Introduction

A major deficiency of MR angiography compared with CT angiography (CTA) has been its inability to identify peripheral vascular calcifications. For instance, dense arterial wall calcifications should be avoided when choosing a percutaneous access site, and their presence is a major determinant of failure for percutaneous endovascular aneurysm repair. Recently, MRI using a proton density-weighted, in-phase stack of stars (PDIP-SOS) technique has proved capable of detecting these calcifications. Just as the high density of calcifications can result in artifactual enlargement (“blooming”) with CT, diamagnetic susceptibility and T2* effects have the potential to cause inaccuracies due to field strength-dependent blooming with MRI. We therefore studied the influence of magnetic field strength on lesion volumes in a two-center clinical study.

Methods

This study was IRB approved and informed consent was obtained. The study population consisted of 44 patients with ilio-femoral vascular calcifications, of whom 24 were imaged at 1.5 Tesla and 20 at 3.0 Tesla. PDIP-SOS images were acquired in an oblique coronal plane centered at the level of the common femoral arteries with scan times of ~7 to 8 minutes. Body and peripheral phased array coils were used for signal reception. Slice thickness was 1.3-mm at 1.5 Tesla and 1.0-mm at 3.0 Tesla, with 128 reconstructed slices per 3D slab and in-plane resolution at both field strengths of 1.0-mm x 1.0-mm prior to interpolation. Between 600 and 660 radial views were acquired. Sampling bandwidth was 300 Hz/pixel at 1.5 Tesla and 460 Hz/pixel at 3.0 Tesla. The PDIP-SOS images were processed into thin (4 to 15-mm) minimum intensity projections for qualitative display of the vascular calcifications. In addition, nonenhanced quiescent-interval slice-selective (QISS) MRA of the ilio-femoral vessels was acquired. Qualitative image analysis as well as quantitative analysis using a semi-automated technique were performed using CTA as the reference standard.

Results

166 vessel segments were analyzed. Qualitatively, all PDIP-SOS MR images showed good-to-excellent confidence to detect vascular calcifications, with good inter-reader agreement (κ=0.73, P<0.001). There was an overall excellent correlation (r=0.98, P<0.001) and agreement (intraclass correlation coefficient=0.96, P<0.001) between PDIP-SOS MRI and CTA measures of calcification volume, with no overt difference in performance at 1.5 Tesla vs. 3.0 Tesla. MRI lesion volumes were slightly lower than those measured for CTA at both field strengths.

Discussion and Conclusion

MRI using a PDIP-SOS pulse sequence is an accurate test for ilio-femoral peripheral vascular calcifications at both 1.5 Tesla and 3.0 Tesla. Lesion volumes were not substantially different despite the two-fold variation in magnetic field strength. Our results suggest that PDIP-SOS MRI provides a reliable alternative to CT for pre-interventional evaluation of calcium burden at both 1.5 Tesla and 3.0 Tesla.

Acknowledgements

NIH grants R01 HL130093 and R21 HL126015.

References

1. Ferreira Botelho MP, Koktzoglou I, Collins JD, Giri S, Carr JC, Gupta N, Edelman RR. MR imaging of iliofemoral peripheral vascular calcifications using proton density-weighted, in-phase three-dimensional stack-of-stars gradient echo. Magn Reson Med. 2017 Jun;77(6):2146-2152. doi: 10.1002/mrm.26295. Epub 2016 Jun 14.