Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Statutory Invention Registration
1995-06-07
2001-06-05
Gregory, Bernarr Earl (Department: 3642)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C324S309000, C600S420000
Statutory Invention Registration
active
H0001968
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention disclosed and claimed herein is generally directed to a technique for magnetic resonance (MR) imaging, wherein a hyperpolarized noble gas or other agent is used to provide the population of magnetized spins or nuclei required for imaging.
The hyperpolarized magnetization technique has been found to be very useful in increasing the signal to noise ratio in certain MR imaging applications. Such applications include imaging of the lungs or other organ or tissue of a subject, wherein the structure to be imaged is proximate to cavities or air spaces. In accordance with one such application of the hyperpolarization technique, a noble gas such as optically pumped Helium-3 or Xenon-129 is hyperpolarized externally to the subject, and then introduced into the cavities or air spaces, such as by inhaling into the lungs. The magnetization of the hyperpolarized gaseous agent is substantially greater than the magnetization predicted by the Boltzmann distribution at the magnetic field strength and temperature of the imaging situation. Accordingly, the strength of acquired MR data signals, used to construct an MR image, tends to be substantially greater than for more well known MR techniques, wherein the magnetized spin population is provided by applying a very strong static or main magnetic field to the subject. Hyperpolarized magnetization is described, for example, in an article entitled “MR Imaging With Hyperpolarized
3
He Gas”, Middleton et al, published in Magnetic Resonance in Medicine, Vol. 33, No. 2, pp. 271-275 (1995).
In alternative implementations of the hyperpolarization technique, the requisite level of magnetization is provided by hyperpolarizing body structure, blood or other body fluid of the subject, as described hereinafter in greater detail.
In the hyperpolarization technique, as in other MR methods, a succession of RF excitation pulses are directed into a volume enclosing the portion of the subject to be imaged. The excitation pulses, together with corresponding magnetic gradient fields, act to generate the MR data signals for imaging. Each excitation pulse diminishes the hyperpolarized magnetization. However, after introduction into the subject, there is no mechanism available to restore the hyperpolarized magnetization lost by successive RF excitation, resulting in substantial reduction in the strength of later-acquired MR data signals. Instead, the longitudinal (T1) relaxation of the magnetization in the patient is to the Boltzmann (rather than hyperpolarized) value, which is much smaller, and can often be considered negligible.
SUMMARY OF THE INVENTION
The invention is directed to a method for providing an MR image of a selected structural portion of a subject, the portion lying within a hypothetical imaging volume. The method comprises introducing a quantity of a magnetically hyperpolarized agent into the subject, proximate to the portion to be imaged, and applying a series of pulse sequences to the imaging volume to generate successive corresponding MR data signals. Each sequence comprises a number of gradient localization pulses and RF excitation pulses. The excitation pulse, which tips longitudinal magnetization into the transverse plane, has an associated flip angle. The associated flip angles are progressively increased to maintain the signal strength of the successive MR data signals at a substantially constant level. After acquisition, the MR data signals are processed to provide the desired image of the structural portion.
In a preferred embodiment of the invention, which neglects T1 relaxation (since the equilibrium magnetization is usually negligible compared to the hyperpolarized value), flip angle is increased by recursively setting the flip angle associated with a given pulse sequence to the inverse sine function of the tangent of the flip angle of the sequence just preceding the given pulse sequence in the series. Such embodiment is considered to simplify the task of progressively increasing the flip angles. In an imaging arrangement wherein the hyperpolarized and equilibrium magnetizations are comparable, the recursive relationship can be easily extended to include T1-relaxation.
It is anticipated that an embodiment of the invention will be particularly useful in imaging body structure such as lungs or heart related cardiac tissue, which surrounds or is proximate to cavities or air spaces which may receive a hyperpolarized gaseous agent. It is further anticipated that the invention will also be useful in imaging structure relating to blood or other fluid if a hyperpolarized agent is dissolved therein. Alternatively, the agent could comprise a quantity of blood or other fluid taken from a subject, externally hyperpolarized, and then reintroduced into the subject.
An object of the invention is to provide an MR imaging arrangement using hyperpolarized magnetization, wherein the signal strength of successive acquired MR data signals is maintained at a substantially constant level.
Another object is to provide an imaging arrangement of the above type, wherein the flip angle of each successive MR sequence is selectively increased over the preceding flip angle.
These and other objects of the invention will become more readily apparent from the ensuing specification, taken together with the accompanying drawings.
REFERENCES:
patent: 5271401 (1993-12-01), Fishman
patent: 5307014 (1994-04-01), Laub
patent: 5337749 (1994-08-01), Shimizu
patent: 5357959 (1994-10-01), Fishman
patent: 5419325 (1995-05-01), Dumoulin et al.
patent: 5499629 (1996-03-01), Kerr et al.
Purdy et al, “The Design of Variable Tip Angle Slab Selection (TONE) Pulses for Improved 3-D MR Angiography”, Book of Abstracts of SMRM, vol. 1, p. 882 (1992).
Middleton et al, “MR Imaging with Hyperpolarized3He Gas”, Magnetic Resonance in Medicine, vol. 330, pp. 271-275 (1995).
Conolly et al, “Prepolarized MRI with a Periodic Bias Field”, Proceedings of the SMRM, vol. 3, p. 750 (1994).
Scott et al, “Body Noise Feasibility Limits of PMRI”, Proceedings of the SMRM, vol. 2, p. 1083 (1994).
Mugler et al, “Shaping the Signal Response During the Approach to Steady State in Three-Dimensional Mag-Prep, Rapid G-E Imaging Using Variable Flip Angle”, Mag. Reson. in Medicine, vol. 28, pp. 165-185 (1992).
Magele et al, “The Effects of Linearly Increasing Flip Angles on 3D Inflow MR Angiography”, Mag. Reson. in Medicine, vol. 31, pp. 561-566 (1994).
Berrier, Jr. Erwin F.
General Electric Company
Gregory Bernarr Earl
Pilarski John H.
Skarsten James O.
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