Lecture 12

Lecture 13: Emission Tomography II
Shahid Younas
NUCLEAR IMAGING
Emission Tomography II
Single Photon Emission Computed Tomography (SPECT)

Introduction
 Nuclear Medicine projection image depicts a two-dimensional projection
of the three-dimensional activity distribution.
 Contribution to the image from structures at different depths overlap.
 Hindering the ability to discern the image of a structure at a particular
depth.

Introduction
 Tomographic imaging attempts to depict the
activity distribution in a single cross section
of the patient.

Types of Tomography
 Conventional Tomography
 Computed Tomography (CT)

Types of Tomography
 Conventional Tomography also called Geometric or Focal Plane.
 Structures out of a focal plane are not removed from the resultant
image.
 They are blurred by an amount proportional to their distance from
the local plane.

Types of Tomography
 Computed Tomography uses mathematical methods to remove
overlying structures completely.
 CT requires the acquisition of a set of projection image from at least
a 180-degree arc about the patient.

Types of Tomography
CT uses mathematical methods; do you know what instrument was
used in nuclear medicine to carry on conventional tomography?
Focused or seven pin-hole collimators

SPECT
Three Rivals of SPECT
 Attenuation of photons in the patient
 Compton scattered photons in the image
 Degradation of spatial resolution with distance from collimator.

SPECT-Design and Principles of Operation
 Single photon emission computed tomography (SPECT) generates transverse
images depicting the distribution of x- or gamma ray emitting nuclides in
patients.
 Standard planar projection images are acquired from an arc of 180 degrees
(most cardiac SPECT) or 360 degrees (most non-cardiac SPECT) about the
patient.

 Most SPECT systems use one or more scintillation camera heads that
revolve about the patient.
 Transverse images are reconstructed using either filtered back-
projection (as in CT) or iterative reconstruction methods.

 If camera heads produced ideal projection images;
 no attenuation by patient
 no degradation of spatial resolution with distance
 then projection images from opposite sides of patient would be mirror
images.

SPECT
 Attenuation greatly reduces number of photons from activity in the half
of patient opposite camera head; this information is blurred by distance.

SPECT-Image acquisition
 SPECT projection images usually acquired in either a 64 x 64 (60 or 64
projections) or a 128 x 128 (120 or 128 projections) pixel format.
 Using too small a pixel format reduces spatial resolution of the projection
images and of the resultant reconstructed transverse images.
 Using too few projections creates radial streak artifacts in the reconstructed
transverse images.

Brain SPECT are acquired over 360o whereas Cardiac SPECT are
acquired at 180o? What is reduced and what is enhanced?
Attenuation is reduced whereas contrast and resolution is enhanced

 Camera heads on older SPECT systems used circular orbits
around the patient while acquiring images,
 Satisfactory for imaging of the brain.
 Loss of spatial resolution in body imaging because of distance
from surface.

Newer systems provide noncircular orbits that keep camera heads in close
proximity to surface of body throughout the orbit.

SPECT-Transverse image reconstruction
 The goal of SPECT image reconstruction methods is to estimate
the true radioactivity distribution in vivo from the measured
projection data.
 After projection images are acquired, they are usually corrected
for axis-of-rotation misalignments.

 The substantial effects of attenuation, scatter , collimator and
detector response are ignored.
 The Following these corrections, transverse image reconstruction
is performed using either filtered back-projection or iterative
methods.

Suppose that a simple test object containing 3 objects with different attenuation
values is scanned and views (attenuation measurements) are obtained at 3
angles.

The attenuation measurements of each view are simply divided evenly along the
path of the ray.

after back projection of only 4 views, an image of the test object is beginning to
appear in Figure 5C.

 Back-projection is efficient each measurement is processed just
once and involves relatively simple calculations but has a serious
flaw,
The resulting images are blurry- poor spatial resolution

Why resulting images are has poor spatial resolution?
The substantial effects of attenuation, scatter , collimator and detector
response are ignored.

 The blurring can be reversed by a mathematic process known as
“Convolution”.
 Consider a scan of a phantom containing a single cylinder with an
attenuation higher than that of its surroundings.

 The attenuation of the cylinder is highest through its center
(where it is thickest) and decreases toward its edges.
 Backprojection builds a cylinder image whose intensity
decreases from the maximum at the center toward the edges.

 To reconstruct a ‘‘deblurred’’ image, a convolution function is
mathematically applied to each view before back-projection.

The mathematic operation is called convolution. Do you know how
the process is referred?
Filtering

 Choice of filter kernel for a particular type of study is determined by
the amount of statistical noise in the projection images
 Mainly determined by injected activity, collimator, and acquisition
time per image

 Their spatial resolution is,
 Determined by collimator and the typical distances from the camera
head(s) from the organ being imaged

Lecture 12

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