Images Obtained With Unenhanced MRA Are Sharper, Less Blurry than Contrast-Enhanced MR Angiography

The objective of a recent study was to compare an unenhanced MR technique with contrast-enhanced (CE)-MR angiography (MRA) for the evaluation of the thoracic aorta.

The study has concluded that in patients who have a contraindication to MR gadolinium-based contrast, an unenhanced 3D SSFP (steady-state free precession) sequence can be used to clearly and reliably depict the thoracic aorta.

Participants
23 patients who were referred for 3D CE MRA of the thoracic aorta.

Methodology
All MRIs were performed with a 1.5-T MRI system. All studies included a CE MRA, which was performed using retrospective ECG triggering. Gadopentetate dimeglumine was administered at 2 mL/s and 0.2 mmol/kg. A 3D FLASH pulse sequence was employed; acquisition time was 20 seconds.

All studies also included an unenhanced MR sequence that was a free-breathing, T2-prepared, segmented 3D steady-state free precession (SSFP) MRA using prospective gating. Imaging parameters used were: TR/TE 2.3/1.0; flip angle 90 degrees; readout bandwidth 980 Hz/pixel; T2 preparation time 40 milliseconds; FOV 400 x 400 mm; matrix 256 x 256; slice thickness 3 mm interpolated to 1.5 mm; lines per heartbeat 51 to 77; number of partitions 88 to 128; and parallel imaging acceleration factor 2.

For quantitative measurements, the aortic diameter was measured in orthogonal pairs at the aortic annulus, the sinuses of Valsalva, the sinotubular junction, the mid-ascending aorta, the proximal aortic arch, the distal aortic arch, and the aortic hiatus using both sequences. For qualitative analysis, image quality was scored on a 5-point scale where 1 equaled nondiagnostic image quality and 5 equaled excellent image quality.

Results of the Study
Diagnoses included normal aortas, dilated or aneurysmal aortic roots or ascending aortas, coarctation, ascending aortic hypoplasia, aortitis, aortic sarcoma, and pulmonary sequestration. The acquisition time of the unenhanced 3D SSFP sequence was 9.3 +/- 4.3 minutes. In all seven areas in which the aorta was measured, no statistically significant measurement differences between the two techniques was found (P <0.05). Qualitatively, the aortic root was better visualized with the unenhanced 3D SSFP technique (score of 4.65 out of 5.0 compared with 3.78 out of 5.0 for CE-MRA).

Conclusions
Unenhanced 3D SSFP sequence is a sharper, more reliable way to image the thoracic aorta than is contrast-enhanced MR angiography.

Reviewer’s Comments
The authors have demonstrated that a 3D SSFP sequence can be used to image the aorta in patients who have contraindications to MR contrast.

Author: Vineet R. Jain, MD

Reference
Francois CJ, Tuite D, et al. Unenhanced MR Angiography of the Thoracic Aorta: Initial Clinical Evaluation.
AJR Am J Roentgenol; 2008; 190 (April): 902-906: