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Published online before print April 29, 2004, 10.1148/radiol.2313030093
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(Radiology 2004;231:906-913.)
© RSNA, 2004

Technical Developments

Carotid Perfusion CT with Balloon Occlusion and Acetazolamide Challenge Test: Feasibility1

Rajan Jain, MD, Ellen G. Hoeffner, MD, John P. Deveikis, MD, Mark R. Harrigan, MD, B. Gregory Thompson, MD and Suresh K. Mukherji, MD

1 From the Departments of Radiology (R.J., E.G.H., J.P.D., S.K.M.) and Neurosurgery (M.R.H., B.G.T.), University of Michigan Health System, University Hospital UH B2 A209, 1500 E Medical Center Dr, Ann Arbor, MI 48109. Received January 21, 2003; revision requested April 8; final revision received October 21; accepted November 6. Address correspondence to E.G.H. (e-mail: hoeffner@umich.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and Methods
Results
 Discussion
 REFERENCES
 
Carotid balloon test occlusion (BTO) is used to assess the collateral circulation and cerebrovascular reserve in patients in whom carotid artery occlusion is contemplated. Eight patients in whom the test was successful were evaluated with perfusion computed tomography (CT) in the resting state and after acetazolamide challenge. Three of the patients showed symmetric blood flow and normal response to acetazolamide. One of them underwent permanent carotid occlusion and did not develop any delayed ischemic stroke. The remaining five patients showed asymmetric blood flow. One of them had markedly low blood flow and abnormal response to acetazolamide. The patient developed ipsilateral hemispheric stroke following permanent carotid occlusion after the superficial temporal artery to middle cerebral artery bypass graft occluded. In the other four patients, the steal phenomenon was seen in ipsilateral and contralateral hemispheres. Although definitive quantitative values for perfusion CT are not yet standardized, it may be feasible to predict that the patients with symmetric blood flow and normal acetazolamide-enhanced challenge test results will do well after permanent carotid occlusion. Patients with asymmetric blood flow and abnormal response to the acetazolamide challenge test may require a revascularization procedure to protect them from delayed ischemic stroke.

© RSNA, 2004

Index terms: Brain, infarction, 17.7219 • Carotid arteries, stenosis or obstruction, 17.721, 17.756 • Cerebral blood vessels, stenosis or obstruction, 17.721, 17.756 • Computed tomography (CT), perfusion study, 17.12119


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and Methods
Results
 Discussion
 REFERENCES
 
Balloon test occlusion (BTO) of the internal carotid artery is performed routinelyto access the collateral circulation in patients in whom acute permanent or prolonged temporary occlusion of the internal carotid artery is indicated or is a risk as part of the endovascular or surgical procedure. Patients in whom BTO is not clinically successful (ie, those who develop any change in their neurologic status during balloon occlusion) are thought to have insufficient collateral circulation or vasodilatory reserve to provide adequate cerebral blood flow (CBF). Almost all patients in whom BTO is not clinically successful develop neurologic deficit if the internal carotid artery is occluded acutely without any revascularization procedure (1,2). Approximately 10% of patients in whom test occlusion is clinically successful (ie, those who do not develop any neurologic changes during balloon occlusion) have ipsilateral low CBF measured at xenon-enhanced computed tomography (CT) (3,4), and 50% of these patients actually develop neurologic deficit that is mostly related to inadequate blood flow or emboli in the ipsilateral hemisphere (5–9).

Perfusion magnetic resonance (MR) imaging, xenon-enhanced CT, positron emission tomography (PET), single photon emission CT (SPECT), and transcranial Doppler sonography are used to evaluate cerebral perfusion during BTO, particularly to identify the subgroup of patients with low CBF who can benefit from a revascularization procedure before permanent internal carotid artery occlusion is performed (10–15). The purpose of our study was to evaluate the feasibility of carotid perfusion CT as an adjunct to BTO for preoperative assessment of delayed stroke (ie, stroke that occurs from within hours to as long as 1 year after permanent carotid occlusion [13]).


