Microwave technology has been used in medical applications for a number of years now. Microwaves are used to generate heat, measure and monitor temperature, and measure motion. Microwave ablation in the form of microwave energy can be applied to a heart muscle by way of a coaxial catheter inserted in a vein.
This approach kills diseased heart cells by delivering lethal levels of heat directly to the damaged areas. A microwave catheter has been developed to provide deep myocardial ablation to treat ventricular tachycardia by restoring appropriate electrical activity within the heart and eliminating irregular heartbeats.
The resulting microwave catheter design, which is now being developed for commercial use in treating ventricular tachycardia, can be modified to treat prostate cancer and benign prostatic hyperplasia (BPH). Given the 350 thousand BPH operations performed each year in the United States alone, this microwave catheter has significant commercial potential.
In clinical medicine, microwave radiometry is used to obtain information about internal body temperature patterns by the measurement of part of the centimetric wavelength component of the natural thermal radiation from the tissues of the body. In appropriate circumstances, knowledge of such thermal patterns can assist clinical disease detection and diagnosis, and may have a role in the monitoring of therapeutic processes. The technique has important practical advantages for clinical medical applications through being noninvasive and inherently completely safe.
The motion detecting capabilities of microwaves have also been applied to medical uses, perhaps most notably with Magnetic Resonance Imaging (MRI). This technique applies a gradient magnetic field and an RF (radio frequency) wave to a subject in a static magnetic field, producing an image based on magnetic resonance signals emitted as an echo from protons in the examination region. Magnetic resonance imaging (MRI) utilizes hydrogen nuclear spins of the water molecules in the human body or other tissue, which are polarized by the strong, uniform, static magnetic field generated by a magnet.
The magnetically polarized nuclear spins generate magnetic moments in the human body. The magnetic moments point in the direction of the main magnetic field in a steady state, and produce no useful information until they are disturbed by an RF excitation.
Radio frequency energy is applied to a specific region of the patient by a transmitter and antenna. The RF energy excites atomic nuclei within the patient's tissues.
The excited nuclei spin at a rate dependent upon the magnetic field. As they spin, they emit faint RF (echo) signals, referred to as magnetic resonance signals. In order to select a specific region, a gradient magnetic field is applied together with the high frequency magnetic field. To provide correct positional information for the measured echo signals, it is necessary to control the application time and intensity of the gradient magnetic field precisely. By applying additional magnetic field gradients, the frequency and phase of the magnetic resonance signals from different locations within the slice can be made to vary in a predictable manner depending upon the position within the slice.
Magnetic resonance imaging (MRI) systems typically include a super conducting magnet capable of producing a strong, homogenous magnetic field; a radio frequency (RF) transmitter and receiver system, including transmitter and receiver coils; a gradient coil system; and a computer processing/imaging system, receiving the signals from the receiver coil and processing the signals into interpretable data, such as visual images.
A magnetic resonance imaging system is equipped with a radio frequency oscillator. This oscillator is used to adjust the carrier frequency of a radio-frequency excitation pulse for selective excitation, depending on a slice-directional position of a specified section (single slice) to be imaged of an object, in cases where imaging is carried out while the object is moved.
The MRI signals are detected using loop antennas known as coils. Magnetic resonance imaging (MRI) collects data in the Fourier domain, typically referred to as k-space, from the magnetic signals of protons processing in a magnetic field. RF component performance is important to the MRI performance. For instance, Microsemi Corporation has developed a new ultra low-magnetic PIN diode that virtually eliminates artifact interference in the surface coil switching of MRI systems.
Artifact interference is created by a "glow" or "halo" surrounding a diode's structure. The greater the artifact, the more difficult it becomes to accurately measure the location of objects, such as tumors, being scanned by the imager's RF magnetic field.
Radio frequency system
The radio frequency (RF) transmission system consists of a RF synthesizer, power amplifier and transmitting coil. This is usually built into the body of the scanner. The power of the transmitter is variable, but high-end scanners may have a peak output power of up to 35 kW, and be capable of sustaining average power of 1 kW. The receiver consists of the coil, pre-amplifier and signal processing system. While it is possible to scan using the integrated coil for transmitting and receiving, if a small region is being imaged then better image quality is obtained by using a close-fitting smaller coil. A variety of coils are available which fit around parts of the body, e.g., the head, knee, wrist, breast, etc.
A recent development in MRI technology has been the development of sophisticated multi-element phased array coils which are capable of acquiring multiple channels of data in parallel. This 'parallel imaging' technique uses unique acquisition schemes that allow for accelerated imaging, by replacing some of the spatial coding originating from the magnetic gradients with the spatial sensitivity of the different coil elements.
However, the increased acceleration also reduces the signal-to-noise ratio and can create residual artifacts in the image reconstruction.
As the MRI systems increase their resolution, the frequency of the RF systems increase, from High Frequency (HF) to Very High Frequency (VHF). This makes new device technology desirable. Freescale Semiconductor has a developed a 50V laterally diffused metal oxide semiconductor (LDMOS) technology that has delivers a notable leap in RF figures of merit for systems above 100MHz.
Freescale offers the MRF6VP11KH that delivers 27 dB of gain and 72% efficiency © 130MHz. In addition, with a pulsed θjc of 0.03ºC/W, the cooling system requirements have been considerably reduced compared to other MOSFET or silicon bipolar technologies. The MRF6VP11KH was introduced in June 2007, a year after Freescale’s initial introduction of 50V LDMOS technology and devices. Freescale now offers a full range of devices for industrial, medical, and broadcast applications, ranging from 10W to 1KW rated power. As medical equipment based on RF and microwave technology continues to evolve, the future capabilities of RF devices will also need to make similar forward leaps in performance.