gradient magnetic fieldSeveral groups of coils located in the magnet cavity are generated by current and attached to the main magnetic field, which can increase or weaken the strength of the main magnetic field, so that the spin protons along the gradient direction have different magnetic field strengths, and thus have different types of resonance frequencies.
The generation of the main magnetic field depends on the magnet, which can have permanent magnet, normal conduction and superconductivity. High field strength is superconducting. Superconductivity is actually a large magnet. Once the current is introduced, there is no need to provide current. The current will hardly be lost at ultra-low temperature. The strong current generates a strong magnetic field, which is mainly used to supplement liquid helium.
gradient magnetic fieldIt is an additional gradient magnetic field on the main magnetic field, which can be single gradient, double gradient, and can be set up on the X,Y and Z axes. A double gradient is a faster gradient transition. The use of gradient magnetic field is mainly in spatial positioning, including phase encoding and frequency encoding, which can be used to determine any position in space.
The RF radio frequency is mainly to transmit signals and collect signals, and to understand the characteristics of tissues through echo signals, mainly T1,T2, protons, and flow signals.
In summary, a large magnetic field is a static magnetic field, and its use is to magnetize the organization to make it orderly; the gradient field is artificially added for spatial positioning.
The energized coil can generate a magnetic field, and a small magnetic field is added to the main magnetic field to form a magnetic field with a certain gradient arrangement. At this time, the precession frequency of protons changes, which can be layered on the Z axis and positioned in plane space on the XY axis.
uniform third order
gradient magnetic fieldAlso known
octopole magnetic field. A uniform three-step magnetic field can be expressed as a three-step magnetic field, B "'= G" = constant. In accelerators, it is used for Lando damping. The core magnet forming such a magnetic field shape is an eight-pole magnet.
1.
octopole electromagnetregular parameters:
Pole face diameter: 42m
Polar clearance: 150mm
Winding specifications: 1.0mm
Single-wire package resistance: 2.8 ohms
2. magnetic field inspection data: (test current is 1A, magnetic field unit is Gs)
| Pole No. |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
| pole face magnetic field |
136.2 |
136.4 |
136.7 |
137.0 |
136.8 |
137.0 |
136.3 |
136.8 |
Pole heads 5 and 1 are connected in series, and pole heads 7 and 3 are connected in series. When the input current is 3.5A, the magnetic field at the horizontal center position is greater than 115Gs.
When pole heads 2,4,6 and 8 input current of 3.5A at the same time, the magnetic field at the vertical center position is greater than 115Gs.
3. other notes:
When the positive and negative poles of each pole head are connected to the positive current, the magnetic field direction of the pole face is N pole.
The four horizontal pole heads are 1,3,5 and 7 respectively, of which 1 and 5 share one power supply and 3 and 7 share one power supply.
Their previous series connection can connect No.1 positive pole to the positive pole of the power supply, No.1 negative pole to No.5 negative pole, and No.5 positive pole to the negative pole of the power supply. The concatenation of numbers 3 and 7 is similar.
