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Abbreviation: RAS, renal artery stenosis.

Abbreviation: RAS, renal artery stenosis.

projections may be required to show the arterial ostia in profile. Oblique views should always be performed in patients with suspected FMD because the dilated segments tend to overlap the weblike stenoses (Fig. 8A,B) (62).

Selective catheterization may be required in these patients to demonstrate distal stenosis and allow pressure measurement. In patients with atherosclerotic disease, this is best avoided because of the risk of dissection and occlusion. However, if a significant stenosis is demonstrated, it is prudent to measure pressures across the lesion. Intra-arterial pressure measurement is regarded as the final reference standard for assessing the clinical significance of any arterial stenosis. This is achieved by passing a selective catheter past the stenosis and measuring paired pressures from beyond the stenosis and within the iliac artery (through the sheath). Alternatively, by steady withdrawal of the catheter, the gradient across the stenosis can be measured. As with change in lumen size, there is still a lack of consensus over the diagnostic thresholds used for intra-arterial pressure changes. An absolute change of between 7 and 34mmHg or 10% to 15% of systolic pressure across the stenosis has been employed, either as a resting value or after vasodilatation. Bonn (63) summarized the data on this subject and concluded that further studies are necessary. The situation has not become any clearer since.

Angiography does have limitations in that eccentric plaques on the anterior or posterior wall of the artery may not be seen. Also ostial stenoses will be projected over the aorta and missed if the artery rises anteriorly (64). Likewise, an ostial stenosis may be missed on selective angiography if the catheter is placed distal to the stenosis. Three-dimensional arterial reconstruction has recently become available, but its value has not yet been defined. Further limitations include the potential for contrast medium-induced nephrotoxicity and the radiation dose rendered to the patient and radiologist.

The issue of contrast medium-induced nephrotoxicity may be addressed by the use of carbon dioxide (CO2) angiography. In this technique CO2 is injected intra-arterially in place of conventional iodinated contrast by means of a closed-system automated CO2 injector (Fig. 7C). This avoids the potential for contamination with air possible during hand injection of CO2 by syringe. CO2 acts as a negative contrast by displacing rather than mixing with blood. It is highly soluble and is excreted via the lungs. Injection is safe but may provoke pain or nausea and vomiting in patients as it fills the mesenteric circulation. Its buoyancy as a gas may lead to underfilling of the renal arteries that travel posteriorly. This problem can be overcome by repeating the run with the patient obliqued to elevate the required side. Although the

contrast opacification is not as good as with iodinated media, images are diagnostic in most cases (65). Opacification of the main artery and primary branches is good, but intrarenal branches and small accessory vessels may not be seen. Schreier et al. (66) reported 83% sensitivity and 99% specificity for CO2 angiography in 100 patients. If CO2 images do not allow confident diagnosis, they can be supplemented by a single iodinated run at an optimized obliquity as judged from the previous CO2 images. CO2 has several advantages over conventional contrast in that it has no nephrotoxic effect and may be used in patients allergic to contrast or those with brittle asthma. In addition, apart from the initial cost of the injector, the CO2 itself is of negligible cost. Nevertheless, because of its inconvenience and significant minor morbidity, CO2 angiography has a restricted place in renal artery imaging.

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