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Patent Application Titled "Method and Apparatus for Acquisition of Magnetic Resonance Data While Avoiding Signal Inhomogeneities" Published Online

February 18, 2014



By a News Reporter-Staff News Editor at Information Technology Newsweekly -- According to news reporting originating from Washington, D.C., by VerticalNews journalists, a patent application by the inventor Paul, Dominik (Bubenreuth, DE), filed on July 25, 2013, was made available online on February 6, 2014.

No assignee for this patent application has been made.

Reporters obtained the following quote from the background information supplied by the inventors: "The present invention concerns a method to acquire magnetic resonance data, as well as a magnetic resonance system and an electronically readable data storage medium for implementing such a method.

"Magnetic resonance (MR) is a known modality with which images of the inside of an examination subject can be generated. Expressed in a simplified manner, the examination subject in a magnetic resonance apparatus is positioned in a strong, static, homogeneous basic magnetic field (also called a B.sub.0 field) with a field strength of 0.2 to 7 Tesla or more, such that nuclear spins in the subject orient along the basic magnetic field. Radio-frequency excitation pulses and possible refocusing pulses (RF pulses) are radiated into the examination subject to trigger nuclear magnetic resonance signals, are detected and entered into a memory in an organization as k-space data, on the basis of which MR images are reconstructed or spectroscopy data are determined. Rapidly switched magnetic gradient fields are superimposed on the basic magnetic field for spatial coding of the measurement data. The acquired measurement data are digitized and stored as complex numerical values in a k-space matrix. For example, by means of a multidimensional Fourier transformation an associated MR image can be reconstructed from the k-space matrix populated with values.

"In the acquisition of magnetic resonance data from a three-dimensional region of an examination subject, signal inhomogeneities result due to inhomogeneities in the basic magnetic field. Such signal inhomogeneities can lead to a decrease of signals in regions of the acquired MR images that are important to a finding. The diagnostic value of the acquired MR images is thereby reduced. Even in the case of an ideally homogeneous basic magnetic field, the problem continues to exist since fat and water protons exhibit different resonance frequencies due to the chemical shift, and under the circumstances the entire region cannot be covered with the bandwidth of one radio-frequency pulse. The greater the field strength of the basic magnetic field, the further apart from one another the resonance frequencies are separated."

In addition to obtaining background information on this patent application, VerticalNews editors also obtained the inventor's summary information for this patent application: "An object of the present invention is to provide a method to acquire magnetic resonance data and a magnetic resonance system and an electronically readable data storage medium to implement such a method that already, during the generation of the spin echo signals that are acquired as magnetic resonance data, avoid signal inhomogeneities in image data reconstructed magnetic resonance data, in spite of inhomogeneities in the basic magnetic field and in spite of effects of the chemical shift.

"The invention is based on the insight that long RF pulses with reduced bandwidth are the most susceptible to inhomogeneities in the basic magnetic field, with the effect that spectral regions are not excited or refocused, since the bandwidth of an RF pulse is inversely proportional to the duration of the RF pulse.

"For example, in magnetic resonance systems with a higher basic magnetic field (for example 3 Tesla or more), the required B1 field is normally realized in a reduced form compared to magnetic resonance systems with a basic magnetic field of a lower strength, so the duration of the RF pulses of a data acquisition sequence is extended and the bandwidth of the RF pulses is reduced. As noted above, such sequences are particularly susceptible to inhomogeneities in the basic magnetic field.

"A method according to the invention for the acquisition of magnetic resonance data with a magnetic resonance system in order to avoid signal inhomogeneities includes radiating an excitation pulse into the examination subject, after a first time period after the radiation of the excitation pulse, radiating a first refocusing pulse into the examination subject, after a second time period after the radiation of the first refocusing pulse, radiating a series of at least two additional refocusing pulses that generate variable flip angles adapted to a predetermined signal curve and are non-selective pulses, acquiring the spin echo signals generated by the radiated pulses as magnetic resonance data, activating gradients for spatial coding the excitation produced by the excitation pulse, the refocusing via the refocusing pulses and the acquisition of the magnetic resonance data, and storing and/or further processing the acquired magnetic resonance data, wherein the center frequency of at least one of the radiated refocusing pulses is adjusted such that it is arranged between the resonance frequency of fat molecules and the resonance frequency of water molecules in the examination subject in the magnetic resonance system.

"Inhomogeneities in the signal intensity of the acquired magnetic resonance data (and therefore signal inhomogeneities) are avoided with the center frequency, adjusted according to the invention, of at least one of the radiated refocusing pulses of the method according to the invention. The non-selective refocusing pulses excite both fat molecules and water molecules as homogeneously as possible via the adjustment according to the invention.

"This applies in principle to every individual radiated refocusing pulse. The selection of at which refocusing pulses an adapted center frequency should be adjusted depends on the type of measurement and the desired influence on the quality of the image data that can be reconstructed from the acquired measurement data. For example, the center frequencies of all radiated refocusing pulses can be shifted according to the invention, although this is not necessary.

