Patent Application
20090309593
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As is understood in the art, surfactants 29 are generally strongly hydrophilic (attracted to polar molecules such as water) on one end and strongly hydrophobic (attracted to non-polar molecules such as hydrocarbons) and in the present invention form a film separating the water 28 and non-hydrogen bearing oil 26 as will be described. In the present invention the surfactant 29 could be sodium octanoate (SOC), sodium decanoate (SDEC), sodium dodecanoate (DODEC), sodium dodecyl sulfate (SDS), sodium succinate or any other surfactant that can be used to create a micelle solution.
The water 28 is preferably doped with a paramagnetic ion and sodium chloride as in conventional phantoms, but the large quantity of non-hydrogen bearing oil 26 significantly reduces the total quantity of water, thereby reducing the dielectric constant of the overall phantom material 24 and thus reducing standing waves or dielectric resonance artifacts. Depending on the ratio of water 28 to oil or oil-like fluid 26, the solution could take on the form of a micelle or reverse micelle solution. In a micelle solution, the surfactant molecules tend to encapsulate the oil in tiny spherical globules in a surrounding matrix of water. In a reverse micelle solution, the surfactant tends to encapsulate the water in tiny spherical globules in the surrounding matrix of oil or oil-like fluid. The main difference between a reverse micelle and micelle phantom is that micelle phantoms have conductive pathways (through the water) that provide increased loading for the coil being tested. As such, the use of a micelle, rather than reverse micelle is preferred.
It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.
The present invention relates to magnetic resonance imaging (MRI) systems, and more particularly, relates to phantoms used to test the performance of MRI systems. Magnetic resonance imaging is used to generate medical diagnostic images by measuring faint radio frequency (RF) signals (magnetic resonance) emitted by atomic nuclei in tissue (for example, water protons) after radio frequency stimulation of the tissue in the presence of a strong magnetic field. The location of the precessing protons is made possible by the application of orthogonal magnetic gradient fields which serve to “encode” the spins according to frequency, phase, and/or slice. The combination of the radiofrequency stimulation and the applied gradient fields is termed a pulse sequence. The acquired signal from the spins (termed a nuclear magnetic resonance (NMR) signal) provides data in “k-space”, a mathematical construction in the frequency domain. A two-dimensional Fourier transform of the k-space data produces the actual image. It will be understood, therefore, that the k-space data does not represent the image itself, but represents the spectral components of the image with the center of k-space representing low frequency spatial components of the image, and the outer portions of k-space representing the high frequency spatial components of the image. The impressing of spatial location information onto the spins of the NMR signal by the applied magnetic gradients makes it extremely important that all applied magnetic fields (including the polarizing magnetic field B0 and the gradient magnetic fields Gx, Gy, and Gz) be well characterized. For this reason, and particularly for the B0 field, it is well known to incorporate shimming coils into the design of a magnetic resonance imaging machine which serve to correct for inhomogeneities in the B0 field through the application of one or more superimposed shimming fields. A number of techniques are known by which to measure inhomogeneities of the magnetic field and thus to calculate the currents needed for the shimming coils. For example, special pulse sequences detecting phase differences in the MRI measurements of a homogenous phantom, for example, a tank of water paramagnetic ion to shorten T1 and T2 and sodium chloride to provide the desired loading, may be used to deduce variations in the magnetic field of the MRI system. One problem in creating phantoms is that water has a high dielectric constant that shortens the wavelengths of radio frequency (RF) energy through the phantom, leading to standing waves and other resonance effects that degrade uniformity of the image at magnetic field strengths greater than 1.5 Tesla. One solution to this problem is the introduction of low dielectric constant material (such as decane) that essentially “dilutes” the water. Unfortunately, this approach, although correcting for standing waves and other resonance effects, does not provide the desired degree of electrical loading due to the relatively low conductivity of the solution. In addition, decane is a highly flammable material, which results in potentially hazardous conditions during the production and use of the solution. Furthermore, decane diffuses readily into most plastics (from which the phantom housing would be constructed). Finally, decane and water resonate at different frequencies which can create imaging problems with some pulse sequences, particularly in the presence of B0 inhomogeneities. Alternatively, phantoms prepared with oil rather than water have been utilized for testing magnetic field strengths of 3 Tesla. The use of oil effectively eliminates the resultant shading created by the dielectric resonance effects in water phantoms. However, oil phantoms do not effectively load the coils so performance assessment of the coil with images is sensitive to effects of normal manufacturing tolerances. The present inventor has recognized that the loading provided by a mixture of low dielectric constant material and water may be increased by promoting the formation of micelles in which islands of the low dielectric constant material are wholly surrounded by conductive water, providing an eddy current path for loading while reducing the average dielectric constant of the phantom. A non-hydrogen bearing oil or oil-like material may be used for the low dielectric material and a surfactant used to create the micelles. The resulting phantom solves the dielectric resonance problem while maintaining the ability to load the coil due to the presence of conductive pathways through the water.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US07/74574 | 7/27/2007 | WO | 00 | 1/28/2009 |
| Number | Date | Country | |
|---|---|---|---|
| 60833958 | Jul 2006 | US |
