Effect of Non-Steady-State Perfusion on Xenon-133 Cerebral Blood Flow Measurements: An Analytical Study

Abstract

Activation studies employing the noninvasive xenon-133 technique are widely used to investigate the cerebral circulation. Typical examples are the investigation of hemispheral specialization of higher cortical function with cognitive activation or the assessment of the hemodynamic reserve in occlusive cerebrovascular disease by CO2 inhalation. Traditionally, in studies using this technique, there is the requirement of a circulatory steady state during the measurement. Due to limitations in the duration of the stimulus or habituation to the stimulus, the basic assumption is often violated. In this study we investigated with the aid of a computer model to what extent blood flow measurement results are affected by non-steady-state blood flow. The findings indicate that cortical activation need not extend throughout the whole measurement to be detectable. Maintenance of activation for at least 5 min is sufficient for a successful measurement. In addition, the results show that the activation should be fully established when the measurement starts to achieve maximal sensitivity. Delay in activating the circulation will result in attenuated responses, especially if the stimulus is delayed beyond 2 min.

Effect of Xenon-Induced Flow Activation on Xenon-Enhanced Computed Tomography Cerebral Blood Flow Calculations

“…To describe blood flow activation, the above equation can be generalized to a non-steady state (Jaggi and Noordergraaf, 1995;Kreyszig, 1972):…”

“…The present study was designed to confirm and extend these findings by generating more realistic uptake curves derived from blood velocity observations. Rather than assuming discrete CBF changes at particular time points, the Kety equation describing s Xe uptake and clearance (Kety, 1951) was generalized to include time-varying changes in blood flow (Jaggi and Noordergraaf, 1995), i.e., CBF = f(t). Analysis of the resulting uptake curves by the traditional Kety equation, in which CBF is con stant, permitted assessment of the influence of s Xe acti vation on routinely calculated CBF values.…”

Cerebral Blood Flow During Spike‐Wave Discharges

“…The EEG showed consistent patterns during each study except during the theta state in session 1, when spike-wave discharges appeared 5 min after the start of the study. Because the IS computation was nearly complete at that time (>92%) (14), the IS measure mainly reflected the theta state, which was treated as a unitary entity for purposes of CBF measurement since the EEG spontaneously fluctuated between frontal and more generalized theta during the course of any single study. All IS values reported were corrected to the patient's mean end-tidal pC02 of 33 mm Hg.…”