Computer-Tomography II: Image reconstruction and applications

Transcription

1 Computer-Tomography II: Image reconstruction and applications Prof. Dr. U. Oelfke DKFZ Heidelberg Department of Medical Physics (E040) Im Neuenheimer Feld Heidelberg, Germany Contents Motivation for CT Basic Physics Hounsfield Units Technical aspects: CT scanner CT reconstruction Spiral- and multi-slice CT Image quality Clinical applications Seite 1 1

2 CT- Reconstruction Radon-transform Backprojection Filtered backprojection Radon-transform Measurement: Line-integrals of attenuation coefficient distribution ( x, y) dl ( r) ( p p ) rnˆ ) d ² r ( A Sinogram: All projections as function of Φ,p: ( p, ) R[ ( r)] ( x, y ) ( p, ) Seite 2 2

3 Backprojection:1st attempt of image reconstruction f b ( r) f ) ( ˆ r r n ( 0 f ( r) d ) B ( r) Summation of projection values for each voxel over all projection angles Backprojection (r) f b = reconstructed image Object Backprojected of profiles Seite 3 3

4 Backprojection = Convolution with PSF Convolution: f b 1 ( r) ( r' ) d ² r' r r' A Kernel, PSF h( r) r 1 f b ( r) B ( r) BR ( r) Deconvolution? f b ( r ) ( h )( r ) Image reconstruction via deconvolution Convolution theorem: F h)( r) F ( r) F h ( ) 2 ( 2 2 r Kernel, PSF h( r) r 1 F h ( r ) H ( ) 2 ) 1 Seite 4 4

5 Theory:Image reconstruction via deconvolution f 1 ( r) F2 F2 BR ( r) 1. Backprojection 2. 2D-Fourier-transform 3. Multiplication with filter 4. Inverse 2D-Fourier-transform Central Slice-Theorem Disadvantage of deconvolution approach: alle projections have to be known Central-Slice-Theorem allows image reconstruction during image acquisition F R 1 F 2 Seite 5 5

6 Image reconstruction: Filtered backprojection R 1 BF 1 1 F d-Fourier transform of projection profile 2. Multiplication with filter (kernel) 3. 1d-inverse Fourier transform 4. Backprojection Filtered backprojection Object Filtered backprojection Seite 6 6

7 Backprojection Ungefilterte of measured Rückprojektionprofiles Filter, kernel Measured profile = Filtered profile Seite 7 7

8 Gefilterte Rückprojektion Backprojection of filtered profiles Object Filtered backprojection Image acquisition Sinogram Filtered Sinogram Tomogramm filtering Backprojection Projektionswinkel Fächerwinkel Seite 8 8

9 Spiral-CT (1989) Continuosly rotating CT-gantry and continous patient transport spiral CT Spiral-CT reconstruction Requires interpolation of data additional reconstruction algorithms Seite 9 9

10 Spiral-CT Additional x-ray power required Extended scanning times, high scanning speed kev, kev 1% x-ray efficiency, 99% heat production at the anode new x-ray tube design: STRATON-x-ray source (2005) Direct cooling of the anode (convection, oil) Increases cooling by a factor of 3 Seite 10 10

11 STRATON-x-ray source z-sharp-technology: B-field focusses / guides e - -beam 2 alternating focal spots at the anode < 0,4 mm resolution at 0,6 mm detector width Multi-slice CT-CT (1998) Several detector rows simultaneous acquisition of several CT-slices Fast volume scans: whole body scans in 25s! Isometric voxel < 1 mm < 1 mm Seite 11 11

12 Cone beam-ct Adaptive array detector Seite 12 12

13 Technical developments min. scan time data per 360 scan data per spiral scan image matrix tube power slice thickness spatial resolution s 5 s 1 s 0.5 s 58 kb 1 MB 2 MB 12 MB - - < 50 MB < 500 MB 80 x x x x kw 10 kw 40 kw 60 kw 13 mm 2-10 mm 1-10 mm mm 3 lp/cm 12 lp/cm 15 lp/cm 24 lp/cm contrast resolution 5 mm / 5 HU / 50 mgy 3 mm / 3 HU / 30 mgy 3 mm / 3 HU / 30 mgy 3 mm / 3 HU / 30 mgy Image quality Seite 13 13

