227P.D. Lyden (ed.), Thrombolytic Therapy for Acute Stroke,DOI 10.1007/978-3-319-07575-4_12, Springer International Publishing Switzerland 2015
Imaging in Acute Stroke Patients
Computed tomography (CT) was the fi rst modality to image the brain and its pathology in vivo. Before the invention of CT in the early 1970s, brain diseases were categorized according their clinical phenotype. The term stroke originates from these old days of medicine, when brain hemorrhages could not be differenti-ated from ischemic brain diseases. It is unfortunate that we still use the term stroke for different diseases like subarachnoidal hemorrhage, cerebral venous thrombosis, spontaneous intracerebral hemorrhages, and focal brain ischemia, which should be distinguished even though each may present as brain attack.
Computerized tomography and magnetic resonance imaging (MRI) are today regarded as safe methods to quickly image the skull and its content. They confi rm the defi nite diagnosis of ischemic stroke and detect the pathology of ischemic brain tissue in individual patients. Moreover, infusion of contrast agent allows the imag-ing of arteries and veins as the assessment of brain perfusion with both modalities. CT angiography (CTA) is now routinely used in many stroke centers, whereas CT brain perfusion imaging (CTP) has not yet shown that it is really needed and has an impact on clinical outcome. Compared to MRI, brain imaging with CT is less expensive, quicker, more practical for severely ill patients, and in general easier to interpret. Computerized tomography is consequently widely used and considered as the method of fi rst choice for differentiating among the stroke syndromes. Moreover, CT imaging of acute stroke pathology is highly predictive of irreversible ischemic injury [ 1 ]. In this chapter, we will discuss the impact of CT tissue and thrombus imaging, CTA, and CTP on reperfusion strategies.
Chapter 12 The Impact of Neuroimaging on Acute Stroke Treatment: Role of Computed Tomography
Rdiger von Kummer
R. von Kummer , M.D. (*) Department of Neuroradiology , Technische Universitt Dresden , Fetscherstr. 74 , Dresden 01307 , Germany e-mail: email@example.com
A CT image of the brain in acute stroke patients is not diffi cult to read; it is, however, not self-evident. Reading of CT needs training and instruction on how to recognize anatomy and pathology, combined with knowledge about the physical conditions of image contrast [ 2 ].
The image provided by CT consists of a limited number of volume units (voxels). A typical matrix has 256 256 voxels or 512 512 voxels. The matrix and the slice thickness determine spatial resolution. The x-ray attenuation of each voxel is elec-tronically detected, grouped in relative attenuation valuesnamed Hounsfi eld Units (HU)between 1,023 and +3,072 calibrated by the attenuation of water (HU = 0) and air (HU = 1,000), and translated into 20 levels of a gray scale, which can be distinguished by the human eye. To enhance contrast resolution, the entire gray scale is used to represent the attenuation of the brain, which normally varies between 0 and 50 HU (Table 12.1 ). Another section (window) of the entire HU scale is used if structures with different attenuation, e.g. the temporal bone, are examined. A typical CT window for the brain has a width of 6080 HU. The mean value of such CT-windows is called level and is responsible for the brightness of the image. X-ray attenuation below the range of the CT window appears as black on the image, above this range as white. A broader window diminishes the contrast between gray and white matter and impairs the detection of subtle changes in x-ray attenuation. If a CT window width of 80 HU is used, each gray level represents 4 HU. The contrast resolution is thus limited to 4 HU and could be enhanced by a reduction of the window width. A smaller window will, however, reduce the signal/noise ratio (Fig. 12.1 ).
The electron densities of the substrate under study attenuate x-rays [ 3 ]. In bio-logical tissue, x-ray attenuation is directly correlated with the tissue specifi c gravity [ 4 ]. The different electron densities of gray and white matter, brain vessels, cerebro- spinal fl uid (CSF), and skull allow to differentiate these structures on CT and to recognize pathological alterations.