   Materials and Methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
 Discussion
 REFERENCES
 
Theory
Perfusion CT is performed by monitoring the first pass of a bolus of standard iodinated contrast material through the cerebral vasculature. The examination can be performed with any standard spiral CT scanner. The contrast agent causes a transient increase in attenuation proportional to the amount of contrast material in a given region. Time-concentration curves for the contrast agent, which have a linear relationship, are generated in an arterial region of interest, a venous region of interest, and each pixel. Deconvolution of arterial and tissue enhancement curves, a complex mathematic process, gives the mean transit time (MTT). The cerebral blood volume (CBV) is calculated as the area under the curve in parenchymal pixels divided by the area under the curve in arterial pixels. CBF is calculated as CBV/MTT.

Patients
From September 2001 through July 2002, eight patients (six men and two women; age range, 32–60 years; mean age, 45.5 years) underwent carotid BTO at our institution. The test was clinically successful in all eight patients, and they then underwent perfusion CT for further evaluation of CBF. These perfusion CT studies were then retrospectively reviewed. Our institutional review board exempted this retrospective study from review; therefore, informed consent was also waived. Six of the eight patients had a lesion on the right side, while the remaining two patients had a lesion of the left side. Three of the eight patients had schwannomas, two had paragangliomas, two had carotid artery aneurysms, and one had medullary thyroid carcinoma metastasis to the cavernous sinus. Three of the eight patients also underwent preoperative embolization of the tumor after BTO.

BTO Technique
All patients underwent angiography and BTO with systemic heparin administration and monitoring of activated clotting times. A 5-F occlusion balloon catheter was used to create temporary occlusion while the patients underwent continuous neurologic testing, stump arterial pressure monitoring, and electrocardiographic monitoring for 30 minutes. The balloon was inflated in the cervical part of the internal carotid artery in all patients. Occlusion of the internal carotid artery was confirmed on the basis of dampening of the stump arterial waveform and stasis of the nonionic contrast material (300 mg of iodine per milliliter) (iohexol, Omnipaque 300; Amersham Health, Princeton, NJ) injected through the guide catheter. BTO was performed in all patients by the same author (J.P.D.), who has 15 years of experience in the performance of BTO procedures.

CT Technique and Data
Test occlusion was clinically successful in all patients. Immediately after BTO, they underwent perfusion CT with a multi–detector row system (LightSpeed; GE Medical Systems, Milwaukee, Wis). Patients were brought to the CT suite with the occlusion balloon in place but deflated. Every attempt was made to ensure that patients did not move their heads during transfer. A heparinized saline flush was maintained constantly through the catheter lumen. The exact inflation volume used during BTO was marked on the inflation syringe, and the same angiographer (J.P.D.) reinflated the balloon during perfusion CT.

After nonenhanced CT of the whole brain, four adjacent 5-mm-thick sections were selected at the level of the basal ganglia that covered all three vascular territories. The balloon was then inflated in the internal carotid artery, and arterial occlusion was confirmed on the basis of dampening of the stump arterial waveform. The contrast material (49 mL) was then injected at a rate of 4 mL/sec. At 5 seconds into the injection, cine (continuous) scanning was initiated with the following technique: 80 kVp, 190–200 mA, 4 x 5-mm-thick sections, and 1 second per rotation for 50 seconds. The 1-second images were reformatted at 0.5-second intervals, and the 5-mm-thick sections were reformatted into two 10-mm-thick sections. Perfusion CT was then repeated with the balloon deflated. Finally, perfusion CT was performed approximately 20 minutes after intravenous bolus injection of 1 g of acetazolamide (Bedford Laboratories, Bedford, Ohio) with the balloon inflated to assess vascular reserve. For all patients, perfusion maps of CBF, cerebral blood volume, and mean transit time were generated by the same author (E.G.H.) by using a workstation (Advantage Windows; GE Medical Systems) with perfusion CT software. We used the anterior cerebral artery as the arterial input and the superior sagittal sinus as the venous input to maintain constant technique in the generation of perfusion maps.

