Acquisition

Sample stage rotation

The stage is assumed to perform a rotation around its $\vec{w}_\textsf{O}$ axis during the scan, resulting in a circular sample trajectory. Other trajectories can be modelled using drifts, especially drifts of the stage geometry parameters. For example, a helix scan can be modelled by starting with a standard circular trajectory of several rotations (as described in the following), and an additional drift of the stage center’s $z$ position. If the scan trajectory is completely described by drifts, the stage rotation described here should be deactivated by setting both "start_angle" and "stop_angle" to 0.

For a circular CT scan, the start and stop angle of the sample stage are defined in this section. An angle of 0 refers to the orientation of the stage as defined in the geometry section (for frame 0). All other angles express a rotation around the $\vec{w}_\textsf{O}$ axis of the stage. The direction of rotation must be mathematically positive in the case of counter-clockwise acquisition direction ("CCW"), and mathematically negative in the case of clockwise acquisition direction ("CW"). The stage reaches the start and stop angle by rotating in the given "direction" around its normal axis (Fig. 4). This means that the "direction" parameter affects both the positions of start and stop angle, as well as the direction of rotation in which the CT scan is performed. It also means that the start angle must always be less than (or equal to) the stop angle. However, negative angular positions and positions greater than 360° are allowed, as well as an angular coverage of more than a full circle, e.g. to perform multiple rotations during one scan.

"start_angle": {"value":   0, "unit": "deg"},
"stop_angle":  {"value": 320, "unit": "deg"}

_images/rotation.svg — Fig. 4 The given start angle and stop angle refer to opposite angular positions in **(a)** counter-clockwise or **(b)** clockwise direction (as seen from “above” the stage). They describe the angular range covered by the CT scan.

The direction of the sample stage rotation can be counter-clockwise ("CCW", mathematically positive) or clockwise ("CW", mathematically negative) around the $\vec{w}_\textsf{O}$ axis:

"direction": "CW",
"direction": "CCW"

The parameter "scan_mode" defines if the rotation stops while a projection is taken, or if it runs continuously.

"scan_mode": "stop+go",
"scan_mode": "continuous"

The property for scan speed should only be used for continuous-motion scans. If undefined, it may be calculated from the detector’s integration and dead time.

"scan_speed": {"value": 360, "unit": "deg/h"}

For stop&go scans, it should be set to null:

"scan_speed": null

Frames and projections

Number of projections

The "number_of_projections" is also given in the acquisition section.

"number_of_projections": 2001

Beginning from the start angle, the necessary number of angular steps is performed (in the case of a stop&go scan). It is assumed that a frame is taken before each step (starting with the first frame at the start angle). The parameter "include_final_angle" can be set to true if the last projection should be taken after the stop angle has been reached, or to false if it is meant to be taken at the last step before the stop angle is reached.

"include_final_angle": true

Frame averaging

The number of frames to be averaged for one projection image can be specified:

"frame_average": 3

Dark field and flat field acquisition and correction

If dark field and flat field images are acquired along with the projections, their numbers and frame averages can be specified. An "ideal" image means that the simulation of noise is to be omitted by the simulation software, possibly in contrast to the noise specification in the detector section. The parameter "correction" tells whether the projection images already come in a corrected form as a result of the scan (true) or if they are taken as uncorrected files (false).

"dark_field": {
  "number": 1,
  "frame_average": 1,
  "ideal": true,
  "correction": false
},
"flat_field": {
  "number": 3,
  "frame_average": 20,
  "ideal": false,
  "correction": false
}

Pixel binning

The number of pixels to bin in directions $\vec{u}$ and $\vec{v}$ of the detector:

"pixel_binning": {"u": 1, "v": 1}

The binning operation is not described here and left to the software.

Scattering

This parameter specifies if X-ray scattering should be simulated or not.

"scattering": false