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ABSTRACT: The aim of the study was to determine and compare the areas of brain activated in response to colorectal distention (CRD) using functional magnetic resonance imaging (fMRI) and c-fos protein expression.
For fMRI study (3.0 T magnet), anaesthetized rats underwent phasic CRD, synchronized with fMRI acquisition. Stimulation consisted of eight cycles of balloon deflation (90 s) and inflation (30 s), at 40, 60 or 80 mmHg of pressure. For c-fos study two sets of experiments were performed on anaesthetized rats: comparing (A) brain activation in rats with the inserted colorectal balloon (n = 5), to the rats without the balloon (n = 5); and (B) rats with inserted balloon (n = 10), to the rats with inserted and distended balloon (n = 10). The pressure of 80 mmHg was applied for 2 h of 30 s inflation and 90 s deflation, alternating cycles.
Functional MRI revealed significant activation in the amygdala, hypothalamus, thalamus, cerebellum and hippocampus. Significant increase in c-fos expression was observed in amygdala and thalamus in the first set of experiments, and hypothalamus and parabrachial nuclei in the second.
The two methods are not interchangeable but appeared to be complementary: fMRI was more sensitive, whereas c-fos had much greater resolution.
Neurogastroenterology and Motility 09/2005; 17(4):548-56. · 3.41 Impact Factor
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ABSTRACT: A numerical model of a female body is developed to study the effects of different body types with different coil drive methods on radio-frequency magnetic (B 1) field distribution, specific energy absorption rate (SAR), and intrinsic signal-to-noise ratio (ISNR) for a body-size birdcage coil at 64 and 128 MHz. The coil is loaded with either a larger, more muscular male body model (subject 1) or a newly developed female body model (subject 2), and driven with two-port (quadrature), four-port, or many (ideal) sources. Loading the coil with subject 1 results in significantly less homogeneous B 1 field, higher SAR, and lower ISNR than those for subject 2 at both frequencies. This dependence of MR performance and safety measures on body type indicates a need for a variety of numerical models representative of a diverse population for future calculations. The different drive methods result in similar B 1 field patterns, SAR, and ISNR in all cases.
Applied Magnetic Resonance 03/2005; 29(1):5-18. · 0.75 Impact Factor
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C M Collins,
Q X Yang,
J H Wang,
X Zhang,
H Liu,
S Michaeli,
X-H Zhu,
G Adriany,
J T Vaughan,
P Anderson,
H Merkle,
K Ugurbil,
M B Smith,
W Chen
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ABSTRACT: Calculations and experiments were used to examine the B(1) field behavior and signal intensity distribution in a 16-cm diameter spherical phantom excited by a 10-cm diameter surface coil at 300 MHz. In this simple system at this high frequency very complex RF field behavior exists, resulting in different excitation and reception distributions. Included in this work is a straightforward demonstration that coil receptivity is proportional to the magnitude of the circularly polarized component of the B(1) field that rotates in the direction opposite to that of nuclear precession. It is clearly apparent that even in very simple systems in head-sized samples at this frequency it is important to consider the separate excitation and reception distributions in order to understand the signal intensity distribution.
Magnetic Resonance in Medicine 06/2002; 47(5):1026-8. · 2.96 Impact Factor
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C.M. Collins,
Q.X. Yang,
J.H. Wang,
X. Zhang,
H. Liu,
S. Michaeli,
X.-H. Zhu,
G. Adriany,
J.T. Vaughan,
P. Anderson,
H. Merkle,
K. Ugurbil,
M.B. Smith,
W. Chen
[show abstract]
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ABSTRACT: Calculations and experiments were used to examine the B1 field behavior and signal intensity distribution in a 16-cm diameter spherical phantom excited by a 10-cm diameter surface coil at 300 MHz. In this simple system at this high frequency very complex RF field behavior exists, resulting in different excitation and reception distributions. Included in this work is a straightforward demonstration that coil receptivity is proportional to the magnitude of the circularly polarized component of the B1 field that rotates in the direction opposite to that of nuclear precession. It is clearly apparent that even in very simple systems in head-sized samples at this frequency it is important to consider the separate excitation and reception distributions in order to understand the signal intensity distribution. Magn Reson Med 47:1026–1028, 2002. © 2002 Wiley-Liss, Inc.