Symmetric regions of interest (2 cm in diameter) were used to obtain data. Six regions were placed on each reference CT scan in either hemisphere to include the maximum amount of cortex at comparable sites that avoided any major cortical vessels. With consensus, two authors (R.J., E.G.H.) evaluated the qualitative and quantitative parameters of CBF, cerebral blood volume, and mean transit time. Specifically, note was made of any symmetry or asymmetry between the two hemispheres and of quantitative results in each region of interest. Images obtained before and after injection of acetazolamide were compared to evaluate normal or abnormal responses to the acetazolamide challenge test. Follow-up clinical information was obtained for all patients.


   Results
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
Discussion
 REFERENCES
 
The Table lists information regarding the demographics, clinical information, and results for the eight patients in this study. For patients 2, 5, and 7, results at perfusion CT showed normal symmetric resting CBF and normal acetazolamide augmentation of CBF in both hemispheres. These findings are suggestive of good collateral vessels and good cerebrovascular reserve, possibly without risk for delayed ischemia (Fig 1). None of these three patients underwent any revascularization procedure prior to definitive treatment of their known neoplasm or aneurysm. Patient 2 underwent permanent carotid artery occlusion with endovascular coils and detachable balloons.


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  		Demographics, Clinical Information, and Results in Eight Patients

 


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  		Figure 1a. Patient 5. (a) Transverse CT image obtained at level of basal ganglia during contrast material administration. Branch of anterior cerebral artery (single arrow) was chosen as reference artery, and superior sagittal sinus (double arrow) was chosen as reference vein. Circles indicate identical regions of interest placed in both cerebral hemispheres in comparable regions that avoid the main cortical vessels. (b) CBF and (c) mean transit time perfusion maps obtained with balloon inflated in left internal carotid artery show normal symmetric blood flow and normal mean transit times, respectively, in both hemispheres. (d) CBF perfusion map obtained with balloon deflated and (e) map obtained after injection of acetazolamide with balloon inflated show normal blood flow augmentation (increase, >5%).

 


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  		Figure 1b. Patient 5. (a) Transverse CT image obtained at level of basal ganglia during contrast material administration. Branch of anterior cerebral artery (single arrow) was chosen as reference artery, and superior sagittal sinus (double arrow) was chosen as reference vein. Circles indicate identical regions of interest placed in both cerebral hemispheres in comparable regions that avoid the main cortical vessels. (b) CBF and (c) mean transit time perfusion maps obtained with balloon inflated in left internal carotid artery show normal symmetric blood flow and normal mean transit times, respectively, in both hemispheres. (d) CBF perfusion map obtained with balloon deflated and (e) map obtained after injection of acetazolamide with balloon inflated show normal blood flow augmentation (increase, >5%).

 


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  		Figure 1c. Patient 5. (a) Transverse CT image obtained at level of basal ganglia during contrast material administration. Branch of anterior cerebral artery (single arrow) was chosen as reference artery, and superior sagittal sinus (double arrow) was chosen as reference vein. Circles indicate identical regions of interest placed in both cerebral hemispheres in comparable regions that avoid the main cortical vessels. (b) CBF and (c) mean transit time perfusion maps obtained with balloon inflated in left internal carotid artery show normal symmetric blood flow and normal mean transit times, respectively, in both hemispheres. (d) CBF perfusion map obtained with balloon deflated and (e) map obtained after injection of acetazolamide with balloon inflated show normal blood flow augmentation (increase, >5%).

 


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  		Figure 1d. Patient 5. (a) Transverse CT image obtained at level of basal ganglia during contrast material administration. Branch of anterior cerebral artery (single arrow) was chosen as reference artery, and superior sagittal sinus (double arrow) was chosen as reference vein. Circles indicate identical regions of interest placed in both cerebral hemispheres in comparable regions that avoid the main cortical vessels. (b) CBF and (c) mean transit time perfusion maps obtained with balloon inflated in left internal carotid artery show normal symmetric blood flow and normal mean transit times, respectively, in both hemispheres. (d) CBF perfusion map obtained with balloon deflated and (e) map obtained after injection of acetazolamide with balloon inflated show normal blood flow augmentation (increase, >5%).

