Computed tomography basics

Information about Computed tomography basics

Published on November 2, 2008

Author: allancavalcanti



Slide 1: Basic Principles of CT Scanning The basics of CT : The basics of CT CT imaging chain System components Acquisition methods Image quality Applications X-ray: The beginning : X-ray: The beginning X-Rays founded in 1895 by Wilhelm Conrad Roentgen CT: The beginning : CT: The beginning CT founded in 1970 by Sir Godfrey Hounsfield Engineer with EMI, LTD. first applications were in neuroradiology CT Scanner : CT Scanner X-Ray modality used to the body in cross section Used to determine extent of trauma location and type of tumors status of blood vessels pre surgical planning CT System : CT System CT System : CT System Basic CT scanner components : Basic CT scanner components Gantry X-Ray Tube Detector Control Console Gantry : Gantry CT X-ray tube High voltage generator Detector array Data acquistion system Slip ring The CT X-ray Tube : The CT X-ray Tube Anode heat capacity 3.5 MHU up to 6.5 MHU Determines maximum mAs Determines volume length Dictates generator size Detector Elements : Detector Elements Capture energy that has not been attenuated by the patient Control console : Control console Set scan parameters kVp, mA, scan time, reconstruction filter, etc. Set scan mode Digital radiograph, axial or volume Houses reconstructor Review and archive images Post-processing CT : CT CT - Computed Tomography CAT Scan - Computerized Axial Tomography Scanning methods : Scanning methods Digital projection AP, PA, Lat or Oblique projection Surview, Scanogram Conventional CT Axial Start/stop Volumetric CT Helical or spiral CT Continuous acquisition Digital Projection : Digital Projection X-ray tube and detector remain stationary Patient table moves continuously With X-rays “on” Produces an image covering a range of anatomy Similar to a conventional X-ray image, e.g. flat plate of the abdomen Image used to determine scan location Axial CT : Axial CT X-ray tube and detector rotate 360° Patient table is stationary With X-ray’s “on” Produces one cross-sectional image Once this is complete patient is moved to next position Process starts again at the beginning Volume CT : Volume CT X-ray tube and detector rotate 360° Patient table moves continuously With X-ray’s “on” Produces a helix of image information This is reconstructed into 30 to 1000 images Volume Scanning: Pitch : Volume Scanning: Pitch Advantages of Volume CT : Advantages of Volume CT More coverage in a breath-hold Chest, Vascular studies, trauma Reduced misregistration of slices Improved MPR, 3D and MIP images Potentially less IV contrast required Gapless coverage Arbitrary slice positioning Fundamentals of Multislice CT : Fundamentals of Multislice CT Multislice Fundamentals : Multislice Fundamentals Everything is better (R)esolution Z-axis, spatial, low contrast (S)peed Temporal - bolus capture, stopped motion (V)olume Thin slice - organ-specific coverage (P)ower Enough photons - uncompromising image quality Multislice Effectiveness : Single Slice = One 10mm slice per rotation Dual Slice = Two 5mm slices per rotation Quad Slice = Four 2.5mm slices per rotation Multislice Effectiveness Everything is better Resolution 2x 4x-8x Speed same same Volume same same Power same same Dual Quad Dual Slice Detector Optimized for 2 Slice Acquisition : . . single detector arc dual detector arc pre-patient collimation post-patient collimation x-ray tube focal spot _ _ _ Mx8000Dual Slice Dual Slice Detector Optimized for 2 Slice Acquisition Quad Detector Technology : Approximately 10% more efficient than matrix detectors Variable Wide Area Detector Asymmetrix™ Variable detector length Fixed detector length Quad Detector Technology Philips patented variable wide area detector Variable slice thickness 4 x 1mm 4 x 5mm 4 x 2.5mm 2 x 0.5mm 2 x 8mm 2 x 10mm Quad Technology : 8 Element 2-D array 4 Slices Quad Technology How it works Asymmetrix™ Technology : Asymmetrix™ Technology Variable slice thickness CT : CT CT attenuation information CT image quality Attenuation : Attenuation X-ray beam passes through patient Each structure attenuates X-ray beam differently According to individual densities Radiation received by detector varies according to these densities Density information : Density information Transferred from detector to CT computer(A to D converter) Reconstructed by computer into a cross-sectional image Displayed on screen Each pixel displayed on monitor has varying brightness The greater the attenuation, the brighter the pixel The less attenuation, the darker the pixel Density information : Density information Density values correspond to a range of numbers Hounsfield scale Window settings : Window settings Window width Determines range of CT numbers displayed on an image Values above this range = white Values below this range = black Window level Sets the center CT number displayed on the monitor Determines the location on the Hounsfield scale about which the window width will be centered CT image quality : CT image quality Spatial resolution Ability to resolve small objects in an image Measured in lp/cm Isotropic Imaging : Isotropic Imaging True 0.5mm Isotropic imaging CT image quality : CT image quality Contrast resolution Ability to differentiate small density differences in an image Post Processing Options : Post Processing Options Visualization ofvasculature in relation to pathology Show course of vessels Show stent placement Define vascular stricture Thin-Slice Spiral Neck : Cervical Spine Spiral Acquisition Rotation – 0.75 sec Coverage – 160 mm Pitch – 0.875 Acq. Time – 36 sec FOV – 250 mm ST – 1.0 mm Rec. Incr. – 0.6 mm Std Res. – 8 lp/cm 120 kV, 200 mAs CTDI100w – 39 mGy CTDIFDAw – 17 mGy Thin-Slice Spiral Neck Neuro-Angiography : Neuro-Angiography Circle of Willis Thin-Slice Spiral Lungs : Thin-Slice Spiral Lungs Renal Arteries : Renal Arteries Extended Spiral Acquisition : Ext. Spiral Acq. Spiral Acquisition UltraFast – 0.5 sec Coverage – 1400mm Pitch – 1.75 Acq. Time – 41.5 sec FOV – 420 mm ST –2.5 mm Rec. Incr. – 1.6 mm Std Res. – 8 lp/cm 120 kV, 96 mAs CTDI100w – 7.3 mGy CTDIFDAw – 4.9 mGy Extended Spiral Acquisition CT Scanners : CT Scanners Provide a window into the body Customer considerations How many patients Referring physicians Budget Upgrade expectations Philips has ALL the answers

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