Magnetic Resonance in Medicine 04/2002; 47(5):1026 - 1028. · 2.96 Impact Factor
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ABSTRACT: A new coil design for simultaneous multiple animal imaging based
on the coaxial cavity is presented. The goal was to improve
B<sub>1</sub> field (radiofrequency magnetic field that is used to
excite the spins) homogeneity, while retaining high signal to noise
ratio (SNR) of the coaxial cavity coil. The difference in B<sub>1</sub>
field magnitude across the coil, measured experimentally as well as the
one predicted on the basis of numerical calculation, is within 10%. The
described design can be utilized to image 6 small animals simultaneously
without great compromise in SNR
Bioengineering Conference, 2002. Proceedings of the IEEE 28th Annual Northeast; 02/2002
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ABSTRACT: Four different end ring configurations of a birdcage coil were
numerically modeled to examine their effects on homogeneity and signal
to noise ratio (SNR) in the coil at 64 MHz. The end ring configuration
with the best homogeneity in the empty coil produces the least
homogeneous B1 field when the coil is loaded with a human head. The coil
configuration that results in the best homogeneity in the loaded
birdcage also has the highest SNR
Bioengineering Conference, 2002. Proceedings of the IEEE 28th Annual Northeast; 02/2002
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J T Vaughan,
M Garwood, C M Collins,
W Liu,
L DelaBarre,
G Adriany,
P Andersen,
H Merkle,
R Goebel,
M B Smith,
K Ugurbil
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ABSTRACT: Signal-to-noise ratio (SNR), RF field (B(1)), and RF power requirement for human head imaging were examined at 7T and 4T magnetic field strengths. The variation in B(1) magnitude was nearly twofold higher at 7T than at 4T ( approximately 42% compared to approximately 23%). The power required for a 90 degrees pulse in the center of the head at 7T was approximately twice that at 4T. The SNR averaged over the brain was at least 1.6 times higher at 7T compared to 4T. These experimental results were consistent with calculations performed using a human head model and Maxwell's equations. Magn Reson Med 46:24-30, 2001.
Magnetic Resonance in Medicine 08/2001; 46(1):24-30. · 2.96 Impact Factor
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ABSTRACT: Technical limitations imposed by resolution and B1 homogeneity have thus far limited quantitative in vivo T2 mapping of cartilage to the patella. The purpose of this study is to develop T2 mapping of the femoral/tibial joint and assess regional variability of cartilage T2 in the knee. Quantitative in vivo T2 mapping of the knee was performed on 15 asymptomatic adults (age, 22-44) using a 3T MR scanner. There is a consistent pattern of spatial variation in cartilage T2 with longer values near the articular surface. The greatest variation occurs in the patella, where T2 increases from 45.3 +/- 2.5 msec at a normalized distance of 0.33-67 +/- 5.5 msec at a distance of 1.0. These results demonstrate feasibility of performing in vivo T2 mapping of femoral tibial cartilage. Except for the superficial 15% where T2 values are lower, the spatial variation in T2 of femoral and tibial cartilage is similar to patellar cartilage.
Journal of Magnetic Resonance Imaging 08/2001; 14(1):50-5. · 2.70 Impact Factor
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J.T. Vaughan,
M. Garwood, C.M. Collins,
W. Liu,
L. DelaBarre,
G. Adriany,
P. Andersen,
H. Merkle,
R. Goebel,
M.B. Smith,
K. Ugurbil
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ABSTRACT: Signal-to-noise ratio (SNR), RF field (B1), and RF power requirement for human head imaging were examined at 7T and 4T magnetic field strengths. The variation in B1 magnitude was nearly twofold higher at 7T than at 4T (∼42% compared to ∼23%). The power required for a 90° pulse in the center of the head at 7T was approximately twice that at 4T. The SNR averaged over the brain was at least 1.6 times higher at 7T compared to 4T. These experimental results were consistent with calculations performed using a human head model and Maxwell's equations. Magn Reson Med 46:24–30, 2001. © 2001 Wiley-Liss, Inc.
