Computer-Tomography I: Principles, History, Technology Prof. Dr. U. Oelfke DKFZ Heidelberg Department of Medical Physics (E040) Im Neuenheimer Feld 280 69120 Heidelberg, Germany u.oelfke@dkfz.de History of CT 1895 W. C. Röntgen: Discovery of x-rays 1917 J.H. Radon: Radon transform, reconstruction algorithm 1963 A.M. Cormack: CT reconstruction algorithm 1972 G.N. Hounsfield und J. Ambrose: first clinical CT 1974 60 clinical CT scanners (head scanners) 1975 First clinical whole body CT 1979 Nobelprize for Hounsfield and Cormack 1989 Spiral CT 1998 Multi-slice CT 2000 > 30000 clinical CT installations Seite 1 1
Contents Motivation for CT Basic Physics Hounsfield Units Technical aspects: CT scanner CT reconstruction Spiral- and multi-slice CT Image quality Clinical applications Motivation Limitations of 2d x-ray imaging: Loss of depth information, superposition of all structures (projection on one plane) Very limited contrast for different soft tissue contrast Seite 2 2
Problems with planar x-ray imaging 2D image: superposition of all structures in depth No differentiation of soft tissues CT vs. planar X-ray imaging X-ray image: 2D-projection Good contrast for bone-air interfaces CT = Reconstruction of anatomical images from x-ray images obtained from different directions Seite 3 3
Principle of CT: Basic physics Absorption of x-rays in matter Intensity Measured: absorption coefficient I ( d ) I 0 exp ( ( E, Z ) ds ) d 0 (, Z ) E Seite 4 4
CT-Numbers: Hounsfield Units HU ( c Wasser Wasser ) 1000 HU = Hounsfield Unit Water: 0 HU; Air: -1000 HU Range of measured HUs: -1024 HU bis +3071 HU 4096 = 2 12 Gray values Hounsfield Units Contrast resolution: ~ 3 HU Seite 5 5
Windowing of CT images Observer can differentiate between 60-80 gray values windowing C = Center W = Width CT: Technical aspects Seite 6 6
Planar -tomography Scanner generations Seite 7 7
Scanner 1. und 2. Generation EMI-Scanner (1. Generation) Seite 8 8
2. Generation scanner Multi-Detector- Translation-rotation Seite 9 9
3. Generation Scanners Realisation of short acquisition time Problem: high voltage supply Solution: sliding ring technology (1987) continuos rotation of gantry Seite 10 10
Scanner 3. und 4. generation Scanner setup - 3rd generation Seite 11 11
First SIEMENS scanner in 1974 - SIRETOM SIRETOM (in 1974) Acquisition time 7 min., image matrix 80x80 pixels, scan field 25 cm, spatial resolution 1,3 mm (4LP/cm) SIEMENS SOMATOM Plus 4 (1996) Tube Detector Seite 12 12
Progress in Image Quality 1972-2000 SIRETOM (1974) SOMATOM Plus 4 UFC (1996) CT: Technical development - detectors Seite 13 13
Detectors Xenon-ionisation chambers Good time response small dead time, small image lag Poor quantum efficiency Detectors Szintillation-detectors (Cs-Iodid, Cd-Wolframat) High quantum efficieny Longer image lag, slower response Seite 14 14
Detectors Today: Ceramic Szintillation- detectors High quantum efficiency Fast response, short image lag Combined advantages of previous detector types Time response of different detectors CT: Basic principle Seite 15 15
Image acquisition "1. Generation" CT-Scanner: Translation Rotation with pencilbeam x Seite 16 16
Seite 17 17
Raw data 1. Projection (Translationsscan) 160 measured values Time (Projections) Line-integrals Raw data 2. Projection (Translation scan after small rotation) 2 x 160 measured values Time (Projections) Line-integrals Seite 18 18
Raw data 180 Rotation Raw data 180 x 160 = 28.800 Time (Projections) Line-integrals Image reconstruction Image Reconstruction Sinogram CT image Seite 19 19