Magnetic resonance imaging (MRI) is an important diagnostic and imaging technique. MRI techniques are based on the absorption and emission of radio frequency (RF) energy by the nuclei of atoms. Typically, an object of interest is placed in a strong magnetic field that causes the generally disordered and randomly oriented nuclear spins of the atoms to become aligned with the applied magnetic field. One or more RF pulses are transmitted into the object, perturbing the nuclear spins. As the nuclear spins relax to their aligned state, the nuclei emit RF energy that is detected by one or more receiving coils disposed about the object. The received RF energy is processed into a magnetic resonance image of the object.
By utilizing non-uniform magnetic fields having gradients in each of three spatial dimensions, the location of the emitting nuclei can be spatially encoded so that the object can be imaged in three dimensions (3-D). The three dimensions are commonly two mutually orthogonal directions x and y defined in a plane denoted as a “slice” with a series of slices defined in a third mutually orthogonal direction z. As used herein, the x-direction is associated with a frequency-encoding (FE) direction, and the y-direction is associated with a phase-encoding (PE) direction. Generally, RF pulses having a range of frequencies are transmitted into the object, and through use of well-known frequency encoding (e.g., for the x-direction) and phase encoding techniques (e.g., for the y-direction), a set of MRI data is received by each of the one or more receiver coils for each slice of the object.
The resulting MR signal is a mix of RF waves with different amplitudes, frequencies, and phases which contain spatial information. The MR signal is digitized and raw data are written into a data matrix called k-space. K-space data are equivalent to a Fourier plane. Therefore, to create an image from k-space data requires use of a 2-D inverse Fourier transform. Thus, MRI data provide a representation of the MRI image in the frequency or k-space domain, where kx and ky are the spatial frequency variables in the x and y directions having units of cycles per unit distance. An image of the slice of the object is obtained by determining the inverse Fourier transformation of the k-space MRI data. In MRI systems having multiple receiver coils (parallel MRI), an image is reconstructed from each receiver coil, and a final image is a combination of the images from each coil. Multiple receiver coil systems can be used to achieve high spatial and temporal resolution, to suppress image artifacts, and to reduce MRI scan time. For example, dissimilarities in the spatial sensitivities of the multiple receiver coils can be used to reconstruct a full field of view (FOV) image with a reduced scan time as known to those skilled in the art.
Thus, in general, portions of the object to be imaged are scanned by a sequence of measurement cycles in which the magnetic field gradients, RF excitation pulse, and signal reading processes are varied based on the MRI imaging protocol selected. For each MRI scan, the resulting signals are digitized and processed to reconstruct the image in accordance with the MRI imaging protocol selected as known to those skilled in the art. For example, an MRI sequence of the MRI imaging protocol may be classified as a spin echo, a gradient echo, or a hybrid echo sequence with multiple variations within each classification. Additionally, a variety of reconstruction algorithms exist the use of which may depend on the MRI sequence selected as known to those skilled in the art. Exemplary reconstruction algorithms include SMASH, SENSE, GRAPPA, GRASE, PILS, SPACE RIP, etc. Additionally, the same line of k-space may be sampled multiple times to increase the signal-to-noise ratio (SNR) with a resulting increased scan time.
As a result, data acquisition for MR imaging can require a time period of several seconds to several minutes depending on the MRI imaging protocol selected. During the data acquisition time period, motion of the object being imaged such as the body of a patient may occur. Without corrective action, the motion produces artifacts that may degrade image quality. There are two types of artifacts due to motion: random movements which produce a blurry and noisy image, mainly in the PE direction, and periodic motion which creates ghost images in the PE direction. Motion artifacts mainly propagate in the PE direction due to movement of the spins between excitations or between the phase-encoding and the reading of the signal echo. Signal sampling and spatial-encoding in the FE direction generally is done so fast that physiological motion produces only a small amount of spatial blurring in the FE direction which can be removed without penalty, for example, using a bandpass filter.
To minimize the motion-related artifacts in the PE direction, a number of techniques have been developed. For example, the MR scan can be synchronized to the anatomical motion if the motion is known. Other techniques include the use of navigator echoes in which two different types of MR data are acquired. The first data type is used to form the MR image. The second data type is used to assess and compensate for the motion that occurs during the MRI data acquisition time period. The second data type may be acquired at regular intervals throughout the MRI data acquisition time period and interleaved with the data acquisition of the first data type. Data of the second data type is referred to as “navigator echoes”.
Motion artifacts can be reduced by using navigator echoes to identify motion-corrupted measurements and to reacquire the identified motion-corrupted measurements when the anatomy is close to a baseline position. Motion correction with navigators can come at the expense of a substantial increase in scan time. A more time efficient method is to extract information of in-plane and through-plane displacements from the navigator echoes so that k-space data can be retrospectively corrected. However, navigator displacement measurements require a priori knowledge of the type of motion so that the navigator can be tailored to the specific type of motion. For example, spherical or orbital navigators may be used to detect bulk translation and rotation. Pencil-beam navigators can be used to detect local translational motion. PROPELLER MRI, proposed by J. G. Pipe, Motion correction with PROPELLER MRI: application to head motion and free-breathing cardiac imaging, Magn Reson Med 1999, 42(5), pp. 963-969, is a type of self-navigated data acquisition technique in which k-space data are acquired in blades to produce oversampling at the center of k-space. The oversampled k-space center acts as an inherent “navigator” to allow correction for in-plane bulk translation as well as rotation. The potential of parallel imaging techniques in removing or reducing motion artifacts has also been investigated. However, each of the current methods for removing or reducing motion artifacts has some drawbacks. What is needed, therefore, is a method and a system for motion correction of imaging data without some of the drawbacks of existing methods.
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