A digital receiver with fast frequency- and gain-switching capabilities for MRI systems.

OBJECT: In this article, two issues pertaining to MRI digital receivers are addressed. One is the maintenance of phase coherence between the transmitter and the receiver-an effective solution is proposed, in which the receiver frequency is switched synchronously with the transmitter frequency. The other is the dynamic range of the receiver-gain-switching technique is utilized to improve the dynamic range. To meet the hardware requirements of these solutions, a digital receiver with fast frequency- and gain-switching capabilities was implemented. MATERIALS AND METHODS: The primary components of the proposed digital receiver are a variable gain amplifier, a high-speed analog-to-digital converter and a single-chip digital receiver core. The radio-frequency magnetic resonance signal is directly sampled by the analog-to-digital converter and processed in the digital receiver core. By pre-storing the receiver waveform in the on-board SDRAM, the frequency and gain of the receiver may be switched very quickly. RESULTS: The performance of the proposed digital receiver is verified by embedding it in an imaging spectrometer. It is then demonstrated by conducting experiments on a home-built 0.3-T magnetic resonance imaging system. CONCLUSION: The results show that the phase coherence between the transmitter and the receiver and the dynamic range of the receiver are greatly improved. Consequently, the proposed digital receiver may be useful for obtaining multiple-slice two-dimensional magnetic resonance images with very high resolution.

Design of active shimming coils on mini-type permanent magnetic resonance imaging system / 生物医学工程学杂志

The homogeneity of magnetic field is a key parameter in nuclear magnetic resonance system (NMRI); it can be refined by active shimming. In this work, the authors analyze the theory of active shimming, carry out data processing and simulation, and design an active shimming system which can work on permanent mini-type MRI system. The shimming coils are placed on both out-sides of the receiving shielding box, so the gap of the coils is constant. The system improves the homogeneity of magnetic field (i. e. the decrease of heterogeneity from 10 ppm to 1.4 ppm) on NMI20 mini-type MRI instrument.

A novel selective-excitation RF pulse for magnetic resonance imaging.

OBJECT: A selective-excitation radiofrequency (RF) pulse that uses hard pulses composed of a sequence of composite pulses with positive and negative phases (P/N pulse) is proposed herein. Because the amplitude of the RF signal is unchanged during the excitation, RF amplification can be accomplished using a nonlinear RF power amplifier (i.e., class C or D type). MATERIALS AND METHODS: In this article, Fourier series have been first used to analyze the equivalence between the proposed P/N pulse and the conventional soft pulse on selective excitation. Subsequently, computer simulations based on density-matrix theory are used to compare the excitation profiles of both the soft and the P/N pulses. RESULTS: The excitation profiles of the P/N pulses have been measured experimentally through a home-built 0.3-T magnetic resonance imaging (MRI) system. In addition, several slices of images have been obtained as proofs by using the multislice two-dimensional spin echo sequence through replacement of the conventional soft pulse by the proposed P/N pulse. CONCLUSION: Because the perfect selectivity of the proposed P/N pulse, it can be used for imaging studies to improve the efficiency of amplification at the lowest cost.

Four-channel magnetic resonance imaging receiver using frequency domain multiplexing.

An alternative technique that uses frequency domain multiplexing to acquire phased array magnetic resonance images is discussed in detail. The proposed method has advantages over traditional independent receiver chains in that it utilizes an analog-to-digital converter and a single-chip multicarrier receiver with high performance to reduce the size and cost of the phased array receiver system. A practical four-channel digital receiver using frequency domain multiplexing was implemented and verified on a home-built 0.3 T magnetic resonance imaging system. The experimental results confirmed that the cross talk between each channel was below -60 dB, the phase fluctuations were about 1 degrees , and there was no obvious signal-to-noise ratio degradation. It is demonstrated that the frequency domain multiplexing is a valuable and economical technique, particularly for array coil systems where the multichannel receiver is indispensable and dynamic range is not a critical problem.

A powerful graphical pulse sequence programming tool for magnetic resonance imaging.

A powerful graphical pulse sequence programming tool has been designed for creating magnetic resonance imaging (MRI) applications. It allows rapid development of pulse sequences in graphical mode (allowing for the visualization of sequences), and consists of three modules which include a graphical sequence editor, a parameter management module and a sequence compiler. Its key features are ease to use, flexibility and hardware independence. When graphic elements are combined with a certain text expressions, the graphical pulse sequence programming is as flexible as text-based programming tool. In addition, a hardware-independent design is implemented by using the strategy of two step compilations. To demonstrate the flexibility and the capability of this graphical sequence programming tool, a multi-slice fast spin echo experiment is performed on our home-made 0.3 T permanent magnet MRI system.