Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2003-02-12
2004-10-12
Arana, Louis (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000
Reexamination Certificate
active
06803762
ABSTRACT:
This disclosure relates to: a pulse train comprising at least an a high-frequency pulse, a preceding 180° pulse, or a preceding 180° and a 90° pulse that precedes the 180° pulse, a slice selection and a k-space line coding and an acquisition module subsequent thereto; a nuclear magnetic resonance scanner; and an imaging method.
Already at the time when the method of magnetic resonance imaging (MRI) was invented, it was anticipated that this method would allow more than just simple, qualitative imaging and that it would, in fact, be suitable for generating quantitative imaging. On the one hand, MRI is a mature method that is used on a daily basis in clinical imaging for simple, qualitative image depictions. On the other hand, MRI is a very important instrument for science and industry in a wide array of application areas such as quality control, pre-clinical evaluation of drugs in the pharmaceutical industry as well as in the determination of the pore size in rock samples in the petrochemical industry. Quantitative imaging is needed for rock samples and this is also done. The MRI signals are rendered more sensitive or weighted by means of appropriate and controlled manipulation of the appertaining parameters such as pulse trains, in order to show the influence of selected parameters. Generally speaking, when a series of differently weighted images are acquired and when suitable models are employed, it is possible to generate quantitative representations of the selected parameters. In this manner, quantitative images of samples can be created for purposes of determining a certain parameter such as diffusion, proton density or spin-lattice relaxation time.
The term “sample” in the case at hand here should be construed in its broadest sense and it encompasses both living and non-living material.
Various methods are known in which a sample is examined by means of an excitation pulse and several rephasing pulses.
In the method of this type, the sample is excited by means of electromagnetic radiation at an energy level that is suitable for the excitation.
It is known procedure in nuclear magnetic resonance tomography to obtain information about a sample by exciting echo signals of the sample.
In nuclear magnetic resonance tomography, atom nuclei possessing a magnetic moment are aligned by applying an external magnetic field, a process in which the nuclei execute a precessional motion having a characteristic circular frequency (Larmor frequency) around the direction of the magnetic field. The Larmor frequency is a function of the strength of the magnetic field and of the magnetic properties of the substance, especially of the gyromagnetic constant &ggr; of the nucleus. The gyromagnetic constant &ggr; is a parameter that is characteristic for each type of atom. The atom nuclei have a magnetic moment &mgr;=&ggr;×p wherein p stands for the spin of the nucleus.
A substance to be examined or a person to be examined is exposed to a uniform magnetic field during nuclear magnetic resonance tomography. The uniform magnetic field is also referred to as the polarization field B
0
, and the axis of the uniform magnetic field as the z-axis. The individual magnetic moments of the spin in the tissue precede with their characteristic Larmor frequency around the axis of the uniform magnetic field.
A net magnetization M
z
is generated in the direction of the polarization field, whereby the randomly oriented magnetic components cancel each other out in the plane perpendicular thereto (x-y plane). After the uniform magnetic field has been applied, an excitation field B
1
is additionally generated. The excitation field B
1
is polarized in the x-y plane and displays a frequency that is as close as possible to the Larmor frequency. As a result, the net magnetic moment M
z
can be tilted into the x-y plane, so that a transverse magnetization M
t
is created. The transverse component of the magnetization rotates in the x-y plane with the Larmor frequency.
Through a variation of the excitation field over the course of time, differing time sequences can be generated for the transverse magnetization. Various slice profiles can be realized in conjunction with at least one applied gradient field.
NMR imaging methods select slices or volumes that yield a measuring signal under the appropriate emission of high-frequency pulses and under the application of magnetic gradient fields; this measuring signal is digitized and stored as a one-dimensional or multi-dimensional field in a measuring computer.
This multidimensional field resulting from the measurement can be depicted in a spatial frequency space, the k-space. The coordinates of this spatial frequency space result from
k
=−&ggr;∫
G
dt. The outer area of the k-space defines the structures of the reconstructed image while the inner area defines the contrast.
The desired image information is then acquired (reconstructed) from the gathered raw data by means of one-dimensional or multi-dimensional Fourier transformation. Before that, there could be a need for the measured data of the multidimensional data field to be arranged in the data memory in such a way that the appertaining k-spaces yield the corresponding slices. Sorting procedures are implemented for this purpose.
A reconstructed slice image consists of pixels, and a volume data record consists of voxels. A pixel (picture element) is a two-dimensional image element, for instance, a square. The image is made up of pixels. A voxel (volume pixel) is a three-dimensional volume element, for instance, a right parallelepiped. The dimensions of a pixel are in the order of magnitude of 1 mm
2
, and those of a voxel are in the order of magnitude of 1 mm
3
. The geometries and extensions can vary.
Seeing that, for experimental reasons, it is never possible to assume a strictly two-dimensional plane in the case of slice images, the term voxel is often employed here as well, indicating that the image planes have a certain thickness.
Little attention has been paid to the representation of the spin-lattice relaxation time, T
1
, since most of the methods presented in the literature require long acquisition times that render these methods unusable for routine clinical examinations.
The advantage of the rapid data acquisition that was attained employing the rapid “Inversion-Recovery (Inversion—Relaxation) EPI (echo-planar imaging) Method” by R. J. Ordidge et al. in Magnetic Resonance in Medicine 16, 238-245 (1990) did not become well established because EPI is not a method in widespread use. In fact, inherent artifacts associated with this method have prevented the utilization of this otherwise elegant method. This is particularly true in those cases where imaging of the highest quality is needed, such as in the segmentation of the human brain. Other quantitative imaging methods (Deichmann et al. in Journal of Magnetic Resonance, 96, 608-612 (1992); Blüml et al., MRM 30, 289-295 (1993); Deichmann et al. in Magnetic Resonance in Medicine, 42: 206-209 (1999)) are slower than IR-EPI and are not fast enough to attain practical significance. The two approaches are based mainly on the spectroscopic Look-Locker methods Look DC and Locker DR (The Review of Scientific Instruments, volume 41, no. 2, 250-251, (1970)), which makes use of successive excitation pulses during a longitudinal relaxation so as to gather numerous points in time during the relaxation. A more effective time-representation scheme that neutralizes movement artifacts can be created in this manner.
The original “snapshot FLASH” method by Deichmann et al. in Journal of Magnetic Resonance, 96, 608-612 (1992) calls for long acquisition times since the initial magnetization has to be completely re-established. In the case of a high spatial resolution, the time resolution is markedly restricted. This especially has an effect in nuclear magnetic resonance scanners without a high-performance gradient system.
In nuclear magnetic resonance tomography of the brain, especially of the human brain, there is need to acquire measuring points over t
Shah Nadim Joni
Steinhoff Sven
Zaitsev Maxim
Arana Louis
Connolly Bove & Lodge & Hutz LLP
Forschungszentrum Jülich GmbH
Hume Larry J.
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