Functional MR imaging of the human brain using FLASH: influence of various imaging parameters.

In this study the influence of a large variety of imaging parameters on the signal increase (DeltaS) and the contrast-to-noise ratio (CNR) of functional magnetic resonance imaging experiments was determined using FLASH imaging at 2 T. During visual stimulation of the brain we detected significant variations of DeltaS as a function of the echo time (30 ms: 3.5 +/- 0.4%, 60 ms: 6.8 +/- 0.7%), slice thickness (2.5 mm: 6.8 +/- 0.7%, 10.0 mm: 3.3 +/- 0.3%), and pixel size (4.69 mm: 3.1 +/- 0.3%, 1.88 mm: 5.9 +/- 0.5%). Significant changes of DeltaS with flip angle occurred for TE = 20 ms (15 degrees : 2.1 +/- 0.2%, 60 degrees : 3.2 +/- 0.5%). At TE = 30 ms there still was a slight increase (15 degrees : 3.0 +/- 0.4%, 60 degrees : 3.8 +/- 0.5%), while at TE = 50 ms no changes of DeltaS could be detected with flip angle. Furthermore, DeltaS decreased with the use of first-order flow and motion compensation (off: 5.8 +/- 0.6%, on: 4.5 +/- 0.5%). The purpose of this study was to identify the optimal imaging parameters for blood oxygenation level dependent contrast using FLASH imaging at 2 T. Relying on a time normalized contrast-to-noise ratio (CNR(n)) we found the following parameters to be optimal: TE approximately 40-50 ms, a rather low spatial resolution (slice thickness approximately 5.0-7.5 mm, pixel size approximately 2.3-4.6 mm, matrix size 64 x 48), and flip angles lower than 30 degrees. Flow compensation should not be applied, and a rather low bandwidth of approximately 2.5 kHz is favorable, as it yields a superior signal-to-noise ratio.

[1]  J. Frahm,et al.  Functional MRI of human brain activation at high spatial resolution , 1993, Magnetic resonance in medicine.

[2]  D. Ortendahl,et al.  Measuring signal-to-noise ratios in MR imaging. , 1989, Radiology.

[3]  R. Turner,et al.  Dynamic magnetic resonance imaging of human brain activity during primary sensory stimulation. , 1992, Proceedings of the National Academy of Sciences of the United States of America.

[4]  R. Turner,et al.  Functional mapping of the human visual cortex at 4 and 1.5 tesla using deoxygenation contrast EPI , 1993, Magnetic resonance in medicine.

[5]  K. Uğurbil,et al.  Spatial and temporal differentiation of fMRI BOLD response in primary visual cortex of human brain during sustained visual simulation , 1998, Magnetic resonance in medicine.

[6]  Ravi S. Menon,et al.  Functional brain mapping by blood oxygenation level-dependent contrast magnetic resonance imaging. A comparison of signal characteristics with a biophysical model. , 1993, Biophysical journal.

[7]  P T Fox,et al.  Quantitative assessment of blood inflow effects in functional MRI signals , 1996, Magnetic resonance in medicine.

[8]  R S Menon,et al.  Investigation of BOLD contrast in fMRI using multi‐shot EPI , 1997, NMR in biomedicine.

[9]  D. Tank,et al.  4 Tesla gradient recalled echo characteristics of photic stimulation‐induced signal changes in the human primary visual cortex , 1993 .

[10]  S. Ogawa.  Brain magnetic resonance imaging with contrast-dependent oxygenation , 1990 .

[11]  M. Raichle,et al.  Focal physiological uncoupling of cerebral blood flow and oxidative metabolism during somatosensory stimulation in human subjects. , 1986, Proceedings of the National Academy of Sciences of the United States of America.

[12]  S. Ogawa,et al.  BOLD Based Functional MRI at 4 Tesla Includes a Capillary Bed Contribution: Echo‐Planar Imaging Correlates with Previous Optical Imaging Using Intrinsic Signals , 1995, Magnetic resonance in medicine.

[13]  R. Deichmann,et al.  Signal intensities in FLASH-EPI-hybrid sequences. , 1999, Journal of magnetic resonance.

[14]  E. Haacke,et al.  Identification of vascular structures as a major source of signal contrast in high resolution 2D and 3D functional activation imaging of the motor cortex at l.5T preliminary results , 1993, Magnetic resonance in medicine.

[15]  K. Uğurbil,et al.  Experimental determination of the BOLD field strength dependence in vessels and tissue , 1997, Magnetic resonance in medicine.

[16]  J H Duyn,et al.  Inflow versus deoxyhemoglobin effects in bold functional MRI using gradient echoes at 1.5 T , 1994, NMR in biomedicine.

[17]  M. Raichle,et al.  Stimulus rate dependence of regional cerebral blood flow in human striate cortex, demonstrated by positron emission tomography. , 1984, Journal of neurophysiology.

[18]  Adrian T. Lee,et al.  Discrimination of Large Venous Vessels in Time‐Course Spiral Blood‐Oxygen‐Level‐Dependent Magnetic‐Resonance Functional Neuroimaging , 1995, Magnetic resonance in medicine.

[19]  Christopher G. Thomas,et al.  Amplitude response and stimulus presentation frequency response of human primary visual cortex using BOLD EPI at 4 T , 1998, Magnetic resonance in medicine.

[20]  D. Tank,et al.  Brain magnetic resonance imaging with contrast dependent on blood oxygenation. , 1990, Proceedings of the National Academy of Sciences of the United States of America.

[21]  X. Hu,et al.  Reduction of signal fluctuation in functional MRI using navigator echoes , 1994, Magnetic resonance in medicine.

[22]  Ravi S. Menon,et al.  Intrinsic signal changes accompanying sensory stimulation: functional brain mapping with magnetic resonance imaging. , 1992, Proceedings of the National Academy of Sciences of the United States of America.

[23]  R Pohmann,et al.  Theoretical evaluation and comparison of fast chemical shift imaging methods. , 1997, Journal of magnetic resonance.

[24]  Xiaoping Hu,et al.  Potential pitfalls of functional MRI using conventional gradient‐recalled echo techniques , 1994, NMR in biomedicine.

[25]  X. Hu,et al.  Fast interleaved echo‐planar imaging with navigator: High resolution anatomic and functional images at 4 tesla , 1996, Magnetic resonance in medicine.

[26]  S. Ogawa,et al.  The sensitivity of magnetic resonance image signals of a rat brain to changes in the cerebral venous blood oxygenation , 1993, Magnetic resonance in medicine.

[27]  R Deichmann,et al.  Calculation of signal intensities in hybrid sequences for fast NMR imaging , 1995, Magnetic resonance in medicine.

[28]  Jean A. Tkach,et al.  2D and 3D high resolution gradient echo functional imaging of the brain: Venous contributions to signal in motor cortex studies , 1994, NMR in biomedicine.

[29]  A. Kleinschmidt,et al.  Brain or veinoxygenation or flow? On signal physiology in functional MRI of human brain activation , 1994, NMR in biomedicine.

[30]  R. Buxton,et al.  Dynamics of blood flow and oxygenation changes during brain activation: The balloon model , 1998, Magnetic resonance in medicine.

[31]  R L Ehman,et al.  Imaging of cerebral activation at 1.5 T: optimizing a technique for conventional hardware. , 1994, Radiology.