 


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  		Figure 1e. Patient 5. (a) Transverse CT image obtained at level of basal ganglia during contrast material administration. Branch of anterior cerebral artery (single arrow) was chosen as reference artery, and superior sagittal sinus (double arrow) was chosen as reference vein. Circles indicate identical regions of interest placed in both cerebral hemispheres in comparable regions that avoid the main cortical vessels. (b) CBF and (c) mean transit time perfusion maps obtained with balloon inflated in left internal carotid artery show normal symmetric blood flow and normal mean transit times, respectively, in both hemispheres. (d) CBF perfusion map obtained with balloon deflated and (e) map obtained after injection of acetazolamide with balloon inflated show normal blood flow augmentation (increase, >5%).

 
Patients 1, 3, 4, 6, and 8 had asymmetric blood flow in the cerebral hemispheres. Patient 4 had globally reduced CBF that was worse on the side of test occlusion (Fig 2). CBF was less than 20 mL/100 g/min, and mean transit time was more than 8 seconds in some of the areas in both cerebral hemispheres. CBF and mean transit time worsened further after balloon inflation during the acetazolamide challenge test; the findings were again more pronounced on the side of test occlusion.



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  		Figure 2a. Patient 4. (a) CBF perfusion map obtained with balloon deflated before acetazolamide challenge test shows low CBF in both cerebral hemispheres. (b) CBF and (c) mean transit time perfusion maps obtained with balloon inflated in right internal carotid artery show globally low blood flow (<20 mL/100 g/min) in many regions of interest and increased mean transit time in both hemispheres (worse on ipsilateral side). (d) After injection of acetazolamide, perfusion CT map shows poor augmentation of CBF. There was further worsening of mean transit time (not shown).

 


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  		Figure 2b. Patient 4. (a) CBF perfusion map obtained with balloon deflated before acetazolamide challenge test shows low CBF in both cerebral hemispheres. (b) CBF and (c) mean transit time perfusion maps obtained with balloon inflated in right internal carotid artery show globally low blood flow (<20 mL/100 g/min) in many regions of interest and increased mean transit time in both hemispheres (worse on ipsilateral side). (d) After injection of acetazolamide, perfusion CT map shows poor augmentation of CBF. There was further worsening of mean transit time (not shown).

 


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  		Figure 2c. Patient 4. (a) CBF perfusion map obtained with balloon deflated before acetazolamide challenge test shows low CBF in both cerebral hemispheres. (b) CBF and (c) mean transit time perfusion maps obtained with balloon inflated in right internal carotid artery show globally low blood flow (<20 mL/100 g/min) in many regions of interest and increased mean transit time in both hemispheres (worse on ipsilateral side). (d) After injection of acetazolamide, perfusion CT map shows poor augmentation of CBF. There was further worsening of mean transit time (not shown).

 


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  		Figure 2d. Patient 4. (a) CBF perfusion map obtained with balloon deflated before acetazolamide challenge test shows low CBF in both cerebral hemispheres. (b) CBF and (c) mean transit time perfusion maps obtained with balloon inflated in right internal carotid artery show globally low blood flow (<20 mL/100 g/min) in many regions of interest and increased mean transit time in both hemispheres (worse on ipsilateral side). (d) After injection of acetazolamide, perfusion CT map shows poor augmentation of CBF. There was further worsening of mean transit time (not shown).

 
Among the five patients with asymmetric blood flow, patients 1 and 3 had normal CBF and normal acetazolamide augmentation in the opposite hemisphere. There was steal phenomenon on the side of the test occlusion that mainly involved the watershed territories (Fig 3). However, CBF was always more than 20 mL/100 g/min and mean transit time was less than 8 seconds, even in the watershed zones. Patients 6 and 8 had low resting CBF and abnormal acetazolamide response on the side of test occlusion, as well as steal phenomenon in the contralateral hemisphere. CBF in the opposite hemisphere, however, always remained above 30 mL/100 g/min with no appreciable increase in mean transit time.



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  		Figure 3a. Patient 3. (a) CBF perfusion map obtained with balloon inflated in right internal carotid artery shows low flow in right middle cerebral artery (arrows) territory. (b) Map obtained with balloon deflated shows more symmetric flow. (c) Map obtained after injection of acetazolamide with balloon inflated shows normal blood flow augmentation in left cerebral hemisphere and reduction in blood flow in right anterior cerebral artery and middle cerebral artery watershed territory (arrows). These findings are suggestive of steal phenomenon. Another map obtained after injection of acetazolamide (not shown) at more superior section level also showed steal phenomenon in right middle cerebral artery and posterior cerebral artery watershed territory.

