Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-03-28
2001-05-08
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C324S309000, C324S307000
Reexamination Certificate
active
06230039
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods and systems for magnetic resonance (MR) imaging of moving parts of a patient with reduced motion artifacts, in particular to MR imaging protocols in which both the magnetic field gradients and the flip angles (&agr;) for exciting nuclear magnetization are adaptively selected.
2. Description of the Related Art
Motion, notably due to or induced by cardiac or respiratory motions, is unavoidable in many clinical MR imaging situations. Without correction, MR images of moving parts of a patient are well-known to contain various confusing artifacts, such as ghost images, which can easily lead to clinical misinterpretation of the MR images.
In one known method for correcting such motion artifacts, after MR signals are collected, the displacement of the moving part is measured, such as by an MR navigator protocol. The collected MR data is then retrospectively discarded, i.e., not used for image reconstruction, if the subsequently measured displacement exceeds a threshold value. See, e.g., Sachs et al., 1994, Magnetic Resonance in Medicine 32:639-645.
However, this method, known as motion “gating”, has the problem of lengthening data collection times in proportion to the fraction of time during which the displacement of the moving part exceeds the threshold value. Also the steady state of nuclear magnetization must be maintained. If the steady state is not maintained, MR signals measured when the displacement is below threshold will have varying signal strengths. Varying signal strengths introduce unpredictable modulations in k-space that are also well-known to lead to artifacts in the visible image.
An improvement to the gating method, referred to herein as motion-adaptive gating (“MAG”), attempts to shorten the data collection period by collecting MR data with less stringent boundary conditions than required by simple gating. According to this method, without significant loss of image quality, data generated from phase encoding gradients with larger time-integrals (stronger gradients) can be acquired at greater displacements, such as at greater respiratory or diaphragmatic displacements, as long as data generated from phase-encoding gradients with smaller time-integrals are acquired at smaller displacements. Then, it is only for a comparatively smaller fraction of time, when the displacement becomes very large, that no useable MR signals at all can be collected. The MAG method is described in U.S. patent application Ser. No. 08/795,119 filed Feb. 7, 1997, now U.S. Pat. No. 5,977,769.
Although the MAG method may reduce data collection times, it can also lead to increased artifacts due to increased, unpredictable signal-strength modulations in k-space. It will be immediately appreciated that in MAG the actual sequence of phase-encoding gradients can be highly disordered. Depending on the motion's course, MR signals adjacent in k-space can be acquired at unpredictable and widely different times during data acquisition. Unless the strength of the MR signals is carefully and smoothly controlled throughout data collection, modulations will be superimposed on k-space having an unknown structures. Random modulation components blurs the point spread function; periodic components produce ghosts, possibly leading to such misinterpretations as focal stenosis. The requisite careful control of MR signal strengths is, however, difficult and problematic.
Therefore, since both gating and, especially, motion-adaptive gating, although reducing motion artifacts, may introduce further artifacts due to uncontrolled and unpredictable modulation of signal strength in k-space, what is needed are simple and reliable MR methods and apparatus for also reducing or eliminating these further artifacts in order to achieve MR images with reduced motion artifacts in reduced times.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicants' invention of the invention subsequently claimed.
SUMMARY OF THE INVENTION
The object of the present invention is to provide methods and apparatus which overcome the above identified problems in, and satisfy the needs of, the art. In particular, it is an object of this invention to provide simple and reliable MR methods and apparatus for reducing or eliminating the artifacts due to uncontrolled or unpredictable modulation of signal strength in k-space introduced by gating, in particular by respiratory gating, and, especially, by motion-adaptive gating. Thereby, the present invention provides MR images of moving parts of a patient with improved image quality and in reduced scan times.
Generally, this object is achieved by methods of MR imaging according to which, where the temporal order of phase-encoding gradients applied to generate MR images is non-deterministic, or unpredictable, or uncertain, or even random for whatever reason, resulting irregularities of the signal strength distribution in k-space are minimized by adaptively selecting the overall signal strength of the generated MR signals in at least partial dependence on the phase-encoding gradients to be applied so that a pre-determined and smooth signal strength distribution is achieved in k-space. MR signal strengths are selected by determining magnetization flip angles, or RF powers, applied to excite the MR signals.
In particular, in the preferred embodiment, non-deterministic temporal ordering of phase-encode gradients results from adaptive gating methods. These methods make use of the fact that lower spatial frequency components of k-space, which provide for overall image definition, are more susceptible to motion disturbance, while higher spatial frequency components, which provide for finer image details, are less susceptible to motion disturbance. Therefore, the methods of this invention, first, determine the instantaneous state of motion of the part of the patient being imaged, and, second, select the spatial frequency of the phase encoding gradients next to be applied so that the lower spatial frequency components are acquired when the state of motion is less, while the higher spatial frequency components can be acquired when the state of motion is greater.
In preferred embodiments, the state of motion is represented by displacements of a moving part of a patient being imaged from a reference position along the directions of the phase-encoding gradients of the MR data acquisition sequence. The spatial frequencies of the data to be acquired are represented by the time integral of the phase-encoding gradients applied.
Next, the strength of the MR signal that will be generated from the next imaging sequence is also adaptively selected in at least partial dependence on the instantaneous state of motion, or on the time-integral of the selected phase-encoding gradient, in order that the signal strength distribution throughout k-space will be have a smooth and slowly-varying pre-determined shape that minimizes artifacts. Flip angles, and RF powers, are determined to achieve the selected MR signal strength.
In preferred embodiments, the flip angles are chosen so that the overall signal strength decreases smoothly and monotonically from the center of k-space to its periphery and is circularly symmetric. Further, for increased accuracy, the flip angles are selected in further dependence on the expected state of longitudinal magnetization just before the RF pulse. Further, the expected state of longitudinal magnetization can be determined by neglecting the effects of residual transverse magnetization, or for increased accuracy, it can be determined by taking these effects into account.
Finally, the following preliminary matters are set forth. Generally, “k-space” is taken herein to be a spatial-frequency domain in which an MR signal is sampled along a trajectory, the sampled values yielding the inverse Fourier transformed values of the magnetization distribution excited in the body. The trajectory in the k-space is determined by
Botnar Rene
Stuber Matthias
Lateef Marvin M.
Mantis Mercader Eleni Maria
Philips Electronics North America Corporation
Vodopia John F.
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