Temporal arteritis, or giant cell arteritis, is a common systemic vasculitis of unknown etiology. In 1890, Hutchinson originally described the condition as inflamed and swollen temporal arteries. In 1932, Horton expanded the definition. In general, temporal arteritis can be thought of as a vasculitis involving medium-to-large arteries originating from the aorta. Although it was originally believed to be a rare entity, it is more commonly recognized today. [1, 2, 3, 4, 5, 6] See the images below.
Currently, temporal artery biopsy is the criterion standard for the diagnosis of temporal arteritis (giant cell arteritis). A negative biopsy finding does not exclude the diagnosis. No radiologic finding is specific for the diagnosis of temporal arteritis alone. Imaging studies are helpful in determining the extent of involvement and in identifying unsuspected areas of involvement.  Angiography can be used when biopsy results are negative, or it can be used to help guide biopsy by demonstrating areas of abnormality. [8, 9]
When performed, angiography is typically directed at the large branch vessels of the proximal aorta and extracranial carotid branch vessels. The temporal arteries are depicted well in almost 90% of patients. In patients with proximal artery stenoses, angioplasty can be used in addition to corticosteroid therapy for symptomatic relief. 
Although angiography is one of the best-studied techniques, it is invasive and inconvenient and has risks associated with the use of contrast material. As a result, less-invasive procedures for evaluating the arterial anatomy have been sought. Magnetic resonance angiography (MRA) has results comparable to those of angiography in evaluating medium to large vessels. In some reported cases, MRA has successfully depicted disease in the temporal arteries. However, MRA is limited in evaluating smaller vessels, and imaging artifacts may result in false-positive results. In addition, larger vessels with mildly thickened walls can be missed.As the sensitivity of MRA continues to improve, it will likely become a more realistic method for evaluating stenotic lesions attributed to temporal arteritis. [11, 12, 13]
Studies have revealed the benefit of ultrasonography in the diagnosis of temporal arteritis. [14, 15, 16, 17] Characteristic changes, including stenoses and occlusions of temporal artery segments and a dark halo around the vessel, have been reliably observed in patients with temporal arteritis. Doppler flow studies have also been performed, with promising results.  Ultrasonography may not depict minor vascular changes or diseased vessels, such as intrathoracic vessels, that are not amenable to ultrasonography.
A study of positron emission tomography (PET) scanning evaluated 18F-glucose uptake and demonstrated a sensitivity of 56%, a specificity of 98%, and a positive predictive value of 93% for the diagnosis of giant cell arteritis or polymyalgia rheumatica. 
Thickening of the arterial walls, stenosis, or occlusion may be demonstrated on contrast-enhanced computed tomography (CT) scans. However, CT scanning commonly fails to depict mild inflammatory changes in the vessels. CT is not useful for the evaluation of small-vessel disease. In older persons, disease processes such as atherosclerotic disease are far more common than temporal arteritis and may result in similar CT findings.
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) findings for temporal arteritis (giant cell arteritis) include loss of the normal flow void in affected vessels from occlusion or slow flow associated with disease. Enhancement of the arterial wall may be observed after the administration of gadolinium-based contrast material. [20, 21, 22, 23, 24, 25] MRA may also demonstrate stenoses, irregularity of the vessel wall, and beading or thickening of the vessel wall.
Contrast-enhanced MRI to diagnose giant cell arteritis was found, in one study, to have a sensitivity of 78.4% and a specificity of 90.4%. In patients in whom temporal artery biopsy was performed, sensitivity and specificity of MRI were 88.7% and 75%, respectively. The authors noted that sensitivity of MRI probably decreases after more than 5 days of systemic corticosteroid therapy, so that imaging should not be delayed. 
In a study of deep temporal artery and temporalis muscle involvement in patients with giant cell arteritis, MRI visualized changes in both the temporalis muscle and the deep temporal artery, and moderate correlation of clinical symptoms with MRI results was observed. Two radiologists assessed the images. They found temporalis muscle involvement in 19.7% and 21.3 % of GCA patients, and it occurred bilaterally in 100%. Specificities were 92% and 97 %, and sensitivities were 20% and 21%. Deep temporal artery involvement was found in 34.4% and 49.2% and occurred bilaterally in 80% and 90.5%; specificities were 84% and 95%, and sensitivities were 34% and 49%. 
In a prospective study of vasculitic changes in patients with giant cell arteritis assessed by 3T MRI, vessel wall enhancement of intradural arteries, mainly the internal carotid artery (ICA), was regularly found. In this study, by Siemonsen et al, 2 independent observers found clear vessel wall enhancement of superficial extracranial and intradural internal carotid arteries in 16 and 10 patients, respectively. Slight vessel wall enhancement of the vertebral arteries was seen. Of 9 patients with giant cell arteritis with vessel occlusion or stenosis, 2 presented with cerebral ischemic infarcts. Vessel occlusion or stenosis site coincided with the location of vessel wall enhancement of the vertebral arteries in 4 patients and of the intradural ICA in 1 patient. 
MRI commonly will miss mild inflammatory changes in vessels. MRI is not useful for the evaluation of small-vessel disease. In the elderly, disease processes such as atherosclerotic disease are far more common than temporal arteritis and may result in similar MRI findings.
Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans.
Ultrasonography can be used to evaluate small vessels, such as the temporal arteries. Findings include stenoses and occlusion of the vessels. A characteristic hypoechoic halo has been described as surrounding the affected vessel that disappears after corticosteroid therapy. Ultrasonography is also useful in guiding biopsy.  Ultrasonography cannot be used to evaluate vessels such as intrathoracic arteries, which are more amenable to angiography and MRI. Findings may be negative in patients with minimal involvement of the temporal arteries. In addition, atherosclerotic disease involving the temporal arteries may have an appearance similar to that of temporal arteritis, although this is unusual.
Positron emission tomography (PET) scanning has been used to evaluate unusual involvement that cannot be evaluated by means of surgical biopsy or ultrasonography. PET scanning cannot be used to distinguish between the increased uptake observed with temporal arteritis and that observed in polymyalgia rheumatica.
FDG PET/CT is increasingly being used to diagnose inflammation of the large arteries in GCA. In one study, the number of vascular segments with diffuse FDG uptake pattern was significantly higher in GCA patients without glucocorticoid use. 
Angiography is an invasive test with inherent risks associated with the procedure and with the administration of contrast material. Findings consist of the involvement of small to moderate vessels. Angiography can demonstrate areas of constriction, beading, and microaneurysm formation that are fairly specific for temporal arteritis (giant cell arteritis). The occlusion of vessels and stenoses that are amenable to treatment may also be observed.
The most common sites for abnormalities to occur anatomically and on imaging studies are in the distal subclavian, proximal axillary, brachial, brachiocephalic, and femoral arteries. Atherosclerotic disease is a common finding in the older population; however, narrowings observed with atherosclerotic disease are typically short, segmental, and irregular, whereas stenoses in temporal arteritis are smooth, long, segmental, and tapered.
Similar findings may be observed in patients with Takayasu arteritis and in those with atherosclerotic disease. Temporal artery biopsy is more definitive than angiography, and it can be guided by the arteriographic findings.
False-negative results may occur in a few patients in whom the temporal arteries are not well visualized.