Computed Tomography: Physical Principles, Clinical Applications, and Quality Control

Purpose: Composition of the coronary artery plaque is known to have critical role in heart attack. While calcified plaque can easily be diagnosed by conventional CT, it fails to distinguish between fibrous and lipid rich plaques. In the present paper, the authors discuss the experimental techniques and obtain a numerical algorithm by which the electron density (rho(e)) and the effective atomic number (Z(eff)) can be obtained from the dual energy computed tomography (DECT) data. The idea is to use this inversion method to characterize and distinguish between the lipid and fibrous coronary artery plaques. Methods: For the purpose of calibration of the CT machine, the authors prepare aqueous samples whose calculated values of (rho(e), Z(eff)) lie in the range of (2.65 x 10(23) <= rho(e) <= 3.64 x 10(23)/cm(3)) and (6.80 <= Z(eff) <= 8.90). The authors fill the phantom with these known samples and experimentally determine HU(V-1) and HU(V-2), with V-1,V-2 = 100 and 140 kVp, for the same pixels and thus determine the coefficients of inversion that allow us to determine (rho(e), Z(eff)) from the DECT data. The HU(100) and HU(140) for the coronary artery plaque are obtained by filling the channel of the coronary artery with a viscous solution of methyl cellulose in water, containing 2% contrast. These (rho(e), Z(eff)) values of the coronary artery plaque are used for their characterization on the basis of theoretical models of atomic compositions of the plaque materials. These results are compared with histopathological report. Results: The authors find that the calibration gives Pc with an accuracy of 3.5% while Z(eff) is found within 1% of the actual value, the confidence being 95%. The HU(100) and HU(140) are found to be considerably different for the same plaque at the same position and there is a linear trend between these two HU values. It is noted that pure lipid type plaques are practically nonexistent, and microcalcification, as observed in histopathology, has to be taken into account to explain the nature of the observed (rho(e), Z(eff)) data. This also enables us to judge the composition of the plaque in terms of basic model which considers the plaque to be composed of fibres, lipids, and microcalcification. Conclusions: This simple and reliable method has the potential as an effective modality to investigate the composition of noncalcified coronary artery plaques and thus help in their characterization. In this inversion method, (rho(e), Z(eff)) of the scanned sample can be found by eliminating the effects of the CT machine and also by ensuring that the determination of the two unknowns (rho(e), Z(eff)) does not interfere with each other and the nature of the plaque can be identified in terms of a three component model. (C) 2015 American Association of Physicists in Medicine.

DECT evaluation of noncalc fied coronary artery plaque

Non contrast Computed tomography of brain scanning is the primary line for exigency evaluation of acute stroke due to its common availability, speed, low cost and accuracy in detecting hemorrhage and cerebral infarct. CT has the ability to quantify the beam attenuation, hence the measurements are expressed in Hounsfield units (HU), which indicates the HU values of various stages of infarct pathologies associated with the brain. This study is based on HU in CT Brain images. First, the scan report of the patient is checked for cerebral infarct and affected location is noted along with the type of infarct. Then the CT images are verified by selecting an axial section in which the infarct is properly visualized and 3 region of interest (ROI) is placed within the infarct for a better average value which enables to get a range of HU value for each type of cerebral infarct which will be evaluated and graded according to the variation seen in the readings. According to this study HU value for diverse sorts of infarct varies. Three set of values obtained in the present study. As per this study the HU for acute infarct is >19.13 HU, Sub-acute infarct 9.55 – 19.13 HU and chronic infarct is < 9.55 HU helps to grade the cerebral infarct which make the diagnosis easier & quicker and it’s useful to the patient those who are not co-operated with MRI.

Grading of cerebral infarction using CT-hounsfield unit to report the hounsfield unit in acute, subacute and chronic stroke

Introduction: The performance of computed tomography is routinely checked using phantoms, which are known as important diagnostic imaging tools. Depending upon the aim of the study, different phantoms are designed, while trying to satisfy certain levels of diversities in their application. Material and Methods: The present study describes the construction of an inexpensive phantom designed for simultaneous measurements of 12 different samples. The body of the phantom, test tube holders, and test tubes were made of materials of low attenuation coefficient. Body of the phantom was filled with water. Test tubes filled with solutions of known chemical compositions were mounted on the test tube holders.  The whole phantom was scanned at 80, 100, 120, 140 kVp to evaluate the performance of the CT system. Using Hounsfield Unit (HU) data from these liquid samples of known electron density, the phantom was calibrated for electron density measurements. Results: The system's accuracy and reproducibility were verified by measuring the HU values for some known materials. According to the results obtained from the experimental datawith liquid samples, the accuracy of the water and noise was within ±3.2 HU and 0.6%, respectively. Moreover, the image uniformity error was less than ±2 HU, and CT system's linearity for calibration was estimated with 99.9% confidence. Conclusion: The present system gives satisfactory results with known samples and can be used with confidence for characterizing unknown materials.

http://ijmp.mums.ac.ir/article_13180_8ecc024d05f158a76e24629081b3ff73.pdf

Design and construction of an affordable phantom for electron density measurement and linearity tests of computed tomography systems

Introduction: In dual-energy computed tomography (DECT), the Hounsfield values of a substance measured at two different energies are the basic data for finding the chemical properties of a substance. The trends of Hounsfield unit (HU) alterations following the changes in energy are different between the materials with high and low Zeff. The present study aimed to analyze the basic principles related to the attenuation coefficient of x-ray photons and a quantitative explanation is given for the mentioned behavior or trend. Material and Methods: A mathematical expression was derived for the HU difference between two different scanner voltages. Attenuation coefficients of diverse substances, such as methanol, glycerol, acetic acid, the aqueous solution of potassium hydroxide, and water were calculated for x-ray scanners operating differently at distinct applied voltages and with diverse inherent or added filters. Results: Findings of the current study demonstrated that the negative or positive outcome of HU(V1) - HU(V2) equation is not determined by the electron density of a substance. However, it is affected by the effective atomic number (Zeff) of the material and machine parameters specified by the source spectrum. Conclusion: According to our results, the sign of HU difference [HU(V1) – HU(V2)] for the variable cases of V2 and V1 gives an indication of the effective atomic number of the material under study. The obtained results might be of diagnostic value in the DECT technique.

http://ijmp.mums.ac.ir/article_14340_92df18cba983745d30cd5952c2720ba3.pdf

Sabysachi Chatterjee

Papers

Design and construction of an affordable phantom for electron density measurement and linearity tests of computed tomography systems

Reversal Trend of Hounsfield Unit Values of Substances with High and Low Effective Atomic Numbers