Attenuation correction for three-dimensional PET using uncollimated flood-source transmission measurements.

An attenuation-correction method for three-dimensional PET imaging, which obtains attenuation-correction factors from transmission measurements using an uncollimated flood source, is described. This correction is demonstrated for two different phantoms using transmission data acquired with QPET, a rotating imaging system with two planar detectors developed for imaging small volumes. The scatter amplitude in the transmission projections was a maximum of 30%; to obtain accurate attenuation-correction factors the scatter distribution was first calculated and subtracted. The attenuation-corrected emission images for both phantoms indicate that their original uniform amplitudes have been restored. The attenuation correction adds only a small amount of noise to the emission images, as evaluated from the standard deviation over a central region. For the first phantom, with maximum attenuation of 48%, the noise added was 2.6%. The second phantom was attenuated by a maximum of 37%, and 1.9% noise was added. Because the transmission data are smoothed, some artifacts are visible at the edges of the phantom where the correction factors change abruptly within the emission image.

Scatter correction for three-dimensional PET based on an analytic model dependent on source and attenuating object.

Three-dimensional positron emission tomography admits a significant scatter fraction due to the large aperture of the detectors, and requires accurate scatter subtraction. A scatter-correction method, applicable to both emission and transmission imaging, calculates the projections of the single-scatter distribution, using an approximate image of the source and attenuating object. The scatter background is subtracted in projection space for transmission data and in image space for emission data, yielding corrected attenuation and emission images. The accuracy of this single-scatter distribution is validated for the authors' small imaging system by comparison with Monte Carlo simulations. The correction is demonstrated using transmission and emission data obtained from measurements on the authors' QPET imaging system using two acrylic phantoms. For the transmission data, generated with a flood source, errors of up to 24% in the linear attenuation coefficients resulted with no scatter subtraction, but the correction yielded an accurate value of mu =0.11+or-0.01 cm-1. For the emission data, the corrected images show that the scattered background has been removed to within the level of the background noise outside the source. The residual amplitude within a cold spot in one of the phantoms was reduced from 21% to 3% of the image amplitude.

Signal-to-noise ratios for attenuation correction in PET imaging.

Attenuation correction is an important part of accurate image reconstruction in positron tomography. The usual correction method involves direct measurement of attenuation correction factors (ACFs). A reconstruct-reproject method, which has been suggested as providing superior noise properties, is sometimes employed; an attenuation image is first reconstructed from the measurement and then ACFs are obtained by reprojection through this image. Here the authors present a model which follows the signal-to-noise ratio (SNR) through the attenuation correction by both the direct and reconstruct-reproject methods. This model is applicable to both 2D and 3D imaging geometry, but applies to the central elements of emission and transmission objects with circular symmetry and constant amplitude. For this simplified geometry, the model predicts that the SNR of the emission image following attenuation correction is the same for both direct and reconstruct-reproject methods, although the SNRs of the ACFs are themselves substantially different. The authors also present the measured SNR at the various steps of attenuation correction for both the direct and reconstruct-reproject methods using simulated transmission and emission data. The measured SNRs agree with the model; no significant difference between the direct and reconstruct-reproject SNRs was observed.

Accurate attenuation correction for a 3D PET system.

An accurate attenuation correction has been developed for a small-volume three-dimensional positron emission tomography (PET) system. Transmission data were measured as twenty-four 2D slices which were reconstructed and combined to form a 3D attenuation image. Emission data were reconstructed using a backproject-then-filter technique, and each event was corrected for attenuation at backprojection time by a reprojection through the attenuation image. This correction restores the spatial invariance of the point response function, thus allowing a valid deconvolution and producing an undistorted emission image. Scattering corrections were not applied to either the transmission or the emission data but simulation studies indicated that scattering made only a small contribution to the attenuation measurement. Results are presented for two phantoms, in which transmission scans of 57,500 and 18,700 events/slice were used to correct emission images of 5.2 and 2.8 million events. Although the attenuation images had poor statistical accuracy and a resolution of 13 mm, the method resulted in accurate attenuation-corrected images with no degradation in image resolution (which was 3 mm for the first emission image), and with little effect on image noise.