Corso di laurea in Fisica A.A. 2007-2008 Fisica Medica 4 TC
Computed Tomography Principles 1. Projection measurement 2. Scanner systems 3. Scanning modes
Basic Tomographic Principle The internal structure of an object can be reconstructed from multiple projections of the object.
Exponential Attenuation of X-ray
Ray-Sum of X-ray Attenuation
Computed Tomography Principles 1. Projection measurement 2. Scanning modes 3. Scanner systems
Projection & Sinogram Computed tomography (CT): image reconstruction from projections P(θ,t) f(x, y)
Computed Tomography Geometry (First CT was developed in 1972)
Matrix Representation of a Tissue Slice in CT detector reconstruction matrix Pixel (Picture Element) H.U. = [µ - µ(water)/µ(water]*1000 focal spot
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
Linear Attenuation Coefficients (60 kev) Tissue Brain - Grey Brain - White Cerebro-Spinal Fluid (CSF) Pancreas Liver Water Fat µ (χµ 1 ).213.215.208 -.213.215.221.205.190
Grey - White Matter Contrast C = (.215 -.213)cm -1 * 1.0 cm =.2 %! CT Number µ(cm -1 ) 39.213 49.215 CT number allows the computer to present the information with a larger grey scale
Variation of Linear Attenuation Coefficients with Energy 0.22 0.2 Water Fat µ 0.18 0.16 0.14 60 70 80 Energy (kev)
Variation of H.U. with Energy Energy (kev) 60 70 80 µ(water).205.193.184 µ(fat).190.179.171 H.U.(fat) -73-73 -73
Image Display µ µ HU( µ ) =1000 µ water water 0 255 CT Number - Hounsfield unit Air: -1024 Water: 0 Bone: +175 to +3071 Viewing Parameters Window level (L) Window width(w) Zoom factor -1024 W +3071 L
Computed Tomography Principles 1. Projection measurement 2. Scanning modes 3. Scanner systems
CT Scanner
Data Acquisition System (DAS)
Data Acquisition System (DAS)
First Generation One detector Translation-rotation 5 minutes/per slice Parallel-beam 160 rays x 180 views
First Generation CT Scanner From Webb, Physics of Medical Imaging
Second Generation Multiple detectors Translation-rotation Small fan-beam 600 rays x 540 views 18 seconds/per slice
Second Generation CT Scanner From Webb, Physics of Medical Imaging
Third Generation Multiple detectors Translation-rotation Large fan-beam 800 rays x 1000 views <1 seconds/per slice
Ring Artifact In 3 rd Generation
Fourth Generation Detector ring Source-rotation Large fan-beam
Fourth Generation Detector fan eliminates ring artifacts.
3 rd and 4 th Generation Scanners From Webb, Physics of Medical Imaging
Krestel- Imaging Systems for Medical Diagnosis
Third & Fourth Generations
Fifth Generations Electron-beam CT for cardiac imaging.
Electron Beam CT
Sixth Generation: Spiral/Helical/Volumetric CT Continuous & Simultaneous : Source rotation Patient translation Data acquisition
Volume Scanning Scan-Translate Patient, Scan-Translate Patient, Continuous Scan, Continuous Patient Translation
Important years in helical CT history Single-slice 1989 Dual-slice 1992 Quad-slice 1998
Single-Slice 8 times faster than single-slice One rotation / sec Quad-Slice Two rotations / sec + 4 slices / rotation
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
Pitch table travel (mm) per gantry rotation Pitch = (1) beam collimation (mm) Information about table travel relative to beam collimation
Pitch = 1 > 1
Volumetric scanning Viewing Volume
Reformatting z y x Reformating multiple slices into a volume produces a volume image with unequal spatial resolution in x, y, and z.
Reformatting with Interpolation Stacked Slices (original images) Interpolated Slices (smoother display)
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