CONTRAST-ENHANCED MAGNETIC-RESONANCE-IMAGING IN A SPINAL EPIDURAL TUMOR-MODEL

RATIONALE AND OBJECTIVES. A spinal epidural tumor model was developed, using the VX-2 adenocarcinoma in rabbits, to assess the strengths and weaknesses of magnetic resonance (MR) as a cross-sectional imaging mo dality for the evaluation of epidural neoplastic disease. High-resolut ion MR images were acquired both before and after intravenous gadolini um chelate injection, assessing lesion detectability and efficacy of i maging technique. METHODS. An adenocarcinoma tumor (VX-2) was produced in the epidural space of six New Zealand White rabbits and subsequent ly studied on a 1.5 tesla whole body MR scanner. VX-2 tumor tissue was removed from the thigh of a carrier rabbit, minced, and screened. Und er fluoroscopic guidance, 0.2 ml of the tumor preparation was then inj ected into the epidural space of the experimental rabbits. The injecti on was performed at the L5-6 level using an epidural needle and polyet hylene tubing sleeved within the needle. The rabbits were imaged using a circular small parts surface coil 5 to 15 days after the epidural i njection. In all six animals, one complete MR exam was obtained within the time frame of days 9 to 11. T1- and T2-weighted axial scans were obtained before contrast injection, with the T1 scans acquired both wi th and without fat saturation. Postcontrast T1 scans also were obtaine d, using fat saturation, after the injection of 0.1 and 0.3 (cumulativ e dose) mmol/kg gadoteridol (Gd HP-DO3A; ProHance(TM)) in all animals. The film images were interpreted in a prospective fashion by a single neuroradiologist who was masked to imaging technique and contrast dos ing. The digital: data was analyzed by region of interest measurement. At the end of the imaging studies, the animals were sacrificed and th e epidural lesion confirmed by gross and microscopic exam. RESULTS. On a prospective masked read of the MR films, epidural tumor was depicte d best on postcontrast fat saturation T1-weighted scans using a cumula tive contrast dose of 0.3 mmol/kg. Substantial contrast enhancement of the tumor was observed in all instances on postcontrast scans. The pr econtrast T1-weighted scan was least efficacious for lesion identifica tion and differentiation from the compressed spinal cord. Depending on the pulse sequence used, one (T2-weighted) to three (T1-weighted with out fat saturation) of the lesions could not be identified prospective ly on precontrast scans. Lesion growth with time after implantation wa s chronicled by MR imaging, accompanied by progression of symptoms. On region of interest analysis, differentiation of epidural tumor from n ormal cord was greatest (11.6 +/- 6.1) on postcontrast scans using a c umulative contrast dose of 0.3 mmol/kg. The level of differentiation a chieved was twice that of postcontrast scans using a contrast dose of 0.1 mmol/kg (5.9 +/- 3.6). These results were superior on statistical analysis to that with all other scan techniques (P = 0.002-0.0005). Co rd and tumor could not be differentiated on the basis of signal intens ity, with any statistical significance, using precontrast T1 and T2 sc ans. The lesions were confirmed in each animal by gross and microscopi c exam. On inspection of the gross specimen, the tumors were noted to be located in the epidural space and to cause cord compression. On mic roscopic exam, the tumor was composed of epithelial cells that were mo derately pleomorphic. CONCLUSIONS. In the New Zealand White rabbit, an epidural tumor could be created consistently using the described perc utaneous approach. These lesions are suitable for MR imaging studies, examining lesion detectability and efficacy of imaging technique. The lesions created in the current study could not be diagnosed prospectiv ely in all cases on precontrast T1 and T2 scans images. Postcontrast s cans were most efficacious for diagnosis and lesion delineation, with high-dose (0.3 mmol/kg) scans superior to standard dose (0.1 mmol/kg).