Required time delay from 99mTc-HMPAO injection to SPECT data acquisition: healthy subjects and patients with rCBF pattern.

Purpose: Functional brain (99m)Tc-HMPAO single-photon emission computed tomography (SPECT) is a useful diagnostic tool for assessment of regional cerebral blood flow, particularly in dementia, cerebrovascular disease and epilepsy. Currently, the European and American Association of Nuclear Medicine Procedure Guidelines for Brain Perfusion SPET using (99m)Tc-labeled Radiopharmaceuticals recommend a time delay of 90 min between injection of (99m)Tc-HMPAO and data acquisition. This time delay is difficult to comply within the daily routine and present a problem, particularly with the elderly or demented patients. This study investigates in patients with perfusion deficits and in healthy subjects if the quality of the SPECT image is affected by lowering the time delay between (99m)Tc-HMPAO injection and data acquisition to 30 or 60 min.

Methods: Thirty-seven healthy subjects (17 females; mean age 65; range 42-84 years) with normal cerebral blood flow distribution and 31 patients (17 females; mean age 67; range 38-84) with reduced rCBF distribution were included. Images were obtained with a three-headed Philips IRIX SPECT scanner with high-resolution collimators. The healthy subjects were scanned 30, 60 and 90 min after (99m)Tc-HMPAO injection, and the patients were scanned 30 and 90 or 60 and 90 min after (99m)Tc-HMPAO injection. For evaluation of differences between the images obtained at various time points after injection, two different methods were used. The z-map method was used to subtract images from each other prior to visual inspection. In addition, principal component analysis was used as a quantitative analysis of the similarity of the images.

Results: Visual inspection of the subtracted images (30 or 60 versus 90 min) revealed that there was no spatial bias. Quantitatively, the average proportion of the total variance explained by the first principal component was 99.5% (range 98.9-99.6) for the healthy subjects and 99.4% (range 98.5-99.8) for the patients.

Conclusion: The time delay from injection of (99m)Tc-HMPAO to the start of the SPECT data acquisition can be reduced from 90 to 30 min without any significant impact on the quality of the acquired image.