Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1998-12-10
2001-06-26
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
Reexamination Certificate
active
06252399
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for determining an image of a first portion of a body placed in a steady magnetic field by means of magnetic resonance (MR), which method comprises the following steps;
excitation of nuclear spins in the first portion,
measurement of a data set of MR signals of the first portion of the body, and
reconstruction of the image from the measured data set, the method also comprising the generation of an RF pulse rotation of a magnetization in a second portion of the body. The invention also relates to an MR apparatus for carrying out such a method, comprising;
means for generating temporary magnetic gradient fields,
means for generating RF pulses,
means for receiving MR signals,
processing means for the reconstruction of an image of the MR signals received,
control means for the generation of control signals for the means for generating temporary magnetic gradient fields and for the means for generating RF pulses,
the control means also being arranged to generate an MR imaging sequence comprising an RF excitation pulse and an RF pulse for rotation of a magnetization within the second portion of the body.
2. Description of Related Art
Such a method is known from U.S. Pat. No. 5,402,785. The known method is used for the magnetic resonance imaging of a part of a body of an animal or human. The MR images obtained may be used as a tool for the diagnosis of a decease of the animal or human by determining perfusion in, for example a part of the brain. Perfusion images show motion of blood in vessels and capillaries in tissue of the human or animal under examination. The known method comprises the determination of perfusion by labelling atoms in a fluid in the second portion located upstream with respect to a flow of liquid towards the first portion, the generating of a steady state in the substance by continuing to label atoms until the effect caused by the labelled atoms perfusing in the substance reaches a steady state, and the generating of image information so as to determine perfusion in the first portion of the body. The labelling involves continuous inversion of spins associated with the atoms in the second portion. In this way the second portion in the form of a slice is selected for which a Larmor frequency of the spins in the slice equals the frequency of the RF pulse. A drawback of the known inversion pulse is that the obtained slice profile deviates from a nominal selection profile, for example a rectangle.
SUMMARY OF THE INVENTION
It is an object of the invention to improve the selection profile, so that the deviation from a nominal selection profile are reduced. To this end, a method in accordance with the invention is characterized in that the RF pulse for 180 degree rotation comprises a concatenation of a first selective RF pulse and a second selective RF pulse. Further advantageous embodiments of the method according to the invention are defined in the dependent Claims. The application of the concatenation of the first selective RF pulse and the second selective RF pulse provides a slice selection with twice a flip angle of a single sequence comprising an RF pulse and a magnetic gradient field. In this way the quality of the slice profile of the RF pulse employed in the single sequences is maintained for slice profile obtained for the concatenation of the first and second selective RF pulses. The slice quality of the selected slice resulting from the concatenation of RF pulses can be compared with that resulting from conventional RF pulses. Such pulses are known per se from the handbook “Magnetic Resonance Imaging” by M. T. Vlaardingerbroek et al, Springer Verlag, page 49. Furthermore, the slice quality is maintained because the axes of rotation describing a magnetization rotation of a single sequence lie in a x,y-plane of a reference co-ordinate system x,y,z for all points in the selected slice. A quality with respect of a selected slice profile can be defined as a ratio of missing magnetic moment inside the slice and a magnetic moment of a perfectly rectangular slice. The slice limits are defined such that a magnetic moment in a z-direction outside the slice is equal to a missing magnetic moment in the z-direction inside the slice. Furthermore, the first portion may coincide with the second portion of the body. The RF deposition of a concatenation of the first and second selective RF pulses is roughly proportional to a nominal flip angle based on the sum of the flip angles of the first and second RF pulses. The RF power reduction can thus be 50% or more for an equal slice selection profile compared to the known RF pulse. In this context the z-direction of a reference co-ordinate system x,y,z corresponds to a direction of the steady magnetic field. For the perfect rectangular slice profile the ratio will be zero percent.
A particular version of the method in accordance with the invention is characterized in that
a first magnetic field gradient pulse is generated with the first selective RF pulse according to a first gradient function of time,
a second magnetic field gradient pulse is generated with the second selective RF pulse according to a second gradient function of time being a time, reversed version of the first gradient function,
a sign of the second gradient function opposes that of the first gradient function,
the first selective RF pulse is applied according to a first amplitude modulation function of time and a first frequency modulation function of time,
the second RF pulse is applied according to a second amplitude modulation function and a second frequency modulation function,
the second amplitude modulation function is a time reversed version of the first amplitude function,
the second frequency modulation function is a time reversed version of the first frequency modulation function, and
a sign of the second frequency modulation function opposes to that of the first frequency modulation function. In this way, for example, a highly selective inversion pulse can be obtained by the concatenation of the first selective RF pulse and the second RF pulse, because a flip angle of the z-magnetization is exactly doubled at all positions within the selected slice. RF-pulses having a large flip angle can thus be derived from RF-pulses having a small angle without loss of profile quality. For the concatenation of the first and the second selective RF pulse a slice quality of for example, 3.5 percent can be obtained. Whereas the slice quality of a slice selected by, for example, a conventional 180 degree inversion RF pulse amounts to 7 percent. An example of such a conventional inversion RF pulse comprises an asymmetric amplitude envelope.
A further version of the method in accordance with the invention is characterized in that the RF-pulse for rotation of the magnetisation is generated before an excitation RF pulse. The concatenated RF pulses are thus used as an inversion pulse. Inversion recovery imaging sequence are known per se from the cited handbook “Magnetic Resonance Imaging”, by M. T. Vlaardingerbroek et al, Springer-Verlag, 1996, page 77. In this way a T
1
-weighted MR image can be acquired from the selected first portion of the body.
A further version of the method in accordance with the invention is characterized in that
the RF pulse for 180 degree rotation of the magnetization is generated after an excitation RF pulse and
in that a further magnetic field gradient pulse is applied in the direction normal to the selected slice. The concatenation of the first selective RF pulse and the second RF pulse can be applied as a refocusing pulse in, for example a known Echo Planar Imaging (EPI) imaging sequence or in a known spin echo imaging sequence or gradient and spin echo sequence (GRASE). These imaging sequences are known per se from international patent application WO 93/01509. In the known spin echo sequence or EPI sequence the concatenation of the first and second RF pulses is generated after the excitation of the nuclear spins for a 180 degree rotation of the magnetizati
Boesiger Peter
Faas Henryk M.
Golay Xavier G.
Pruessmann Klaas P.
Stuber Matthias
Oda Christine
U.S. Philips Corporation
Vargas Dixomara
Vodopia John F.
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