Login with Facebook

Principle and parameters of Volumetric computed tomography

Computed tomography has become the standard thoracic imaging technique after chest radiography performed on the first line. Technological developments have made it possible to shorten the duration of image acquisition to a few seconds and to image the entire thorax in patients who cannot hold a long apnea. The purpose of this article is to present the fundamental principles of the formation of the CT image, the different embodiments, and the constraints of this examination.

Image formation

CT is used to measure the absorption of X-rays within an explored volume, using multiple incidences of an X-ray beam. The CT scanner, commonly called "scanner," consists of a rotating assembly composed of an X-ray tube facing rows of detectors, this assembly rotating around the patient. The number of rows (“bars”) of these detectors is increasing with each new generation of detectors. The digital information from the different angular incidences during the rotation is processed by a computer to define the value of the X-ray attenuation coefficient of each pixel of the axial image obtained (the axial plane being perpendicular to the head – feet axis) of the patient). The image matrix is ​​a table composed of n rows and n columns defining a number of elementary squares or pixels. The current matrices are most often in 512×512. Whatever the sectional imaging modality (CT, MRI, ultrasound, etc.), an image always has a certain thickness, and each pixel on the screen, therefore, represents a unit of an elementary volume called a voxel. In CT, each voxel in the reconstruction matrix corresponds to an attenuation coefficient.

 The attenuation coefficient (often called density) of a voxel is measured in Hounsfield units (UH) on a scale from −1000 for air to+3000, the 0 correspondings to the density of the water, +1000 being the density of the compact bone. The density of a tissue structure varies between 20 and 80 HU and a fatty structure between - 40 and - 120 HU. 

 The human eye distinguishing only 16 gray levels, the range of 2000 densities of the human body (from –1000 to +1000 HU) cannot be seen simultaneously on the screen. Among this wide range of densities, we define a window of densities that will be effectively translated into gray levels on the screen. A window is defined by its width (W for width), and it's level or center (C).

For the study of the mediastinum, where the structures have close densities (tissue: 20 to 80 HU; pure liquid structures: 0 to 20 HU; fatty structures: −120 to −40 HU), a "tight" window of width w=400 HU, and centered by the average density of the mediastinal structures C=0 HU). The densities shown range from −200 to +200 UH. 

•X-ray computed tomography is a radiological imaging technique for obtaining axial sections.

•"Strips" are the number of rows of detectors that pick up the residual X-ray beam.

•The pixel is the elementary square at the origin of the formation of the image.

•The voxel is the unit of volume corresponding to a pixel.

•The attenuation coefficient (or density) of a voxel is measured in Hounsfield units (UH).

•The size of the window used depends on the density of the tissue studied.

Volume acquisition

Until 1989, only the sequential acquisition existed, a single axial cut (perpendicular to the patient's axis) is acquired with each rotation. Then the examination table advances and stops again for the realization of the cut next. With this mode, in an apnea, only a few cuts are made, and several apneas are necessary to cover the chest. There is the spacing between each of these cuts (example: 1 cut of 1mm thick every 10mm).

The volume acquisition was developed in the early 1990s, the principle is the displacement of the examination table, during the continuous rotation of the tube – detector pair. The tube is describing a helix shape around the patient; this type of acquisition is also called "helical." It is the most commonly used acquisition mode today, and it allows you to image the entire chest in less than 15 seconds on most current machines and requires only one apnea.

Image quality

The quality of an image depends on the spatial resolution, the contrast resolution, and the possible presence of artifacts.

The resolution, in contrast, is the ability to differentiate two structures whose density is close (low natural contrast, which is the case in the mediastinum). The contrast resolution is altered by image noise, which visually translates into a granularity on the image. This noise disturbs much more the visualization of structures with low natural contrast as in the mediastinum than the visualization of the lung. Noise can be measured on an image by the standard deviation of the densities measured within a homogeneous structure. This noise increases with:

•The decrease in the dose of X-rays, hence sometimes a discreet granularity on a thoracic scanner "low dose of irradiation," carried out within the framework of systematic screening.

