Monday, May 1, 2023 1CT PHYSICS - II
CT PHYSICS – II Dr. Archana Koshy
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OVERVIEW 1. IMAGE RECONSTRUCTION
2. HOUNDSFIELD UNITS
3. IMAGE QUALITY
4. MULTIPLE SLICE COMPUTED TOMOGRAPHY
SPECIAL APPLICATIONS OF CT :
4. 3D CT IMAGING 5. CT FLUOSCOPY
6. CARDIAC CT
7.CT ANGIO
8. CT BONE DENSITOMETRY
9. ARTEFACTS
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IMAGE RECONSTRUCTION
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(i) Correction for the heterochromatic nature of the beam (ii)Weighing factor to compensate the differences between
the size and shape of the scanning beam and picture matrix .
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ALGORITHMS FOR IMAGE RECONSTRUCTION
• For the purpose of reconstruction, complex mathematical equations called algorithms are required.
• The following three methods will be discussed :
(i) Back projection
(ii)Iterative methods
(iii)Analytical methods
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BACK PROJECTION • Also known as Summation method ; one of the oldest
methods .
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ITERATIVE METHODS Depending on whether the correction sequence involves the whole matrix , one ray or a single point .
-Simultaneous reconstruction –
• All projections for the entire matrix are calculated at the beginning of the iteration.
• All corrections are made simultaneously for each iteration.
-Ray by ray correction-
• One ray sum is calculated and corrected and these are incorporated into the future ray sums .
-Point by point correction -
• Calculations and corrections are made for all rays passing through one point .
• These corrections are used in ensuing calculations m again with the process being repeated for every point .
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ANALYTICAL METHODS
• Differ from iterative methods in that exact formulas are utilized for the analytical reconstruction .
• 2 most popular methods :
A) Filtered back projection
B) Two dimensional Fourier analysis – Any function of time or space can be represented by the sum of various frequencies and various amplitudes of sine and cosine waves.
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FILTERED BACK PROJECTION
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CT NUMBERS • Linear transformation of the original linear
attenuation coefficient measurement of each pixel .
• CT NUMBER = K (µp- µw) µw• The original EMI scanner used a magnification
constant of 500 and the values ranged from -500 ( air ) to + 500 ( dense bone ) .
• In contemporary CT untis , these values are arranged on a scale from -1024 HU to +3071 HU. CT PHYSICS - II
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IMAGE QUALITY
QUANTUM MOTTLE ( NOISE )
• Variation in the number of X-ray photons absorbed by the detector .
• The only way to decrease noise is to increase the number of photons absorbed by the detector ; thereby increasing X-ray dose to the patient .
• In order to avoid statistical fluctuation , iterative logarithms were employed , noise would be smoothed out and the image would appear quite homogenous .
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• The image on the left has a higher degree of image noise, measured as a standard deviation in the range of HU of 48.
• On the right, the image has a more homogeneous appearance; less noise, which is measured as 9.4 HU.
• Noise also affects spatial resolution.
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SPATIAL RESOLUTION
• Ability of the Ct scanner to display separate images of two images placed close together .
• Determined by -Scanner design -Computer reconstruction -Design • Expressed in line pairs/ cm .
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CONTRAST RESOLUTION
• Defined as the ability of an imaging system to display the image of a relatively large object , which only slightly different in density from the surroundings .
• Low contrast visibility is determined by noise .
• Contemporary CT scanners can display objects about 3 mm in diameter with density difference of 0.5 % or less .
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MULTIPLE SLICE COMPUTED TOMOGRAPHY • Conventional CT scanners use a single row of detector
elements and acquire a single slice per rotation .
• MDCT uses multiple rows and can therefore take multiple slices per rotation .
• Speed of gantry rotation is increased resulting in an overall increase in scan speed .
• Allows larger volumes to be scanned in the same time .
• Functions both in high speed and high resolution mode promises to improve performance of spiral CT dramatically .
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DETECTOR DESIGN
• More than 30000 detector elements are placed in a 2D array .
