MR EVALUATION OF ACOUSTIC SCHWANNOMA WITH FRACTIONAL CONTRAST DOSES

Objective: To investigate the utility of lower contrast medium doses f or the detection and conspicuity of acoustic schwannomas. Materials an d Methods: The L/B (L, lesion; B, background) ratios or lesion contras t of 17 pathologically proven acoustic schwannomas studied with a stan dard dose (0.1 mmol/kg) of gadopentetate dimeglumine was measured. In addition, 22 patients with acoustic schwannomas were studied prospecti vely with fractional doses using the incremental dose technique. Each patient received an initial bolus injection of one-eighth the standard dose (0.0125 mmol/ kg) followed by an injection of one-eighth, one-fo urth, and one-half the standard dose at 5 min intervals to achieve a c umulative dose of one-fourth, one-half and full dose, respectively. Im aging was performed immediately after each injection. Results: Standar d dose-The L/B ratios of pathologically proven acoustic schwannomas to mastoid air cells ranged from 14.8 to 41.2 (mean +/- SEM, 28.0 +/- 1. 95), which were approximately 17 times more than those of intraparench ymal lesions. Fractional cumulative dose-Qualitative visual analysis d emonstrated that all acoustic schwannomas showed apparent enhancement at one-fourth dose. Intense enhancement was noted at one-half and full dose. Quantitative analysis demonstrated the mean L/B ratios between the acoustic schwannomas and mastoid air cells of the precontrast and one-eighth, one-fourth, one-half, and full dose studies were 8.33 +/- 0.52, 11.21 +/- 0.75, 13.02 +/- 0.83, 15.38 +/- 0.98, and 18.03 +/- 1. 36, respectively. Conclusion: The L/B ratios or lesion contrast of aco ustic schwannomas at various fractional contrast medium doses was sign ificantly higher compared with that of intraparenchymal lesions. Thus, the standard contrast medium dose may not be necessary for detection of acoustic schwannomas, and a fractional dose may be sufficient. Alth ough the optimal fractional dose remains to be determined, one-half of the standard dose (0.05 mmol/kg) appears to be sufficient because of intense enhancement at this dose.