Application of intravenous contrast in PET/CT: does it really introduce significant attenuation correction error?

UNLABELLED: The current perception of using contrast-enhanced CT (CECT) for attenuation correction (AC) is that of caution, as it might lead to erroneously elevated (18)F-FDG uptake on the PET scan. This study evaluates in vivo whether an intravenous iodinated contrast agent produces a significant AC artifact in the level of standardized uptake value (SUV) changes in PET/CT. METHODS: Fifty-four patients referred for whole-body (WB) PET/CT scans were enrolled and subdivided into 2 groups. In part I, 26 patients had a single WB PET scan that was corrected for attenuation using noncontrast and intravenous CECT obtained before and after the emission data, respectively. The final PET images were compared for any visual and SUV maximum (SUV(max)) measurement difference. This allowed analysis of the compatibility of the scaling processes between the 2 different CTs and the PET. The SUV(max) values were obtained from ascending aorta, upper lung, femoral head, iliopsoas muscle, spleen, liver, and the site of pathology (total, 193 regions). Part II addressed whether intravenous contrast also influenced the PET emission data. For that purpose, the remaining 28 patients underwent a limited plain CT scan from lung base to lower liver edge, followed by a 1-bed PET scan of the same region and then a WB intravenous contrast CT scan in tandem with a WB PET scan. SUV(max) values were obtained at the lung base, liver, spleen, T11 or T12 vertebra, and paraspinal muscle (total, 135 regions). The data obtained from pre- and post-intravenous contrast PET scans were analyzed as in part I. RESULTS: There was no statistically significant elevation of the SUV level in the measured anatomic sites as a whole (part I: mean SUV(max) difference = 0.06, P > 0.05; Part II: mean SUV(max) difference = -0.02, P > 0.05). However, statistically significant results as a group (mean SUV(max) difference = 0.26, P < 0.05)--albeit considered to be clinically insignificant--were observed for areas of pathology in the part I study. No abnormal focal increased (18)F-FDG activity was detected as a result of the intravenous contrast in both parts of this examination. CONCLUSION: No statistically or clinically significant spuriously elevated SUV level that might potentially interfere with the diagnostic value of PET/CT was identified as a result of the application of intravenous iodinated contrast.

The CT motion quantitation of lung lesions and its impact on PET-measured SUVs.

UNLABELLED: We previously reported that respiratory motion is a major source of error in quantitation of lesion activity using combined PET/CT units. CT acquisition of the lesion occurs in seconds, rather than the 4-6 min required for PET emission scans. Therefore, an incongruent lesion position during CT acquisition will bias activity estimates using PET. In this study, we systematically analyzed the range of activity concentration changes, hence SUV, for lung lesions. METHODS: Five lung cancer patients were scanned with PET/CT. In CT, data were acquired in correlation with the real-time positioning. CT images were acquired, in cine mode, at 0.45-s intervals for slightly longer (1 s) than a full respiratory cycle at each couch position. Other scanning parameters were a 0.5-s gantry rotation, 140 kVp, 175 mA, 10-mm couch increments, and a 2.5-mm slice thickness. PET data were acquired after intravenous injection of about 444-555 MBq of (18)F-FDG with a 1-h uptake period. The scanning time was 3 min per bed position for PET. Regularity in breathing was assisted by audio coaching. A commercial software program was then used to sort the acquired CT images into 10 phases, with 0% corresponding to end of inspiration (EI) and 50% corresponding to end of expiration (EE). Using the respiration-correlated CT data, images were rebinned to match the PET slice locations and thickness. RESULTS: We analyzed 8 lesions from 5 patients. Reconstructed PET emission data showed up to a 24% variation in the lesion maximum standardized uptake values (SUVs) between EI and EE phases. Examination of all the phases showed an SUV variation of up to 30%. Also, in some cases the lesion showed up to a 9-mm shift in location and up to a 21% reduction in size when measured from PET during the EI phase, compared with during the EE phase. CONCLUSION: Using respiration-correlated CT for attenuation correction, we were able to quantitate the fluctuations in PET SUVs. Because those changes may lead to estimates of lower SUVs, the respiratory phase during CT transmission scanning needs to be measured or lung motion has to be regulated for imaging lung cancer in routine clinical practice.

Four-dimensional (4D) PET/CT imaging of the thorax.

We have reported in our previous studies on the methodology, and feasibility of 4D-PET (Gated PET) acquisition, to reduce respiratory motion artifact in PET imaging of the thorax. In this study, we expand our investigation to address the problem of respiration motion in PET/CT imaging. The respiratory motion of four lung cancer patients were monitored by tracking external markers placed on the thorax. A 4D-CT acquisition was performed using a "step-and-shoot" technique, in which computed tomography (CT) projection data were acquired over a complete respiratory cycle at each couch position. The period of each CT acquisition segment was time stamped with an "x-ray ON" signal, which was recorded by the tracking system. 4D-CT data were then sorted into 10 groups, according to their corresponding phase of the breathing cycle. 4D-PET data were acquired in the gated mode, where each breathing cycle was divided into ten 0.5 s bins. For both CT and PET acquisitions, patients received audio prompting to regularize breathing. The 4D-CT and 4D-PET data were then correlated according to respiratory phase. The effect of 4D acquisition on improving the co-registration of PET and CT images, reducing motion smearing, and consequently increase the quantitation of the SUV, were investigated. Also, quantitation of the tumor motions in PET, and CT, were studied and compared. 4D-PET with matching phase 4D-CTAC showed an improved accuracy in PET-CT image co-registration of up to 41%, compared to measurements from 4D-PET with clinical-CTAC. Gating PET data in correlation with respiratory motion reduced motion-induced smearing, thereby decreasing the observed tumor volume, by as much as 43%. 4D-PET lesions volumes showed a maximum deviation of 19% between clinical CT and phase- matched 4D-CT attenuation corrected PET images. In CT, 4D acquisition resulted in increasing the tumor volume in two patients by up to 79%, and decreasing it in the other two by up to 35%. Consequently, these corrections have yielded an increase in the measured SUV by up to 16% over the clinical measured SUV, and 36% over SUV's measured in 4D-PET with clinical-CT Attenuation Correction (CTAC) SUV's. Quantitation of the maximum tumor motion amplitude, using 4D-PET and 4D-CT, showed up to 30% discrepancy between the two modalities. We have shown that 4D PET/CT is clinically a feasible method, to correct for respiratory motion artifacts in PET/CT imaging of the thorax. 4D PET/CT acquisition can reduce smearing, improve the accuracy in PET-CT co-registration, and increase the measured SUV. This should result in an improved tumor assessment for patients with lung malignancies.