Scanning principle of ct by mohit BRIT 4TH SEMESTER
1. Scanning principle of CT
Mohit
BRIT 4th
Semester
2315693
Submitted to:- Mr Ashish Yadav
2. CONTENTS
1. BASIC CONCEPT OF CT
2. COMPONENTS OF A CT SCANNER
3. CT SCANNING PROCESS
4. DATA ACQUISITION
5. IMAGE RECONSTRUCTION
6. TYPES OF CT SCANNING TECHNIQUE
7. CT SCAN ARTIFACTS AND LIMITATIONS
3. BASIC CONCEPT OF CT SCANNING
Ct uses X-rays and detectors to obtain multiple projections from
different angles, which are then reconstructed into a detailed 3D
image. Unlike conventional X-ray imaging, which provides a 2D
image, CT provides slice-by-slice imaging of body.
4. COMPONENTS OF CT
I. X ray tube- emits x ray beam
II. Detectors- measures the intensify of x-rays passing through the patient
III. Gantry- houses the x-ray tube and detectors and rotate around the patient
IV. Patient table- moves the patient through the scanner
V. Computer system- processes the data and reconstructs images.
5. Ct scanning process
Step 1: X-ray Generation and Emission
The X-ray tube inside the gantry emits a narrow, fan-shaped or cone-
shaped X-ray beam.
The X-ray beam penetrates the patient’s body, and different tissues
absorb X-rays to varying degrees (attenuation).
Dense tissues (e.g., bone) absorb more X-rays appear white.
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6. Step 2: X-ray Detection
After passing through the body, the remaining X-rays are captured by detectors
positioned opposite to the X-ray source.
The detectors measure the intensity of transmitted X-rays, converting them into electrical
signals.
Step 3: Data Acquisition (Projection Data Collection)
The gantry continuously rotates 360° around the patient, collecting multiple projections
at different angles.
Each projections represents a line of X- r
7. Image Reconstruction
Types of image Reconstruction
1. Simple back projection
2. Iterative method
3. Analytic method
(a) Fourier transformation
(b) filtered back Projection
8. 1. SIMPLE BACK PROJECTION METHOD
∆ Also called as “summation method” or “linear superposition method” first used by O
(1961), Khul and Edwards (1963),
∆ Not used in commercial CT scanners.
∆ This involves “smearing back the projection across the image at the angle it was
acquired
∆ Rays from two or more projections are superimposed or back projected they produce a
chate reproduction of original object.
∆ Some produced images are “starred” and “blurred’ that makes it unsuitable for medical
13. Product demo
You can replace the image on
the screen with your own
work. Just right-click on it and
select “Replace image”
14. 3. Analytical method
Current commercial scanner uses this method
A mathematical technique known as Convolution or filtering is used.
Technique employs a spatial filter to remove blurring artifacts.
1. Filtered back projection
►Filtered back projection is also referred as Convolution method.
► The projection profile is filtered to remove the typical star like blurring that is
characteristic of simple back projection.
15. ► Filtering refers to altering the projection data before we do the back-projections.
►This type of filter picks up sharp edges within the projection (and thus, in the underlying slice) and
tends to ignore flat areas. Because the high pass filter actually creates negative pixels at the edges, it
subtracts out the extra smearing caused by back projection.
► The optimal way of eliminating the star like pattern is by use of Ramp filter.
► Ramp filter has the effect of filtering out low frequencies and passing high frequencies, with a
linear behaviour in between. Thus with this filter, contrasting features (high-frequencies) are
accentuated, while blurring (low-frequencies) is minimized.
► The combination of ramp filter and back projection is filtered back projection.
16. 2. Fourier Reconstruction Algorithms
► A property of Fourier transform.
► Relates the projection data in spatial location domain to spatial frequency domain.
► Used in MRI image reconstruction.
► Unlike filtered back projection, this algorithm does not use any filtering as
interpolation does the work of rearranging the image components in rectangular grid.
► Based on Fourier Slice Theorem.
18. 2 FOURIER TRANSFORM:-
►Developed by a mathematician Baron Jean-Baptiste-Joseph Fourier in 1807.
► Used in Radiology for image reconstruction in CT and MRI.
► Fourier transform is a “mathematical function that converts a signal in spatial domain to
a signal in frequency domain”.
∆Divides a waveform into series of sine and cosine functions of different frequency and
amplitude, which later can be separated
Fourier transform! Why?
► The image in frequency domain can be manipulated (edge enhancement or smoothing) by
changing amplitude of frequency components without losing the actual signal intensity.
► Computer can perform manipulations (digital image processing) i.e. MPR, VRT, MIP etc.
► Frequency information can be used to measure image quality through the point spread
function, line spread function and modulation transfer function.
20. ∆. Types of CT Scanning Techniques
1. Axial (Step-and-Shoot) Scanning
The patient table remains stationary while a single slice is captured.
The table then moves slightly, and the process repeats.
2. Helical (Spiral) Scanning
The patient table moves continuously while the gantry rotates.
Produces a continuous helical dataset, improving speed and resolution.
3. Multislice CT (MSCT)
Uses multiple detector rows to capture several slices per rotation.
Provides faster scanning and higher resolution.
4. Cone Beam CT (CBCT)
Uses a cone-shaped X-ray beam and a flat panel detector.
Often used in dental and orthopaedic imaging
