[A new approach to eliminate background count from gated blood pool image (iterative constant subtraction method)....

It is important for obtaining a left ventricular volume curve and ejection fraction from a multigated blood pool image (MGBP image) to accurately subtract the background (BG) from the image. We devised a new method of accurately subtracting the BG with good reproducibility. This method (tentatively called the ICS method) is as follows. An image, on which n times (n = 0, 1, 2, 3,..., N-1) an adequately low constant (k) is subtracted from the MGBP image at every pixel of the image matrix, is prepared. When n equals 0, constant is not subtracted from the image, therefore it is original MGBP image. Then the original (n = 0) and constant (nk: n = 1, 2, 3,..., N-1) subtraction MGBP images were prepared. The constant was defined as about 5% of the maximum pixel count in the end-diastole of the left ventricle. Rough region of interest (ROI) were set at the left ventricle on these MGBP images, and time activity curves (TACs, left ventricular volume curves) on each image were prepared. These N kinds of curves are shifted in parallel for normalization in the first frame (end-diastole). The first frame count on the original MGBP image is normalized. As a result, TACs obtained by inadequate subtraction in which subtraction of the constant does not reach the BG count, overlap completely, while TACs obtained by subtraction over the BG gradually separate from the overlapping curves. The MGBP image including a curve located in the border between the overlapping curve and the separated curve is regarded as the image accurately subtracting the BG, and the constant of subtraction (nk) as the BG count for each pixel. The border is obtained by analyzing changes in count of the end-systolic frame of each TAC normalized. The ICS method provides computer software that is performed automatically within a short time by setting a rough ROI at the left ventricle on the end-diastolic MGBP image.