Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1998-12-10
2001-09-04
Smith, Ruth S. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S419000, C324S306000, C324S309000
Reexamination Certificate
active
06285900
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for determining a perfusion image of a portion of a body placed in a static magnetic field by means of magnetic resonance (MR), which method comprises the following steps of:
generation of a control pulse sequence in a first portion of the body and measurement of a control data set by generation of an MR-image sequence for imaging of an third portion of the body to be imaged.
generation of a labelling pulse sequence in a second portion of the body wherein a fluid flows towards the third portion, and measurement of the labelled data set by generation of the MR-image sequence for imaging the third portion of the body, reconstruction of the perfusion image of the fluid in a mass of the third portion of the body from a combination of the control data set and the labelled data set. The invention further relates to an MR apparatus for carrying out such a method.
2. Description of the Related Art
Such a method is known from U.S. Pat. No. 5,402,785. In the known method, after the control preparation by the control pulse sequence, the control data set is measured from the third portion of the body by the MR image sequences, and after labelling the fluid by the inversion of the proton spins of the fluid by the labelling pulse sequence in the second portion of the body, the labelled data set is measured from the third portion of the body, for example, a slice of the head of a body. From the control data set and the labelling data set, respectively, a control image and a labelled image are reconstructed. The perfusion image is then determined by a combination of the control image and the labelled image, for example by subtraction of the control image from the labelling image. In the known method, magnetisation transfer effects suppress magnetic resonance signals, whereby the rate of suppression varies for different types of tissues and blood. Also magnetic resonance signals from blood, are reduced by the magnetic transfer effects. As a result, the contrast of the perfusion image is affected. In order to reduce magnetic transfer effects in the known method, the first portion and the second portion are positioned symmetrically with respect to the third portion of the body. A drawback of the known method is that geometric restrictions exist in the choice of the third portion of the body.
SUMMARY OF THE INVENTION
It is inter alia an object of the invention to reduce the geometric restrictions in the choice of the third portion of the body. To this end, a method in accordance with the invention is characterised in that the labelling pulse sequence comprises a first selective RF pulse and a second RF pulse, the control pulse sequence comprises the first selective RF pulse and a third RF pulse, a phase of the second RF pulse being opposite to that of the third RF pulse. In this way a magnetisation transfer-insensitive labelling technique can be performed, wherein the positions of the first and second portions coincide and the geometric restriction in the choice of the imaging portion is reduced. The invention is based on the insight that the control pulse sequence and the labelling pulse sequence both effect a z-magnetisation of the bound water in the same way by taking advantage of a difference between a transverse relaxation time T
2
of bound water and of free water. Because of the relatively short transverse relaxation time T
2
of bound water compared to that of free water, the z-magnetisation of the bound water, resulting from the application of the first selective RF pulse and the second RF pulse and the first selective RF pulse and the third RF pulse, does not depend on a phase relation between the first selective RF pulse and the second RF pulse or a phase relation between the first selective RF pulse and the third RF pulse. When the interval between the first selective RF pulse and the second RF pulse and between the first selective RF pulse and the third RF pulse is long enough with respect to the transverse relaxation time T
2
of bound water, the magnetic transfer effects due to the labelling and control pulse sequences are equal and the magnetic transfer effects can be cancelled by combining data of the control image and the labelling image.
A further advantage is that the method can be combined with multi-slice and angulated MR imaging because the magnetisation transfer compensation is not restricted geometrically. Furthermore, because the first portion coincides with the second portion no distal labelling is induced in the first portion and distal inflow from the first portion into the second portion does not impair perfusion assessment.
A particular version of the method in accordance with the invention is characterised in that the first selective RF pulses and the second RF pulse are applied according to a first modulation function of time and the third RF pulse is applied according to a second modulation function, which is the same as the first modulation function, but of an opposite sign. In this way, the labelling and control preparations yield a maximum difference in the magnetisation of free water within the desired second portion and no difference in the magnetisation outside the second portion.
A further version of the method in accordance with the invention is characterised in that the labelling and control sequences comprise second magnetic field gradient pulses, the first magnetic field gradient pulses being applied with the first selective RF pulse according to a first gradient function of time, the second magnetic field gradient pulses being applied with the second RF pulse according to a second gradient function of time, being a time-reversed version of the first gradient function, and the sign of the second gradient function being opposite to that of the first gradient function, the first selective RF pulse being applied according to a first amplitude modulation function of time and a first frequency modulation function of time, the second RF pulse being applied according to a second amplitude modulation function and a second frequency modulation function, the second amplitude modulation function being a time-reversed version of the first amplitude function and the second frequency modulation function being a time-reversed version of the first frequency modulation function, the sign of the second frequency modulation function being opposite to that of the first frequency modulation function,
the third RF pulse being applied according to a third amplitude modulation function and the second frequency modulation function, the third amplitude modulation function being the same as the second amplitude modulation function, but of opposite sign.
The result of the concatenation of the first selective RF and the second RF pulses is that a quality of the second portion is improved, for example, a highly selective inversion pulse can be obtained by a concatenation of the first selective RF pulse and second RF pulse, because a flip angle of the z-magnetisation is exactly doubled at all positions within the second portion. The concatenation of the first selective RF pulse and the third RF pulse results in a flip angle of zero degrees. Furthermore, an advantage of a high-quality labelling slab is that a minimum gap between the second portion and the third portion can be reduced.
A further version of the method in accordance with the invention is characterised in that the method comprises a further step of generating a refocussing RF pulse between the first selective RF pulse and the second RF pulse and between the first selective RF pulse and the third RF pulse, respectively. The effect of the refocussing pulses, for example a centred 180 degrees refocussing pulse, is that a reduction of the effects of magnetic field inhomogeneities is obtained.
A further version of the method in accordance with the invention is characterised in that a flip angle of the first selective RF pulse and the flip angle of the second and third RF pulses are about 90°. Concatenations of 90° pulses can be advantageo
Boesiger Peter
Golay Xavier G.
Pruessmann Klaas P.
Scheidegger Markus B.
Stuber Matthias
Smith Ruth S.
U.S. Philips Corporation
Vodopia John F.
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