Ct Basics

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This is a presentation that i did as part of my neurology residency programme.

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Ct Basics

  1. 1. Computerised Tomography: Basics
  2. 2. WC Roentgen Allan Cormack History
  3. 3. Sir Godfrey Newbold Hounsfield. 1919-2004. CT for brain: 1972-73. CT for whole body: 1975. Worked on NMR. EMIDEC 1100. Magnetic films for information storage. Nobel prize in 1979. Knighted in 1981.
  4. 4. History <ul><li>Electrical and Musical industries (EMI). </li></ul><ul><li>No formal university education. </li></ul><ul><li>Produced improved versions with reduced radiation exposure, better resolution. </li></ul><ul><li>Others had considered and abandoned the idea because of the phenomenal mathematics involved. </li></ul><ul><li>Allan Cormack worked independently. </li></ul><ul><li>Both had no training in biology or medicine. </li></ul>
  5. 5. Initial sketch of the CT machine by GN Hounsefield
  6. 6. Experimental CT machine in the lab of GN Hounsefield
  7. 7. GN Hounsfield with his initial working prototype CT machine
  8. 8. <ul><li>Original axial CT image from the dedicated Siretom CT scanner circa 1975. This image is a coarse 128 x 128 matrix </li></ul>
  9. 9. Structure of a standard CT machine Generator Generator Array of Array of detectors Patient table a Aperture Gantry X-ray tube Scan frame
  10. 10. CT apparatus
  11. 11. Components for data acquisition <ul><li>Scan frame: rotating frames, slip rings </li></ul><ul><li>X-ray generators: 80- 140 kV </li></ul><ul><li>located on rotating scan frames within CT gantry </li></ul><ul><li>X ray tubes: </li></ul><ul><li>Fixed anode, oil cooled X ray tubes </li></ul><ul><li>Rotating anodes with unique cooling methods </li></ul><ul><li>X ray beam filtration systems </li></ul><ul><li>Detector system </li></ul>
  12. 14. Principles <ul><li>Disadvantages of routine radiography: </li></ul><ul><li>Qualitative. </li></ul><ul><li>Cannot differentiate subtle differences in object contrast. </li></ul><ul><li>Superimposition of structures on film. </li></ul><ul><li>CT abolished: </li></ul><ul><li>Pneumoencephalogram </li></ul><ul><li>Most angiograms </li></ul><ul><li>Most head skiagrams </li></ul>
  13. 15. Principles <ul><li>Source of ionising radiation. </li></ul><ul><li>Object of interest. </li></ul><ul><li>Recording apparatus. </li></ul><ul><li>I=Io( e(-µx) ) </li></ul><ul><li>I= transmitted intensity of X-ray </li></ul><ul><li>Io= incident intensity of X-ray beam on the surface </li></ul><ul><li>x= thickness of the object </li></ul><ul><li>e= Euler’s constant (2.718) </li></ul><ul><li>µ= linear attenuation coefficient </li></ul>
  14. 16. Io= incident ray I=transmitted ray µ=100 µ=50 Tissue X-ray film 6x 4x 2x 6x X-ray source µ=100
  15. 17. Conventional radiography Vs CT µ1 µ2 µ3 µ2 µ2 µ2 Io I= x I= x Io
  16. 18. X-rays <ul><li>Central 4 blocks represent an internal object. </li></ul><ul><li>Measured attenuation is proportional to the number of blocks in each row/ column. </li></ul>4 2 4 4 4 4 2 2 2 Voxel
  17. 19. 8 4 4 4 4 4 4 8 2 2 6 6 6 6 6 6 6 6 4 4 4 4 8 8 8 8 Attenuation measurements are added Numerical representation of the object (matrix) 4 4 2 2 2 2
  18. 20. A. Applying grey scale B. Perfect representation C. Imperfect representation Pixel 8 6 4 6 8 4 6
  19. 21. Technical terms <ul><li>Voxel: Unit attenuating volume of material </li></ul><ul><li>Ray projection: Single attenuation measurements. </li></ul><ul><li>Matrix: numerical representation of an object. </li></ul><ul><li>Pixel: unit picture element. </li></ul><ul><li>Window: Choosing the number of grey shades for display. </li></ul><ul><li>Window level: The number at which the window setting is centered. </li></ul>
  20. 22. 3 step process of CT imaging
  21. 23. Data Acquisition
  22. 24. A ray is the pathway of a portion of the x-ray beam from one specific focal-spot position to a specific detector position. A view is made-up of many individual rays.
