Response to Letter by Sohn et al
We appreciate the comments by Sohn et al concerning our study comparing preperfusion and postperfusion magnetic resonance angiography (MRA) in acute stroke.1 After studying 10 patients with postperfusion MRA and digital subtraction angiography, Sohn et al suggested that most cases of better middle cerebral artery flow after contrast are attributable to collateral flow. We would like to make some observations regarding their findings.
The results we reported suggest a rather more complex scenario. All the patients in Sohn’s study had TIMI 0-I flow before contrast administration and TIMI II after contrast (10/10), as compared with only 29% (9/31) of the patients in ours. Most of our patients (16/31) did not show flow improvement, maintaining a TIMI score of 0 to 1. Furthermore, a small group (5/31) showed a large improvement in flow, from TIMI 0-I to TIMI III. This fact suggests the existence of real anterograde flow and makes retrograde-collateral flow unlikely.
As the authors state, the time period between the 2 examinations (MRI and digital subtraction angiography) was an average of 76 minutes, and it is possible that progression from stenosis with distal flow to complete occlusion with collateral flow could occur during this time.
Sohn et al do not mention whether patients were examined with transcranial Doppler immediately after MRI. Transcranial Doppler2 or even a fast phase contrast sequence3 can be used to demonstrate middle cerebral artery flow reversal.
Sohn’s observation that postcontrast-enhanced MRA flow voids represent complete occlusion with distal retrograde collateral filling does not concur with the reported results in severe stenosis of the internal carotid artery,4 where contrast-enhanced MRA presented a tendency to overestimate carotid stenoses and digital subtraction angiography showed better flow.
We agree with the authors that collateral flow must be an explanation for the distal flow after proximal middle cerebral artery stenosis in some cases. Further studies with contrast-enhanced MRA, phase contrast MRA and transcranial Doppler are needed to better understand the hemodynamics of cerebral arterial flow in acute stroke.
The most important conclusion, which we all agree on, is that postperfusion MRA is highly useful for accurate assessment of middle cerebral artery flow in acute stroke.
Sources of Funding
This work was supported by a grant from the Fondo de Investigaciones Sanitarias (PI021083) and by the Fundación Doctor Josep Trueta.
Pedraza S, Silva Y, Mendez J, Inaraja L, Vera J, Serena J, Davalos A. Comparison of preperfusion and postperfusion magnetic resonance angiography in acute stroke. Stroke. 2004; 35: 2105–2110.
Burgin WS, Malkoff M, Felberg RA, Demchuk AM, Christou I, Grotta JC, Alexandrov AV. Transcranial Doppler ultrasound criteria for recanalization after thrombolysis for middle cerebral artery stroke. Stroke. 2000; 31: 1128–1132.
Borisch I, Horn M, Butz B, Zorger N, Draganski B, Hoelscher T, Bogdahn U, Link JA. Preoperative evaluation of carotid artery stenosis: comparison of contrast-enhanced MR angiography and duplex sonography with digital subtraction angiography. AJNR Am J Neuroradiol. 2003; 24: 1117–1122.