 


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  		Figure 3b. Patient 3. (a) CBF perfusion map obtained with balloon inflated in right internal carotid artery shows low flow in right middle cerebral artery (arrows) territory. (b) Map obtained with balloon deflated shows more symmetric flow. (c) Map obtained after injection of acetazolamide with balloon inflated shows normal blood flow augmentation in left cerebral hemisphere and reduction in blood flow in right anterior cerebral artery and middle cerebral artery watershed territory (arrows). These findings are suggestive of steal phenomenon. Another map obtained after injection of acetazolamide (not shown) at more superior section level also showed steal phenomenon in right middle cerebral artery and posterior cerebral artery watershed territory.

 


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  		Figure 3c. Patient 3. (a) CBF perfusion map obtained with balloon inflated in right internal carotid artery shows low flow in right middle cerebral artery (arrows) territory. (b) Map obtained with balloon deflated shows more symmetric flow. (c) Map obtained after injection of acetazolamide with balloon inflated shows normal blood flow augmentation in left cerebral hemisphere and reduction in blood flow in right anterior cerebral artery and middle cerebral artery watershed territory (arrows). These findings are suggestive of steal phenomenon. Another map obtained after injection of acetazolamide (not shown) at more superior section level also showed steal phenomenon in right middle cerebral artery and posterior cerebral artery watershed territory.

 
Two of the eight patients underwent permanent occlusion of the carotid artery, with surgical clipping in patient 4 and endovascular occlusion in patient 2 with Guglielmi detachable coils (Boston Scientific/Target, Fremont, Calif) and detachable balloons. In patient 4, perfusion CT demonstrated the risk of delayed ischemia; therefore, superficial temporal artery to middle cerebral artery bypass was performed to prevent delayed ischemia. Two weeks after the bypass procedure, the patient underwent resection of the right cavernous sinus tumor and clipping (sacrifice) of the internal carotid artery. On the first postoperative day, he developed left upper extremity weakness that was blood pressure dependent. Cerebral angiography performed on the same day revealed poor blood flow through the superficial temporal artery to middle cerebral artery bypass graft, and the patient finally developed right middle cerebral artery territory infarct. In patient 2, perfusion CT demonstrated symmetric blood flow and normal acetazolamide augmentation. These findings are suggestive of good collateral vessels and good cerebrovascular reserve with no risk for delayed ischemia. Therefore, the patient underwent permanent carotid artery occlusion without any revascularization procedure. At 9 months after permanent carotid occlusion, he had not developed any delayed ischemia or neurologic deficit. None of the remaining six patients underwent permanent carotid artery occlusion or prolonged temporary occlusion of the carotid artery during surgery.

Patient 1 developed hemiparesis contralateral to the side of BTO immediately after perfusion CT. Angiography revealed embolic occlusion of the anterior cerebral artery and middle cerebral artery branches on the side of test occlusion. This occlusion was treated with local intraarterial thrombolysis with 27 mg of tissue plasminogen activator. The patient did not have residual neurologic deficit. No serious reaction to acetazolamide was observed in any of the patients.


   Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
 Discussion
REFERENCES
 
In the past, various methods have been tried to acquire and use absolute quantitative CBF measurements to predict stroke after permanent carotid artery occlusion. Perfusion MR imaging, xenon-enhanced CT, xenon 133 (133Xe)-enhanced SPECT, oxygen 15 H2O PET, technetium 99m hexamethyl-propyleneamine oxime (or, HMPAO) SPECT, and transcranial Doppler sonography have been used to evaluate cerebral perfusion during BTO (10–15). During BTO with the intracarotid xenon injection and washout method, an absolute CBF value of less than 30 mL/100 g/min was used to predict stroke in 50% of patients within 1 year (13). In a study performed in 1966 with 133Xe, a decrease of more than 25% from baseline values was thought to be associated with delayed stroke (14). Although this threshold reduced the risk of delayed stroke, it had a low positive predictive value, and treatment was denied to some of the patients who might have tolerated permanent occlusion (15). Those authors revised the recommendations on the basis of absolute CBF values measured with 133Xe. The revised recommendations included the following: with CBF of more than 40 mL/100 g/min, proceed to permanent carotid artery occlusion; with CBF of less than 20 mL/100 g/min, avoid permanent carotid occlusion; with CBF of 20–40 mL/100 g/min, proceed to permanent carotid occlusion if the relative decrease in CBF is less than 25% (15). However, definition of a threshold for tolerance with high positive predictive value and negative predictive value is still elusive.

