Comparison of conventional MR images with diffusion-weighted images makes it possible to narrow the time point of the infarction a bit better, which may be important if the stroke symptoms appear on waking (“wake-up stroke”) or the patient is found unconscious or with global aphasia. If the DWI-marked infarct is not yet visible on T2 and/or FLAIR images, it is relatively fresh and there is more of an indication for acute thrombolytic or mechanical thrombectomy treatment.
1.4.4.3 CT angiography (CTA) and MR angiography (MRA)
Imaging of the cerebral arteries from the aortic arch up to the peripheral branches of the middle cerebral, anterior and vertebrobasilar territories is obligatory in cases of ischemic infarction, and with modern equipment it takes only a few minutes. Intravenous contrast medium is injected to demonstrate the vessels, and imaging is started on the first passage of contrast through the aortic arch and cervical vessels. Standardized postprocessing programs allow selective three-dimensional display of the vessels (lumenography).
1.4.4.4 Perfusion CT imaging (PCT) and perfusion magnetic resonance imaging (PMRT)
Magnetic resonance and computed-tomographic perfusion imaging are procedures used for diagnostic demonstration and quantification of organ perfusion. They allow at least semiquantitative measurement of cerebral perfusion, can be carried out within a few minutes, and display hypoperfused areas of the brain immediately after vascular occlusion has occurred. Positron-emission tomography (PET) and single-photon emission computed tomography (SPECT), like ultrasound and Doppler ultrasonography, have no role in the modern diagnostic work-up for acute stroke.
Contrast-enhanced perfusion imaging is based on the indicator dilution theory: the passage of an intravenously administered contrast bolus, as compact as possible, through the cerebral circulation is displayed at a frame rate of no less than 1 image per second, if possible.
On the CT, the radiographic density of the normally perfused brain increases transiently during passage of the contrast (Fig. 1.4-17). On perfusion MRI, either T1-weighted imaging is used to determine the signal increase, or T2-weighted gradient imaging (T2*) is used to measure the signal decrease that occurs when the MR contrast flows through the capillaries, leading to local magnetic field changes (susceptibility disturbances) (Fig. 1.4-18).
Arterial spin labeling (ASL) is another elegant MR perfusion technique, and it does not require any contrast administration. Blood flowing into the brain is marked, and the blood itself serves as an endogenous marker during its passage through the brain. Due to the longer measurement time of approximately 5 minutes, in comparison with 1 minute for contrast-enhanced measurements, this technique is only used in special cases (contrast intolerance, renal problems) in patients with acute stroke (Fig. 1.4-19).
Functional parameters for cerebral perfusion are calculated from the signal curves using various mathematical models and algorithms and are presented in the form of parameter images. Changes in the mean transit time (MTT) and time to peak (TTP) parameters are the easiest to interpret and detect, and they allow perfusion to be described with a high level of sensitivity.
Cerebral blood flow (CBF) describes how much blood per unit of time is flowing through the cerebral tissue. Normal findings are 50–70 mL per 100 g tissue per minute. Neurological deficits occur starting from 20 mL/100 g/min. Irreversible cell damage occurs below a threshold of approximately 15 mL/100 g/min. This applies especially to the core of the infarct, although it is usually surrounded by tissue that is still temporarily receiving sufficient blood from collaterals. In a model that is not uncontroversial and which is of little assistance in treatment planning, this tissue is described as the penumbra, or “tissue at risk.”
Cerebral blood volume (CBV) is the percentage proportion of the blood (arterial, capillary, and venous) within a defined quantity of brain (usually also 100 g).
In cases of acute stroke, autoregulation can initially keep the cerebral perfusion constant. A decline in perfusion leads to dilation of the cerebral vessels. This has the effect that during a stroke, the CBV increases as long as the affected cerebral tissue is still receiving blood via collaterals. Hemodynamically, the penumbra is characterized by a reduction in CBF but with normal or increased CBV, whereas in cerebral tissue that has already suffered infarction or in the core of the infarct, the CBV and CBF are both reduced—the latter to values below 15 mL/100 g cerebral tissue/min.
Fig. 1.4–17a–c CT perfusion. (a) Noncontrast CT in a 54-year-old patient with acute left-sided hemiparesis, with no clear pathological findings. (b) Interactive measurement of the arterial contrast increase in the anterior cerebral artery and of venous outflow in the superior sagittal sinus. (c) The CT perfusion image calculated from the data shows that hypoperfusion in the right central region is the cause of the left-sided hemiparesis.
Fig. 1.4–18a-e Perfusion MRI. A signal intensity–time curve (a) is measured for each pixel, and on the basis of the indicator dilution theory a concentration–time curve (b) is adapted to it. The perfusion parameters are calculated using the concentration–time curve: the relative cerebral blood volume (RCBV) corresponds to the area under the concentration–time curve (c); the mean transit time (MTT) corresponds to the first moment of the concentration–time curve (d). The regional cerebral blood flow is calculated from the CBV and MTT (RCBF = RCBV / MTT) (e). The corresponding parameter maps are calculated for each of these parameters. The perfusion parameters (c-e) show hypoperfusion in the circulation area of the middle cerebral artery.
Fig. 1.4–19 The principle of arterial spin labeling (ASL).
1.4.4.5 Catheter digital subtraction angiography (DSA)
Catheter angiography is only used as a primary diagnostic tool in exceptional cases—e.g., to discover whether an occlusion or pseudo-occlusion is present, or to examine vessels using high spatial and temporal resolution—e.g., when vasculitis is the suspected cause of cerebral perfusion disturbances. On the other hand, imaging of the entire cerebral circulation is obligatory in the context of endovascular therapy in order to detect the morphology of the vascular occlusion and the extent of the collateral supply, particularly now that 3D images are available using rotation angiography and dyna-CT can be used on the angiography table as well to depict the brain parenchyma and measure CBV.
1.4.5 Treatment
Acute ischemic stroke is treatable. Rapid reopening of the occluded cerebral vessel leads to a reduction in the mortality rate and can help avoid disability in one in three patients. Spontaneous recanalization occurs in approximately 25% of cases, but mainly in occlusions of small cerebral vessels and usually too late to salvage the downstream cerebral tissue.
Many treatment approaches for revascularizing cerebral vessels have been borrowed from the field of cardiology. The main difference between infarction in the brain and in the heart is that cerebral infarction is usually caused by an embolism that occludes an otherwise healthy cerebral vessel. In contrast, occlusion in the coronary vessels usually takes place against the background