Transcript Single-Photon emission computed tomography (SPECT )

Chapter-4
Single-Photon emission computed
tomography (SPECT)
Planar images (2D) Vs. Single-Photon emission computed tomography
(SPECT)(3D)
Advantages of tomographic images
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A basic problem in conventional radionuclide imaging is that the images
obtained are 2D projections of 3D source distributions.
Images of structures at one depth in the patient thus are obscured by
superimposed images of overlying and underlying structures.
One solution is to obtain tomographic imaging (3D images= multiple 2D slices of
a 3D object).
Tomographic images are capable of providing more accurate quantitation of
activity at specific locations within the body
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Single-Photon emission computed
tomography (SPECT)
Tomographic images
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Tomos is Greek word for cut or section
Modern computed tomography (CT) techniques, including positron emission
tomography (PET) and single photon emission tomography (SPECT), and x-ray CT,
use detector systems rotated around the object so that many different angular
views (known as projections) of the object are obtained.
Mathematical algorithms then are used to reconstruct images of selected planes
within the object from these projection data.
Reconstruction of images from multiple projections of the detected emission
from radionuclides within the body is known as emission computed tomography
(ECT)
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Tomographic images (SPECT camera)
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
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SPECT cameras acquire multiple planar views of the radioactivity in an organ.
The sequential planar views acquired during tomographic acquisition are called
Projection views
The data are then processed mathematically to create cross-sectional views of
the organ
SPECT utilizes the single photons emitted by gamma-emitting radionuclides such
as 99mTc, 67Ga, 111In and 123I. This is in contrast to positron emission
tomography (PET), which utilizes the paired 511-KeV photons arising from
positron annihilation.
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Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Types of cameras
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The simplest camera design for SPECT imaging is similar to that of a
planar camera but with two additional features
First; the SPECT camera is constructed so that the head can rotate
either stepwise or continuously about the patient to acquire multiple
views.
Second; it is equipped with a computer that integrates the multiple
images to produce the cross-sectional views of the organ
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Types of cameras
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The more advanced SPECT camera designs have more than one head
or are constructed with a ring of detectors.
In the case of the single and multiple head cameras, the heads are
mechanically rotated around the patient to obtain the multiple
projections views.
Ring detectors have a ring of individual small crystals or a single,
donut-shaped crystal that does not rotate.
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Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Types of cameras
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Angle of rotation
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Single-headed cameras must rotate a full 360 degree to obtain all
necessary views of most organs.in contrast, each head of a doubleheaded camera need rotate only half as far, 180 degree, and a tripleheaded camera only 120 degree to obtain the same views.
The cost of the additional heads must be balanced against the
benefits of increased speed of acquisition.
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Angle of rotation
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Arc acquisition
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Tomographic projection views are most often acquired over an arc of
360 or 180 degree. The 360 arc of rotation of the camera heads is
regularly used for most organs. The 180 degree arc is used for organs
that are positioned on one side of the body, such as the heart.
Views of the heart are obtained in a 180 degree arc extending from
the right anterior oblique position to the left posterior oblique
position.
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Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Equipment
Arc acquisition
The data from this 180 degree is
considered adequate
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Equipment
Number of projection tomographic views
Over a full 360 degree arc, 64 or 128 tomographic projections are usually
collected; similarly 32 or 64 views are generally obtained over a 180
degree arc.
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Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Collection times
for a given dose of radiopharmaceutical, better images are generated
using the higher count statistics from longer acquisitions. However,
patient comfort and cooperation limit imaging times. Acquisition times of
20 to 40 seconds per projection views are standard.
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Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Collection methods
Steps-and-shoots vs. continuous acquisition
• One method for collection of tomographic projection views is called
step-and-shoot acquisition.
• In this technique, each projection view is acquired in entirely at each
angular stop (position). There is a short pause of a few seconds
between views to allow for the automatic rotation of the camera head
to the next stop. The camera makes a single rotation around the
patient.
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Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Collection methods
Steps-and-shoots vs. continuous acquisition
• In continuous acquisition, data are collected over one or several
sequential 360 degree rotations. There are no pause; rotation is
continuous.
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Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed
tomography (SPECT)
Collection methods
Steps-and-shoots vs. continuous acquisition
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Collection methods
Steps-and-shoots vs. continuous acquisition
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Collection methods
Circular Vs. Elliptical orbits
• Most acquisitions are performed with a circular orbit. The camera
head is rotated at a fixed distance from the center of the body. Since
the body is more nearly elliptical than circular in cross section, the
camera does not come as close to the organ as possible over a
significant portion of its rotation. Because image resolution is better if
the camera is as close to an organ as possible, some cameras are
designed to rotate in elliptical orbits, which allow the camera head to
more closely follow the contour of the body and therefore stay closer
to the organ being imaged)
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Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Collection methods
Circular Vs. Elliptical orbits
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Vidoes
(SPECT) https://www.youtube.com/watch?v=7BEqUGFDqwM
https://www.youtube.com/watch?v=nqnrO6x45SQ
(heart) https://www.youtube.com/watch?v=l6V6VLxQlkY
(QA) https://vimeo.com/88175239
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Single-Photon emission computed
tomography (SPECT)
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Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction
• Reconstruction is the process of creating trans-axial slices from
projection views.
• There are two basic approaches to creating the trans-axial slices.
