The Theoretical Study Of Using The EPI To Improve Susceptibility-weighted Imaging

PurposeSusceptibility weighted imaging (SWI) is a recently developed new magnetic resonance imaging. It is based on the characteristic that the human body in different tissues under the influence of external magnetic field has different magnetic susceptibility, using T2* weighted 3D gradient echo sequence (GRE), also obtained the phase images and amplitude images in the same part, the phase image after high-pass filter made of the phase mask, then the amplitude image multiplied the phase mask to obtain the final SWI images with high resolution and high contrast. SWI provides an excellent imaging method of the brain and vascular lesions relating to iron deposition and has good prospects in clinical diagnosis. However, SWI imaging is slow and can only be used in brain imaging, which greatly limits the development of SWI. This study starting with the improving of sequence, combining the fastest technique of current clinical imaging echo planar imaging (EPI) with SWI, theoretically proves the feasibility, in order to find a kind of practical way to shorten the imaging time of SWI.Materials and MethodsThis study analyzed the imaging features of SWI firstly, its imaging features are summarized in five aspects, namely:3D imaging, GRE sequence, T2* weighted, the combination of phase and the magnitude image, and magnetic susceptibility sensitivity of the human tissues. According to the five characteristics that can be demonstrated from the following three, EPI sequence can be used to improve SWI:1, EPI sequence sensitive to magnetic susceptibility; 2, EPI sequence is T2* weighted; 3, EPI sequence is a frequency encoding. In determining the EPI sequence can be used to improve SWI imaging, we need to classify and select the existing EPI sequence. The existing EPI sequences according the form of pulses can be divided into SE-EPI, GRE-EPI and IR-EPI; according the K space filling mode can be divided into three types of the original EPI, BEST series, and spiral sequence. Where, because of magnetic susceptibility is not sensitive and SE-EPI and IR-EPI sequence are not suitable for improving the SWI imaging, non-adoption. Therefore, to retain the original GRE pulse sequence form, we combined different K space-filling model of the EPI form with several different GRE-EPI sequences. During analyzing the timing diagrams and K space-filling models of these several different GRE-EPI sequences, the BEST series is selected to combine with GRE sequence to improve SWI. Comparethe original 3D GRE sequence of SWI and the selected GRE-EPI sequence from the four aspects of image quality of imaging time, SNR, spatial resolution, and contrast-noise ratio. In the process of contrast, firstly the existing formula is derived, and assuming the same instrument, overcoming external environmental factors and factors of the condition of equipment under the approximate formula, the formula is (?) approximate retention and post-imaging sequence related to a number of factors(?)ne parameters used in clinical imaging and individual equipment-related physical quantities are taken into the approximate formula, to obtain the contrast of the image quality between 3D GRE sequences and GRE-EPI sequence in different conditions.ResultsIn the imaging time, GRE-EPI sequences are mostly in 30s, while the 3D GRE sequence is about the majority reached the 500s; in SNR, since different organizations have different SNR, in this study, SNR of two main organizations of brain:the gray matter (GM) and white matter (WM), on different field intensity were compared,3D GRE sequence relative SNR is between 0.78 to 1.05, while the GRE-EPI sequence SNR is a larger range, from 0.71 to 4.24; for each parameter set or selected range of imaging and imaging matrix is determined, so the same set of parameters for the two sequences in terms of spatial resolution is the same; in GM/WM CNR,3D GRE sequences and GRE-EPI sequences were 0.01～0.15 and 0.66～1.44.DiscussionAfter comparison shows that, in the conditions of the same spatial resolution, compared with the 3D GRE sequence, GRE-EPI sequence can greatly reduce the imaging time; SNR of GRE-EPI sequence generally higher than that of 3D GRE sequence; the CNR in the area, GRE-EPI sequence is greater than 3D GRE sequences. From this, using the EPI sequence to improve SWI imaging is feasible in theory.