”
Magn. Reson. Med.
37, 331-335, 1997; R. T. Constable, D. D. Spencer, “Composite image formation in z-shimmed functional MR imaging,”
Magn. Reson. Med.
42, 110-117, 1999; and V. A. Stenger, F. E. Boada, and D. C. Noll, “Gradient compensation method for the reduction of susceptibility artifacts for spiral fMRI data acquisition,”
Proc. ISMRM,
p. 538, 1999.
Because the superimposed gradient field across the slice is often not linear, one compensatory gradient is generally not sufficient to compensate the entire slice. Thus, a set of linear gradients is typically needed to compensate the nonlinear field segment-by-segment to achieve satisfactory results. When the number of the linear gradients increase, i.e., increments become finer, the nonlinear field can be better compensated. Despite the effectiveness in recovering signal, the time-consuming nature of such techniques may limit their practical value in routine fMRI experiments. In practice, as many as sixteen repetitions may be needed to sum up to a uniform image. Most of cognitive fMRI experiments cannot be performed this way.
More sophisticated methods were also proposed that showed promise in reducing the number of compensating gradients to a much more tolerable level. The efficiency is much increased by using high-order field compensation. Cho et al. proposed tailored pulse with a quadratic profile that has shown improved tolerance toward field inhomogeneity. Z. H. Cho, and Y. M. Ro, “Reduction of susceptibility artifact in gradient-echo imaging,”
Magn. Reson. Med.
23, 193-196, 1992. Glover et al. also presented a method using high order phase compensation by obtaining a field profile for each subject and incorporating it into the phase profile of the excitation pulse. G. Glover, S. Lai, “Reduction of susceptibility effects in fMRI using tailored RF pulses,”
Proc. ISMRM,
p.298, 1998. A similar concept was used in a recent report using a two-shot technique with explicitly matched RF excitation. N. K. Chen, A. M. Wyrwicz, “Removal of intravoxel dephasing artifact in gradient-echo images using a field-map based RF refocusing technique,”
Magn. Reson. Med.
42, 807-812, 1999. Another recent method used a two-shot technique combining a quadratic excitation pulse and the compensatory gradient. J. Mao, and A. W. Song, “Intravoxel rephasing of spins dephased by susceptibility effect for EPI sequences,”
Proc. ISMRM,
p.1982, 1999. The resultant phase profile can be used to better match the susceptibility-induced gradients when an appropriate compensatory gradient is used. The two excitations can be implemented back-to-back within one run to allow fMRI experiments to be carried out; however, the effective repetition time is still doubled. Images from the two excitations can then be combined to achieve

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