Published ahead of print on August 11, 2011
doi: 10.3174/ajnr.A2685

American Journal of Neuroradiology 32:E163-E164, September 2011
© 2011 American Society of Neuroradiology

M. Groth^a, J. Fiehler^a and N.D. Forkert^b
aDepartment of Diagnostic and Interventional Neuroradiology
^bDepartment of Medical Informatics 
University Medical Center Hamburg-Eppendorf 
Hamburg, Germany

After endovascular coil embolization of an intracranial aneurysm, coil compaction, aneurysm growth, or migration of the coil into intraluminal thrombus might lead to recurrence. In this case, interventional neuroradiologists have to decide whether to treat the recurrent aneurysm. Although there are several factors influencing the individual treatment decision, such as aneurysm shape, initial aneurysm size, dynamic aneurysm growth, patient preference, individual treatment risk, patient age, and so forth, previousobservations suggest that mainly the recurrence volume determines the treatment decision.¹

We have read the article of Daugherty et al entitled “Observer Agreement Regarding the Necessity of Retreatment of Previously Coiled Recurrent Cerebral Aneurysms”² with special interest. The authors evaluated observer agreement with respect to the question of whether to treat recurrent aneurysms. The experienced interventionalists (some were even from the same “school”) revealed substantial variability in their opinions about the indication for treatment of recurrent aneurysms.

First, this observation reflects the notorious difficulties of clinical therapy decision-making in these conditions. A certain morphology or size suggesting definite need for retreatment is unclear because hemorrhage from residual or recurrent aneurysms is a rare event. Second, the authors put the use of retreatment as an objective end point in clinical trails into question on the basis of the considerable disagreement. Therefore, it would be desirable to define reliable parameters that help to decide whether retreatment is indicated and to consequently assess objective end points in clinical trails (eg, for the evaluation of new coils). We believe that the general use of 2D digital subtraction angiography images for the evaluation of the genuine 3D structure “recurrent aneurysm appearance” is one major source of heterogeneity. The personal calibration of visual preconception by a specific “school” could be another explanation.

In our opinion, evaluation by visual inspection as used in core labs could be replaced by quantitative volumetric parameters.Recently, such a method for the extraction of a quantitative parameter characterizing recurrent aneurysms was proposed and validated.³ Briefly, the aneurysm-recurrence volume is estimated by rigid coregistration of two 3D time-of-flight MR angiography (TOF-MRA) datasets acquired at 2 discrete time points after therapy. For this estimation, the vascular structures are automatically segmented⁴ in both datasets and used for a masked intensity-based registration. After registration, the aligned vessel segmentations are subtracted such that the aneurysm-recurrence volume is left over, which can be used for quantification purposes (Fig 1).

TOF-MRA is a standard follow-up method after endovascular aneurysm therapy in many centers worldwide. The key difficulties in the application of 3D MRA image sequences for aneurysm-recurrence volume quantification have been the statistical basis of possible outcome variables as well as the development of an algorithm that allows the precise coregistration of routine MRA scans in 3D space. Such a method needs to be both robust and precise, even if the image data are from different MR imaging scanners.

When applied correctly, such volume measurements can be used as reproducible end points in clinical trails. However, to this point, these measurements are not suitable for clinical decision-making.

A volumetric surrogate end point could substantially decrease sample size by limiting the variability in the assessment asdescribed by Daugherty et al.² Such analysis might allow a more timely estimation of possible benefits of new materials at a fraction of the cost of a study with hundreds of patients that is normally required, at least in part, to reduce the variabilityintroduced by visual assessment.

References

1. Ries T, Wulff A, Wegscheider K, et al. Unexpected kink in aneurysm recurrence volume distribution is associated with retreatment decision. Neuroradiology 2010;52:657–58. Epub 2010 Apr 22[CrossRef][Medline]
2. Daugherty WP, Rad AE, White JB, et al. Observer agreement regarding the necessity of retreatment of previously coiled recurrent cerebral aneurysms. AJNR Am J Neuroradiol 2011;32:566–69[Abstract/Free Full Text]
3. Ries T, Wegscheider K, Wulff A, et al. Quantification of recurrence volumes after endovascular treatment of cerebral aneurysm as surrogate endpoint for treatment stability. Neuroradiology 2011;53:593–98[CrossRef][Medline]
4. Forkert ND, Schmidt-Richberg A, Fiehler J, et al. Fuzzy-based vascular structure enhancement in time-of-flight MRA images for improved segmentation. Methods Inf Med 2011;50:74–83[Medline]

Reply

Published ahead of print on August 11, 2011
doi: 10.3174/ajnr.A2688

American Journal of Neuroradiology 32:E165, September 2011
© 2011 American Society of Neuroradiology

D.F. Kallmes^a, H. Cloft^a, W.P. Daugherty^b and P.M. Meyers^c
aDepartment of Radiology
^bDepartment of Neurosurgery 
Mayo Clinic 
Rochester, Minnesota
^cDepartments of Radiology and Neurosurgery 
Columbia University 
New York, New York

We greatly appreciate the suggestions by Dr Groth and colleagues about the potential use of volumetric analysis in the evaluation of recurrent aneurysms. Unfortunately, this sort of volumetric analysis would very likely add little in terms of providing objective guidance about re-treating a given aneurysm, as the authors of the letter acknowledge. The use of volumetric analysis might have some value in the context of a clinical trial but very likely would offer scant benefit to clinical practice. We do, however, applaud any attempt to provide improved analytic tools to the aneurysm-research community and wish Dr Groth and colleagues success in their chosen line of investigation.