[Determination of blood circulation time for optimizing contrast medium administration in CT angiography]

CT angiography of intracranial vascular lesions requires a constant and high intra-arterial concentration of the contrast medium over the period of data acquisition. The volume of the injected contrast medium can be minimized by exact timing of the bolus injection. Non-predictable interindividual variance of veno-arterial circulation time, e.g., in patients with impaired cerebral hemodynamics after subarachnoidal hemorrhage, requires determination of the appropriate prescan delay.

We used the CT scanner (Philips SR 7000) with a low-dose scan parameter to determine the veno-arterial circulation time with a bolus injection of 20 ml contrast medium IV and compared these values with the results of an algorithm (t = n x 60 x f-1) based on the duration of n = 12 heart actions. On CT seven scans (1-s scan time) without table feed were applied at the level of the circle of Willis 4 s after the beginning of contrast medium bolus injection (5 ml/s). An interscan delay of 4 s was used. A region of interest was placed exactly on the most evident part of one of the cerebral base arteries contralateral to pathology.

In 35 patients we measured circulation times between 10 and 25 s, all of which were confirmed by CT angiography, which followed. There was no correlation with systolic or diastolic blood pressure, heart rate or body length, body weight or body surface. Differences of up to 18 s were seen between the measured and calculated circulation times.

Transit time to peak concentration in cerebral vessels following antecubital bolus injection of contrast medium cannot be calculated on the basis of hemodynamic parameters or body measurement, even in patients without cardiopulmonary or occlusive vessel disease. The quick method presented for measurement of the veno-arterial circulation time with a test bolus of contrast medium using the CT scanner and scan parameters with low-radiation exposure provided an exact time setting and saved on the amount of contrast medium needed in CT angiography.