The detector array: the key component at the heart of a CT scanner
A computed tomography (CT) scanner is a powerful and sophisticated medical diagnostic tool. CT is commonly used today in the diagnostic examination of the head/neck, chest, CT angiography, abdomen and pelvis.
In a CT scanner, multiple ‘slices’ or cross-sectional images of the patient’s body are captured in sequence, as described in How a CT scanner works.
A key component of a modern multi-slice CT scanner is the detector array: this senses the X-ray radiation which is partially attenuated by tissues in the patient’s body, and converts them into a digital signal. This digital signal contains all the information required by the powerful image signal processor, which reconstructs slices from contiguous projections of X-ray image data acquired at multiple rotation angles of the CT gantry, and then renders them as 3D volumes of the patient’s organs and tissues.
Low-noise and high-precision operation
The detector array is a sophisticated assembly of optoelectronics components. Its main elements are:
The scintillator, a material which converts X-ray photons to light photons
Photodiode array – a 2D arrangement of photodiode elements or pixels, each generating a current in response to the absorption of light originating from the scintillator. That is, the photodiode converts light into an electrical current.
Readout IC – a highly integrated semiconductor device which converts the current generated by each photodiode pixel at many angular positions (as the detector rotates around the patient) into a digital signal representation. The acquired data is then further processed by an image signal processor or image reconstructor. The readout IC is basically a high performance multi-channel analog-to-digital converter.
To produce an accurate digital representation of the radiation transmitted through the patient, the active components in a detector array – the photodiodes and readout IC – must feature certain critical characteristics: high sensitivity, low noise, high resolution, and high speed.
High sensitivity – in order to produce images covering the variety of tissues and densities in the body, the detector must be able to acquire data across a wide dynamic range of X-ray intensities with the lowest possible dose of radiation. Soft tissue has low radiodensity and absorbs little X-ray radiation, and bone has high radiodensity and absorbs a high amount of radiation. A detector system with high sensitivity provides a high-contrast image which enables the physician to clearly distinguish between body tissues.
Low noise – a low noise floor enables the detector to capture weak inputs reliably, and to maintain a high contrast between weak and strong intensities. Because a low-noise detector can capture weak signals, it allows the use of lower doses of radiation, making the scanner safer for the patient.
High resolution – the higher the detector’s resolution, the smaller the features it can detect, allowing it to distinguish finer structures, for instance in chest or lung examinations.
High speed enables the detector to support high imaging speeds, for use in the scanning of dynamic phenomena such as the beating of the heart, and reducing motion artefacts by allowing ever increasing rotation speeds of the CT gantry.
Choice of assembly configurations
The size of the CT scanner’s imaging area determines the size of the photodiode array, and hence the number of imaging readout ICs required to convert the light sensor input to a digital output.
In a detector assembly, multiple readout ICs are mounted on a carrier substrate. They can be mounted either contiguously on three sides (a three-side buttable solution), or on all four sides (a four-side buttable solution). The method for assembling the readout ICs affects the performance of the CT system.
Three-side buttable solutions have wire bonds on the one side, with the remaining three sides available for butting with adjacent ICs. Such solutions are best suited for value CT systems with moderate detector coverage.
Four-side buttable solutions, used for high-end CT systems, require more sophisticated process steps during manufacturing and assembly, but offer the advantage of allowing the detector’s imaging area to be extended in all directions. This allows the system to offer a large detector coverage, ultimately enabling organs such as the heart to be scanned in a single rotation.
ams offers high performance solutions addressing both three-side and four-side buttable options, exhibiting low noise, ultra-low power and high readout speeds. Such solutions range from stand-alone analog-to-digital converters allowing maximum flexibility for our customers, to highly integrated ICs combining both photodiode arrays and readout circuits in a single silicon package, producing the best possible performance.