Brain, Vol. 127, No. 2, 452-454, 2004
© 2004 Guarantors of Brain
doi: 10.1093/brain/awh024
Book Review |
IMAGING IN STROKE
IMAGING IN STROKEBy Michael G. Hennerici
2003 London: Remedica Publishing
Price £30. ISBN 1-901346-25-0.
Imaging has revolutionized the approach to managing stroke patients. Previously it was taught that the brain without oxygen survives for only a few minutes, and that the adult brain, unlike the developing brain, has limited powers of plasticity to recover from any acquired insult. These two dictums have been partially responsible for the all-pervasive feeling of nihilism previously surrounding stroke management. Imaging has been fundamental in exploding this dogma, allowing new concepts to evolve about aspects of acute neuronal damage and functional recovery/plasticity. With the introduction over the last 20 years of various diagnostic imaging techniques (structural imaging with CT in the 1970s and MRI in the 1980s, and especially the functional neuroimaging techniques of PET and perfusion/diffusion MR, which allows imaging of brain physiology), stroke has been redefined as a dynamic and evolving process with potential openings for acute therapy and longer-term rehabilitation.
Imaging, therefore, has become extremely important in the management of individual patients, as an accurate diagnosis is essential for safe, rapid and appropriate management. Also, importantly, the newer functional imaging techniques allow individual patients to be classified according to pathophysiology rather than clinical or structural imaging alone. Thus, an understanding of the neuroimaging techniques and what they tell us about pathophysiology is required of all professionals who hope to provide optimal management of acute stroke patients and of patients in the rehabilitation phase.
Imaging in Stroke is an excellent multi-author overview that provides this understanding. As would be expected from the prestigious authors, it is a well-written, concise and comprehensive reflection of current concepts of stroke imaging. The 216 pages are easily readable and are broken down into eight chapters, all written by eminent experts in their fields, and the book is edited by the well-known stroke neurologist Hennerici. There may be an over-representation of the editor’s colleagues from Germany, but their expertise cannot be doubted. One of the strengths of this book, as a consequence of its being mainly technique-based, is that each chapter stands alone as a very good reference in itself, and gives extensive and important references for the topic it covers. All the chapters review information derived from basic research as well as well as from clinical and pathological studies, and so are of general interest.
The opening chapter is dedicated to the classification of stroke. This is an important chapter, particularly in stroke research, especially for trials looking into the pathophysiology and natural history of stroke. There is an explanation of why stroke classification is important and its limitations regarding the definition of pathophysiological subtypes in the hyperacute stage (e.g. less than 6 h), which is important for current therapeutics. A very succinct and excellent outline of the main classifications, along with their shortcomings, is given. This is ideal for anyone designing or involved in stroke trials, such as stroke research nurses.
The next four chapters are dedicated to the CT, MRI and neurosonology techniques used in the assessment of acute stroke [computed tomographic angiography (CTA) and magnetic resonance angiography (MRA) are dealt with in a separate chapter]. In practical terms, CT is likely to remain the imaging technique of first choice for stroke worldwide because of its ease of use and accessibility. Stroke physicians therefore need to be thoroughly trained in all features of its use, including the recognition of the early changes of ischaemia. The CT chapter adequately covers early CT signs of ischaemia, with some good images as examples. Also briefly covered with examples are watershed infarcts, the dense basilar artery sign and acute haemorrhagic strokes, and there is a separate section on the increasingly used technique of CT perfusion. Judging by the overall renaissance of CT (with the introduction of newer, faster scanners along with CTA and CT perfusion), I felt that this subject was if anything a little under-represented, given the current climate.
The MRI chapter was written by some of the founding fathers of the newer MRI techniques of diffusion and perfusion MRI; unsurprisingly, there is a good succinct introduction and explanation of the technical aspects in very simple terms. An important feature of this chapter is its emphasis on the pathophysiological insights gained in acute ischaemic stroke using these techniques (compared with, say, CT or T2/T1-weighted MRI). It expertly outlines the concepts of the DWI–PWI (diffusion-weighted–perfusion-weighted imaging) mismatch and there is an excellent figure showing the proposed relationship between the mismatch and various cerebral blood flow thresholds for core, penumbra and oligaemia. This helps explain why MRI can be essential in identifying patients suitable for therapies such as thrombolysis, possibly outside the current limit of 3–6 h. Importantly, the limitations of absolute quantification with perfusion imaging at present are mentioned. There is also discussion of hyperacute haemorrhage detection using T2*- or susceptibility-weighted sequences, which have now been shown to be highly sensitive and in some centres now replace CT in excluding intraparenchymal bleeds. Although diffusion tensor imaging may be regarded as not being clinically relevant in acute stroke imaging at the moment, I would have thought there is sufficient interest in it in the literature and in the research world to justify its explanation and exploration in a little more detail, particularly with the potential relevance of tract-tracing/tract disruption in monitoring stroke outcome and rehabilitation. This is a very exciting topic.
The CTA and MRA chapter again is a very good summary of current techniques, outlining their pros and cons. There are some beautiful images outlining the differences seen with time-of-flight and contrast-enhanced MRA techniques, emphasizing the caution with which we should interpret each method. However, the advances that have been made are such that these will probably replace conventional angiography as diagnostic tools.