Magnetic Resonance in Medicine 06/2001; 46(1):24 - 30. · 2.96 Impact Factor
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ABSTRACT: Calculations of the radiofrequency magnetic (B(1)) field, SAR, and SNR as functions of frequency between 64 and 345 MHz for a surface coil against an anatomically-accurate human chest are presented. Calculated B(1) field distributions are in good agreement with previously-published experimental results up to 175 MHz, especially considering the dependence of field behavior on subject anatomy. Calculated SNR in the heart agrees well with theory for low frequencies (nearly linear increase with B(0) field strength). Above 175 MHz, the trend in SNR with frequency begins to depend largely on location in the heart. At all frequencies, present limits on local (1 g) SAR levels are exceeded before limits on whole-body average limits. At frequencies above 175 MHz, limits on SAR begin to be an issue in some common imaging sequences. These results are relevant for coils and subjects similar to those modeled here. Magn Reson Med 45:692-699, 2001.
Magnetic Resonance in Medicine 05/2001; 45(4):692-9. · 2.96 Impact Factor
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ABSTRACT: Calculations of the RF magnetic (B(1)) field as a function of frequency between 64 and 345 MHz were performed for a head model in an idealized birdcage coil. Absorbed power (P(abs)) and SNR were calculated at each frequency with three different methods of defining excitation pulse amplitude: maintaining 90 degrees flip angle at the coil center (center alpha = pi/2), maximizing FID amplitude (Max. A(FID)), and maximizing total signal amplitude in a reconstructed image (Max. A(image)). For center alpha = pi/2 and Max. A(image), SNR increases linearly with increasing field strength until 260 MHz, where it begins to increase at a greater rate. For these two methods, P(abs) increases continually, but at a lower rate at higher field strengths. Above 215 MHz in MRI of the human head, the use of FID amplitude to set B(1) excitation pulses may result in apparent decreases in SNR and power requirements with increasing static field strength. Magn Reson Med 45:684-691, 2001.
Magnetic Resonance in Medicine 05/2001; 45(4):684-91. · 2.96 Impact Factor
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ABSTRACT: Calculations of radiofrequency magnetic (B1) field and specific energy absorption rate (SAR) distributions in a sphere of tissue and a multi-tissue human head model in a 12-element birdcage coil are presented. The coil model is driven in linear and quadrature modes at 63, 175, 200, and 300 MHz. Plots of B1 field magnitude and SAR distributions, average SAR, maximum local SAR, and measures of B1 field homogeneity and signal-to-noise ratio are given. SAR levels for arbitrary pulse sequences can be estimated from the calculated data. Maximum local SAR levels are lower at lower frequencies, in quadrature rather than in linear coils, and in linear fields oriented posterior-to-anterior rather than left-to-right in the head. It should be possible to perform many experiments in the head at frequencies up to 300 MHz without exceeding standard limits for local or average SAR levels.
Magnetic Resonance in Medicine 01/1999; 40(6):847-56. · 2.96 Impact Factor
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ABSTRACT: The electromagnetic fields induced by a surface coil in a spherical phantom, having a wide range of electrical properties, is studied using numerical methods of calculation. The specific absorption rate (SAR), radiofrequency magnetic field (B1), magnetic field energy within the phantom (EB), and the volume-averaged SAR ( ) are calculated at 10, 63, and 200 MHz. They are analyzed with respect to dielectric constant, wavelength, and skin depth effects, which become increasingly important in high field magnetic resonance imaging (MRI) where safety and field homogeneity issues need further study. Particular attention is given to solutions representing neural tissue at each frequency. In general, the data at high field strengths have local maxima, with a quasi-harmonic behavior, when the following two resonant conditions are satisfied: 1) skin depth becomes comparable to, or larger than, the sample diameter Ds; and 2) Ds is near an integral multiple of the wavelength. These are also the solutions with maximum EB values and the least homogeneous B1. Samples undergoing resonance at 200 MHz are shown to have important off-axis B1 maxima (affecting field homogeneity) and large values. Some non-resonating 200-MHz phantoms, including simulations consistent with neural tissue, contain larger SAR maxima than the resonating samples, posing safety concerns in high field imaging of biologic tissue.
Magnetic Resonance Imaging 09/1998; 16(7):787-98. · 1.99 Impact Factor
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ABSTRACT: Results from numerical calculations of the radiofrequency
electromagnetic fields induced in the human head during magnetic
resonance imaging (MRI) of the head at frequencies from 63 to 200 MHz
are presented. Numerical values are given in a form that can easily be
used to determine specific SAR levels for any MRI procedures at these
frequencies. Maximum local SAR levels are lower at lower frequencies, in
circularly- than in linearly-polarized fields, and in linearly-polarized
fields polarized posterior-to-anterior than left-to-right in the head
model. Magnetic field homogeneity and signal-to-noise ratio are greater
at lower frequencies and in circularly-polarized fields
Bioengineering Conference, 1998. Proceedings of the IEEE 24th Annual Northeast; 05/1998
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ABSTRACT: The design and performance of an inductively fed low-pass birdcage radiofrequency (RF) coil for applications at 9.4 T are described where tuning is accomplished by mechanically moving a concentric RF shield about the longitudinal axis of an RF coil. Moving the shield about the RF coil effectively changes the mutual inductance of the system, providing a mechanism for adjusting the resonant frequency. RF shield tuning eliminates adjustable capacitors on the legs of the RF coil, eliminates current imbalances and field distortions, and results in improved B1 field homogeneity and high quality (Q) factors. RF shield tuning and inductive matching provide an isolated resonance structure which is both physically and electrically unattached. Experimental analysis of shield position on both B1 field homogeneity and resonant frequency is provided. Computer simulations of B1 field homogeneity as a function of shield position and shield diameter are also presented. Magnetic resonance microimaging substantiates the usefulness of this design.
Journal of Magnetic Resonance 03/1998; 131(1):32-8. · 2.14 Impact Factor
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ABSTRACT: For a birdcage coil with elliptical cross-section, a sinusoidal current pattern does not provide a homogeneous B1 field. A simple theory was developed to create an optimized current distribution for elliptical birdcage coils. This optimized current pattern can create a perfectly homogeneous B1 field inside any elliptical shape. To verify the theory, a 16-element high-pass elliptical birdcage coil was built inside a circular RF shield. The current was optimized by using the inductance characteristics of the coil components to calculate the end-ring capacitances. The B1 field was theoretically calculated and experimentally mapped for the optimized elliptical bird-cage coil and a nonoptimized coil. The results demonstrate that by optimizing the current distribution, a very homogeneous B1 field is produced. This method can be directly applied in design and construction of elliptical birdcage coils for imaging of the naturally occurring elliptical cross-sectional geometries in the human body.
Magnetic Resonance in Medicine 05/1997; 37(4):600-8. · 2.96 Impact Factor
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ABSTRACT: Finite element analysis was used to calculate the static magnetic field within the three-dimensional head model. Localized field distributions were evaluated by using the magnetic field histogram technique. Experimental field maps and histograms of the human head were also obtained to validate the simulation results. Field deviations and gradients inside the human head cause NMR signal frequency shifts and line broadening, respectively. Voxels 2 x 2 x 0.5 cm may have frequency differences of more than 2.0 ppm. The linewidth of a single voxel may be broadened by more than 0.5 ppm. Calculated and experimental field maps are in excellent agreement. The global field distortion in the human head is primarily due to the susceptibility difference between air and tissues and their corresponding geometrical shapes.
Magnetic Resonance in Medicine 12/1996; 36(5):705-14. · 2.96 Impact Factor
