Nuclear Cardiology: Nuclear Cardiology:

258 Nuclear Cardiology, The BasicsBecause the required number of counts for these high-count floods isvery high (30 million counts), they are often acquired overnight andanalyzed in the morning. For a multiple-headed camera system, it maytake a number of hours to acquire the uniformity flood.QC of daily uniformity flood (2–6 million counts) can be doneextrinsically or intrinsically.An extrinsic flood is usually done with a Co-57 sheet source, orfillable flood source with Tc-99m, Tl-201, or any other isotope usedin the laboratory. The source is placed on the detector’s collimator. Astatic image is acquired and then analyzed for uniformity. Each vendormay have different acquisition parameters, and one should checkwith the manufacturer for specific recommendations. Most systemshave an automatic program for analyzing the static image for percentuniformity. Again, each manufacturer will have different uniformitytolerance limits. It is recommended that one always visually inspectthe image for hot and cold areas, indicating nonuniformity. A dailylog should kept to record the percent uniformity daily. This allows oneto look for slow drifts in uniformity.Intrinsic floods are done with a Tc-99m point source with a smallvolume (0.5 mL) and low activity (100–200 µCi) acquired with thecollimator off the gamma camera detector head. A static image isacquired and then analyzed for uniformity. Just as with extrinsic floodsit is recommended that one visually inspect the image for hot and coldareas indicating nonuniform areas. Daily percent uniformity shouldbe recorded in a log. If any problems with the uniformity are noted,camera service should be called in immediately.Two parameters are used to measure and document flood uniformity:Integral uniformity: This is a global parameter that measures contrastover an extended area of the detector and is expressed as percentage[100% × (max–min)/(max+min)].Differential uniformity: This is a regional parameter that measurescontrast over a small area. The measurement is performd using a 5 × 1pixel area in both the X and Y directions and is expressed as percentage(100% × largest deviation (max–min)/(max+min).Both percentages must be < 5%. (Max = maximal count found in anypixel within the specified area; min = mimimal count found in any pixelwithin the specified area.)