What is CT Scanning?
Computed Tomography or CT Scanning is a method of using a focused X-ray beam and a detector traveling around an object or having the object spin between the focused x-ray beam and detector while capturing several individual X-ray imaging data so that the radiographer can visualize a volumetric image. To achieve the most accurate and usable CT Image, the CT scanner must be designed to provide stabile and accurate movements as well and enough energy and detection capability to penetrate the object.
According to Wikipedia, "The history of X-ray computed tomography goes back to at least 1917 with the mathematical theory of the Radon transform. In October 1963, William H. Oldendorf received a U.S. patent for a "radiant energy apparatus for investigating selected areas of interior objects obscured by dense material". The first commercially viable CT scanner was invented by Sir Godfrey Hounsfield in 1967."
Since the invention of CT the entire stakeholder community has been working to make the CT imaging modality faster, accurate, less expensive, operator friendly etc... In most cases the X-ray tube and detector will travel around the patient for medical examinations so that the patient does not have to be moved. CT exam studies provide a far more imaging data because the entire region or interest (ROI) is examined and presented as a volumetric image that can be explored without harming the patient. However, we will focus our discussions on industrial or NDT CT.
Industrial CT, makes use of accurate turntables to rotate the part or object while having the needed flexibility to adjust several axis that control FDD, FOD, turntable height and centering, tube and detector height, collimation and filtering.
Axis Label Stroke Movable Component
1. Zd ≥ 1,300 mm Detector
2. Zs ≥ 1,300 mm Source/ X-ray Tube
3. Yd ≥ 1,000 mm Detector
4. Yo ≥ 1,200 mm Object/ Turntable
5. Xo +/- 250 mm Object/ Turntable
6. Xs customizable Source/ X-ray Tube
7. Xd +/- 250 mm Detector
8. Rotation endless Object/ Turntable
CT Image Acquisition and Display
There are different tomographic movements that can be discussed. However, to keep to a limited scope pertaining to typical NDT CT we will discuss the use of the circular tomographic motion. Application engineering for each part requires that the industrial CT operator is creative in positioning and having a good handle on geometry and understanding how to obtain the best results with the fasted scan times. For purposes of explanation, when we put an object on the CT turntable we try to center it and use prescribed methods to limit scatter radiation and artifacts.
The CT System X-ray Generator technique is set up so that the optimum tube output is achieved whereas the attenuated X-ray beam is captured by the pixel matrix. If using a LDA the number of acquisitions will increase compared to a flat panel detector that will capture several lines at one time.
The simplest way to show the CT image acquisition is to show the LDA acquisition. The picture shows a medical CT with spinning detector and tube gantry around the skull phantom. With the tube and detector spinning around the skull phantom, a slice or series of images are taken in that plane. The attenuated X-ray data is collected in a matrix. Because the collection is in a circle, the date can be visualized as a sinogram but it is not of any diagnostic quality to a clinician or radiographer. Each square contains a finite volume element of the pixel data or Voxel. The size of your matrix depends on the number of pixels used in the acquisition. Interpolation Process - Each pixel is assigned a CT number or bit depth representing the density after X-ray attenuation. Each Voxel has a (for 12 bit) CT number between -1024 (air) to +3071 (dense material). 3D reformatting requires each original voxel shaped as rectangular parallel piped or solid as dimensionally altered into multiple cuboidal voxels.
According to Wilipedia, "The projection of an object, resulting from the tomographic measurement process at a given angle, is made up of a set of line integrals (see Fig. 1). A set of many such projections under different angles organized in 2D is called sinogram (see Fig. 3). In X-ray CT, the line integral represents the total attenuation of the beam of x-rays as it travels in a straight line through the object. As mentioned above, the resulting image is a 2D (or 3D) model of the attenuation coefficient." There are different types of reconstruction and several mathematical formulas that are used to reconstruct the CT image. We discuss these in depth studies in our training courses.
Fig. 2: Phantom object, two kitty-corner squares.
Fig. 3: Sinogram of the phantom object (Fig.2) resulting from tomography. 50 projection slices were taken over 180 degree angle, equidistantly sampled (only by coincidence the x-axis marks displacement at -50/50 units).
Fig.4: Algebraic Reconstruction Technique based tomographic reconstruction of the sinogram of Fig.3, presented as animation over the iterative reconstruction process. The original object could be approximatively reconstructed, as the resulting image has some visual artifacts.
Figure 1: Parallel beam geometry utilized in tomography and tomographic reconstruction. Each projection, resulting from tomography under a specific angle, is made up of the set of line integrals through the object.