Corso di laurea in Fisica A.A Fisica Medica 4 TC

Transcription

1 Corso di laurea in Fisica A.A Fisica Medica 4 TC

2 Computed Tomography Principles 1. Projection measurement 2. Scanner systems 3. Scanning modes

3 Basic Tomographic Principle The internal structure of an object can be reconstructed from multiple projections of the object.

4 Exponential Attenuation of X-ray

5 Ray-Sum of X-ray Attenuation

6 Computed Tomography Principles 1. Projection measurement 2. Scanning modes 3. Scanner systems

7 Projection & Sinogram Computed tomography (CT): image reconstruction from projections P(θ,t) f(x, y)

8 Computed Tomography Geometry (First CT was developed in 1972)

9 Matrix Representation of a Tissue Slice in CT detector reconstruction matrix Pixel (Picture Element) H.U. = [µ - µ(water)/µ(water]*1000 focal spot

10 CT Display Scale reconstructed image linear attenuation coefficient, µ(x,y,z) displayed image Houndsfield units, H.U. H.U. = [µ - µ(water)/µ(water]*1000

11 Linear Attenuation Coefficients (60 kev) Tissue Brain - Grey Brain - White Cerebro-Spinal Fluid (CSF) Pancreas Liver Water Fat µ (χµ 1 )

12 Grey - White Matter Contrast C = ( )cm -1 * 1.0 cm =.2 %! CT Number µ(cm -1 ) CT number allows the computer to present the information with a larger grey scale

13 Variation of Linear Attenuation Coefficients with Energy Water Fat µ Energy (kev)

14 Variation of H.U. with Energy Energy (kev) µ(water) µ(fat) H.U.(fat)

15 Image Display µ µ HU( µ ) =1000 µ water water CT Number - Hounsfield unit Air: Water: 0 Bone: +175 to Viewing Parameters Window level (L) Window width(w) Zoom factor W L

16 Computed Tomography Principles 1. Projection measurement 2. Scanning modes 3. Scanner systems

17 CT Scanner

18 Data Acquisition System (DAS)

19 Data Acquisition System (DAS)

20 First Generation One detector Translation-rotation 5 minutes/per slice Parallel-beam 160 rays x 180 views

21 First Generation CT Scanner From Webb, Physics of Medical Imaging

22 Second Generation Multiple detectors Translation-rotation Small fan-beam 600 rays x 540 views 18 seconds/per slice

23 Second Generation CT Scanner From Webb, Physics of Medical Imaging

24 Third Generation Multiple detectors Translation-rotation Large fan-beam 800 rays x 1000 views <1 seconds/per slice

25 Ring Artifact In 3 rd Generation

26 Fourth Generation Detector ring Source-rotation Large fan-beam

27 Fourth Generation Detector fan eliminates ring artifacts.

28 3 rd and 4 th Generation Scanners From Webb, Physics of Medical Imaging

29 Krestel- Imaging Systems for Medical Diagnosis

30 Third & Fourth Generations

31 Fifth Generations Electron-beam CT for cardiac imaging.

32 Electron Beam CT

33 Sixth Generation: Spiral/Helical/Volumetric CT Continuous & Simultaneous : Source rotation Patient translation Data acquisition

34 Volume Scanning Scan-Translate Patient, Scan-Translate Patient, Continuous Scan, Continuous Patient Translation

35 Important years in helical CT history Single-slice 1989 Dual-slice 1992 Quad-slice 1998

36 Single-Slice 8 times faster than single-slice One rotation / sec Quad-Slice Two rotations / sec + 4 slices / rotation

37 Why is faster better? Improved temporal resolution Faster scanning causes less motion artifacts Breath holding time is reduced Improved spatial resolution Narrower collimation leads to higher resolution in the z-axis (MPR) Narrower collimation reduces partial volume effect Improved contrast media concentration Higher contrast media concentration due to faster infusion Better separation of arterial and venous phases Increased power (mas) The widened x-ray beam and sampling of multiple slices for each rotation allows for raised mas Decreased image noise A direct effect of raised mas Efficient x-ray tube utilization Faster scanning causes markedly less waiting for tube cooling More images from x-ray tube during tube life cycle

38 Pitch table travel (mm) per gantry rotation Pitch = (1) beam collimation (mm) Information about table travel relative to beam collimation

39 Pitch = 1 > 1

40 Volumetric scanning Viewing Volume

41 Reformatting z y x Reformating multiple slices into a volume produces a volume image with unequal spatial resolution in x, y, and z.

42 Reformatting with Interpolation Stacked Slices (original images) Interpolated Slices (smoother display)

43 Scan a volume view a volume 25, 50, 100, 400, 1000 images can be reconstructed from volume. The image data from the volume can be merged into single images. Coronal Reformat