Synopsis
The main aim of this course is to demonstrate MRI researchers/students the basic procedures for phased-array construction and show an optimized protocol for constructing, tuning and decoupling a highly parallel array coil. The goal is to provide a better understanding of the basic experimental RF tools and procedures to facilitate the efficient design and construction of highly parallel MRI receive-arrays. We demonstrate the protocol with the construction of a 16-channel coil array of overlapped surface coil elements.
Introduction
In the last decade, parallel detection of the Magnetic Resonance Imaging (MRI) signal with multiple receive-only surface coils has proven valuable for increasing image sensitivity and acquisition speed. The success of parallel imaging methods has driven the design of MR receive arrays with an increasing number of elements [1–5]. The high degree of parallelism requires systemized design, construction and testing in order to implement the large number of tuned receive circuits with minimal mutual interaction and has altered the workflow for how we construct receive arrays [1].The main aim of this course is to demonstrate MRI researchers/students the basic procedures for phased-array construction and show an optimized protocol for constructing, tuning and decoupling a highly parallel array coil. The goal is to provide a better understanding of the basic experimental RF tools and procedures to facilitate the efficient design and construction of highly parallel MRI receive-arrays. We demonstrate the protocol with the construction of a 16-channel coil array of overlapped surface coil elements.Array Layout
Given the geometrical constraints of the coil former and knowledge of the extent of anatomical coverage desired, the first step is to layout the pattern of overlapping coils which geometrically “tile” this 3D space.Loop Design
The diameters of the loop coils corresponding to hexagon tiles on the coil former are determined from the size of the hexagon tiles; the loop diameter is slightly larger than the diameter of the circle which inscribes the vertexes of the hexagon. Next, we construct a pair of test loops to determine the values of the needed capacitors and fine-tune the final size for each coil element.Coil Quality Factor
After the determination of the loop sizes and needed components, we calculate the coil quality factor (Q), which describes how much the reactive impedance dominates over the resistive impedance in the loop coil. It informs us of the power dissipated in loss mechanisms relative to the energy stored in the tuned circuit.Tuning and Active Decoupling
An LC trap together with a PIN diode, detune the loop during transmit with the body RF coil. To test that the detuning is adequate, we measure first the tuning of the LC circuit using a single pickup loop (“sniffer probe”) connected to a network analyzer S11. The pre-tuning of trap circuit is done without the main coil loop being tuned (variable capacitor was still missing) and with the PIN diode forward biased (conductive state). After initial tuning of the trap circuit with a small sniffer probe, we attach a variable capacitor to close the circuit of the main loop and tune the coil to 123.25 MHz using a double-probe controlled via the S21 measure.Preamplifier Decoupling
Preamplifier decoupling is used to reduce coupling between next-nearest and further neighbors. It has become one of the most important tools in constructing Rx arrays [6, 7]. Optimization of the preamplifier decoupling is a critical step in constructing highly parallel arrays. The goal is to design the preamplifier/coil circuit so that the preamplifier performs a voltage measurement across the loop.Determining the Critical Overlap
In this step the overlap between neighboring elements is adjusted to null their mutual inductance [8]. We estimated the critical overlap ratio of a pair of loops by carefully moving those loops towards each other while measuring the S12 parameter between the tuned coils. We monitor the S12 interaction between the two neighboring coils using a test probe that plugs into the circuit board at the position of the preamplifier.Array Assembly
After successfully testing the first loops, we assemble the whole array: First, all the drive points and capacitor solder pads, made out of FR4 circuit material, are mounted on the coil former. Second, a 16-awg thick tin-plated copper wires are cut, bent and assembled over the whole array. Third, the preamplifier board including the 42 mm long coaxial cable were mounted to the designated holder, as well as wiring up the preamplifier and bias tee circuitry to the plug cables. All preamps were carefully orientated along the z-direction to minimize Hall effect issues [9–11]. Fourth, the detuning traps on the drive are adjusted to be resonant at the Larmor frequency when the PIN diode was forward biased.Geometrical Nearest-Neighbor Decoupling of the Array
The goal of the geometrical decoupling is to find a critical overlap of all adjacent coils in order to minimize the mutual decoupling between any pair of neighboring coils. To achieve this, we monitor the S12 interaction between two neighboring coils using the probe that plugs into the circuit board at the position of the preamplifier. This optimization of all next neighbors is the most time-consuming adjustment procedure during the phased-arrayconstruction.Tuning and Matching
After the geometrical decoupling, each element needs to be retuned and matched to the preamplifier’s noise- matched condition. This is done using a direct S11 measurement with cables directly connected to the preamplifier sockets of the element under test. It is important that the electrical delay produced by the probe is accurately calibrated. When adjusting the tuning and matching all other unused elements of the phased-array are detuned. This procedure is done with the array loaded with a lossy phantom.Final Bench Touch Up
The final step of phased-array construction is a careful check-up of the loop tuning, active detuning, and preamplifier decoupling. This step is done with all of the preamplifiers present and the array plugged into a simulator that passes the PIN diode biases and DC power to each preamplifier from the scanner. Starting with all of the array elements in the detuned state, the bias voltage for a given loop is toggled on and off while the detuning is monitored using a double inductive probe and the S12 measurement. A fine adjusting of the detuning inductor is sometimes needed to achieve this. After this step, we verify the preamplifier decoupling by viewing the S12 versus frequency for the coil in the tuned state using the same decoupled double-probe, but with reduced power output from the network analyzer (-25dBm). Finally, the cable traps on each plug were checked and adjusted via an S12 measurement using current probes.Acknowledgements
No acknowledgement found.References
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