Magnetic field homogeneity is an important factor in the MRI system. What becomes important here is not the field homogeneity with human body out, but the field homogeneity with human body in. Echelon incorporates HOSS (High Order Shim System) in order to maintain the magnetic field homogeneity in the state with human body in.
HOSS realizes not only the compensation of the first order shimming which has been used in conventional MRI systems, but realizes the higher order shimming which can correspond with the further complex disorder of the magnetic field. The development of the Hitachi’s advanced high order shimming automatic algorithm program allows more effective and simpler use of the high order shimming function.
In addition, Echelon has realized the HOSS regional shimming function which allows not only HOSS application toward a wide FOV, but also applying high precision shimming to junctions and breast locally, providing more expectations toward wider applications. HOSS has allowed the development of higher image quality for the sequences which are apt to be influenced by magnetic field homogeneity such as BASG and EPI, the improvement of the fat suppression effect in regions far from the magnetic center such as shoulders and knees as well as the application to such high functional applications as MRS.
In MRI imaging, the problem of artifact is inevitable at all. The artifact problem becomes more apparent as the magnetic field becomes higher.
Hitachi developed RADAR (RADial Acquisition Regime) as an imaging method to suppress motion artifacts. The feature of RADAR exists in the advanced k-space filling method. Different from the conventional k-space filling method, the k-space is filled radially around the center of the k-space as its axis as if it were rotating. This advanced k-space filling method allows the decrease of artifacts produced in specific directions.
RADAR decreases not only motion artifacts when the examinee moves, but also is effective in decreasing artifacts which are derived from such involuntary movements such as pulsation, breathing and CSF.
In order for higher speed imaging, more powerful gradient magnetic field and more powerful RF irradiation system have been materialized. This has realized a high-speed scan while other problems such as SAR and dB/dt came out. Hitachi has developed RAPID (Rapid Acquisition through a Parallel Imaging Design) as measures for the rapid scan and SAR. (SAR: High frequency Absorption Rate of a living body)
RAPID eliminates the aliasing in the phase encode number decreasing of images by utilizing the sensitivity distribution of the RAPID receiver coils. A scan time can be shortened as compared with a normal scan, but the SN ratio has the characteristics of decreasing proportionally. RAPID enables selection out of the pre-scan calibration method in which sensitivity distribution data are acquired before measurement and the self-calibration method in which sensitivity distribution data are acquired during the scan.