14 Image quality indicators Indicators: spatial resolution contrast noise, homogeneity artefacts Spatial resolution There are 7 or 8 mice... No, there are 10 of them! The ability to resolve High Contrast Objects, - High Contrast Resolution Seite 14 14

15 Spatial resolution Edge enhancement filter Smoothing filter Contrast resolution When Small Contrast differences are questioned - Low Contrast Resolution Is there a cat?! There could be a cat! A cat! Seite 15 15

16 Contrast resolution Edge enhancement filter Smoothing filter Contrast detail diagram Seite 16 16

17 Noise Poisson statistics of photons Depends on x-ray current I, slice thickness d I = 50 ma d = 4 mm I = 200 ma d = 4 mm I = 200 ma d = 16 mm CARE-Technology Volumes with low attenutation contribute less to image noise potential reduction of dose without loss of image quality Dynamic adaptation of output of the x-ray source to the expected attenuation dose reduction of up to 68% Seite 17 17

18 Artefacts Artifacts Movement artifacts Partial volume effects Seite 18 18

19 Influence of different reconstruction kernels Partial volume effect CT-slice Soft tissue Bone Seite 19 19

20 Partial volume effect CT-slice Soft tissue Bone Partial volume effect CT-slice Soft tissue Bone Seite 20 20

21 Clinical applications Routine applications Bones & Lungs Soft Tissues Seite 21 21

22 Bone/Lung high contrast Superior Vena Cava Thrombosis (Emotion) 130kV, 150mA, 0.8s 5 mm, Pitch 1.5, 50% overlap Seite 22 22

23 Liver Hemangioma Study (Esprit) 4-Phase spiral study Excellent Enhancement Precontrast Arterial Portal-venous Delayed phase 8 mm, Pitch kV, 120mA, 1.5s 120 ml / flow rate 3 ml/s Low dose imaging Seite 23 23

24 Broken heel 1mm Spiral CT Emotion - Sagittal Reformat Volume Access - VRT Spine (Emotion/Emotion Duo) 2 mm, Pitch 2 Low dose ( 90 ma) Low Dose of 100 mas 2 x 1.5 mm collimation 2 mm slice width 99 s spiral duration Seite 24 24

25 Pediatrics Head (Emotion Duo) 1.25mm axial slice 2 x 1mm, 60 mas CT Angiography Seite 25 25

26 CT-Angiography 23cmin5s < 0,4 mm Schichtdicke 0,5 s Rotation Cerebrale arteries 1 mm, Pitch 2, Emotion 2 x 0.5mm, Pitch 3, Volume Zoom Seite 26 26

27 Carotids (Emotion/Emotion Duo) 2 x 1 mm, Pitch 3, 1.25mm SW 130kV, 100mA, 0.8s, 2 mm, Pitch 2 Abdominal Aortic Stent Follow-up (Emotion Duo) 35cm 130kV, 143mAs, 0.8s, 2 x 1.5 mm, Pitch 4 Seite 27 27

28 Mehrschicht-CT Intrafractional Movement by 4D-MSCT on a Siemens Sensation open Lung cancer T4N3 paralysis of rt diaphragm Seite 28 28

29 Siemens SOMATOM Definition Dual-CT Dual CT 2 x-ray sources 2 detectors Siemens SOMATOM Definition Dual-CT Slice thickness < 0,4 mm 32 row-detektor + z-sharp = 64 slices per rotation Min. scanning time: 83ms (90 rotation) CT of the moving heart Seite 29 29

30 Siemens SOMATOM Definition Dual-CT "Dual Energy" Scanning with two x-ray sources at different energies Energy dependence of attenuation coefficient additional contrast Toshiba 256-slices CT 128 mm scan-width at 0,5 mm slice thickness whole heart imaging in one rotation Seite 30 30

31 4D-CT Toshiba 256-slice-CT Seite 31 31