Computed Tomography Principles and Techniques

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Questions and Answers

What is the objective of CT image reconstruction?

To determine how much attenuation of the narrow x-ray beam occurs in each voxel of the reconstruction matrix.

How are the calculated attenuation values represented?

As gray levels in a dimensional image of the slice.

What does the Z dimension of the voxels correspond to?

The slice thickness.

What factors do the X and Y voxel dimensions depend on?

The size of the area over which the x-ray measurements are obtained as well as on the size of the matrix (the number of rows and columns) into which the slice is imagined to be divided. Signup and view all the answers

What is the Radon Transform widely applicable to?

Tomography, the creation of an image from the Projection Data associated with cross-sectional scans of an object. Signup and view all the answers

What does the Radon Transform represent?

The projection data obtained as the output of a Tomographic scan. Signup and view all the answers

What is the Inverse of the Radon Transform used for?

To reconstruct the original density from the Projection Data. Signup and view all the answers

What term is also known as Image Reconstruction?

Tomographic Reconstruction. Signup and view all the answers

What is a Ray in CT Image Reconstruction?

An imaginary line between Tube & Detector. Signup and view all the answers

What does Multiplanar Reformatting (MPR) allow?

Images to be created from the original axial plane in either the coronal, sagittal, or oblique plane. Signup and view all the answers

What are the four basic factors that influence CT image quality?

Image Contrast, Spatial Resolution, Image Noise, and Artifacts Signup and view all the answers

CT Display Contrast is not arbitrary and is dependent on the subject itself.

False Signup and view all the answers

What mainly determines CT Subject Contrast?

Differential Attenuation, which is the differences in X-Ray Attenuation by Absorption or Scattering in different types of tissue. Signup and view all the answers

What type of scattering events are overwhelmingly common in CT?

Compton-scattering events. Signup and view all the answers

What is CT image noise associated with?

The number of x-rays contributing to each detector measurement. Signup and view all the answers

What is the ability to distinguish small, closely spaced objects on an image?

Spatial Resolution. Signup and view all the answers

What does increasing the peak Kilovoltage in CT do?

It increases the number of x-rays penetrating the patient and reaching the detectors. Signup and view all the answers

What does changing the mA value in CT do?

Higher mA values increase the beam intensity, resulting in more x-rays being emitted by the tube. Signup and view all the answers

What is the term for the increase in the mean energy of the x-ray beam when it passes through an object?

Beam Hardening. Signup and view all the answers

What are artifacts in CT?

Any structure that is seen on an image but is not representative of the actual Anatomy. Signup and view all the answers

What type of artifacts are associated with third-generation scan geometry?

Ring or Partial Ring Artifacts. Signup and view all the answers

What type of artifacts may occur in all scanners and are often caused by inconsistent or bad detector measurements?

Streak Artifacts. Signup and view all the answers

What is the primary reason that streaks occur from metal objects?

They exceed the maximum attenuation value that a CT system can image. Signup and view all the answers

What causes Partial -Volume Effects?

When a voxel contains many types of tissue. Signup and view all the answers

What is the most critical region to produce partial volume artifact?

The posterior cranial fossa. Signup and view all the answers

Thinner slices and computer algorithms can help reduce partial volume artifacts.

True Signup and view all the answers

What kind of artifacts are caused by anatomy that is outside of the selected scan field of view?

Out of Field Artifacts. Signup and view all the answers

What is the most common type of Shading Artifact?

Beam-Hardening Effects. Signup and view all the answers

What is the primary reason that bone causes more beam hardening than an equivalent thickness of soft tissue?

Bone has a higher attenuation than soft tissue. Signup and view all the answers

What happens when the x-ray beam passes through an object, in terms of CT numbers?

CT numbers of certain structures change. Signup and view all the answers

How can insufficient X-ray intensity affect CT images?

Leading to high random errors. Signup and view all the answers

Out of field artifacts can be avoided by ensuring that scan field of view is larger than the object to be scanned.

