Published online before print November 14, 2002, doi: 10.1161/01.STR.0000041999.64363.B2
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
(Stroke. 2002;33:2736.)
© 2002 American Heart Association, Inc.
Letters to the Editor |
CT or MRI for Imaging Patients with Acute Stroke: Visualization of "Tissue at Risk"?
Department of Neuroradiology
Department of Neurology
Department of Neuroradiology, Massachusetts General Hospital, Boston, Massachusetts
To the Editor:
In a recent Stroke editorial weighing the relative merits of CT versus MRI for imaging patients with acute stroke, Tatlisumak raises a number of important and compelling points.1 Among these, however, is the statement—with regard to using MR perfusion-diffusion mismatch in order to identify treatable "tissue at risk"—that a "similar approach does not yet exist for CT imaging." We wish to call attention to preliminary results suggesting that such an approach may, indeed, currently exist using CT perfusion imaging.2,3 Specifically, pilot studies in patients with middle cerebral artery stroke have revealed that, like diffusion-weighted imaging, CT cerebral blood volume maps may delineate ischemic regions with a high probability of being already infarcted by the time of image acquisition.4–6 Quantitation of cerebral blood flow (CBF) and mean transit time with the bolus tracking technique should be more accurate using CT, rather than MR methodology, because signal change is linearly proportional to dye concentration with CT, but not with MR, and because CT images typically have higher spatial resolution. The poor specificity of MR perfusion maps has taught us that quantitative CBF thresholds are necessary to identify tissue that is still salvageable but destined for infarction without early reperfusion.2 The restriction of CT CBF and mean transit time maps to a predetermined slab of tissue (typically 2 cm of coverage per contrast bolus) remains CT’s major limitation. Diffusion-weighted imaging also remains necessary to identify early brain stem and small infarcts.6 We would argue that, given the current published capability of a quick contrast CT study (which includes CT angiography, whole brain perfused blood volume, and single slab quantitative CT perfusion), replacing CT with MR as the workhorse in emergency stroke evaluation is not necessary, leads to unnecessary delays in treatment, and is likely not cost effective.7
References
1. Tatlisumak T. Is CT or MRI the method of choice for imaging patients with acute stroke? Why should men divide if fate has united? Stroke. 2002; 33: 2144–2145.
2. Wintermark M, Reichhart M, Thiran JP, Maeder P, Chalaron M, Schnyder P, Bogousslavsky J, Meuli R. Prognostic accuracy of cerebral blood flow measurement by perfusion computed tomography, at the time of emergency room admission, in acute stroke patients. Ann Neurol. 2002; 51: 417–432.[CrossRef][Medline] [Order article via Infotrieve]
3. Koroshetz WJ, Lev MH. Contrast computed tomography scan in acute stroke: "you can’t always get what you want but ... you get what you need." Ann Neurol. 2002; 51: 415–416. Editorial.[CrossRef][Medline] [Order article via Infotrieve]
4. Lev MH, Segal AZ, Farkas J, Hossain ST, Putman C, Hunter GJ, Budzik R, Harris GJ, Buonanno FS, Ezzeddine MA, Chang Y, Koroshetz WJ, Gonzalez RG, Schwamm LH. Utility of perfusion-weighted CT imaging in acute middle cerebral artery stroke treated with intra-arterial thrombolysis: prediction of final infarct volume and clinical outcome. Stroke. 2001; 32: 2021–2028.
5. Wintermark M, Reichhart M, Cuisenaire O, Maeder P, Thiran JP, Schnyder P, Bogousslavsky J, Meuli R. Comparison of admission perfusion computed tomography and qualitative diffusion- and perfusion-weighted magnetic resonance imaging in acute stroke patients. Stroke. 2002; 33: 2025–2031.
6. Berzin TM, Lev MH, Goodman D, Koroshetz WJ, Hunter GJ, Hamberg L, Buonanno F, Putman C, Budzik R, Eskey C, Schwamm L, Gonzalez RG. CT perfusion imaging versus MR diffusion weighted imaging: prediction of final infarct size in hyperacute stroke. Stroke. 2001; 32: 317. Abstract.
