Technical principles and applications of multislice CT

Abstract

The introduction of spiral computed tomography (CT) in the early 1990s constituted a fundamental evolutionary step in the development and ongoing refinement of CT-imaging techniques [1,2]. For the first time, volume data could be acquired without the danger of misregistration or double registration of anatomical details. Images could be reconstructed at any position along the patient axis (longitudinal axis, z-axis) and overlapping image reconstruction could be used to improve longitudinal resolution. Volume data became the very basis for applications such as CT angiography [3], which has revolutionized noninvasive assessment of vascular disease. The ability to acquire volume data also paved the way for the development of three-dimensional image processing techniques such as multiplanar reformations (MPR), maximum-intensity projections (MIP), surface-shaded displays (SSP), or volume-rendering techniques (VRT) [4], which have become a vital component of medical imaging today. The main drawbacks of single-slice spiral CT are either insufficient volume coverage within one breath hold time of the patient or missing spatial resolution on the z-axis due to wide collimation. With single-slice spiral CT the ideal of isotropic resolution, i.e., of equal resolution in all three spatial axes, can only be achieved for very limited scan ranges [5]. Larger volume coverage and improved longitudinal resolution may be achieved by simultaneous acquisition of more than one slice and by faster gantry rotation. In 1998, all major CT manufacturers introduced multislice CT (MSCT) systems, which typi-cally offered simultaneous acquisition of four slices at a rotation time of 0.5 s, thus providing considerable improvement of scan speed and longitudinal resolution and better utilization of the available X-ray power [6-9]. Simultaneous acquisition of M slices results in an M-fold increase in speed if all other parameters, such as slice thickness are unchanged. The increased performance allowed for the optimization of a variety of clinical protocols. The examination time for standard protocols could be significantly reduced, which proved to be of immediate clinical benefit for the quick and comprehensive assessment of trauma victims and of uncooperative patients. Alternatively, the scan range that could be covered within a certain scan time was extended by a factor of M, which is relevant for oncological staging or for CT angiography with extended coverage, for example of the lower extremities [10]. The most important clinical benefit, however, proved to be the ability to scan a given anatomic volume within a given scan time with substantially reduced slice width at M times increased longitudinal resolution. This way, for many clinical applications the goal of isotropic resolution was within reach with 4-slice CT systems. Examinations of the entire thorax [11] or abdomen could now routinely be performed with a 1-mm or 1.25-mm collimated slice width (Fig. 1.1). MSCT also dramatically expanded into areas previously considered beyond the scope of third-generation CT scanners based on the mechanical rotation of the X-ray tube and detector, such as cardiac imaging with the addition of ECG gating capability. With a gantry rotation time of 0.5 s and dedicated image reconstruction approaches, the temporal resolution for the acquisition of an image was improved to 250 ms or less [12,13], which proved to be sufficient for motion-free imaging of the heart in the mid-to end-diastolic phase at slow to moderate heart rates (i.e., up to 65bpm [14]). Despite all these promising advances, clinical challenges and limitations remained for 4-slice CT systems. True isotropic resolution for routine applications had not yet been achieved, since the longitudinal resolution of about 1 mm does not fully match the in-plane resolution of about 0.5-0.7 mm in a routine scan of the chest or abdomen. For large volumes, such as CT angiography (CTA) of the lower extremity run-off [10], even thicker (i.e., 2.5-mm) collimated slices had to be chosen to complete the scan within a reasonable timeframe. For ECG-gated coronary CTA, stents or severely calcified arteries constituted a diagnostic dilemma, mainly due to partial volume artifacts as a consequence of insufficient longitudinal resolution [15], and reliable imaging of patients with higher heart rates was not possible due to limited temporal resolution. As a next step, the introduction of an 8-slice CT-system in 2000 enabled shorter scan times, but did not yet provide improved longitudinal resolution (the thinnest collimation was 8x1.25 mm). The latter was achieved with the introduction of 16-slice CT [16,17], which made it possible to routinely acquire substantial anatomic volumes with isotropic submillimeter spatial resolution. Improved longitudinal resolution goes hand in hand with considerably reduced scan times that enable high-quality examinations in severely debilitated and severely dyspneic patients (Fig. 1.1). CTAs in particular benefit from the gain in spatial resolution, and clinical praxis suggests the potential of 16-slice CT to replace conventional catheter examinations for many indications. ECG-gated cardiac scanning is enhanced by both, improved temporal resolution achieved by gantry rotation times down to 0.375 s and improved spatial resolution [18,19]. As a consequence of the increased robustness of the technology, characterization and classification of coronary plaques is becoming feasible even in the presence of calcifications. Currently, the race for more slices is ongoing. In 2004, all major CT manufacturers introduced MSCT-systems with 32, 40 or 64 simultaneously acquired slices, which brought about a further leap in volume coverage speed. Some of these scanners use double z-sampling, a refined z-sampling technique enabled by a periodic motion of the focal spot in the z-direction (z-flying focal spot), to further enhance longitudinal resolution and image quality in clinical routine [20]. With the most recent generation of CT systems, CT angiographic examinations with submillimeter resolution in the pure arterial phase become feasible even for extended anatomical ranges. The improved temporal resolution due to gantry rotation times down to 0.33 s has the potential to increase clinical robustness of ECG-gated scanning at higher heart rates, thereby significantly reducing the number of patients requiring heart rate control and facilitating the successful integration of CT coronary angiography into routine clinical algorithms. Very useful up-to-date information regarding MSCT is readily available on the Internet, for example on the UK MDA CT Web site www.medical-devices.gov.uk or at www.ctisus.org. © Springer-Verlag Berlin Heidelberg 2006.