This book reviews the merits and describes the tech- nical details for using CT and MRI to rapidly evaluate the patient who presents with symptoms of acute stroke. Both modalities are widely available and pro- vide information on the state of the brain parenchy- ma, the vessels, and brain tissue perfusion. Since many have the capability to perform either or both, the question arises as what to use. The question is practical, but the answer is complex and depends on many factors including the state of the patient, the time since stroke onset, the logistical constraints of using one or the other modality, and the capability of a facility to perform advanced therapy such as intra- arterial thrombolysis.
In clarifying this issue, it is important to consider the fundamental questions that require rapid an- swers in the clinical setting we are considering. There are four key questions including: (1) is there a hem- orrhage?, (2) is a large vessel occluded?, (3) what part of the brain is irreversibly injured?, and (4) is there a part of the brain that is viable but underperfused and in danger of infarcting? Both CT and MR provide in- formation to answer each of these questions. Howev- er, they are not equal, and it is illuminating to consid- er the relative strengths of each modality. Let us con- sider each question in turn, and, assuming state-of- the-art CT and MRI instruments, estimate which modality is superior.
1. Is there a hemorrhage? CT is superior. Both can detect clinically significant parenchymal hemor- rhages; however, CT remains superior in detecting acute subarachnoid hemorrhage, which is an im- portant consideration in the acute stroke patient.
MRI is less sensitive because the high oxygen lev- els in cerebrospinal fluid results in erythrocytes maintaining near-normal levels oxyhemoglobin.
2. Is a large vessel occluded? CTA is superior. The question of whether the internal carotid artery (ICA), middle cerebral artery (MCA) M1 or M2 branch or the basilar artery is occluded is ex- tremely important especially if the capability ex- ists for intra-arterial intervention. Under optimal conditions, MR angiography (MRA) is equivalent to CT angiography (CTA), but conditions are sel- dom optimal when dealing with the acute stroke patient. The longer time required for MRA is an important consideration, but of greater impor- tance is the vulnerability of MRA to motion arti- fact that commonly results in an MRA of poor quality. CTA more reliably produces high-quality angiographic information of both the head and neck vessels in less than 2 min.
3. What part of the brain is irreversibly injured? Dif- fusion MRI is superior. CTA source images provide an estimate of irreversible injury, but are less sen- sitive than diffusion-weighted images.
4. Is there a part of the brain that is viable but un- derperfused and in danger of infarcting? Perfu- sion MRI is superior. Both methods provide infor- mation of similar quality. However, MRI systems can evaluate a larger proportion of the brain.
These last two questions together are estimating the infarct core and the operational penumbra, a clinically highly relevant determination well illus- trated by the success of the Desmoteplase in Acute Ischemic Stroke (DIAS) Trial discussed elsewhere in this book.
The answers to each question are based on review of the literature detailed in the six chapters on CT and MRI found in this book, and the practical, everyday clinical experience of dealing with stroke patients
Epilogue: CT versus MR in Acute Ischemic Stroke
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R. Gilberto González
264 R.G. González
that come to Massachusetts General Hospital (MGH).
So what do we use at the MGH? The answer is both whenever possible. The imaging triage of patients is shown in Fig. 13.1.
We will perform both in the majority of cases be- cause we are fortunate to have in our emergency de- partment state-of-the-art CT and MRI systems that are located close to each other. The studies are also rapid with CT/CTA/CTP requiring 10 min or less and the MRI restricted to diffusion, perfusion and per- haps one additional sequence taking no more than 10 min of imaging time. Many institutions will have CT and MRI scanners in different locations, raising logistical issues. The optimal solution will obviously vary from institution to institution, and from patient to patient. It is our hope that this book will help each institution to develop the optimal solutions to maxi- mize the number of good clinical outcomes in the acute stroke patient.
Fig. 13.1
MGH stroke imaging protocol
