TECHNICAL FIELD

The present disclosure concerns motion correction techniques and, in particular, techniques that compensate for motion-induced phase offsets in accordance with the use of blipped Simultaneous Multi-Slice (SMS) and blipped Controlled Aliasing in Parallel Imaging Results in Higher Acceleration (CAIPIRINHA) imaging techniques used in magnetic resonance imaging (MRI) scans.

BACKGROUND

Several types of motion correction techniques for MRI scans are conventionally used to eliminate or reduce artifacts caused by the motion of an imaged object. Some examples of such conventional techniques include using image data (2D slice stacks or 3D volumes), or using additional detection techniques such as skin surface detection or tracking applied landmarks (e.g. markers).

However, these conventional approaches cannot be combined with blipped parallel imaging techniques such as blipped SMS or blipped CAIPIRINHA, as such methods make use of an additional gradient (referred to as a blip or blipped gradient) to apply supplemental encoding in three-dimensional k-space. Furthermore, conventional motion correction techniques ignore the additional gradient implemented by the aforementioned parallel imaging techniques, leading to inconsistent gradient moment accumulation depending on the motion state of the imaged object during image acquisition, which results in corrupted image reconstruction. As a result, conventional MRI motion correction techniques have several drawbacks.

SUMMARY

As noted above, conventional motion correction techniques for MRI scans cannot be combined with parallel imaging techniques such as blipped SMS or blipped CAIPIRINHA without introducing artifacts into the reconstructed images. As further discussed below, blipped SMS and blipped CAIPIRINHA utilize an additional blipping gradient to provide a phase offset within a 3D slab or between jointly acquired MR data slices, which are recorded as raw data in k-space along a specific k-space trajectory. In other words, the blipped gradients provide a predetermined phase dispersion within a volume of interest, which leads to field of view (FOV) shifts of the aliased slices/voxel groups. Assuming motion can be ignored during the short time a gradient is applied, the effect of motion is that the volume is shifted to a different position relative to the origin of the gradient system (isocenter). Therefore, the desired phase dispersion within the volume is still achieved.

However, in addition to this “inter-slice” phase dispersion or phase ramp, which results from the use of the blipping gradient, the separate k-space entities/lines are also phase shifted depending on the motion state of the scanned object while the MR signals are acquired. In other words, each of the acquired MR data slabs or slices have an inter-slice phase offset (i.e., the aforementioned phase ramp or dispersion) with respect to one another that is a result of encoding of the blipped gradient. In addition to this, the movement of the object during the MR signal acquisition phase introduces a “motion-induced” phase offset to sequentially acquired k-space lines of the acquired MR data. For 3D acquisition techniques, encoding an additional phase offset is introduced in the same way by movement in the case of CAIPIRINHA blips. In the following, the concepts will be discussed for the 2D case using SMS blips; these apply analogously for the 3D case using CAIPIRINHA blips.

The aspects described herein compensate for the aforementioned motion-induced phase offset in various ways, as further discussed in detail below. For instance, the motion-induced phase offset may be accounted for during the MR signal acquisition phase by adapting the MR data acquisition parameters such as sampling rate and/or phase settings. Optionally, this may be implemented automatically as part of the MRI system's motion correction (MOCO) framework. As another example, the motion-induced phase offset may be accounted for during the image reconstruction phase. In any event, information regarding the motion states may be acquired via suitable prospective tracking mechanisms, such as camera systems, for instance, or additional imaging data, such as navigators, for instance. Advantageously, compensation of the motion-induced phase offset during the image reconstruction phase may also allow for other, supplementary phase offsets to accounted for during the image reconstruction phase in case of more sophisticated phase profiles (e.g. as caused by the Wave technique).

