Ciné acquisition, in contrast to helical acquisition, is conducted with a stationary couch. The CT scanner rotates and acquires projection data over a longitudinal span that nearly matches the radiation field length. The length of this span depends on the detector layout and is usually between 2 and 4 cm (see Sect. 2.2.2). The number of acquisitions per couch position is programmed in advance and the CT rotation time determines the temporal resolution of the scan. The number of scans multiplied by the scan period is the minimum amount of time that scans can be acquired for, but pauses between scans can be programmed to lengthen the effective scan time to assure that at least one breathing period occurs at each couch position. In fact, the amount of time can easily be stretched to multiple breathing periods to reduce the chance that no motion data are acquired because of breathing irregularities.

Ciné CT acquisition provides many benefits over helical acquisition. Principally, the number of scans is selectable and not constrained by couch speed limitations. Prospective gating is also possible with ciné-CT [4]. Ciné-CT protocols can be programmed to be started by an external trigger, set for example to specific phases of the breathing cycle. Sophisticated analysis of the breathing cycle could be conducted to assure that only images that will provide the required breathing motion data will be acquired, reducing the radiation dose to the patient and improving the usability of the CT scan data.

CT images are reconstructed using x-ray projections from up to . In helical multislice CT, the projection angles also include an out-of-plane angle. Ideally these projections would be acquired over a limited period of time. In order to minimize image noise, however, when reconstructing an image at a specific breathing phase, manufacturers utilize each projection acquired during a time consistent with that breathing phase. The projections may therefore be acquired during different breaths. If the user wants to use the commercial sorting algorithm to reconstruct images, there is no problem, but if the user wants to control the specific timepoints from which projection data are taken, this process is not useful. An advantage of ciné CT is that the projections used to acquire the CT data are acquired only during the gantry rotation time for the specific ciné acquisition. This allows the user to precisely control the timepoints at which the data are acquired for subsequent reconstructions. Pat et al. [7] compared data sufficiency conditions for helical and ciné acquisitions using a 20 cm coverage, 4 s breathing cycle, half-scan reconstruction. He found that the helical acquisition benefited from more rapid scan time (by 10 %) than the ciné acquisition. The helical slice sensitivity profile was 1.8 times broader than the ciné profile and required an additional breathing cycle of scanning at the beginning and end of the scan to assure adequate sampling at the ends of the scans. The ciné scans also delivered between 4 and 8 % less dose than the helical acquisition.

Table 3.1 shows a summary of the comparison between helical and ciné acquisitions.

Ciné CT scan acquisition yields a series of CT slice scans (collections), each acquired at different times and breathing phases. Between 10 and 25 repeated CT scans are acquired per table position before the table is moved to the next position, and this process is repeated until the entire volume of interest is scanned. The process of generating useful 4D CT images that encompass the organs of interest involves the building of 3D CT images at different breathing phases using the acquired ciné images.

There are two current approaches to generate 4D CT images from ciné CT acquisitions and a breathing surrogate measured at acquisition time. They are termed amplitude-based sorting and phase-based sorting. Figure 3.2 shows an example of the amplitude-based process as described by Low et al. [5]. They selected the tidal volume measured with spirometry as the method for describing the breathing cycle. To create a 3D CT image at a specific tidal volume, they selected the ciné scans that were obtained at each couch position with a tidal volume closest to the desired volume. The result was a full 3D CT that represented the patient geometry as though the patient had been scanned at that tidal volume. This process could be repeated for any tidal volume for which there was sufficient scan data.

General Electric used a phase-based approach with their commercial ciné 4D CT scanners [8, 10]. Figure 3.3 shows the process that GE uses to acquire their 4D CT scans. The scans are acquired at the same time as a breathing surrogate is measured. The breathing cycles are divided in phases that are equal in time between inhalations. The images acquired in each phase are collected for each table position to form single-phase CT images, similar to what is done in Fig. 3.2 for breathing amplitude gating.