Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2001-02-21
2003-04-08
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S410000, C600S428000, C324S307000, C324S309000, C324S314000
Reexamination Certificate
active
06546274
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a magnetic resonance imaging apparatus (referred to hereafter as MRI), and in particular to a magnetic resonance imaging apparatus which performs imaging by assigning magnetic tags, and a magnetic resonance imaging method.
The terminology used in the following description of this specification is summarized below.
[Tagging Sequence]: Sequence for applying magnetic field which performs magnetic tagging (pulse sequence).
[Imaging Sequence]: Sequence for applying magnetic field which performs magnetic resonance imaging (MRI) (pulse sequence).
[Amplitude Modulated Burst Pulse]: Radiofrequency burst pulse comprising plural sub-pulses formed at equal intervals on the time axis whereof the amplitudes are modulated by a sinc function (the plural sub-pulses comprise a sub-pulse of amplitude value 0).
[Period in Cardiac Cycle]: The times when one cardiac cycle is divided into for example, approximately, 4-12 parts when extracting the amount of movement of the heart wall.
[Echo Signal Gathering Efficiency]: Number of echo signals gathered in unit time.
First, the prior art technology will be described regarding analysis of cardiac function by performing magnetic tagging to image the motion of the heart wall (Ref. 1: L. Axel et al., MR Imaging of Motion with Spatial Modulation of Magnetization: Radiology, vol. 171, p. 841-845 (1989)).
FIG. 17
is a diagram describing the principle of assigning magnetic tags disclosed in Ref. 1.
FIG. 17A
is a diagram describing a tagging sequence.
FIG. 17B
is a diagram describing the behavior of magnetization vectors in the tagging sequence of
FIG. 17A
, and
FIG. 17C
is a diagram describing the spatial intensity distribution of the z direction component of the nuclear magnetization vector.
In
FIG. 17
, a static magnetic field is applied in the z direction. As shown in
FIG. 17B
, in the initial state (time
0
) shown in
FIG. 17A
, the nuclear magnetization vector is oriented in the z direction, and as shown in
FIG. 17C
, the intensity of the z direction component of the nuclear magnetization vector is M
0
(constant value). As shown in
FIG. 17A
, when a radiofrequency magnetic field pulse RF is irradiated at a time a, the nuclear magnetization vector rotates around the x axis and inclines at an angle &thgr; in the yz plane, and the intensity of the z direction component of the nuclear magnetization vector is M
0
cos &thgr;. Next, as shown in
FIG. 17A
, when a gradient magnetic field Gx is applied at a time b, the nuclear magnetization vector is phase-modulated corresponding to the position coordinates.
FIG. 18
is a diagram describing the intensity of the x direction component of the nuclear magnetization vector in a tagging sequence according to the prior art. As shown in
FIG. 18
, the intensity of the x direction component of the nuclear magnetization vector after applying the inclined magnetic field Gx is modulated relative to the x direction in which the gradient magnetic field Gx is applied. However, as shown in
FIG. 17C
, the intensity of the z direction component of the nuclear magnetization vector is M
0
cos &thgr; (constant value). The nuclear magnetization vector precesses around the z axis.
Subsequently, as shown in
FIG. 17A
, when the radiofrequency magnetic field pulse RF is again irradiated at a time c, the nuclear magnetization vector rotates around the x axis, and inclines at an angle &thgr; in the yz plane. As a result, the modulation in the x direction of the nuclear magnetization vector shown in
FIG. 18
is reflected in the z direction component of the nuclear magnetization vector. Due to the tagging sequence described above, as shown in
FIG. 17C
, at a time d before the imaging sequence is implemented, the z direction component of the nuclear magnetization vector can be modulated corresponding to the x coordinate.
When the imaging sequence is implemented, the above spatial modulation of the z direction component of the nuclear magnetization vector is reflected in the signal intensity of the acquired image, and stripes are generated perpendicular to the x direction on the image. Specifically, magnetization is suppressed in the peripheral part of the stripes on the image obtained by the tagging sequence shown in FIG.
17
A. By a combination of the applied amount and applied direction of the gradient magnetic field, the direction of the stripes and interval of the stripes can be controlled, and stripes can also be generated in the vertical and horizontal directions. These stripes are tags.
FIG. 19
is a diagram describing the tagging sequence in the prior art. In
FIG. 17
, the simplest example has been shown to describe the principle of tagging, but in general, a binomial SPAMM (Spatial Modulation of Magnetization) pulse which makes the amplitude ratio of the radiofrequency magnetic field pulses RF
3
′,
5
′ a binomial coefficient is often used (Ref. 2: L. Axel, et al., Heart Wall Motion: Improved Method of Spatial Modulation of Magnetization for MR Imaging, Radiology, vol. 172, p. 349-350 (1989)), as shown in FIG.
19
. The numbers above the radiofrequency magnetic field pulse RF shown in
FIG. 19
are amplitude ratios of radiofrequency magnetic field pulses.
In the example shown in
FIG. 19
, a gradient magnetic field Gx
4
′ in the x direction is applied alternately with a radiofrequency magnetic field pulse RF
3
′, a gradient magnetic field Gx
6
′ in the y direction is applied alternately with a radiofrequency magnetic field pulse RF
5
′, and tags are assigned in the x direction and y direction. As described hereabove, if the method disclosed in Ref. 1 is used, the nuclear magnetization is suppressed at equal intervals in straight lines in the vertical and horizontal directions, i.e. in a grid shape. As a result, if the imaging sequence is implemented immediately after completion of the tagging sequence, an MRI image is obtained having bright points arranged in a lattice.
In general, a pulse sequence according to a fast imaging method is used as the imaging sequence, particularly the pulse sequence in the fast spin echo technique (Ref. 3: D. Matthaei et al., Cardiac and Vascular Imaging with an MR Snapshot Technique, Radiology, vol. 177, pp. 527-532 (1990)). The fast spin echo technique is suitable for extraction of the heart wall. It may be mentioned that the echo planar (EPI) technique is suited to extraction of blood circulation, but not to extraction of the heart wall.
FIG. 25
is a diagram describing the imaging sequence in the prior art.
FIG. 25A
is fast spin echo type pulse sequence wherein a radiofrequency burst pulse, comprising plural sub-pulses formed at equidistant intervals on the time axis whereof the amplitudes are modulated by a sinc function, is applied.
FIG. 25B
shows a pulse sequence according to the fast spin echo method.
In the pulse sequence shown in
FIG. 25A
, after a first amplitude modulated burst pulse is irradiated, a slice gradient magnetic field Gs is applied, a &pgr; pulse is irradiated, and slice selection and inversion of magnetization are performed. Next, a readout gradient magnetic field Gr is applied, the phase of the nuclear magnetization is provided to generate an echo signal, and the echo signal is measured. Subsequently, inversion of magnetization due to irradiation by the &pgr; pulse and application of the readout gradient magnetic field following the &pgr; pulse are repeated to generate echo signals on plural occasions, and measurement of the echo signal is repeated. The number of echo signals generated by one irradiation with the &pgr; pulse is equal to the number of sub-pulses in the first amplitude modulated burst pulse. It should be noted that different phase encodings are assigned to the echo signals by a phase encoding gradient magnetic field Gp.
In the pulse sequence (imaging sequence) shown in
FIG. 25A
, after a second amplitude modulated burst pulse is irradiated wherein the carrier frequency of the first amplitude modulate
Itagaki Hiroyuki
Ochi Hisaaki
Antonelli Terry Stout & Kraus LLP
Hitachi Medical Corporation
Lateef Marvin M.
Pass Barry
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