Power spectral density imaging to expand the Nyquist limit

Power spectral density imaging (PSDI), a new Magnetic Resonance Imaging (MRI) method, is introduced for the unaliasing of high frequency signal components which are usually mixed with the low frequency components in conventional MRI. In PSDI, k-space is scanned in several different ways, so the temporal and spatial components are aliased in different patterns. With the correct reconstruction, the power spectral density for each pixel is acquired with a much higher Nyquist frequency limit than could be otherwise achieved. In two-spatial-dimension (2D) PSDI, the Nyquist limit is determined by the time spent scanning every k_x line. In the case of three-spatial-dimension (3D) PSDI, it is the time spent on scanning every (k_x, k_y) plane which determines the limit. Signals of full-brain functional MRI (FMRI) and functional connectivity studies are always contaminated with temporally aliased respiratory and cardiac signals, which are caused by hardware limitations on the TR time. With a higher Nyquist limit, PSDI can increase the accuracy of function detection in FMRI and functional connectivity studies by elimination of the contamination from the cardiac and respiratory signals. Conversely, unambiguous detection of the cardiac-band signal fluctuations could provide a new type of MR angiography. The tradeoff of this method is that only statistics can be acquired for each pixel, such as the mean and the power spectral density. In addition, a relatively long time is required for the collection of all the required data.