General RF-coil construction methods and useful home-made accessories and tools are demonstrated through constructing a simple Rx array coil. Alternative decoupling techniques for design and construction are illustrated. Imaging results using the constructed array coil in the demonstration on a human wrist are presented.

• Scientists and MR community members wishing to acquire basic RF engineering skills and learn about design and construction of simple RF coils/arrays.

• The presented design and construction methods are applicable to construct similar arrays for both human and animal MRI systems particularly useful for array coil construction which do not rely on preamplifier decoupling, including Tx/Rx arrays.
• We hope this course will motivate less experienced researchers and students to build their own hardware and become passionate about RF coil design and construction.

Multiple radiofrequency (RF) coils arranged in an array configuration have become an essential component of modern clinical MRI systems. Relatively small receive-only (Rx) surface coils have been combined to form large coil-arrays, achieving remarkable results in terms of increase in image sensitivity, improvement of signal-to-noise ratio (SNR) over a large field of view (FOV) and reduced imaging time [1-2].

On the other hand, the associated increased complexity of constructing large arrays has presented new engineering challenges and the production cost has increased. Innovative solutions to overcome some of the challenges can be found in recent literature and very useful advice and guidance to practical construction has been presented during past ISMRM educational courses. However, some labs or research groups do not have sufficient resources and experience to pursue such sophisticated construction methods. In this course, we will present an alternative array design, which is easy and less costly to construct, with some unique features and potentially new applications.

Tools and accessories

Before we start with the live construction of an Rx array we would like to quickly introduce some essential tools and accessories which we use on a daily basis in our RF lab. While the utmost important piece of equipment in a RF lab is most likely a commercially available Vector Network Analyzer (VNA), the majority of RF engineers have developed their own preferences when it comes to ways of tuning and testing MRI coils. Well known examples are all kind of pick-up or “sniffer” probes, single or double loop versions and a few of them are shown in Fig. 1.

In Fig. 1 some “tuning sticks” as we call them are also shown. These are essentially plastic tubes of various sizes, which have a copper or brass insert on one end and a small piece of RF-ferrite material, as commonly found in variable inductors, on the other end. They prove very useful during tuning of any LC circuits, since inserting the one or other end into air-core wire inductors will easily indicate whether a higher or lower inductor is needed to tune the circuit to the desired frequency. This method saves unnecessary bending or breaking of inductors and is especially useful for locations difficult to reach in the arrays.

Tools with more complexity such as a 50 ohm 8-port termination box and high power attenuator are also included in Fig.1 and Fig.2. When working with large arrays it can be quite cumbersome to handle many individual commercial terminations. This also applies for the high power 8-port attenuator, necessary when testing individual Tx or Tx/Rx coils of an array. We have tested the shown version with up to 1kW of pulsed RF power and have not damaged any port.

Another item included in Fig. 2 is a customized interface box with special coil plug to fit a particular MRI scanner. This allows initial testing and evaluation of different coil geometries /combinations in the scanner. Modular design allows combining any number of these items to match the channel number of the array/scanner. A biasing box which splits/combines RF signals and DC detuning and/or LNA power supply enables sending both these signals over the same coaxial transmission line, very practical for bench testing as well as for live measurements in the scanner.

Finally, a specially home-made item – a matrix switch with expanded eight ports from the two ports of a VNA for coil testing. All unused ports are automatically terminated, so another great advantage for faster testing of large arrays. The shown model is portable, battery operated and usable close to an MRI system. A new version with four-to-sixteen port extension capability is currently being developed.

Construction of the Rx array
The design of the Rx array we are going to present and construct during this course is rather unconventional and not so well-known. Some popular designs have already been covered by excellent presentations over the past ISMRM educational sessions and many recent publications. We thought it might be interesting to show general Rx array building techniques while highlighting the particularities of a modified 3D orthogonality phased array coil (PAC) [4]. A 3-element orthogonality PAC for imaging of the wrist (radiocarpal region) for a 1.5T clinical MR system was constructed and has been extensively used as a research-only device for more than two years to image human wrists and hands of volunteers.

Materials, design and part manufacturing: the main coil former was machined using an acrylic tube with ID=100mm. Three elliptic coil elements were arranged 120° apart azimuthally and tilted to an angle of 54.7° with respect to the XY plane or 35.3° with respect to the Z axis relative to a cylindrical structure (Fig.3 and Fig.4). A length (along Z axis) of 100mm was chosen to image the desired region of interest.
Blade-like conductors [5] were used to minimize detuning and mismatching of the PAC due to change in wrist size and positioning as they are less susceptible to capacitive coupling between the coil and sample (coil-sample coupling effects). These were designed and manufactured using conventional PCB processes, same as for the three circular flanges (Fig.3). All PCB's and mechanical parts of the array housing were designed using 3D CAD software. Therefore, rework of parts was kept at minimum, as complex assemblies were first simulated and the design optimised before commencing the manufacturing.

Electrical design of the array: the schematic of one coil element is given in Fig.5. Each loop is composed of two longitudinal “blades”, connected together by circular circuit boards on each end, which accommodate most of the fixed ceramic capacitors, passive and active detuning circuits. Variable tuning/matching capacitors and biasing RF chokes are partly located on a third circular board. Short semi-rigid coaxial cable is used to connect the matching points to female BNC connectors located on the service side of the array housing. Having a closer look at the schematics of one loop the classical circuitry used in Rx surface coils can be easily recognized. The two blades form the main inductors of the coil, aligned along the longitudinal axis of the imaging space. The loops are divided symmetrically to allow insertion of capacitors C4, C5, C6 and C7 on the blades (all same value) and C1, C2 and C3 on the circular PCB’s. Then, the active detuning circuit is formed by C2, C3, L2 and D1. The value of L1 is chosen to form a high impedance resonant circuit when connected in parallel to C2 and C3, during the forward biasing cycle of D1. This corresponds to the excitation (transmit of RF power from the body/volume coil) cycle, during which the Rx coil is ideally ‘transparent’ to allow uniform B1+ field distribution in the sample. The passive detuning circuit composed of C1, L1 and the anti-parallel diode pair D2 and D3 take over as an additional safety circuit and switch the coil off by self-biasing D2 and D3 by the RF pulse of the transmit coil, should the main detuning feature fail due to any reason, i.e. missing biasing signal from the scanner, broken biasing circuits etc... Accurate tuning of these two circuits is crucial, both for correct excitation and loss-less receiving of signals from the sample. Details of the correct procedure can be found in the literature and will be covered during the live demonstration in this course. Biasing current and voltage (forward/reverse) of D1 is provided through the two RFC inductors from the scanner.
Matching to 50 ohm impedance is achieved by adjusting C-match, while fine tuning to the desired frequency (considering the loading of the sample and MRI scanner environment) is possible by adjusting C-tune (the two capacitors, C-t connected in series with C-tune help to set the perfect tuning range).

A receive-only 3-element wrist PAC, based on a modified version of the orthogonality design method, was constructed and applied successfully in a series of pilot MRI studies at 1.5T on a phantom, volunteers and patients wrist. Patient comfort was maintained due to its unique ability to orientate the coil at any required angle within the magnet bore. We have demonstrated that the coil is very easy to construct, excellent coil decoupling can be straightforwardly achieved and the design can be readily adapted to construct other Rx or Tx or Rx/Tx coils, since coil decoupling does not rely on preamplifier decoupling.
One could also think of using this design to build multi nuclei coils, and explore designing special coils taking advantage of the Magic Angle Phenomena [7-9].