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1 From the Department of Radiology, Section of Vascular and Interventional Radiology, Harborview Medical Center, University of Washington, 325 Ninth Ave, Box 359728, Seattle, WA 98104. Received February 28, 2001; revision requested March 29; revision received June 1; accepted June 7.
| ABSTRACT |
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MATERIALS AND METHODS: Retrospective review was performed of 35 consecutive cases that were positive for true or false aneurysm, arteriovenous fistula or malformation, or hemorrhage when a lesion was located beyond a first-order branch of the aorta. An artery that potentially supplied the lesion was considered positive if it filled with contrast material, emptied, and filled again while adjacent vessels demonstrated progressive opacification.
RESULTS: Nine (26%) of the 35 cases demonstrated intermittent opacification of an injured artery. All were confirmed as true-positive with superselective catheterization or additional projections, and seven were successfully treated with transcatheter embolization. Intermittent opacification was associated only with false aneurysm and hemorrhage. No congenital arteriovenous malformations or congenital aneurysms demonstrated intermittent opacification.
CONCLUSION: If present, the intermittent opacification of an artery is a valuable finding that assists in superselective transcatheter embolization of the arterial branch that supplies a false aneurysm or hemorrhage.
Index terms: Arteries, injuries, 9.4 • Arteries, therapeutic embolization, 9.1264
| INTRODUCTION |
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After initial aortography, selective catheterization and contrast material–enhanced imaging can lead to rapid selection of the injured artery. However, if multiple arteries lead to the region of extravasation, false aneurysm, or arteriovenous fistula, identification of the contributing arterial branch may require either imaging with multiple magnified and oblique views or selection and contrast-enhanced imaging of multiple suspect branches (5). Identification of the injured artery may also be difficult if the act of catheterization diminishes blood flow to the abnormality, due either to the size of the catheter or to the spasm induced by the guide wire or catheter (10). The purpose of this study was to evaluate the prevalence of intermittent opacification, a finding previously described as diagnostic of active bleeding (11), that allows identification of an injured vessel at initial aortography or first-order selective angiography.
| MATERIALS AND METHODS |
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Cases were reviewed by at least two of the authors (by consensus), with the knowledge that an abnormality had been identified; in some cases the exact type of lesion was known. All studies were performed with digital subtraction angiography. Reviews of positive cases were performed at weekly intervals so that every image and all sequences of a particular study were available for evaluation at the workstation. The aortograms or first-order selective image sequences were evaluated for imaging plane, catheter position, type of arterial injury, and presence of intermittent opacification. Contrast-enhanced images obtained after further selective catheterization were reviewed to identify the specific artery injured and results of embolization, if any.
Intermittent opacification is the opacification, then decreased opacification, then increased opacification, then decreased opacification of a single vessel in consecutive arterial phase images at angiography (Figure) (11). For this study, the artery had to be in an area of interest as identified by a site of extravasation, aneurysm, or arteriovenous connection. Adjacent vessels an equal distance distal from the point of contrast agent injection and of similar caliber had to progressively opacify on the same images.
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If an artery with all components of intermittent opacification could be identified (by consensus of the reviewers) at initial aortography or first-order selective arteriography, it was considered positive. If the same artery was confirmed as the artery that supplied the abnormality after further catheterization and selective injection, it was considered true-positive. If the vessel proved not to contribute to the abnormality, it was considered false-positive.
A MEDLINE literature search was performed to obtain prior descriptions of intermittent opacification. Search terms included the following: artery, injury, trauma, false aneurysm, arteriovenous fistula, hemorrhage, pulsation, and intermittent.
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Alternating opacity was seen in one nonbleeding artery; it was identified in the proximal portion of an occluded inferior gluteal artery. This was distinguished from a positive finding in that contrast material never filled the entire vessel, and no distinct extravasation or false aneurysm was identified in that vascular territory.
Since all the cases were selected because they were positive for arterial injury, specificity could not be calculated. Although the prevalence of intermittent opacification was only 26%, there were no false-positive cases as defined by our criteria for a positive finding. An incidental finding was that no other arteries in the reviewed cases (from which the positive cases were differentiated) demonstrated intermittent opacification. Intermittent opacification was identified only in association with false aneurysm and extravasation; it was not seen with congenital aneurysm, congenital malformation, or arteriovenous fistula.
The MEDLINE search revealed a prior description of the finding of diastolic clearance of angiographic contrast medium selectively from the bleeding vessel, which the authors identified as a sign of active bleeding (11). Even without an associated collection of contrast material or false aneurysm, the "systolic/diastolic vascular flash" was considered an "absolute, definitive and diagnostic" sign of arteriographic bleeding (11).
| DISCUSSION |
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Arterial lesions associated with hemorrhage may be identified on an angiogram by the presence of false aneurysm or extravasation. These are often amenable to transcatheter embolization (1–9). The percutaneous approach has substantial advantages over open repair, particularly when the surgical risks are high because of patient comorbidity or complicated access to the lesion. The findings of arterial injury are well described, and while there is some controversy over the indications for intervention on minimal injuries, those that are clearly transmural (eg, false aneurysm, arteriovenous fistula, or hemorrhage) warrant intervention in the clinical setting of hemodynamic instability or blood loss (1,2).
