9. In CT a cross- sectional layer of the body is divided in
to many tiny blocks.
Each block is assigned a number proportional to the
degree the block attenuated the x-ray beam.
“Voxels and Pixels”.
Their composition & thickness along with the quality of
the beam - linear attenuation coefficient (µ).
Image
reconstructi
on
10. Pixel ( 2D ) - Each cell of information
Numerical information contained in each
pixel = CT no. / Hounsfield Unit.
Size – determined by computer program.
Voxel ( 3D) – tissue volume
Product of pixel size & thickness of CT scan
slice.
Size – determined by width of x-ray beam.
11.
12. Image display
A Ct image is usually displayed on a television monitor for immediateviewing
and recordedon a filmfor interpretation.
The displaymatrix was on average 512x512 some have1024x1024.
Had 256 shades of gray – should range in CT no. +1000 to -1000.
14. The center CT number is called Windowlevel.
The range of CT number above and below are calledwindowwidth.
15. Motion artifacts
Patient motionhas devastating effects on image quality.
This is prevented by developing faster scanUnits .
The reconstructedimage will display an object in motionas a streak in the direction
of motion.
Artifacts
17. Beam hardening
artifactsHeterogeneous x-ray beampasses through the patient – low energy
photons are absorbed.
X-raybeamexiting the pt has lower energy of photons than the beam
entering the pt. – beamhardening effect.
18. Ring artifact
Result of miscalibrationof one detector.
It will record incorrect data in every projection.
This misinformation is reconstructedas a ring in the image.
Radius of ringdetermines the
positionof the faultydetector.
A collimated x-ray beam is directed on the pt, and the attenuated remnant radiation is measured by a detector whose response is transmitted to computer. The computer analyzes the signal from the detector, reconstructs the image and displays the image on television monitor. This image can be photographed for further evaluation.
Reconstruction matrix. Hounsfield envisioned scanned slice as being composed of matrix of small boxes of tissue called voxels, each with attenuation coefficient μ. x-Ray transmission measurements (Ni) can be expressed as sum of attenuation values occurring in voxels along path of ray for Ni.
8 CT no.s are assign to same shade of gray.
Reconstructed image has less dark in center than periphery.
The spiral-CT technique was demonstrated successfully on a prototype by Kalender in year 1989.
Slice thickness:single detector array scanners
•Determined by the physical collimation of the incident x-ray beam with two lead jaws
Slice thickness:MDA scanners
•In axial scanning (i.e., with no table movement) where, for example, four detector arrays are used, the width of the two center detector arrays almost completely dictates the thickness of the slices
•For the two slices at the edges of the scan, the inner side of the slice is determined by the edge of the detector, but the outer edge is determined either by the outer edge of the detector or by the collimator penumbra, depending on collimator adjustment
Carlos
Figure 4: Plain and contrast enhanced CT scan. (a) Hyperdense midline vein of Galen and enlarged straight sinus noted. (b) Intense enhancement of the malformation is seen on contrast administration
