MEDICAL EQUIPMENT & DEVICES ANALOG DEVICES
Figure 2: CT detector module signal chain
produce high resolution 3D images of blood vessels, soft tissue, and more.
The CT detector is the central component of the entire system architecture and, indeed, is the heart of the CT system. It is comprised of multiple modules, which are depicted in Figure 2. Each module transforms the incident X-rays into electrical signals routed into the multichannel analogue data acquisition system (ADAS). Each module contains a scintillating crystal array, a photodiode array, and the ADAS, which contains multiple integrator channels that are multiplexed into the ADCs. The ADAS must have very low noise performance to maintain good spatial resolution with reduced X-ray dosage and to achieve high dynamic range performance with extremely low current outputs. To avoid image artifacts and ensure good
contrast, the converter front end must have highly linear performance and offer low power operation to relax cooling requirements for the temperature sensitive detector. The ADC must have high resolution of at least 24 bits to achieve better and sharper images, and a fast sampling rate to digitise detector readings that can be as short as 100μs. The ADC sampling rate must also enable multiplexing, which would allow the use of fewer converters as well as the reduction of the size and power of the entire system.
POSITRON EMISSION TOMOGRAPHY Positron emission tomography (PET) involves ionising radiation resulting from a radionuclide introduced into a human body. It emits positrons that collide with electrons in tissue, generating pairs of gamma rays radiated roughly in opposite directions. These pairs of high energy photons simultaneously strike opposing PET detectors aligned in a ring around a gantry bore. The PET detector, schematically shown in Figure 3, consists of an array of scintillators and photomultiplier tubes (PMTs) converting gamma rays into electrical currents that are translated into
20 October 2024 Irish Manufacturing (VGAs). The resulting signal is split between ADC and
comparator paths to provide energy and timing information used by the PET coincidence processor to reconstruct a 3D image of a radioactive tracer concentration within the body.
energies are around 511keV and if their detection times differ by less than one ten-billionth of a second. The photons’ energy and the detection time difference impose strict requirements on the ADC, which must have good resolution of 10 to 12 bits and fast sampling rates typically better than 40MSPS. Low noise performance to maximise the dynamic range and low power operation to reduce heat dissipation are also important for PET imaging.
MAGNETIC RESONANCE IMAGING Magnetic resonance imaging (MRI) is a non-invasive medical imaging technique that relies on the phenomenon of nuclear magnetic resonance and does not use ionising radiation, which distinguishes it
from DR, CT, and PET systems. The carrier frequencies of the MR signals scale
the frequencies ranging in commercial scanners from 12.8MHz to 298.2MHz. Signal bandwidth encoding direction and can vary from a few to several dozens of kHz.
receiver front end, which is typically based on a super-heterodyne architecture (see Figure 4) with lower speed SAR ADCs. However, recent advancements in analogue- to-digital conversion enabled fast and low power multichannel pipeline ADCs for direct digital conversion of the MR signals in the most common frequency ranges at conversion rates exceeding 100MSPS at a 16-bit depth. The requirement for dynamic range is very demanding – it typically exceeds 100dB. Enhanced image quality is achieved by oversampling the MR signal, which improves resolution, increases SNR, and eliminates aliasing
Figure 3: PET electronic front-end signal chain
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