1. Filtered backprojection
2. Iterative reconstruction
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Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction
Sinogram
• For the purpose of analysis, it is convenient to introduce a new
coordinate system that is stationary with respect to the
gamma camera detector.
• This is denoted as the (r,𝝓) coordinate system a
representation of this matrix, generically known as a sinogram
• Each row across the matrix represents an intensity display
across a single projection.
• The successive rows from top to bottom represent successive
projection angles.
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction
Sinogram
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction
Sinogram
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction
Sinogram
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction
Sinogram
• The sinogram provides a convenient way to represent the full set
of data acquired during a scan and can be useful for determining
the causes of artifacts in SPECT and PET
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Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction
Sinogram is useful to study motion artifacts
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction
Simple and Filtered back projection
• The general goal of reconstruction tomography is to generate a 2D crosssectional image of activity from a slice within the object, using the
projection profiles obtained for that slice.
• The most basic approach for reconstructing an image from the profiles is
by simple backprojection
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction (Simple and Filtered back projection)
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The concepts are
illustrated for a point
source object
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Projections profiles are
acquired from different
angles around the source
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Approximation for the
source distribution within
the plane is obtained by
projecting (or distributing)
the data from each
element in a profile back
across the entire image grid
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction (Simple and Filtered back projection)
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The counts recorded in a
particular projection profile
element are divided
uniformly amongst the
pixels that fall within its
projection path (this
operation is called
backprojection)
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When the backprojections
for all profiles are added
together, an approximation
of the distribution of
radioactivity within the
scanned slice is obtained
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction (Simple and Filtered back projection)
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Projecting the data back
toward the center that
gives the term
backprojection
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction (Simple and Filtered back projection)
• This figure is a representation of data
obtained in the acquisition of the
projection views of a thin radioactive disk.
• In this figure, an imaginary grid is placed
over the disk (A). The disk is imaged (B),
and the counts for each pixel are recorded
(C) [ideally]
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction (Simple and Filtered back projection)
• The counts in each of the cells of a column
are summed and stored in an array (D)
[during collection].
• In a similar manner, all of the rows are
summed and stored in an array of sums to
the right of the matrix (E ) [during
collection].
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction (Simple and Filtered back projection)
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During the backprojection, the
two arrays of projection data
are used to recreate the original
disk.
• The upper array is spread or
backprojected across the
columns of a blank matrix so
that each of the values in
any single column are
identical.
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction (Simple and Filtered back projection)
• The array to the right of the
matrix is backprojected
across the rows, and these
values are added cell by cell
to the values of the
preceding set
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction (Simple and Filtered back projection)
• If the counts in each
pixel are represented
by dots (for ease of
illustration each dot
represents 5 counts)
one begins to see a
relatively dense central
area that corresponds
generally to the size
and the location of the
original disk
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction (Simple and Filtered back projection)
• The wide bans of dots
extending in four
directions from this
central density are an
artifact of the
backprojection process;
they are residual counts
from the
backprojection of the
arrays.
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Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction (Simple and Filtered back projection)
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The image built up by a simple
backprojection resembles the
true source distribution.
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However, there is an obvious
artifact in that counts inevitably
are projected outside the true
location of the object, resulting
in a blurring of its image.
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Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction (Simple and Filtered back projection)
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A type of noise peculiar to the reconstruction
process is called star artifact, so named because a
star composed of the backprojection “rays”
surround each object (A)
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The quality of the image can be improved and the
star artifact can be reduced by increasing the
number of projection angles and the number of
samples along the profile.
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The star artifact can also be reduced by a
mathematical technique called “filtering”
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Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Signal vs. Noise
• The signal is that part of the information that
produces the actual image.
• Noise is extraneous data and may have no
direct relation to the actual image.
• Noise reduces the quality of the image.
• Photon scattering, statistical variation, and
random electronic fluctuation are among the
sources of noise, which can be reduced by
improved collimation, longer acquisition times
and better design of the circuitry.
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction (Simple and Filtered back projection)
Filtering
• Filtering is a mathematical technique applied during reconstruction to improve
the appearance of the image.
• Filters have different types
• Two types of filter s will be discussed
• Filters used to reduce the effects of the star artifact
• Filters used to remove noise due to photon scattering and statistical
variations in counts
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction (Simple and Filtered back projection)
Filtering
Domain of Images
Domain of Images
Spatial domain
• When image data is represented
in counts per pixel, this data is
said to be in spatial domain
• Filtering proves to be
computationally burdensome
Frequency domain
• This is when the data is represented as a
series of sine waves.
• The data is said to be transformed into
the frequency domain
• It is easier to perform filtering in this
domain
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Reconstruction (Simple and Filtered back projection)
Filtering
Spatial and Frequency domains of Images
• These two domains are not entirely independent.
• In fact, they only represent different views of the underlying data
Chapter-4
Single-Photon emission computed
tomography (SPECT)
Single-Photon emission computed tomography (SPECT)
Vidoes
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FBP (https://www.youtube.com/watch?v=ddZeLNh9aac)
FBP (https://www.youtube.com/watch?v=8V2QBD8nh_s)
• Reconstruction (https://www.youtube.com/watch?v=MTBhqcVjQ8Q)
• FBP (https://www.youtube.com/watch?v=ZtRsVPXrmSc)