Takayasu arteritis is a granulomatous vasculitis of unknown etiology that commonly affects the thoracic aorta and its branches, the pulmonary arteries, and the coronary arteries. It causes intimal fibroproliferation, which ultimately leads to segmental stenosis, occlusion, dilatation, and aneurysmal formation in these vessels. [1, 2, 3, 4, 5, 6, 7, 8] In North America, Takayasu arteritis has an incidence of 2.6 per million population. [9, 10] Ninety percent of patients with Takayasu arteritis are younger than 30 years. [4, 11, 12]
Imaging is considered the cornerstone of the diagnosis of Takayasu arteritis.  Historically, angiography has been the criterion-standard imaging tool for the diagnosis and evaluation of Takayasu arteritis. Computed tomography angiography (CTA) and magnetic resonance angiography (MRA) have become equally valuable tools. Their large fields of view, as well as the fact that they are noninvasive, make them far more attractive as diagnostic tools. The increasing resolution of multidetector-row CT arrays increases the diagnostic value. The soft tissue differentiation possible with MR techniques is valuable in distinguishing active forms of Takayasu disease from quiescent forms. [14, 15, 16, 17, 18, 19, 20, 21]
Mavrogeni and colleagues suggested a practical approach for the diagnosis of Takayasu arteritis and selection of imaging methods, as follows  :
CT and MRI for luminography are less invasive than angiography and depict similar results
Use MRI if possible over CT because of lack of radiation
Evaluate the heart; MRI is superior
Ultrasound is excellent for evaluating extracranial carotid arteries
Use PET if the main complaint is fever of unknown origin
Choose PET/MRI over PET/CT or PET
Correlate with imaging findings with clinical and laboratory findings
See the image of Takayasu arteritis below.
Although not a primary tool for diagnosis, radiographic manifestations have been historically described in patients with Takayasu arteritis. Loss of sharp definition and a wavy or scalloped appearance of the descending thoracic aorta have been described.  Linear calcifications tend to be present in the aortic arch (porcelain aorta) and in the descending thoracic aorta in chronic disease, sparing the ascending aorta.  Cardiomegaly, decreased pulmonary vessels, and rib notching can also be found. 
B-mode ultrasonography may depict homogeneous circumferential thickening of affected vessels, characterized by a diffuse hypoechoic or isoechoic thickening of the affected wall on longitudinal scanning, classically described as the “macaroni sign,” which may represent edema, increased vascularity, or both  . B-mode morphologic study has further been utilized to monitor the progression of disease. In a follow-up study of 20 patients,  wall thickness and outer diameter of the carotid artery increased in patients who relapsed and decreased in patients who remained in remission. Vascular stenosis, occlusions, and dilation with increased flow velocity beyond the stenotic lesions are common findings on spectral Doppler ultrasound.  Dissections of the aorta, celiac artery, subclavian artery, vertebral artery, brachial artery, and femoral artery have been described.  Collateral formation of occluded carotid arteries frequently occur and have incidentally suggested the diagnosis of Takayasu arteritis. 
A scoring system that correlates color Doppler ultrasound with clinical activity has been described, with statistically significant results in a study of 19 patients. A correlation was also found with angiographic findings in selected arterial sites.  . Further evaluation of this tool is needed.
Contrast-enhanced ultrasound (CEUS) has permitted the identification of flow within the thickened arterial wall, which corresponds to neovessels on the adventitial side during the initial inflammatory phase. [30, 31] In a study of 31 patients with large vessel vasculitis that compared CEUS vascularization grade with grade of vascular inflammation on FDG/PET as the gold standard, carotid CEUS showed a 100% sensitivity and 92% specificity in the detection of active vascular disease. 
In early disease, contrast CT demonstrates mural thickening, with intramural calcifications identified as the disease progresses. 
On CT angiography (CTA), the precontrast scan shows a high attenuation of the aortic wall. The arterial phase demonstrates luminal changes such as stenosis, and the venous phase shows a double ring pattern of the aortic wall.  Cardiac CTA has also detected silent coronary involvement characterized by nonostial stenosis, ostial stenosis, and coronary aneurysms. 
The CT differential diagnosis includes giant cell arteritis, polyarteritis nodosa, and atherosclerosis  .
One disadvantage of CT, as compared to conventional angiography, is that pressure differentials cannot be measured across lesions in which imaging findings regarding hemodynamic significance are inconclusive.
Magnetic Resonance Imaging
Currently, MRI is one of the methods most often used to evaluate Takayasu arteritis. It offers reproducibility and excellent luminography and lacks radiation. The utility of this noninvasive technique is particularly high in pediatric patients, in whom the biological costs and complications of angiography are potentially greater than in adults.
