How demanding are medical imaging systems?

The physical principles of digital radiography (DR) are the same as those of all conventional absorption radiography systems . X-rays passing through the body are attenuated by tissues with varying degrees of radiotransmittance and projected onto a flat-panel detector system. This principle is illustrated in Figure 1.

Figure 1. Digital X-ray detector signal chain.

Detectors convert X-ray photons into an electrical charge proportional to the energy of the incident particle. The resulting electrical signal is amplified and converted to the digital domain to produce an accurate digital representation of the X-ray image. Image quality depends on signal sampling in both the spatial and intensity dimensions. In the spatial dimension, the minimum sampling rate is defined by the detector's pixel matrix size and the update rate of real-time fluoroscopic imaging.

Flat-panel detectors with millions of pixels and typical update rates of 25 fps to 30 fps use channel multiplexing and multiple ADCs with sampling rates up to tens of MSPS to meet the minimum conversion time requirements without sacrificing accuracy. In the intensity dimension, the ADC's digital output signal represents the integrated number of X-ray photons absorbed by a given pixel during a specific exposure time. This value is grouped into a finite number of discrete levels defined by the ADC's bit depth.

Another important parameter is the signal-to-noise ratio (SNR), which defines the system's inherent ability to faithfully represent the anatomical features of the imaged body. Digital X-ray systems employ 14-bit to 18-bit ADCs, with SNR levels ranging from 70 dB to 100 dB, depending on the type of imaging system and its requirements. A wide variety of discrete ADCs and integrated analog front ends are available, enabling various types of DR imaging systems to achieve higher dynamic range, finer resolution, higher detection efficiency, and lower noise.

Computed tomography (CT) also uses ionizing radiation, but unlike digital X-ray technology, it relies on a sector-shaped detector system that rotates synchronously with the X-ray source and utilizes more sophisticated processing techniques to produce high-resolution 3D images of blood vessels, soft tissue, and other structures.

The CT detector is a core component of the entire system architecture, effectively the heart of the CT system . It consists of multiple modules, as shown in Figure 2. Each module converts incident X-rays into electrical signals and routes them to a multi-channel analog data acquisition system (ADAS). Each module consists of a scintillator array, a photodiode array, and an ADAS with multiple integrator channels multiplexed to an ADC. The ADAS must offer extremely low noise performance to maintain good spatial resolution and reduce X-ray dose, as well as very low current output to achieve high dynamic range performance.

Figure 2. CT detector module signal chain.

To avoid image artifacts and ensure good contrast, the converter front end must have excellent linearity performance and provide low-power operating modes to reduce cooling requirements for heat-sensitive detectors. The ADC must have a high resolution of at least 24 bits to obtain better and clearer images, while also having a fast sampling rate (as low as 100 μs) to digitize the detector readings. The ADC sampling rate must also support multiplexing, which allows the use of fewer converters and reduces the size and power consumption of the entire system.

Positron emission tomography (PET) involves the use of ionizing radiation produced by a radionuclide introduced into the body. The positrons it emits collide with electrons in tissue, producing pairs of gamma rays radiating in roughly opposite directions.

These high-energy photon pairs simultaneously strike opposing PET detectors, which are arranged in a ring around the port of the stent. The PET detector (shown in Figure 3) consists of an array of scintillating crystals and photomultiplier tubes (PMTs), which convert the gamma rays into an electric current, which is then converted into a voltage. This is then amplified and compensated for amplitude variations by a variable gain amplifier (VGA). The resulting signal is then separated between the ADC and comparator paths to provide energy and timing information used by the PET coincidence processor to reconstruct a 3D image of the radiotracer concentration in the body.

Figure 3. PET electronic front-end signal chain.

If two photons have energies around 511 keV and their detection times differ by less than a nanosecond, they are classified as correlated. The photon energy and detection time difference place stringent demands on the ADC, which must have high resolution of 10 to 12 bits and a fast sampling rate typically exceeding 40 MSPS. Low noise performance maximizes dynamic range, while low-power operation reduces heat dissipation, both of which are important for PET imaging.

Magnetic resonance imaging (MRI) is a non-invasive medical imaging technique that relies on the phenomenon of nuclear magnetic resonance and does not require the use of ionizing radiation, which distinguishes it from DR, CT, and PET systems.

The carrier frequency of the MR signal is directly proportional to the main magnetic field strength and ranges from 12.8 MHz to 298.2 MHz in commercial scanners. The signal bandwidth is defined by the field of view in the frequency encoding direction and can vary from a few kHz to tens of kHz.

