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• Computed Tomography (CT)


  An imaging procedure that uses special x-ray equipment to create detailed pictures, or scans, of areas inside the body. Also called computerized tomography, or computerized axial tomography (CAT).

  
• Tomography


  Comes from the Greek words tomos (a cut, a slice, or a section) and graphein (to write or record). Computed tomography (CT) is noninvasive and produces cross-sectional images of the body.

  
• Each cross-sectional image represents a "slice" of the person being imaged, like the slices in a loaf of bread. These cross-sectional images are used for a variety of diagnostic and therapeutic purposes.
• Limitations of Film-Based Radiography

  • The superimposition of all structures on the film makes it difficult and sometimes impossible to distinguish a particular detail
  • Radiography is a qualitative rather than quantitative process, making it difficult to distinguish between a homogeneous object of nonuniform thickness and a heterogeneous object of uniform thickness
• Limitations of Conventional Tomography


  • Persistent image blurring that cannot be completely removed
  • Degradation of image contrast because of the presence of scattered radiation created by the open geometry of the x-ray beam
  • Inability to adequately demonstrate slight differences in subject contrast, which are characteristic of soft tissue

  
• Radiographic film is not sensitive enough to resolve small differences in tissue contrast, as typical film-screen combinations can only discriminate x-ray intensity differences of 5% to 10%.
• The limitations of radiography and tomography result in the inability of film to image very small differences in tissue contrast. Contrast cannot be adjusted after it has been recorded on the film.
• Digital imaging modalities such as CT

  Can alter the contrast to suit the needs of the human observer (radiologists and technologists) by use of various digital image post-processing techniques.
• CT scans can be performed on every region of the body for a variety of reasons (e.g., diagnostic, treatment planning, interventional, or screening). The cross-sectional images generated during a CT scan can be reformatted in multiple planes, and can even generate three-dimensional images which can be viewed on a computer monitor, printed on film or transferred to electronic media.

  
• Although most common in medicine, CT is also used in other fields, such as nondestructive materials testing, to study biological and paleontological specimens.
• CT differs from the conventional radiography in two significant ways
• CT scan


  Computed tomography scan, a medical imaging technique that uses X-rays to generate cross-sectional images of the body

  
• CT scan


  • Generates cross-sectional images of various internal structures (organs, blood vessels, bones, soft tissue etc.)
  • Can be used for diagnostic purposes, guidance for treatment, and detection/monitoring of conditions

  
• CT scan technique


  1. Digital geometry processing used to generate 3D image from 2D X-ray images

  2. Patient lies on flat table that moves into gantry

  3. X-ray tube rotates around patient, X-rays pass through to detectors

  4. Computer system acquires measurements and generates viewable image

  
• One cross-sectional slice of the body is obtained for each complete rotation of the scanner
• Within one rotation, about 1,000 profiles (2D images) are acquired
• A 2D image (slice) is formed when the full set of profiles from each rotation are compiled and analyzed by a computer
• Generations of CT scanners

  • First-generation
  • Second-generation
  • Third-generation
  • Fourth-generation
  • Fifth-generation (electron beam CT - EBCT)

  
• First-generation CT scanners


  • Characterized by single X-ray source (pencil beam or parallel-beam geometry)
  • Multiple measurements of X-ray transmission obtained using single highly collimated pencil beam and detector

  
• First-generation CT scan process

  1. Source and detector translate simultaneously in scan plane

  2. Beam translated linearly across patient to obtain projection profile

  3. Process repeated for given number of angular rotations to obtain multiple projection profiles

  
• Major adjustments in CT scanner technology include: tube orientation and beam shape, number of detectors, and detector arrangement

  
• Multiple x-ray transmission measurements

  1. Using a single highly collimated x-ray pencil beam

  2. Detector directing across the patient isocenter

  
• Scanning process


  1. Source and detector translate simultaneously in a scan plane

  2. Beam is translated in a linear motion across the patient

  3. To obtain a projection profile

  
• Scanning process repeated


  1. For a given number of angular rotations

  2. Approximately 1 degree

  3. Another projection profile is obtained

  4. Until the source and detector have been rotated by 180 degrees
• Advantages of this design

  • Simplicity
  • Good view-to-view detector matching
  • Flexibility in the choice of scan parameters (such as resolution and contrast)
  • Highly collimated beam provides excellent rejection of radiation scattered in the Patient
• This scanner was limited because
• Limitations of the scanner

  • Only head scans could be performed
  • Generates a lot of heat, therefore, require an elaborate cooling system
  • Scan time was very slow. About 1 minute per slice therefore the duration of scan (average): 25-30 mins
• Second-generation CT systems use the same translate/rotate scan geometry as the first Generation

  
• Difference in second-generation


  • Pencil beam is replaced by a fan beam
  • Single detector is replaced by multiple detectors (5-30)
  • A series of views can be acquired During each translation
  • Leads to correspondingly shorter scanning times, about 20 Seconds per slice therefore duration of scan (average): less than 90 sec

  
• Objects of Wide range sizes can be easily scanned with the second-generation scanners
• Reconstruction algorithms are slightly more complicated than those for first-generation Algorithms because they must handle fan-beam projection data
• Third-generation scanning process


  1. A fan beam of x-rays is rotated 360 degrees around the isocenter

  2. No translation motion is used

  3. Fan beam must be wide enough to completely contain the patient

  4. A curved detector array consisting of several hundred independent detectors (500-1000) is mechanically coupled to the x-ray source, and both rotate together

  
• Rotate-only motions in third-generation

  Acquire projection data for a single image in as little as 1 s
• Third-generation systems are typically faster than second-generation systems
• The detectors in third-generation have incorporated bigger amount of sensors in the detector array
• Fourth-generation scanning process


  1. X-ray source and fan beam rotate about the isocenter

  2. Detector array remains stationary

  3. Detector array consists of 600 to 4800 independent detectors in a circle that completely surrounds the patient

  
• Scan times are less to those of third-generation scanners (~ 2sec.)
• The number of views is equal to the number of detectors
• Detector geometries used in fourth-generation systems

  • Rotating x-ray source inside a fixed detector array
  • Rotating x-ray source outside a nutating detector array

  
• Both third- and fourth-generation systems are commercially available with advanced configurations