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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_3734_Библиотеки_им_академика_М_И_Перельмана
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Long-Term Imaging oftheAorta: Considerations andComparison ofModalities
https://t.me/med1917
imaging surveillance in the acute phase given its superior spatial resolution and
better ability to detect mediastinal/periaortic soft tissue abnormalities associated
with leak/contained rupture, magnetic resonance angiography (MRA) is a reasonable alternative for long-term annual imaging surveillance of patients with stable
and uncomplicated dissections, and avoids the cumulative risk of repeated radiation exposure.
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Intramural Hematoma (IMH) andPenetrating Atherosclerotic
Ulcer (PAU)
Published recommendations for imaging surveillance of patients with IMH and
PAU largely mirror those for patients with aortic dissection, although data on longterm imaging are even more limited than for dissection. Frequent imaging followup in the early phase is of particular importance to patients with IMH, as the
majority of the complications occur within the rst year, and patients with nonresolving IMH have been shown to be at the greatest risk [7]. In addition to maximal aortic dimeter of the affected segment, the degree of mural thickness should be
assessed to track IMH evolution. PAUs are occasionally incidentally detected in
asymptomatic patients, and in this setting annual imaging surveillance is generally
performed. While many PAUs can be managed non-operatively, lesions that are
painful or associated with other complications such as intramural hematoma, pseudoaneurysm or signs of rupture, open or endovascular repair may be necessary [8,
9]. Long-term imaging follow-up is important in PAUs as approximately 43% of
symptomatic PAUs and 16% of asymptomatic PAUs were noted to progress on
follow-up imaging [10]. Although specic imaging surveillance recommendations
are not supported by strong evidence and are subject to signicant variability, a
general summary of standard imaging follow-up intervals in several clinical scenarios is presented in Table1.
Comparison ofImaging Modalities
The three primary imaging modalities for long-term monitoring and follow-up
imaging in the thoracic aorta are CT angiography (CTA), MR angiography (MRA),
and echocardiography. Each modality has strengths and weaknesses for imaging the
thoracic aorta. The selection of the appropriate imaging test should be driven by
specic aortic pathology, surgical history, co-morbidities such as aortic valve disease or extension of dissection or aneurysm into the abdominal aorta, and patient
related factors, particularly renal function and age.
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Table 1 Summary of standard imaging surveillance intervals
Clinical
Scenario Early Follow-Up (0–12months)
Aortic
dissection
Type A
(repaired)
Type B
(OMT)
PostEndograft
IMH/PAU Before discharge and @1 month*
† Consider MRI/MRA if appropriate for patient-specic factors
‡ Addition of delayed phase imaging is recommended
a
CTA is preferred modality
Abbreviations: OMT optimal medical therapy, IMH intramural hematoma, PAU penetrating atherosclerotic ulcer, ULP ulcerlike projection
— —
Before discharge or @1 month*
– If enlarging: 3, 6 & 12months*
– If stable: 6 & 12months
Before discharge and @1 month*
– If enlarging: 3, 6 & 12months*
– If stable: 6 & 12months
Before discharge or @1 month*‡
– If enlarging and/or signicant endoleak:
3, 6 & 12months*‡
– If stable/no endoleak: 6 & 12months
– If non-resolving: 3, 6 & 12months*
– If resolving: 6 & 12months
Recommend careful image analysis for
development of intimomedial defects or
ULP [11]
Long-Term Follow-up (12months
+)
If stable: Annual intervals†
If enlarging: 6month intervals
– After ≥3years stability,
consider change to 2 or 3year
intervals†.
If stable: Annual intervals†
If enlarging: 6month intervals
– After ≥3years stability,
consider change to 2year
intervals†.
If stable/FL regression: Annual
intervals†
If enlarging: 6month intervals
– After ≥3years stability,
consider change to 2 or 3year
intervals†.
