Lect 2 CT Technique PDF

# Lect 2: CT Introduction & Historical Outline ## What is Conventional Tomography? Tomo = layer or section or cut or slice Conventional Tomography: A special type of imaging used to obtain a diagnostic image of a specific layer of tissue that is superimposed by other tissues. This is achieved by...

# Lect 2: CT Introduction & Historical Outline ## What is Conventional Tomography? Tomo = layer or section or cut or slice Conventional Tomography: A special type of imaging used to obtain a diagnostic image of a specific layer of tissue that is superimposed by other tissues. This is achieved by a moving source and image receptor assembly to avoid superimposition. ## What are the disadvantages of Conventional X-rays? 1. Travels in straight lines. X-ray tissue interaction just absorbed and scattered. 2. X-ray depicts only Two Dimension, we cannot get Three Dimensions. This results in Tissue overlying & superimposition on the image & Blurring. All reduce the visibility to find details & reduce Image Quality. 3. Not differentiate between two tissues that have the same density. 4. Not accuracy detection of the soft tissue lesions. 5. X-ray Photons depict a specific part, which interacts with X-ray photons, i.e. not slices. ## Define the Terminology of Computed Tomography. * Tomography: Tomo, meaning cut, section, slice or layer. * Computed: Computerized Methods & Processors used to obtain data and transform into “Images”. * Computed Tomography: Imaging Modality based on X-ray Tissue Interaction to get Multiple Cross-section (Axial or Transverse) slices. * Computed Axial Tomography (CAT scan) avoiding tissues Superimposition. ## What is a slice? Slice = Cross sectional part of the body scanned to produce a CT image. Slice has 3 Dimension therefore is volume (length, width, and depth). Depth of the slice = Slice thickness & determined by the width of the X-ray beam. ## What is the Idea of CT (How Axial cuts Available)? As x-ray travels in straight lines conventional Tomography Scans has (Low Contrast Resolution & Long Time). Solution: X-ray tube rotated by 360 degrees with patient motion simultaneously to get axial cuts. ## Define the following Specific Terminology. * Photon = Unite of all electromagnetic waves. * Ray: Imaginary line between Tube & Detector. * Beam: A group of rays with a different angle. ## Beam Geometry: Parallel, Fan &Cone as in (Dental scanning) ## Projections: A series of X-ray beams pass through the patient at the same orientation with different angles is called Projections or Transmissions. In other words, Projections mean information derived from the transmitted x-ray photons when an object is scanned from different angles and directions. ## Write Hints about Scientists' efforts in CT imaging. * **1917 Radon:** If we have an Infinite Number of Projections through the body, from different angles and directions. So. reconstructing cross-sectional images available. In other words, if the human body scans the Internal Structures can be reconstructed from different projections. * **1963 Cormack:** Mathematical Equations for Projections & Photons Transmission Measurements & calculating the Absorption Coefficient for each organ. Absorption Coefficient = Amount of x-ray photons absorbed or attenuated by specific organ or tissue. * **1969 Hounsfield:** Design the prototype. 1971 First CT head scanner. 1973 Clinically used. 1975 Whole Body Scanning. 1979 Nobel Prize Cormark & Hounsfield. ## What is the Main Principle of Computed Tomography? CT Cross Sectional Images created from Projections information of X-ray beams passed through the body from different Points &Angles (X-ray Tube Rotation). ## Why are Axial Sections Needed? Axial sections needed to avoid the Superimposition, to give Three Dimensions slices for more accurate lesions detection, to differentiate small differences in densities of anatomic structures & Increasing Image Quality. ## What are Common CT INDICATIONS? * Detection of the following * Blood Clotting (Infarctions) * Bone Fractures * Brain Tumors * Sinus Infections * Internal Organs Ruptures * Cardiac Imaging (Coronaries) * Biopsy ## Radiation Therapy Planning: * Monitoring Treatment. ## Summary of the Lecture 2: Since the first CT scanner was developed in 1972 by Sir Godfrey Hounsfield, the modality has become established as an essential radiological technique applicable in a wide range of clinical situations. CT uses X-rays to generate cross-sectional, three-dimensional images of the body. Images are acquired by rapid rotation of the X-ray tube 360° around the patient. The incidence of X-ray photons on the patient is called projections. Typically, 1000 projections are measured during each 360° rotation. The transmitted radiation is then measured by a ring of sensitive radiation detectors located on the gantry around the patient. The final image is generated from these measurements utilizing the basic principle that the body's internal structure can be reconstructed from multiple X-ray projections. Early CT scanners acquired images a single slice at a time (sequential scanning). However, during the 1980s, significant technological advancements heralded the development of slip ring technology, which enabled the X-ray tube to rotate continuously in one direction around the patient. Allan MacLeod Cormack, In the early 1960s Cormack showed how details of a flat section of soft tissues could be calculated from measurements of the attenuation of X-rays passing through it from many different angles. He thus provided the mathematical technique for the CAT scan, in which an X-ray source and electronic detectors are rotated about the body, and the resulting data is analyzed by a computer to produce a sharp map of the tissues within a cross-section of the body. ## What is CT? CT is a method for acquiring and reconstructing an image of a thin cross-section of an object. It is based on measurements of X-ray attenuation through the section using many different projections. This is achieved by rotating both the x-ray tube and detectors around the patient. ## How is CT different from conventional radiographs? CT differs from conventional radiography in two significant ways: 1. CT forms a cross-sectional image, eliminating the superimposition of structures that occurs in plain film imaging because of the compression of three-dimensional (3D) body structures onto the two-dimensional recording system. 2. The sensitivity of CT to subtle differences in x-ray attenuation is at least a factor of 10 higher than normally achieved by film screen recording systems because of the virtual elimination of scatter. ## What are the basic principles of CT? Fundamentally a CT scanner makes many measurements of attenuation through the plane of a finite thickness cross section of the body. The system uses these data to reconstruct a digital image of the cross-section.