Regional multiparameter estimation from tomographic diffusible tracer clearance curves: modification of the double-integral method.

ABSTRACT

Measurement of regional CBF in transverse section using inert, diffusible tracers can be carried out using a double-integral form of the Kety-Schmidt equation. An implementation of this form proposed by Kanno and Lassen (the K-L estimator) is utilized by the Tomomatic 64, a dynamic single-photon computed tomography system that records washin-washout data during and following inhalation of 133Xe gas. Advantages of the algorithm include noninvasive calibration of the input function and excellent depiction of ischemia disadvantages are sensitivity to errors in input function delay (delta), the inability to estimate the partition coefficient (lambda) (hence, only the clearance index, k, is estimated), and a noise sensitivity proportional to k. A modification of the method is proposed that not only accounts for delta variations, but also provides an estimate of relative lambda (hence, the perfusion, f, is estimated). The proposed estimator is shown to be robust in the presence of noise with error variances equal to or better than those with the K-L estimator, yet estimates of both relative f and lambda are provided by the modification. New simulation results implicate the Compton scatter fraction as a major contributor in the overestimation of white matter perfusion values using both the K-L and proposed estimators, and illustrate the need for hardware and software scatter fraction reduction and control.