CT : image formation and reconstruction

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Questions and Answers

What is the primary purpose of taking two perpendicular projections in radiology?

To reduce the exposure to X-ray radiation

To increase the accuracy of the CT scan

To visualize internal structures from different angles (correct)

To reduce X-ray transmission

What happens to the X-ray beam when it passes through an object?

It is amplified

Some of the photons are absorbed or scattered (correct)

It is redirected

It is reflected

What is the purpose of the X-ray detectors in a CT scanner?

To rotate the X-ray source

To produce X-rays

To collect and convert X-ray photons to electrical signals (correct)

To absorb X-rays

What happens to the electrical signals collected by the X-ray detectors?

They are converted into digital data Signup and view all the answers

What is the significance of the rotation of the X-ray tube and detectors around the patient?

It allows for the collection of many projections from consecutive angular orientations Signup and view all the answers

What is the ultimate goal of the data acquisition process in a CT scanner?

To reconstruct a 3D image of the internal structures Signup and view all the answers

What is the primary goal of CT scanning in a patient's body?

To obtain a spatially resolved map of absorption coefficients in one slice Signup and view all the answers

How many independent equations are required to solve for an arbitrary object composed of n-by-n different materials?

n^2 equations Signup and view all the answers

Which law is used to describe the attenuation of X-rays in CT scanning?

Lambert Beer's law Signup and view all the answers

What is the name of the procedure used in today's CT systems to reconstruct images from X-ray measurements?

Convolution-back projection Signup and view all the answers

What is the minimum number of projections required to solve for the absorption coefficients in a simple system with 4 unknowns?

4 projections Signup and view all the answers

What is the purpose of taking multiple projections at different angles in CT scanning?

To solve the linear equation system for the absorption coefficients Signup and view all the answers

What is the primary objective of Computed Tomography (CT)?

To obtain a spatially resolved map of absorption coefficients in one slice of the patient's body Signup and view all the answers

What is the result of the reconstruction of images from X-ray measurements?

A two-dimensional matrix of preselected size and detail Signup and view all the answers

What is the typical shape of the X-ray beam used in Computed Tomography (CT)?

A fan Signup and view all the answers

What is the unit of measurement used to represent the absorption coefficients of body tissues?

Hounsfield units Signup and view all the answers

What is the primary purpose of data acquisition in Computed Tomography (CT)?

To obtain X-ray measurements Signup and view all the answers

What is the representation of a single element of the two-dimensional matrix resulting from image reconstruction?

A voxel Signup and view all the answers

What is the primary reason for taking multiple projections at different angles in CT scanning?

To enable the reconstruction of a spatially resolved map of absorption coefficients Signup and view all the answers

What is the mathematical concept used to describe the relationship between the intensity of the X-ray beam and the absorption coefficients?

Lambert Beer's law Signup and view all the answers

What is the unit of measurement used to represent the absorption coefficients of body tissues in CT scans?

Hounsfield units Signup and view all the answers

What is the primary purpose of the convolution-back projection procedure in CT image reconstruction?

To enable the reconstruction of a 3D image from 2D projections Signup and view all the answers

What is the representation of a single element of the two-dimensional matrix resulting from image reconstruction in CT scanning?

A voxel Signup and view all the answers

What is the primary factor that affects the spatial resolution of a reconstructed CT image?

The sampling rate of the data acquisition system Signup and view all the answers

What is the primary challenge in solving the equation system for the absorption coefficients in CT scanning?

The large number of unknowns in the equation system Signup and view all the answers

What is the primary advantage of using the convolution-back projection procedure in CT image reconstruction?

It reduces the computational complexity of the reconstruction process Signup and view all the answers

What is the term used to describe the small volume of tissue represented by a pixel in a cross-sectional image?

Voxel Signup and view all the answers

What is the range of Hounsfield units (HU) used to express the density value of each pixel?

-1024 to +3071 HU Signup and view all the answers

What is the typical resolution of a modern tomographic axial image?

1024 × 1024 Signup and view all the answers

What is the purpose of calculating the attenuation measurements in a CT scanner?

To reconstruct the image Signup and view all the answers

What is the number of grayscale values that must be stored for each cross-sectional image with a resolution of 512 × 512?

Over 260,000 Signup and view all the answers

What is the primary imaging modality used in CT scanning?

Volumetric imaging Signup and view all the answers

What is the relationship between the density value of each pixel and the composition of the tissue it represents?

The density value depends on the composition of the tissue Signup and view all the answers

What is the purpose of dividing the selected 'field of view' into small image elements, called pixels?

To reconstruct the image Signup and view all the answers

What is the purpose of collimating the X-ray beam in Computed Tomography?