  23. 25. <ul><li>The projection of the fan-shaped x-ray beam from one specific x-ray tube focal spot position produces one view . </li></ul><ul><li>Many views projected from around the patient's body are required to acquire the necessary data to reconstruct an image. </li></ul>A Single view Single ray Single detector X-ray focal spot A single detector A single ray X-ray focal spot A single view
  24. 26. As the x-ray beam is scanned around the body, forming many views, the data recorded by the detectors are stored in computer memory for later image reconstruction.
  25. 27. <ul><li>A complete scan is formed by rotating the x-ray tube completely around the body and projecting many views. </li></ul><ul><li>Each view produces one &quot;profile&quot; or line of data. </li></ul><ul><li>The complete scan produces a complete data set. </li></ul><ul><li>In principle, one scan produces data for one slice image. </li></ul><ul><li>However, with spiral/helical scanning this is not true. </li></ul>Multiple views
  26. 29. 2 types of motions of the x-ray beam relative to the patient's body. 1. Movement of the beam around the body. 2. Movement of the beam along the length of the body.  This is achieved by moving the body through the beam as it is rotating. 2 types of beam motion
  27. 30. <ul><li>Translate/rotate principle (Scan and step): 1st scanning mode. </li></ul><ul><li>One complete scan around the immobile body; table moved to next slice position. </li></ul><ul><li>The principle characteristic (and limitation) of this mode is that the data set is fixed to a specific slice of tissue ie. the slice thickness, position, and orientation is &quot;locked in&quot;. </li></ul>Scan and Step
  28. 31. <ul><li>Spiral scanning: </li></ul><ul><li>The patient's body is moved continuously as the x-ray beam is scanned around the body. </li></ul><ul><li>Controlled by operator selected value of the pitch factor . </li></ul><ul><li>If the body is moved 10 mm during one rotation, and the beam width is 5 mm, the pitch will have a value of 2. </li></ul>
  29. 32. Spiral CT: X ray tube rotation and table movement 3 rd gen Slip ring technology
  30. 33. Slip ring technology <ul><li>Conventional CT systems use a mechanism to deliver electrical power to the X-ray tube that allows it to rotate through perhaps 400-600 degrees before it has to stop. </li></ul><ul><li>Slip-ring allows the continuous rotation of the X-ray tube (and the detector assembly if appropriate). </li></ul><ul><li>Can attain sub-second image acquisition rates, zero interscan delay, and are compatible with spiral CT scanning. </li></ul>
  31. 36. <ul><li>When the pitch is increased , the x-ray beam appears to move faster along the patient's body, and X-ray beam will be spread over more of the body. </li></ul><ul><li>This has three major effects: </li></ul><ul><li>Scan time will be less to cover a specific body volume. </li></ul><ul><li>The radiation is less concentrated so dose is reduced. </li></ul><ul><li>Image quality will be reduced. </li></ul>
  32. 37. <ul><li>A major advantage of spiral/helical scanning it that it produces a continuous data set extending over some volume of the patient's body. </li></ul><ul><li>This volume data set can be sliced many ways later during the image reconstruction phase. </li></ul>
  33. 38. Spiral CT: Advantages <ul><li>Fast scan times and large volume of data collected. </li></ul><ul><li>Minimizes motion artifacts . </li></ul><ul><li>Less mis-registration between consecutive slices. </li></ul><ul><li>Reduced patient dose (upto 33%). </li></ul><ul><li>Improved spatial resolution in the Z axis. </li></ul><ul><li>Enhanced multiplanar (MPR) or 3D renderings. </li></ul><ul><li>Image can be reconstructed at any point along the effective path of the X-ray tube. </li></ul><ul><li>Improved temporal resolution- CT angiography. </li></ul>
  34. 39. A major advantage of spiral scanning is that the thickness, position, and orientation of image slices can be adjusted during the reconstruction phase. Images of overlapping slices can be created. The reconstruction can be repeated to produce images with different spatial characteristics.