Many of these additional methods are cumbersome and are not widely available. The use of stable xenon even in subnarcotic doses may cause some side effects, although they are mostly not dangerous (16). The need to use a tightly applied face mask also makes the experience a little unpleasant. The image acquisition time in SPECT is usually 10–20 minutes, which can lead to substantial motion artifact. Perfusion MR imaging allows qualitative CBF evaluation; however, hardware requirements can be a problem when BTO is performed in routine practice. In contrast, perfusion CT can be performed quickly with currently available CT technology (even with a third-generation scanner without spiral capability) and standard computer software (17). Single-section CBF maps can help predict all territorial infarcts larger than 10 mL (17). In addition, as opposed to MR imaging, there is a linear relationship between the attenuation changes at CT and the contrast agent concentration; this relationship allows more robust kinetic analysis (18). The amount of contrast agent used in perfusion CT, however, could be a concern in patients with deranged renal function. Second, the area covered at perfusion CT is limited compared with that covered at xenon CT because only 10-mm-thick sections can be examined. This limitation is mainly a result of the much quicker kinetics of the iodinated contrast medium. This deficiency can be partly overcome, however, by selecting imaging sections at or near the basal ganglia level that cover part of the three major vascular territories.

In addition to assessment with quantitative CBF, assessment with many other adjunctive methods—such as induction of hypotension with acetazolamide (19–21), monitoring of middle cerebral artery blood flow velocities with transcranial Doppler sonography (22–24), electroencephalography (25–27), angiography of the collateral vessels in the circle of Willis (28), visualization of venous phase hemispheric asymmetry (29), and continuous sustained attention testing (13)—have been shown to enhance the predictive value of BTO. We used acetazolamide in the present study to increase the predictive value of perfusion CT and BTO. Measurement of CBF in resting state alone is considered inadequate for evaluation of hemodynamic compromise. Acetazolamide penetrates the blood-brain barrier slowly by means of diffusion, inhibits carbonic anhydrase, and induces acidosis. Therefore, acetazolamide induces a considerable increase in CBF that is similar to that evoked with CO2 inhalation, with levels of about 70% of CBF in healthy control subjects (30). This increase is attributed to compensatory dilation of small arterioles as a result of decrease in tissue pH. The acetazolamide-reactive mechanism functions as an autoregulator at the lower end of the autoregulatory range and is useful for estimating the decrease in cerebral perfusion pressure and the presence of hemodynamic compromise (31). It is thought that the CBF increase induced with acetazolamide will be reduced if compensatory vasodilation associated with an increase in cerebral blood volume has already occurred (31).

In the present study, only three (38%) of the eight patients in whom BTO was clinically successful had symmetric resting blood flow and normal acetazolamide challenge response in both hemispheres. These findings are suggestive of good cerebrovascular reserve and collateral vessels. Only one (12%) patient underwent permanent carotid artery occlusion without the protection of any revascularization procedure, and he did not experience delayed stroke in the 9 months of follow-up. Five (62%) patients had asymmetric blood flow at perfusion CT despite having undergone clinically successful BTO. One (12%) patient had abnormally low CBF and abnormal response to the acetazolamide challenge test (CBF, <20 mL/100 g/min; mean transit time, >8 seconds). These findings are suggestive of poor collateral vessels, poor cerebrovascular reserve, and thus risk for delayed stroke. The patient with abnormally low CBF developed ischemic stroke in the ipsilateral hemisphere after his superficial temporal artery to middle cerebral artery bypass graft occluded. Thus, perfusion CT may be able to help predict the risk of delayed stroke in patients who undergo permanent carotid artery occlusion, and results may help identify those patients with asymmetric low blood flow and abnormal response to the acetazolamide challenge test.