The practical identification of the path to the abnormality and determination of whether it is amenable to transcatheter therapy are less well described. The finding of contrast material extending beyond the confines of a vessel, which persists into the venous phase, or the finding of early filling of veins in association with malformation or fistula is diagnostic but does not assist in the identification of the arterial supply to the lesion. Though previously described as an indirect finding of arterial bleeding (11), intermittent opacification of the supply artery is a unique, direct sign of a course of altered arterial flow that indicates a pathway for selective catheterization of a lesion.
In some vascular beds (ie, the internal iliac arteries), precise localization of the injury may not be necessary, as nonselective embolization may incur minimal morbidity and may be expeditious (1–3). Other vessels, particularly distal arteries in nonexpendable structures (eg, the kidney or bowel), benefit from a most distal level of embolization, which requires identification of the terminal abnormal branch (5–8). The presence of intermittent opacification may minimize the number of additional oblique or magnified images required to target a vessel for selective catheterization. This has the potential to reduce both the contrast material dose and the procedure time. Since catheterization of normal arterial branches would be reduced, it follows that spasm or injury to these vessels may be diminished.
The mechanism of intermittent opacification is unclear. Arteries with normal peripheral beds have substantial blood flow resistance, such that blood flow markedly slows or reverses in diastole (12). The presence of arteriovenous fistula, false aneurysm, or extravasation is associated with an abnormal, decreased outflow resistance in the artery that supplies the lesion. This is a manifestation of the law of Poiseuille: The conductance in a conduit increases in proportion to the fourth power of the radius. The defect in the arterial wall (due to aneurysm or unconstrained space) would remove the distal resistance and could result in continuous antegrade flow (11). If flow in the upstream artery stops or reverses during diastole (due to the arterial walls and the majority of outflow branches being intact) while flow in the injured branch artery continues, that branch could empty of opacified blood. Although some passive diastolic antegrade flow into the injured branch artery or retrograde flow from the intact vessel beyond the injured branch may occur, this flow could disperse distally and may not create substantial opacity. Also, the volume of blood flowing retrograde from the intact branch would decrease due to the volume lost during systole through the injured branch. On the radiographic image, the net result would be an increase and decrease in opacity with regard to the cardiac cycle.
Because the majority of positive cases were clearly associated with false aneurysm, an alternative or partial explanation for the finding of intermittent opacification is that while opacified blood fills the artery during systole, unopacified blood from the false aneurysm may flow retrograde into the supplying branch during diastole. However, this explanation would not account for the two instances of extravasation (in patients 1 and 7) in which retrograde flow would not be expected.
Another possible explanation for the appearance of intermittent opacification is intermittent spasm. Vessels in spasm may be markedly narrowed and may completely occlude (13). There was no evidence that the vessels we observed were either occluded or narrowed, however. Although spasm may vary over time and produce intermittent narrowing, it would be unusual for spasm to vary from second to second.
Another consideration is collateral inflow of unopacified blood. Indeed, intermittent opacification of pancreaticoduodenal vessels during gastroduodenal or superior mesenteric artery contrast-enhanced imaging or intermittent opacification of a contralateral anterior cerebral artery after injection into the internal carotid artery is commonly observed due to the dual blood supply in those distributions. However, in the cases we observed, the effect of such collateral flow was excluded by the persistent opacification of adjacent and more distal vessels. Collateral inflow could be expected to produce an alternating opacity in all branches equidistant from the point of inflow. In addition, there was no evidence for atherosclerotic disease or other stenoses that would compromise inflow or produce collateral vessels; none were observed at nonselective injection.
Identification of intermittent opacification is dependent on rapid imaging, since the variation of arterial blood flow with regard to the cardiac cycle permits visualization of this finding. The image acquisition rate must be greater than the pulse rate to avoid aliasing and to ensure visualization of the intermittent opacity if it exists. Although some of our cases illustrated the finding with slower image acquisition rates, an acquisition rate that is double the pulse rate would ensure visualization if intermittent opacification is present. Similarly, the injection of contrast material must be of adequate duration to ensure that the column of contrast material is present for two cardiac cycles (ie, a 2-second injection if the pulse is 60 beats per minute). The image acquisition timing requirement was not considered in these cases (as this review was retrospective), however, and a standard two-image-per-second imaging sequence may have been inadequate in the often hypotensive, tachycardic patients who were evaluated for active hemorrhage. Therefore, the prevalence of the finding in this study may be underestimated.
The true prevalence of intermittent opacification and its use as an aid to embolization is unclear, as our study was unblinded and retrospective. Determination of the predictive value of intermittent opacification to identify an embolization target would require a prospective study in which imaging parameters were optimized, and assessment of the value of intermittent opacification would require comparison with embolization cases in which intermittent opacification is absent or not sought.
We did not see intermittent opacification in any of the congenital arteriovenous malformations or congenital intracranial aneurysms that we reviewed. Although these lesions demonstrated high flow, their chronic nature could theoretically lead to a compensatory increase in the size of the inflow vessels such that the low resistance outflow does not exceed inflow, even in diastole (14).
The absence of intermittent opacification does not provide diagnostic information, and it does not exclude an artery from supplying an abnormality. When present, however, the finding of intermittent opacification may expedite the analysis of whether a lesion is amenable to embolization and may minimize the time and manipulation necessary for selective catheterization of the injured vessel.
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