Siemens Sensation 64 Scanner and How it Works
Thomas Flohr, Siemens Medical Solutions
The SOMATOM Sensation 64 makes use of a periodic motion of the focal spot in the longitudinal direction (z-flying focal spot) to double the number of simultaneously acquired slices with the goal of improved longitudinal resolution and elimination of spiral artifacts. By permanent electromagnetic deflection of the electron beam in the X-ray tube the focal spot is wobbled between two different positions on the anode plate. Due to the anode angle of 7° this translates into a motion both in the radial direction and in the z-direction. The radial motion is a side-effect which is taken care of by the image reconstruction algorithms. The amplitude of the periodic z-motion is adjusted such that two subsequent readings are shifted by half a collimated slice width in the patient's longitudinal direction, see below.
Therefore, the measurement rays of two subsequent readings with collimated slice-width 0.6 mm interleave in the z-direction, and every two 32-slice readings are combined to one 64-slice projection with a sampling distance of 0.3 mm. With this technique, 64 overlapping 0.6 mm slices per rotation are acquired. The z-coverage is 32x0.6 mm = 19.2 mm, and the sampling scheme is identical to that of a 64x0.3 mm detector. This fine sampling is the reason for the improved spatial resolution and the elimination of spiral artifacts. The improved sampling is obtained at any spiral pitch. Hence, resolution is improved and spiral artifacts are eliminated at any pitch. The improved sampling is furthermore not restricted to the iso-center, but is maintained in a wide range of the scan field of view (SFOV), see the figure above. This is a major difference to conventional approaches which attempt to improve longitudinal sampling by the choice of optimized small pitch values (so-called "High Quality" pitches), so that data acquired in different rotations interleave in the z-direction. In this case, a sampling distance of half the collimated slice width can be achieved close to iso-center only, see below, and improved resolution will only be achieved close to iso-center, which is a clinical drawback. Free selection of the pitch is not possible, and the user is restricted to special modes with low table feed and low volume coverage.
The clinical benefits of optimized z-sampling with the z-flying focal spot technique are two-fold: firstly, longitudinal resolution is improved at any pitch and in a wide range of the SFOV by establishing narrow, well-defined slice sensitivity profiles (SSPs). Secondly, spiral artifacts are suppressed at any pitch. Typical spiral artifacts present as hyper- or hypo-dense "windmill" structures surrounding z-inhomogeneous high-contrast objects such as bones, which rotate when scrolling through a stack of images. Using conventional MDCT scan and reconstruction techniques spiral artifacts can be reduced by either decreasing the pitch and/or increasing the reconstruction slice width relative to the collimation. Both approaches aim at improving the spiral sampling along the z-direction, but at the expense of reduced table feed and/or reduced spatial resolution. Using conventional MDCT systems, demanding applications such as neuro scanning need low pitch protocols to reduce artifacts and to improve image quality. CTAs of the carotid arteries and the circle of Willis for instance require a careful optimization of the spiral pitch to ensure sufficient volume coverage speed on the one hand and avoid intolerable spiral artifacts on the other. The z-flying focal spot technique maintains a low artifact level up to high pitch values even for critical applications, thus increasing the maximum volume coverage speed that is clinically useful.
To sum up:
The z-flying focal spot technique improves longitudinal resolution and eliminates windmill artifacts at any pitch. Even the most demanding clinical applications can be performed at maximum pitch without degradation of image quality or resolution. This is a major difference to conventional CT-systems which claim to have a larger detector coverage but are restricted in table feed ("High Quality" Modes), if good image quality is required.