To shape the beam into a fan or cone Signup and view all the answers

What is the ultimate result of the reconstruction of images from X-ray measurements in CT?

A two-dimensional matrix of preselected size and detail Signup and view all the answers

What is the main goal of obtaining a spatially resolved map of absorption coefficients in CT?

To provide an image of the patient's body in one slice Signup and view all the answers

What type of measurements are obtained from the X-ray beam as it passes through the patient's body in CT?

Attenuation measurements Signup and view all the answers

What is the representation of the absorption coefficients of body tissues in CT imaging?

Hounsfield units Signup and view all the answers

What is the primary factor that determines the resolution of a CT image?

The sampling rate of the X-ray measurements Signup and view all the answers

What is the term for the smallest unit of a 3D image in CT?

voxel Signup and view all the answers

What is the process of reconstructing images from X-ray measurements in CT?

Convolution-back projection Signup and view all the answers

What is the main advantage of Computed Tomography (CT) over projection X-ray imaging?

Better soft tissue contrast Signup and view all the answers

What is the unit of measurement used to represent the absorption coefficients of body tissues in CT scans?

Hounsfield units Signup and view all the answers

What is the primary purpose of the convolution-back projection procedure in CT image reconstruction?

To reconstruct the image from X-ray measurements Signup and view all the answers

What is the representation of a single element of the three-dimensional matrix resulting from image reconstruction in CT scanning?

Voxel Signup and view all the answers

What is the primary factor that affects the spatial resolution of a reconstructed CT image?

Detector size Signup and view all the answers

What is the significance of the rotation of the X-ray tube and detectors around the patient in CT scanning?

To allow for multiple projections at different angles Signup and view all the answers

What is the primary challenge in solving the equation system for the absorption coefficients in CT scanning?

Computational complexity Signup and view all the answers

What is the ultimate goal of the data acquisition process in a CT scanner?

To measure the X-ray attenuation coefficients Signup and view all the answers

What is the primary factor that determines the reduction of X-ray transmission through an object?

Atomic composition of the crossed tissues Signup and view all the answers

What is the purpose of converting X-ray photons to electrical signals in a CT scanner?

To calculate the attenuation value Signup and view all the answers

What is the result of the X-ray beam passing through an object in a CT scanner?

The X-ray beam is partially attenuated Signup and view all the answers

What is the significance of taking multiple projections at different angles in CT scanning?

To reconstruct images of the body Signup and view all the answers

What is the purpose of the X-ray tube and detectors rotating around the patient in a CT scanner?

To collect many projections from consecutive angular orientations Signup and view all the answers

What is the primary purpose of data acquisition in Computed Tomography (CT)?

To collect X-ray transmission measurements Signup and view all the answers

What is the result of the conversion of X-ray photons to electrical signals in a CT scanner?

Digital data Signup and view all the answers

What determines the attenuation value of an object in a CT scanner?

All of the above Signup and view all the answers

What is the primary goal of obtaining a spatially resolved map of absorption coefficients in CT?

To reconstruct images from X-ray measurements Signup and view all the answers

What is the minimum number of independent equations required to solve for an arbitrary object composed of n-by-n different materials?

n^2 Signup and view all the answers

What is the unit of measurement used to express the density value of each pixel in a CT image?

Hounsfield units Signup and view all the answers

What is the process of reconstructing images from X-ray measurements in CT?

Convolution-back projection procedure Signup and view all the answers

What determines the attenuation value of an object in a CT scanner?

The absorption coefficients of the object Signup and view all the answers

What is the primary factor that affects the spatial resolution of a reconstructed CT image?

The size of the image elements (pixels) Signup and view all the answers

What is the term for the smallest unit of a 3D image in CT?

Voxel Signup and view all the answers

What is the purpose of calculating the attenuation measurements in a CT scanner?

To obtain a spatially resolved map of absorption coefficients Signup and view all the answers

What is the primary purpose of the convolution-back projection procedure in CT image reconstruction?

To reconstruct the image from the X-ray measurements Signup and view all the answers

What is the term used to describe the small volume of tissue represented by a pixel in a cross-sectional image?

Voxel Signup and view all the answers

What is the unit of measurement used to represent the absorption coefficients of body tissues in CT scans?

Hounsfield units (HU) Signup and view all the answers

What is the primary factor that affects the spatial resolution of a reconstructed CT image?

The number of projections taken Signup and view all the answers

What is the purpose of calculating the attenuation measurements in a CT scanner?

To calculate the absorption coefficients Signup and view all the answers

What is the result of the convolution-back projection procedure in CT image reconstruction?

A reconstructed image Signup and view all the answers

What is the term used to describe the process of reconstructing images from X-ray measurements in CT?

