Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-03-09
2002-06-25
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S309000, C324S318000
Reexamination Certificate
active
06411088
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a magnetic resonance device which is able to obtain a transparency image of an examination subject. As used herein, the term “transparency image” means an image which simulates a radiographic image produced by penetrating radiation, i.e., all organs are superimposed in the image and are visible dependent on their respective radiographic transparency. The term “transparency direction” means the direction from which the image is “seen,” which, in the context of a radiographic image, would be the direction of propagation of the penetrating radiation.
2. Description of the Prior Art
Magnetic resonance tomography device is a known technique for generating images of the inside of a body of an examination subject. For that purpose, rapidly switched gradient fields, which are generated by a gradient coil system, are superimposed onto a static basic magnetic field, which is generated by a basic field magnetic system, in a magnetic resonance tomography device. Further, the magnetic resonance tomography device has a high frequency system, which, for producing magnetic resonance signals, radiates high frequency signals into the examination subject and picks up the generated magnetic resonance signals on the basis of which magnetic resonance images are prepared.
Magnetic resonance tomography devices have been clinically utilized in an increasing extent since 1983. Due to the extremely high initial cost and operating expenses, as well as due to the extremely long measuring periods, magnetic resonance tomography was initially reserved for specific clinical examinations in the area of the central nervous system. Although magnetic resonance tomography currently is clearly the image-diagnostic method of choice for many different pathologies, magnetic resonance tomography still is a last resort, if used at all, for most everyday diagnostic imaging, since the initial cost is still high and the measuring periods are comparatively long. Currently, X-ray devices are mainly utilized as a first imaging modality within the clinical routine for answering everyday questions. Such clinical everyday questions are examinations of the inner organs, such as the lung. A contrast representation (shadow image) of an organ or of an entire body section is most commonly produced.
Since its introduction, further developments in the field of the magnetic resonance tomography have had the goal as shortening measuring periods and improving the resolution and the contrast properties of the tomograms. For that purpose, the efficiency of the components of a magnetic resonance tomography device has increased with an increasing technical outlay.
Among other things, the reduction of measuring periods is the goal of several known measuring sequences. For example, U.S. Pat. No. 4,769,603 and an article by J. L. Duerk et al. “Remember True FISP? A High SNR Near 1-Second Imaging Method for T2-Like Contrast in Interventional MRI at 0.2T”, vol. 8, No. 1, 1998, pages 203 through 208, describe a refocused gradient echo sequence by means of which magnetic resonance tomograms can be picked up with a high image contrast given a short measuring time, known by the acronym FISP, or True-FISP. Characteristic of the measuring sequence described in the above-cited documents is that a slice selection gradient is generated at the same time as a high frequency pulse for a duration T, a readout gradient and the slice selection gradient with reversed polarity are subsequently generated for a duration of approximately T/2, a readout gradient with reversed polarity is subsequently generated for a duration of approximately T, and these steps are multiply repeated. A dynamic equilibrium magnetization is maintained with a high frequency pulse sequence of high symmetry.
In addition to techniques, such as the above for imaging slices of an examination subject, volume imaging techniques are known for achieving high-quality magnetic resonance images from a magnetic resonance signal with a high signal-to-noise ratio. Apart from the frequency encoding, two phase encoding gradients are successively switched in two spacial directions that are orthogonal to one another in volume imaging. This requires a correspondingly efficient gradient system. Further, a dataset must be processed that is many times larger than for a slice, corresponding to an expansion in a third dimension. This requires increased efficiency in the measured value processing and in the image reconstruction, as well as an extended measuring time.
There is still a need to have an inexpensive magnetic resonance device for obtaining transparency images in order to replace, for example, X-ray-based devices, whose damaging effect on living tissue of patients as a result of the use of ionizing radiation is known for the everyday imaging purposes described above.
German PS 35 04 734 discloses a method for the fast pickup of slice tomograms with a high spacial resolution, which is a gradient version method with a small flip angle and a small high frequency output. The method is described as being particularly advantageous given high magnetic flux densities of a basic magnetic field that is generated by asuperconducting basic field magnetic system. In an embodiment of the method, a pickup of a transparency-like tomogram ensues without slice selection.
German OS 196 28 951 discloses a method for magnetic resonance angiography with which transparency-like angiograms can be picked up with a short measuring time and with a T
1
-weighting. For that purpose, a contrast medium is injected into the pulsatile vessels to be imaged. In the pickup of the angiogram, either no slice selection takes place, or a thick slice is selected with a weak slice selection gradient, this slice encompassing the entire target volume. Among other things, a disadvantage of the above-cited methods is that transparency-like images of vessels that are filled with the contrast medium can be picked up. Visualization of anatomical structures in an environment of the vessels is not possible.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a magnetic resonance device with which meaningful transparency images can be picked up with short measuring times, comparable to those obtained using X-ray devices, and which reduces above cited disadvantages of the prior art. As used herein, thus, a “meaningful transparency image” means an image having a diagnostic clarity and resolution comparable to an x-ray transparency.
This object is inventively achieved in that a magnetic resonance device for generating a transparency image in a selected direction, the device having basic field magnetic system for generating a basic magnetic field that exhibits a high homogeneity and a low magnetic flux density in an imaging volume, and a gradient coil system and a controller therefor for, in a first version, executing a refocused gradient echo sequence, wherein two gradient fields, whose gradients are perpendicularly directed to the selected direction, can be switched. In a second version the controller executes a refocused gradient echo sequence with a large slice thickness in the selected direction.
In the literal sense, such a magnetic resonance device is not a tomography device wherein, for example, refocused gradient echo sequences are employed to image a slice having a slice thickness of a maximum of a few millimeters. Therefore the device is referred to as magnetic resonance transparency device in the following. The magnetic resonance transillumination device differs from devices that operate using the above cited volume technique not only because merely a simple phase encoding is required, but also because the dataset is many times smaller and the measuring time is many times shorter.
The invention is based on the recognition that transparency image, which is diagnostically extremely meaningful, since it is not distorted by artefacts, can be surprisingly obtained with a short measuring time by means of the combined action of the fol
Kuth Rainer
Rupprecht Sabine
Rupprecht Thomas
Fetzner Tiffany A.
Lefkowitz Edward
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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