•Reducing the reconstructed cutting thickness. Therefore, in addition to the series of fine images of the pulmonary parenchyma, we reconstruct a series of thicker images for the mediastinum, less noisy.

•The image reconstruction filter: The mediastinal series is reconstructed with a high-resolution contrast filter, which reduces noise (smoothed image). As a corollary, the spatial resolution decreases, and even by applying a parenchymatous windowing to the mediastinal series. The pulmonary structures will appear less precisely than on the pulmonary series with a high spatial resolution filter.

Spatial resolution is the distance below which CT can no longer separate two points. The spatial resolution is defined by the size of the voxel. In high resolution "standard" mode, the voxel size is 0.625×0.625mm in the axial plane, the third dimension of the voxel being the reconstructed cutting thickness (0.7 to 1mm for fine analysis of the pulmonary parenchyma).

It depends on factors intrinsic to the machine, but also on selected factors:

•The thickness of the reconstructed image: the finer the cut, the smaller the thickness of the voxel. Reconstructed cutting thickness of 3mm or more is insufficient to analyze invasive pneumonia;

•The size of the matrix: most often 512×512. Very high spatial resolution with a voxel size of 0.3–0.4mm can be obtained using a 768 2 matrix, but it is exceptionally useful in thoracic imaging and requires a large capacity for storing digital data;

•The reconstruction filter: the choice of a high spatial resolution filter increases the visibility of details while increasing noise. The (moderate) increase in noise is not a problem for viewing the pulmonary parenchyma, which has high natural contrast. On the other hand, even by applying mediastinal windowing to the series of parenchymal images, the noise will make the mediastinal structures much less visible;

•The reconstruction field the width of the thorax.

•The dose of X-rays.

•Volume (or helical) acquisition replaced sequential acquisition.

•The quality of the image depends on the spatial resolution, the contrast resolution, and any artifacts.

•Spatial resolution is the distance below which CT can no longer separate two points.

•The resolution, in contrast, is the ability to differentiate two structures whose density is close.

•Noise is an alteration of the image, which visually translates into a granularity.

Conclusion

The CT allows us to know the density of each voxel of the patient. The first step is always the reconstruction of transverse axial images, the number of which now reaches several hundred for the study of the thorax alone. Only a selection of these images is usually reprographic on film, and increasingly on paper. These digital images that can be viewed on-screen can have several supports, CD-ROM and DVD, and Picture Archiving and Communicating System (PACS), allowing the precise comparison of a patient's scanners, and to judge the evolution of his pathology. This digital data of axial images called "native" can be processed by a computer to improve the visualization of certain signscomputed tomography, but also to quantify the extension of certain pathologies.


Comments





Send Comment

Need a custom
essay?

We will write it for you.
Order now

Free Essay Examples

Free essays:

