Calcifications Technical Principles

EBCT has been very well validated in several experimental and clinical studies and has been extensively used in the past for coronary artery calcium scoring. EBCT was therefore considered the reference method (so-called 'gold-standard') for coronary artery calcium scoring against which all new methods must be judged. The heart of the EBCT scanner is a stationary electron gun that runs at a constant tube current of 625 mA and a tube voltage of 130 kV. The EBCT unit generates an electron beam that is deflected towards and focused onto one of four target tungsten rings (210°). The emanating radiation fan meets one of two detector rings (216°) on the opposite side. The invariable tube current yields a fixed mAs product that cannot be adapted to the individual patient's body constitution. For coronary artery imaging the so-called ECG-triggered 'single-slice mode' is used, which utilizes 100 ms sweeps along one of the four target rings and acquires a stepwise volumetric data set by moving the patient incrementally along the z axis. Using a standard EBCT scanning protocol, a stack of 40 contiguous slices with a thickness of 3.0 mm is acquired with prospective ECG triggering in diastole.

MSCT is characterized by multiple detector rows that are concurrently targeted by the rotating X-ray tube which allows simultaneous acquisition of multiple axial slices. The first generation of MSCT scanners with 4 detector rows was introduced into clinical practice in the late 1990s. Nowadays MSCT scanners with up to 64 detector rows are in widespread use.

MSCT scanning allows an alternative approach to ECG-synchronized data acquisition, the so-called retrospective ECG gating. For retrospective gating a spiral MSCT scan is performed while the patient's ECG is simultaneously recorded. MSCT data and ECG are synchronized afterwards and images are created at a particular time point within the RR interval. This technique allows retrospective reconstruction of images at any phase of the cardiac cycle. Retrospective ECG gating involves irradiation throughout the entire cardiac cycle and requires a slow table feed for data oversampling to ensure complete phase-consistent coverage of the heart.

The accuracy of coronary artery calcium scoring crucially depends on the occurrence of motion artifacts. To prevent significant motion artifacts a high temporal resolution, namely a short acquisition time and the selection of the optimal phase within the cardiac cycle, is of utmost importance. EBCT achieves a short acquisition time of 100 ms for coronary artery calcium scanning by its technical design that does not involve rotating mechanical components. In contrast, MSCT reaches an acquisition time of only 115-250 ms depending on the scanners maximum gantry speed (330-500 ms/rotation) if a conventional half-scan image reconstruction technique is used. However, the temporal resolution can theoretically be increased by using so-called multi-cycle reconstruction algorithms which combine the raw data required for halfscan image reconstruction not from a single rotation within one cardiac cycle but instead collect data from multiple partial rotations over several cardiac cycles to create images. The factor by which this algorithm improves temporal resolution is equal to the number of cardiac cycles used. Selection of the optimal phase within the cardiac cycle is also needed to minimize motion artifacts. In general, reports in the literature suggest that the optimal cardiac phase for coronary artery imaging is a time window during early diastole (at 40-50% of the RR interval) and another during late diastole (at 80% of the RR interval). However, it is difficult to predict the most suitable time window for an individual examination. Moreover, it has to be noted that each coronary artery has its own optimal time window and that the length and position of the window change with the heart rate. Thus, choosing the optimal cardiac phase by using retrospective ECG gating (where any number of data sets at any time point within the RR interval can be reconstructed afterwards) allows a more effective minimization of motion artifacts when compared with prospective ECG triggering (where the time point within the RR interval has to be selected beforehand).

Spatial resolution and image noise is a crucial parameter for visualization of the coronary arteries and the detection of small calcium deposits. An EBCT standard protocol typically acquires a stack of contiguous slices with a thickness of 3 mm, whereas MSCT now allows acquisition of overlapping slices with a thickness of less than 1 mm. Despite a thinner slice thickness, MSCT scans have a lower image noise and thus a more favorable signal-to-noise ratio compared with EBCT. High image noise levels in EBCT may prevent the detection of small calcium deposits (false-negative result) or may lead to a considerable overestimation of the calcium burden (false-positive result) [11]. Here, the fact that the mAs product of the electron beam scanner cannot be changed by selecting a different tube current is clearly a disadvantage since the scanner operates at a constant tube current and voltage.

Consequently data in the literature demonstrated that MSCT achieves calcium scoring results with at least similar or superior accuracy and reproducibility than EBCT [12-18].

0 0

Post a comment