In cases of Takayasu arteritis, gadolinium-enhanced MRA may demonstrate concentric or crescent-like thickening of the arterial wall.  Inversion recovery–prepared echo-gradient pulse sequences with fat suppression depict indistinct outlines. Delayed hyperenhancement is seen on delayed contrast-enhanced MRI in the inflamed aortic arterial walls.  Mural thrombi can be more easily detected on MRI than on conventional angiography  .
Assessment of disease activity is one of the major challenges for physicians. The soft tissue differentiation obtained with MR techniques may help distinguish the active or acute phase from the chronic phase of the disease. This capability may be important in the timing of catheter-based or other interventions.
Discerning between active disease and inactive disease with fibrosis has been a matter of extensive study, as the 2 entities have similar characteristics on contrast MRI. New contrast agents that compartmentalize to the intravascular space, such as gadofosveset trisodium, may be the answer to this problem. Already approved in Europe, this contrast agent has demonstrated the capacity to make this distinction. Significant enhancement of the arterial wall was identified in active disease, whereas no such enhancement was detected in fibrotic tissue.  Further studies are required before this agent can be considered for approval by the FDA.
Cine MRI is fundamental to detect cardiovascular and hemodynamic changes in patients with associated aortic regurgitation. 
Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see Nephrogenic Fibrosing Dermopathy.
Limitations include the possibility of misdiagnosing stenosis as occlusions in vascular branch points, as well as the inability to identify and characterize calcifications. 
Karapolat and colleagues investigated F-18 FDG PET-CT scanning as a new tool in assessing disease activity in Takayasu arteritis. They concluded that the findings were consistent with clinical disease status.  Intense linear uptake of FDG is characteristic of active vasculitis.  This imaging method may have potential for confirming remission or assessing disease activity. [43, 44] This modality depicted positive findings in all patients with active disease in a retrospective analysis of patients with and without immunosuppression  .
Thallium-201 (201Tl) myocardial scintigraphy with dipyridamole (DPM) as a vasodilator agent has been utilized to identify patients with Takayasu arteritis who have asymptomatic myocardial involvement. 
Angiography has been considered the gold standard for diagnosis of Takayasu arteritis.  However, because of its invasiveness, angiography is not the first-line study in most patients, particularly in pediatric patients. Gadolinium-enhanced MRA findings may be diagnostic or strongly suggestive of the disease (see the images below). 
Categorized by the vessels involved, a new angiographic classification has been proposed,  which divides the disease into 5 types:
Type I involves only the branches of the aortic arch
Type IIa involves the ascending aorta, the aortic arch, and its branches; type IIb involves type IIa vessels plus the descending aorta
Type III involves the descending aorta, abdominal aorta, and/or renal arteries
Type IV involves the abdominal aorta and/or renal arteries
Type V involves type IIB and IV combined
In 75% of patients, the sites of vascular involvement include the aortic arch and its branches. The most commonly involved aortic branches are the left subclavian artery, which is affected in 55% of patients, followed by the right subclavian artery (38%), the left common carotid artery (30%), and the right common carotid artery (15%).
Aortography most frequently reveals focal, smooth, symmetric narrowing of the aorta and multiple branch vessel stenosis or occlusion, with secondary collateral vascularity or systemic-pulmonary shunts. Stenosis is the most common finding and tends to affect the thoracic aorta, abdominal aorta, subclavian artery, common carotid arteries, and renal arteries. Arterial dilatation and fusiform aneurysms are often found in the ascending aorta and the right-sided brachiocephalic and subclavian arteries. When Takayasu arteritis involves the subclavian artery, the lesion is a smoothly tapered stenosis; it begins within a few centimeters of the arch and extends to the origin of the vertebral artery. When the condition involves the carotid artery, regions of multisegmental dilation are alternated with regions of normal-appearing arterial wall  .
Serial angiography is not only helpful for the initial diagnosis but also helpful for follow-up. In a prospective study, up to 61% of patients with clinically inactive disease developed angiographic changes consistent with persistent inflammatory disease at angiography follow-up. 
Pressure measurements should be obtained in the ascending aorta and compared with measurements in the extremities.
Fibromuscular dysplasia (FMD) is part of the differential diagnoses of Takayasu arteritis. Classic FMD has a beaded appearance and usually does not affect the aorta, as evidenced on angiograms. FMD is rare in the subclavian artery.
Angiography has also been used in interventional radiology procedures, such as percutaneous transluminal angioplasty (PTA) in patients with quiescent disease and clinical stenosis. PTA without stent placement has demonstrated a long-term patency of 50% at 5 years.  Detailed assessment, including disease activity and optimal immunomodulatory treatment, should be determined before undergoing these procedures, as restenosis is frequent in patients with active disease.