Guidelines and standards
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ill patients or those with relative contraindications to MRI such as per- manent pacemakers and defibrillators. Multidetector CT (MDCT) pro- vides extensive z-axis coverage (in the long axis of the body), with high spatial resolution images acquired at modest radiation exposure within a scan time lasting a few seconds. 1,64 Furthermore, CTA allows simultaneous imaging of vascular structures, including the vessel wall and of solid viscera. 65 The minimization of operator variability and the capacity of delayed reprocessing of source images make it an ideal technique for comparative follow-up studies. 1,64 The latest innovations in clinical practice include electrocardio- graphically gated 66 aortic computed tomographic studies leading to high-quality, precise imaging of the ascending aorta, as well as simul- taneous
evaluation of the coronary arteries
67 ( Figure 15 ). Electrocardiographically gated CTA adds valuable information in the study of aortic pathology involving the aortic root and valve, 68 in congenital heart disease, 69 for simultaneous aortocoronary evalua- tion, 66
68,70 for imaging of the postsurgical ascending aorta, 71 and to show dynamic changes of true luminal compression in aortic dissection. 72 The main drawbacks of CTare the use of ionizing radiation and iodin- ated contrast media (ICM). 73 Using optimal acquisition methods, large reductions in ionizing radiation dose can be achieved. These include the use of tube current modulation, prospective electrocardiographically triggered acquisitions, or tube voltage reductions to 80 to 100 kV. Radiation dose becomes most relevant in younger men and premeno- pausal women. Contrast-associated nephropathy 74 may be avoided or significantly decreased by proper patient hydration and use of the minimum volume of low- or iso-osmolar ICM. 75 The rate of adverse reactions to low-osmolar ICM in CT is approximately 0.15%, with most cases self-resolving and mild. 76 Among patients with renal insufficiency, the rates of contrast-associated nephropathy are low. Pooled data from recently published prospective studies have shown an overall rate of contrast-associated nephropathy of 5% af- ter intravenous injection of ICM in 1,075 patients with renal insuf- ficiency, with no serious adverse outcomes (dialysis or death). 74 The current generation of computed tomographic scanners is able to significantly decrease the effective radiation dose and the total vol- ume of ICM required for aortic imaging. 77 Additionally, CT for follow-up of aortic expansion may be performed without ICM, relying on noncontrast images only. 1. Methodology. a. CTA.–The combination of wide multidetector arrays with short gantry rotation times in $64-detector computed tomographic scanners results in standard acquisition times of 3 to 4 sec for the thoracic aorta and <10 sec for the thoracoabdominal aorta and the iliofemoral arteries. The minimal technical characteristics of state-of-the-art CTA are a slice thickness of #1 mm and homogeneous contrast enhancement in the aortic lumen. Complete examination of the aorta from the Figure 16 Sagittal multiplanar reformatted (A) and double oblique images (B, C) from a computed tomographic aortogram in a patient with AAS. The anatomic locations of the planes of (B) and (C) are marked on (A). Note the transition from a type B acute IMH involving the descending thoracic aorta to a type B aortic dissection involving the abdominal aorta. 132 Goldstein et al Journal of the American Society of Echocardiography February 2015
supra-aortic vessels to the femoral arteries is needed for evaluation before transthoracic endovascular aortic repair (TEVAR), but as a gen- eral rule the scan length (anatomic scan range) on CTA should be indi- vidually tailored to avoid unnecessary exposure to ionizing radiation. 1
pected AAS, it is important to initiate the protocol by a noncontrast thoracic computed tomographic scan to rule out IMH. This scan iden- tifies concentrated hemoglobin in recently extravasated blood within the aortic wall that shows a characteristically high computed tomo- graphic density (40–70 Hounsfield units), 78 facilitates the character- ization of the hematoma, 79 can identify vascular calcifications, and provides a baseline examination for postcontrast evaluation. ii. Electrocardiographically Gated CTA Motion artifacts involving the thoracic aorta are evident in most (92%) standard nongated computed tomographic angiograms. Because of the limited temporal resolution of CT, imaging artifacts arising from the peduncular motion of the heart, the circular distension of the pulsewave, aortic distensi- bility, and the hemodynamic state may appear as a ‘‘double aortic wall’’ on standard nongated CTA. 8,23,24,80,81 This finding may also lead to a false-positive diagnosis of a dissection flap 64,67,80 and impair accurate measurement of the aortic root and ascending aorta. 67,82
