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computed Tomography

introduction, generation, spiral CT, artefacts.

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computed Tomography

  1. 1. Presented by Moderator Sujan Poudel Mr. Mukesh Kr. Jha B.Sc. MIT 2015 Demonstrator Department of Radio- diagnosis and imaging B.P Koirala Institute of Health Sciences Dharan Computed Tomography
  2. 2. Introduction  Word tomography literally means ‘tomo’ meaning to cut, section , ‘graphy’ meaning drawing. In case of CT a sophisticated computerized method is used to obtain data and transfer them into cross sectional slice of human body.  Earlier scanner produced only axial cuts so it was called computerized axial tomography (CAT) scan.  CT combines rotating X radiation and radiation sensitive detectors coupled with a computer to create cross sectional image of any part of the body.
  3. 3. ADVANTAGE OF COMPUTED TOMOGRAPHY OVER CONVENTIONAL RADIOGRAPHY  To overcome superimposition of structures.  Ability to distinguish between two tissue with similar density. i.e. it have higher contrast resolution.  For an object to be visible on image produced from screen film radiography it must have a least 5% difference in contrast from its background material whereas for CT it is 0.5%  In CT 1% contrast difference correspond to difference of 10 HU.
  4. 4. PRINCIPLE OF TOMOGRAPHY  The internal structure of the object can be reconstructed from multiple projections of the object.  A source of ionizing radiation is transmitted through an object to recreate an image of the object based on its x-ray absorption.  I=IOe-μx  Attenuation depend on Atomic number and density of material so linear attenuation coefficient will be higher for bone than soft tissue.  Difference in linear attenuation are responsible for contrast of CT image.  To produce image, attenuation is expressed in HU,water is given HU number zero and attenuation less than water is given negative number and vice versa and different shade of grey is assigned for each HU. There are more than 2000 shade of grey.
  5. 5. Terminology used in CT  SLICE  MATRIX  PIXEL  VOXEL  CT NUMBER  WINDOW WIDTH  WINDOW LEVEL
  6. 6. Generation of CT  Generation is the order in which CT scanner design has been introduced , and each has a number associated with it.  Classification based on arrangement of components and mechanical motion required to collect data.  Higher generation number doesn’t necessarily indicate higher performance system.
  7. 7. FIRST GENERATION CT  x ray tube was mounted in a fixed relationship with two detector (NaI)  80 parallel rays were acquired as assembly translated across FOV then system rotated 1 degree and x ray tube and detector translated in another direction. This was continued until 180 degree of data were acquired. Large number of projection are generated and image is reconstructed.  It uses pencil beam.  Scan required 4 min and reconstruction ran overnight.
  8. 8. FIRST GENERATION CT
  9. 9. SECOND GENERATION OF CT SCAN  Narrow fan beam  Linear detector array(5 to30)  Translate-Rotate movements of Tube- Detector combination • Fewer linear movements are needed as there are more detectors to gather the data.  Between linear movements, the gantry rotated 30o  Scan time~30secs(advantage over first generation.
  10. 10. THIRD GENERATION OF CT SCAN  Rotate(tube)Rotate(detectors) Motion.  Pulsed wide fan beam. Generally 60 degree.  Arc of detectors(600-900)  Detectors are perfectly aligned with the X-Ray tube  Both Xenon and scintillation crystal detectors can be used  Scan time< 5secs  Disadvantage: Due to rigid alignment of x ray source and detector array when CT detectors are not properly calibrated with respect to each other Ring artefacts arise.
  11. 11. FOURTH GENERATION OF CT SCAN  Complete circular array of about 1200 to 4800 stationary detectors  Single x-ray tube rotates with in the circular array of detectors due to which alignment of x ray tube changes with respect to detector array and CT data are computed from so called detector fan.  Wide fan beam to cover the entire patient  Scan time of newer scanners is about ½ s or, <2s.  Each detectors serves as its own reference detection, changes in detector sensitivity are forced out in computation of projection data sets thus eliminating ring artefacts.  Disadvantage: High cost.
