Information about SPECT

Published on December 1, 2008

Author: david.s.graff

Source: slideshare.net

Lecture notes on SPECT technology for nuclear medicine technologists

• Tomography • Filtered Back Projection • Iterative Reconstruction • Quality Control

Tomography τομοσ: a cut or a slice atom: cannot be cut anatomy: a cutting (of the patient)

A Gamma Camera measures the total activity along a line of sight* *Ignoring scatter, attenuation, noise, blurring

We want to know the activity at every point in space, but we measure the sum total attenuation along the line of sight All these systems will look the same

To get more information: measure sums along rays at different angles

To get full information, measure at all angles In order to see a sharp edge, we must have rays parallel to the edge

A sinogram shows all the angles in one slice 0º Head Angle 180º360º Position on detector

• Tomography requires lines of sight through 180º • Filtered Back Projection • Iterative Reconstruction • Quality Control

Collect projections (data) at wide range of angles

Backproject from all projections Edges are lost. Image is fuzzy

Enhance edges of projections (ﬁlter) Backproject ﬁltered projections

Filter increases high spatial frequencies and decreases low frequencies

A ramp ﬁlter would increase noise too much The ﬁlter is modiﬁed to reduce high frequencies

Filters combine blurring and ramp to reduce noise a mp Hann R lp/mm

Filtered Back Projection

Object

Projections

Filtered

Backprojected

• Tomography requires lines of sight through 180º • Filtered Back Projection enhances edges and backprojects to reconstruct images • Iterative Reconstruction • Quality Control

Filtered Back Projection assumes • No noise • No attenuation • Perfect resolution • No scatter These assumptions are much worse for SPECT than for CT Iterative algorithms don’t make these assumptions

Collect projections (data) at wide range of angles

Collect projections (data) at wide range of angles

Assume a simple smooth model of patient

Collect projections (data) at wide range of angles Assume a simple smooth model of patient

Project model of patient Include attenuation, resolution, scatter in projection

Collect projections (data) at wide range of angles Assume a simple smooth model of patient Project model of patient (including physics)

Compare projections of model with actual projections Derive a correction for each projection ÷ =

Backproject corrections

Collect projections (data) at wide range of angles Assume a simple smooth model of patient Project model of patient (including physics)

Collect projections (data) at wide range of angles Assume a simple smooth model of patient Project model of patient (including physics) Compare model projections to measured projections Backproject correction to update model

• proper reconstruction requires attenuation and scatter. • How do we know how much attenuation?

SPECT-CT

• Create projections from model of patient • Compare projections with data from patient • Improve model

• Tomography requires lines of sight through 180º • Filtered Back Projection enhances edges and backprojects to reconstruct images • Iterative Reconstruction accounts for noise, attenuation, scatter, resolution. Compare model of patient with data to improve model. • Quality Control

Bullseye artifact results from poor homogeneity

• Tomography requires lines of sight through 180º • Filtered Back Projection enhances edges and backprojects to reconstruct images • Iterative Reconstruction accounts for noise, attenuation, scatter, resolution. Compare model of patient with data to improve model. • Quality Control to verify center of rotation, uniformity, ability to pick up cold spheres and rods

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