Factors affecting signal intensity: proton density, T1 value, T2 value, chemical shift, liquid flow, water molecule diffusion, etc. We can adjust the parameters to determine what factors play a decisive role in tissue signal strength and image contrast. The main imaging parameters to be adjusted are: 1, RF pulse: bandwidth (frequency range), amplitude (intensity), when to apply, duration; 2, gradient field: application direction, field strength, when to apply, duration; 3, signal acquisition moment. We call the setting of each parameter related to the RF pulse, gradient field and signal acquisition time and their arrangement in the time sequence the pulse sequence of MRI. The basic construction of the pulse sequence: consists of five parts: RF pulse, level-selected gradient field, phase-encoded gradient field (applied after the 90-degree pulse and before the 180-degree pulse), frequency-encoded gradient field (also called readout gradient field, which must be applied during the echo generation), and MR signal. TR: Repetition time. TE: Echo time. Effective TE: effective echo time, in FSE or EPI sequences, there are multiple echoes generated after one RF pulse excitation, which are filled at different locations in K-space, the TE of each echo is different, in these sequences, we call the time interval from the midpoint of the RF pulse to the midpoint of the one filling the center of K-space as effective TE. ELT: echo chain length, appears in ELT is called the time factor of a fast imaging sequence. ES: echo spacing, the time gap between the midpoints of two adjacent echoes in the echo chain. the smaller the ES, the less time is needed for the whole echo chain acquisition, which can indirectly speed up the acquisition speed. Inversion time: Only appears in pulse sequences with 180-degree inversion pre-pulse: inversion recovery sequence, fast inversion recovery sequence, inversion recovery EPI sequence, and the time interval from the midpoint of 180-degree inversion pre-pulse to the midpoint of 90-degree pulse is generally referred to as TI. Number of excitation (NEX): The average number of signal or signal acquisition, which is the number of repetitions of each phase encoding step in the pulse sequence. An increase in NEX is good for reducing artifacts and increasing signal-to-noise ratio, but increases time. General sequences need NEX>2, while fast sequences, especially the sequence of breath-holding NEX is mostly 1, or even less than 1 (partial spatial times). Acquisition time (TA acquisition time): also called scan time, single excitation EPI: tens of milliseconds; SE T2WI tens of minutes. Two-dimensional MRI acquisition time TA=TR*n*NEX (n is the number of times TR needs to be repeated when NEX=1) For sequences without echo chains such as SE or GRE, n is the number of steps of phase encoding, for sequences with echo chains such as FSE or EPI, n is the number of steps of phase encoding divided by ELT. Three-dimensional is volumetric acquisition, which requires additional layers of phase encoding in the direction of the volume. The volume needs to be divided into several layers then the phase encoding of the same step level is required, so its acquisition time TA=TR*n*NEX*S (S is the number of layers in the volume range) Layer thickness determinants: the gradient field strength of the layer selection, the bandwidth of the RF pulse. In two-dimensional images, the layer thickness is the thickness of the excited layer. The thinner it is, the higher the spatial resolution, but the signal-to-noise ratio is reduced. Layer spacing: CT: the spacing between the centers of the thickness of two adjacent layers, such as layer thickness = 1 and layer spacing = 1, is equivalent to no spacing. But MRI is different: layer thickness = 1, layer spacing = 0.5, then it is equivalent to no image of 0.5 cm of tissue between two layers. Influenced by the linearity of the gradient magnetic field and the frequency characteristics of the RF pulse, there is actually interlayer interference and a certain layer spacing is often required. Matrix: that is, the number of pixels in the frequency coding and phase coding directions, the number of pixels in the frequency coding direction does not directly affect the image acquisition time; while the number of pixels in the phase coding direction is determined by the number of steps in the phase coding, and thus the larger the number, the longer the time required.