Table 12.1 X-ray attenuation in cranial CT
X-ray attenuation (HU)
Gray matter 3545 White matter 2030 Cerebro-spinal-fl uid 48 Skull 1001,000 Large vessels 4050 Tissue calcifi cation 80150 Hematoma 7090 Fat 60 to 70 Air 1,000
R. von Kummer
Technical Note: How to Perform CT in Acute Stroke
If the imaging facility is informed in advance, the scanner can be kept free for the patient with acute stroke. Emergent life support should be continued during the patients imaging test if necessary. Direct transportation of the patient to the scanner will save time. Neurological examination can be performed on the scanner. While a detailed neurologic examination is not needed before the brain imaging is done, localizing the stroke to the posterior fossa or the cerebral hemisphere will assist in optimizing the imaging techniques selected.
CT should be fi rst performed without contrast with a rapid scan time to reduce motion artifact. A correct head position is crucial to avoid obliquity of the sections. If necessary due to motion artifact, specifi c section cuts can be repeated. The high density of bone often causes an artifact, which may impair visualization of the lower part of the brain stem in the posterior fossa. Single slice scanning should be preferred to continuous spiral scanning. Thin (e.g. 0.625 mm) transaxial sections with 2.5 mm and 5 mm reconstructions can minimize artifacts and enhances sensitivity for thrombus detection [ 5 ]. Image windows should be adjusted so that gray and white matter can be easily distinguished and subtle hypodensities are detected. A window width of 7080 HU at a center of 3540 HU is recommended. A close communication
Fig. 12.1 Two representations of the identical section of a CT in a patient with acute stroke. Both sections are displayed with the same level, but with a different window width. With a window width of 80 HU (Panel a ), the contrast between normal cortical density and cortical hypodensity is less obvious than with a window width of 31 HU (Panel b ). The area of hypodensity covers part of the temporal lobe, the entire insular cortex, and the lateral rim of the putamen and is better outlined with the narrow CT window ( arrows ). Note the increase of noise in Panel b
12 The Impact of Neuroimaging on Acute Stroke Treatment
between the stroke physician and the radiologists or technician performing the scan will enhance the information gained from CT. The image should be optimized for spatial and contrast resolution in the region of interest. Additional bone win-dow views can be performed if head trauma is a possibility, in order to search for skull fractures, subdural air or blood, or effusions in nasal sinuses and middle ear. A contrast-enhanced scan might be obtained after, if there is suspicion for neoplasm or localized infection and no MRI available.
In case of suspected major cerebral artery occlusion that may require intra- arterial intervention, CT angiography (CTA) should be prepared and performed immediately after non-enhanced CT. Immediate interpretation of the non-enhanced scan may allow the start of thrombolytic infusion in parallel to CTA.
Advanced CT technology provides the opportunity of CTA and CT perfusion imaging with one contrast injection only [ 6 , 7 ]. A conventional CTA is performed after bolus injection of 130 ml non-ionic contrast (injection rate: 45 ml/s) with a spiral scan of the brain base and 3-dimensional reconstruction of the Circle of Willis on a workstation. Perfusion imaging with CT requires repeated imaging of one or more sections to measure the contrast uptake and clearance curve in each voxel. Parameter images are then calculated for cerebral blood volume (CBV), mean tran-sit times (MTT), cerebral blood fl ow (CBF), or time intervals to the peaks of con-trast enhancementtime to peak (TTP) (Fig. 12.2 ) [ 8 ]. Multidetector CT technique now allows whole brain perfusion imaging.
Interpretation of CT Findings
Pathological fi ndings on non-enhanced CT (NCT) in acute stroke patients may be thrombo-embolic occlusion of large vessels, focal brain tissue swelling caused by vasodilatation, ischemic edema, intracranial hemorrhage, focal brain tissue infl am-mation, or tumor like lesions. Computed tomography identifi es these fi ndings by detecting changes in x-ray attenuation of normal brain structures, a shift or replace-ment of brain structures by pathological substrates, or pathological contrast enhancement. A pathological increase in x-ray attenuation is called hyperdensity, a pathological decrease hypodensity. These terms are somewhat confusing because they do not defi ne a fi xed degree of x-ray attenuation. They are commonly used to characterize the attenuation of a structure in comparison to other tissue, e.g. in saying that a parenchymal hematoma is identifi ed by its hyperdensity if compared with gray matter. These terms are best used, however, to characterize a change in x-ray attenuation by comparing the density of an affected structure to its normal density. The symmetry of the brain structures in transaxial planes facilitates this comparison. e.g., the putamen is best evaluated by comparing its attenuation to that of the contralateral