Sensor Technology – Ceramic Detectors

Computed tomography (CT) has become an indispensable tool for visualizing anatomic structures in the human body. CT generates three-dimensional images of internal organs in seconds, giving physicians precise information regarding disease conditions. A few years ago, Siemens introduced the first CT scanner capable of generating images of the beating heart (see Pictures of the Future, Spring 2003,  Before Illness Strikes). This feat will be even easier to achieve with the launch of the new Somatom Sensation 64 this year, as the device is even faster than its predecessor and its images have a higher resolution. As a consequence, it will be able to visualize even tiny deposits in heart vessels. Alongside sophisticated electronics, one of the main reasons for the new device’s high image quality is an inconspicuous ceramic. This component forms part of the detector, which is located opposite the X-ray tube in a ring measuring 1.5 m in diameter. This ring, which is known as the gantry, rotates quickly around the patient. By doing so, it creates individual images that are combined to create a 3D picture. In the detector, the ceramic converts X-rays into light signals, which are then transformed into electrical pulses by photodiodes.

The more effectively the detector transforms the X-rays, the smaller the dosage required for an examination. Detector materials therefore need to be very good at absorbing X-ray quanta. This requirement is met by the ceramic developed by Siemens Medical Solutions (Med) in Forchheim, Germany, and Siemens Corporate Technology in Munich. In addition, the material works extremely quickly, taking only a fraction of a second to react to changes in radiation intensity. This is particularly important in cases where the X-ray beam first penetrates soft tissue and then bone. Since soft tissue allows more radiation to pass through than bone does, this transition is visible in the CT image as a light-dark contrast. The sharpness of the image is dependent on the material used for the detector, because each substance has some afterglow, which means it emits fluorescent light longer than desired. For sharp contrast between tissue and bone, the afterglow must be minimized. Just as the radiation reaching the detector is reduced abruptly at the transition from soft tissue to bone, the afterglow of the detector material should also cease immediately. In other words, the shorter the afterglow, the sharper the image. Although the new ceramic has been used in Siemens CT systems since 1996, it achieves its full potential only at the extremely fast rotation times found in the Somatom Sensation 64, where the gantry circles the patient in 0.33 s.

Secret Recipe. "The afterglow of our special ceramic decays about 400 times faster than is the case with yttrium-gadolinium oxide, which has been used for some time by other manufacturers," says Frank Berger, head of Ceramic Manufacturing at Med. "That’s why we dubbed the material UFC—or UltraFastCeramic." In addition to the rare earth element gadolinium, this ceramic contains sulfur, oxygen and other additives. "Our manufacturing process results in a substance with extremely pure, precise crystalline structures—one of the preconditions for high luminous efficiency," says Berger. The end product is a hard, yellow substance that weighs about as much as gold and is just about as valuable. The recipe for producing the ceramic remains a secret. "It’s the same with Coca-Cola," says Dr. Thomas von der Haar, Head of CT Detector Development. "The ingredients are known, but aside from the manufacturer, nobody knows the full recipe." Since the new material has a significant effect on CT system characteristics, it creates considerable competitive advantages for Siemens—one of the reasons for the in-house UFC development.

A Heart in Nine Seconds. Furthermore, UFC has additional advantages: It can be easily cut with tools from the silicon industry into roughly one-millimeter-thick, stamp-sized plates— approximately the size of a detector element. In the Somatom Sensation 64, 42 detector elements are arranged next to each other over a distance of approximately one meter. This is wide enough to image a patient from shoulder to shoulder. Each detector element is divided into a precise pattern of millimeter-wide rows and columns of tiny pixels. Until a few years ago, CT scanners were equipped with only one detector line, which meant that only a single slice could be acquired per revolution of the gantry.

These devices were followed by multi-slice systems, which feature several detector lines located next to each other. During one rotation, several adjoining slices are acquired. As a result, a wider body region can be imaged within the same time period. Each of the detector elements in the Somatom Sensation 64 has 40 pixel rows as well as 16 pixel columns set at right angles. Since the focus of the X-ray tube shifts in a fraction of a second, 64 slices can be measured during each rotation. As a result, the system has about 43,000 pixels altogether, which means it can achieve an unparalleled resolution of 0.4 mm. Thanks to this development, physicians can study many structures in detail that were previously impossible to see.

When the gantry rotates, electronics process 2.5 billion signals every second. Due to the fast rotation, examinations can now be performed more quickly; in fact, the new CT can depict the heart in just nine seconds. "The ultra-fast ceramic can handle this challenge with relative ease," says Berger. "There’s still a long way to go before we reach the limits of its potential."

Tim Schröder