Abstract
Ultra-low field magnetic resonance imaging (ULF MRI) is an effective imaging technique that applies the ultrasensitive detector of superconducting quantum interference device (SQUID) sensor to detect the MR signal at a microtesla field range. In this work, we designed and developed a SQUID-based ULF MRI system with a frequency-adjustable measurement field, the performance of which was characterized via water phantoms. In order to enhance the MR signals, a 500 mT Halbach magnet was used to prepolarize the magnetization of the sample prior to excitation. The signal-to-noise-ratio (SNR) of the spin-echo- (SE-) based pulse sequence can reach up to 70 in a single scan. The images were then reconstructed successfully by using the maximum likelihood expectation maximization (MLEM) algorithm based on the backprojection imaging method. It was demonstrated that an in-plane resolution of 1.8 × 1.8 mm2 can be achieved which indicated the feasibility of SQUID-based MRI at the ULF.
Highlights
Conventional magnetic resonance imaging (MRI) uses high magnetic fields, gradient fields, and radio frequency pulses to generate images of the organs in the body as one of the most important methods in clinical investigations
To evaluate the SNR of the designed system, a free induction decay (FID) signal was obtained in a single scan using the FID-based pulse sequence (Figure 6)
Similar to the pulse sequence discussed above (Figure 5), a π/2 pulse was applied for 2 ms after prepolarization. e cryogenic switch was turned on during the π/2 pulse to avoid the large field shock to the superconducting quantum interference device (SQUID) sensor, and subsequently the signal was acquired
Summary
Conventional magnetic resonance imaging (MRI) uses high magnetic fields, gradient fields, and radio frequency pulses to generate images of the organs in the body as one of the most important methods in clinical investigations. It has been widely used in physics, chemistry, biology, and medicine [1,2,3,4]. Ere are, many MRI applications where the ultra-high field is not an optimal choice, for example, imaging performed in the presence of metal or where it is impractical to employ a large and expensive magnet [10]. ULF magnetic resonance technology offers a wide range of applications in chemistry, physiology, and biomedicine, such as imaging of water, the human brain, the human forearm, and the human wrist [12,13,14]
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