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Computed TomographySCANCO User Meeting 2005

Dr. Bruno KollerSCANCO Medical AG

www.scanco.ch

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Overview X-Ray Basics CT Hardware Components Measurement Reconstruction Artefacts

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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

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Introduction

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Whole Body CT

2 cm

Good S/N Good contrast bone/soft tissue Slice thickness 2-5 mm

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Peripheral CT Good Contrast Bone/Soft tissue Voxelsize 100 m Limited FOV (130 mm)

1 cm

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Microtomography Excellent contrast bone/soft

tissue Slice thickness and in plane

resolution <10 m More noise in images

1 mm

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3D Microtomography

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CT-Basics Based on measurement of attenuation of X-rays (Beer-

Lambert):

Measurement of a projection value (Sample):

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Source Detector

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Measurement of one projection

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Measurement of one projection

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Measurement of one projection

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Measurement of one projection

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Projection Value Measurement

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Object Detector

X-rays

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Source X-Ray Tubes (most common)

Continuous, steady output (high flux) Small focal spot (< 10 m) Variable energy and intensity Polychromatic beam

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Attenuation coefficient [1/cm] Attenuation coefficient changes with material:

Attenuation coefficient changes with energy:

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Beam Hardening Soft X-rays are attenuated more than hard X-rays Depending on object, spectrum changes

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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

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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

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1st generation scanner Single Detector System Translation-Rotation 5 min. per slice

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2nd generation scanner multichannel-Systems (4, 6, 8,

16) Translation-Rotation 20 sec. per slice

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3rd generation scanner Fan-Beam-Geometry multichannel-system (500+

detectors), angle > 180o

Rotation of tube and detektorsystem

no translation 1 – 10 sec. per slice

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Parallel Beam (Synchrotron) Parallelbeam Rotation of object only No collimators required 2D-Detector arrays

A. Kohlbrenner, ETH Zürich

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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

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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

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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...

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Reconstruction Direct method: The measured projections are backprojected under the

same angle as the measurement was taken. All projections are summed up

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Reconstruction

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Reconstruction

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Convolution-Backprojection

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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

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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

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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) ...

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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