In the absence of indications for
repair:
If non-resolving IMH:
6–12months intervals
If resolved IMH:
– Annual intervals if residual
aortic dilation†
– Unclear role of continued
imaging if no residual aortic
dilation
Echocardiography Versus Cross-Sectional Imaging
While echocardiography is not the preferred modality for comprehensive thoracic
aorta evaluation, all standard echocardiography examinations can provide information on multiple aortic segments and it has been recommended as a primary screening tool in aortic disease. The aortic valve, sinuses and proximal ascending aorta can
usually be well assessed with transthoracic echocardiography (TTE), while evaluation of the descending aorta requires transesophageal echo for evaluation (TEE). On
TTE, using a combination of left and right parasternal long axis views and basal
short axis views, it is possible to measure the aortic annulus, sinuses of Valsalva
(SOV), and sinotubular junction (STJ). Right parasternal and apical long axis views
Long-Term Imaging oftheAorta: Considerations andComparison ofModalities
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are used to measure ascending aorta diameters (AAo). Suprasternal views allow
visualization of the aortic arch and branch vessels although this sonographic window can be limited due to patient body habitus or emphysema. TEE allows for
short- and long-axis evaluation of the descending aorta, but the invasive nature of
this test limits its usefulness as a standard follow-up or monitoring tool [12].
An important advantage of the TTE compared to other cross-sectional modalities
is the relative ease of acquisition without the need for intravenous contrast or exposure to ionizing radiation. This feature makes TTE a useful tool in serial monitoring
post-operative complications following ascending aorta repair. Additional key
advantages of echocardiography for aortic evaluation include the opportunity to
assess any co-morbid aortic valve disease and measure biophysical properties such
as aortic distensibility and pulse wave velocity with Doppler echocardiography
[13]. For follow-up imaging in ascending aortic disease, an important disadvantage
of TTE is that aortic diameters are measured only in the long-axis plane, and this
approach has been shown to underestimate aortic diameter when compared to
double- oblique measurements [14]. While there are several possible measurement
techniques, the “leading edge to leading edge” technique has been shown to have
excellent reproducibility and the best agreement with CTA measurements [15].
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CTA Acquisition
CT Angiography is considered the primary imaging modality for diagnosing and
monitoring thoracic aorta disease and for follow-up imaging after aortic interventions. Key advantages of CTA are that image acquisition is fast, less complex than
MRI/MRA and yields high resolution images. CTA is generally performed with
spatial resolutions on the order of 0.7mm3, and its volumetric nature allows for 3D
analysis and multi-planar reformats of aortic anatomy. Depending on scanner specic features such as detector size, CTA acquisitions of the chest can be performed
in a single breath-hold of 5–10seconds, with some modern scanners in 1–2seconds
[16]. An important disadvantage of CTA is that the exam most often provides static
snapshot of the anatomy and lacks hemodynamic information. CTA can generate
dynamic “cine” images with the use of retrospective electrocardiograph (ECG) gating techniques, however, this comes at the cost of higher radiation dose and should
thus be used sparingly.
CTA requires intravenous injection of iodinated contrast with timing of image
acquisition such that aortic opacication is maximal. Poor contrast timing can signicantly limit the diagnostic performance of the test. Patients are generally asked
to hold their breath during the scan to minimize respiratory motion. For evaluation
of aortic root size or ascending aorta diameter, ECG gating is usually employed.
ECG-gating limits cardiac and aortic root motion to decrease motion-related artifacts which can result in inaccurate measurements and limit the assessment of the
dissection aps. Prospective ECG gating involves only acquiring images during a
portion of the cardiac cycle, most often in mid-late diastole (70–75% R-R interval),
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and this approach requires signicantly less radiation than retrospective gating.
ECG-gating can fail in the setting of arrhythmia due to inconsistent R-R intervals
resulting in motion/pulsation artifact. ECG-gating is generally not required for evaluation of the arch and descending aorta where pulsatile motion is less pronounced.
N. S. Burris et al.
MRI/MRA Acquisition
MR angiography has similar spatial resolution to CTA, usually in the range of
0.7–1.2mm3, but patient related factors such as body habitus may necessitate changing the eld of view which can lower image resolution. Scan times for MRA tend to
be longer, usually due to the acquisition of multiple sequences during each study.
However, MRA offers several advantages relative to CTA including lack of ionizing
radiation, opportunity to acquire hemodynamic information through time-resolved
techniques, and ability to evaluate luminal/intraluminal structures with non-contrast
MRA techniques.
Similar to CTA, contrast-enhanced MRA (CE-MRA) acquisition requires image
acquisition timing to be optimized to contrast opacication of the aorta. The
CE-MRA technique employs a 3D T1 weighted sequence which leverages the T1
shortening properties of gadolinium—rather than the x-ray attenuating properties of
iodine—to provide high contrast within the aortic lumen. Similar to CTA, MRA of
the aorta is ECG-gated to reduce artifacts at the root and ascending aorta due to
cardiac motion. Breath-holding is also necessary at MRA to limit respiratory motion
artifact. In addition to CE-MRA images, most studies will include pre- and/or postcontrast T1 and T2 weighted sequences, which can help with identication of intramural hematoma, mediastinal abnormalities, and vessel wall inammation [17].
