Abstract
BackgroundFor therapeutic applications of several isotopes (e.g., 131I, 153Sm, 177Lu) in nuclear medicine, the high activities typically applied require accurate dead time correction in early time point imaging. We present a novel, straightforward dead time correction method using the Lambert W function, which is in principle exact for the paralyzable detector model with a single parameter τ (i.e., dead time).ResultsAs a proof of concept, the method is validated with a simple model: a commonly used isotope, 99mTc, with a single photopeak. We measured count rates of a gamma camera both intrinsically and extrinsically (i.e., with collimators) with point sources in air and in a scatter phantom (extrinsic only). τ was estimated for both open window (τOW) and a 99mTc photopeak window (τTc), using a “graphical” method for fitting the count rate of decaying sources. These values for τ were subsequently used for dead time correction.τ varied significantly between the different geometries for both energy windows, but τOW was more reproducible than τTc, particularly for the scatter phantom measurements.τOW measured from the phantom measurements was approximately 30% lower than τOW from the intrinsic measurement but corresponded within 15% with the extrinsic point source measurements. Accordingly, using the intrinsic τOW led to an overcorrection of 8% at high count rates; τOW from the extrinsic point source measurements corrected the phantom measurement to within 2%.However, significant differences were observed between τTc values. All measured τTc values underestimated dead time losses in a second independent phantom measurement, with even τTc from the first phantom measurement underestimating activity with 5–10% at the highest count rates. Based on measurements of the effect of energy window settings and geometry, we tentatively attribute the added dead time losses to pulse pile-up.ConclusionsAnalytic dead time correction based on the Lambert W function is accurate for the range in which gamma detectors behave as paralyzable systems. However, further investigation indicated measured τ values to be variable with geometry as well as window fraction. We propose that dead time correction should be based on the open window value, τOW, corrected for window fraction.
Highlights
For therapeutic applications of several isotopes (e.g., 131I, 153Sm, 177Lu) in nuclear medicine, the high activities typically applied require accurate dead time correction in early time point imaging
For quantification of nuclear medicine images, the number of detected counts within a region of interest (ROI) or image is assumed to be proportional to the activity A within the region
The value of τOW, measured from the extrinsic scatter phantom measurement with the open energy window, was for both detectors approximately 30% lower than that determined from the intrinsic measurement, while it corresponded quite well with the extrinsic point source measurement (< 15%)
Summary
For therapeutic applications of several isotopes (e.g., 131I, 153Sm, 177Lu) in nuclear medicine, the high activities typically applied require accurate dead time correction in early time point imaging. Á ; ð1Þ with CEW the sensitivity and Rt,EW the count rate for a chosen energy window EW, and A the activity within the imaged region of interest. For a point source in air, with negligible absorption and background, the denominator in Eq (1) can be ignored, as it is unity. This results in the count rate Rt,EW being proportional to activity A in the imaged region, with proportionality constant or linear response coefficient [2], CEW 1⁄4 Rt;EW
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