To locate the imaging slice, the hydrogen protons within the selected slice are excited by the coaction of radio-frequency pulse (RF pulse) and slice-selection gradient during the process of magnetic resonance imaging (MRI). However, the absolute consistency of the time between the RF transmitting system and gradient system can’t be guaranteed in an actual magnetic resonance system, which makes the slice profile not ideal enough. Imperfect slice profile will lead to poor imaging quality. In this paper, Shinnar-Le Roux (SLR) theory is used to simulate the slice profile with different relative delays between RF and slice-selection gradient to solve this problem. By matching the actual slice profile with the simulation results, the relative delay is measured in an actual 0.35T permanent magnet MRI system. The practicability of the method is verified for the improvement of image quality after applying the measured delay to correct the actual MRI system.
This paper analyzes the design method of multi-position progressive die which adopts the existing product length and longitudinal overlap as the step, and proposes the design method of multi-position progressive die which is targeted at feature and adopts the longitudinal length of feature that appears most frequently in the products as one step, in order to deal with all kinds of limitation and weakness of process arrangement in the complex products with medium size. It also briefly explains the controlling procedure of punch and the classification principles of feature. The proposed method shortens the total length of the die, saves the die materials, reduces the difficulty of manufacturing the die, manufacturing cost and production cycle, and improves the stability of feeding materials and manufacturing precision of the product.
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