Imaging inflammatory Large Vessel Vasculitides (LVV)

Young lady (22 years) came to the vascular surgery clinic with low-grade fever, loss of weight, body pains, tenderness in the carotid triangle and weak radial pulsations in both the hands. There were no vascular risk factors or joint pains. Giant-cell arteritis (GCA) and Takayasu’s arteritis (TA) are the commonest large-vessel vasculitides (LVV) in such patients. Imaging studies are increasingly being used to diagnose and to monitor LVV. The aim of this presentation is to discuss a case and review the literature on imaging studies.

Imaging studies play a central role in diagnosing and monitoring the large vessel vasculitides (giant-cell and Takayasu’s arteritis). CT or MRI, TEE can examine the deep and central large vessels, while color Doppler ultrasound and MRI have been used with promising results to investigate the temporal arteries. Positron emission tomography is very sensitive in detecting large-vessel inflammation, although it does not delineate the vessel wall. Imaging procedures can also be used to monitor the disease course. However, imaging signs of inflammation may sometimes persist despite clinical remission and, conversely, seemingly unaffected vessels may develop alterations later on. The commonest angiographic findings in Takayasu’s disease are long, smooth vascular stenoses and sometimes occlusions and aneurysms. However, angiography cannot demonstrate early vasculitic lesions such as vessel wall alterations and is thus not useful for early diagnosis. 

In early Takayasu’s disease, CT may show arterial wall thickening with mural enhancement and low-attenuation ring on delayed images . On the other hand, in inactive TA the arterial wall is slightly thickened or normal with a high attenuation or calcifications on unenhanced phase images, and absent or only slight mural enhancement without low-attenuation ring on delayed images. CT angiography may have a role in monitoring the disease course. Aortic wall enhancement was shown to resolve after immunosuppressive therapy in 7 of 13 Takayasu’s disease patients by Paul JF et al (2001). Wall thickening decreased after immunosuppressive therapy in 56% of the lesions, albeit 25% of the initial lesions showed evidence of progression. In contrast, in another study Yoshida et al failed to show a reduction in wall thickening of the supra-aortic branches after glucocorticoid therapy, possibly due to the limited sensitivity of CT angiography in depicting lesions in arteries smaller than the aorta.

            The data suggest that 1.5T–3T MRI is much more sensitive for diagnosing GCA than 1T MRI. There is limited knowledge on the role of MRI in the follow-up of GCA patients. In a patient, marked improvement of MRI inflammatory changes of the temporal arteries was observed 8 weeks after commencing glucocorticoid therapy.

Increased glucose uptake by active cells can be detected by administration of a fluorine-labelled glucose analogon (¹⁸Fluorodeoxyglucose, FDG). Large-vessel FDG uptake is usually graded on a 4-point scale: none (grade 0), lower than liver uptake (grade 1), similar to liver uptake (grade 2) and higher than liver uptake (grade 3). Grades 2–3 are relatively specific for vasculitis, while grade 1 (rarely 2) has been observed in atherosclerotic vessels. PET may be more sensitive than MRI in detecting vessel inflammation in early-stage LVV, probably because inflammatory cell infiltration (revealed by PET) is likely to precede the development of vessel wall oedema (depicted by MRI). PET also shows the extent of vascular involvement, although some arteries, such as the temporal and renal arteries, cannot be visualized. Finally, since PET is very sensitive in revealing areas of active inflamm-ation, it has also been proposed to evaluate response to therapy. PET is minimally invasive and involves a very small dose of radiation.Andrews et al. compared PET and MRI in the assessment of disease activity after immunosuppression in six TA patients. The ¹⁸FDG vessel wall uptake was judged to be much more reliable in defining disease activity than gadolinium enhancement on MRI.

Discriminating between atherosclerotic and vasculitic lesions may be challenging, but a number of characteristics may point towards one condition and away from the other. First, vasculitic lesions are usually characterized by a more intense FDG uptake. Second, involvement of vessels usually spared by atherosclerosis would point to vasculitis. Third, atherosclerotic plaques show as ‘hot spots’, whereas vasculitic lesions are usually smooth–linear. In doubtful cases, the involved vessels should be imaged by MRI or US. Practical limitations of PET include its high costs and restricted availability.

Imaging studies can assist in diagnosing early vasculitis by demonstrating vessels lesions even when angiography is negative. Imaging procedures are also useful for monitoring purposes. Signs such as increased vessel wall thickness, a halo on US or vessel wall edema and mural enhancement on CT or MRI, are usually considered evidence of active disease. Second, inflammatory changes on imaging may persist despite clinical remission. Vessel wall thickness usually gradually decreases as inflammation subsides, but it is difficult to confidently discriminate between active inflammatory from chronic fibrotic lesions on the basis of vessel wall thickness alone. Vessel stenosis may be due to active vasculitis, but also to scarring after inflammation has abated. PET may be a more reliable procedure in the evaluation of disease activity. A thorough knowledge of the possibilities and limits of the various imaging procedures places the clinician in the best position not only to diagnose early vasculitis, but also to fine-tune the treatment to the requirement of the individual cases.