Research assistant, University of Minnesota/ Center for Magnetic Resonance Research (CMRR)
A loop+dipole transceiver antenna array was designed for 447 MHz /10.5 tesla human head imaging.
Magnetic resonance imaging (MRI) is one of the invasive techniques for human imaging without any radioactive materials. Especially, ultra-high field (UHF) MRI, such as 10.5 tesla (T) MRI, provides a high signal-to noise ratio (SNR) which allows high resolution anatomical MRI and functional MRI (fMRI). To transmit and receive the signal for MRI, radiofrequency (RF) coil plays the role as the medium between the MRI machine and the human subject. The magnetic field called “B field” is generated by RF coils. Without RF coil development, none of MRI researches could have shown such substantial results we see nowadays. One thing we have to accept is that at 10.5T, the human body shows a short wavelength, and due to this, a significantly non-uniform B field distribution of RF coils results. Arrays consisting of transmit antennas have the ability to mitigate these non-uniformity through optimal combinations of phase and amplitude of the RF excitation waveform. For UHF applications, arrays are essentially required to achieve acceptable B field uniformity and optimized transmit efficiency. A combined loop and dipole antenna structure we developed is predicted to yield optimal RF transmitter performance for 10.5T human brain imaging. Loop is one of the efficient coil types for MRI. Recently, a new design of array coil based on a radiative antenna, such as a dipole antenna array, has been proposed to address the challenges of currently available loop array coil. In my research, I am combining a loop array and a dipole antenna array. This concept of combination is possible from the orthogonal B field generation of loop and dipole antenna. To investigate this for various geometries, we evaluated a geometrically adjustable 16-channel loop+dipole antenna array both through simulations and experiments on a human head size phantom. This combined loop+dipole antenna array includes benefits from both loop and dipole antenna array. Since RF coil is the electrical machine, going through simulation process is essential for the safety issue. For the safety, we calculated specific absorption rate (SAR) of this coil at 10.5T by simulation using the finite-difference time-domain (FDTD) method.
Abstract: To propose a new Extended Monopole antenna Array with individual Shields (EMAS) coil that improves the B1 field coverage and uniformity along the z-direction.To increase the spatial coverage of Monopole antenna Array (MA) coil, each monopole antenna was shielded and extended in length. Performance of this new coil, which is referred to as EMAS coil, was compared with the original MA coil and an Extended Monopole antenna Array coil with no shield (EMA). For comparison, flip angle, signal-to-noise ratio (SNR), and receive sensitivity maps were measured at multiple regions of interest (ROIs) in the brain.The EMAS coil demonstrated substantially larger flip angle and receive sensitivity than the MA and EMA coils in the inferior aspect of the brain. In the brainstem ROI, for example, the flip angle in the EMAS coil was increased by 45.5% (or 60.0%) and the receive sensitivity was increased by 26.9% (or 14.9%), resulting in an SNR gain of 84.8% (or 76.3%) when compared with the MA coil (or EMA).The EMAS coil provided 25.7% (or 24.4%) more uniform B1+ field distribution compared with the MA (or EMA) coil in sagittal. The EMAS coil successfully extended the imaging volume in lower part of the brain. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.
Pub.: 23 Jul '15, Pinned: 01 Jul '17
Abstract: To investigate intracranial microvascular images with transceiver radio-frequency (RF) coils at ultra-high field 7 T magnetic resonance imaging (MRI).We designed several types of RF coils for the study of 7 T magnetic resonance angiography and analyzed quantitatively each coil's performance in terms of the signal-to-noise ratio (SNR) profiles to evaluate the usefulness of RF coils for microvascular imaging applications. We also obtained the microvascular images with different resolutions and parallel imaging technique.The overlapped 6-channel (ch) transceiver coil exhibited the highest performance for angiographic imaging. Although other multi-channel coils, such as 4- or 8-ch, were also suitable for fast imaging, these coils performed poorly in homogeneity or SNR for angiographic imaging. Furthermore, the 8-ch coil was poor in SNR at the center of the brain, while it had the highest SNR at the periphery.The present study has demonstrated that the overlapped 6-ch coil with large-size loop coils provided the best performance for microvascular imaging or angiography with the ultra-high-field 7 T MRI, mainly because of its long penetration depth together with high SNR.
Pub.: 06 Aug '14, Pinned: 01 Jul '17