Tham khảo tài liệu 'advances in optical and photonic devices part 19', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Ultra-Sensitive Optical Atomic Magnetometers and Their Applications 349 cost of conventional MRI machines. Such scanners can lead to wider spread of MRI diagnostics around the world. Normally in NMR MRI simple pick-up coils are used but the coils which output signal according to Faraday s law is the time derivative of the magnetic flux loose sensitivity at low frequency and do not perform well in the ULF regime. Apart from this the polarization of NMR and MRI spins is also weak in this regime. On the other hand the standards of MR imaging are set very high with resolution on the order of 1 mm and SNR on the order of 30 setting demands on the sensitivity. To enhance a weak NMR signal in the ULF regime the method of prepolarization was proposed Macovski Conolly 1993 . In this method relatively large field whichever is practical to generate with a coil is applied to polarize protons or other nuclear spins and is turned off during the NMR MRI measurements. The process is repeated many times. With prepolarization method the NMR signal is enhanced more than 1000 times compared to that would be generated in microTesla fields at which MRI enconding and readout are actually carried out. One advantage of the ULF readout and encoding is that gradients arising from the ULF coil are quite small and no shimming is necessary. Still even with prepolarization enhancement the SNR and resolution are quite inediquate if simple pick-up coils are used at low frequency and some solution of this problem is necessary. One solution is to replace the coil with SQUIDs or AMs to achive additional gain in sensitivity. ULF MRI with SQUIDs is now a conventional way to do ULF MRI. For example recently an airport security scanner have been built and tested based on a multi-channel SQUID detector. However in all low-Tc SQUID applications the main drawback the requirement for cryogenics exists. The alternative solution to avoid cryogenics can be an atomic magnetometer. The most potentially useful AM
