Leading Concept in High-End CT Imaging

For almost a decade now, Siemens’ Dual Source concept has spearheaded computed tomography imaging and proven its value in exceptional products. During this time, Dual Source CT (DSCT) has continually set new standards in CT imaging. In 2013, the latest pinnacle was reached with the introduction of SOMATOM® Force.

Over the past four decades, experts from a range of scientific fields – physicists, clinicians, IT experts, engineers – have been working on ideas to take CT imaging forward, aiming at better, more reliable diagnostic results that will ultimately help physicians to determine the optimal treatment path for the patient.

In developing their computed tomography systems, the various vendors have taken a variety of approaches. Among these, three distinct concepts can be identified in the arena of high-end CT imaging:

Dual layer detector design:

• Two different layers of detector material absorb either low and high energy photons of the X-ray beam to evaluate two different energy spectra
• Wide detector coverage enabled by a large detector array beyond the established 64-row design, with up to 16 cm coverage
• Dual Source CT with two tube detector pairs integrated at an approximate 90-degree angle

Dual layer detectors were only introduced as a clinical product in 2013 so practical experience is limited, even though prototypes and concepts reach back to 2005. However, initial results indicate that Dual Source CT is a better method of acquiring Dual Energy information.[1]

The general idea is that the X-ray beam is absorbed by a two-layered detector after it has penetrated the patient, delivering two different energy spectra from the respective layers, thus enabling Dual Energy imaging. But a natural drawback of this design is that in every other non-Dual Energy examination, users have to live with the system’s drawbacks (e.g. higher electronic noise, fixed kV settings, etc.), making it a specialized niche scanner.

The other two concepts, wide detector coverage and Dual Source CT have been used much more extensively in clinical routine. The extent of clinical and scientific results that have been published on these two concepts therefore allows a reliable comparison. The initial motivation for wide detector coverage beyond the established 64-row detector design was to cover larger anatomies (e.g. whole organs or vascular structures) using a fast scan mode at high spatial resolution. Another field was dynamic studies such as perfusion or long-range angiographies.

Although the available wide detector designs enable coverage of organs, such as the heart, the concept itself had several drawbacks: In 2012, a paper concluded that “…for a 140 mm system, 24.5% of imaging volume exhibits more severe cone beam artifacts than a 64-slice system, which also poses a patient dose concern. In addition, this system may suffer from a 36% peak power (flux) loss, which is equivalent to about 20% image noise increase. Therefore, a wide coverage CT system using a single X-ray source is likely to face some severe challenges in IQ and clinical accuracy.”[2]

It is important to note that the wide detector further is limited by its temporal resolution (resulting in motion artifacts) and also by its capabilities regarding spiral acquisition modes for volumes larger than 16 cm. Based on advanced and comprehensive cardiac studies, such as studies on electron beam CT scanners,[3] a temporal resolution far below 100 ms is favored when patients with high and irregular heart rates are to be scanned. Since no conventional single source CT scanner can deliver the required rotation speed, reconstruction approaches were introduced to somewhat reduce motion artifacts and compensate for the lacking temporal resolution. But although these methods have been available for several years now, their true impact in regular clinical routine still remains to be seen and has thus far only been shown for lower and stable heart rates.[4] In dynamic studies, for example in myocardial perfusion assessments, temporal resolution below 100 ms is also required.

In contrast to this, Dual Source CT has shown that it can compensate these limitations and actually achieve the intended goal of fast large volume coverage. With hundreds of clinical publications based on the Dual Source concept, this approach has clearly established itself as the pinnacle in high-end CT. Taking the cardiac arena in particular, several studies have demonstrated that Dual Source CT avoids of the issue of heart rate control[5], makes it possible to image children even when they are not sedated[6], and facilitates significant dose reduction to far below one mSv, not only in selected cases but also for large patient populations.[7]

When it comes to Dual Energy imaging, only Dual Source truly scans at two independent individual kV settings or energy levels, making it the gold standard in this arena. And now, the unique capabilities of SOMATOM Force allow clinical researchers to take CT imaging into new clinical fields where CT previously could not be applied, for instance scanning patients with renal insufficiencies or conducting long-range dynamic studies for treatment planning (Fig. 1) or vascular surgeries (Fig. 2).

Fig. 2: Dynamic 4D study conducted with SOMATOM Force – 796 mm acquired using dynamic 4D spiral mode at 70 kV with 1 mSv. MIP images show nicely the dynamic flow of the vascular details in the lower extremities. In comparison to an one-time Runoff CTA, a dynamic 4D scan provides more diagnostic information in multiple acquisition phases.

Consequently, Dual Source CT has not only established itself firmly as the leading technological approach with more than 1,500 Dual Source installations since the initial introduction of Dual Source scanning with SOMATOM Definition. It also continues to set the clinical gold standard in CT imaging with SOMATOM Definition Flash and pushes innovation in high-end CT with SOMATOM Force.

Jan Freund is head of the Product Marketing team at Computed Tomography, Siemens Healthcare, Forchheim, Germany.