Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1999-01-22
2001-08-07
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C324S307000, C324S309000
Reexamination Certificate
active
06272369
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of the invention is nuclear magnetic resonance imaging (“MRI”) methods and systems. More particularly, the invention relates to the suppression of signal from fat by optimizing the chemical shift selective (“CHESS”) MRI technique.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B
0
), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B
1
) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M
z
, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment M
t
. A signal is emitted by the excited spins after the excitation signal B
1
is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G
x
G
y
and G
z
) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The majority of commercial MRI systems excite and image hydrogen nuclei. Hydrogen is present in the human body in many different molecules, and due to the different molecular level interactions, the Larmor frequency of the hydrogen is shifted in frequency. This “chemical shift” of the Larmor frequency results in a spectrum of frequencies in the acquired NMR signals. For example, in an MRI system with a polarizing field of 1.5 Tesla the major NMR signal component produced by hydrogen in fat molecules is shifted about 220 Hz from the signal produced by hydrogen in water molecules. This chemical shift is often expressed independently of field strength as 3.5 parts per million.
In some applications it is desirable to produce images only of the water molecules. In the article “H NMR Chemical Shift Selective (CHESS) Imaging”, Haase, et al.,
Phys. Med. Biol.,
1985, vol. 30, No. 4, pp. 341-344, a technique is described in which the undesired signal component (e.g. fat) is first excited by a 90° RF excitation pulse which selectively saturates the spin magnetization prior to each imaging pulse sequence. The longitudinal magnetization of the undesired spins is thus maintained at a low level throughout the image acquisition and the NMR signals which they produce are suppressed. The CHESS sequence is comprised of an RF excitation pulse which is frequency selective to the undesired spin Larmor frequency, followed by a spoiler gradient pulse which dephases the resulting transverse magnetization prior to commencing the imaging pulse sequence.
In most applications of the CHESS method the fat signal to be suppressed is assumed to be a single signal component offset in frequency from the desired water signal. The frequency selective saturation pulse (“ChemSat pulse”) produces a 90° flip angle for this signal component. In the publication “Optimization of Chemical Shift Selective Suppression of Fat”, Proc., ISMRM, 6th Annual Meeting, pg. 1981 (1998) K. Kuroda, et al. describe a technique for calculating an optimal flip angle that takes into consideration the many different signal components in fat signals. While this work establishes that the flip angle can be optimized for the multiple fat signal components that are to be suppressed, it does not take into consideration the particular scan parameters used to acquire the image data.
SUMMARY OF THE INVENTION
The present invention is a method for practicing the CHESS technique in which the flip angle of the RF saturation pulse is optimized for the prescribed scan parameters prior to performing the scan. More particularly, the method includes: inputting the scan parameters, including parameters indicative of the TR period and the number (N) of frequency selective RF saturation pulses (ChemSat pulses) to be produced during each TR period; calculating an optimal flip angle for the ChemSat pulse based in part on the number (N) of ChemSat pulses during each TR; and acquiring MR image data with an imaging pulse sequence preceded by a CHESS pulse sequence that employs a ChemSat pulse having the optimal flip angle.
One of the many scan parameters input prior to acquiring image data is the number of slices to be acquired during each TR. Multi-slice acquisitions are common, and the number of slices varies from scan to scan. It is a discovery of the present invention that because ChemSat pulses are not spatially selective, the number of them applied during each TR has a substantial effect on the flip angle to be used for optimal suppression of selected frequency components. The optimal flip angle is thus calculated just prior to image acquisition after all the relevant scan parameters have been input.
REFERENCES:
patent: 4748409 (1988-05-01), Frahm et al.
patent: 5079504 (1992-01-01), Machida
patent: 5079505 (1992-01-01), Deimling et al.
patent: 5329231 (1994-07-01), Hatta et al.
NMR Chemical Shift Selective (CHESS) Imaging, Phys. Med. Biol., 1985, vol. 30, No. 4, 341-344, Haase, et al.
Optimization of Chemical Shift Selective Suppression of Fat, p. 1981, Kuroda, et al.
Optimization of Chemical Shift Selective Suppression of Fat, MRM 40:506-510 (1998), Kuroda, et al.
Improved Water Suppression for Localized in Vivo1H Spectroscopy, JMR Series B, 106(1005) Feb., No. 2, Ernst, et al.
Improved Water and Lipid Suppression for 3D Press CSI Using RF Band Selective Inversion with Gradient Dephasing (Basic), MRM 38:311-321 (1997), Star-Lack, et al.
Cabou Christian G.
GE Medical Systems Global Technology Company LLC
Lateef Marvin M.
Qaderi Runa Shah
Quarles & Brady LLP
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