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