A 0.3 mm Na-22 point source encapsulated in Lucite with activity of 132
μCi was scanned in an Inveon preclinical PET scanner
(Siemens, Knoxville, TN) at (−15 mm,−15 mm) in the central axial
plane as shown infigure 1(a) . The crystal
size of the Inveon scanner is about 1.59 mm and the 2D sinogram size is 128
(radialbins) × 160 (angles). The total number of events collected in the
2D sinogram was 73 312 and the estimated random fraction was less than 0.1%.
To evaluate the effect of point source contrast on measured spatial
resolution, we simulated the background sinogram by forward projecting a uniform
background with different activity concentrations using the system matrix, and
then superimposed the background projection data onto the point source data
before reconstruction. Since the purpose of adding the background is to reduce
the nonlinear response and the effect of non-negativity constraint, there is no
need to introduce Poisson noise in the background data. Maximum-likelihood image
reconstruction was then performed using 5000 iterations of the preconditioned
conjugate gradient (PCG) algorithm (Qi
et al 1998 ). Only standard normalization was
applied in the reconstruction. The system matrix used during forward projection
and reconstruction includes a geometric projection matrix and a sinogram
blurring matrix (Tohme and Qi 2009 (link)). The
image matrix size was 1005 × 1005 and the pixel size was 0.1 mm. A small
pixel size was used to increase the accuracy of FWHM measurements.
Reconstruction of the background sinogram was also performed, and was subtracted
from the result of the combined reconstruction to produce the point source only
image. Profiles were drawn through the peak of the point source and linear
interpolation was performed between neighboring pixels to measure the FWHM along
the radial and tangential directions. No curve fitting was performed because the
pixel size was much smaller than the expected FWHM. Negative values were not
considered in the FWHM calculation. In our implementation, the point source
contrast was defined as Here the reconstructed point peak intensity refers to the peak
intensity of the point source in the reconstructed image before subtracting the
background. For comparison, 2D FBP reconstruction using the standard software on
the scanner was performed with a pixel size of 0.097 mm. To show the 2D results
presented here are applicable to other conditions, fully 3D maximum-likelihood
expectation-maximization (MLEM) reconstruction of a point source at the center
of the FOV was also performed with different background values. The image matrix
size in the 3D reconstruction was 256 × 256 × 161 with a voxel
size of 0.194 × 0.194 × 0.796 mm3.
μCi was scanned in an Inveon preclinical PET scanner
(Siemens, Knoxville, TN) at (−15 mm,−15 mm) in the central axial
plane as shown in
size of the Inveon scanner is about 1.59 mm and the 2D sinogram size is 128
(radialbins) × 160 (angles). The total number of events collected in the
2D sinogram was 73 312 and the estimated random fraction was less than 0.1%.
To evaluate the effect of point source contrast on measured spatial
resolution, we simulated the background sinogram by forward projecting a uniform
background with different activity concentrations using the system matrix, and
then superimposed the background projection data onto the point source data
before reconstruction. Since the purpose of adding the background is to reduce
the nonlinear response and the effect of non-negativity constraint, there is no
need to introduce Poisson noise in the background data. Maximum-likelihood image
reconstruction was then performed using 5000 iterations of the preconditioned
conjugate gradient (PCG) algorithm (
et al 1998
applied in the reconstruction. The system matrix used during forward projection
and reconstruction includes a geometric projection matrix and a sinogram
blurring matrix (Tohme and Qi 2009 (link)). The
image matrix size was 1005 × 1005 and the pixel size was 0.1 mm. A small
pixel size was used to increase the accuracy of FWHM measurements.
Reconstruction of the background sinogram was also performed, and was subtracted
from the result of the combined reconstruction to produce the point source only
image. Profiles were drawn through the peak of the point source and linear
interpolation was performed between neighboring pixels to measure the FWHM along
the radial and tangential directions. No curve fitting was performed because the
pixel size was much smaller than the expected FWHM. Negative values were not
considered in the FWHM calculation. In our implementation, the point source
contrast was defined as Here the reconstructed point peak intensity refers to the peak
intensity of the point source in the reconstructed image before subtracting the
background. For comparison, 2D FBP reconstruction using the standard software on
the scanner was performed with a pixel size of 0.097 mm. To show the 2D results
presented here are applicable to other conditions, fully 3D maximum-likelihood
expectation-maximization (MLEM) reconstruction of a point source at the center
of the FOV was also performed with different background values. The image matrix
size in the 3D reconstruction was 256 × 256 × 161 with a voxel
size of 0.194 × 0.194 × 0.796 mm3.
