Pulmonary Magnetic Resonance Angiography: The direct and indirect findings of pulmonary embolism and their mimics
John Bisges1, Scott K. Nagle1, Christopher J. François1, Peter Bannas2, Michael D. Hope3, J. Paul Finn4, Karl Vigen1, Thomas M. Grist1, Scott B. Reeder1, and Mark L. Schiebler1

1Radiology, UW-Madison, Madison, WI, United States, 2Radiology, University of Hamburg-Eppendorf, Hamburg, Germany, 3Radiology, University of California at San Francisco, San Francisco, CA, United States, 4Radiology, University of California at Los Angeles, Los Angeles, CA, United States

Synopsis

The use of pulmonary magnetic resonance angiography (MRA) is playing an increasingly important role for the primary diagnosis of pulmonary embolism (PE) and other causes of acute chest pain. We will define appropriate imaging scenarios for the clinical use of this test. Then, using a pictorial essay approach, we will demonstrate the various imaging features that: (A) directly indicates the presence of PE; (2) indirectly suggests the presence of PE; (3) findings that directly show right heart strain; (4) indirect findings suggesting elevated central venous pressure and most importantly; (5) those findings that can mimic PE. After review of these teaching cases, imaging physicians will be able to confidently make the diagnosis of PE on pulmonary MRA examinations.

Background and Introduction

The use of pulmonary magnetic resonance angiography (MRA) is playing an increasingly important role for the primary diagnosis of pulmonary embolism (PE) and other causes of acute chest pain.(Ref 1,2) However, for accurate diagnosis it is of critical importance to understand its appropriate use in the patient population, the normal variation in intraluminal signal intensity related to the Gibbs truncation artifact,(Ref 3) phase of contrast enhancement and route of opacified venous inflow. (Ref 4) Here we describe when to use this test along with the direct and indirect findings of PE that can be found in daily practice. Also, the pitfalls in diagnosis of this disorder using MRA are addressed. These teaching points can help the reader to reach a confident diagnosis of this acute disease using MRA.

Appropriateness Criteria

Pulmonary MRA for the primary diagnosis of PE is most effective when used in patients with the following criteria: (A) a low to intermediate pretest probability for venous thromboembolic disease; (B) patients with iodinated contrast allergies; (C) female subjects less than 30 years of age that are potentially at slightly higher risk from medical radiation; (D) borderline renal function patients wherein the use of Ferumoxytol as an MRA contrast agent may be considered.(Ref 5) This test is not recommended for ill patients with significant dyspnea at high risk for PE, as the MRI room is not suitable environment for cardiopulmonary resuscitation.

Choice of MRA contrast agent

There is limited data on the use of non contrast MRA methods in the clinical setting of PE. Currently available GBCA's are all potentially of use for this procedure, however, those agents with higher relaxivity are preferred to maximize the intraluminal signal intensity. When the patient is unable to hold their breath, purely intravascular agents that have a long residence time are helpful to obtain exams during free breathing. For those patients that are in renal failure, the use of Ferumoxytol is an option.(Ref 5)

Technique

There has been some work showing an advantage for an initial perfusion examination which is then followed by a higher resolution MRA.(Ref 2) Another important feature to consider is the length of time for the bolus administration. We have found that having contrast diluted to a volume of 30 cc allows the bolus to be administered for the entire length of the acquisition and thus helps to limit Maki artifacts.(Ref 4)

Pictorial Essay

Direct findings of Pulmonary Embolism at MRA: (1) Occlusive intraluminal filling defect with a vessel "cutoff sign", (2) non-occlusive intraluminal filling defect, (3) non-occlusive filling defect with dilation of the affected pulmonary artery, (4) webs of non-occlusive clot from resolving PE, (5) double bronchus sign (wherein the hypointense occlusive thrombus next to a bronchus creates a double barrel shotgun in cross-section appearance) (Figure 1) (6) high T1 signal intensity from met­ hemoglobin intralumenally before IV contrast administration.

Indirect findings of PE: (7) Pulmonary infarction with "atoll" sign (Figure 2) (8) atelectasis, (9) pleural effusion, (10) White-black-white sign of a focal perfusion defect (black) surrounded on both sides by enhancing lung (white) (11) high signal intensity draining pulmonary vein, (12) perfusion defect, (13) enhancing pleural surfaces.

Indirect findings of elevated pulmonary artery pressure: (14) enlarged pulmonary trunk (>3.0 cm),

Direct finding of Right ventricular dysfunction:(15) increased right ventricular short axis/left ventricular short axis ratio (RV/LV).

Indirect findings of right ventricular dysfunction: (16) Inferior vena cava reflux in centimeters, (17) Oval shape of the Inferior vena cava, (18) Bowing of interatrial septum towards the left atrium.

Mimics/Pitfalls of PE diagnosis at MRA: (19) Gibbs truncation artifact and the use of the Bannas 50% signal dropout rule (Figure 1), (20) truncation artifact ringlets, (21) Maki artifact from bolus timing error, (21) "Pseudo PE" appearance from unenhanced venous inflow (Transient interruption of the bolus and extra cardiac venous shunts (Glenn and Fontan)).

Disclosures

Pulmonary MRA is an off-label use of both Gadolinium based contrast agents (GBCA's) and the intravascular iron oxide agent Ferumoxytol.

Acknowledgements

Departmental Research and Development funds, GE Healthcare, and Bracco Diagnostics.

References

(1) Schiebler JMRI 2013 October;38:914-925

(2) Kalb Radiology 2012 Apr;263(1):271-8

(3) Bannas Eur Radiol 2014 Aug;24(8):1942-9

(4) Maki JMRI 1996;6:642-51

(5) Hope AJR 2015;205:W366-W373

Figures

MRA of PE and Gibbs artifact: (A) SPGR Fat SAT post contrast showing the occlusive embolus (dashed arrow) next to the low signal intensity bronchus (arrow) -"Double bronchus" sign; (B) MRA showing the same area as (A) with the dashed arrow pointing to the embolus and the solid arrow pointing to the interlobar bronchus; (C) Gibbs ringing artifact (arrow) in the center of the left lower lobe pulmonary artery; (D) emboli in two right lower lobe pulmonary arteries (arrows).

Indirect findings of PE: (A) Perfusion defect; (B) Fontan inflow (small horizontal arrow) meeting Glenn shunt ( Large horizontal arrow) resulting in perfusion defect in the right lower lobe (vertical arrow); (C) Pre-contrast scan showing high T1 signal of pulmonary infarct (arrow); (D) Post-contrast 'atoll' sign of small pulmonary infarct.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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