Conventional Arteriographic and Computed Tomography Arteriographic Diagnosis of Renovascular Hypertension



Conventional Arteriographic and Computed Tomography Arteriographic Diagnosis of Renovascular Hypertension



Peter S. Liu and David M. Williams


Despite increased awareness of and treatment for hypertension in the United States since the 1970s, hypertension remains a major public health problem. Survey data suggest that approximately 50 million Americans have an elevated blood pressure that merits treatment, and worldwide hypertension prevalence is estimated at nearly 1 billion people. The prevalence of hypertension increases with advancing age, affecting approximately 50% of patients older than 60 years old and nearly 75% of patients older than 70 years. Hypertension is a major determinant of future cardiovascular events, such that a 20 mm Hg increase in systolic blood pressure or 10 mm Hg increase in diastolic blood pressure results in a doubling of mortality for ischemic heart disease and stroke.


Although most cases of hypertension are primary (essential hypertension), a small percentage of cases can be attributed to a secondary process. Renal artery stenosis represents approximately 1% to 5% of all hypertension cases in the general population. These patients might have some component of primary hypertension as well, and thus they might not be fully cured after revascularization. However, restoration of normal inflow to the kidney can improve blood pressure control with fewer medications. Therefore, renovascular disease is seen as an important treatable cause of secondary hypertension.



Catheter Angiography


Conventional renal angiography has benefited from advances in cross-sectional imaging, often allowing the operator to forgo the preliminary aortogram and proceed to selective arteriography. Catheter angiography remains important for detecting stenosis using magnification techniques in branch arteries in the pediatric population and assessing hemodynamic significance of stenosis in kidney transplants and in aortic dissections extending into the renal artery. Catheter angiography is crucial to renal artery interventions, including measurement of poststenotic pressures, delivery of balloons and stents, delivery of sclerosants to ablate parenchyma beyond segmental stenoses, and to position radiofrequency (RF)-ablation catheters for techniques to achieve renal sympathetic denervation.



Computed Tomography Arteriography


The advent of modern computed tomography (CT), including helical scanning technique and multidetector CT (MDCT) technology, has made noninvasive vascular imaging a clinical reality. Helical scanning combines a revolving gantry and detector element with a continuously moving table. Although initial setups suffered from differential through-plane resolution versus in-plane resolution, the continuous data acquisition resulted in a volumetric slab of data rather than a stack of interrupted slices. MDCT relies on the use of several detector-row elements placed adjacent to one another. These tiny elements individually facilitate higher through-plane resolution, but in aggregate they allow faster scanning through greater coverage per rotation.


State-of-the-art detector setups use 64 elements or more. As a result of these technological advances, modern MDCT scanners can acquire a complete volumetric data set in a single breath hold, providing submillimeter isotropic data resolution that can be reconstructed into any desired plane. Iodinated contrast material is required for vascular opacification, often introduced through large-bore peripheral intravenous access using a power injector device at 3 to 5 mL/sec. This rapid delivery of contrast material creates a relatively compact bolus that makes the abdominal aorta and renal arterial system opaque for a short duration. Because blood flow through the kidneys is brisk, accurate scan timing is critical to ensure peak arterial opacification with relatively little venous contamination.


Although several different timing mechanisms can be used in CT arteriography (CTA), the most reproducible method is an automated bolus-tracking technique that triggers acquisition from a threshold level of contrast delivery in a large vessel, such as the abdominal aorta. For bolus tracking in renal CTA, the suprarenal aorta is often used to monitor delivery of contrast. Once the imaging data have been acquired, review and diagnosis are often aided by postprocessing techniques, which can help delineate the three-dimensional (3-D) relationships of various structures. In particular, maximum-intensity-projection images and volume-rendered images are often generated for CTA studies to highlight and segment the arterial anatomy. Studies have shown similar sensitivities for arterial stenoses between the two techniques, though volume-rendered techniques have shown increased specificity. Whereas the 3-D reformatted imaging may be performed by a technologist, complete review of both the native axial imaging and postprocessed 3-D imaging is critical for establishing the correct diagnosis, because some features can be erroneously highlighted or overlooked based solely on postprocessed imaging.


Evaluation of suspected renal arterial narrowing has become a major indication for renal CTA. Depiction of renal artery stenosis on CTA is relatively straightforward, particularly with reformatted imaging, where the arteries may be segmented or highlighted versus the background tissues.


Atherosclerotic disease and fibromuscular dysplasia can be well demonstrated on CTA (Figures 1 and 2). Narrowings may be graded qualitatively or quantitatively. Eccentric narrowings, such as the webs associated with fibromuscular dysplasia, are often best demonstrated in an orthogonal plane or using 3-D reformatted images. Initial reports on the use of CTA for investigation of suspected renal artery stenosis were strongly positive. Early reports showed a high accuracy for hemodynamically significant stenoses outside of the renal hilum, and subsequent studies demonstrated high sensitivity and accuracy in the evaluation of fibromuscular dysplasia. A large meta-analysis in 2001 that investigated CTA for use in suspected renovascular hypertension revealed a high level of diagnostic performance, including an area under the summary receiver operating characteristic (ROC) curve of 0.99.


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Aug 25, 2016 | Posted by in CARDIOLOGY | Comments Off on Conventional Arteriographic and Computed Tomography Arteriographic Diagnosis of Renovascular Hypertension

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