Image reconstruction Signup and view all the answers

What is the primary advantage of using the convolution-back projection procedure in CT image reconstruction?

It allows for accurate reconstruction of images Signup and view all the answers

What is the primary advantage of CT imaging over projection X-ray imaging?

Vastly exceeding soft tissue contrast Signup and view all the answers

Who is credited with the invention of CT imaging?

Sir Godfrey Hounsfield Signup and view all the answers

What is the typical unit of measurement used to represent the absorption coefficients of body tissues in CT scans?

Hounsfield units (HU) Signup and view all the answers

What is the result of the reconstruction of images from X-ray measurements in CT scanning?

A 3D absorber map Signup and view all the answers

What is the primary factor that affects the spatial resolution of a reconstructed CT image?

Number of projections taken Signup and view all the answers

What is the purpose of the convolution-back projection procedure in CT image reconstruction?

To reconstruct a 2D or 3D absorber map Signup and view all the answers

What is the representation of a single element of a 3D matrix resulting from image reconstruction in CT scanning?

A voxel Signup and view all the answers

What is the primary purpose of calculating attenuation measurements in a CT scanner?

To reconstruct a 2D or 3D absorber map Signup and view all the answers

What determines the reduction of X-ray transmission through an object?

The atomic composition of the crossed tissues Signup and view all the answers

What is the purpose of converting X-ray photons to electrical signals in a CT scanner?

To detect the X-ray photons Signup and view all the answers

What is the unit of measurement used to express the density value of each pixel in CT imaging?

Hounsfield units Signup and view all the answers

What is the result of the reconstruction of images from X-ray measurements in CT?

A 3D image Signup and view all the answers

What is the primary factor that affects the spatial resolution of a reconstructed CT image?

The number of X-ray projections Signup and view all the answers

What is the term used to describe the small volume of tissue represented by a pixel in a cross-sectional image?

Voxel Signup and view all the answers

What is the purpose of calculating the attenuation measurements in a CT scanner?

To determine the X-ray transmission through the object Signup and view all the answers

What is the representation of a single element of the three-dimensional matrix resulting from image reconstruction in CT scanning?

Voxel Signup and view all the answers

What is the typical number of voxels in a tomographic axial image?

1024 × 1024 Signup and view all the answers

What is the unit of measurement used to express the density value of each pixel?

Hounsfield units (HU) Signup and view all the answers

What is the primary factor that affects the resolution of a reconstructed CT image?

Number of voxels Signup and view all the answers

What is the range of Hounsfield units (HU)?

–1024 to +3071 Signup and view all the answers

What is the result of dividing the selected 'field of view' into small image elements?

Pixels Signup and view all the answers

What is the primary purpose of attenuation measurements in a CT scanner?

To calculate the density value of each pixel Signup and view all the answers

What is the relationship between the attenuation measurements and the composition of the tissue?

The attenuation measurements are directly proportional to the density of the tissue Signup and view all the answers

What is the representation of a small volume of tissue in a cross-sectional image?

Voxel Signup and view all the answers

Study Notes

Computed Tomography (CT)

A volumetric imaging modality based on X-ray absorption
Invented by Sir Godfrey Hounsfield in the early 1970s
Allows reconstruction of a two or three-dimensional absorber map
Exceeds projection X-ray imaging in soft tissue contrast
Spatial resolution of a clinical CT scanner is lower than plain X-ray imaging

Data Acquisition

Refers to the collection of X-ray transmission measurements through the patient
Requires an X-ray source collimated into a fan or cone shape
Geometry: source and arc-shaped detector array rotate in tandem, recording projections through a single plane within the body for many different angles

Image Formation

Aim: obtain a spatially resolved map of absorption coefficients in one slice of the patient's body
Map is sampled at a finite resolution to provide an image
Reconstruction of images involves various steps
Result is a two-dimensional matrix of preselected size and detail

Image Reconstruction

Involves convolution-back projection procedure
Selected 'field of view' is divided into small image elements called pixels
Pixels represent a small volume of tissue called a voxel (for volume element)
Density value of each pixel depends on the composition of the tissue it represents and is expressed in Hounsfield units (HU)
HU range from -1024 to +3071

Historical Background

First clinical CT scan on a patient: October 1st, 1971 at Atkinson Morley's Hospital, London, England
CT was invented by Sir Godfrey Hounsfield, who received the Nobel Prize in medicine in 1979
Since the introduction of the first CT scanners, major progress has been made
Modern clinical CT scanners are very fast and can produce a 2D cross-sectional image in less than a second

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Description

Learn about the process of data acquisition in CT scanners, including the use of X-ray transmission measurements and perpendicular projections. Understand the role of the X-ray source and detector array in capturing images of the body.