The chapter on neurosonology is particularly comprehensive and very informative, especially for a UK neurology trainee (even one with an interest in stroke), as neurosonology is somewhat neglected in the education of neurology and even stroke trainees (whereas on the Continent it is generally an optional part of their formal training, and neurologists developing an interest in stroke are required to become competent in using these techniques). Anyone working with stroke patients in the future will have to have a good working knowledge of these ultrasonographic techniques. The opening section describes the up-to-date ultrasonographic imaging techniques available and where and why they are used. Examples of the use of these techniques that are described include (i) the identification of extracranial sources of stroke (there is an interesting section on the role of ultrasonography in plaque morphology and its relationship with ‘plaque activity’, which may be relevant in the future in the still highly debatable subject area of asymptomatic stenosis); (ii) the role of transcranial Doppler in the evaluation of stroke (of great interest here is the detection of high-intensity signals by transcranial Doppler; these correspond to both gaseous and solid microembolic signals, which can occur during procedures such as angiography, carotid angioplasty, open heart surgery and carotid endarectomy, as well as in the assessment of symptomatic and ‘asymptomatic’ carotid stenosis); and (iii) stroke monitoring (recanalization times are important after adminstration of tissue plasminogen activator, as they are correlated to clinical outcome). The last section deals with new, emerging technologies. Written with an obvious expert’s understanding, this section explains the limitations of contrast harmonic imaging and discusses its further development. Many interesting points can be gleaned from this chapter, such as the use of contrast for improving diagnostic confidence in the detection of carotid artery stenosis.
Although a separate, isolated chapter on primary intracerebral haemorrhage written by the editor seems a bit odd at first glance, it is probably required in order to fill the knowledge gap left by the technique-based chapters and the discussions in all chapters regarding imaging and the pathophysiology of ischaemic stroke. Special attention is given to cerebral amyloid angiopathy (CAA) as this is becoming a hot topic in imaging research. Gradient echo MR can now pick up asymptomatic microbleeds, which can be associated with white matter disease in some patients, indicating the possible coexistence of CAA and therefore the possibility that these patients are at a higher risk of intracerebral haemorrhage from the use of warfarin and tissue plasminogen activator (or even aspirin). This is an authoritative short summary, and it emphasizes the need for further clinical trials in this condition.
After expertly summarizing some of the methodology behind PET, W.-D. Heiss introduces the PET chapter by emphasizing the unique advantages of PET over other clinical investigations in determining pathophysiology in acute stroke, and outlines the importance of the two critical flow thresholds that define the penumbra. Tissue with blood flow within this range (i.e. penumbra) exhibits neuronal functional failure without structural damage and can be prevented from becoming irreversibly infarcted by reperfusion if it is done early enough. Thus PET has been fundamental in the development of the rationale behind thrombolysis (and neuroprotection). There is an excellent section reviewing the experimental PET studies behind the development of the penumbral concept and a section on new radioligands, such as 11C-flumazenil, which are being used currently to detect irreversibly damaged tissue in the acute setting (as well as possibly to detect selective neuronal loss). Exciting and important sections then follow on the use of PET in investigating functional recovery. It is pointed out that neurons remaining within and surrounding the infarct, as well as compensatory mechanisms in functionally connected networks, may make important contributions to the eventual outcome. The discussion of recovery of motor function and from dysphasias gives a flavour of the emerging understanding of recovery—and also the controversies.
The final chapter, ‘Recovery and plasticity imaging’, is on a topic that is of huge interest and is a rapidly expanding research field. In the last two decades, functional brain imaging has contributed immensely to our understanding of the pathophysiological mechanisms underlying recovery or deterioration after stroke. Understanding these mechanisms would certainly result in better-designed pharmacotherapy and rehabilitation. Functional MRI and PET activation studies are now assessing the subacute and chronic stages, when synaptic changes and repair phenomena, as well as the reorganization of cortical maps and networks, are taking place. This knowledge will hopefully lead to intervention treatments/monitoring in damaged brains. Brain areas involved in, for example, simple motor finger/hand movements reveal networks involving the SMA (supplementary motor area), bilateral prefrontal and inferior parietal cortices etc., knowledge of which may lead to an understanding of recovery. There is still much debate regarding the overactivation of the normal hemisphere, and this chapter highlights this controversy (which is fundamental in the development of newer physiotherapy techniques, such as constraint therapy). It appears for now, however, that spontaneous re-establishment of the use of the cortical area normally involved in the affected function appears to be associated with best outcome, and promoting this should be one major goal of rehabilitation and pharmacotherapy. However, major gaps remain in our knowledge of these and related phenomena, and these are outlined in this chapter.
This book, while not a textbook, is a comprehensive and excellent resource which brings into focus the most relevant and up-to-the-minute aspects of stroke research affecting clinical stroke management via the combined authoritative lens of acknowledged experts in there respective fields. The chapters are generally of the right length and depth and are well-written. There is good balance between providing enough detail to stimulate interest but not so much to make it indigestible. It will appeal to a broad readership, including stroke physicians, neurologists, neurosurgeons, rehabilitation specialists and others with a clinical or research interest in stroke. I would certainly recommend it to all my fellow trainee neurologists.
Deparment of Neurology, Addenbrooke’s Hospital, Cambridge, UK
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