New CT System Utilizes Less Radiation with Faster Results

A new computed tomography (CT) system requires only a fraction of the radiation dose that systems previously required to scan even the smallest anatomic details, and does that faster than ever before.

The Somatom Definition Flash, developed by Siemens Healthcare (Erlangen, Germany), is a new dual-source CT from Siemens, featuring two X-ray tubes that simultaneously revolve around the patient's body. A 43 cm/s scanning speed in CT and a temporal resolution of 75 ms, enable, for example, complete scans of the entire chest region in just 0.6 seconds. Therefore, patients are no longer required to hold their breath during the exam the way they had to in the past. At the same time, the Somatom Definition Flash operates at an extremely reduced radiation dose. For example, a spiral heart scan can be performed with less than 1 millisievert (mSv), whereas the average effective dose required for this purpose usually ranges from 8 mSv to 40 mSv. The new CT scanner will be available for sale in the first quarter of 2009.

This new generation of dual-source CT scanners, featuring two X-ray tubes and two detectors, will provide an innovation boost to dual-source technology. The enormous speed of the Somatom Definition Flash offers decisive advantages, especially regarding examinations of moving structures, such as the thorax and the heart. The gantry (i.e., the X-ray detector system surrounding the bore) rotates about its own axis in just 0.28 seconds. This extraordinary rotational rate enables very fine temporal resolutions and a scan speed never before attained in CT.

The patient is moved through the CT tube more than twice as fast as with any conventional system. At the same time, scans acquired with the system require a much lower radiation dose than conventional scans. While the average effective dose for a heart CT scanner ranges from 8 mSv to 40 mSv, the new Siemens CT scanner's dose is less than 1 mSv. In comparison, the natural-source X-ray radiation that humans are exposed to each year, range from 2 mSv to 5 mSv. The dose values of this scanner lie far below those of an intracardiac catheter examination, thus opening up possibilities for using CT scanners for routine cardiologic examinations.

Due to the high scanning speed, it is now possible to acquire scans of the thorax, heart, or both at the same time in fractions of a second. For example, thorax examinations now only require a scan time of 0.6 seconds. As a result, patients no longer have to hold their breath during the scan, providing substantial advantages, particularly in cases involving the elderly, children, emergency, and intensive care unit (ICU) patients.

It is also possible to perform extremely fast whole-body scans: For example, a person with a height of 198 cm can be scanned in less than 5 seconds. Until now, such whole-body examinations took more than 10 minutes to perform from patient preparation to diagnosis. With this system, this process is completed in just a few minutes. This represents an advantage, particularly for emergency medicine since, until now, physicians often had to forego this examination method due to time pressure. Furthermore, it is no longer necessary to sedate children prior to the examination, since they no longer have to remain still. The high scanning speed also makes it possible to cover large areas measuring up to 48 cm with four-dimensional (4D) imaging (3D plus time). The areas scannable using conventional systems are limited to a maximum of 16 cm due to the detector size involved.

A scan of the entire heart can be performed in only 250 milliseconds, which is less than half a heart beat in duration. Moreover, it is possible for clinicians to effectively display a heart with a fast pulse or an irregular heart beat without using beta-blockers, thus simplifying the workflow.

The contrast in CT scans will be increased without having to apply the higher radiation dose previously required. This is achieved via a new, selective photon shield that blocks unnecessary parts of the energy spectrum. It thereby provides improved separation of the two simultaneous scans with low and high photon energy, without causing a higher radiation exposure than would result from an individual, conventional CT examination with a single energy source. Thus, the Somatom Definition Flash can always provide a double contrast, which for the first time ever, can also be used to classify the chemical composition of tissues via a CT scan in routine daily work. Subsequently, it could also be used to reconstruct unenhanced CT images without contrast media not having to perform an additional examination.

Another technical development for keeping the patient's radiation exposure as low as possible is the X-Care application. This application selectively reduces the radiation exposure of dose-sensitive anatomic regions, such as the female breast. This is done by switching the X-ray tube assemblies off during the rotation phase in which the anatomic regions concerned are most directly exposed to radiation. In this manner, it is possible to reduce the radiation exposure of individual anatomical regions by up to 40%.

Furthermore, an adaptive dose shield blocks irrelevant prespiral and postspiral radiation with dynamic diaphragms, thus ensuring that only a minimum and clinically essential radiation exposure occurs. This enables an additional 25% reduction of the dose required for routine examinations. The Care Dose4D software, which analyzes the individual cross-sectional anatomy in real time and adapts the emitted X-ray dose accordingly, also helps to reduce radiation exposure.

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