Asian vs. western leadership styles
All you need to know about the epidemiology
Applications of BIOMEMS
All you need to know about metabolic pathways
Broadcasting: How to Be Good In Front of the Camera
Bioconjugates for the Drugs Delivery
Big data analysis in evolutionary biology
Brain-Computer Interface systems with EEG signals
Current challenges to the health of women
Current challenges to the health of Adolescence
Competitive strategies for modern businesses
Computed Tomography (CT) images and their reconstruction
Doppler ultrasound or simply eco Doppler
DNA extraction
Dynamic Systems Modeling with Machine Learning
Digital Imaging: What Is It?
Everything you need to know about the capital market
Entrepreneurship: Conceptual Framework for Evaluating Technology
Entrepreneurship: Opportunity and Design Thinking
Everything you need to know about enzymatic kinetics
Factors about The Savings Investment Process
Financial intermediaries and its Types
Fascias: all about fibrous membranes
Funding sources for non-profit organizations
Genetic regulation networks: modeling in biology
How to make your business survive at economic crisis
How to choose the topic of the senior capstone project
Healthcare technologies that help patients with better self-management
Introduction of Microfabrication techniques
Introduction to Writing a TV Script
Introduction to Journalism
Importance of modern trade policy
Journalism: What is News?
Journalism: Broadcast media and Television Presenters
Journalism: Sources of News
Journalism and Law
Kinetic models in biology and Related fields
Know about the different forms of traditional African dances
Know about the history of science
Latest technology trends
Microfluidics and microfluidic chips
Molecular Imaging
Modern conservatism in America
Modeling and simulation of genetic regulatory networks
Nanomedicines to target tumors
Nonlinear Dynamics in Biology & Biomedicine
NLS (Nonlinear System) - Revolutionary system in diagnostic medicine and therapy
Neural networks: what they are and what they are for
Overcoming Hiring Challenges for Nonprofit Organization
Principle of Optical coherence tomography (OCT)
Politics and Religion: Relations and Conflict Issues
Principle and parameters of Volumetric computed tomography
Role of technology in the modern business environment
Role of Homeostasis in Human Physiology
Role of a digital artist
Role of gender studies
Skills in Journalism: Effective Interview
Systems biology - An understanding the functioning of biological systems
Self-assembly for biological systems
School of American Ballet
The Structure of Interest Rates and the Yield Curve
The New Media: Emerging Trends
The technical basis of optical coherence imaging
The Principles of MRI Contrast agents
Ultrasound: physical principles and image formation
Understanding social entrepreneurship
Understand the importance of investigative journalism
Urban planning in the US
Various theoretical perspectives of sociology
Virtual reality, what it is and how it works
What is the purpose of deep brain stimulation?
What is transition elastography?
4 Facts about Origin of Mathematics!
5 techniques to create an animation
13 Best colleges for political science in the world
3d Model Of Building
All you need to know about the ACA Code of ethics
Architecture and Democracy: An Introduction
Architecture and Democracy: Democratic Values
Architecture and Democracy: Democratic Procedures
All You Need to Know About a Synthesis Essay
An essential guide to understanding Film Theory
Application of Artificial Intelligence in Cyber Security
Applications of electrical engineering
Augmented reality: what it is, how it works, examples
Advantages And Disadvantages Of Social Networking
All you need to know about Cryptography
Applications of astrophysical science
All you need to know about architecture engineering
Applications of geological engineering
Artificial intelligence and medicine: an increasingly close relationship
An insight into Computational Biology
ACA code of conduct
A Rose for Emily
Applications of Mathematics in daily life
Architecture mistakes to avoid
All you need to know about Toxicology
All you need to know about Holistic Medicine
All you need to know about linguistics
An introduction to Linguistics and its subfields
All you need to know about Anxiety disorder
All you need to know about Drones
A Brief Insight into Political Science
Assumptions related to feminism
All you need to know about Byzantine emperors
All you need to know about labour economics
An insight into xenobots -the first-ever robots
An ultimate guide about Biomaterials
A Comprehensive Introduction to the Mona Lisa
Analysis methods of Transport through biological membranes
An ultimate guide about biochemical reactions
Analysis of brain signals
Artificial Gene Synthesis
Application to synthetic biology of CAD methods
All you need to know about metabolic pathways
Applications of BIOMEMS
All you need to know about the epidemiology
Asian vs. western leadership styles
How to Write a Personal Essay
Housing Needs in America
How to Write a Description Essay
How to Create an Excellent Scholarship Essay?
How to write a cause and effect essay
How to Hire the Best Essay Writing Service Provider?
