Rapid MRI acquisition is typically achieved by acquiring all or most lines of k-space after one RF excitation. Parallel imaging techniques can further accelerate data acquisition by acquiring fewer phase-encoded lines and utilizing the spatial sensitivity information of the RF coil arrays. The goal of this study was to develop a new MRI data acquisition and reconstruction technique that is capable of reconstructing a 2D image using highly undersampled k-space data without any special hardware. Such a technique would be very efficient, as it would significantly reduce the time wasted during multiple RF excitations or phase encoding and gradient switching periods.
The essence of this new technique is to densely sample a small number of projections, which should be acquired at an angle other than 0 degrees or multiples of 45 degrees. This results in multiple rays passing through a voxel and provides new and independent measurements for each voxel. Then the images are reconstructed using the unique information coming from these projections combined with RF coil sensitivity profiles. The feasibility of this new technique was investigated with realistic simulations and experimental studies using a phantom and compared with conventional nonuniform fast Fourier transform technique. Eigenvalue analysis and error calculations were conducted to find optimal projection angles and minimum requirements for dense sampling.
Pictured Above: Axial image of the ACR phantom acquired with full-scan SPGR pulse sequence (left). The images were reconstructed from the undersampled projection data using HAPI and conventional NUFFT methods. Np is the number of projections. HAPI was able to reconstruct high resolution images with negligible artifacts with only 16 projections while conventional techniques still had substantial streaking artifacts.
We have developed a new inverse-solution approach to design RF coils optimized for parallel imaging techniques in MRI. Parallel imaging improves the speed of image acquisition. However, it introduces spatially varying degradation in signal to noise ratio (SNR) in images when conventional RF coils are used. In order to address this problem, we derived an inverse problem in which SNR was formulated as a function of coil geometry. This expression is calculated by a Least Squares approach to find the RF coil geometry that maximized the SNR of images. We hold a patent for this approach (Patent no: 7362101) and received the 1st place award in Engineering Category in the 2006 scientific meeting of the International Society of Magnetic Resonance in Medicine (ISMRM) (abstract # 26).
Pictured Above: (a) One element of the phased array coil optimized for brain imaging at 1.5T MRI. (b) and (c) are the transverse component of the magnetic flux density in axial and sagittal planes, respectively. (d) shows 1/g factor and (e) and (f) are the full and SENSE accelerated SNR, respectively.
We have also developed various new RF coil designs to achieve better sensitivity for multinuclear MRI. One of those RF coils can be tuned automatically inside the MRI using LabView to achieve the best performance for variations in the loads (http://www.ncbi.nlm.nih.gov/pubmed/11945031). We also developed a PIN diode controlled multinuclear RF coil that improved the SNR by more than two-fold compared to conventional coil designs (http://www.ncbi.nlm.nih.gov/pubmed/20393229). These RF coils are essential to study anatomy and metabolism simultaneously. For instance, changes in sodium concentration were implicated in tumors. Similarly, localized sodium deficits were reported in the brain in several cognitive disorders. Thus, one can obtain information about the disease metabolism from the sodium images while hydrogen images provide high-resolution structural information.
Multi-nuclear phased array receive coil inserted into a quad-port fed dual-tuned birdcage transmit coil along with associated electronics.
In this study, the performance of linogram acquisition was investigated for the reconstruction of images from undersampled data using parallel imaging methods . Generalized auto-calibrating partially parallel acquisition was implemented for this new sampling scheme and images were reconstructed with high acceleration rates. A schematic representation of the method is shown in Figure 1. The results demonstrated that the PSF was sharper and the mean artifact power was lower in linogram sampling compared with radial sampling. In vivo human study performed at 7T demonstrated that linogram sampling could provide high-quality images of anatomy, even at high acceleration rates (Figure 2). Linogram sampling not only possesses the advantages of radial sampling, such as reduced sensitivity to motion and higher acceleration rates, but it also provides sharper images with fewer artifacts. Moreover, it is less prone to off-resonance artifacts compared to radial sampling.
 Ersoz A, Muftuler LT. Undersampled linogram trajectory for fast imaging (ULTI): experiments at 3 T and 7 T. NMR Biomed. 2016 Mar;29(3):340-8.