Some techniques can be used in conventional imaging methods, but also in special imaging methods
Adipose tissue not only has high proton density, but also has very short T1 values (1.5T 200-250ms) and long T2 values, so it presents very high signal in T1WI and higher signal in T2WI, and its signal intensity will be further increased on the currently commonly used FSE T2WI.
①Adipose tissue causes motion artifacts. The higher the signal intensity, the more obvious the artifact is. (ii) Chemical artifacts appear at the water-lipid interface. ③Decrease the contrast. ④Decrease the enhancement scan result. Zhang Chengjun, Department of Radiology, Chaoyang Central Hospital, Liaoning Province
On T1WI, fat, protein-containing fluid, and hemorrhage can show high signal, and compression of lipids can identify whether they contain fat. renal vascular smooth muscle lipoma, teratoma, etc.
II. Tissue properties associated with fat suppression techniques.
1. Chemical shift phenomenon and the concept of homo/anti-phase: the same magnetic nucleus, if the molecule it is in is different, the distribution of its surrounding electron cloud will differ, then even in the same uniform external magnetic field environment, due to the different shielding of the magnetic nucleus by the electron cloud, the magnetic field strength felt by the nucleus will differ, and its incoming frequency will certainly differ. In magnetic resonance physics, this phenomenon is called chemical shift phenomenon. The degree of chemical shift is proportional to the strength of the main magnetic field, and the higher the magnetic field, the more pronounced the chemical shift.
The clinical MRI object is the hydrogen proton, and the frequency of hydrogen protons in the water molecule and in the fat molecule also differ because of the chemical shift effect, with the hydrogen protons in the fat molecule moving 3.5 ppm slower than the hydrogen protons in the water molecule.
When the RF pulse has just finished excitation, the macroscopic transverse magnetization vectors (Mxy) of fat and water are in the same phase, which is called the same phase, and if the signal is collected at this time, the signal intensity of the pixel is the sum of the water signal and the fat signal; after the RF pulse is turned off, the Mxy of water will be 180 degrees faster than the Mxy of fat at a certain moment because the incoming frequency of hydrogen protons in water molecules is faster than that of hydrogen protons in fat. degree, that is, the Mxy of water and fat Mxy difference of 180 degrees, known as the anti-phase, when the acquisition signal then the signal intensity of the pixel for the water signal and fat signal subtraction.
2, the longitudinal relaxation characteristics of adipose tissue: in the human normal tissue, the longitudinal relaxation speed of fat is the fastest and the T1 value is the shortest, and the T1 value of the tissue will change under different field strengths.
Third, the fat suppression technique commonly used in MRI.
Different methods for different field strengths; the same field strength, different methods for different parts.
1, frequency selective saturation method. Also called chemical shift selection saturation technique. The use is the chemical shift effect of fat and water. We know that the fat in the proton into the frequency is 3.5ppm slower than the water molecules, if the excitation pulse in the imaging sequence before the application, first applied one or several narrow bandwidth fat saturation pre-pulse, the frequency of these pre-pulse and the fat in the proton into the frequency, so that the fat tissue will be continuously excited and saturation phenomenon, the pre-pulse generated by the Mxy can be eliminated using gradient technology;. And the water molecules in the proton is not excited due to the different incoming frequency, then apply the real imaging RF pulse, the fat tissue because of saturation can no longer receive energy, and therefore do not generate signals, while the water molecules in the proton can be excited to generate signals, so as to achieve the purpose of fat suppression. By the same token, the frequency-selective saturation method can also perform water suppression and obtain images of fat signals.
Advantages: ①High selectivity or specificity. ②Can be used for a variety of sequences. SE T1WI or T2WI; FSE T1WI or T2WI, GRE sequences. ③Good fat inhibition can be obtained above 1.0T device.
Disadvantages: ①The field strength dependence is large. The degree of chemical displacement is proportional to the main magnetic field strength. low field machines below 0.5T, the difference between the fat and water feed frequencies is very small, and it is difficult to use the frequency selection saturation method for fat suppression. ② High requirements for the uniformity of the magnetic field. Automatic or manual homogenization of the main magnetic field is previously required, along with the removal of any items in the body or on the body surface that may affect the homogeneity of the magnetic field. ③The poor effect of fat compression around the large FOV is related to the reduction of the uniformity of the peripheral magnetic field. ④Increase the energy of RF absorption by the body. ⑤ fat pre-saturation pulse occupies a period of time in the TR interval, so it will reduce the number of layers that can be acquired within the same TR, such as the need to maintain a certain number of scanned layers will need to extend the TR, which will inevitably prolong the TA, and may image contrast.
2. STIR technique. Based on the short T1 characteristics of adipose tissue, currently commonly used, available IR or FIR, currently more FIR sequences are used. Short TI, 1.5T generally 150~170ms.
Advantages: ① low field strength dependence, low field can also achieve good results. ② Compared with the frequency-selective saturation method, the STIR technique requires less uniformity of the magnetic field. ③Large FOV scan can also achieve a good pressure lipid effect.
Disadvantages: ①The selectivity of signal suppression is low, if the T1 value of a certain tissue such as hematoma is close to fat, its signal is also suppressed. ②The scanning time is also long due to the long TR. ③ Generally, it cannot be used for enhancement scan because the T1 value of the enhanced tissue may be shortened to be similar to that of fat tissue and the signal is suppressed, which may affect the judgment of the degree of enhancement.
3.Frequency selective inversion pulse fat suppression technique. It is actually a combination of the above two fat suppression methods. Before the real imaging pulse is applied, a pre-pulse is applied. The bandwidth of this pulse is narrow, and the center frequency is the incoming frequency of protons in fat, so that only fat tissue is excited and the angle can be adjusted at will. After the pre-pulse, longitudinal relaxation occurs in fat tissue, and the real imaging pulse is applied when Mz passes the zero point, and the fat tissue signal is suppressed. Currently the most widely used.
4.Selective water or fat excitation technique. A frequency and spatially selected binomial pulse is usually used. This pulse is actually a combination of multiple pulses with different deflection angles and deflection directions.
This selective excitation technique can be used in SE, FSE and gradient echo sequences, and can be used in both 2D acquisition mode and 3D acquisition mode, which requires a high degree of magnetic field uniformity, so it needs to be homogenized. Clinically, the selective water excitation technique is more widely used than the selective fat excitation technique, and is used in the examination of orbits, nerve roots, abdomen, bones and joints, etc.
5.Dixon technique. It is a water-lipid separation imaging technique, using pulse shift technique in SE or FSE sequence, or double echo technique in gradient echo, both can obtain water-lipid phase consistent (in-phase) image or water-lipid phase opposite (anti-phase) image. By adding or subtracting the two kinds of image information, we can obtain the water quality sub image and fat proton image.
Fourth, GE pressure lipid features
①STIR. need to choose IR-FSE sequence, and then set the appropriate TI according to the different field strength.
Fat. can be selected at the sequence positioning interface, which is equivalent to the frequency-selective saturation method of fat compression technology, but no longer uses four consecutive 90-degree pulses for saturation, but pulses slightly larger than 90 degrees for fat saturation.
③Fat Classic. also selected at the sequence positioning interface, has a similar effect to the Fat option, but with a weaker degree of fat compression.
④SPECIAL. A frequency selection pulse greater than 90 degrees is used to excite the 3D or 2D level, and after a time delay (automatically set by the device), the imaging pulse is then applied, which can achieve a good fat compression effect. It is mainly used for 3D gradient-echo type sequences, and now it can also be used for 2D FIESTA sequences.