We sought to assess the performance of post contrast ultra-short echo time (UTE) MR compared with contrast enhanced pulmonary magnetic resonance angiography (MRA) for depiction of the vasculature and other anatomic structures in the chest.

This pilot study was IRB-approved and HIPAA-compliant. Twelve patients were prospectively recruited to undergo an additional post contrast free-breathing 3D radial UTE acquisition performed immediately after an MRA performed for clinical reasons. Eleven subjects received 0.03 mmol/kg gadofosveset trisodium (Ablavar). One subject received 0.1 mmol/kg gadobenate dimeglumine (MultiHance). Nine exams were performed at 1.5 T (MR 450w, GE Healthcare), and three were performed at 3.0 T (MR 750w, GE Healthcare).

Breath-held MRA was performed using a previously published, commercially available method^1,2. Scan time for the UTE acquisition was approximately 5 minutes, during which patients were instructed to breathe normally. UTE imaging parameters included: slab selection, variable density readout³, retrospective respiratory gating with a 40% acceptance window using respiratory bellows, flip angle=8°, resolution=1.25mm isotropic, TE=0.08 ms, TR=2.8 ms at 1.5T and 3.9 ms at 3T.

Images were evaluated on 16 clinically relevant anatomic features (6 vascular, 10 non-vascular) in the chest. To create a composite score, each feature was assigned a weighting factor based upon our perception of the feature’s importance for clinical diagnosis when imaging a patient with symptoms suspicious for pulmonary embolism (Figure 1). Two cardiothoracic radiologists then scored each MRA and UTE exam in an independent, blinded fashion in randomized order. The readers rated each feature on a 4-point ordinal scale: 0=Poor, 1=Fair, 2=Good, 3=Excellent. We calculated total scores (0-96) and pulmonary artery subscores (0-24) by a weighted sum of the feature scores. We assessed the inter-observer variability of the total scores using the intra-class correlation coefficient (ICC). We performed paired 2-tailed t-tests to compare UTE and MRA total scores and pulmonary artery subscores, using the average of the 2 readers’ scores for each acquisition.

The inter-observer ICC was 0.58 for MRA (moderate agreement) and 0.78 for UTE (strong agreement). Total scores were significantly better for UTE (74.4) than for MRA (62.9) with a p-value of 0.018 (Figure 2A). Pulmonary artery sub-scores for UTE (17.8) did not significantly differ from MRA (17.9) with p=0.94 (Figure 2B). Figures 3 and 4 show examples in which UTE allowed identification of clinically important findings not visible on the MRA examsOverall, UTE outperformed MRA for depiction of chest structures. There were several examples of specific pathology (bone and mediastinal lymphadenopathy) seen on the UTE examinations that were not detected by the same readers on the MRA exams. However, there were also several examples of MRA outperforming UTE for the aorta and pulmonary arteries due to the breath hold nature of that acquisition (Figure 2). A limitation of this study was the absence of pulmonary emboli in any of the subjects. However, a prior animal study showed excellent efficacy of UTE relative to MRA in the detection of pulmonary embolism⁴. These results support a larger prospective clinical study of UTE in patients suspected of pulmonary embolism, focusing not only on diagnostic efficacy for detecting pulmonary embolism but also on the ability of UTE to depict alternative diagnoses that may be the cause of the patient’s symptoms.UTE imaging is complementary to conventional MRA imaging of the chest and is superior to MRA in depicting non-vascular structures in clinical patients. The addition of a UTE scan following MRA may be of particular value in patients who have difficulty breath-holding or in whom pulmonary embolism is only one of several possible diagnostic concerns.Contrast enhanced pulmonary magnetic resonance angiography is an off label use of gadolinium based contrast agents.