CT scanners: what are they for, and how do they operate?
A computed tomography (CT) scanner is one of the most powerful diagnostic tools available to medical practicioners. In traditional digital radiography, an X-ray flat panel detector provides a 2D image of dense material such as bone and other tissues overlaid in a single plane. This form of medical imaging remains in widespread use in domains such as dentistry and mammography, providing scans of specific organs or parts of the body.
CT scanning, clinically available since the early 1970s, substantially changed the practice of medicine, as it introduced the ability to display three-dimensional images of organs such as the head, heart or lungs. Images generated by a CT scanner also show a high contrast ratio between bone, tissue, blood vessels and other parts of the body.
So how does a CT scanner produce the images used by medical experts in the diagnosis and treatment of conditions such as cancer, heart disease, bronchitis and pneumonia, and injury to muscles or the skeleton?
How a CT scanner ‘slices’ the body
A CT scanner uses a combination of sophisticated hardware and software technologies to reconstruct a 3D image of internal organs of the human body.
The scanner consists of an X-ray tube and a detector diametrically opposite each other, and rotating around the patient to provide a complete 360° axial scan. X-rays emitted by the tube pass through the body. The detector array captures the attenuated X-ray photons on the opposite side and collects numerous data points from many angles while the patient moves into the scanner.
The X-ray tube and detector array are both mounted on a rotating ring (or gantry). The gantry allows for the rotation speed to be controlled, and the angular position at which each of the projections are made to be accurately measured.
The detector array spins rapidly around the patient and captures projection images at many angles. These projections are used to create three-dimensional slices of the body. Image slices can be stacked together by sophisticated algorithms to generate a rendered 3D image. This explains the name ‘computed tomography’: the word tomography denotes that the images are obtained by ‘cutting’ the body into virtual slices; and the technique is ‘computed’ because the raw image data from hundreds of slices have to be compiled in software to render an image that the physician can view and analyze.
The information in this rendered image is so useful because it can achieve a high level of contrast between the different parts of the body, with no overlay. The contrast is the result of the different X-ray attenuation characteristics of soft tissue, bone and blood, and of the effect of scanning with or without contrast agents. In addition, the acquisition of projections at multiple angles allows the CT scanner’s software to resolve the overlaying stack of materials in the body by displaying 3D rendered images or slices.
Critical factors affecting CT scanner performance
In an ideal world, a physician would wish a CT scan to be minutely detailed, sharp and clear. It should faithfully represent the body with no errors. It should be unaffected by motion artefacts such as the beating of the patient’s heart, or the movement of their chest as they breathe. And for safety reasons it should apply the lowest possible dose of radiation required to acquire clinically relevant data.
Today’s scanners get remarkably close to this ideal. But technology is always advancing, and CT scanner manufacturers are continually researching new techniques and technologies to:
- Capture thinner slices to create more detailed images benefiting from higher spatial resolution.
- Wider detection area, allowing the scanner to cover larger portions of the body in a single rotation.
- Reduce the effect of electronic noise and increase the signal-to-noise ratio. This improves image quality and allows for a reduction in dose.
- Lower system power consumption. This lowers heat dissipation, and thus reduces the requirement for cooling operations, which affect the performance of the detector array.
- Increase the speed at which an image slice can be captured and processed, to reduce the impact of motion artefacts.
- Increase sensitivity and resolution to enable detection of smaller features in greater detail.
ams inside the CT scanner
ams has been developing high-end sensors for CT detector arrays since the 1990s. The function of the detector array is to capture a clean and precise image signal. This calls for high-resolution, low noise, mixed-signal semiconductor technology which consumes very little power – a domain in which ams is a world leader.
ams standard products for CT scanners include sensors and readout ICs such as the AS5950, AS5951 and AS5900. They offer high performance for detector arrays used in the value CT segment, targeting 16- to 64-slice CT machines. The highest-performance detector arrays for the most advanced scanners featuring 128, 256 or more slices require extremely low noise and high linearity. ams serves this high-end segment by supplying CT sensors as application-specific ICs (ASICs) to the world’s leading manufacturers of CT machines.