7. Gleason S, Furie KL, Lev MH, O’Donnell J, McMahon PM, Beinfeld MT, Halpern E, Mullins M, Harris G, Koroshetz WJ, Gazelle GS. Potential influence of acute CT on inpatient costs in patients with ischemic stroke. Acad Radiol. 2001; 8: 955–964.[CrossRef][Medline] [Order article via Infotrieve]
Department of Neurology, Helsinki University Central Hospital, Helsinki, Finland
Response
I thank Dr Lev et al for their interest and constructive criticism1 on my editorial.2 Unfortunately, at the time I processed the editorial, the most recent articles cited by Dr Lev and his colleagues3–5 were yet not available to me, and the rest I considered preliminary.6–8 Indeed, I have anticipated this development on the basis of preliminary publications, personal communications, and some very limited personal experience, and suggested the development of a "stroke CT package" including conventional CT images, perfusion CT, and CT angiography.2 This is a good (and a very delightful) example of how rapidly stroke research is progressing before our eyes, and our statements are becoming obsolete before they are even published. It took only few years for diffusion-weighted and perfusion-weighted MRI to become generalized after the early milestone reports,9,10 and this may be the case for CT perfusion studies.11 A "stroke CT package" will increase the dose of radiation to which the patient is exposed by 3-fold or more. However, the disadvantages of radiation exposure are likely to be overstated unless the same patient is imaged repeatedly in a short period of time.
CT perfusion imaging may deliver more accurate information than MR perfusion imaging, but it is limited to a lesser number of slices and requires extensive postprocessing. Currently, whole brain perfusion CT imaging requires abundant doses of radiation and contrast agent, and if the lesion cannot be visualized by conventional CT imaging, placement of the perfusion CT slice to the correct region of interest relies on clinical judgment only, and furthermore, perfusion CT cannot yet be used for volumetric measurements, but decision-making must be based on a single or few slices. On the other hand, neither hindrance is unlikely to be solved in the close future, as Mayer et al6 have already reported a fast image postprocessing software that prevents loss of the time for image analysis that is gained by faster imaging with CT. However, the statement by Lev and his colleagues that "the poor specificity of MR perfusion maps has taught us that quantitative CBF thresholds are necessary to identify tissue that is still salvageable but destined for infarction without early reperfusion" is very much an issue of debate. First, perfusion MRI allows for calculation of relative maps of MTT, CBV, and CBF,10 and attempts have been made to establish a methodology for absolute measurements of the same parameters.12 Second, thresholds for irreversibility have already been calculated from perfusion MRI images13 and can predict the final infarct size better than mere visual evaluation of perfusion maps. Third, flow heterogeneity measurements may deliver a more precise estimate of final infarct size,14 but constructing maps is timely, unless quick methods are available to all. Fourth, perfusion MRI can be combined with diffusion-weighted MRI that has the advantage of depicting the ischemic lesion, a clear advantage over CT imaging. DWI is used as the gold standard for determination of final infarct size in 1 study3 and was found to be 100% sensitive for detection of parenchymal stroke detection, whereas the same value was 83% with perfusion CT in another study,8 both cited by Lev et al in their letter.1 Many studies reporting the use of novel MRI and CT techniques, however, suffer from small numbers of examined subjects. Incorporating these methods into routine emergency evaluation and/or multicenter trials in the near future will deliver firmer data and allow for firmer conclusions.
A CT scanner within the premises of the emergency room is a very good investment, especially if it is an ultrafast helical scanner capable of performing perfusion CT and CT angiography. Still, it may be an even better investment to have an MRI scanner next to it, but it is seldom feasible.2 Regarding the cost-effectiveness of MRI in hyperacute stroke, it is likely to be too early to make conclusions given that only a small fraction of all costs in management of a stroke patient is imaging cost and precise evaluation of cost-effectiveness requires carefully designed prospective studies. Besides, the cost of scanners has been decreasing for years and this trend will likely continue.
In conclusion, I believe that both CT and MRI have some advantages and some disadvantages, and considering the present evidence, an "ideal" imaging method for acute stroke patients does not yet exist. As many will agree, it is of benefit for all to continue developmental research both for CT and MRI.
References
1. Lev MH, Koroshetz WJ, Schwamm LH, Gonzales RG. CT or MRI for imaging patients with acute stroke: Visualization of "tissue at risk"? Stroke. 2002; 33: this issue. Letter.
2. Tatlisumak T. Is CT or MRI the method of choice for imaging patients with acute stroke? Why should men divide if fate has united? Stroke. 2002; 33: 2144–2145. Editorial.