Among the five patients with asymmetric blood flow depicted at perfusion CT, the steal phenomenon was depicted in the ipsilateral hemisphere in two (40%) and in the opposite hemisphere in two (40%). However, none of these patients had CBF of less than 20 mL/100 g/min and mean transit time of more than 8 seconds. Because none of them underwent permanent or prolonged temporary carotid artery occlusion during surgery, we do not know the exact importance of these findings. These patients could have been at higher risk for delayed ischemia, however, if permanent carotid occlusion had been performed without any revascularization procedure. The relatively high percentage of patients with asymmetric flow (five [62%] of eight patients) may be related to differences in methods among the various techniques used to measure blood flow. At perfusion CT, an intravascualar tracer is used to measure blood flow; findings may reflect a physiology different from that measured with other techniques such as xenon CT and PET, which involve use of a diffusible tracer (32).

Quantitative parameters measured at perfusion CT are dependent on the placement of the regions of interest and the choice of the reference artery; hence, there is also operator dependence. In the present study, all the perfusion CT maps were generated by one operator (E.G.H.) to minimize the interoperator variability. Selection of the reference artery is also very important in the generation of perfusion maps. We used the anterior cerebral artery as the reference artery in all patients to maintain homogeneity in the study. In addition, at or near the level of the basal ganglia, the anterior cerebral artery is visualized in cross section, which eliminates any errors due to partial volume effects. We used 2-cm-diameter regions of interest to generate perfusion maps because we have also used them in xenon-enhanced CT perfusion studies at our institution. The same diameters were used because perfusion CT values have good correlation with those at xenon-enhanced CT (33).

Definitive quantitative values generated at xenon-enhanced CT, however, may not be exactly applicable to perfusion CT (33). There are only a few studies in the literature in which perfusion CT was compared with other modalities (33), and the quantitative parameters at perfusion CT are still not standardized. The small number of patients in our study and the small fraction of them who underwent permanent carotid artery occlusion are limitations of our study. A prospective controlled trial with data from a larger number of patients would likely provide more firm conclusions. In the present study, patient 2 underwent uneventful permanent carotid occlusion without any revascularization procedure despite the xenon-enhanced CT finding of low CBF (<25 mL/100 g/min) in the bilateral middle cerebral artery and posterior cerebral artery watershed territories. Normal quantitative CBF values may also vary in different patients; hence, comparison of ipsilateral with acetazolamide-enhanced perfusion CT maps obtained in the contralateral hemisphere provides better evaluation of the risk of delayed stroke.

In conclusion, perfusion CT is an easily available imaging modality that can be performed relatively quickly with BTO as an adjunctive method to measure CBF. In addition, an acetazolamide challenge test can be added to evaluate the cerebrovascular reserve. Definitive quantitative CBF values at perfusion CT are still not standardized, which makes accurate prediction of the risk of delayed stroke difficult. It seems feasible, however, to predict that patients with symmetric blood flow and normal response to the acetazolamide challenge test will do well after permanent carotid artery occlusion, while patients with asymmetric blood flow and abnormal response to the acetazolamide challenge test will require a revascularization procedure for protection from delayed ischemic stroke.


   FOOTNOTES
 
Abbreviations: BTO = balloon test occlusion, CBF = cerebral blood flow

Author contributions: Guarantors of integrity of entire study, E.G.H., R.J.; study concepts, R.J., E.G.H., M.R.H., J.P.D.; study design, R.J., E.G.H., J.P.D., S.K.M.; literature research, R.J., E.G.H., M.R.H.; clinical studies, R.J., E.G.H., J.P.D.; data acquisition, R.J., E.G.H., J.P.D.; data analysis/interpretation, all authors; statistical analysis, R.J., E.G.H.; manuscript preparation, revision/review, R.J., E.G.H., S.K.M.; manuscript definition of intellectual content, R.J., E.G.H., J.P.D., S.K.M.; manuscript editing, R.J., E.G.H.; manuscript final version approval, R.J., E.G.H., J.P.D., S.K.M., B.G.T.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results
 Discussion
 REFERENCES
 

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