Contrast enhanced MRI of the liver with SPIO at 3 T. Does the higher field strength translate into higher contrast?

Purpose: Small particles of iron oxide (SPIO) are successful used as a contrast agent in liver MRI for lesion detection and characterization. The contrast effect is a result of the shortened T2 relaxation time due to the increased susceptibility of the iron particles. Susceptibility increases proportional with the magnetic field strength. Aim of the study was to analyse if the higher field strength at 3T compared to 1.5T translate into a higher contrast.

Methods: An intra-individual comparative study on 22 consecutive patients (mean age 57 (34–73) m/f: 13/9) was performed on a 1.5T and a 3T Intera System (Philips MS, Best NL) using a SENSE capable, 6 elements synergy cardiac coil. Respiratory triggered T2 weighted images (TSE) were acquired with spectral fat saturation (SPIR/SPAIR) after administration of SPIO (Resovist ®, Schering, Berlin – Germany). Both examinations was done within one hour. 19/22 Patients revealed focal liver lesions of different origin (HCC, metastasis, haemangioma, CLL, FNH). Image analysis was done by ROI measurements in the liver, kidneys, muscle, fluid collections as well as in liver lesions. Contrast ratios were calculated according to (SNR(A) – SNR(B))/(SNR(A) + SNR(B)). Statistical analysis was done with the paired t-test.

Results: There are no statistical significant differences between 3T and 1.5T concerning the image contrast between liver tissue and fluid, kidney as well as liver lesions. In contrast there is a significant difference regarding the contrast of liver and muscle tissue. Whereas the calculated increase in SNR at 3T showed to be about 1.4–1.5 fold compared to 1.5T, the SNR increase for the muscle tissue was about 2.5 fold. The calculated contrast values showed a strong correlation between 1.5T and 3T (r=0.76–0.89)

Conclusion: SPIO enhanced liver imaging at 3T does not result in an increase in contrast between liver and liver lesions as well as between liver and fluid or kidney parenchyma. A possible explanation may be the already strong shortening of the T2 relaxation at 1.5T which may not be significantly increased at 3T under the given imaging setup. Surprisingly we found a significant increase in contrast between liver and muscle tissue which was due to a marked increase in SNR of the muscle tissue at 3T compared to 1.5T. The reason therefor is the non uniform alteration of T1 and T2 relaxation times of different tissues comparing 1.5T and 3T.