This places special demands on the receiver front end, which is typically based on a superheterodyne architecture with a relatively slow SAR ADC (see Figure 4). However, recent advances in analog-to-digital conversion have enabled fast, low-power, multichannel pipeline ADCs to directly digitize MR signals with 16-bit depth and conversion rates exceeding 100 MSPS across the most common frequency ranges. The dynamic range requirements are very stringent, often exceeding 100 dB.

Figure 4. MRI superheterodyne receiver signal chain.

Oversampling the MR signal can improve resolution, increase SNR, and eliminate aliasing artifacts in the frequency encoding direction, thereby enhancing image quality. To achieve fast scan acquisition time, compressed detection technology based on undersampling can be applied.

Sonography, or medical ultrasound, is based on a different physical principle than all other imaging modalities discussed in this article. It uses pulses of sound waves with frequencies ranging from 1 MHz to 18 MHz. These sound waves scan internal tissues and reflect as echoes of varying intensities. These echoes are acquired in real time and displayed as a sonogram, which can contain different types of information, such as acoustic impedance, blood flow, tissue activity over time, or its degree of stiffness.

The key functional blocks of a medical ultrasound front end (shown in Figure 5) are represented by an integrated multichannel analog front end (AFE), which includes a low-noise amplifier, a variable-gain amplifier, an anti-aliasing filter (AAF), an ADC, and a demodulator. One of the most important requirements for the AFE is dynamic range. Depending on the imaging mode, this requirement may need to reach 70 dB to 160 dB to distinguish blood signals from background noise generated by probe and body tissue motion.

Figure 5. Medical ultrasound front-end signal chain.

Therefore, the ADC must have high resolution, high sampling rate, and low total harmonic distortion (THD) to maintain the dynamic fidelity of the ultrasound signal. The high channel density of the ultrasound front end also requires low power consumption. A range of integrated AFEs available for medical ultrasound equipment can achieve optimal image quality while reducing power consumption, system size, and cost.

Medical imaging places extremely demanding demands on electronic design. The requirements for modern medical imaging systems discussed in this article dictate low power consumption, low noise, high dynamic range, and high resolution at low cost and in compact packages. ADI addresses these demands, providing highly integrated solutions for key signal chain functional blocks, enabling the realization of state-of-the-art clinical imaging equipment that is becoming an increasingly integral part of today's global healthcare system.

Finally, let me share ADI's products suitable for the various medical imaging modes mentioned in this article:

ADAS1256 : This highly integrated analog front end contains 256 channels with low-noise integrators, low-pass filters, and correlated double samplers multiplexed into a high-speed 16-bit ADC. It is a complete charge-to-digital conversion solution designed for DR applications that can be mounted directly on a digital X-ray panel.

For discrete DR systems, the 18-bit AD7960 PulSAR® ADC offers 99 dB SNR and a 5 MSPS sampling rate, providing unmatched performance to meet the noise and linearity requirements of the highest dynamic ranges. The 16-bit, dual-channel AD9269 and 14-bit, 16-channel AD9249 pipeline ADCs offer sampling rates up to 80 MSPS and 65 MSPS, respectively, to enable high-speed fluoroscopy systems.

ADAS1135 and ADAS1134 : These highly integrated 256-channel and 128-channel data acquisition systems consist of a low-noise/low-power/low-input-current integrator, a simultaneous sample-and-hold device, and two high-speed ADCs with configurable sampling rates and up to 24 bits of resolution. These systems deliver excellent linearity to maximize image quality in CT applications.

AD9228 , AD9637 , AD9219 , and AD9212 : These 12-bit and 10-bit multichannel ADCs offer sampling rates from 40 MSPS to 80 MSPS and are optimized for excellent dynamic performance and low power consumption to meet PET requirements.

AD9656 : This 16-bit, quad-channel pipeline ADC offers conversion rates up to 125 MSPS and is optimized for traditional direct digital conversion MRI system architectures, with excellent dynamic performance and low power consumption.

AD9671 : This 8-channel integrated receiver front end is designed for low-cost, low-power medical ultrasound applications. It features a 14-bit ADC and operates at sampling rates up to 125 MSPS. Each channel is optimized for high dynamic performance of 160 dBFS/√Hz and low power of 62.5 mW in continuous wave mode, making it suitable for applications requiring a small package size.