As with all industries that are heavily dependent on technological progress, medical imaging equipment manufacturers are continually driven to improve their products -- chiefly, the image quality of their systems. Most medical imaging techniques involve arrays of sensors receiving a signal from the patient, whether a reflected acoustic wave in ultrasound, magnetic field disturbances in magnetic resonance imaging (MRI), or positron emissions in positron emission tomography (PET). The most straightforward way to increase image quality is to simply increase the size of these sensor arrays. But with the addition of more sensors to the machines, there is a requisite addition of more electronics in the signal chain transporting the signal to the processing engine.

At the same time, manufacturers are driven to improve other measures of their systems such as size, power consumption, and performance of the specific electronic components. Performance improvements in one area can cause challenges in others. Simply by adding sensors and signal chains, one can trigger the unwanted effect of increasing both system size and power consumption. But, the newest generation of signal chain components enables designers of medical systems to improve signal chain density and power consumption without compromising dynamic performance, simultaneously allowing systems to improve their image quality, power consumption and size.

Elements of the Medical Imaging Receive Chain
For most typical medical imaging applications, each element in the sensor array requires its own signal chain to convey and convert the small signal responses of the sensor into one fit for digital signal processing. Because of the varying nature of the signal responses for the sensors used in imaging applications, there are usually three main active elements involved in the signal’s conversion process. The first is a low-noise amplifier (LNA) whose primary purpose is to fix the noise figure (NF) of the analog system at as low a level as possible. Following the LNA is another amplifier stage to gain the signal to optimally match the input range of the final stage, the analog-to-digital converter (ADC).

Figure 1: Power vs. Noise Performance

Applications such as MRI, which usually do not have large swings in signal amplitude, can use fixed gain stages. However, systems with high variance in signal strength, such as ultrasounds, require variable gain amplifiers (VGA), and potentially programmable gain amplifiers (PGA) to precede the ADC. After the ADC, the signal is in digital format and ready to be sent to the system’s digital signal processor (DSP) usually via a field programmable gate array (FPGA) for processing and conversion into a final image. For MRI there also can be a series of mixing stages between the LNA and amplifier to down-convert the radio frequency (RF) energy of the magnet to low frequencies. With three or more devices required per element, a two-fold increase in sensors may require as much as a six- to 10-fold increase in analog components just for the receive signal chain. This says nothing of the increased power requirements. Therefore, it is not surprising that system designers are pressing component suppliers to innovate new integrated circuit (IC) designs to address such problems of scale.

Integration: More for Less
One main area of innovation has been to integrate more and more active devices onto a single chip, thereby reducing the number of ICs required in a system. Consider a typical ultrasound receive chain. Potentially, there are four devices per sensor, and three are amplifiers. With modern design and processing, IC suppliers now offer devices that combine the LNA, VCA and PGA into one variable gain amplifier (VGA), cutting the number of chips by one-third. Moreover, designs now typically contain multiple signal chain channels per chip. Devices such as the VCA8617 from Texas Instruments offer up to 8 VGA channels per chip. By integrating devices, system designers can allow for the optimization of designs to balance the power versus performance equation, as depicted in Figure 1.

Lower Power and Better Performance
The remaining ADC portion has undergone similar integration as the amplifiers. Many current designs offer eight high-speed ADC channels to match eight-channel VGAs. All the while, ADCs have had a dramatic reduction in power consumption without affecting their performance in the operational envelope of the typical medical imaging application. Because of the noise and linearity requirements imposed by medical imaging applications, the amplifier stages usually are built-in processes such as silicon germanium (SiGe). These processes offer the best balance of low noise, low power and high linearity for typical response frequencies from DC to 20 MHz. Conversely, high-speed ADCs generally are built using CMOS processes because this technology offers a good balance between power and performance for 10- to 14-bit resolution converters.

Due to the advancements in CMOS technology, the power consumption characteristics and footprint of ADCs have been significantly reduced while their performance has improved. These increases in ADC performance with respect to input frequency (IF) have allowed for completely new system architectures in MRI. The main magnets of MRI machines produce a narrow-band IF residing in a range from 30 MHz to 140 MHz. Traditional architecture mixes the IF down to near DC where it can be sampled by a precision delta-sigma (ΔΣ) ADC. Now the newer 14- and 16-bit ADCs can easily sample IFs in this range. With digital decimation, these ADCs can achieve similar SNR to that achieved by using the traditional architecture, saving both board space and system cost, while improving imaging performance.

With the increased use of imaging for medical applications, equipment manufacturers will continue to design new systems with ever-higher quality images. To aid manufacturers in the pursuit of the perfect image, leading semiconductor firms will continue to work, develop and produce the needed technology to meet the needs of higher quality imaging products that reside in smaller forms and consume less power.

Charles (Chuck) Sanna is a product marketing engineer with high-speed analog-to-digital converters (ADCs) at Texas Instruments. He received a bachelor of science in electrical engineering from Northwestern University, and a master’s of science in electrical engineering at The University of Texas at Dallas.