3.2 Factors required for neurosurgery endoscopy 18
3.3 3 D limitations for early endoscopes 18
3.4 "Insect-eye" technology in neurosurgery 18
3.5 Conclusion 20
4. 3 D in Neuroimaging 21
4.1 Basics of MRI 21
4.1.1 Magnetism of body 22
4.1.2 Weightings 25
4.1.3 Slice selection 26
4.1.4 Phase and frequency encoding 27
4.1.5 Sequences 27
4.1.6 K-space 28
4.2 3 D MRI 29
4.3 3 D model for neurosurgery 30
4.4 Basics of CT 30
4.4.1 Scanning 32
4.4.2 Reconstruction 34
4.5 3 D CT 36
4.5 3 D CT vs 3 D MRI 36
4.5 Conclusion 37
The term 3 D simply means 3 dimensions and it includes length, breadth and depth (height). The dimension of a space or object is informally defined as the minimum number of coordinates needed to specify any point within it. Any one-dimensional entity will require only one co-ordinate to represent a point on it. For example a line is having one dimension. A point on a plane needs two co-ordinate system to represent it and so it is having two dimensions. A three-dimensional system like a cube requires three co-ordinates to represent it.
The three-dimensional systems in the world are represented in two-dimension using an image. The images which are formed separately in the two eyes (left and right) are processed in the brain and it helps us to perceive things in the world in three- dimension. The normal image which is obtained using a camera is of two- dimension and there is no information about how far the objects are from the point of capture and in other words there is no depth perception. The depth perception is the visual ability to perceive the world in three dimensions (3D) and the distance of an object.
The neurosurgery makes use of imaging techniques for planning the surgery and instruments like microscopes and endoscopes while performing surgery. All these provide the representation of the deep structures present in the body in the form of two-dimensional images. The neurosurgery demands for high precision and good depth perception. Therefore, incorporating the 3-dimensional information with all these is very important for an efficient and safe neurosurgical operation.
The word stereoscopy is derived from the Greek words stereos which means, "firm, solid" and skopeō, "to look" or "to see”. The stereoscopy is concerned with, or relating to, seeing space three-dimensionally as a result of binocular disparity. The human eyes are approximately two-and-a-half inches or 6cm apart (‘interocular distance’ (Fig 1.1)), so they see the same image from slightly different angles and perspectives. The brain then combines these two images in order to gauge distance. This is called binocular vision. Stereo vision, or ‘Stereopsis’, is a result of good binocular vision, wherein the separate images from two eyes are successfully combined into one 3D image in the brain. The images in Stereoscopic 3D are obtained and displayed in such a way that it is an attempt to replicate what we see with our own two eyes. So, stereoscopy creates the illusion of three-dimensional depth from images on a two-dimensional plane.
The human eyes provide different lines of sight and it leads to a difference in the image obtained. Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight. The Fig 1.2 shows the change in the position of the object when viewed from viewpoint A and viewpoint B. From viewpoint A the object appears to be in front of blue background whereas from viewpoint B it appears to be in front of the red background. This clearly illustrates that there is a change in the apparent position also in the line of sight of the left and right human eyes which leads to the different images from a particular vision point. This parallax is the basis for the perception of depth by the brain.
Fig 1.2: parallax
The human eye has the ability to adjust the focal length according to the distance of the object and both the eyes work simultaneously to accommodate the object. When the eyes move laterally, in the same direction, this is called a version. When the eyes move in opposite directions, to an object closer than where the eyes are pointing or farther than where the eyes are pointing, this is called a vergence. When the eyes move in, it is a convergence eye movement; when the eyes move out, it is a divergence eye movement. The effect of convergence and divergence eye movements is to be considered for the good 3 D effect to be created.