briefly describes data acquisition, image digitization and image reconstruction in CT imaging system.

1. IMAGE RECONSTRUCTION IN
COMPUTED TOMOGRAPHY
Prepared by: Manoj Bhatta
B.Sc.MIT, 1st batch
Gandaki Medical CollegeTHRC

2. This presentation includes;
• Data acquisition and modes of data acquisition
• Digitizing an image
• Pixel and voxel
• CT numbers
• Image reconstruction and reconstruction algorithms

3. Data acquisition in CT
•A method by which the patient is scanned, and obtains
enough data for reconstruction.

4. Modes of data acquisition
1)Scanned projection
2)Basic axial acquisition
3)Helical (spiral) acquition

5. Digitizing an image
• The primary objective of digitizing an image is to convert an analog image
into numerical data for the processing by the computer.
• It includes following steps:
I. Scanning
II. Sampling
III. quantization

6. Pixels &Voxels
pixel voxel

7. CT number
• Each pixel is displayed on the monitor as a level of brightness which
correspond to a range of CT numbers from -1000 to +3000.
• CT number = 𝑘
𝜇 𝑡−𝜇 𝜈
𝜇 𝑤
where,
• µt = attenuation coefficient of the tissue in the voxel under analysis
• µw = attenuation coefficient of water
• ‘k’ is a constant that determines the scale factor for the range of CT numbers
• When k is 1000, the CT numbers are called Hounsfield Units and range from -
1000 to +1000

8. CT numbers

9. Image reconstruction
•The process of using raw data to create an image is
called reconstruction.
•Image reconstruction is done with the combination of
complex computer algorithms, mathematical equations
and physics.

10. Reconstruction algorithms
• An algorithm is a finite set of unambiguous steps performed in a
prescribed sequence to solve a problem.
• In CT, reconstruction algorithms are used by the computer to
solve the many mathematical equations necessary for the
conversion of the information detector arrays to the information
suitable for image display.

11. Reconstruction algorithms
1. Backprojection: The process of converting the data from the
attenuation profile to a matrix is known as backprojection.

12. Reconstruction algorithms
2. Filtered backprojection:
• Simple backprojection technique results in characterstic star-like
artifact.
• To eliminate this artifact, a mathematical filter is used in scanned
data before backprojection, and hence k/a filtered backprojection.
• The process of applying a filter function to a scanned data is
called convolution.

13. Reconstruction algorithms
3. Fourier transformation:
• Developed by 17th century mathematician Baron Jean Baptiste Joseph
Fourier.
• The Fourier transformation is a mathematical procedure for breaking down
a waveform into a series of sine and cosine functions of different
frequencies and amplitude.

14. Reconstruction algorithms
4. Ierative reconstruction:
•An iterative reconstruction starts with an assumption
and compares this assumption with measured values,
makes corrections to bring the two into agreement, and
then repeats this process over and over until the
assumed and measured values are same or within
acceptable limits.