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This Article Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Dewaraja, Y. Articles by Koral, K. PubMed Articles by Dewaraja, Y. Articles by Koral, K.

Effects of dead time and pile up on quantitative SPECT for I-131 dosimetric studies

¹ Radiology, University of Michigan, Ann Arbor, Michigan; ² Medical Radiation Physics, University of Lund, Lund, Sweden

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Objectives: To evaluate quantitative SPECT at the high count-rates typical for patient imaging following I-131 tositumomab therapy.

Methods: For a SPECT/CT system dead time measurements were carried out for triple energy windows (TEW) using a phantom with 18 mCi of I-131. In addition, three (M1, M2, M3) SPECT and energy spectra measurements were performed of the same phantom over one month under conditions that were identical apart from the count rate (21, 9 and 2 kcps respectively in main window, typical for imaging at day 1 post-therapy, day 3 post-therapy and 1 hr post-tracer). Data was reconstructed using previously developed 3D OSEM with 1) TEW scatter correction and no dead time correction 2) TEW correction and dead time correction of the main window 3) Monte Carlo (MC) based scatter correction and dead time correction of the main window.

Results: The main window data was fitted well with a paralyzable model (dead time constant of 2.5 µs), but due to pile up effects the model was not applicable to the adjacent scatter windows. The spectra showed that under identical scatter conditions, the proportion of photons detected in the scatter windows increased significantly (up to 29%) with count rate. Hence, at high count rates scatter is overestimated and activity is underestimated with TEW correction. For the 3 reconstructions the errors (compared to truth) in SPECT derived phantom activity ratios M1/M2, M1/M3, M2/M3 were: 1) 8%, 15%, 8%; 2) 4%, 8%, 4%; 3) -2%, -1%, 1%.

Conclusions: At high count rates, I-131 activity is underestimated because of dead time and pile up. Although quantification was improved with dead time correction, further improvements were achieved when the TEW correction was replaced by a MC correction, since it utilizes only the main window data.

Research Support: NIH 2R01 EB001994

This Article Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Dewaraja, Y. Articles by Koral, K. PubMed Articles by Dewaraja, Y. Articles by Koral, K.