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SCANCOMEDICAL Computed Tomography SCANCO User Meeting 2005 Dr. Bruno Koller SCANCO Medical AG

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Presentation on theme: "SCANCOMEDICAL Computed Tomography SCANCO User Meeting 2005 Dr. Bruno Koller SCANCO Medical AG"— Presentation transcript:

1 SCANCOMEDICAL Computed Tomography SCANCO User Meeting 2005 Dr. Bruno Koller SCANCO Medical AG

2 SCANCOMEDICAL BK/ 2 Overview X-Ray Basics CT Hardware Components Measurement Reconstruction Artefacts

3 SCANCOMEDICAL BK/ 3 Introduction 3D distribution of tissue- properties Density (absorption of X- rays, speed of sound…) Chemical composition Temperature... Imaging of these local tissue properties using grayscale or color mapping

4 SCANCOMEDICAL BK/ 4 Introduction

5 SCANCOMEDICAL BK/ 5 Whole Body CT 2 cm Good S/N Good contrast bone/soft tissue Slice thickness 2-5 mm

6 SCANCOMEDICAL BK/ 6 Peripheral CT Good Contrast Bone/Soft tissue Voxelsize 100  m Limited FOV (130 mm) 1 cm

7 SCANCOMEDICAL BK/ 7 Microtomography Excellent contrast bone/soft tissue Slice thickness and in plane resolution <10  m More noise in images 1 mm

8 SCANCOMEDICAL BK/ 8 3D Microtomography

9 SCANCOMEDICAL BK/ 9 CT-Basics Based on measurement of attenuation of X-rays (Beer- Lambert): Measurement of a projection value (Sample): SourceDetector d  IoIo I

10 SCANCOMEDICAL BK/ 10 Measurement of one projection

11 SCANCOMEDICAL BK/ 11 Measurement of one projection

12 SCANCOMEDICAL BK/ 12 Measurement of one projection

13 SCANCOMEDICAL BK/ 13 Measurement of one projection IoIo t I t

14 SCANCOMEDICAL BK/ 14 Projection Value Measurement I Source Object Detector X-rays I0I0 I m

15 SCANCOMEDICAL BK/ 15 Source X-Ray Tubes (most common) Continuous, steady output (high flux) Small focal spot (< 10  m) Variable energy and intensity Polychromatic beam I E

16 SCANCOMEDICAL BK/ 16 Attenuation coefficient  [1/cm] Attenuation coefficient changes with material: Attenuation coefficient changes with energy: bone muscle fat m E

17 SCANCOMEDICAL BK/ 17 Beam Hardening Soft X-rays are attenuated more than hard X-rays Depending on object, spectrum changes I E I E m(E) d

18 SCANCOMEDICAL BK/ 18 Detectors Usually detect visible light only Counting Systems (Photomultipliers) Integrating Systems (CCD, Diode Arrays, CMOS-Detectors) They all need Scintillators Convert X-rays into light NaI, CsI, CdTe... The thicker, the more efficient, but the thiner, the better the spatial resolution (tradeoff between high output or high res) Fiber optics (straight or tapered) in between to protect from remaining X-rays

19 SCANCOMEDICAL BK/ 19 CT-Measurement For a CT measurement one needs an certain number of single projection measurements at different angles (theoretically, an unlimited number is required) In realized Tomography-Systems one usually finds a geometrically ordered detector configuration

20 SCANCOMEDICAL BK/ 20 1 st generation scanner Single Detector System Translation-Rotation 5 min. per slice

21 SCANCOMEDICAL BK/ 21 2 nd generation scanner multichannel-Systems (4, 6, 8, 16) Translation-Rotation 20 sec. per slice

22 SCANCOMEDICAL BK/ 22 3 rd generation scanner Fan-Beam-Geometry multichannel-system (500+ detectors), angle > 180 o Rotation of tube and detektorsystem no translation 1 – 10 sec. per slice

23 SCANCOMEDICAL BK/ 23 Parallel Beam (Synchrotron) Parallelbeam Rotation of object only No collimators required 2D-Detector arrays  A. Kohlbrenner, ETH Zürich

24 SCANCOMEDICAL BK/ 24 Cone Beam Tube with focal spot Linear, 2-D Detector (e.g x 1024 Elements, CCD) Single rotation Artefacts due to improper scanning scheme (would require to different movements)  A. Kohlbrenner, ETH Zürich

25 SCANCOMEDICAL BK/ 25 Spiral scanning Continuous movement of patient during rotation Volumetric measurement Slicewise reconstruction with variable slice thickness by interpolation As scanner can continuously rotate, one can achieve much faster scan speeds Latest models (clinical scanners) with parallel detector rings (Multirow, currently up to 64) 40 slices per second (150 rpm) No need in current MicroCT systems as the rotation speed is low

26 SCANCOMEDICAL BK/ 26 Reconstruction Iterative reconstruction ART (Arithmetic Reconstruction Technique) Assume image (base image) Calculate projections of this base image Modify image after comparing calculated projections with measured Projections Strategy...

27 SCANCOMEDICAL BK/ 27 Reconstruction Direct method: The measured projections are backprojected under the same angle as the measurement was taken. All projections are summed up

28 SCANCOMEDICAL BK/ 28 Reconstruction

29 SCANCOMEDICAL BK/ 29 Reconstruction

30 SCANCOMEDICAL BK/ 30 Convolution-Backprojection

31 SCANCOMEDICAL BK/ 31 Artefacts Beam Hardening Attenuation coefficients depend on energy soft X-rays are much more absorbed than harder X-rays Distribution changes when beams penetrate object Segmentation problems

32 SCANCOMEDICAL BK/ 32 Artefacts Object outside of FOV Inconsistent set of projection data (only partially within the beam at some angles, completely in the beam at other angles) Local Reconstruction: only for geometry

33 SCANCOMEDICAL BK/ 33 Artefacts Motion Object moves during scan May be eliminated by external gating (respiratory, heart beat) Total absorption of X-Rays e.g. Caused by metallic implants (division by 0 in reconstruction) Other Artefacts Wrong geometry (fan-beam-angle) Centers artefact Mechanical alignment Insufficient no. of projections (sampling)...

34 SCANCOMEDICAL BK/ 34 Resources Volume of 1024 x 1024 x 1200 requires 2.4 GB (short integer) Doubling the resolution requiers 8x more time to calculate Doubling the resolution requiers 8x more disk space

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