"In one embodiment, at least the center frequency of the first refocusing pulse is shifted according to the invention to a position between the resonance frequencies of fat and water. Particularly if the first refocusing pulse is a 180.degree. pulse in order to generate a pure spin echo with high signal intensity, this first refocusing pulse is longer than the additional refocusing pulses that normally generate smaller flip angles. For both of these reasons (generation of a pure spin echo and longer duration of the first refocusing pulse relative to the additional refocusing pulses), this first refocusing pulse is most sensitive with regard to the problem addressed above and also has the greatest influence on the quality of the image data that can be reconstructed from the acquired measurement data.

"Through the series of at least two refocusing pulses after one excitation pulse, an echo train of similarly many spin echoes is generated. Because the refocusing pulses generate variable flip angles adapted to a predetermined signal curve, particularly long echo trains can be generated via correspondingly many refocusing pulses without the signal intensities of the echoes declining too severely. Appropriate methods to determine and implement the variable flip angles are known from, for example: Mugler, Kiefer and Brookeman: 'Three-Dimensional T2-Weighted Imaging of the Brain Using Very Long Spin-Echo Trains', Proc. ISMRM 8 (2000) P. 687; Mugler, Meyer and Kiefer: 'Practical Implementation of Optimized Tissue-Specific Prescribed Signal Evolutions for Improved Turbo-Spin-Echo Imaging', Proc. ISMRM 11 (2003) P. 203; Mugler and Brookeman: '3D Turbo-Spin-Echo Imaging with up to 1000 Echoes per Excitation: From Faster Acquisitions to Echo-Volumar Imaging', Proc. ISMRM 11(2004) P. 2106; and Mugler and Brookeman: 'Efficient Spatially-Selective Single-Slab 3D Turbo-spin-Echo Imaging', Proc. ISMRM 11 (2004) P. 695.

"In contrast to older sequences, for example a TSE sequence ('Turbo Spin Echo') or an FSE sequence ('Fast Spin Echo'), the readout module of the pulse sequence according to the invention advantageously corresponds to a SPACE sequence ('Sampling Perfection with Application optimized Contrasts using different flip angle Evolutions'). For example, this SPACE sequence has proven preferable to the older TSE and FSE sequences due to the variable flip angles and the longer echo train lengths that are possible from this sequence, than are normal in practice. SPACE ('Sampling Perfection with Application optimized Contrasts using different flip angle Evolution') allows high-resolution, three-dimensional (3D) image exposures to be created in a shorter period of time. The SPACE sequence is a single slice 3D turbo spin echo (TSE) sequence with application-specific variable flip angles.

"A magnetic resonance system according to the invention for the acquisition of magnetic resonance data in a selected region within an examination subject has a basic field magnet, a gradient field system, at least one RF antenna, and a control device to control the gradient field system and the at least one RF antenna, to receive the measurement signals acquired by the at least one RF antenna, and to evaluate the measurement signals and to create the magnetic resonance data, and a computer to determine the center frequency of the refocusing pulses and to determine flip angles adapted from a predetermined signal curve. The magnetic resonance system is operated by the control unit to radiate an excitation pulse into the examination subject and after the excitation pulse, to radiate a first refocusing pulse into the examination subject after a first time period, with the center frequency of at least one of the radiated refocusing pulses being set by means of the computer so that it is between the resonance frequency of fat molecules and the resonance frequency of water molecules in the examination subject. The control unit further operates the magnetic resonance system; such that, after a second time period, a series of at least two refocusing pulses is radiated in order to generate spin echo signals in the examination subject. The refocusing pulses generate variable flip angles adapted to a predetermined signal curve, and the refocusing pulses of the series of at least two refocusing pulses are non-selective pulses. The control unit operates the magnetic resonance system to acquire the generated spin echo signals as magnetic resonance data and such that the magnetic resonance system activates gradients for spatial coding before and after the radiation of the excitation pulse, the radiation of the refocusing pulses and during the data acquisition. The magnetic resonance system stores and/or displays the acquired magnetic resonance data.

"In general, the magnetic resonance system is designed to implement the method according to the invention as described herein.

"An electronically readable data medium according to the invention has electronically readable control information stored thereon that cause a computerized control device of a magnetic resonance system to implement the method according to the invention.

"The advantages and embodiments specified with regard to the inventive method apply analogously to the magnetic resonance system and the electronically readable data medium.

BRIEF DESCRIPTION OF THE DRAWINGS

"FIG. 1 schematically illustrates a magnetic resonance system according to the invention.

"FIG. 2 shows an example of a pulse sequence suitable for use in the method according to the invention.

"FIG. 3 schematically shows an adjustment according to the invention of the center frequency of the first refocusing pulse of the pulse sequence that is used.

"FIG. 4 is a flowchart of an embodiment of the method according to the invention."

For more information, see this patent application: Paul, Dominik. Method and Apparatus for Acquisition of Magnetic Resonance Data While Avoiding Signal Inhomogeneities. Filed July 25, 2013 and posted February 6, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=5074&p=102&f=G&l=50&d=PG01&S1=20140130.PD.&OS=PD/20140130&RS=PD/20140130

Keywords for this news article include: Patents, Information Technology, Information and Data Storage.

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Source: Information Technology Newsweekly


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