Abstracts

Rationale: Subtraction ictal single-photon emission computed tomography (SPECT) co-registered to MRI (SISCOM) is a valuable non-invasive neuroimaging tool in the presurgical evaluation of patients with medically intractable focal epilepsy (MIFE). Complex hyperperfusion patterns observed in SISCOM maps can help elucidate areas of seizure generation and subsequent propagation and aid in localization of the epileptic focus, thereby contributing to surgical planning for intracranial evaluation and surgical resection (1, 2). Ictal SPECT injection relies on the timely intravenous administration of radioactive tracer during an epileptic seizure. An early ictal SPECT injection is critical for the accurate localization of the relevant seizure networks. Although the time of injection is observed from video recordings, the actual arrival time for the nuclear tracer to reach the brain is not known precisely and has not been investigated rigorously. In this study, we detect the uptake period tracer in the brain by placing sensors on patients undergoing their routine interictal SPECT studies that were subtracted from the corresponding ictal SPECT data. Measurement of tracer kinetics will provide an objective tool for accessing the quality of SPECT studies and potentially will improve the interpretability of the observed perfusion pattern.

Methods: After obtaining IRB approval, we recruited ten adult patients undergoing interictal SPECT investigations. Their routine medical care involved the injection of the radiotracer (Tc-99m Neurolite) followed by SPECT imaging one hour later. Four external radiation detectors (Lara System, Lucerno Dynamics) were mounted before tracer injection: one on the arm proximal to the injection site, one on the contralateral arm (serving as a reference), and two on the scalp. Detected radioactivity in counts per second (cps) was recorded for each detector at a 1-second sampling rate.

Results: The Lara System detectors had sufficient sensitivity to measure the time-activity curve at the brain, typically detecting 2500 cps (net) from a nominal 20 mCi injection. The optimal location of the brain detectors was at the vertex, which minimized background counts (typically 300 cps) from radioactive tracer located outside the brain. The start and end of the brain uptake period were determined from the measured time-activity curve (Figure 1). Initial detection of the tracer at the brain ranged from 19 to 23 secs (average 21 sec) after the start of injection. The time to reach 80% of maximum tracer uptake ranged from 25 to 43 sec (average 32 sec) after the start of injection. Outlying cases indicated potential variability from tracer injection technique (e.g., timing of saline flush), patency of intravenous line, and patient-dependent blood flow.

Conclusions: The feasibility of measuring the brain tracer uptake period in a clinical setting was demonstrated in interictal studies. This technique has potential benefits in quality assurance and optimization of tracer injection and in the interpretation of SISCOM studies.

References:
1. W. Van Paesschen, Epilepsia. 45, 35–40 (2004).
2. B. Krishnan et al., Neuroimage (2021).

Funding: Please list any funding that was received in support of this abstract.: Lucerno Dynamics (loan of Lara System for this research study).