Computed tomography (CT) is a medical imaging technique that produces three-dimensional images of internal human body parts from a large series of two-dimensional X-ray images taken around a single axis of rotation. When compared with a conventional X-ray radiograph, which is an image of many planes superimposed on each other, a CT image exhibits significantly improved contrast.
With the advent of diagnostic imaging systems like CT, where complex and intensive image processing is required, semiconductors play a very important role in developing systems with increased density, flexibility and high performance.
X-ray slice data is generated using an X-ray source that rotates around the object with X-ray detectors positioned on the opposite side of the circle from the X-ray source. The whole rotating structure is called gantry and every x-ray shot from a given angle is called profile. Of the order of 1000 profiles per revolution are taken progressively as the object is gradually passed through the gantry. The data acquisition system usually consists of a number of channel cards that have an array of scintillator-photodiode solid state detectors follow by the readout electronics.
Each photodiode produces a current proportional to the x-ray intensity that the pixel receives. Traditionally, the channel card has a front-end where the current from the detector is integrated and converted to digital values by ADCs. TI’s DDC products are single-chip solutions for Directly Digitizing low-level Currents from photodiode arrays in CT scanners. Each DDC channel provides a dual switched integrator front-end to process the current coming from one photodiode. This configuration allows for continuous current integration (avoiding any input signal loss): while one integrator output is being digitized by the on board A/D converter, the other is integrating the input current.
The digital data from all channel cards is transferred by high-speed link (LVDS interface) to the controller card and onto the image conditioning cards. The image conditioning card is connected to the host computer where the CT images can be viewed. Here, the digital data are combined by the mathematical procedure known as tomographic reconstruction.
Within the controller cards, TI DSPs with advanced VelociTI™, very-long-instruction-word (VLIW) architecture developed by Texas Instruments (TI), are an excellent choice for medical imaging applications. DSPs can be used to provide accurate control of the gantry rotation, the movement of the table (up/down and in/out), tilting of the gantry for angled images, and other functions such as turning the X-ray beam on and off. Another important DSP control functionality is ECG gating used to reduce motion artifacts caused by heart movement. Here, the data acquisition is carefully synchronized with the heartbeat. For interfacing with a PC, gigabit Ethernet transceivers allow for high-speed full-duplex point-to-point data transmissions. The PCI Express™ PHY interfaces the PCI Express Media Access Layer (MAC) to a PCI Express serial link.
The CT Scanner application may have ultra-fast transient requirements for high performance DSP and/or FPGAs, where TI’s high-performance non-isolated power modules are well suited. If high PSRR, fast start-up, and low noise are concerns, low-dropout (LDO) linear regulators are available. TI’s portfolio includes voltage supervisors, DC/DC converters, power modules, and LDOs that allows complete flexibility for the user to configure a power solution that meets the sequencing requirements for the system.
Recommended Resources
Medical Imaging Brochure
Medical Imaging Brief
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