Prospective or retrospective synchronization of data acquisition with the electrocardiographic tracing eliminates these artifacts, thereby improving the accuracy of diagnosis and reproducibility of aortic size measurements. 67 Low-dose prospective electrocardiographically gated CT protocols have the advantage of decreased radiation expo- sure compared with the standard technique. 83
minal scan ( $50 msec after bolus injection) improves the detection of visceral malperfusion in the acute setting of aortic dissection, 65 de-
tects slow endograft leaks, 84 distinguishes slow flow from thrombus in the false lumen, 85 and allows alternative abdominal diagnoses in the absence of acute aortic pathology. Figure 17 The methodology of double oblique aortic images. Multiplanar reformatted images (A–F) in different planes obtained from a computed tomographic aortogram (C) corresponds to the axial source image and shows an elliptical descending aorta. The sagittal (A) and coronal (B) correlates show the reference planes of (C) as well as the tortuosity of the aortic segment, which results in a dis- torted shape. The plane is corrected in both the sagittal (D) and coronal (E) images to achieve perpendicularity to the aortic flow, re- sulting in a corrected true transversal image of the aortic lumen, which is circular in this case. Figure 18 Image from a 44-year-old man with BAV. Single end-systolic image from cine steady-state free precession (SSFP) sequence depicts a bicuspid valve (yellow arrows) with normal leaflet thickness and unrestricted opening. Journal of the American Society of Echocardiography Volume 28 Number 2 Goldstein et al 133
iv. Exposure to Ionizing Radiation Radiation minimization protocols include limiting scan range, prospectively electrocardiographically triggered acquisitions, 66 and using low tube voltage (80–100 kV) for low–body weight patients (<85 kg) without risking loss of diag- nostic quality. 77 Application of iterative reconstruction algorithms provide the opportunity for even larger reductions in scan acquisition parameters. Despite progress in radiation reduction, the use of alter- native methods such as MRI and echocardiography remains a consid- eration for serial studies. 1
CTA depicts the aortic wall, thereby permitting measurement of both the inner-inner (luminal) and outer-outer (total) aortic diameters. Imaging artifacts from the highly contrasted lumen frequently impair the visualization of a thin and healthy wall in the ascending aorta. 24 Multiplanar reconstruction of the axial source data can create aortic images in a plane perpendicular to the aortic lumen direction (double-oblique or true short-axis images of the aorta; see Figures 16 and 17 ). This method corrects shape distortions introduced by aortic tortuosity. 8,86,87
In cases of noncircular aortic shape, both major and minor diameters should be measured. The manual procedure of double-oblique images is time consuming and may add observer variability. 88 Automated aortic segmentation software is available at many institutions but, like most automated software, has limitations and requires manual adjustment. The measurement technique must be highly reproducible to correctly assess follow-up studies. Accurate assessment of aortic morphologic changes can be achieved by side-by-side comparison of source axial images from two or more serial computed tomographic aortographic examinations with anatomic landmark synchronization and a slice thickness #1 mm. Electrocardiographically gated or triggered imaging is an additional refinement that further reduces variability, with a maximum interobserver variability of 61.2 mm in the ascending aorta. 89
with a circular shape and craniocaudal axis, like the midascending and the descending aorta. 1 Distortion in the axial image introduced by aortic tortuosity may be minimized by measuring the lesser diameter. 90 Interobserver variability is always higher than intraobserver vari- ability, 91,92
suggesting that follow-up of aortic disease in a specific patient should be performed by a single experienced observer. 88,89 In summary, CTA is one of the most used techniques in the assess- ment of aortic diseases. Advantages of CTA over other imaging Figure 19 Images from a 54-year-old woman with an elevated sedimentation rate and dilatation of the descending aorta. Wall thick- ening is well depicted in dark (black) blood images (left, yellow arrow). Short tau inversion recovery (STIR) images (right) demonstrate bright signal in the aortic wall (yellow arrow), a result consistent with edema. Surgical repair was performed in this patient, and his- tology was consistent with GCA. Figure 20 Images from a 60-year-old man with moderately severe aortic insufficiency. Three-chamber (left) and coronal oblique (right) cine steady-state free precession (SSFP) images depict dark signal (yellow arrows) caused by intravoxel dephasing associated with the posteriorly directed jet of insufficiency. 134 Goldstein et al Journal of the American Society of Echocardiography February 2015