Multiple non-contrast MRA techniques have been developed that can be useful
in patients where contrast imaging is either limited or not possible. Steady-state free
precession (SSFP) is an MR acquisition technique which results in high signal
intensity of the blood without the need for intravenous contrast. Three-dimensional,
ECG-gated, non-contrast SSFP MRA provides excellent image quality and accurate
aorta measurements relative to CE-MRA, but requires signicantly longer scan
times [18, 19]. Other options for non-contrast assessment of the thoracic aorta
include spin-echo, time-of-ight (TOF), and phase contrast techniques.
MRI/MRA also provides the ability to evaluate aortic hemodynamics using timeresolved MRA (TR-MRA), as well as two-dimensional and three-dimensional (“4D
Flow”) phase contrast techniques (Fig. 1). Time-resolved MRA is a contrastenhanced technique which acquires images rapidly during contrast injection yielding dynamic images of contrast transit similar to angiography, and can provide
information such as the location of endoleaks after TEVAR and false lumen lling
patterns in aortic dissection. Phase-contrast MRA can be used as a non-contrast
technique which measures differences in magnetic spin phase shifts that occur with
owing blood. These phase shifts are proportional to ow velocity, thus allowing
for quantication of aortic blood velocity and ow rates. This technique can be
Long-Term Imaging oftheAorta: Considerations andComparison ofModalities
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abc
Fig. 1 Anatomic Versus Hemodynamic Assessment with Computed Tomography Angiography
(CTA) and Magnetic Resonance Angiography (MRA): Sagittal view of a patient with TBAD using
CTA (a) demonstrates a proximal entry tear (arrow head) and distinct true and false lumen.
Similarly, sagittal MRA images also clearly depict the entry tear with similar anatomic detail to
CTA (b). MRI also allows for measurement of blood ow velocity, and three-dimensional maps of
blood ow can be generated using 4D Flow MRI techniques, allowing for clear visualization of the
ow jet at the proximal entry tear (c), which impacts the opposite wall of the false lumen (asterisk)
applied in 2D or 3D and has been used to assess valve-related aortopathy, pulsewave velocity and many other advance hemodynamic parameters [20].
Artifacts andTechnical Limitations
Several imaging related artifacts can limit CTA image quality. One of the most commonly encountered CT artifacts is beam-hardening or “streak” artifact which results
from dense structures (bones, surgical implants, wires, pacemaker/debrillator generators, iodinated contrast in the superior vena cava) interfering with normal image
formation [21]. Newer dual-energy CT scanners have metal artifact reduction algorithms which can be employed to reduce the impact of such artifacts [22]. To image
the entire chest with CTA, the CT table must translate during the scan, and with
ECG-gating this can mean different portions of the chest are imaged at different
R-R intervals. Motion during imaging can lead to a linear “step-off” or “stair step”
artifact, which is most visible on coronal or sagittal reformatted series. Such stepoffs can blur the aortic wall, lead to inaccurate aortic measurements and be mistaken
for dissection (Fig.2) [23].
With MRA, distortions in the magnetic eld caused by metallic objects in the
body often can produce susceptibility artifact, which present as areas of dark signal void surrounding the metallic object and are worse in 3T compared to 1.5T
scanners. Potential sources of artifact include pacemaker/ICD generators or electrodes, spinal hardware, and sternal wires. Endografts composed of nitinol can be
imaged with MRI/MRA, whereas stainless-steel endografts result signicant
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Fig. 2 Motion/pulsation artifact on non-ECG gated CT scans can result in blurring of the margins
of the aorta and main pulmonary artery (a, arrow head) and image artifacts that simulate aortic
dissection. In this representative case, a dissection ap in the ascending aorta (a, arrow) was
described at an outside hospital CT performed without ECG-gating, however, on repeat CT scan
with ECG-gating the ap was conrmed to be artifactual in nature (b)
N. S. Burris et al.
artifacts and are poorly evaluated [24]. Patient specic factors can degrade image
quality, with the most important factors being arrhythmia (failure of ECG-gating),
difculty with breath holding, and claustrophobia. Claustrophobia and breathholding difculties tend to be more severe in MRA than CTA due to longer image
acquisition times, a longer scanner bore, and need for multiple breath-holds during
the exam.
