Strictly speaking, the only preparation that is absolutely necessary for CT scanning is to ensure that the object fits inside the field of view and that it does not move during the scan. Because the full scan field for CT is a cylinder (i.e., a stack of circular fields of view), the most efficient geometry to scan is also a cylinder. Thus, when possible it is often advantageous to have the object take on a cylindrical geometry, either by using a coring drill or drill press to obtain a cylindrical subset of the material being scanned, or by packing the object in a cylindrical container with either X-ray-transparent filler or with material of similar density. For some applications the sample can also be treated to enhance the contrasts that are visible. Examples have included injecting soils and reservoir rocks with NaI-laced fluids to reveal fluid-flow characteristics (Wellington and Vinegar, 1987; Withjack, 1988), injecting sandstones with Woods metal to map out the fine-scale permeability, and soaking samples in water to bring out areas of differing permeability, which can help to reveal fossils (Zinsmeister and De Nooyer, 1996).
Calibrations are necessary to establish the characteristics of the X-ray signal as read by the detectors under scanning conditions, and to reduce geometrical uncertainties. The latter calibrations vary widely among scanners; as a rule flexible-geometry scanners such as the ACTIS scanner at UTCT require them, whereas fixed-geometry scanners geared towards scanning a single object type may not.
The two principal signal calibrations are offset and gain, which determine the detector readings with X-rays off, and with X-rays on at scanning conditions, respectively. An additional signal calibration, called a wedge, used on some third-generation systems (including the UTCT ACTIS scanner) consists of acquiring X-rays as they pass through a calibration material over a 360º rotation. The offset-corrected average detector reading is then used as the baseline from which all data are subtracted. If the calibration material is air, the wedge is equivalent to a gain calibration. A typical non-air wedge is a cylinder of material with attenuation properties similar to those of the scan object. Such a wedge can provide automatic corrections for both beam hardening and ring artifacts, and can allow utilization of high X-ray intensities that would saturate the detectors during a typical gain calibration. Although widely employed in medical systems, which use phantoms of water or water-equivalent plastic to approximate the attenuating properties of tissue, the wedge calibration is relatively uncommon in industrial systems.
The principal variables in collection of third-generation CT data are the number of views and the signal-acquisition time per view. In most cases, rotation is continuous during collection, and each rotation is for a full 360º, although for some systems smaller rotations may be used. On the UTCT ACTIS scanner, the number of views used ranges from 600 to 3600 or more. Each view represents a rotational interval equal to 360º divided by the total number of views. The raw data are displayed such that each line contains a single set of detector readings for a view, and time progresses from top to bottom. This image is called a sinogram, as any single point in the scanned object corresponds to a sinusoidal curve. Second-generation CT data are collected at a small number of distinct angular positions (such as 15 or 30), but the progression of relative object and source-detector position combinations allows these data to complete a fairly continuous sinogram.
Reconstruction is the mathematical process of converting sinograms into two-dimensional slice images. The most widespread reconstruction technique is called filtered backprojection, in which the data are first convolved with a filter and each view is successively superimposed over a square grid at an angle corresponding to its acquisition angle. The primary convolution filter used at UTCT is the Laks filter (Ramachandran and Lakshminarayanan, 1970), which is preferred when high-resolution images are desired; also available is the Shepp-Logan filter (Shepp and Logan, 1974), which is used more frequently in medical systems and reduces noise at some expense in spatial resolution (ASTM, 1992).
During reconstruction, the raw intensity data in the sinogram are converted to CT numbers or CT values that have a range determined by the computer system. Most medical and older industrial systems use a 12-bit scale, in which 4096 values are possible, while most more recent systems use a 16-bit scale, which allows values to range from 0 to 65535. On most industrial scanners, these values correspond to the grayscale in the image files created or exported by the systems. Although CT values should map linearly to the effective attenuation coefficient of the material in each voxel, the absolute correspondence is arbitrary. Medical systems generally use the Hounsfield Unit (HU), in which air is given a CT number of -1000 and water is given a value of 0, causing most soft tissues to have values ranging from -100 to 100 and bone to range from 600 to over 2000 (Zatz, 1981). Industrial CT systems are sometimes calibrated so that air has a value of 0, water of 1000, and aluminum of 2700, so the CT number corresponds roughly with density (Johns and others, 1993). The calibration of CT values is straightforward for fixed-geometry, single-use systems, but far less so for systems with flexible geometry and scanning modes, and multiple uses each requiring different optimization techniques.
Although a link to a reference scale can be useful in some circumstances, the chemical variability of geological materials and the wide range of scanning conditions used precludes any close correspondence to density in most cases. Furthermore, because material components can range from air to native metals, a rigid scale would be counterproductive. Given the finite range of CT values, a single scale may be insufficiently broad if there are large attenuation contrasts, or needlessly desensitize the system if subtle variations are being imaged. For geological purposes, it is commonly more desirable to select the reconstruction parameters to maximize the CT-value contrast for each scanned object. This can be done by assigning arbitrary low and high values near the limits of the available range to the least and most attenuating features in the scan field. In general we try to ensure that no CT value is generated beyond either end of the 16-bit range, lest some dimensional data be lost. For example, the boundary of an object being scanned in air is usually taken to correspond to the CT-value average between the object and air. If air is assigned to a CT value below zero, the apparent boundary of the object may shift inward.