modalities include the short time required for image acquisition and processing, the ability to obtain a complete 3D data set of the entire aorta, and its availability. Moreover, MDCT permits a correct evalua- tion of the coronary arteries and aortic branch disease. Its main draw- backs are the radiation exposure and need for contrast administration. G. MRI
MRI is a versatile tool for assessing the aorta and aorta-related pathol- ogies. This imaging modality can be used to define the location and extent of aneurysms, aortic wall ulceration, and dissections and to demonstrate areas of wall thickening related to aortitis or IMH. MRI can also be used for preoperative and postoperative evaluation of the aorta and adjacent structures. Additionally, MRI can provide functional data, including quantification of forward and reverse aortic flow, assessment of aortic wall stiffness and compliance, and aortic leaflet morphology and motion ( Figure 18 ). All of this information is obtainable without the burden of ionizing radiation and, in some in- stances, without the need for intravenous contrast. MR images are based on the signal collected from hydrogen nuclei, 93
when a patient enters the scanner. This precession can be altered by applying magnetic field pulses in a controlled fashion to create ‘‘pulse sequences.’’ After these pulse sequences are applied, the signal from the hydrogen nuclei is measured and then processed to produce MR images. The versatility of MRI can be attributed to the multiple types of pulse sequences that can be used to define structure, characterize tissue, and quantify function. 1. Black-Blood Sequences. Black-blood MRI sequences, acquired with spin-echo techniques, and often including inversion recovery pulses, are useful for defining morphology across a spec- trum of aortic conditions without the need for intravascular contrast medium.
94-97 With
these sequences, the use
of multiple
radiofrequency pulses nulls the signal from moving blood, causing the dark blood appearance; mobile protons in stable or slowly moving structures (e.g., aortic wall) provide the signal in the image. Aortic wall morphology can be defined and tissue characterized with T1- and T2-weighted sequences and their vari- ants, including T2-weighted dark-blood techniques and T2 turbo spin-echo and
short-tau inversion recovery sequences ( Figure 19 ). 98-100
Each of these imaging protocols has relative strengths and limitations; for example, T2-weighted MRI is sensitive to areas of increased water content, as is often noted in pathologic conditions, but is limited by relatively low signal-to-noise ratio. MRI of the thoracic aorta can be obtained with high spatial resolu- tion,
with in-plane
resolution typically in the
range of 1.5 Â 1.5 mm and submillimeter acquisition achievable with more specialized MRI sequences. 101,102 2. Cine MRI Sequences. Bright-blood imaging with approaches such as steady-state free precession and gradient-echo techniques is use- ful for obtaining high–temporal resolution cine images of flow in the aorta. In these images, the blood pool is bright compared with the adja- cent aortic wall, which is typically intermediate in signal. Cine imaging can demonstrate flow within aortic lumens (true or false), and areas of low signal caused by intravoxel dephasing can be seen with complex flow pat- terns associated with valvular stenosis or regurgitation ( Figure 20 ). 103 3. Flow Mapping. Velocity-encoded phase-contrast imaging can be used to quantify aortic flow. The phase-contrast technique is based on the fact that protons undergo a change in phase that is proportional to velocity when they pass through a magnetic field gradient consisting of equal pulses that are of opposite polarity and slightly offset in time. Blood flow can be quantified by integrating these measured velocities within the aortic lumen throughout the cardiac cycle with values that have shown strong agreement with phantom models and other mea- surement approaches. 104 Phase-contrast imaging of the aorta can be used to assess forward flow and stenotic and regurgitant valves 105,106
and can aid in assessment of congenital heart disease. 107
Phase- contrast imaging is typically acquired in a single in-plane or through-plane direction, with some applications allowing flow encod- ing in multiple directions. 108,109 4. Contrast-Enhanced MR Angiography (MRA). Contrast- enhanced MRA can provide a 3D data set of the aorta and branch vessels, allowing complex anatomy and postoperative changes to be depicted through postprocessing techniques such
as maximum-intensity projection and
