As the acquisition starts immediately, a

center out, non-Cartesian, sampling of k-space is required as there is no time for a phase encode gradient or de-phasing read BLZ945 clinical trial gradient [24]. Typically k-space is sampled radially however, spiral sampling has also been used for samples with a somewhat longer signal lifetime [6]. A center out sampling pattern is desirable as it minimizes the echo time and ensures maximum signal sampled at the center of k-space. A drawback of non-Cartesian sampling is that it prevents the use of the fast Fourier transform (FFT), and therefore image reconstruction becomes prohibitively time consuming for many images. To overcome this limitation, “re-gridding” techniques have been developed to interpolate the measured signal onto a regular Cartesian grid which can then be transformed using the FFT [27]. It is important to choose the convolution function for this interpolation process accurately. Theoretically, a sinc function of infinite extent should be used, however, this is not practical. Common alternative convolution functions include truncated sinc interpolation, Kaiser–Bessel interpolation

and min–max interpolation [28] and [29]. Such re-gridding techniques permit image reconstruction in almost the same time as with Cartesian sampling. check details Non-Cartesian sampling, especially radial sampling, acquires data non-uniformly throughout k-space. In the case of radial sampling, many more points are acquired at the center of k-space (i.e. in the low spatial frequency region). If all data points are weighted equally, the Fourier transform would be biased to these low frequency data resulting in a low spatial resolution, or heavily blurred, image. Density compensation is used to overcome this limitation [30]. Density compensation considers the sampling density throughout k-space

and uses a weighting function to correct for this. For radial sampling the weighting function will increase the contribution of the points around the edge of k-space prior to re-gridding and Fourier transformation. Re-gridding with density compensation alone can produce blurring and artifacts in the reconstructed image, especially if the number of lines in the radial sampling pattern is small. An alternative approach is to iteratively reconstruct the image based Parvulin on the a priori assumption that the unknown spin proton density image is sparse with respect to a specific representation. This assumption results in nonlinear optimization methods such as CS [3], [16], [17], [18] and [19]. All experiments were performed using a Bruker, AV400 spectrometer, operating at a 1H resonance frequency of 400.23 MHz. A three-axis, shielded gradient system with a maximum strength of 146 G cm−1 was used for gradient encoding, and a 25 mm diameter birdcage r.f. coil was used for excitation and signal detection.