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Three-Window Transformation Cross-Talk Correction for Simultaneous Dual-Isotope Imaging

Karin Kneaurek and Josef Machac

Division of Nuclear Medicine, Mount Sinai Medical Center, New York, New York

Correspondence: For correspondence or reprints contact: Karin Kneaurek, PhD, Division of Nuclear Medicine, Box 1141, One Gustave L. Levy Place, New York, NY 10029.

ABSTRACT

We developed and tested a new transformation cross-talk correction method for simultaneous dual-isotope SPECT imaging, which uses information from three energy windows in a simultaneous ¹⁸F/^99mTc cardiac phantom study. Methods: The method combines the previously reported "three-window" technique and transformation cross-talk correction methods. In the three-window technique, the images from the third energy "scatter" window are usually multiplied by a constant factor to obtain the estimates of the cross-talk. However, such an approach neglects differences in the spatial distribution between cross-talk photons in different energy windows. The transformation method is based on the assumption that the transformations, which convert the images from one energy window into the other energy windows, are known. These transformations were found by measuring the point-spread functions in different energy windows for both isotopes in water. The transformation method takes into account the different spatial distributions of the primary and scatter cross-talk photons in the different energy windows. Here, we are assuming that the imaging system and the image transformations between different energy windows are shift-invariant and linear. Thus, the new method is described by two convolution equations applied in frequency space. In addition to the cross-talk correction, the restoration filters were also applied to the resulting corrected images. The new method was performed on the simultaneous ^99mTc cardiac phantom study. Three separate studies were acquired in our phantom study: two single-isotope studies and one dual-isotope study. The single-isotope images were used as references. The contrast between the left ventricle cavity and the myocardium was used in transaxial slices as a parameter to evaluate results of the dual-isotope correction method with restoration. Results: The contrast improvement in the dual-isotope corrected images in both energy windows, i.e., the ^99mTc primary window (140 keV) and the ¹⁸F primary window (511 keV), was significant. The corrected 511-keV, dual-isotope image had a contrast of 0.74 compared to 0.60, which was the value in the noncorrected dual-isotope image. The improvement of the contrast in the corrected, dual-isotope 511-keV image was exclusively a result of the restoration correction. The restoration-corrected, 511-keV, single-isotope ¹⁸F image had the same contrast (0.74). For the dual-isotope, 140-keV transaxial slice,first,the contrast improved from 0.78 to 0.85 after cross-talk correction, and, then, it finally reached 0.92 after additional restoration correction.The contrast in the 140-keV, single-isotope ^99mTc image after restoration correction improved from 0.87 to 0.95. Conclusion: The three-window transformation dual-isotope correction method with restoration significantly improves the contrast between the left ventricle cavity and the myocardium of the simultaneous ^99mTc SPECT imaging.

Key Words: simultaneous dual-isotope imaging • cross-talk correction • SPECT