Recently, a number of methods have been demonstrated for large field of view MR imaging using continuous table motion. As with conventional, fixed-table MRI, the spatial encoding is performed using magnetic field gradients. However, it is demonstrated in this work that as a consequence of every measurement being made at a slightly different displacement between the object and the gradient field, gradient nonlinearities are manifest as blurring in addition to spatial distortion. Moreover, the blurring is spatially dependent. It is also shown that correcting all phase-encoding steps individually or in groups can reduce these effects. (1)(2)(3)(4)(5)(6)(7)(8)(9). With these techniques the data must be spatially registered based on the table position. Because the pixel value at each location in the reconstructed image is formed from data acquired at multiple positions of the object within the gradient field, one might assume that artifacts from gradient nonlinearities manifest themselves differently for these types of acquisitions than for those with a fixed table position. To our knowledge, these gradientrelated effects have not been directly addressed in moving table imaging. Rather, the severity of these effects has been reduced by minimizing the FOV of the data acquisition.In MR image reconstruction it is assumed that there is a direct linear relationship between position and precessional frequency based on the gradient field strength. Any deviation from linearity results in a spatial distortion of the reconstructed image. Correction for this spatial distortion in imaging a fixed FOV is well understood and involves both a spatial remapping as well as an intensity correction (10,11). The technique used in this work is from Ref. 10 and is commonly referred to as "GradWarp."In one method of continuous moving table imaging in which frequency-encoding is along the direction of motion (4), 3D imaging volumes with a limited FOV are repeated multiple times while the table is moved continuously to extend the total imaging FOV. In this case, each phaseencoding step is first Fourier-transformed along the frequency-encoding direction and then registered along the same direction based on the distance the table has moved. This step is repeated for all phase-encoding steps in each acquisition. The resulting hybrid data that has been Fourier-transformed along the frequency-encoding direction can then be transformed along the other two dimensions to generate an imaging volume with a reconstructed FOV larger than the frequency-encoding FOV.In this work we describe the impact of gradient nonlinearity on continuous table motion acquisitions. First, we provide a qualitative description of the effects, followed by a more quantitative derivation of the point-spread function. We then present a technique for correction of such effects and demonstrate the performance of the correction in the specific application of table motion along the frequency-encoding direction. DESCRIPTIONThe effect of gradient nonlinearity in continuous table mot...

The feasibility of reconstructing three-dimensional (3D) MRI data sets from limited-view projections is investigated in phantom and in vivo animal studies to improve the temporal resolution of magnetic resonance angiography without sacrificing spatial resolution. Thirty-two pairs of orthogonal biplane projections are acquired in an interleaved manner during the first pass of a contrast agent. The full data set is reconstructed as a priori 3D information. Each pair of projections is then reconstructed into an individual 3D data set based on a correlation analysis with the a priori data set. In this way, time-resolved 3D data sets at 1-to 2-s time intervals are reconstructed with submillimeter spatial resolution. Three-dimensional (3D) contrast-enhanced magnetic resonance angiography (MRA) has been widely applied as a minimally invasive vascular imaging modality (1). To avoid venous enhancement and to improve arterial signalto-noise ratio (SNR) of the 3D data set, elliptical centricordered (EC) sequences are normally used to acquire central regions of k-space at the peak arterial enhancement (2). This requires accurate timing of the contrast arrival at the region of interest and also a good estimate of the contrast distribution time through the region on a patient-by-patient basis to optimize the acquisition parameters such as the injection rate, repetition time (TR), and data matrix size (3). Recessed EC sequences (4) were developed to minimize artifacts associated with timing errors and successfully improved the robustness of contrast-enhanced MRA in clinical applications. However, in some particular situations, such as among patients with arteriovenous fistula or arteriovenous malformations, the blood flow pattern may be significantly altered by irregular pathways and the time for artery-vein contrast passage may be shortened to only a few seconds (5). In such circumstances, venous contamination cannot be avoided even with an accurate timing of the contrast arrival.Time-resolved MRA has been developed for the applications where temporal resolution is of critical importance. Techniques such as time-resolved imaging of contrast kinetics (TRICKS) and projection-reconstruction TRICKS (6 -8) are able to produce 3D data sets at a high temporal rate (2-4 s/3D set) by undersampling data and using view-sharing reconstructions. Therefore, difficulties associated with contrast timing are avoided, and pathologic information can potentially be observed. However, these techniques are not without limitations. Due to the view-sharing (or sliding window) reconstructions, temporal information is smoothed. Therefore, rapid signal changes are not expected to be captured well. Furthermore, the undersampling of high-resolution data both spatially and temporally introduces concerns about the accuracy of the dynamic signals in small vessels.Alternatively, high temporal resolution can be achieved by trading off the spatial resolution in the through-plane dimension (9). This works well clinically when the signal overlapping in the throu...