Clot Characterization by Noncontrast CT to Predict IV tPA Failure

Letters to the Editor Letters to the Editor

Published online before print February 9, 2012, doi: 10.3174/ajnr.A3041
AJNR 2012 33: E63

C.H. Riedela
aDepartment of Neuroradiology
University of Kiel
Kiel, Germany

A.J. Yoob
bDivision of Interventional and Diagnostic Neuroradiology
Massachusetts General Hospital Harvard Medical School
Boston, Massachusetts

We have read the recent article entitled “Quantification of Thrombus Hounsfield Units on Noncontrast CT Predicts Stroke Subtype and Early Recanalization after Intravenous Recombinant Tissue Plasminogen Activator” by Puig et al with great interest.1 We congratulate the authors on their analysis, in which they found that relative Hounsfield unit attenuation of MCA clot was the best predictor of IV tPA failure, with an optimal threshold <1.38. Furthermore, they found that thrombus length was not associated with IV tPA response (see On-line Table 1 in Puig et al1). This last finding is at odds with a recent study supporting a strong effect of clot length on the likelihood of IV tPA recanalization.2 In that study of 138 patients, clots longer than 8 mm (as determined by thin-section NCCT) had a <1% chance of opening.

We offer a likely explanation for this disagreement. Puig et al1 measured clot length on axial thick-slab maximum-intensity-projection data from the CTA datasets. We believe that this method is prone to considerable measurement error because it relies on contrast to opacify the distal end of the clot. The degree to which contrast approaches the distal clot face, in turn, is dependent on the strength of the pial collateral circulation, which is often compromised and, in many cases, does not reach the occluded vessel segment. Indeed, in an angiographic study by Christoforidis et al,3 only 27 of 53 (51%) patients demonstrated sufficient collaterals to opacify the distal clot face. Therefore, it is highly likely that Puig et al overestimated clot length in many instances. To address this problem, we suggest that the authors determine clot length by using their NCCT data with a section thickness no greater than 2.5 mm (which they can achieve given their 0.75-mm collimation). On the basis of recent data,4 partial volume effects introduce significant error in clot-length determination at greater section widths. For similar reasons, it may be better to measure Hounsfield attenuation by using section thicknesses smaller than the 3-mm sections used in their study. It would be interesting to see whether this re-analysis will confirm the importance of clot length on IV tPA responsiveness and whether clot attenuation provides additional prognostic information.

References

  1. Puig J, Pedraza S, Demchuk A, et al. Quantification of thrombus Hounsfield units on noncontrast CT predicts stroke subtype and early recanalization after intravenous recombinant tissue plasminogen activator. AJNR Am J Neuroradiol 2011;33: 90–96. Epub 2011 Dec 8 » Medline
  2. Riedel CH, Zimmermann P, Jensen-Kondering U, et al. The importance of size: successful recanalization by intravenous thrombolysis in acute anterior stroke depends on thrombus length. Stroke 2011;42: 1775–77 » Abstract/FREE Full Text
  3. Christoforidis GA, Mohammad Y, Kehagias D, et al. Angiographic assessment of pial collaterals as a prognostic indicator following intra-arterial thrombolysis for acute ischemic stroke. AJNR Am J Neuroradiol 2005;26:1789–97 » Abstract/FREE Full Text
  4. Riedel CH, Jensen U, Rohr A, et al. Assessment of thrombus in acute middle cerebral artery occlusion using thin-slice nonenhanced computed tomography reconstructions. Stroke 2010;41: 1659–64 » Abstract/FREE Full Text

Reply

Published online before print February 9, 2012, doi: 10.3174/ajnr.A3056
AJNR 2012 33: E64

J. Puiga, S. Pedrazaa and G. Blascoa
aDepartment of Radiology-IDI
Girona Biomedical Research Institute
Hospital Universitari de Girona Dr Josep Trueta
Girona, Spain

A. Demchukb
bDepartment of Clinical Neurosciences
Hotchkiss Brain Institute
University of Calgary
Calgary, Alberta, Canada

We thank Drs Riedel and Yoo for their interest in and pertinent comments about our article “Quantification of Thrombus Hounsfield Units on Noncontrast CT Predicts Stroke Subtype and Early Recanalization after Intravenous Recombinant Tissue Plasminogen Activator.”1

Early recanalization is the phenomenon that has the greatest impact on clinical outcome in stroke.2 Little is known about the factors that determine the success of intravenous thrombolysis. Recently, Riedel et al3 demonstrated that intravenous thrombolysis has nearly no potential to recanalize occluded vessels when thrombus length exceeds 8 mm. In our analysis, we found no differences in length between unrecanalized and recanalized thrombi (median 13.7 mm [interquartile range, 8.4–21.6] and 11.7 mm [interquartile range, 6.8–13.5], respectively), probably because our sample (45 patients) was small for a stroke study.1 We agree that the compromise of pial collateral circulation could result in poor enhancement of the distal end of the clot and thereby introduce a measurement error into the thick-slab maximum-intensity-projection data from the CTA datasets. However, we showed the thrombus on 3 or 4 axial NCCT sections in nearly all cases, and we did not have the impression that clot length had been overestimated when we assessed the CTA in correlation with the NCCT before IV rtPA. In a second phase, we will tackle this issue.

On the other hand, we recommend against measuring any quantitative variable of thrombus directly on NCCT. In our study, thrombi were isoattenuated or hypoattenuated in 12 cases (26.67%), and this circumstance could lead to underestimation of the real length of the clot on NCCT. Moreover, slow blood flow immediately proximal to the thrombus can be mistaken for the classic hyperattenuated MCA sign, resulting in an overestimation of thrombus length (see our On-line Figure).1 Finally, as we stated in the limitations of the study, we also agree that thinner reconstructions of NCCT data with a section width smaller than the 3 mm, which we used in our study, might improve the accuracy of Hounsfield unit quantification in the cerebral artery.

Using noninvasive imaging tools to determine the characteristics of thrombi to predict the success of early recanalization in patients with acute stroke will very likely remain the subject of further research. A model incorporating CT information about both the composition and length of the thrombus will probably be more accurate in predicting rtPA failure and more useful for determining the best recanalization strategies.

References

  1. Puig J, Pedraza S, Demchuk A, et al. Quantification of thrombus Hounsfield units on noncontrast CT predicts stroke subtype and early recanalization after intravenous recombinant tissue plasminogen activator. AJNR Am J Neuroradiol 2012;33: 90–96 » Abstract/FREE Full Text
  2. Molina CA, Alexandrov AV, Demchuk AM, et al. Improving the predictive accuracy of recanalization on stroke outcome in patients treated with tissue plasminogen activator. Stroke 2004;35: 151–56 » Abstract/FREE Full Text
  3. Riedel CH, Zimmermann P, Jensen-Kondering U, et al. The importance of size: successful recanalization by intravenous thrombolysis in acute anterior stroke depends on thrombus length. Stroke 2011;42: 1775–77 » Abstract/FREE Full Text
Clot Characterization by Noncontrast CT to Predict IV tPA Failure