Sung Min Sohn

SoC design of an auto-focus driving image signal processor for mobile camera applications

This paper presents a CMOS system on a chip (SoC) solution to capture focused images for mobile camera phones. In this work, auto focus (AF) lens has been formed by voice coil motor (VCM) type and the image signal processor (ISP) includes a built-in AF control block and AF driver to fully implement AF with fast and accurate performance. The proposed ISP with AF driver can meet constraints such as small size and low cost for mobile camera applications.

A CMOS image sensor (CIS) architecture with low power motion detection for portable security camera applications

This paper presents a low power motion detection algorithm and its system architecture. The proposed architecture uses just one bit data of each selected pixel for the motion detection, where less than a few percentages of the total pixels will be selected. Since only small amount of image data is used for the motion detection, extreme low power consumption is possible. Therefore, this architecture is useful for portable battery-operated security cameras.

RF Head Coil Design with Improved RF Magnetic Near-Fields Uniformity for Magnetic Resonance Imaging (MRI) Systems.

Higher magnetic field strength in magnetic resonance imaging (MRI) systems offers higher signal-to-noise ratio, contrast, and spatial resolution in magnetic resonance (MR) images. However, the wavelength in ultrahigh fields (7 T and beyond) becomes shorter than the human body at the Larmor frequency with increasing static magnetic field (B0) of the MRI system. At short wavelengths, an interference effect appears, resulting in nonuniformity of the RF magnetic near-field (B1) over the subject and MR images may have spatially anomalous contrast. The B1 near-field generated by the transverse electromagnetic RF coils microstrip line element has a maximum near the center of its length and falls off towards both ends. In this study, a double trapezoidal-shaped microstrip transmission line element is proposed to obtain uniform B1 field distribution by gradual impedance variation. Two multi-channel RF head coils with uniform and trapezoidal shape elements were built and tested with a phantom at 7-T MRI scanner for comparison. The simulation and experimental results show stronger and more uniform B1+ near-field with the trapezoidal shape.

Novel multi-channel transmission line coil for high field magnetic resonance imaging

Radiofrequency (RF) coils are the antenna-like devices used in magnetic resonance imaging (MRI) to inductively excite and receive the nuclear magnetic resonance (NMR) signal in anatomy. This nuclear magnetic induction is most efficient at the field strength dependent Larmor frequency for a nuclear species. Coils must resonate at Larmor frequencies of 300 MHz or more to take advantage of the signal-to-noise benefits of 7T+ MRI. In high water content tissue dielectrics however, the wavelengths at these frequencies are 12cm and less, significantly shorter than human anatomic dimensions. One consequence of these short wavelengths is a highly non-uniform RF excite field. In this investigation, we aim to mitigate this problem through a novel coil element design. The traditional microstrip line element is modified into a multi-section alternating impedance configuration to homogenize the magnetic field over the coil length. Feasibility of this approach is numerically simulated, and then empirically validated by phantom and human imaging.

Stepped Impedance Resonators for High-Field Magnetic Resonance Imaging

Multi-element volume radio-frequency (RF) coils are an integral aspect of the growing field of high-field magnetic resonance imaging. In these systems, a popular volume coil of choice has become the transverse electromagnetic (TEM) transceiver coil consisting of microstrip resonators. In this paper, to further advance this design approach, a new microstrip resonator strategy in which the transmission line is segmented into alternating impedance sections, referred to as stepped impedance resonators (SIRs), is investigated. Single-element simulation results in free space and in a phantom at 7 T (298 MHz) demonstrate the rationale and feasibility of the SIR design strategy. Simulation and image results at 7 T in a phantom and human head illustrate the improvements in a transmit magnetic field, as well as RF efficiency (transmit magnetic field versus specific absorption rate) when two different SIR designs are incorporated in 8-element volume coil configurations and compared to a volume coil consisting of microstrip elements.

