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November 2010
Trends in Emergency Aortic Imaging
Determining optimal imaging techniques for each patient.
By Perry Choi, MD, and Hamid Mojiban, MD
Computed tomography (CT) and magnetic resonance imaging (MRI) play vital roles in both the diagnosis and management of patients with aortic emergencies. Due to its wide availability and limited contraindications, helical CT is considered the fastest and the standard method of imaging for acute aortic pathologies such as traumatic injuries, acute aortic syndromes, and aortic aneurysms. The imaging features of these conditions have been well described in the literature.1-3 For patients who are unstable, such as those with traumatic injury or clinical concern for ruptured aneurysm, non–electrocardiography (ECG)-gated helical multidetector (MD) CT with contrast is often ordered. Given the accessibility, speed, and ease of patient monitoring, this is a favorable option. However, the one big disadvantage of non–ECGgated CT study is the lack of dynamic evaluation of a very dynamic and mobile structure. The most obvious consequence of this problem is motion artifact, especially at the root and ascending aorta, which can lead to a misdiagnosis of aortic dissections or other pathologies in a significant number of patients (Figures 1 and 2).4
A less-recognized aspect of the dynamic nature of the aorta is the change in size and shape during the cardiac cycle and in different hemodynamic statuses. As shown in several recent publications, the aortic diameter can vary up to 15% during diastole and systole.5 Aortic size also changes in patients with hemodynamic instability, as shown in both human and animal studies.6,7 The aortic branches also move significantly during the cardiac cycle.8 None of these variables can be evaluated with a non–ECG-gated CT angiography (CTA) or magnetic resonance angiography (MRA) examination. During the past few years, the number, extent, and complexity of percutaneous aortic interventions have exponentially increased; not considering the variables of device designs and a lack of planning for many of these interventions may result in poor outcomes or complications.
To reduce motion artifact from cardiac pulsations, ECG gating was introduced. With retrospective ECG-gated studies of the aorta, the patient is scanned helically with a low pitch while continuous ECG recording is performed.9 After the scan, the data can be reconstructed during a specific phase of the cardiac cycle, or the entire cardiac cycle can be used for a comprehensive dynamic evaluation. In many instances, the coronary arteries and cardiac function can also be evaluated at the same time, eliminating the need for further invasive cardiac studies.10 In one study in which patients had a heart rate of < 75 bpm, 98% of the coronary artery segments were well visualized for assessment when imaging was performed for the aorta.11 Coronary artery occlusion from dissection flaps, coronary artery disease, aberrant coronary artery origin, and other pathologies can be more accurately evaluated with ECG-gated studies dedicated to the aorta (Figures 3 through 5). The main disadvantage of retrospectively gated studies is the high radiation dose to the patient, because a low pitch value is used. The radiation dose is particularly high when the chest, abdomen, and pelvis are included in the scan. The radiation dose may reach 40 mSv, which is almost equivalent to 3,000 chest radiographs.
REDUCING EXPOSURE
One solution would be to image the chest portion with
retrospective gating and the abdomen/pelvis portion with
the nongated technique. This would help minimize the radiation
dose (Figure 6). The recent introduction of prospectively
ECG-triggered CT scanning provides a low-radiation
method for aortic evaluation. With the prospective technique,
the CT tube current is turned on for a short time
during a defined range of the R-R interval, often centered in
the mid-to-late diastolic phase, during which heart motion
is minimized. Based on the average length of previous R-R
intervals, the scan is triggered after an R wave. Studies have
shown a significant reduction in radiation dose when
prospective ECG triggering is used compared with retrospective
gating. In one study, the average radiation from prospective scanning was 14 mSv compared with 43 mSv
with retrospective gating.12 At Yale-New Haven Hospital, we
have been able to optimize our studies to yield a high diagnostic
accuracy with radiation dose in the range of 3 to 6
mSv in the average-sized patient. This method works best
for patients with normal rhythms. The disadvantage of this
prospective technique occurs in patients with irregular
heart rates. This variability of the R-R interval makes prediction
of the next R-R interval length difficult and can lead to
scanning during systole when there is relatively more
motion and a resultant decrease in image quality. Another
disadvantage is that only a limited number of phases of the
cardiac cycle can be reconstructed, and dynamic evaluation
of the aorta during the entire cardiac cycle is not possible.
MR VERSUS CT
In stable patients, MRI offers several advantages over CT.
The main advantage is the lack of ionizing radiation with
similar accuracy to CT.13 Also, for patients allergic to iodinated
contrast, gadolinium-based contrast agents used for
MRA are a viable alternative. Many protocols and techniques
are also available for MRI.14 ECG gating is recommended
in all patients undergoing MRI, as this produces relatively
motion-free images. Studies examining the optimal
sequences for MRI of the aorta suggest that a combination
of imaging techniques is required to most accurately characterize
the range of aortic pathologies. For example, one
study compared the diagnostic yield of cine steady-state
free precession (SSFP), two-dimensional double inversionrecovery
fast spin-echo (IR-FSE), and contrast-enhanced
MRA in the diagnosis of aortic pathologies.15 Emergent conditions
were found to occur equally among the three individual techniques, but the investigators concluded that a
combination of the techniques provided the best overall
diagnosis accuracy. There is some institutional variability in
the MRI protocol, but most include both pre- and postcontrast
images. At Yale, we routinely use both black-blood and
white-blood imaging. Additionally, cine- balanced SSFP or
gradient-recalled echo images are also important, along
with gadolinium-enhanced MRA.
For patients with poor renal function, there is concern for iodinated contrast-induced nephrotoxicity with CT and gadolinium-associated nephrogenic systemic sclerosis with MRI. In these patients, noncontrast MR techniques such as cine SSFP and double IR-FSE can play an important role. Many noncontrast MRA techniques are also available. Studies have confirmed that noncontrast MRI demonstrates similar efficacy in diagnosis of most aortic emergencies including aortic dissection.16
Regardless of technique, CTA or MRA of the aorta can provide additional information that is invaluable for patient management. In patients with aortic aneurysms, identification of the artery of Adamkiewicz can be important for preoperative management. CTA and MRA can identify this artery in a majority of patients.17
CONCLUSION
The optimal imaging technique depends on the pathology
in question and stability of the patient. In stable patients,
ECG-gated MRI provides an accurate dynamic assessment
of the aorta with no radiation. If CT is used, prospective
ECG gating should be used in most instances given a reduction
in radiation dose and improved diagnostic ability of
adjacent structures, such as the coronary arteries. In the
near future, ECG-gated techniques for the aorta will
become the standard of care.
Perry Choi, MD, is an Advanced Cardiovascular Imaging Fellow, Department of Radiology, Yale-New Haven Hospital in New Haven, Connecticut. He has disclosed that he holds no financial interest in any product or manufacturer mentioned herein. Dr. Choi may be reached at perry.choi@yale.edu.
Hamid Mojibian, MD, is Assistant Professor of Radiology and Section Chief of Cardiac Imaging, Department of Radiology, Yale-New Haven Hospital in New Haven, Connecticut. He has disclosed that he holds no financial interest in any product or manufacturer mentioned herein. Dr. Mojibian may be reached at hamid.mojibian@yale.edu.
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