Prodrive Technologies has been developing critical components in the Medical market for over 15 years. Most recently they have pushed the envelope in one of the most important components in Magnetic Resonance Imaging (MRI), the RF amplifier. Using latest device technologies, design techniques, and several years of designing critical electronics, Prodrive has created an advanced, efficient and reliable RF solution for MRI.
The RF amplifier in MRI
Different medical imaging modalities come with their share of advantages. For MRI there are two distinct and major advantages over others; the ability to create amazingly detailed medical images without exposing the patient to ionizing radiation and having the best imaging capabilities of soft tissue deep within the body.
A complex imaging modality like MRI can be a bit simplified into four major functional blocks: nuclear alignment, excitation, spatial location, and finally image processing. Excitation is the ‘R’ in MRI; essentially using Radio Frequency(RF) to ‘excite’ or increase energy through ‘R’ (for Resonance) of particles within the body and the subsequent relaxation of those particles. This part of the process and overall imaging is not possible without the RF amplifier.
The RF amplifier therefore is absolutely critical within the imaging chain of MRI and must come with stable and repeatable performance. Making use of a proprietary hybrid control loop as a part of the design, Prodrive is able to achieve above market fidelity performance in their NGRF series of products. While continuing to build upon and improve those characteristics, Prodrive also envisioned latest technology to drive better efficiency and, an in turn, reliability; improving total cost of ownership (TCO) which is critical in a more and more cost conscious medical market.
RF Power Device: LDMOS vs VDMOS
Prodrive’s experience, market and technology research has shown that two solid-state structures are well applicable for a MRI RF amplifier: vertical and laterally double diffused metal oxide semiconductors (VDMOS and LDMOS respectively).
VDMOS finds extensive application as MOSFET in switching power supplies. Disadvantages are the low gain, the maximum power per FET and the robustness. Due to the low amplification factor, much power is required for the transistor control, which also becomes complex. In addition, many transistors are required to realize the required output power, which directly influences the reliability and efficiency of the product.
LDMOS has an approximately ten times greater gain, a much larger maximum power per FET and is many times more robust. In order to achieve the power levels per channel (~20kW) in modern MRI machines you historically needed a substantial amount of devices due to their effciencies. The number of devices and thermal management of those have not only, cost but, subsequently, reliability consequences. LDMOS gives you the ability to drastically reduce the number of devices in general and then again at higher operating frequencies, like in 3T. LDMOS allows for a more linear relationship with number of devices going from 1.5T to 3T, for example. Overall this has enormous advantages in component reduction, cost and overall reliability, driving Prodrive’s decision towards LDMOS and are deploying it across the NGRF platform.
Additional Noteworthy Design
With about 50 V the operating voltage of LDMOS FETs is relatively low but the current is higher for this combination of output power and low voltage. During the momentary peak powers, it is very important that the voltage remains very stable; any voltage drop in the power supply will result in a lower output power, undesirable for MRI applications. In addition, in step responses, the current can rise in less than a microsecond, requiring a low inductive design with a very fast response. Prodrive solved these challenges with a proprietary and specially designed DC-DC converter.
With such complex components, it is a good thing to be able to use diagnostics at the system level. All our amplifiers; NGRF series amplifiers, NG series –Gradient amplifiers and NGS-shim Amplifiers are ‘connected’ and with the same easy to use GUI tool suite they can be read remotely and used to setup, control and diagnose multiple key components along the imaging chain. This functionality can be used many ways from initial setup and tuning of the amplifier(s), to built-in predictive maintenance and providing insight into how our partners’ customers use the system(s), allowing for optimization.
Putting it all together
With its completely organic/‘everything in house’ approach in manufacturing and development, deploying latest technology and design techniques across multiple RF products, Prodrive Technologies has created an advanced, efficient and extremely reliable RF amplifier platform solution