Medical imaging: shrinking form factor and improving performance

Like all industries that rely heavily on technological advances, medical imaging equipment manufacturers have to continually improve their products—primarily improving the imaging quality of their systems. Whether it is ultrasound reflected sound waves, magnetic resonance imaging (MRI) magnetic field perturbations or positron emission from positron emission tomography (PET), most medical imaging techniques require a patient signal receiving sensor array. The most straightforward way to improve image quality is to expand the size of the sensor array. But because more sensors are added to the device, the signal chain that transmits the signal to the processing engine must add electronics.

At the same time, vendors must shrink their system size, reduce power consumption, and improve performance. Performance enhancements in one aspect of the system may present challenges to others. Simply adding sensors and signal chains can cause adverse effects including system size and power consumption. However, the latest generation of signal chain components for medical imaging systems enables medical system designers to improve signal chain density and power consumption without compromising dynamic performance—that is, the system achieves higher image quality and lower power at the same time. Consumption and smaller size.

Figure 1 Functional structure of the ultra low power VGA

Components of a medical imaging receiver

For most typical medical imaging applications, each component of the sensor array requires its own signal chain to transmit and convert the small signal response of the sensor into a matched small signal response for digital signal processing. Because the signal response properties of imaging application sensors are not the same, three main active components are usually indispensable during signal conversion. The first is the Low Noise Amplifier (LNA), whose main function is to fix the noise figure (NF) of the analog system as low as possible. The second amplifier is typically the signal that swings after the LNA to best match the final stage of the analog-to-digital converter (ADC) input.

Figure 2 Comparison of noise figure and selected VGA performance

Applications such as MRI (which typically have little swing in signal amplitude) can use a fixed gain stage. However, if the system differs greatly in signal strength (such as ultrasound), then the system requires a variable gain amplifier (VGA) and requires a programmable gain amplifier (PGA) before the ADC. After the ADC, the analog signal is converted to a digital signal and ready to be sent to the system's digital signal processor (DSP), which typically completes the final stage of signal processing and conversion through a field programmable gate array (FPGA). For MRI, there may also be a series of mixing stages between the LNA and the amplifier to convert the radio frequency (RF) energy of the magnet into low frequency energy. Because each component requires three or more devices, each doubling of the sensor, the number of analog components that only receive the signal chain may need to be increased by a factor of 6 to 10! In addition, the increase in power requirements is not to mention. No wonder system designers are constantly asking component suppliers to innovate their new integrated circuit (IC) designs to address size-related issues.

High integration: more signal chains, less space and lower power consumption

A major improvement is the integration of more and more analog devices on a single chip, which reduces the number of ICs required for the system. In the case of a typical ultrasonic receiving chain, each sensor may require four devices, three of which are amplifiers. With modern design and process, IC vendors now offer devices that integrate LNA, VCA, and PGA into a single variable gain amplifier, ultimately reducing the number of chips by a third. In addition, current designs typically include multiple signal chain channels in each chip, and an IC package in a 64-pin QFN package contains up to eight VGA channels. This allows the VGA output to go directly to the input of the ADC without the need for external passive or active components, saving more board space. In Figure 1, other functional modules, such as continuous-wavelength array switches and clamp circuits, especially for medical imaging systems, are also integrated into the device.

Integrating multiple channels in a single device has many other advantages besides the size advantage. Typically, the first component is designed to achieve a balance of power and performance as a separate entity. Although designed to work together, the performance of each component looks better than the performance required by the system. So when the components work together, each component distort the power and performance balance toward performance, resulting in higher power than expected.

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