Approximation of arterial input curve data in MRI estimation of cerebral blood-tumor-barrier leakage: Comparison between Gd-DTPA and 99mTc-DTPA input curves

Accurate and reproducible MRI measurements of the blood-brain-barrier transport rate constant Ki require measurements of the arterial Gd-DTPA concentration to characterise the arterial input function to the brain. Methods of measuring [Gd] in human plasma are not common practice in clinical departments. In this study we present a fast and easy method of substituting Gd-DTPA with 99mTc-DTPA as the ‘tracer’ to be measured for the characterisation of the arterial input curve in MRI blood-brain-barrier (BBB) measurements. Gd-DTPA and 99mTc-DTPA was injected intravenously as a bolus into six brain tumor patients during dynamic, fast gradient-echo MRI. Arterial blood samples were collected with an interval of 10 s during imaging. 99mTc-DTPA and Gd-DTPA were measured in the same blood samples by scintillation counting and Inductively Coupled Plasma Atomic Emission Spectroscopy respectively. The two methods yield the concentrations of both the tracers in mmol/l Gd-DTPA and hence the results are directly comparable. The data results showed no statistically significant difference between the two methods. However, the 99mTc-DTPA method had fewer ‘bad points’ than the direct Gd-DTPA measurement method. The input curves were in agreement. Conclusively, we found that Gd-DTPA and 99mTc-DTPA were equivalent with regard to distribution and elimination kinetics during the first 10 min after administration and, therefore, characterisation of arterial input curves may be done using simultaneous injection of 99mTc-DTPA and Gd-DTPA in facilities where means of measuring Gd-DTPA is not available.