  35. 40. The radiation detectors are very small elements that are arranged in rows that span and intercept one view. A specific CT machine can be designed to have either a single row of detectors or multiple rows. There are advantages in having multiple rows. MDCT
  36. 41. A body section can be scanned faster with a multiple row detector system with multiple fan beams scanning simultaneously. Crucial for covering a large body section with thin beams for producing thin, high-detail slice images or 3-D volume images. MDCT
  37. 42. Multislice/ Multidetector Spiral CT <ul><li>3 rd generation geometry. </li></ul><ul><li>Better z axis resolution. </li></ul><ul><li>Larger coverage and faster scan times. </li></ul><ul><li>Less radiation dose. </li></ul><ul><li>Less motion artifacts. </li></ul><ul><li>Better utilization of contrast. </li></ul><ul><li>8, 16, 64, 256 slice CT machines are available. </li></ul>
  38. 43. Reconstruction
  39. 44. <ul><li>&quot;Filtered&quot; refers to the use of the digital image processing algorithms that are used to improve image quality or change certain image quality characteristics, such as detail and noise. </li></ul><ul><li>&quot;Back projection&quot; is the actual process used to produce or &quot;reconstruct&quot; the image.  </li></ul>
  40. 45. <ul><li>Scan view through a section containing 2 objects.  The data produced is not a complete image, but a profile of the x-ray attenuation by the objects. </li></ul><ul><li>This profile is used to draw an image by &quot;back projecting&quot; the profile onto the image surface. </li></ul><ul><li>There is only enough information in the profile to allow drawing the image as streaks like shadows across the image area. </li></ul>
  41. 46. <ul><li>X-ray beam is rotated by 90 deg for another view. </li></ul><ul><li>On back projection onto the image area the beginnings of an image of the two objects are seen. </li></ul><ul><li>Several hundred views are used to produce clinical CT images. </li></ul>X-ray beam
  42. 47. <ul><li>X-ray linear attenuation coefficient for each individual voxel is first calculated by the reconstruction process and then used to calculate the CT number values (Hounsfield Units). </li></ul><ul><li>Water is the reference material for CT numbers: value of zero . Attenuation (density) > water: positive CT numbers.  </li></ul><ul><li>Density < water : negative CT numbers. </li></ul><ul><li>X-ray attenuation and CT number depends on the density . </li></ul>Z= atomic number
  43. 48. <ul><li>A volume data set can be used to reconstruct 3-D images. </li></ul><ul><li>This is achieved by scanning with thin beams and relatively low pitch values . </li></ul>
  44. 49. Display
  45. 50. <ul><li>A matrix of pixels with each pixel having a CT number, is converted into a visible image of grey shades. </li></ul><ul><li>Depends on window width and level , zoom control, other factors </li></ul><ul><li>The area of the digital image that is actually displayed is controlled by the zoom control. </li></ul>
  46. 51. Window level and width <ul><li>Window width: range of CT numbers displayed with shades of gray, ranging from black to white. </li></ul><ul><li>CT numbers > window: white, and below: black. </li></ul><ul><li>Width control adjusts the range of CT numbers displayed with contrast. Reducing window width increases image contrast. </li></ul><ul><li>Window level: describes the center of the scale. </li></ul><ul><li>CT has very high contrast sensitivity as a window can be set to enhance very small tissue density differences. </li></ul>
  47. 52. <ul><li>Contrast appreciation of human eye: 10% </li></ul><ul><li>CT machine can detect difference of <1% </li></ul><ul><li>Small density resolution difference measured by the CT scanner must be exaggerated to permit viewing. </li></ul>
  48. 53. CT number <ul><li>Hounsfield units of some tissues/ material. </li></ul><ul><li>Air: -1000 </li></ul><ul><li>Water: 0 </li></ul><ul><li>Bone: 1000 </li></ul><ul><li>Blood: 40 to 70 </li></ul><ul><li>CSF: 15 </li></ul><ul><li>Calcification: 140 to 200 </li></ul><ul><li>Fat: -50 to -100 </li></ul><ul><li>Grey matter: 32 to 45 </li></ul><ul><li>White matter: 20 to 30 </li></ul>
  49. 54. Image quality <ul><li>Spatial resolution: ability to distinguish two small high contrast objects located very close to each other under noise free conditions. </li></ul><ul><li>Contrast/ density resolution: ability to differentiate attenuation coefficients of adjacent areas of tissue. </li></ul><ul><li>Noise: variation in the computation of pixel value. </li></ul><ul><li>Temporal resolution: ability to capture objects that change shape or position over time. </li></ul><ul><li>Dose: increasing the dose improves image quality. </li></ul><ul><li>Pitch: Decreasing pitch improves image quality. </li></ul>
  50. 55. Improving picture quality: 1. Reducing pitch. 2. Small detector size and therefore small ray size. 3. Small voxel size.
  51. 56. Small voxel size increases the noise because fewer protons are absorbed or captured in each voxel. Noise can be decreased by increasing the dose to the patient. Selected filter algorithms can decrease or increase the noise.
  52. 57. Decreasing Slice Thickness (To Improve Detail)- small voxels- more noise- will require increased dose to reduce noise level. Hence, thin slices should be requested only when necessary. Effect of Slice thickness on Noise and Dose Voxel + Voxel -
  53. 58. Increasing pitch: 1. Faster scanning 2. Reduced dose (the radiation is less concentrated) 3. Limiting factor is reduced image detail.
