Electrocardiographic gating, pulse gating, respiratory gating, navigation echo technology I. Electrocardiographic gating technology Utilizes the signals from the electrocardiogram (ECG) or the heart vectorgram (VCG) as the basis for the heart’s motion cycle movement, thus ensuring the synchronization of the acquisition process with the heart’s beat cycle. Purpose: ① Remove the beating artifacts of the cardiac great vessels; ② Using the gating technology in conjunction with the rapid imaging technology, information on the physiological function of the cardiac great vessels can be obtained. 1, ECG acquisition and cardiac cycle: clean skin; leadwire direction and the direction of the main magnetic field, try to avoid bending; calm breathing, minimize respiratory motion on the cardiac and image quality; arrhythmia can make the cardiac gating failure, if necessary, with sedation; systole from the beginning of the R wave to the end of the T wave, diastole from the end of the T wave to the emergence of the P wave before the MRI signal acquisition is generally in the mid- and late diastole, this period of cardiac motion is generally relatively static. MRI signals are generally acquired in mid- to late-diastole, a time when cardiac motion is generally relatively static. 2, cardiac gating technology: often refers to retrospective cardiac gating, in the entire cardiac cycle of MR radiofrequency excitation and signal acquisition are carried out at the same time, while the electrocardiographic information fused to the MRI system, each cardiac cycle of similar time-phase MRI signals for the reconstruction of an image, significantly reducing the motion artifacts. If multi-temporal reconstruction is selected, MRI signals from the entire cardiac cycle can be utilized, and MRI signals from different temporal phases are used to reconstruct images from different temporal phases. If you choose 20 temporal phases per cardiac cycle, each level in a cardiac cycle can be divided into 20 images to display, using the movie form can be observed throughout the cardiac cycle of each atrial contraction and diastole, and can calculate the ejection fraction and other physiological indicators. 3, electrocardiographic trigger technology: also known as prospective electrocardiographic gating technology, in the R wave crest is detected, after a delay, equivalent to enter the ventricular diastole in the middle of the moment, the MR sequence is triggered to start, radio frequency excitation and signal acquisition, to the next ventricular contraction of the eve of the MR sequence is suspended, so as to basically ensure that in the middle and late ventricular diastole MR signal acquisition, because this period of time the heart movement is relatively quiescent, which can significantly reduce motion artifacts. When using electrocardiographic triggering, in order to make each acquisition located in the same phase of the cardiac beat cycle, the parameters of the sequence should be set depending on the patient’s heart rate, and the effective TR should be 1 to several R-R intervals. The delay time (TD) is the time interval between the detection of the R wave and the start of the imaging sequence sampling, i.e., the start point of the imaging sequence sampling; another important parameter is the termination point of the sampling, and the correct setting of the sampling start and termination points ensures that the acquisition of MR signals takes place at the end-diastolic phase. It is mainly used for cardiac morphology examination. Second, pulse gating technique Cardiac gating is susceptible to interference from RF pulses and gradient field changes. III.RESPIRATION GATED TECHNOLOGY Elastic respiratory belt, pressure detection sensor. 1, Respiratory compensation technique: mainly used for body SE T1WI sequence (RC, inimaging options), the MR signal has been acquired throughout the respiratory cycle. 2, Breath-triggered technique: It is a prospective breath-gating technique. Although it can also be used for gradient echo T1WI sequences, it is more commonly used for FSE-like T2WI sequences. GE: Select respiratory triggering in the imaging options, and then set the relevant parameters in the Gating /triggering interface. Among them, #Resp intervals indicates the number of respiratory cycles, which will determine the length of TR, and is often set to 1~3 according to the respiratory rate, the number of scanning layers, and the different ELT of FSE sequences, and is usually set to 2 in most cases; Triggering point is the time period between the detection of the peak of the end of the inhalation to the start of scanning, and is expressed as a percentage of respiratory cycles, which determines the start point of sequence scanning, and is usually set to 2. The start point of the sequence scan is usually set to 25~35%; Trigger windows is the time period between the end point of the scan and the start point of the next scan, which is also expressed as a percentage of the respiratory cycle and determines the end point of the sequence scan in each respiratory cycle, and is usually set to 30~40%. The time period between the start point and the end point is the acquisition period, and the correct method is to place the acquisition period in the plateau period after the end of expiration. Since the respiratory frequency of each case is different, in addition to setting trigger point and trigger window, it is necessary to coordinate the number of respiratory cycles with the ELT, and the higher the respiratory frequency, the number of respiratory cycles can be increased or set when the ETL is shortened. Navigator. 1. For cardiac imaging, especially for coronary artery imaging, the use of electrocardiographic triggering technology to The cardiac trigger technique is used to control the effect of cardiac motion on the image, and Navigator is used to control the effect of respiratory motion on the image. 2. Free-breathing epigastric imaging is equivalent to the respiratory trigger technique.