SPECTSingle Photon Emission Computed Tomography

David S. Graff PhD

• Tomography

• Filtered Back Projection

• Iterative Reconstruction

• Quality Control

Tomographyτομοσ: a cut or a slice

atom: cannot be cutanatomy: 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

Position on detector

Hea

d A

ngle

0º18

0º36

0º

• 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 muchThe filter is modified to reduce high frequencies

Filters combine blurring and ramp to reduce noise

lp/mm

Ramp Hann

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

rat heart