Magnetic resonance imaging method

Details Magnetic resonance imaging method Magnetic resonance imaging method

2002-08-19

2004-03-16

Arana, Louis (Department: 2862)

Electricity: measuring and testing

Particle precession resonance

Using a nuclear resonance spectrometer system

C324S309000

Reexamination Certificate

active

06707299

ABSTRACT:

The invention relates to an imaging method used to examine substances, whereby through indirect nuclear spin—spin interaction, a precession of at least some nuclear spins having an additional phase angle &Dgr;&phgr;=−&ggr;
I
-&Dgr;B
z
T relative to an already existent precession is created in an external magnetic field, so that a transverse magnetization fans out perpendicular to the external magnetic field, as a result of which a relaxation of the transverse magnetization having a relaxation time T
2
is generated.
A method of this generic type is known from European Patent Application EP 0 803 740 A1. In this method, a high-resolution image is taken and subsequently a calibration sequence is taken after one or more rephasing pulses.
International patent application WO 99/14616 discloses a magnetic resonance imaging method in which a keyhole technique is used. Here, first of all, a high-resolution image is taken and subsequently, several images having a lower resolution are made by means of a fast EPI sequence.
The article titled “Functional Imaging by I
0
−
and T
2
* Parameter Mapping Using Multi-Image EPI” describes an echo-planar imaging method (EPI) in which, in order to achieve an especially high time resolution in the functional imaging, a sequence of EPI activations after one single excitation.
Taking a high-resolution image and several images during one single relaxation is not known from any of the documents found.
The invention relates especially to a method for determining the spin—spin relaxation time T
2
.
In nuclear magnetic resonance tomography, nuclear spins are excited. The excited nuclear spins relax in states of equilibrium. Energy transfer is needed for this purpose. In the case of spherically symmetrical magnetic nuclei, which have no electric nuclear quadropole moment, only an interaction with time-variable magnetic fields &Dgr;
B
(t) is a possibility in this context. They have vertical components.
The vertical components, &Dgr;B
x
(t) and &Dgr;B
y
(t), which oscillate at a Larmor angular frequency &ohgr;
L
=−&ggr;
I
B
0
, induce transitions between the split nuclear levels and ultimately lead to a non-adiabatic, irreversible relaxation of the longitudinal magnetization M
z
. The main causes for these interference fields are known to be:
1. anisotropy of the shielding: &Dgr;B
shift
(t),
2. fluctuations due to direct nuclear dipole—dipole interaction: &Dgr;
B
dipolar
(t),
3. spin-rotation interaction: &Dgr;
B
spin-rot
(t),
4. interaction with paramagnetic atoms or molecules: &Dgr;
B
para
(t).
This relaxation process takes place on a microscopic scale (1 to 10 A) and is described by the longitudinal or also spin-lattice relaxation time T
1
. The latter designation refers to the notion that the molecular movements effectuate a thermal “phonon bath”, the so-called lattice. The closer the resonance frequency of the nucleus {overscore (&ohgr;)}L/(2&pgr;) is to the phonon frequency—~1 MHz for biological tissue—the more readily and more often the lattice is present in a suitable configuration to dissipate nuclear spin energy in a resonant manner. The resonance frequencies used in medicine are well above 1 MHz, so that weaker main fields B
0
lead to a greater exchange of energy between the nuclear spin and the phonon bath, resulting in a shorter T
1
, T
1
=T
1
(B
0
). Typical values for T
1
in liquids are between 10
−4
s and 10 s. Due to the much slower movement of the atoms in the crystal lattice, T
1
in solids is longer by about two orders of magnitude (10
−2
to 10
3
).
Primarily the indirect nuclear spin—spin interaction makes a special—this time adiabatic—contribution to the relaxation on the microscopic level in that it enlarges the local magnetic field by &Dgr;B
z
at the location of a nucleus for a short “exposure time”&tgr;. Then the nuclear spin precedes with an additional phase angle &Dgr;&phgr;=−&ggr;
I
&Dgr;B
z
&tgr; relative to the precession in the external magnetic field
B
0
. All in all, this effect causes the phase relationship of equivalent nuclear spins to be lost and the transverse magnetization to fan out in the x-y plane. This relaxation of the transverse magnetization is characterized by a transversal relaxation time T
2
, which is also designated as spin—spin relaxation time. The presence of paramagnetic foreign atoms can likewise result in a spin density transfer to the nucleus location, thus diminishing T
2
. In microscopic dimensions (1 to 100 &mgr;m), the diffusion and perfusion of the nuclear spin in local susceptibility gradients reduce the T
2
time.
By virtue of its origins, T
2
is almost independent of the strength of the main magnetic field B
0
. However, a relaxation of the transverse magnetization is always associated with the T
1
relaxation. Consequently, T
2
can never be larger than T
1
. In liquids, T
2
is approximately one order of magnitude below T
1
, whereas in solids, the “exposure time” &tgr; is much longer, so that normally T
2
is <<T
1
here.
Nuclear magnetic resonance tomography is employed, among other things, to obtain spectroscopic information or image information about a given substance. A combination of nuclear magnetic resonance tomography with the techniques of magnetic resonance imaging (MRI) provides a spatial image of the chemical composition of the substance.
Magnetic resonance imaging is, on the one hand, a tried and true imaging method that is employed clinically worldwide. On the other hand, magnetic resonance imaging constitutes a very important examination tool for industry and research outside the realm of medicine as well. Examples of applications are the inspection of food products, quality control, pre-clinical testing of drugs in the pharmaceutical industry or the examination of geological structures, such as pore size in rock specimens for oil exploration.
The special strength of magnetic resonance imaging lies in the fact that very many parameters have an effect on nuclear magnetic resonance signals. A painstaking and controlled variation of these parameters allows experiments to be performed that are suitable to show the influence of the selected parameter.
Examples of relevant parameters are diffusion processes, probability density distributions of protons or a spin-lattice relaxation time.
In nuclear resonance tomography, atom nuclei having a magnetic momentum are oriented by a magnetic field applied from the outside. In this process, the nuclei execute a precession movement having a characteristic angular frequency (Larmor frequency) around the direction of the magnetic field. The Larmor frequency depends on the strength of the magnetic field and on the magnetic properties of the substance, particularly on the gyromagnetic constant &ggr; of the nucleus. The gyromagnetic constant &ggr; is a characteristic quantity for every type of atom. The atom nuclei have a magnetic momentum &mgr;=&ggr;×p wherein p stands for the angular momentum of the nucleus.
In nuclear resonance tomography, a substance or a person to be examined is subjected to a uniform magnetic field. This uniform magnetic field is also called a polarization field B
0
and the axis of the uniform magnetic field is called the z axis. With their characteristic Larmor frequency, the individual magnetic momentums of the spin in the tissue precede around the axis of the uniform magnetic field.
A net magnetization M
z
is generated in the direction of the polarization field, whereby the randomly oriented magnetic components cancel each other out in the plane perpendicular to this (the x-y plane). After the uniform magnetic field has been applied, an excitation field B
1
is additionally generated. This excitation field B
1
is polarized in the x-y plane and it has a frequency that is as close as possible to the Larmor frequency. As a result, the net magnetic momentum M
z
can be tilted into the x-y plane in such a way that a transverse magnetization M
t
is created. The transverse component of the magnetization rotates in the x-y plane wit

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