• Two approaches to the detector design :
a) Fixed matrix detector
b) Adaptive array detector
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IMAGE RECONSTRUCTION
• In multi slice CT , the outer most rays of the X-ray beam are tilted with respect to the imaging plane by the “cone angle “.
• The artefact level depends on the ratio of the cone angle and slice collimation .
• If higher number of detector rows are activated , visible artefacts appear .
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SPECIAL APPLICATIONS OF
CT IMAGING
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• Transverse images are stacked to form a 3 D data set ,which can be rendered as an image .
• Mostly used in Multispiral CT
• The algorithms include –
(i) Maximum Intensity projection (MIP)
(ii) Minimum Intensity Projection ( MinIP)
(iii)Surface shaded display volume rendering technique.
MULTIPLANAR RECONSTRUCTION
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CT FLUOROSCOPY • Computed tomographic fluoroscopy is a technical
advance resulting from slip-ring technology, x-ray tubes with improved heat capacity, high-speed array processors, and partial reconstruction algorithms .
• The images can be reconstructed at a rate of approximately 6 frames per second, allowing near real-time visualization similar to that of ultrasonography.
• Safe and effective guidance tool for percutaneous interventional procedures .
• An additional concern is the scattered radiation to the consulting doctors .
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CARDIAC COMPUTED TOMOGRAPHY
• Synchronisation of the data acquisition to the cardiac cycle and fast speed of the data acquisition to freeze the motion of the heart .
• Achieved by ECG gating • A particular ECG signal triggers the initiation of the CT
scan • Or Data acquired is reconstructed relative to a
selected cardiac phase .
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CT ANGIO
• Utilises the principle of using narrow collimation to scan the region and reconstructing both thin and thick slices .
• Superior quality tomographic images .
• The high resolution slices allow luminal views of the vessel . (“fly through”)
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DUAL ENERGY CT • Relatively new technique which uses two different x-
ray tubes in a single CT unit.• Additional applications include tissue differentiation
and visualization of tendons and ligaments.
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• Dual Source CT uses two rotating tubes to acquire both high and low voltage images.
• Since the images are dependent on the attenuation of the x-ray beam, which depends on the voltage applied across the tube, each image acquired is energy dependent.
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APPLICATIONS OF DUAL CT • Angiography
• Renal calculi differentiation by determining the specific properties of the calculus .
• Plaque distribution in the vessels which have calcified can be viewed to diagnose atherosclerosis.
• Differentiation of thick ligaments and tendons.
• Ventilation and perfusion images
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QUANTITATIVE COMPUTED TOMOGRAPHY
• CT numbers or x-ray attenuation of a tissue is properly referenced to a calibration standard and then used to quantify some property of the tissue.
• Measures trabecular bone and is highly sensitive to changes in skeletal density.
• Both single energy and dual energy QCT can be used.
• QCT scanning done with dual energy eliminated the effect of marrow fat ; increased accuracy .
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ARTEFACTS
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MOTION ARTEFACTS • The reconstructed image will
display the object as a streak in the drection of motion .
• Motion of objects that have densities much different from their surroundings produces more intense artefacts .
• The intensity of the streak artefact will depend on the density of the object in motion .
• Motion of metallic or gas containing structures produce striking artefacts .
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STREAK ARTEFACTS • At every position , each detector will absorb some
transmitted radiation . • If a high density material severely reduces the
transmission , the detector may not record any image .• Hence streaks appear in the image
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BEAM HARDENING ARTEFACT • As a heterogenous xray beam passes through the
patient , the low energy protons are rapidly absorbed .
• Therefore the xray beam exiting the patient contains a lower percentage of energy photons .
• Reconstruction programs anticipate and correct the variation in linear attenuation co efficients , but are not precise .
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RING ARTEFACT
• Result of miscalibration of a detector .
• Records incorrect data in every position .
• The misinformation is reconstructed as a ring in the image .
• The radius of the ring determined by the position of the faulty detector .
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