  54. 59. Image artifacts <ul><li>Streak and ring artifacts </li></ul><ul><li>Metal and bone artifacts </li></ul><ul><li>Beam hardening artifacts </li></ul><ul><li>Partial volume artifacts </li></ul><ul><li>Motion artifacts </li></ul><ul><li>Stair stepping artifacts </li></ul><ul><li>Spiral pitch artifacts </li></ul><ul><li>Cone beam artifacts </li></ul>
  55. 60. Metal artifact
  56. 61. Ring artifacts
  57. 62. Eg of artifacts
  58. 63. Artifacts <ul><li>Beam hardening artifact: </li></ul><ul><li>Corrected by processing during the reconstruction process. </li></ul><ul><li>Partial volume artifact: </li></ul><ul><li>Occurs when a voxel contains two very different materials, like bone and soft tissue. </li></ul><ul><li>The resulting CT number will be somewhere between the correct values for the different materials.  </li></ul><ul><li>Depending on the window setting, a structure such as bone, can appear thinner or thicker than it's actual dimension. </li></ul>
  59. 64. First Generation <ul><li>First CT scan design. </li></ul><ul><li>Single X-ray source, single X-ray detector. </li></ul><ul><li>A pencil beam is translated across the pt to obtain a set of parallel projection measurements at one angle. </li></ul><ul><li>Source/detector pair is rotated slightly and another set of measurements are obtained during a translation past the patient. This is repeated once for each projection angle. </li></ul><ul><li>Translate/rotate scanner. </li></ul>
  60. 66. Second Generation <ul><li>Multiple detectors which all lie within the scan plane but not necessarily contiguous nor do they span the entire diameter of the object. </li></ul><ul><li>Source/ detectors are translated as in a first generation system. </li></ul><ul><li>Each translation step generates multiple parallel ray projections. </li></ul><ul><li>More efficient and faster. </li></ul><ul><li>Translate/rotate scanner. </li></ul>
  61. 68. Third generation <ul><li>High spatial resolution detector cells allow simultaneous measurement of a fan-beam projection of the entire patient cross-section. </li></ul><ul><li>In view of the large number of detectors there is no need for the tube system to translate. </li></ul><ul><li>Tube-detector assembly rotates around the object. </li></ul><ul><li>Faster than 1st or 2nd generation systems. </li></ul><ul><li>Rotate/rotate scanner geometry. </li></ul>
  62. 70. Fourth Generation <ul><li>Stationary detector ring and a rotating X-ray tube. </li></ul><ul><li>Attempt to reduce ring artifacts common with 3 rd generation systems. </li></ul><ul><li>Requires a larger number of detector cells and electronic channels (at a potentially higher cost) to achieve the same spatial resolution and dose efficiency as a 3rd generation system. </li></ul><ul><li>Rotate-stationary or rotate only geometry. </li></ul>
  63. 72. Wide angle fan beam geometry 50-55 degree
  64. 73. Electron beam CT/ Cine CT/ 6 th generation, Ultrafast CT <ul><li>No mechanical scanning motion: both the detector and X-ray tube anode are stationary. </li></ul><ul><li>X-ray beam is moved electronically. </li></ul><ul><li>Anode is a large fixed tungsten arc of 210 deg, radius of 90 deg fitted on the scan frame. </li></ul><ul><li>2 sets of stationary detectors (432, 864) with arcs of 216 deg; may be used singly or in combination. </li></ul>
  65. 74. Electron beam CT (modified 4 th generation CT) Calcium tungstate crystal with silicon photodiode 216 deg 210 deg
  66. 75. Electron beam CT <ul><li>Calcium tungstate crystal with silicon photodiode. </li></ul><ul><li>As the beam can be moved very rapidly, image acquisition rates are extremely fast. </li></ul><ul><li>Stationary-stationary scanner. </li></ul><ul><li>Superior temporal resolution: cardiac imaging. </li></ul><ul><li>Image quality shortfalls for routine imaging. </li></ul>
  67. 76. Primary Indications (ACR) <ul><li>Acute head trauma. </li></ul><ul><li>Suspected acute intracranial hemorrhage. </li></ul><ul><li>Vascular occlusive disease or vasculitis </li></ul><ul><li>Aneurysm evaluation. </li></ul><ul><li>Detection or evaluation of calcification. </li></ul><ul><li>Immediate postoperative evaluation. </li></ul><ul><li>Treated/untreated vascular lesions. </li></ul><ul><li>Suspected shunt malfunctions, or shunt revisions. </li></ul><ul><li>Mental status change. </li></ul>
