The three-dimensional models are created based on the parameters obtained form multi-slice images and they are used in neuronavigation techniques for planning the neurosurgery. The tumour is highlighted in the model with respect to the surrounding depending on the MRI signals as shown in the Fig 4.13.
Fig 4.13 3D Model Basics of CT
CT enables the acquisition of two-dimensional X-ray images of thin “slices” through the body. Multiple images from adjacent slides can be obtained in order to reconstruct a three-dimensional volume. The basic principle behind CT is that the two-dimensional internal structure of an object can be reconstructed from a series of one-dimensional “projections” of the object acquired at different angles. In order to obtain an image from a thin slice of tissue, X-ray is collimated to give a thin beam as shown in Fig 4.14. The detectors, which are situated opposite the X-ray source, record the total number of X-rays that are transmitted through the patient, producing a one-dimensional projection. The signal intensities in this projection are dictated by the two-dimensional distribution of tissue attenuation coefficients within the slice. The X-ray source and detectors are then rotated by a certain angle and the measurements are repeated. This process continues until sufficient data have been collected to re-construct an image with high spatial resolution. Reconstruction of the image involves a process termed back projection. The reconstructed image is displayed as a two-dimensioanl matrix, with each pixel corresponding to the CT number of the tissue at the spatial location.
Fig 4.14: Components of CT
The signal intensity recorded at each detector depends on the attenuation coefficient and the thickness of each tissue that lies between the X-ray source and that particular detector and it can be represented as:
Where I=transmitted intensity of X-ray, Io=incident beam of X-ray on the surface, x=thickness of the object, e=Euler’s constant(2.718), μ= linear attenuation coefficient
There are three basic steps involved in obtaining CT and they are:
The motion of the X-ray source and the detector occur in two-ways: linear and rotational. M-linear steps were taken with the intensityof the transmitted X-rays being detected at each step. This produced a single projection with M data points as shown in Fig 4.15. Then both the source and detector were rotated by 180/N degrees, where N is the number of rotations in the complete scan, and a further M translational lines were acquired at this angle. The total data matrix acquired was therefore M X N points.