Routine 18F-2-deoxy-2-fluoro-D-glucose (18F-FDG) myocardial tomography using a normal large field of view gamma-camera

Comparison of 99mTc-sestamibi-18F-fluorodeoxyglucose dual isotope simultaneous acquisition and rest-stress 99mTc-sestamibi single photon emission computed tomography for the assessment of myocardial viability

Dual isotope simultaneous acquisition single photon emission computed tomography (DISA SPECT) offers the advantage of obtaining information on myocardial perfusion using Tc-sestamibi ( Tc-MIBI) and metabolism using F-fluorodeoxyglucose ( F-FDG) in a single study. The prerequisite is that the Tc-MIBI images are not degraded by scattered 511 keV photons or poor count statistics due to the lower efficiency of the extra high energy (EHE) collimator. Therefore, we compared the registered Tc-MIBI uptake and image quality of DISA and single isotope acquisition. Furthermore, we investigated whether DISA yields additional information for the assessment of myocardial viability in comparison with rest-stress Tc-MIBI. Nineteen patients with known coronary artery disease and irreversible perfusion defects on previous rest-stress MIBI test studies were investigated. After oral glucose loading and simultaneous injection of 600 MBq of Tc-MIBI and 185 MBq of F-FDG at rest, DISA was performed using energy windows of 140 (+/-15%), 170 (+/-20%) and 511 keV (+/-15%). Planar 140 keV images were corrected for scatter by subtraction using the 170 keV window. The single and dual isotope Tc-MIBI images were both displayed in a polar map with 128 segments normalized to maximum counts. F-FDG and Tc-MIBI images were visually scored for a perfusion-metabolism mismatch pattern using nine regions per heart. There was an excellent correlation (r =0.93, P<0.0001) between the Tc-MIBI uptake detected in the single and dual isotope acquisition. The average difference between the dual and single isotope Tc-MIBI uptake was -1.2% (not significantly different from zero) and the coefficient of variation of the difference was 8.7%. Of the 79 regions with irreversible perfusion defects on previous rest-stress Tc-MIBI, six regions in five patients showed a perfusion-metabolism mismatch pattern. We conclude that DISA does not affect the quality of the Tc-MIBI images. Furthermore, F-FDG- Tc-MIBI DISA may show viability in a small but significant (7.6%, P<0.0034) number of regions with irreversible perfusion defects on rest-stress Tc-MIBI.

Improvement of the fluorine-18 fluorodeoxyglucose images in simultaneous F-18 FDG/Tc-99m collimated spect imaging

Collimated F-18 FDG SPECT imaging has been shown to be an acceptable alternative to F-18 FDG PET imaging for evaluating injured but viable myocardium. Ultra-energy (UHE) imaging is usually performed in simultaneous F-18 FDG/Tc-99m MIBI studies. The main limitations of this technique are degradation of the Tc-99m MIBI images due to F-18 downscatter to the Tc-99m window, and loss of resolution in Tc-99m images caused by using a UHE rather than a low-energy collimator. The quality of F-18 images has not been addressed. In our clinical and phantom studies we have found that F-18 images are inferior to simultaneously acquired Tc-99m MIBI images. This paper compares two correction methods for F-18 FDG images in a realistic cardiac phantom study. One approach is subtractive scatter correction, which employs a third 410 keV energy window image to estimate scatter. The other approach is based on restoration. The phantom acquisition was performed with 7.2 MBq of F-18 and 22.2 MBq of Tc-99m injected into the left ventricular (LV) wall. Three inserts, 3 cm, 2 cm, and 1 cm in diameter, were placed in the LV wall to simulate infarcts. Circumferential profiles were drawn from three successive short-axis slices and compared with true phantom data. The differences were calculated as root-mean-square error (rmse). Scatter correction improved rmse only 4.5 +/- 0.3%, while restoration improved rmse 16.1 +/- 0.4%, when compared with raw data. The same differences, measured as rmse, were 9.5 +/- 0.5, 6.8 +/- 0.4, and 5.1 +/- 0.5 for raw, scatter corrected, and restored F-18 data, respectively, when compared with Tc-99m window 140 keV data. The amount of noise, measured as root-mean-square % (rms%) was 5.3 +/- 0.5% for the Tc-99m image, 4.9 +/- 0.7% for the F-18 restored image, 6.2 +/- 0.6% for the raw F-18 image, and 6.5 +/- 0.9% for the scatter corrected F-18 image. The contrast measured for 2 cm and 3 cm inserts was 0.17 +/- 0.07 and 0.26 +/- 0.06 for F-18 raw data, 0.19 +/- 0.08 and 0.29 +/- 0.06 for the scatter corrected F-18 image, and 0.28 +/- 0.06 and 0.43 +/- 0.07 for the restored F-18 image. The contrast was 0.20 +/- 0.07 and 0.46 +/- 0.05 for the Tc-99m 140 keV window image. The restoration approach provided F-18 images of better contrast and detectibility than uncorrected or scatter corrected F-18 images. Restored F-18 images match better with the simultaneously acquired Tc-99m images.

Influence of high and low plasma insulin levels on the uptake of fluorine-18 fluorodeoxyglucose in myocardium and femoral muscle, assessed by planar imaging

The aim of this study was to optimize the metabolic conditions for planar myocardial fluorine-18 fluorodeoxyglucose (FDG) imaging. The effects of high and low insulin levels during euglycaemic clamping on myocardial and femoral muscle FDG uptake were compared since insulin plays a major role in glucose metabolism. FDG uptake in 11 patients was studied using planar scintigraphy. Patients were studied twice: in the low-dose insulin protocol (LDI), insulin was infused at a rate of 20 mU/kg per hour, starting 1 h before FDG administration, while in the high-dose insulin protocol (HDI) it was infused at a rate of 100 mU/kg per hour. Glucose infusion rate was adjusted according to frequently determined blood glucose levels. Somatostatin was infused to block endogenous insulin release. Planar images were obtained from the thorax region and femoral muscles. Regions of interest were drawn over normal and abnormal myocardial areas (based on angiographic and thallium-201 data) and over lung, liver and muscle areas. After clamping, insulin levels during LDI and HDI at t = 60 were 30.6 +/- 13.3 and 129.6 +/- 30.5 mU/l respectively (P < 0.0001). Femoral muscle uptake was significantly higher during HDI (P < 0.001). Uptake in normal and abnormal myocardial areas did not differ between the two protocols. Heart/lung ratios (NS) and heart/liver ratios (P < 0.05) increased during HDI. It may be concluded that planar FDG imaging is influenced by plasma insulin levels. The euglycaemic hyperinsulinaemic clamp technique, although more demanding, gives an adjustable metabolic steady state without significantly altering the FDG uptake patterns in normal and abnormal myocardial regions. The image quality of planar FDG images improves due to lower background uptake of FDG during clamping with high plasma insulin levels.