CTA Radiation Exposure
Radiation exposure if a common concern of patients and physicians, and it is well
documented that ionizing radiation is associated with a risk of malignancy, particularly in radiosensitive organs in the chest such as the breasts, lungs, and thyroid.
Estimates of cancer risks due to radiation doses below 100 millisievert (mSv), a
dose corresponding to approximately 1000 chest radiographs or 10 CTAs, are not
well validated but are estimated at approximately 1% lifetime risk [25]. As scanner
technology and imaging processing algorithms have improved, CT doses have dramatically decreased. Currently, an ECG-gated CTA of the chest has an effective
radiation dose of ~5–8mSv. In most patients with thoracic aortic disease, the benet
of undergoing optimal imaging far outweighs the risk of radiation-induced malignancy. Considering the typical latency period of ≥10 years, the signicance of
radiation- induced malignancy should be considered in the context of patient life
expectancy [26]. However, children, younger adults and pregnant patients exposed
to similar doses of radiation have a considerably higher risk, and alternative imaging strategies should be considered in these groups [27].
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Contrast Safety Concerns
Both iodinated contrast for CTA and gadolinium contrast for MRA have patient
safety concerns that may determine which modality is should be considered when
selecting the best imaging modality for a given patient. Iodinated contrast has historically been associated with acute kidney injury, a phenomenon termed contrast
induced nephropathy (CIN), with guidelines suggesting reduced contrast dose or
the withholding contrast in patients with renal insufciency to reduce the risk of
renal failure in these patients. Increasingly, data has suggested that the risk of CIN,
even in patients with GFR<30mL/min is very low or possibly non-existent [28].
Institutional guidelines should be reviewed to determine the locally accepted iodinated contrast dosing practice.
Gadolinium-related nephrogenic systemic brosis (NSF) has been a concern
in patients with low renal function (GFR < 30 ml/min) undergoing contrastenhanced MRI/MRA. However, gadolinium-based contrast agents have now
been developed with safety proles which make the likelihood of NSF exceedingly low and routinely checking renal function prior to contrast enhanced MRI
is not required in most patients [29]. Over the last several years there has been
increasing evidence of MRI-related gadolinium deposition in the brain and other
organs. While the signicance of this nding remains uncertain and no clear
clinical sequela have been identied, it is a dose dependent phenomenon, and
therefore patients undergoing frequent MR studies, particularly younger patients,
should carefully consider the potential risks and benets of contrast administration [30]. This phenomenon may increase the importance of non-contrast MRA
techniques in the future. The strengths and limitations of CTA and MRA are
summarized in Table2.
Post-Endograft Imaging
Imaging Follow-Up
There are no standardized guidelines for imaging follow-up of patients after endovascular repair of aortic dissection or other acute aortic syndromes, partly owing to
the wide degree of variability in repair complexity and the rapidly evolving nature
of endovascular techniques. However, there are some principles of post-endograft
surveillance that are widely accepted and have been summarized in recently published appropriateness criteria [31]. CTA is generally considered the optimal modality for imaging surveillance, at least within the rst 6–12months after repair, given
the superior imaging resolution, ability to evaluate the integrity of metallic stent
frames, and improved evaluation for potential mediastinal or other intra-thoracic
complications. Assessment of aortic remodeling after TEVAR is most commonly
performed by aortic diameter measurement (either overall diameter or false lumen
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Table 2 Comparing CTA and MRI
Characteristic CT Angiography (CTA) MR Angiography (MRA)
Radiation Ionizing radiation (X-ray)
Spatial resolution 0.5mm
Number of
acquisitions
Set-up and scan
time
Acquisition
complexity
Patient
participation
Strength Anatomy Soft tissue characterization and
Contrast risk Iodinated contrast:
– DNA damage
3
(minimal) ~ 0.7–1.2mm3 (variable)
Usually single Usually multiple
Short (5–10min) Long (45–60min)
Easy More difcult
Minimal
– Single breath hold
– Hold still for
~10–30seconds
1. Contrast-induced
nephropathy (CIN)
– Rare
2. Severe allergy (~1:1000)
Non-ionizing (radiofrequency)
– No DNA damage
Signicant—Multiple breath hold
– Multiple breath holds
– Hold still for at least 5–10minutes
hemodynamic/functional assessment
Gadolinium contrast:
1. Nephrogenic systemic brosis (NSF)
– Extremely rare
2. Gadolinium deposition in brain
(unclear signicance)
2. Severe allergy (~1:100,000)
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diameter). False lumen volumetric assessment has been proposed as a more sensitive marker of false lumen remodeling in a variety of studies and trials, and while
there are clear theoretical benets of volumetric measurements, such measurement
techniques remain poorly standardized and the benet over diameter measurements
has not been formally established [32].
