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Three Dimensional Computed Tomography: Basic Concepts

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Title: Three Dimensional Computed Tomography: Basic Concepts


1
Three Dimensional Computed Tomography Basic
Concepts
  • Chapter 17
  • Seeram

2
Why 3-D?
  • Can be used to aid in the study of AIDS,
    Huntingtons disease, and schizophrenia
  • This is done by using the 3D model as a map to
    determine areas most affected by disease
    processes
  • Models can be used to more accurately show tumor
    shape and size for radiation therapy planning
  • 3D imaging is beginning to gain acceptance as a
    tool for virtual colonoscopy by allowing the
    viewer to fly through the colon
  • The downside is that no tissue samples can be
    obtained during this procedure

3
Why 3D?
  • 3D imaging has been used to study Egyptian
    mummies without destroying the plaster or
    bandages
  • 3D imaging aids in the diagnosis of vascular
    pathology
  • 3D imaging can be used to plan surgery and is
    often used during surgical procedures
  • Real time 3D information shows the surgeon where
    the cuts are being made in relation to critical
    anatomy and pathology

4
History of 3D
  • Greenleaf et al produced a motion display of the
    ventricles using biplane angiography
  • Greenleaf JF, Tu TS, Wood EH (1970)
    Computer-generated three-dimensional
    oscilloscopic images and associated techniques
    for display and study of the spatial distribution
    of pulmonary blood flow. IEEE Trans Nucl Sci
    NS-17 353-359
  • Using the information gained from Greenleaf et
    al, it was clear that contiguous CT images could
    be stacked in a fashion that would create a 3D
    image

5
History of 3D
  • Soon software and hardware became available to
    ease the production of 3D images
  • Along with the hardware and software came
    algorithms for 3D imaging
  • By the 1980s many CT scanners offered 3D
    software as an optional package

6
Early History of 3D medical imaging
  • 1969 Hounsfield and Cormack develop the CT
    scanner
  • 1970 Greenleaf and colleagues report first
    biomedical 3D display computer-generated
    oscilloscope images relating to pulmonary blood
    flow
  • 1972 - First commercial CT scanner introduced
  • 1975 Ledley and colleagues report first 3D
    rendering of anatomic structures from CT scans
  • 1979 Herman develops technique to render bone
    surface in CT data sets collaborates with Hemmy
    to image spine disorders

7
Early History of 3D Medical Imaging
  • 1980 A CT scanner manufactured by General
    Electric features optional 3D imaging software
  • 1980 1982 Researchers begin investigating 3D
    imaging of craniofacial deformities
  • 1983 Commercial CT scanners begin featuring
    built-in imaging software packages
  • 1986 Simulation software developed for
    craniofacial surgery
  • 1987 First international conference on 3D
    imaging in medicine organized in Philadelphia
  • 1990 1991 First textbooks on 3D imaging in
    medicine published atlas of craniofacial
    deformities illustrated by 3D CT images published
  • Taken from Seeram E Computed Tomography Physical
    Principles, Clinical Applications, and Quality
    Control, 2001

8
Fundamental 3D Concepts
  • The following rules should be followed when
    acquiring data sets
  • Field of view, matrix size, and centering must be
    the same for all images
  • Angulation/orientation must be the same for all
    slices
  • There should not be any duplicate images within
    the dataset
  • Thinner slices are typically better
  • Usually a standard algorithm is best for
    acquiring data sets Edge algorithms are often
    too noisy

9
Fundamental 3D Concepts
  • Resolution 3D images and reconstructions appear
    best in the planes they were acquired
  • 3D images in the acquisition plane have the same
    resolution as the original image set (256x256 or
    512x512)
  • 3D images in any plane other than the original
    data set, the resolution will depend on the
    inter-slice distance

10
Fundamental 3D Concepts
  • When a voxel has the same dimensions in all
    planes, it is said to be isotropic
  • Isotropic voxels will allow the model to
    approximately the same resolution in all planes

11
Modeling
  • The generation of a 3D object using computer
    software is called modeling
  • Models can be rotated and viewed from many
    different angles
  • Several modeling techniques exist
  • The most common is called extrusion
  • Extrusion uses computer software to transform a
    2D profile into a 3D object
  • An example is a square being changed into a box

12
Modeling
  • Several modeling techniques exist
  • The most common is called extrusion
  • Extrusion uses computer software to transform a
    2D profile into a 3D object
  • An example is a square being changed into a box
  • Extrusion can also be used to create a wireframe
    model
  • Wireframes were more common in the early days of
    medical 3D, but are still commonly used in other
    applications

13
Wireframe Model of an Embryo
14
Modeling
  • After the wireframe model is created, a surface
    is created by placing a layer of pixels and
    patterns on top of the wireframe
  • The technologist can control various attributes
    such as color and texture

15
Shading and Lighting
  • Shading and lighting help to add realism to the
    model
  • Several different types of shading algorithms
    exist
  • A few examples are
  • Constant shading
  • Faceted shading
  • Gouraud shading
  • Phong shading
  • Each technique has its own advantages and
    disadvantages

16
Constant Shading
  • One shade or color is assigned to an entire
    object (http//www.siggraph.org/education/material
    s/HyperGraph/scanline/shade_models/constant.htm)

17
Faceted Shading
  • Simple and quick but not very realistic
    (http//www.siggraph.org/education/materials/Hyper
    Graph/scanline/shade_models/shadfaceted.htm)