3. Wintermark M, Reichhart M, Thiran J-P, Maeder P, Chalaron M, Schnyder P, Bogousslavsky J, Meuli R. Prognostic accuracy of cerebral blood flow measurement by perfusion computed tomography, at the time of emergency room admission, in acute stroke patients. Ann Neurol. 2002; 51: 417–432.[CrossRef][Medline] [Order article via Infotrieve]
4. Koroshetz WJ, Lev MH. Contrast computed tomography scan in acute stroke: "you can’t always get what you want but ... you get what you need." Ann Neurol. 2002; 51: 415–416. Editorial.[CrossRef][Medline] [Order article via Infotrieve]
5. Wintermark M, Reichhart M, Cuisenaire O, Maeder P, Thiran JP, Schnyder P. Bogousslavsky J, Meuli R. Comparison of admission perfusion computed tomography and qualitative diffusion- and perfusion-weighted magnetic resonance imaging in acute stroke patients. Stroke. 2002; 33: 2025–2031.
6. Mayer TE, Hamann GF, Baranczyk J, Rosengarten, B, Klotz, E, Wiesmann M, Missler U, Schulte-Altedorneburg G, Brueckmann HJ. Dynamic CT perfusion imaging of acute stroke. AJNR Am J Neuroradiol. 2000; 21: 1441–1449.
7. Lev MH, Segal AZ, Farkas J, Hossain ST, Putman C, Hunter GJ, Budzik R, Harris GJ, Buonanno FS, Ezzeddine MA, Chang Y, Koroshetz WJ, Gonzales RG, Schwamm LH. Utility of perfusion-weighted CT imaging in acute middle cerebral artery stroke treated with intra-arterial thrombolysis: prediction of final infarct volume and clinical outcome. Stroke. 2001; 32: 2021–2028.
8. Berzin TM, Lev MH, Goodman D, Koroshetz WJ, Hunter GJ, Hamberg L, Buonanno F, Putman C, Budzik R, Eskey C, Schwamm L, Gonzales RG. CT perfusion imaging versus MR diffusion weighted imaging: prediction of final infarct size in hyperacute stroke. Stroke. 2001; 32: 317. Abstract.
9. Warach S, Gaa J, Siewert B, Wielopolski P, Edelman RR. Acute human stroke studied by whole brain echo planar diffusion-weighted magnetic resonance imaging. Ann Neurol. 1995; 37: 231–241.[CrossRef][Medline] [Order article via Infotrieve]
10. Hamberg LM, Macfarlane R, Tasdemiroglu E, Boccalini P, Hunter GJ, Belliveau JW, Moskowitz MA, Rosen BR. Measurement of cerebrovascular changes in cats after transient ischemia using dynamic magnetic resonance imaging. Stroke. 1993; 24: 444–450.
11. Hamberg LM, Hunter GJ, Halpern EF, Hoop B, Gazelle GS, Wolf GL. Quantitative high-resolution measurement of cerebrovascular physiology with slip-ring CT. AJNR Am J Neuroradiol. 1996; 17: 639–650.[Abstract]
12. Lia TQ, Guang Chen Z, Ostergaard L, Hindmarsh T, Moseley ME. Quantification of cerebral blood flow by bolus tracking and artery spin tagging methods. Magn Reson Imaging. 2000; 18: 503–512.[CrossRef][Medline] [Order article via Infotrieve]
13. Rohl L, Ostergaard L, Simonsen CZ, Vestergaard-Poulsen P, Andersen G, Sakoh M, Le Bihan D, Gyldensted C. Viability thresholds of ischemic penumbra of hyperacute stroke defined by perfusion-weighted MRI and apparent diffusion coefficient. Stroke. 2001; 32: 1140–1146.
14. Simonsen CZ, Rohl L, Vestergaard-Poulsen P, Gyldensted C, Andersen G, Ostergaard L. Final infarct size after acute stroke: prediction with flow heterogeneity. Radiology. 2002; 225: 269–275.
This article has been cited by other articles:
 |
|
 |
|
  E.S. Rosenthal, L.H. Schwamm, L. Roccatagliata, S.B. Coutts, A.M. Demchuk, P.W. Schaefer, R.G. Gonzalez, M.D. Hill, E.F. Halpern, and M.H. Lev Role of Recanalization in Acute Stroke Outcome: Rationale for a CT Angiogram-Based "Benefit of Recanalization" Model AJNR Am. J. Neuroradiol., September 1, 2008; 29(8): 1471 - 1475. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 L. R. Caplan Stroke Thrombolysis: Slow Progress Circulation, July 18, 2006; 114(3): 187 - 190. [Full Text] [PDF]
|
|
|
|
|
|
M. H. Lev CT versus MR for Acute Stroke Imaging: Is the "Obvious" Choice Necessarily the Correct One? AJNR Am. J. Neuroradiol., November 1, 2003; 24(10): 1930 - 1931. [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||