  12. 12. THIRD AND FOURTH GENERATION CT
  13. 13. FIFTH GENERATION OF CT SCAN (ebct)  Stationary/stationary  Developed specifically for cardiac tomographic imaging  No conventional x-ray tube; large arc of tungsten encircles patient and lies directly opposite to the detector ring  Electron beam steered around the patient to strike the annular tungsten target  Capable of 50-msec scan times; can produce fast frame- rate CT movies of the beating heart.
  14. 14. FIFTH Generation CT
  15. 15. SIXTH Generation CT  Sixth generation are dual energy source ( two x ray tube) that have two sets of detector that are offset by 90 degree . A typical approach would be to operate one tube at 80 kV and the other tube at 140 kV.  The key advantage of dual-energy and spectral CT techniques is that they can be used to probe the attenuation arising from density and atomic number separately by making two different measurements of the same sample, object, or body part.  The dual source CT scanners provide improved temporal resolution needed for imaging moving such as heart  The most promising applications for dual-energy and spectral CT capabilities are virtual noncontrast (VNC) exams, iodine quantification, and calcium quantification
  16. 16. COMPONENT OF HELICAL CT  Gantry : mechanical support for the X- ray tube, tube collimator and data measurement system (DMS) has to be designed so as to withstand the high gravitational forces associated with fast gantry rotation (~17 g for 0.42 s rotation time, ~33 g for 0.33-s rotation time).  X-ray source and generator : should provide a peak power of 60–100 kW, usually at various,user-selectable voltages, e.g., 80 kV, 100 kV, 120 kV and 140 kV.  Heat storage capacity: typically of 5 to 9 MHU, realized by thick graphite layers attached to the backside of the anode plate detector and detector electronics,
  17. 17. Continue….  Data transmission systems (slip rings) : contactless transmission technology is generally used for data transfer, which is either laser transmission or electro-magnetic trans- mission with a coupling between a rotating trans-mission ring antenna and a stationary receiving antenna .  computer system for image reconstruction and manipulation : divergence of the fan beam along the longitudinal axis (z-axis),creates a cone-beam shape due to source-detector geometry; there is a significant difference between the distance from the x-ray focal spot to the center detector row and the distance from the focal spot to the outer detector rows  The most commonly used reconstruction algorithm in MDCT systems is a modification of the filtered back-projection method called the Feldkamp
  18. 18. Requirement for helical CT  X ray gantries with slip ring : for continuous gantry rotation.  More efficient tube cooling  Higher X- ray output  Smoother table movement  More efficient detectors
  19. 19. Single Row Detector System  Contain many detector elements aligned in single row.  Detector element is quite high in z direction (approx 15mm) .  Opening and closing of collimator controls slice thickness.  Width of detectors in a single detector array place upper limit on slice thickness. Drawbacks  insufficient volume coverage within one breath-hold time of the patient or missing spatial resolution in the z-axis due to wide collimation.  The ideal isotropic resolution, i.e., of equal resolution in all three spatial axes, can only be achieved for very limited scan ranges
  20. 20. Multi detector row system  Uses many detector elements in multiple parallel rows.  Single detector can produce multiple slices.  Slice thickness is determined by combination of x ray beam width and detector configuration.  Each detector element consists of a radiation- sensitive solid-state material (such as cadmium tungstate, ceramics, CSI), which converts the absorbed X-rays into visble light The light is then detected by a Si photodiode.  The resulting electrical current is amplified and converted into a digital signal.