18
Gouraud Shading
  • Better than faceted, looks smoother
    (http//www.siggraph.org/education/materials/Hyper
    Graph/scanline/shade_models/shadgou.htm)

19
Phong Shading
  • Makes images appear smooth and shiny
    (http//www.siggraph.org/education/materials/Hyper
    Graph/scanline/shade_models/shadphong.htm)

20
Rendering
  • Final step in the process of generating a 3D
    object
  • Rendering is a computer program that converts the
    anatomic data collected from the patient into the
    3D image seen on the computer screen
  • Rendering adds lighting, texture, and color to
    the final 3D image

21
Rendering
  • Two types of rendering are used in radiology
  • Surface rendering Uses only contour data from
    the data set. Creates an external surface that
    is hollow. Less memory intensive than volume
    rendering
  • Volume rendering Uses the entire data set to
    create a 3D image. Produces a better image than
    surface rendering, but uses more computing power

22
Classification of 3D Imaging Approaches
  • The primary approaches to 3D imaging have been
    identified
  • Slice imaging
  • Projective imaging
  • Volume imaging

23
Slice Imaging
  • Simplest method of 3D imaging
  • Also known as multiplanar imaging (MPR)
  • Slice imaging doesnt produce a true 3D image but
    rather a 2D image displayed on a computer monitor
  • MPR is available on all CT and MR scanners
  • MPR produces coronal, sagittal, and oblique images

24
MPR
  • Oblique sagittal reconstruction

25
Projective Imaging
  • Most popular 3D imaging approach
  • Still doesnt offer a true 3D model
  • Some people classify projective imaging as 21/2 D
    or 2.5D
  • Projective imaging uses the axial stack obtained
    from a CT exam to create projections of what
    various anatomical structures would look like
    from many different angles

26
Projective Imaging
  • Axial MRI of the circle of willis has been
    subjected to a projective imaging technique.

27
Projective Imaging
  • Central kangaroo is projected at several
    different angles into the 2D viewing space

28
Volume Imaging
  • Volume imaging should not be confused with Volume
    rendering
  • Volume rendering (often seen in MRI and CT) is a
    class of projective imaging
  • Volume imaging produces a true 3D visualization
    mode

29
Volume Imaging
  • Various methods of volume imaging include
  • Holography
  • Stereoscopic displays
  • Anaglyphic methods
  • Varifocal mirrors
  • Synthanalyzer
  • Rotating multidiode arrays

30
Picture of a Hologram
31
Generic 3D Imaging System
  • Four major elements are noted for any 3D imaging
    system
  • Input
  • Workstation
  • Output
  • User

32
Input
  • Devices that acquire the data
  • CT scanner, MR scanner
  • The acquired data is sent to a workstation

33
Workstation
  • The workstation is the heart of the 3D system
  • The workstation is a powerful computed that
    handles the various 3D imaging operations
  • Preprocessing
  • Visualization
  • Manipulation
  • Analysis

34
Output
  • Once processing is completed, the results are
    displayed for viewing and recording

35
User
  • The user interacts with each of the three
    components to optimize use of the system

36
4 Steps to Create 3D Images
  • 1. Data acquisition slices, or sectional
    images, of the patients anatomy are produced.
    Methods of data acquisition in radiology include
    CT, MRI, ultrasound, PET, SPECT, and digital
    radiography
  • 2. Creation of 3D space or scene space. The
    voxel information from the sectional images is
    stored in the computer
  • Scene is defined as a multidimensional image
    rectangular array of voxels with assigned values

37
4 Steps to Create 3D Images
  • 3. Processing for 3D image display. This is a
    function of the workstation and includes the four
    operation listed above
  • 4. 3D image display. The simulation 3D image is
    displayed on the 2D computer screen

38
Maximum Intensity Projection
  • Maximum Intensity Projection (MIP) is a volume
    rendering technique that originated in magnetic
    resonance angiography and is now used frequently
    in computed tomography angiography. MIP does not
    require sophisticated computer hardware because,
    like surface rendering, it makes use of less than
    10 of the data in 3D space

39
Steps Involved in MIP
  • A mathematical ray is projected from the viewers
    eye through the 3D space
  • This ray passes through a set of voxels in its
    path
  • The MIP program allows only the voxel with the
    maximum intensity to be selected

40
Stand Alone Workstations
  • Picker, Siemens, General Electric, and several
    other manufacturers provide 3D packages. Most
    workstations offer a variety of 3D processing
    features

41
3D Processing Features
  • Multiplanar Reconstruction (MPR)
  • Can demonstrate the entirety of a curved
    anatomical structured in one image. This feature
    could be useful in demonstrating the entire
    length of the descending aorta in one view
  • Surface Rendering
  • Slice Plane Mapping
  • Allows two tissue types to be viewed at the same
    time

42
3D Processing Features
  • Slice Cube Cuts
  • This is a processing technique that allows the
    operator to slice through any plane to
    demonstrate internal anatomy
  • Transparency Visualization
  • This technique allows the operator to view both
    surface and internal structures at the same time

43
3D Processing Features
  • Maximum Intensity Projection
  • 4D Angiography
  • This shows bone, soft tissue, and blood vessels
    at the same time to allows the viewer to see
    tortuous vessels with respect to bone
  • Disarticulation
  • This shaded surface display technique allows the
    viewer to enhance the visualization of certain
    structures by removing others

44
3D Processing Features
  • Virtual Reality Imaging
  • Some workstations are capable of virtual
    endoscopy. This allows the viewer to fly
    through various anatomical structures including
    the colon and bronchus
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