CT Scan Analysis - 3 PDF

Computed Tomography  COMPUTED TOMOGRAPHY  CT image formation: BY AHMED JASEM ABASS  MSC of Medical Imaging Steps of CT image formation  A computed tomography (CT) image is a display of the anatomy of a thin slice of the body developed from multiple x-ray absorption measurement...

Computed Tomography  COMPUTED TOMOGRAPHY  CT image formation: BY AHMED JASEM ABASS  MSC of Medical Imaging Steps of CT image formation  A computed tomography (CT) image is a display of the anatomy of a thin slice of the body developed from multiple x-ray absorption measurements made around the body's periphery.  All CT systems use a three step process: 1. Scan or Data Acquisition. 2. Image Reconstruction. 3. Image Display. Scan or Data Acquisition  The scanning process begins with data acquisition.  Data acquisition refers to a method by which the patient is systematically scanned by the X-ray tube and collect the enough information by the detector for image reconstruction.  The total X-ray transmission measured by each detector is the result of ray sum.  The collection of X-ray sum for all the detector at a given tube position is called a projection.  Projection data sets are acquired at different angle around the patient. Scan or Data Acquisition  Two projections geometries have been used in CT imaging:  Parallel beam geometry with all rays in projection parallel to one another.  Fan beam geometry in which the rays at a given projection diverge with different angles.  Cone beam geometry. Image reconstruction  The process of using raw data to create an image is called reconstruction.  Image reconstruction is done with the combination of complex computer algorithms, mathematical equations and physics. Image reconstruction  When source-detector makes one sweep across the patient, the internal structures of the body attenuate the X-ray beam according to their mass density and effective Z.  The intensity of radiation detected varies according to this attenuation pattern and an intensity profile or projection is obtained.  These projections are not displayed visually, stored in digital form in the computer.  The computer processes the projections that involve super imposition of each projection to reconstruct an image of the anatomic structures within that slice. Image reconstruction  The individual value of the matrix elements are obtained by solving the simultaneous equations.  The matrix of values that are obtained represents cross sectional anatomy.  Dedicated array processor is used to do calculation and instantaneous image display.  The image reconstruction algorithm are:  Iterative technique.  Back projection.  Filtered back projection. Image reconstruction  Prospective reconstruction:- reconstruction that is automatically produced during scanning  Retrospective reconstruction:- using the same raw data later to generate a new image.  Image display  Image display includes all of the system components necessary to convert the digital data created from the reconstruction process to electrical signals needed by the CT display monitor.  The reconstructed image is displayed either on a cathode ray tube or some form of flat panel such as a TFT LCD , by allotting shades of gray to each CT number.  There are 256 shades of gray in the system and each CT number is allotted one shade of gray. Image Reformation  Image reformation is also known as image rendering ; is a image post processing technique in CT.  Raw data:- an initial measurement or projections obtained by the CT scanner after scanning the patient.  Image data:- the processed and reconstructed form of raw data after applying mathematical transformations and algorithms.  DFOV:- (zoom/ target) determines how much of the collected raw data is used to create an image.  SFOV:-determines the area, within the gantry, from which the raw data are acquired ; determines the number of detector cells collecting data.  Image center:- it is the geometric midpoint of the reconstructed CT image, representing the center of the field of view (FOV).  Anisotropic voxel:- A voxel with unequal dimensions in the three axes, often with thinner slices in one dimension.  Isotropic voxel:- A voxel with equal dimensions in all three axes (x, y, z). Facilitates accurate 3D reconstructions and multiplanar reformation (MPR). CT Image Reformatting Techniques Two- dimensional formats Multiplanar reformation (MPR). Curved planar reformation (CPR). Three- dimensional formats Surface shaded displays (SSD). Volume rendering (VR). Average intensity projection (AIP). Maximum intensity projection (MIP). Minimum intensity projection (MinIP). Endoluminal imaging. Multiplanar Reformation (MPR) Reformation that is done to show anatomy in various planes is referred to as multiplanar reformation (MPR). 2D in nature. can create coronal, sagittal, and paraxial images from a stack of contiguous transverse axial scans If voxels are isotropic, the reformatted image is virtually identical in quality to the original axial images. If voxels are anisotropic then the image quality can be improved by using overlapping images. MPR in Coronal Plane MPR in Sagittal Plane MPR in Oblique Plane Curved Planar Reformation  Also called curved multi- planar reformation or cMPR  Allows Vessel- Tracking  CPR allows the images to be created along the centerline of tubular organs (eg vessels, CBD, ureters etc.) in a single image. Intensity Projection Rendering The term sliding thin slabs is known for this technique. The algorithms for sliding thin slabs (STS) include Average intensity projection (AIP), Maximum intensity projection (MIP), and Minimum intensity projection (MinIP) Average Intensity Projection AIP images represents average attenuation values within the voxel With AIP images is achieved, an appearance similar to traditional axial low contrast resolution. This may be useful for the characterization of the internal structures of a solid body or walls of hollow structures such as blood vessels or intestine. Maximum Intensity Projection ( MIP )  MIP images show only the highest attenuation values within the voxels and especially useful to create angiographic and urography images  Enables the detection of highly intense structure.  MIP displays the higher CT number in a volume of interest when projected into a new plane.  Two significant limitations of MIP are:  The presence of other high-attenuating voxels may obscure evaluation of vasculature  3D relationships among the structures in the display are not visible. Maximum Intensity Projection ( MIP ) Maximum Intensity Projection ( MIP )  Raw data (stacks of 2D axial slices); each slice contains pixel intensity values corresponding to density of structure of body. For each ray, the ray tracing algorithm identifies the maximum intensities value. The result is a single 2D image where each pixel’s value is highest intensity encountered along the corresponding ray. Maximum Intensity Projection  MIP is mainly used to show the vessels with contrast material in CT angiography to provide clear view of lesions. Minimum Intensity Projection ( MinIP )  MinIP images shows only the lowest attenuation values.  It enables detection of low- density structures.  2D image of a selected volume is generated where each pixel is represented by displaying the lowest attenuation value in each voxel.  It is used to represent structures containing air such as tracheobronchial tree. Use of Minimum Intensity Projection It is mainly used to diagnose lung diseases. For example:- traction of bronchiectasis and emphysema. MinIP vs AIP vs MIP MinIP AIP MIP Thank You