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Advanced Biomedical Imaging Lecture 6 Basic physical principles of computed tomography & Image formation Dr. Azza Helal A. Prof. of Medical Physics Faculty of Medicine Alexandria University Points to be covered Linear Tomography Components of computed tomography unit Basic data acquisition CT scanner generations Principle of CT imaging CT number & its clinical application Windowing. 1. Linear Tomography: (single slice imaging) Method of imaging single slice of object parallel to film and placed at a specific point (fulcrum) which is adjusted to height of anatomy of interest. Tube and film moves from 1st position to the 2nd , all points in focal plane project to same position on the film Points above or below the focal plane do not project to the same film position and are blurred. By changing the relative motion of the film and tube the focal plane can be adjusted upward or downward It is useful in IV urography Body imaging tomography (Computed tomography) (CT): Reconstruction by a computer of an image of a plane or slices of an object. 2. Components of CT unit X-ray Tube X-ray Beam CT Table Detectors Table, scanning gantry (x-ray source & detector array) x-ray generator, computer and viewing consoles. 5 3. Basic data acquisition Probing patient from different directions during 360 rotations with x-ray beam of known intensity (I0) & measuring it after it has passed the pt (I) using detectors. Detector (Scintillator / ionization chambers) measures exiting x-ray beam (I) & converts it into a proportional signal current. From I & I0, (U) is calculated (reflect intensity of photon beam attenuated) I=I0e-ux u is different for different tissue density. Image is developed from multiple measurements of x-ray u detected from exterior of the patient. By solving a system of linear equations for several projections, value of u can be computed. 4. 4. CTCT scanner generations scanner generations 1st: single beam & detector. Translate - rotate 2nd : narrow fan beam / detector array. 3rd: wide fan beam / detectors array, both rotate 4th: x ray tube alone rotates / stationary detectors. 5th : multiple x ray tubes & multiple stationary detectors to scan multiple slices simultaneously. 4. CT scanner generations 2nd 1st 3rd 4th 5. Principle of CT imaging • To allow the computer to present this information with a large scale, a reference material is used (water). Why? its µ is close to those of soft tissues. • CT number is calculated as a relative comparison of x-ray attenuation of a voxel of tissue to equal volume of water. • CT number of each voxel (average of individual CT no of the contents of corresponding voxel) is computed & stored in computer memory. Hounsfield Units large amount of data presented as grey scale (whiteness is α average µ of contents /voxel). Tissues appear as shades of gray from black & white. Tissues with high u (bone) appear white Tissues with low u (air) appears black. The Hounsfield scale bone= 1000 water= 0 air= -1000 . CT number (reflect density) Air = -1000, Lung -550 to -950 Fat=-80 to -100 Water = 0 White matter = 20-30, Gray matter = 35-45 soft tissue +40 to +60 depends on KV Acute Hge = 70 to 100 HU Calcification = 200 to 300 HU Bone +500 to +1500 Contrast agent +3000 CT tissue characterization CT Numbers or Hounsfield Units Windowing Selective display of a restricted range of gray scale of selected tissues (tissues of interest). Tissues with CT no outside this window un displayed. • Manipulated selection of : by • window center. • window width • Window level is CT number selected for centre of the range of numbers displayed on the image. •Window width is total range of values selected. •Width determines contrast. • A narrow window enhances inherent contrast. •Window level determines the brightness Centre <centre >centre average gray lighter gray darker gray W = 80 C = 30 W = 80 W = 80 W = 200 W = 50 C = 50 C = 20 C = 30 C = 30 In routine work •Brain is visualized at level 30 and width 80. •Soft tissue is visualized at level 40 and width 250. •Bones are visualized at level 1000 & width 2000. •Lungs are visualized at level -600 & width 1500. As CT no reflects u and so different tissues densities, So it is used to characterize normal tissues & pathologies as calcification & lesion diameter. 1. Define; window level & width? 2. What are the detectors used in CT unit? 3. What is the CT number of bone, air and water? 4. What is the difference between u and CT number of the tissue?