nuclear imaging, pet ct medical physics

Lecture 12: Emission Tomography I

Shahid Younas

NUCLEAR IMAGING

Emission Tomography I

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.



What is PSD? What is its role in SPECT imaging?

Pressure Sensitive Device.

Detect nearest position w.r.t.

patient body without touching

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 have 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.

Back-projection 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

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