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¹⁶⁹Yb-DTPA Distribution and Dosimetry in Cisternography

Veterans Administration Hospital and University of Florida College of Medicine, Gainesville, Florida

Correspondence: For reprints contact: Valerie A. Brookeman, University of Florida College of Medicine, Dept. of Radiology, Box 219, Gainesville, Fla. 32610.

ABSTRACT

Following intrathecal administration of 1 mCi ¹⁶⁹Yb-DTPA to nine patients, spinal counts were obtained from 2 to 48 hr with a gamma camera/computer system. Utilizing computer outputs, the length of the spine was divided into six equal segments approximately 2 in. wide. The counts in each segment at each time were corrected for the surrounding blood background and converted to the percent of administered activity by reference to a standard. The cumulated activity in each segment was determined by graphic integration of mean activity-time curves. Absorbed radiation doses were computed for cylindrical geometry utilizing appropriate reduction coefficients for the dose contribution from electrons. Maximum doses to the surfaces of the cord and nerve roots are 31 ± 18 and 46 ± 20 rads, respectively, for 1 mCi ¹⁶⁹Yb-DTPA administered. By a depth of 0.01 cm, the thickness of the pia, these doses decrease to the average values of 5 ± 3 and 8 ± 3 rads, respectively. Surface and average doses to the cord and nerve roots from intrathecal administration of 400 µCi ¹⁶⁹Yb-DTPA are about the same as those from 100 µCi ¹³¹I-IHSA.