Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2001-06-29
2004-03-30
Ruhl, Dennis (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C128S898000, C324S307000, C324S309000, C324S312000, C324S314000, C382S128000
Reexamination Certificate
active
06714807
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to magnetic resonance imaging systems, and specifically to systems using steady-state free precession techniques.
Magnetic resonance imaging (MRI) images nuclei having a magnetic moment, usually hydrogen nuclei, by measuring a signal generated by the nuclei precessing in a magnetic field. The angular frequency of precession &ohgr;
0
is directly dependent on the magnetic field B
0
within which the nuclei are positioned, according to the Larmor equation:
&ohgr;
0
=&ggr;·B
0
(1)
wherein &ggr; is a constant termed the gyromagnetic ratio.
The magnetic field is set to vary in a known, spatially-dependent manner within the region being imaged, so that the corresponding precession frequency will vary in the same spatially-dependent manner. The spatially-dependent field is generated by imposing a plurality of magnetic fields having known gradients on the homogeneous “underlying” magnetic field B
0
. Most preferably, three orthogonal, substantially linear gradients G
x
, G
y
, and G
z
are imposed, so that the magnetic field at any point (x, y, z) is given by the equation:
B
(
x,y,z
)=
B
0
x·G
x
+y·G
y
+z·G
z
(2)
In order to cause nuclei to precess, the nuclei are shifted from their equilibrium thermal state by a pulsed radio-frequency (RF) excitation field, whose magnetic component is in a direction orthogonal to the spatially-dependent magnetic field imposed on the nuclei, herein assumed to be along the z-axis. The frequency is approximately equal to the Larmor frequency, so that the RF pulse acts as a resonant driver of the nuclei. At the conclusion of the RF driving pulse, the nuclei will have been “flipped” towards the x-y plane, by an angle dependent on the length and amplitude of the RF pulse. The nuclei then relax towards their thermal equilibrium state, by precessing about the magnetic field, and thus generate a precession signal. The intensity of a specific frequency of precession signal will be a function of the numbers of nuclei precessing at that frequency, and thus the intensity gives a measure of the density of those nuclei at the position defined by the frequency.
Steady-state free precession (SSFP) is a technique for generating MRI signals which is well known in the MRI art, wherein the hydrogen nuclei do not completely return to their thermal equilibrium state. SSFP pulse sequences are described in
Magnetic Resonance Imaging
by Haacke et al., published by Wiley-Liss. The technique relies on achieving a quasi-steady-state of magnetization in a subject being scanned, usually a human subject, by applying an SSFP pulse sequence at repetition times (TR) significantly shorter than the spin-lattice (T1) and the spin—spin (T2) relaxation times of hydrogen nuclei within the subject. The SSFP pulse sequence comprises a series of RF excitation pulses. The SSFP sequence also comprises a plurality of magnetic gradient pulses which reverse the magnetic field gradients in a predetermined manner, in order to enhance the signal, by methods which are known in the art. Each set of pulses has the same overall repetition time TR. Using SSFP pulse sequences achieves high signal-to-noise ratios within short scan times. However, images produced by some SSFP sequences are very sensitive to motion.
An article titled “Motion-Insensitive, Steady-State Free Precession Imaging,” by Zur et al., in
Magnetic Resonance in Medicine
16 (1990), which is incorporated herein by reference, describes a method for overcoming problems associated with SSFP sequences caused by motion of the region being scanned. The method comprises generating the magnetic field gradients so that a time integral of each of the gradients during a TR period is substantially zero.
The method further comprises changing a phase of a transverse magnetization of the nuclei in a sequential manner, most preferably by changing a phase of the excitation pulses. For a series of N scan sequences, a phase shift of
2
π
(
j
-
1
)
N
radians is added, as explained in appendix B, to the spins in each TR during the j
th
sequence (j=1, 2, . . . , N). The signals from these scans are linearly combined to obtain a final image. The authors state that to avoid aliasing, it is necessary to use N≧6, and in order to reach steady-state it is necessary to wait T1 seconds between sequences. The authors further state that the SSFP signals are strongly dependent on the angle of precession, &phgr;, where &phgr; is the total precession angle over one TR period.
In addition to determining the density of hydrogen nuclei at different sections of a region being imaged, the ability to differentiate between molecular species within which the hydrogen is a component is important. Methods for generating MRI scans which differentiate between species, such as water and fat, in an image are known in the art. For example, in an article titled “Linear Combination Steady-State Free Precession MRI” by Vasanawala et al., in
Magnetic Resonance in Medicine
43 (2000), which is incorporated herein by reference, the authors describe a method for differentiating between water and fat by performing a series of SSFP scans. A first scan sequence is set to be a standard SSFP sequence, and generates raw data termed D
0-0
. In a second scan sequence a phase of 180° is added to even numbered RF excitation pulses, generating raw data termed D
0-180
. A water image is obtained from D
0-0
+i·D
0-180
; a fat image is obtained from D
0-0
−i·D
0-180
. Unfortunately, the separation of water from fat is affected both by the value of
T1
T2
of the sample and by the RF flip angle. Furthermore, in this method, the value of TR is restricted to:
TR
=
1
2
Δ
WF
(
3
)
wherein &Dgr;
WF
is a difference between water and fat resonant frequencies, and the method is unable to determine water and fat content in a single voxel.
In general, in an imaging volume strong banding artifacts are generated if &Dgr;&PHgr;, the variation in precession angle &PHgr; within the volume, is greater than about &pgr; radians. Since &Dgr;&PHgr;=2&pgr;·≢f·TR, artifacts do not occur if:
TR
<
1
2
Δ
f
(
4
)
wherein &Dgr;f is the resonance frequency variation in the imaging volume.
While values of TR satisfying inequality (4) are possible at low fields, at higher fields, i.e., approximately 1.0 T and above, the required short TRs cause severe practical problems of implementation. The short TRs necessitate very short gradient switching times and very short image signal acquisition times. Thus, the known advantages of higher-field MRI are difficult to implement with short values of TR, which also has the effect of generating peripheral nerve stimulation and an increase in RF specific absorption rate (SAR).
Disadvantages of short TR include 1) High gradient demand. The maximum available in-plane resolution and slice width is very restricted. 2) Sub-optimal SNR per unit time, because the time allotted for data acquisition in each TR is short. 3) Efficient k-space acquisition strategies such as spiral and multi-shot EPI cannot be used. 4) Fat signal suppression is difficult. 5) SAR is high.
SUMMARY OF THE INVENTION
It is therefore desirable to provide apparatus and methods for generating magnetic resonance images without a restriction of repetition time.
In preferred embodiments of the present invention, a magnetic resonance imaging (MRI) system is implemented using radio-frequency (RF) and magnetic gradient pulses in a set of SSFP sequences. Each SSFP sequence comprises a short repetition time (TR) gradient echo with fully balanced gradients in the sequence. A set of MRI generating signals comprises two to five, most preferably two or three, SSFP sequences with RF excitation pulses having high flip angles. The repetition time for each sequence is not limited to short values. By applying specific signal acquisition and analysis techniques, described hereinbelow, and by using flip angles close to 90°, inaccuracies caused by n
Della Penna Michael A.
GE Medical Systems Global Technology Co. LLC
Horton Carl B.
Lin Jeoyuh
Ruhl Dennis
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