  21. 21. DETECTORS  To select different slice widths, scanners combine several detector rows electronically to a smaller number of slices according to the selected beam collimation and the desired slice width  All recently introduced 16-slice CT systems employ adaptive array detectors. The SOMATOM Sensation
  22. 22. DETECTORS  16 (Siemens, Forchheim, Germany) as a representative example uses 24 detector rows. The16 central rows define 0.75-mm collimated slice width at isocenter; the 4 outer rows on both sides define 1.5-mm collimated slice width. The total coverage in the transverse direction is 24 mm at isocenter. By appropriate combination of the signals of the individual detector rows, either 12 or 16 slices with 0.75- mm or 1.5-mm collimated slice width can be acquired simultane-ously.
  23. 23. PITCH IN SDCT  Pitch is defined as travel distance of CT scan table per 360 rotation of x ray tube divided by x ray beam collimation width  Pitch is inversely proportional to dose  Pitch les than 1 is overlapping pitch and pitch greater than 1 is extended pitch.  Increasing pitch will result more anatomic coverage and reduce radiation dose to patient.
  24. 24. Contd..
  25. 25. PITCH IN MDCT  In MDCT slice thickness is not controlled by beam width but by detector configuration.  So for calculation of pitch in MDCT beam width should be determined by multiplying the number of slices by slice thickness.  For Example : In 4 slice scanner at 1.25 slice thickness and table feed 0f 6mm per rotation then  Pitch =  Beam width = 4 X 1.25 = 5 putting value in formula we get  Pitch = 6/5 = 1.2
  26. 26. Advantage of helical CT over AXIAL CT  The ability to minimize motion artifacts owing to faster/shorter acquisitions.  Decreased incidence of misregistration between consecutive axial slices  Reduced patient dose  Improved spatial resolution in the z-axis  Enhanced multiplanar or 3D renderings
  27. 27. Advantage of Axial CT OVER HELICAL CT  Highest image quality than helical methods because of axial nature i.e. slices are perpendicular not slanted and patient table remains stationery during data acquisition.
  28. 28. Multiplaner reconstructions  Surface renderings : also known as shaded surface display (SSD)  Image are created by comparing the intensity of each voxel in data set to some predetermined threshold CT value  Thus software will include or exclude the voxel depending on whether its CT number is above or below the threshold and uses this information to create a surface of an object USES  Tubular structures like airways, colon , and bone surface etc
  29. 29. VOLUME RENDERINGS  It is 3D semitransparent representation of the imaged structure in which all voxels contribute to the image.  VR images display multiple tissue and show their relationship with one another  It sums the contributions of each voxel along line and each voxel is assigned an opacity value based on its HU.  Opacity value determine the degree to which each voxel will contribute to final image.  Pixels in final image can be assigned a colour, brightness and degree of opacity. Generally soft tissue have high transparency bone strong opaqueness due to difference of HU.  Virtual endoscopy, virtual bronchoscopy and virtual colonoscopy is form of VR.
  30. 30. Curve planer reformations  Allows image to be created along centerline of tubular organs like CBD, ureters.  Curved planar reformations were obtained using a cursor to draw a curved line along a special anatomic structure on a stack of axial, sagittal, coronal section at workstations.
  31. 31. Maximum intensity projection  Voxels with higher HU number are displayed  Generally used in angiography to see blood vessel. DISADVANTAGE  Depth information is minimal.  Increased in mean background intensity.
  32. 32. Artefacts in MDCT  Spiral Pitch Artifacts : Spiral artifacts exhibit stepping in reformatted images, the steps appear as a spiral groove. It have a unique appearance in axial scans; a star pattern is seen off of sharp edges, where the number of spokes in the star is directly related to the number of multislice detector row this is because each row contributes only a portion of its projection
  33. 33. ARTEFACTS continue…  Windmill artefacts : black/white patterns that spin off of features with high longitudinal gradients. The number of black/white pairs matches the number of slices (detector rows) in the multi-slice detector.
  34. 34. Zebra artefacts  Faint stripes may be apparent in multiplanar and three-dimensional reformatted images from helical data because the helical interpolation process gives rise to a degree of noise inhomogeneity along the z axis

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