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

Abstract

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.

Comparison of 18F SPECT with PET in myocardial imaging: A realistic thorax-cardiac phantom study

“…In the SPECT study (Fig. 3 ) the 99m Tc images have better resolution and contrast than the corresponding 18 F images [ 5 , 11 ]. However, in the PET study (Fig.…”

“…The standard deviation was less than 0.5 mm in all measurements and was obtained from three measurements. This shows that planar resolution for the UHE collimator is better for 99m Tc than for 18 F. Septal penetration for the UHE collimator is 6.5% and for the low-energy-high-resolution collimator is 0.50% [ 5 ]. The quoted septal penetration values were calculated for only single septi.…”

“…The main limitations of this technique are poor resolution of the 18 F and 99m Tc SPECT images and, when acquired simultaneously, degradation of the 99m Tc MIBI images due to 18 F downscatter to the 99m Tc window. The 18 F images are also influenced by high septal penetration and poor sensitivity, due to the limited stopping power of the detector crystal in standard gamma cameras [ 5 ]. In PET centres without a cyclotron, sequential Rubidium-82 ( 82 Rb) and 18 F FDG perfusion/viability myocardial imaging are the most appropriate, because of the availability of both isotopes [ 6 - 10 ].…”