Imaging surveillance in the post-endograft generally involves the rst postoperative study being performed before discharge or at least within the rst month,
with subsequent studies typically occurring at 3–6 and 12 months and annually
thereafter in the absence of growth or other complications. Long-term follow-up
studies of TEVAR patients have shown that late complications can occur (e.g. development of endoleaks, stent fracture, stent graft migration), and lifelong imaging
surveillance of patients with aortic endografts is therefore recommended [33].
Imaging protocols for CTA/MRA vary by institution, however, generally postendograft studies are performed with multiple phases of contrast including noncontrast, arterial phase and delayed phase (30–60seconds after arterial phase) (Fig.3).
Delayed phase imaging is a unique feature of post-endograft studies, and is necessary for the detection of low ow endoleaks [34].
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Fig. 3 Post-Endograft CTA Technique: Standard post-endograft CTA consists of a 3-phase imag-
ing protocol. First, a non-contrast scan is performed (a) to clearly identify any dense objects such
as the metallic endograft, surgical material (asterisk) or calcium (arrowhead). Second, contrast is
administered and images are acquired in the early arterial phase (b), producing maximal aortic
opacication and allowing identication of endoleaks (arrow). Lastly, a delayed phase is acquired
(30–60seconds after arterial phase) to allow detection of slow lling endoleaks and better depict
the full extent of any endoleaks (arrowhead)
Patient-Specic Considerations forPost-Endograft
Imaging Surveillance
Two common patient-specic considerations that arise with post-endograft imaging
surveillance are the cumulative radiation exposure of CT among young patients
(<50–60), particularly those with connective tissue disease or traumatic aortic
injury, and contrast-induced nephropathy among patients with renal insufciency.
Both of these situations can often be managed effectively by utilizing noncontrast
imaging, MRI/MRA or a combination of both. Studies have shown that cumulative
radiation doses accrued during post-endograft CT imaging surveillance can often
reach the level of 350 mSieverts, a level at which the estimated rate of radiationinduced malignancy would be 2.5% [35]. Radiation concerns are less over
65–70years of age or when the expected lifespan is less than 10–15years, given the
latency period of radiation-induced malignancy. Options to limit radiation dose
include limiting the number of phases acquired (e.g. noncontrast phase if already
acquired in prior studies) or utilizing MRI/MRA techniques, which do not utilize
employ ionizing radiation. Magnetic artifact prevents MR imaging of stainless steel
endografts, however, newer nitinol endografts do not produce signicant artifact
and can be adequately assessed with MRI/MRA (Fig.4) [36]. In the setting of renal
insufciency, noncontrast CT imaging can be performed to assess the overall aortic
dimensions and device stability/integrity, although assessment of endoleaks and
individual lumen dimensions and intraluminal pathology is not possible. Noncontrast
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Fig. 4 Post-Endograft Surveillance with MRA: Sagittal images of a patient with penetrating ath-
erosclerotic ulcer of the descending aorta who underwent TEVAR using a nitinol endograft. The
metallic endograft can be clearly seen on CTA images (a, arrow head). While the endograft material appears dark/black on MRA images (b), the nitinol doesn’t create any signicant artifact in the
image and detailed evaluation of the lumen and surrounding anatomy is possible
MRI/MRA techniques have the unique ability to delineate blood, thrombus and the
aortic wall, and can thus be a powerful tool for long-term imaging surveillance for
patients with nitinol-based endografts.
Post-Processing andMeasurement
Measurement Techniques andLimitations
Maximal aortic diameter is the primary metric of aortic disease severity, and is measured in imaging surveillance to monitor disease progression, estimate risk of complications and to determine surgical candidacy [37]. While aortic diameter is simple
to measure and has a well-dened biomechanical relationship with wall tension
(i.e., Laplace’s law), accurate and reproducible diameter measurements can be challenging for technical and anatomic reasons. Traditionally aortic diameter measurements were performed on axial CT images; however, it is well-recognized that this
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