The construction principle and clinical application of γ immune counter

Immunological counters and liquid scintillation counters are essential tools for radioimmunoassay techniques, and the immunological counters used to measure iodine-labeled kits are the most widely used. After decades of development, the immune counter has a series of mature products. The use of computer-controlled immunological counters with automatic sample change and automatic on-line data processing is widely used in clinical practice. This article briefly describes the construction principle of the immune counter and the precautions for the maintenance of clinical common faults. Focus on the scintillation detector. Take the GC-1200 dual probe immune counter of Zhongjia Optoelectronics Co., Ltd. as an example.

1 immune counter composition and basic working principle

1.1 Composition Introduction: The immune counter is a kind of solid scintillation counter, which is shown in Figure 1. It is mainly composed of solid scintillation detector, electronic circuit, mechanical transmission system and computer.

1.1.1 solid scintillation detector: usually the scintillator, photomultiplier tube and preamplifier are installed in a light-shielding cassette, and the scintillator is filled or coated with a reflective layer. The scintillator and the photomultiplier tube are added between Optical coupling agent, if necessary, can also add light guide. The high voltage power supply applies a voltage to each pole of the photomultiplier tube through a voltage dividing resistor welded to the stem. as shown in picture 2. Currently, in the immunological counter, sodium iodide (é“Š) crystals are most used, and its chemical symbol is NaI (T1). It is a neodymium-activated sodium iodide transparent single crystal. Its characteristics: high density, high average atomic number; high energy conversion efficiency; better resolution for gamma rays, easier preparation, cheaper price, but extremely deliquescent, must be sealed in aluminum containers with optical glass sheets Or used in other metal containers. Solid scintillation detectors are also widely used in other nuclear medicine equipment, such as SPECT, PET-CT and so on.

1.1.2 electronic circuit: generally consists of amplifier, discriminator, single channel pulse analyzer, scaler, A / D conversion, high voltage power supply, computer interface and other auxiliary electronic circuits.

1.1.3 Mechanical transmission system: generally consists of servo motor, servo motor control circuit, box drive mechanism, positioning sensor and so on.

1.1.4 Computer: General personal computer, can meet the application of radioimmunoassay application software, data processing software and meet various measurement requirements and data processing work.

1.2 The working process of the immune counter can be divided into five interrelated steps:

(1) The radiation enters into the scintillator to interact with the scintillator to ionize and excite the atoms and molecules of the scintillator. (2) When the ionized and excited atoms and molecules are de-excited, part of the ionization and excitation energy are released in the form of light radiation. (3) Most of the photons generated in the scintillator are collected in one direction by the reflective layer around the scintillator, and are incident on the photocathode of the photomultiplier tube through the light guide and optical coupling agent between the scintillator and the photomultiplier tube. Hit the optoelectronics. (4) The photoelectrons are multiplied in the photomultiplier tube, and the multiplied electron beam is collected on the anode to generate an output signal. (5) The electrical signal is sent to an electronic instrument for recording and analysis to detect the nature, energy or intensity of the nuclear radiation. The components of an electronic instrument vary depending on the application. In the measurement of nuclear radiation intensity, circuits such as amplifiers, discriminators and scalers are generally required. When measuring nuclear radiation energy spectrum, a single-channel pulse amplitude analyzer or multi-channel pulse analyzers and other auxiliary circuits are generally required.

2 clinical application notes

Since the amplification factor of the power supply and the instrument will drift, the operating point of the scintillation counter will drift. Therefore, the scintillation counter should work in the pad area to make the counting stable. The flat length of the scintillation counter, in addition to the scintillator, is also related to the total sensitivity of the photomultiplier tube, noise, high voltage instability, and magnification of the instrument. Therefore, these parameters must be reasonably selected during the measurement, so that the counter has a longer working frequency band, and the counting efficiency is relatively stable. The GC-1200 dual probe 丫 immune counter is computer controlled, and all parameters are sent and changed using keyboard operation. The machine has a more complete measurement function, which can complete automatic counting measurement, background measurement, efficiency measurement, automatic measurement of 丫 energy spectrum, automatic measurement of 丫 spectrum, automatic correction of double detector, which brings great convenience to use and maintenance.

Due to the error caused by the inconsistency of the two photomultiplier tubes, when the same sample is measured in different amounts in the two probes, and the mutual deviation is greater than 5, the result error and repeatability of the measured specimen will be poor. . Since there is a certain error in the use of the dual probe, quality control is required at ordinary times. For example, when the above error occurs in the GC-1200 dual probe 丫 immune counter, the quality control software carried by the random test can be used to correct the detector. At the same time, it should be checked whether there is any contamination inside the probe. If it cannot be solved, consider whether the machine hardware is faulty. When disconnecting the circuit board inside the machine, be sure to turn off the power of the main unit to avoid damage to the integrated circuit components due to the impact of current. When any fault starts to be repaired, first check whether the power supply of each group is normal (including fuse, cable and input voltage, etc. The environment where the instrument is placed should be dry and clean. If the sample drive fails, check whether the micro switch on the drive platform is checked first. The contact is good. The mechanical failure may cause the sample tube to rupture, causing the background to rise. At this time, the position of the lifter head and the like may be thoroughly cleaned by soapy water, but the high-voltage current deviation may also cause the background to rise, and the specific situation is analyzed. In addition, do not judge and eliminate the fault. In short, do not easily open the detector for overhaul. When repairing the detector, pay attention to protect from the light and moisture to avoid damage to the crystal. Since the machine has two separate circuits, the exchange method can be used in the repair process. Law to narrow the scope of maintenance.

( FJ MedicalJourna Sun Liming )