Design of an Electrically Automated RF Transceiver Head Coil in MRI

Magnetic resonance imaging (MRI) is a widely used nonionizing and noninvasive diagnostic instrument to produce detailed images of the human body. The radio-frequency (RF) coil is an essential part of MRI hardware as an RF front-end. RF coils transmit RF energy to the subject and receive the returning MR signal. This paper presents an MRI-compatible hardware design of the new automatic frequency tuning and impedance matching system. The system automatically corrects the detuned and mismatched condition that occurs due to loading effects caused by the variable subjects (i.e., different human heads or torsos). An eight-channel RF transceiver head coil with the automatic system has been fabricated and tested at 7 Tesla (T) MRI system. The automatic frequency tuning and impedance matching system uses digitally controlled capacitor arrays with real-time feedback control capability. The hardware design is not only compatible with current MRI scanners in all aspects but also it operates the tuning and matching function rapidly and accurately. The experimental results show that the automatic function increases return losses from 8.4 dB to 23.7 dB (maximum difference) and from 12.7 dB to 19.6 dB (minimum difference) among eight channels within 550 ms . The reflected RF power decrease from 23.1% to 1.5% (maximum difference) and from 5.3% to 1.1% (minimum difference). Therefore, these results improve signal-to-noise ratio (SNR) in MR images with phantoms.

8-Channel RF head coil of MRI with automatic tuning and matching

The Radio Frequency (RF) coil is an essential part of hardware in Magnetic Resonance Imaging (MRI) systems and microstrip transmission line (TEM) RF coils have been widely used for high-field applications to excite and receive the nuclear magnetic resonance signals. These coils are typically terminated by capacitors. On one end two variable capacitors, the matching capacitor (Cm) and tuning capacitor (Ct), and on the other end a fixed value capacitor (Cf) form a capacitively tuned, matched, and foreshortened half-wave resonator. These resonant coil elements have narrow bandwidth due to their high quality factors (Qs). High transmit power efficiency and receive Signal-to-Noise Ratio (SNR) depend on a well-tuned and matched coil element. Conversely, the variable body loading of these coil elements can adversely impact both tuning and matching, and therefore power efficiency and SNR of transmit/receive signals. Loading effects are problematic and a manual tuning is a time-consuming adjustment. It, however, is the only method to avoid loading problems at present. This study demonstrates the automatic frequency tuning and impedance matching technique for the optimal coil efficiency. An 8-channel RF head coil has been successfully built and tested with a fully automatic tuning and matching function at 7tesla (T). It offers real-time fast operation (less than 550ms per channel) and accurate frequency tuning and impedance matching (less than -20dB in the reflected coefficient, S11, at the Larmor frequency) resulting in the high power efficiency (4% ~ 21% improvement for each channel).

Alternating impedance multi-channel transmission line resonators for high field magnetic resonance imaging

In high field MRI systems, microstrip transmission line elements have been successfully implemented as magnetic field generating elements in multi-channel volume coils. However, at these field strengths, short in vivo wavelengths and greater sample losses lead to RF in-homogeneities, as well as, RF inefficiencies. Optimizations of these elements are required to overcome these challenges and to perform a variety of MR applications. In this study, two different microstrip designs with varying impedance lines along the length of the coil -- one producing peak magnetic field in the center and the other extending the length of usable magnetic field along the length of the coil-- are investigated. Simulation and image results for 8- channel volume coils incorporating these element designs were obtained using a phantom in a 7 Tesla MRI system.

RF multi-channel head coil design with improved B 1 + Fields uniformity for high field MRI systems

In ultra high field MRI systems (7 tesla and higher), the wavelength inside the body is short and smaller than the human anatomy at the Larmor frequency. At these shorter wavelengths, interference effects appear; the uniformity of the RF excitation B1+ field over the whole subject becomes inhomogeneous. The RF B1+ field generated by the RF coil is a maximum in the center along the length of the coil microstrip element and falls off towards both ends of the coil element and it leads to anomalous contrast in the MR images. In this study, double trapezoid-like shape along the length of the microstrip resonator is proposed to obtain gradual impedance variation and flatten transmission near-field profile along the length of the coil. The conventional and proposed 8 channel head coils were investigated with a phantom in a 7 tesla MRI scanner. The results show very flat field distribution with about 35% increased local transmission magnetic field strengths at the end sides as well as about 13% improvement at the center and the Q (quality factor) ratio between unloaded and loaded is also increased about 45% (from 1.46 to 2.13) compared to the conventional structure.

In vivo MR imaging with simultaneous RF transmission and reception.

To present a practical scheme of a simultaneous radiofrequency (RF) transmit (Tx) and receive (Rx) (STAR) system for MRI, discuss the challenges and solutions, and show preliminary in vivo MR images obtained with this new technique.