Abstract
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Introduction: Availability of highly efficient digital PET/CT systems have led to faster imaging Rb-82 myocardial perfusion PET/CT protocols and lower patient radiation dosages. Though there are several advantages to these rapid protocols, shorten9ng the time between the rest and stress dosages may introduce contamination stress dynamic study with counts from the resting dosage. Because of the lower extraction fraction at high flow rates, this small contribution of counts from the resting study can have a disproportionate effect on the blood flow calculation. We tested a methodology for measuring the impact of rest dosage that is present in the stress blood flow study and performed a correction of the stress blood flow study. We then examined the impact of these corrections on stress myocardial blood flow and blood flow reserve.
Methods: A group of 25 consecutive clinical patients (n(male)=12) were imaged using a Siemens Vision 450 digital PET/CT. Patient were first imaged using a low-dose CT for attenuation correction followed immediately by an injection of 25 mCi of Rb-82. Patients were then imaged in list mode for 7 minutes. Patients were then immediately stressed and injected at the peak of stress with a second dose of 25 mCi of Rb-82 followed by an additional 7 minutes of PET imaging. Following PET imaging, patients received a low dose stress CT for attenuation correction. Data was rebinned into dynamic (8 x 12 secs, 2 x 27 secs), summed (90 second post injection delay) and gated data (8 time gates). Data were reconstructed using time-of-flight reconstruction. Myocardial blood flow and rest spillover correction was performed using the ImagenQ software package (CVIT, Kansas City, MO). This blood flow package uses a net retention model for blood flow calculation. This spillover correction uses a first frame measurement of the residual activity and applies that activity measurement to correct later uptake frames for rest residual resting dosage.
Results: The average time between the start of the rest scan and start of the stress scan was 577±32 seconds. This corresponds to an average decay factor of 0.0052±0.0013. Measurements of the residual dose at the start of stress imaging were 1.1±0.7% of the activity in the myocardium at 136 seconds into imaging (mean of the uptake frame used in the retention model).
Conclusions: This simple initial activity spillover correction technique can be employed to remove the influence of rest spillover activity for the calculation of blood flow in highly efficient digital PET. Because the rest spillover correction only needs to be applied on the final frame, this initial activity spillover correction is robust when using the retention model. Furthermore, though the effect of spillover in Rb-82 imaging is relatively low, quantitative studies using longer lived radionuclides like F18 flurpiridaz and Tc-99m SPECT tracers may benefit from this initial activity spillover correction.
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