A simplified reference tissue model with linear regression and spatial constraint (SRTM-LRSC; Zhou et al.
Neuroimage 2007) is a sensitive method for detection of subtle changes in distribution volume ratios in nondemented
older adults using a 90 min dynamic acquisition. As a shorter scan time is an important consideration
in PET studies of older adults, we investigate whether 70 min dynamic data acquisition provides an acceptable
alternative to 90 minutes using SRTM-LRSC.
Methods: Sixty-five non-demented participants in the Baltimore Longitudinal Study of Aging (age 78.2±6.6 years;
6 with CDR=0.5, 26 with follow-up [11C]PIB study) had dynamic [11C]PIB PET scans analyzed for (0,90 min; DVR90)
and (0,70 min; DVR70) timeframes. Fifteen regions of interest were defined on co-registered MRIs, and cerebellum
was used as a reference tissue. Regional DVRs were obtained via SRTM-LRSC using (0,90 min) and (0,70 min)
data. The mean cortical DVR (mcDVR) of 8 cortical regions was calculated for both timeframes. Correlations
between DVR70 and DVR90, possible systematic differences in mcDVRs for the two measures, as well as changes
in DVR over time for DVR70 and DVR90, were evaluated.
Results: DVR70 is highly correlated with DVR90 (DVR70=0.0155+0.953 * DVR90, r=0.998). DVR70 is lower than
DVR90 by 3.3% with mean difference of 0.04 (SD 0.02) DVR. DVR70 for posterior cingulate showed the greatest
underestimation (4.1%). The difference between DVR70 and DVR90 was not related to mcDVR in older adults with
elevated mcDVR (mcDVR>1.1, p=0.51). Changes in mcDVR over time differ by 0.002 (SD=0.013).
Conclusions: DVR obtained using 70 min data is highly correlated with the DVR obtained using 90 min data.
Underestimation of mcDVR obtained using 70 min data is within the test-retest variability of [11C]PIB and does not
increase with increasing DVR, providing an alternative timeframe for dynamic [11C]PIB studies analyzed by SRTM.
Support: This research was supported by the Intramural Research Program of the NIH, National Institute on Aging
N01-AG-3-2124 and K24 DA000412(DFW).