Low-field interventional MRI in neurosurgery: finding the right dose of contrast medium

MRI is increasingly being used as an interventional tool in neurosurgery. T he field strength of "intraoperative" MR systems is usually lower than that of imagers commonly used for diagnostic purposes. However, lesion enhancem ent and apparent lesion extent depend on field strength. The aim of this st udy was to compare the contrast between intracranial, contrast-enhancing sp ace-occupying lesions and the surrounding white matter obtained with low-fi eld (0.2 T) and high-field (1.5 T) MR imaging and to find the contrast medi um dosage for low-field MRI that produces the same lesion-to-white-matter c ontrast as the one obtained with high-field MRI after the administration of a standard dose of the contrast medium. A total. of 38 patients with intra cranial metastases or high-grade glioma were enrolled in this study. TI-wei ghted spin-echo sequences were acquired. High-field (1.5 T) studies were pe rformed after the i.v, administration of 0.1 mmol gadolinium-DTPA /kg body weight. For low-field MRI (0.2 T) a dose escalation technique was used. T1- weighted sequences were repeated after each of three i.v. injections of 0.1 mmol gadolinium-DTPA/kg body weight. Thus, at the low-field examinations t hree T1-weighted sequences with a contrast medium dosage of 0.1, 0.2 and 0. 3 mmol gadolinium-DTPA /kg body weight were obtained. Lesion-to-white-matte r contrasts were calculated and compared. The average lesion-to-white-matte r contrast obtained with high-field MR examinations was 1.63 (standard devi ation 0.32), In the low-field MR examinations the average lesion-to-white-m atter contrast was 1.34 (0.2) after a single dose, 1.57 (0.2) after a doubl e dose, and 1.71 (,19) after a triple dose of contrast medium. The lesion-t o-white-matter contrast of the high-field MR examination after a single dos e of contrast medium was significantly higher than that of the low-field st udy after a single dose (P < 0.0001), but did not differ significantly from the low-field studies after a double (P = 0.28) or a triple dose (P = 0.17 ) of contrast medium. In a series of patients with contrast-enhancing space occupying brain lesions low-field MRI (0.2 T) after a double dose of contr ast medium yielded the same lesion-to-white-matter contrasts as high-field MRI (1.5 T) after a standard dose. This is an important finding to avoid er rors in intraoperative MRI due to the immanently lower degree of lesion enh ancement in low-field MR imaging.