It is difficult to obtain a strong magnetic field, especially a highly stable and uniform magnetic field. 1) First of all, this requires low temperature technology, and the temperature of liquid helium is-269; In order to keep a large amount of liquid helium in the instrument, the usual method is to add a liquid nitrogen (-196) jacket outside. 2) The machining accuracy of parts needs to ensure that the magnetic field intensity in the area where the sample is located is constant, because it is related to the resolution. 3) The stability of the electronic system is very high, so it is necessary to suppress the voltage fluctuation, otherwise it will destroy the sensitivity and even the accuracy.
Whether it is a nuclear magnetic resonance spectrometer in chemical field or a nuclear magnetic resonance imaging instrument in medical field, the basic principle is the same: under the action of magnetic field, the nuclear energy level splits, and under the action of radio frequency pulse, the energy level jumps, thus generating signals. In addition, the signal detector needs to be able to detect the macroscopic magnetic moment generated by the nucleus, which is very weak, and even the thermal noise of the coil will affect its sensitivity, so there will be a low-temperature probe cooled by liquid nitrogen.
For the above reasons, it is a complicated technology to manufacture nuclear magnetic resonance instrument. The general nuclear magnetic resonance spectrometers are Brooke and Varian, and the nuclear magnetic resonance imaging instruments are mainly Siemens, General Electric and Hitachi. This leads to: 1) nuclear magnetic resonance instrument is very expensive. The price of MRI machine is 500,000-1, which is the cost and price guidance of MRI machine. Therefore, depreciation is expensive, and the cost allocated to testing is also expensive. 2) The maintenance cost of nuclear magnetic resonance is high. Nuclear magnetic vibrator needs liquid nitrogen and liquid helium to maintain the strong magnetic field generated by superconducting magnet. Even if it stops, it needs to consume liquid nitrogen and liquid helium. Although the price of liquid nitrogen is not high (usually less than 10 yuan per liter), the consumption rate is 0.4 liters per hour, so the consumption is very high. Liquid helium is very expensive, generally per liter of 200 yuan. Liquid helium shall be added once every 3-4 months at a cost of 1 1,000 yuan.