Comparison of chemical shift imaging sequences at 3T (2d and 3d dual gradient echo, 3D dual gradient echo dixon) and 1.5t (2d dual gradient echo) for characterization of adrenal lesions

Aims and Objectives: Incidental adrenal lesions are frequently encountered in daily practice of radiology. Majority of these lesions are asymptomatic adenomas. However, accurate differentiation of benign adenomas and non-adenomas is crucial in patients with a known primary malignancy. The use of chemical shift imaging for characterization of adrenal lesions at 1.5T is well established, but there is paucity of data at 3T. Furthermore there is no consensus in the literature about the optimal sequence parameters for chemical shift MRI at 3T. The purpose of this study was to compare three chemical shift MRI sequences (two dimensional (2D) spoiled gradient echo (sGRE), 3D VIBE and 3D VIBE-Dixon) at 3T and 2D sGRE chemical shift imaging at 1.5T to assess the ability of detecting microscopic fat in adrenal adenoma and their usefulness for differentiation between adrenal adenomas and non-adenomas. Methods and Materials: We evaluated 97 adrenal masses (78 adenomas, 19 non-adenomas) in 78 patients. There were 52 female and 25 male raging in age from 32 to 77 years (mean 59.05). All of the patients underwent both 1.5T and 3T chemical shift MRI. After localization imaging, T1W/T2W and chemical shift imaging were acquired. Chemical shift MRI was performed with 2D sGRE at 1.5T and 2D sGRE, 3D VIBE, 3D VIBE-Dixon at 3T MR unit. For each lesion, in/opposed phase and water/fat only images were viewed on the same screen to detect the slice that was used in quantitative image analysis and signal intensity (SI) values were measured. Using mean SI values of adrenal lesions on the in/opposed phase and water/fat only images, the signal intensity index (SII) and fat fraction values were calculated. The Wilcoxon signed-ranked test was used to determine if there was statistically significant difference between the SII values of the each sequence to assess their ability of detecting microscopic fat in adrenal adenomas. ROC analysis was performed for the SII values of each sequence and the fat fraction values of 3D VIBE-Dixon sequence to identify optimal thresholds that maximizing the average of sensitivity and specificity. Results: In 66 of 77 patients, 78 adrenal lesions were diagnosed as adenoma. Diagnostic confirmation of the 67 adrenal adenomas in 58 patients was based on less than 10% variation in the lesion's maximum transverse diameter during MR imaging follow up for a minimum duration of 12 months. 11 adrenal lesions in 8 patients were classified as a benign adrenal adenoma on the basis of unenhanced CT attenuation values of less than 10 HU. The remaining 19 lesions in 11 patients were diagnosed as non-adenoma which included 18 metastases and 1 pheochromocytoma. Among the non-adenomas, 17 lesions in 9 patients were classified as metastasis due to new occurrence in follow up MR images. Proof of diagnosis was obtained on the basis of histological confirmation in two lesions (1 metastasis, 1 pheochromocytoma) after surgery. The mean SII values (± standard deviation of mean) of adrenal adenomas in 2D sGRE technique at 1.5T and 2D sGRE, 3D VIBE, 3D VIBE-Dixon techniques at 3T were calculated as 72.32±13.66, 58.44±11.25, 69.11±11.39, 57.76±13.17 respectively. The mean SII value of 2D sGRE technique at 1.5T was significantly higher than all of the chemical shift imaging techniques at 3T (p=0.001). There was no significant difference between the mean SII values of 2D sGRE and 3D VIBE-Dixon techniques at 3T (p=0.685). The sensitivity/specificity/accuracy values of 2D sGRE technique at 1.5T and 2D sGRE, 3D VIBE, 3D VIBE-Dixon techniques at 3T were calculated respectively as %100/100/100, %97.4/89.5/95.9, %98.7/94.7/98, %98.7/94.7/98 for the SII and %98.7/100/99 for the fat fraction values of 3D VIBE-Dixon technique at 3T. Conclusion: Although there is no significant difference between diagnostic performance of chemical shift imaging techniques at 1.5T and 3T for differentiation adrenal adenomas from non-adenomas, the sensitivity/specificity/accuracy values were higher at 1.5T which has well established sequence parameters.