SCANCOMEDICAL Computed Tomography SCANCO User Meeting 2005 Dr. Bruno Koller SCANCO Medical AG www.scanco.ch SCANCOMEDICAL Overview X-Ray Basics CT Hardware Components Measurement Reconstruction Artefacts BK/ 2 SCANCOMEDICAL Introduction 3D distribution of tissueproperties Density (absorption of Xrays, speed of sound…) Chemical composition Temperature ... Imaging of these local tissue properties using grayscale or color mapping BK/ 3 SCANCOMEDICAL Introduction BK/ 4 SCANCOMEDICAL Whole Body CT Good S/N Good contrast bone/soft tissue Slice thickness 2-5 mm 2 cm BK/ 5 SCANCOMEDICAL Peripheral CT Good Contrast Bone/Soft tissue Voxelsize 100 mm Limited FOV (130 mm) 1 cm BK/ 6 SCANCOMEDICAL Microtomography Excellent contrast bone/soft tissue Slice thickness and in plane resolution <10 mm More noise in images 1 mm BK/ 7 SCANCOMEDICAL 3D Microtomography BK/ 8 SCANCOMEDICAL CT-Basics Based on measurement of attenuation of X-rays (BeerLambert): Source m Io d Detector I I Io e md Measurement of a projection value (Sample): I P (t ) ln o mdl I t L BK/ 9 SCANCOMEDICAL Measurement of one projection BK/ 10 SCANCOMEDICAL Measurement of one projection BK/ 11 SCANCOMEDICAL Measurement of one projection BK/ 12 SCANCOMEDICAL Measurement of one projection Io t I t BK/ 13 SCANCOMEDICAL Projection Value Measurement I I I0 m X-rays Source Object Detector BK/ 14 SCANCOMEDICAL Source X-Ray Tubes (most common) Continuous, steady output (high flux) Small focal spot (< 10 mm) Variable energy and intensity Polychromatic beam I E BK/ 15 SCANCOMEDICAL Attenuation coefficient m [1/cm] Attenuation coefficient changes with material: Io m x, y dl P (t ) ln I t L Attenuation coefficient changes with energy: m bone muscle fat E BK/ 16 SCANCOMEDICAL Beam Hardening Soft X-rays are attenuated more than hard X-rays Depending on object, spectrum changes I m(E) I d E E BK/ 17 SCANCOMEDICAL Detectors Usually detect visible light only They all need Scintillators Counting Systems (Photomultipliers) Integrating Systems (CCD, Diode Arrays, CMOS-Detectors) 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 BK/ 18 SCANCOMEDICAL 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 BK/ 19 SCANCOMEDICAL 1st generation scanner Single Detector System Translation-Rotation 5 min. per slice BK/ 20 SCANCOMEDICAL 2nd generation scanner multichannel-Systems (4, 6, 8, 16) Translation-Rotation 20 sec. per slice BK/ 21 SCANCOMEDICAL 3rd generation scanner Fan-Beam-Geometry multichannel-system (500+ detectors), angle > 180o Rotation of tube and detektorsystem no translation 1 – 10 sec. per slice BK/ 22 SCANCOMEDICAL Parallel Beam (Synchrotron) Parallelbeam Rotation of object only No collimators required 2D-Detector arrays A. Kohlbrenner, ETH Zürich BK/ 23 SCANCOMEDICAL Cone Beam Tube with focal spot Linear, 2-D Detector (e.g. 1024 x 1024 Elements, CCD) Single rotation Artefacts due to improper scanning scheme (would require to different movements) A. Kohlbrenner, ETH Zürich BK/ 24 SCANCOMEDICAL 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 BK/ 25 SCANCOMEDICAL 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... BK/ 26 SCANCOMEDICAL Reconstruction Direct method: The measured projections are backprojected under the same angle as the measurement was taken. All projections are summed up BK/ 27 SCANCOMEDICAL Reconstruction BK/ 28 SCANCOMEDICAL Reconstruction BK/ 29 SCANCOMEDICAL Convolution-Backprojection BK/ 30 SCANCOMEDICAL 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 BK/ 31 SCANCOMEDICAL 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 BK/ 32 SCANCOMEDICAL 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) ... BK/ 33 SCANCOMEDICAL 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 BK/ 34