ct physics – ii

Monday, May 1, 2023 1CT PHYSICS - II

CT PHYSICS – II Dr. Archana Koshy

Monday, May 1, 2023CT PHYSICS - II 2

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


IMAGE RECONSTRUCTION


(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 .


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


BACK PROJECTION • Also known as Summation method ; one of the oldest

methods .


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 .


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.


FILTERED BACK PROJECTION

Monday, May 1, 2023 10

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


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 .


• 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.


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 .


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 .


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 .


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


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 .


SPECIAL APPLICATIONS OF

CT IMAGING


• 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


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 .


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 .


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”)


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.


• 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.


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


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 .


ARTEFACTS


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 .


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


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 .


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 .

ct physics – ii

Healthcare