True Signup and view all the answers

The ______ represents the collection of Ray Sum information from different angles as the x-ray source rotates around the patient.

View Signup and view all the answers

The ability to distinguish small, closely spaced objects on an image is called ______.

Spatial Resolution Signup and view all the answers

Which of the following is NOT a type of 3D display used in CT?

Digital Subtraction Angiography (DSA) Signup and view all the answers

Study Notes

Computed Tomography (CT) Image Reconstruction

CT image reconstruction aims to determine the attenuation of narrow X-ray beams through voxels in the reconstruction matrix.
Attenuation values are translated into grayscale levels in a two-dimensional image of a slice.
Volumetric data acquisition using spiral CT allows for post-processing tailored to clinical needs.

CT Voxel Structure

Hounsfield's approach involves dividing a slice into three-dimensional rectangular boxes (voxels).
X and Y directions are within the slice plane.
Z direction aligns with the subject's axis (slice thickness).
X and Y voxel dimensions depend on the area and matrix size where X-ray measurements are taken.

Radon Transform in CT

The Radon Transform significantly applies to Tomography, transforming projection data into images from cross-sectional scans.
The Inverse Radon Transform is instrumental in reconstructing original densities from projection data in Computed Tomography.
This process, also known as image reconstruction, forms the mathematical foundation of CT imaging.

Ray, Ray Sum, View, and Attenuation Profile

A ray is an imaginary line between the X-ray tube and detector.
Ray Sum represents the attenuation along a single ray.
View encompasses a collection of ray sums.

Multiplanar Reformatting (MPR)

Creates images from the original axial plane in coronal, sagittal, or oblique planes.
Helps in a more holistic patient analysis.

Three-Dimensional Displays

Three-dimensional (3D) displays present a scan volume within a single image, enhancing specific characteristics.
Successful display depends on high-contrast structures (e.g., skeleton).
Common 3D display techniques include Shaded Surface Displays (SSD), Minimum Intensity Projection (MinIP), Maximum Intensity Projections (MIP), Volume Rendering (VR), Perspective Volume Rendering Technique (pVRT)/Virtual Endoscopy (VE), and Curved Plane Reconstructions.

CT Image Quality

Fundamentally, CT image quality rests on four factors: Image Contrast, Spatial Resolution, Image Noise, and Artifacts.
These factors interact to influence sensitivity (perceiving low-contrast structures) and detail visibility.

CT Image Contrast

Subject Contrast and Display Contrast influence CT image contrast.
Display Contrast, being arbitrary, is not discussed further in this context.
Subject contrast hinges on differential attenuation arising from tissue absorption or scattering differences impacting X-ray intensity detected.
Compton scattering dominates in soft tissues due to high kilovoltage and beam filtration.

CT Spatial Resolution

Spatial resolution relates to the ability to differentiate small, closely spaced structures on an image.
Factors affecting resolution include motion producing blurring, and the matrix size and pixel size. Larger matrices with smaller pixel sizes typically offer sharper images.

CT Image Noise

CT image quality's noise is related to the number of X-Rays contributing to each detector measurement.
Noise is influenced by factors like X-ray tube amperage (mA), scan time, slice thickness, and peak kilovoltage.

Image Artifacts

Artifacts represent structures on images that do not correspond to true anatomical features.
Common artifact types include:
- Shading (Beam Hardening): Caused by differing attenuation of bone and soft tissue.
- Ring/Partial Ring: Arising from errors in detector-array measurements, and
- Streak: Associated with motion and metal objects.
- Partial Volume
- Out-of-Field: due to anatomy not being encompassed in the field of view.
Insufficient X-Ray Intensity: leading to a noisy image.
Malfunctions (tube arcing): system malfunctions introduce artifacts.

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Description

Explore the fundamental concepts of computed tomography (CT) including image reconstruction techniques, voxel structure, and the Radon Transform. This quiz covers the principles that underlie CT imaging and how data is processed to create images from X-ray scans.