Download Three Dimensional Computed Tomography: Basic Concepts

Three Dimensional Computed
Tomography: Basic Concepts
Chapter 17
Seeram
Why 3-D?
Can be used to aid in the study of AIDS,
Huntington’s 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
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
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
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 1980’s many CT scanners offered
3D software as an optional package
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
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
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
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
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
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
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
Wireframe Model of an Embryo
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
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
Constant Shading
One shade or color is assigned to an
entire object
(http://www.siggraph.org/education/materials/HyperGraph/scanline/shade_models/constant.htm)
Faceted Shading
Simple and quick but not very realistic
(http://www.siggraph.org/education/materials/HyperGraph/scanline/shade_models/shadfaceted.htm)
Gouraud Shading
Better than faceted, looks smoother
(http://www.siggraph.org/education/materials/HyperGraph/scanline/shade_models/shadgou.htm)
Phong Shading
Makes images appear smooth and shiny
(http://www.siggraph.org/education/materials/HyperGraph/scanline/shade_models/shadphong.htm)
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
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
Classification of 3D Imaging
Approaches
The primary approaches to 3D imaging
have been identified
– Slice imaging
– Projective imaging
– Volume imaging
Slice Imaging
Simplest method of 3D imaging
Also known as multiplanar imaging (MPR)
Slice imaging doesn’t 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
MPR
Oblique sagittal
reconstruction
Projective Imaging
Most popular 3D imaging approach
Still doesn’t 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
Projective Imaging
Axial MRI of the circle
of willis has been
subjected to a
projective imaging
technique.
Projective Imaging
Central kangaroo is
projected at several
different angles into
the 2D viewing space
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
Volume Imaging
Various methods of volume imaging
include
– Holography
– Stereoscopic displays
– Anaglyphic methods
– Varifocal mirrors
– Synthanalyzer
– Rotating multidiode arrays
Picture of a Hologram
Generic 3D Imaging System
Four major elements are noted for any 3D
imaging system
– Input
– Workstation
– Output
– User
Input
Devices that acquire the data
– CT scanner, MR scanner
The acquired data is sent to a workstation
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
Output
Once processing is completed, the results
are displayed for viewing and recording
User
The user interacts with each of the three
components to optimize use of the system
4 Steps to Create 3D Images
1. Data acquisition – slices, or sectional
images, of the patient’s 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
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
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
Steps Involved in MIP
A mathematical ray is projected from the
viewer’s 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
Stand Alone Workstations
Picker, Siemens, General Electric, and
several other manufacturers provide 3D
packages. Most workstations offer a
variety of 3D processing features
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
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
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
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