  68. 77. Primary Indications <ul><li>Increased intracranial pressure. </li></ul><ul><li>Headache. </li></ul><ul><li>Acute neurologic deficits. </li></ul><ul><li>Suspected intracranial infection. </li></ul><ul><li>Suspected hydrocephalus. </li></ul><ul><li>Congenital lesions eg. craniosynostosis, macrocephaly, microcephaly. </li></ul><ul><li>Evaluating psychiatric disorders. </li></ul><ul><li>Brain herniation. </li></ul><ul><li>Suspected mass or tumor. </li></ul>
  69. 78. Secondary Indications (ACR) <ul><li>MRI imaging is unavailable or contraindicated. </li></ul><ul><li>Diplopia, Cranial nerve dysfunction. </li></ul><ul><li>Seizures. </li></ul><ul><li>Apnea. </li></ul><ul><li>Syncope. </li></ul><ul><li>Ataxia. </li></ul><ul><li>Suspicion of neurodegenerative disease. </li></ul><ul><li>Developmental delay. </li></ul><ul><li>Neuroendocrine dysfunction. </li></ul><ul><li>Encephalitis. </li></ul><ul><li>Drug toxicity. </li></ul><ul><li>Cortical dysplasia, migration anomalies, or other morphologic brain abnormalities. </li></ul>
  70. 79. Technique of CT brain <ul><li>Performed with 15- 20 degree angulation to canthomeatal line at 5-10 mm increments. </li></ul>
  71. 80. Patient preparation <ul><li>Informed consent. </li></ul><ul><li>Remove all metallic accessories, eyeglasses, jewelry, hearing aid, dentures. </li></ul><ul><li>Enquire about pregnancy, diabetes, renal dysfunction, allergies, asthma, cardiac and other medical illness. </li></ul><ul><li>Avoid food for atleast 4 hrs prior to a contrast study. </li></ul><ul><li>Adequate hydration: pre and post contrast. </li></ul>
  72. 81. Precautions <ul><li>Sedation: children, uncooperative patients. </li></ul><ul><li>Avoid breastfeeding for 24 hrs after contrast study. </li></ul><ul><li>Equipped to deal with anaphylactic reaction. </li></ul><ul><li>Children are more sensitive to radiation than adults. </li></ul><ul><li>The effective radiation dose is about 2 mSv , which is what an average person receives from background radiation in 8 months. No proven adverse effects. </li></ul><ul><li>ALARA concept. </li></ul>
  73. 82. Contrast <ul><li>Improves detection and characterization of intra cranial lesions. </li></ul><ul><li>Opacifies blood vessels and detects areas of abnormal BBB break down. </li></ul><ul><li>Dosage –2 ml/kg (upto 100 ml) of iodinated contrast / iv/ at 350 mg/ml gives excellent vascular opacification. </li></ul><ul><li>Renal function tests: only in selected patients. </li></ul><ul><li>Ionic vs non-ionic contrast. </li></ul>
  74. 83. Contrast <ul><li>Physiological contrast enhancement </li></ul><ul><li>Pitutary gland and its stalk. </li></ul><ul><li>Dural structures. </li></ul><ul><li>Arteries and venous structures, esp deep veins and sinuses. </li></ul><ul><li>Choroid plexus. </li></ul><ul><li>Contra indications </li></ul><ul><li>Renal dysfunction. </li></ul><ul><li>Allergy to contrast. </li></ul><ul><li>Acute trauma. </li></ul>
  75. 84. Contrast nephropathy <ul><li>Increase of 25% or more or absolute increase of 0.5 mg/dl or more of S. Creatinine from the base line value occurring within 3 days of administration of iodinated contrast in the absence of other causes. </li></ul><ul><li>Risk factors – Dehydration, CCF, age>70 yrs and concurrent administration of nephrotoxic drugs. </li></ul><ul><li>Women with S. Creatinine <1.2 mg/dl and men with value <1.4 mg/dl have almost no risk for developing nephropathy. </li></ul>
  76. 85. References <ul><li>1. Haaga’s CT and MR imaging of whole body. </li></ul><ul><li>2. Osborn text book of neuro radiology. </li></ul><ul><li>3. Grainger and Allison’s diagnostic radiology. </li></ul><ul><li>4. Practical approach to Cranial CT: Uday Kumar Makwane. </li></ul><ul><li>5. Essential Physics of Medical Imaging: Jerrold T. Bushberg. </li></ul><ul><li>6. Neuroimaging by Zimmerman. </li></ul>

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