Gradient System is one of the most important parts of Magnetic Resonance Image System.It is a key factor in the image quality.Therefore,during the whole installation process,advanced gradient system is strictly needed.Firstly,this article gives a clear explanation about the theory and function of the gradient system.Then,it discusses how to make rational use of gradient system to acquire the best image quality.

Objective To compare theγpassing rate between measurements at actual degree gantry angle and zero degree gantry angle for dose verification of intensity?modulated radiotherapy ( IMRT) in the treatment of nasopharyngeal carcinoma ( NPC) and cervical carcinoma. Methods Thirty patients with NPC and thirty patients with cervical carcinoma were randomly chosen from 87 patients with NPC and 54 patients with cervical carcinoma, respectively. Using a gamma criterion of 3 mm/3%, the γ passing rates at actual gantry angle and zero degree gantry angle were measured using ArcCHECK and compared by paired t test. Results The γ passing rate was significantly lower at actual gantry angle than at zero degree gantry angle in patients with NPC or cervical carcinoma ((93.8±3. 6)% vs. (97.8±1. 1)%, P=0. 00;(96.3±2. 1)% vs. (98.2±1. 0)%, P=0. 00). Moreover, the variation range of the γ passing rate at actual gantry angle was larger than that at zero degree gantry angle. Bothγpassing rates at actual gantry angle and zero degree gantry angle were lower in the patients with NPC than in the patients with cervical carcinoma . Conclusions Compared with that at zero degree gantry angle, theγpassing rate at actual gantry angle is closer to reality. Therefore, the actual gantry angle is recommended for dose verification. In order to meet the clinical requirement, a higher standard of γ passing rate should be proposed when zero degree gantry angle is used for dose verification.