Frequency selective pulse

Details Frequency selective pulse Frequency selective pulse

1997-08-22

2002-08-27

Arana, Louis (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Using a nuclear resonance spectrometer system

C324S307000

Reexamination Certificate

active

06441613

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to magnetic resonance imaging and in particular to the use of adiabatic pulses for changing magnetic moments.
BACKGROUND OF THE INVENTION
FIG. 1
is a schematic diagram of an MRI (Magnetic Resonance Imaging) system as well known in the art. A patient
20
is placed into a powerful, axial, homogenous magnetic field generated by a magnet
22
. The tissues which make up patient
20
contain many hydrogen nuclei, some of which have magnetic moments aligned with the magnetic field. When an aligned hydrogen atom is irradiated with RF radiation of a suitable frequency, transmitted by an RF transmitter
24
, its magnetic moment changes direction relative to the field. The new direction of the magnetic moment is not stable, so after a time the magnetic moment reorients itself back to the magnetic field direction. During this reorientation, the hydrogen atom emits RF radiation, which radiation is detected by RF receiver
30
.
The RF radiation is generated by hydrogen nuclei which have a magnetic moment at an angle to the magnetic field, which moment precesses about the axial field direction. The RF irradiation tends to align the magnetic moments so they are all in phase with each other such that their individual contributions to the RF signal are additive.
As can be appreciated, tissues which have a high concentration of hydrogen nuclei will emit a higher amplitude of RF energy than tissues which have a low concentration of hydrogen nuclei. In addition, different body tissues can be characterized by their T
1
and T
2
relaxation times, which can be imaged using various MRI imaging sequences. Also, the emission frequency depends on the chemical bonds of the hydrogen atom, so the emission frequency can be analyzed to detect different chemicals in the body, in what is known as MRI spectroscopy.
In order to generate an image of a body portion, the spatial origin of the acquired RF signals must be determined. In a typical coordinate system the Z coordinate is in the direction of the longitudinal axis of patient
20
(and the direction of the DC magnetic field) and the X and Y axes are perpendicular to it. Some methods of MRI imaging limit the image acquisition to one slice in the X-Y plane at a time, by what is called slice selection. The RF frequency, which causes the magnetic moments of hydrogen nuclei to change their orientations, is functionally dependent on the intensity of the magnetic field. A Z gradient coil
26
imposes a gradient, in the Z direction, on the Z directed field generated by magnet
22
, so that each axial slice of patient
20
is subjected to a different Z-directed magnetic field. RF transmitter
24
transmits a narrow band RF signal, which affects the orientation of hydrogen nuclei in only a narrow body portion (slice)
27
and hence causes emission of RF radiation from portion
27
alone.
In order to differentiate between RF signals emitted by different segments of portion
27
, a Y gradient coil
32
is typically used to momentarily create a Y gradient in the Z directed magnetic field of magnet
22
. This Y gradient is created before readout, described below. Nuclei in segments of portion
27
which have a momentary increase in the locally applied magnetic field advance in phase, since the precession speed is dependent on the field strength, while nuclei which have a momentary decrease in the locally applied magnetic field retreat in their phase. Thus, portion
27
is divided into first parallel strips (in the Y direction), each of which emits RF radiation at a different phase.
The X coordinate of an RF emission is determined during a stage called readout. To perform readout, the Z directed magnetic field including portion
27
is varied in the X direction, using an X gradient coil
28
, so that portion
27
is substantially divided into second parallel strips (in the X direction) each of which has a different local magnetic field strength. Since the frequency of the emitted RF radiation is directly dependent on the local field intensity, each of the second parallel strips emits radiation at a different frequency. An image can be reconstructed by acquiring a plurality of RF signals during readout, each of which has a different Y gradient phase encoding and a X gradient frequency encoding, and performing a two-dimensional FFT (fast Fourier transform) on the acquired signals. It should be appreciated that many different types of MRI imaging sequences are known in the art, including sequences where the imaged nucleus is not Hydrogen.
FIG. 2A
is a schematic diagram showing the interaction between a magnetic moment
40
of a sample and a static Z magnetic field
42
having a magnitude B
0
. Field
42
causes magnetic moment
40
to become aligned with the +Z direction and to spin at an angular velocity &ohgr;
0
=&ggr;B
0
around the Z direction, where y is the gyromagnetic ratio of the inspected nucleus. &ohgr;
0
is also called the Larmor frequency.
FIG. 2B
illustrates an example where moment
40
is also subjected to an RF magnetic field
44
having a magnitude B
1
and a frequency &ohgr;. The most convenient frame of reference in which to describe the effect of RF field
44
is a rotating frame of reference, having an origin coinciding with moment
40
and rotating in the X-Y plane at an angular velocity &ohgr;. The following description is based on such a rotating frame. The total interaction between RF field
44
, static field
42
and moment
40
may be described by an effective magnetic field vector B
eff
which interacts with moment
40
. B
eff
created by RF field
44
comprises an X-component &ggr;B
1
(which defines the component of B
eff
in the X-Y plane) and a Z component &Dgr;&ohgr;=&ohgr;−&ohgr;
0
. The interaction between B
eff
and moment
40
is such that moment
40
precesses around B
eff
at an angular frequency |&ggr;B
eff
|. RF field
44
has X, Y and Z components. However, the RF field component in the Z direction generally has a smaller magnitude than B
0
, so it may be ignored.
The effect of a (varying) RF magnetic field on a magnetic moment is generally described by a well known differential equation, the Bloch equation, which is described in detail in C. P. Slichter, “Principles of Magnetic Resonance,” 3rd ed. Springer-Verlag, Berlin, 1992, the disclosure of which is incorporated herein by reference. The following description is correct for short time scales (pulse duration is shorter than a typical T
2
of the human tissues), where the relaxation terms in the Bloch equation may be ignored.
When combining the effects of RF field
44
and static field
42
, notice must be taken of the relationship between &ohgr; and &ohgr;
0
.
FIG. 3
shows the case where &ohgr;=&ohgr;
0
. In this case, the B
eff
interacting with moment
40
is wholly in the X direction (with an amplitude determined by the amplitude of RF field
44
). The effect of such a vector on moment
40
is that moment
40
precesses around the X axis (i.e., in the Z-Y plane in the case that moment
40
was originally aligned with the +Z direction).
In MRI terminology, a pulse using an RF field with &ohgr;=&ohgr;
0
is called an AM pulse. This type of pulse has a frequency, equal to the Larmor frequency (&ohgr;
0
). Its amplitude changes with time. An AM pulse may be used to effect inversion of a moment
40
, which moment is originally aligned with the Z direction, in the following manner: an RF field
44
having a known strength is applied to moment
40
; and once moment
40
has made a 180° rotation (in the Y-Z plane), i.e., moment
40
is inverted, RF field
44
is turned-off.
The precession velocity of moment
40
about rotating axis X is linearly dependent on the amplitude of RF field
44
. An important result of this linear dependency is the sensitivity of the AM pulse to RF field inhomegenities, which may be characteristic of an RF coil which generates RF field
44
or by local inhomegenities caused by the sample being imaged. As an example of this sensitivity to fi

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