multiplanar reformatting ( Figure 21 ). In patients with contraindications to contrast or in cases of difficult intravenous access, a 3D angiogram of the aorta can still be obtained with unenhanced segmented steady-state free precession angiography. 110 When precise dimensions of the aortic root and proximal ascending aorta are needed, electrocardiographically gated techniques can be used. 101,110 Improved scanning speed allows time- resolved MRA. 111
Although contrast timing for contrast-enhanced MRA can be a challenge, particularly in the concurrent assessment of the aorta and pulmonary arteries or veins, the use of newer blood-pool contrast agents can circumvent the limitations of tradi- tional interstitial gadolinium contrast agents and in conjunction with Figure 21 MIP image obtained from MRA in a 60-year-old man with a dilated ascending aorta (large yellow arrow). There was suspicion of coarctation of the descending aorta raised by sur- face echocardiographic imaging; however, MRA revealed a mild kink in the isthmus without significant stenosis (large red arrow) and normal-sized intercostal (small red arrow) and internal mam- mary (small yellow arrow) arteries, results consistent with pseu- docoarctation. Journal of the American Society of Echocardiography Volume 28 Number 2 Goldstein et al 135
electrocardiographic and respiratory gating has been shown to in- crease vessel sharpness and reduce artifacts. 112 MRI may also be used as a tool to investigate aortic physiology. Quantification of stiffness, an important predictor of cardiovascular outcome, can be obtained with pulsewave measurements from high–temporal resolution cine imaging. 113
MRI can provide insight into the elastic properties of the aorta, quantify the resultant blood flow, 114
and estimate aortic wall shear stress. 115
5. Artifacts. Similar to echocardiographic imaging, MRI artifacts occur. Consequently, consistently recognizing artifacts can prevent misinterpretation. The reader is referred to two excellent reviews for a detailed discussion of these. 116,117
H. Invasive Aortography Once considered the reference standard for the diagnosis of acute aortic diseases, invasive catheter-based aortography has largely been replaced by less invasive techniques, including CT, MRI, and TEE. 42,118-124 These noninvasive imaging modalities provide higher sensitivity and specificity for detecting AAS and enable the assessment of aortic wall pathologies that are not seen on lumenograms (as obtained by contrast aortography). In addition, CT, MRI, and TEE also provide greater sensitivity in detecting supporting findings such as pericardial or pleural hemorrhage or effusion. Moreover, aortography is time consuming and incurs a risk for contrast-induced nephropathy. Thus, invasive aortography Figure 22 Contrast aortogram (left) before (A) and after (B) endovascular repair showing relief of malperfusion syndrome. Three-dimensional CTA (right) shows corresponding computed tomographic angiographic reconstructions before (C) and after (D) repair.
Figure 23 Sensitivity of imaging modalities in evaluating sus- pected aortic dissection in a meta-analysis of 1,139 patients. Ó Massachusetts General Hospital Thoracic Aortic Center; re- produced with permission. Figure 24 Specificity of imaging modalities in evaluating sus- pected aortic dissection in a meta-analysis of 1,139 patients. Ó Massachusetts General Hospital Thoracic Aortic Center; re- produced with permission. 136 Goldstein et al Journal of the American Society of Echocardiography February 2015
no longer has a role as a primary diagnostic modality for AAS.
42,120-122,124 Although invasive aortography has been replaced for diagnostic purposes, it continues to be useful to guide endovascular procedures and to screen for endoleakage. Intraprocedural contrast aortography is often essential to identify aortic side branches and provide impor- tant landmarks during the endovascular procedure. Figure 22 reveals
the resolution of distal dynamic aortic obstruction after a stent graft was placed in a type B dissection. IVUS is an alternative imaging tech- nique during endovascular procedures. 59,60,62,125,126 I. Comparison of Imaging Techniques With advances in imaging technology, there are now multiple modal- ities well suited to imaging the thoracic aorta, including CTA, MRA, echocardiography, and aortography. 1,127 No single modality is preferred for all patients or all clinical situations. Instead, the choice of imaging modality should be individualized on the basis of a patient’s clinical condition, the relevant diagnostic questions to be answered, and local institutional factors such as expertise and availability. A few pertinent comments follow. When assessing broadly for the presence of thoracic aortic aneu- rysms (TAAs), or to size such aneurysms, CTA or MRA is preferred, as all segments of the thoracic aorta are well visualized. As well, the Yüklə 9,02 Mb. Dostları ilə paylaş:
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