Concepts in Magnetic Resonance Part B-Magnetic Resonance Engineering最新文献

A novel method for excitation of RF B₁ field in high-field (3-T) magnetic resonance imaging (MRI) systems using a subject-loaded quadrifilar helical antenna as an RF coil is proposed, evaluated, and demonstrated. Design, analysis, characterization, and evaluation of the novel coil when situated in a 3-T MRI bore and loaded with different phantoms are performed and cross-validated by extensive numerical simulations using multiple computational electromagnetics techniques. The results for the quadrifilar helical-antenna RF body coil show (a) strong field penetration in the entire phantoms; (b) excellent right-hand circular polarization (RCP); (c) high spatial uniformity of RCP RF magnetic field, B₁⁺, throughout the phantoms; (d) large field of view (FOV); (e) good transmit efficiency; and (f) low local specific absorption rate (SAR). The examples show that the new RF coil provides substantially better B₁⁺-field uniformity and much larger FOV than any of the previously reported numerical and experimental results for the existing RF coil designs at 3 T in literature that enable comparison. In addition, helical RF body coils of different lengths can, for instance, easily provide an excellent RCP and highly uniform B₁⁺-field within the MRI maximum FOV length of 50 cm, and even 100 cm. The proposed MRI RF coil yields a remarkable improvement in the field uniformity in the longitudinal direction, for various phantoms, with comparable efficiency and SAR levels.

Cylindrical quadrature radio frequency (RF) coils are widely used in magnetic resonance imaging and spectroscopy due to their high sensitivity and field uniformity. However, the field geometry is unsuitable for use in low-field open magnetic resonance imaging (MRI) systems with vertical B₀ field configurations. Therefore, a new design is proposed. A quadrature RF coil that combines Alderman-Grant and Helmholtz designs was constructed to produce two independent modes, both orthogonal to the main magnetic field. The coil provides good RF homogeneity over a 20 × 15 × 15 cm volume and operates as both a transmit and receive coil. The application of the coil for 0.2 Tesla permanent magnet with a vertical B₀ field is shown. The proposed coil may be applied to MR imaging of larger objects at low vertical magnetic fields.

The purpose of this study was to develop a dedicated high signal-to-noise ratio (SNR) radio frequency coil for cervical spinal cord (CSC) imaging without using the preamp decoupling technique. A novel eight-channel CSC array was constructed using butterfly, loop (circular), and rectangular elements. The adjacent elements were decoupled by the critical geometrical overlapping, and most non-adjacent elements were decoupled using the loop and butterfly elements. The performance of the proposed CSC coil was compared with the performance of the standard manufacturer's coil (Siemens' head, neck, and spine array) at 3T MRI system in T₂-weighted images, diffusion tensor images, and ultrahigh-b diffusion-weighted images. In T₂-weighted images, the SNR improvement of the eight-channel CSC coil was 1.4–2.0 times over the manufacturer's coil at the different levels of the CSC vertebrae. Higher contrast between white matter and gray matter was observed in the diffusion-weighted (b = 500 s/mm²) images and the fractional anisotropy maps obtained using the eight-channel CSC coil compared with the manufacturer's coil. The eight-channel CSC coil yielded 2.0 times higher SNR compared with the manufacturer's coil from the white matter region of the ultrahigh-b (b = 7348 s/mm²) radial diffusion-weighted images.

When magnetic resonance (MR) thermometry is performed for temperature monitoring during time-consuming thermal therapy like hyperthermia, tiny main magnetic field (B₀) drifts may cause significant errors in temperature readings inside the human subject. We propose a correction method of B₀ drift effects in MR thermometry, which is based on temperature-dependent proton resonance frequency shift (PRFS) of water molecules. We placed magnetic field monitoring (MFM) probes around the subject and we read the center frequency of MFM signals. By interpolating the center frequencies of MFM signals on the imaging slice, we computed phase correction maps for MR thermometry. We intermittently acquired MFM signals with performing MR thermometry at 3 Tesla during radiofrequency (RF) heating of a tissue-mimicking phantom. With the B₀ drift effect correction, the temperature readings of MR thermometry maps became similar to the temperature readings of an optic fiber temperature sensor embedded at the center of the phantom. We believe the proposed correction method can be used for MRI-guided thermal therapy in which precise temperature monitoring is critical.

The magnetic field gradient waveform monitor (MFGM) technique permits characterization of the temporal evolution of magnetic field gradients in magnetic resonance (MR) instruments (MRIs). Knowledge of the gradient waveform performance permits the development of further techniques, such as gradient waveform pre-equalization, that correct and optimize gradient waveform distortions due to eddy currents induced during the application of switched magnetic fields and other system limitations. The accuracy of the MFGM technique is important since the overall uncertainty of the gradient waveform measurement will propagate into an uncertainty in corrected gradient waveforms impacting the precision of the resulting MR/MRI measurements. The accuracy of MFGM is investigated through a treatment of the noise present in a MRI. A noisy receiver model provides the basis for characterization of the noise and permits examination of the overall impact of noise on the phase accumulated in a pure-phase encoded MR signal. Ultimately, a relationship between the signal-to-noise ratio of a measurement and the corresponding MFGM uncertainty is developed. The theoretical development is supported through simulation in conjunction with experimental results. The propagation of uncertainties to gradient waveform pre-equalization is also discussed.

This work presents the design and development of a general-purpose T/R switch for MR imaging and spectroscopy, compatible with single and double-tuned RF coils using linear, quadrature, or both modes at a low cost and with minimum development time using simple electronic circuits. This T/R switch was demonstrated with a custom double-tuned ¹H/³¹P transmit receive surface RF coils built for ³¹P MRS of a lamb's heart with surgically created congenital heart defects. Two passive trap circuits tuned to ¹H and ³¹P frequencies fed from a common drive point were built to filter two different frequency T/R switches. The T/R switch was built for a ¹H linear RF coil and ³¹P quadrature RF coil. This T/R switch design is easily modifiable to linear, quadrature and double-tuned RF coils of relatively wide range of frequencies, so as to enable imaging of proton and X-nuclei at 3T field. The performance of the custom T/R switch was tested with bench measurements of its isolation, insertion loss and switching time, under pulsed conditions, and on a phantom with a 3T magnet by comparing the signal-to-noise ratio with a commercial T/R switch. A high isolation, low insertion loss and fast switching time in the range of nanoseconds were obtained on the bench. Identical isolation was observed under pulsed conditions. An improvement of about 29% gain in signal-to-noise ratio was obtained with this T/R switch compared to the commercial T/R switch.