How to Write a College Application Essay?
How to get the most out of your English lectures
How to write Expository Essay
How to succeed in your psychology class?
How to Write an Academic Essay in the Shortest Time?
History of Journalism
How Different Sectors are Using Artificial intelligence (AI)
How to write an informative essay
How to deliver persuasive essays?
How to Give a Convincing Presentation
How to write an essay on leadership?
Historical Art Still Around Today
Humanoid robot: what it is, how it works and price
History of Chemistry
Healthcare Advanced Computer Power: Robotics, Medical Imaging, and More
Healthcare AI: Game Changers for Medical Decision-Making and Remote Patient Monitoring
How to understand different types of English
How to Cope with Chronic Pain
How African American choreographers and dancers have influenced American dance
How mobile robot can do in logistics or in production
How To Become a Successful Entrepreneur
History of the Philosophy of Feminism
How is the climate changing?
How to Track Your Content Marketing ROI
How to Gun control In the USA?
Historical and contemporary role of labour in the modern world
How breast cancers are classified?
How the cells of our body communicate?
How the Lymphatic System Works?
How Digestive System Works
How to complete your capstone projects effectively?
How to write a research project
Healthcare technologies that help patients with better self-management
How to choose the topic of the senior capstone project
How to make your business survive at economic crisis
The Looming Energy Crisis in America
Top benefits of performance-based engineering
The More Languages You Know, The More Times You Are a Man
Things to consider while writing an Argumentative Essay
Top Ways to Improve Your Academic Writing Skills
Tips to Excel in Creative Writing
The origins of films in the early 19th century
Top career options in Architecture
The Elevator Pitch
Top finance trends 2020
The basic Structure and functionality of robots
The Way to Success
The election system of the President in the United States of America
Two-party System in United States of America
Top trends in urban design
The history and theory of African American filmmaking
Top benefits of creative writing
Tinnitus Guide: Common Symptoms and Treatment Options
The language of dance
The digital image processing management
Top famous politicians of the World
Top methods of political science!
The history of the feminist movement
The blood flow in cardiovascular system - Biofluid Mechanics
The best of Leonardo Da Vinci
The Structure and Function of Macromolecules
The structure of cell: a research on the bricks of the human body!
Tissue and organ construction: Adhesion and recognition between cells
The kinetics of the transformation processes
The Modeling of Biological Systems
Tips for writing a great thesis statement
The Defense mechanisms against infections
The impact of the technological innovations in medicine
Top journalism trends to know about
The relation between mass media & politics
Theranostics: Diagnosis and Care through Nanoparticles
The practical Applications of X-rays
The applications of Ultrasound in medicine
Transfer mechanisms of genetic information in Bacteria
The regulation of cellular metabolism in the diagnosis
The Principles of MRI Contrast agents
The technical basis of optical coherence imaging
The New Media: Emerging Trends
The Structure of Interest Rates and the Yield Curve
What is a Definition Essay?
What are diagnostic essays?
What is the relation between art structural engineering?
What is a Narrative Essay
What are robotics and intelligence systems?
What are the benefits of studying health sciences?
What is artificial intelligence and why it matters?
What is comparative Literature?
Why study neuroscience
What is Wi-Fi and how does it works
What is French history famous for?
What are Humanistic Studies?
What is covered in Biophysics?
What is modern journalism?
What is Virtualization? Benefits & Applications
What are modern public relations?
What is plasma physics?
What is teacher preparation?
What is rapid prototyping for 3D printing?
What is contemporary European Politics?
Why should you learn American Ballet?
What is engineering physics?
What is the purpose of African American Literature?
Ways to learn the Rhythm
What is digital art used for?
What are Enzymes and how do they work
Who is the father of political science?
Why Study Political Science - Job?
What is the Philosophy of Feminism?
What is a quantum computer?
Ways B2B Startups Streamline Their Conversion Strategies
Why do biomedical signals need processing?
What are the long term effects of climate change?
Why study labour relations
What is Holoprosencephaly?
What is antisocial disorder?
What are the important principles of evolution?
What is the cytoplasm and its function?
What is biopolymers?
What Makes a Good Leader
Women empowerment in modern generation
What is the history of political thought?
What is Gene recombination
What is synthetic biology
What is business cost analysis?
What is Inflation
What are the consequences of unemployment?
What is lithotripsy and its types?
What is transition elastography?
What is the purpose of deep brain stimulation?