*2.4.4.2 Magnetic resonance imaging (MRI) and diffusion weighted imaging (DWI)- fluid attenuated inversion recovery (FLAIR) mismatch*

Another method of determining mismatch is by using magnetic resonance imaging (MRI). This is a newer, more sensitive form of imaging compared to CT. Its primary advantages are its high spatial resolution and versatility in use. Its main disadvantages are that the images take longer than CT to acquire and the scanners are not nearly as common or accessible as for CT. Nevertheless, specific MRI sequences can provide crucial information for the diagnosis of AIS. A diffusion-weighted imaging (DWI) measures the diffusion of water molecules across the cellular membrane [44]. In patients with AIS, due to disruption of the electrochemical gradient, this diffusion is disrupted. This shows up at hyperintense or bright on imaging. This bright signal can be detected for at least 2 weeks after the initial event [45]. The apparent diffusion coefficient (ADC) image is the inverse of the DWI and has low intensity during this period. The DWI and ADC image are useful in demonstrating whether there is any ischemia to a region, reversible or irreversible. The T2 fluid attenuated inversion recovery (FLAIR) image is the sequence that is used to see the structure and anatomy of the brain tissue. In the hyperacute phase, the FLAIR has variable intensity and increases in intensity after 6 hours. The FLAIR image demonstrates structural damage and irreversible infarcted tissue [44]. To be a candidate for endovascular thrombectomy, the DWI volume must be greater than 70 mL [40]. In situations of uncertain onset, positive DWI with negative FLAIR, known as a DWI-FLAIR mismatch, strongly suggest that the stroke is hyperacute and there is significant mismatch (**Figure 4**). Using DWI-FLAIR mismatch, once can predict if an AIS is presenting within 4·5 h of LKNT with 62% (95% CI: 57–67) sensitivity, 78% (95% CI: 72–84) specificity, 83% (95% CI: 79–88) positive predictive value, and 54% (95% CI: 48–60) negative predictive value [46]. Therefore, in patients in whom LKNT is unknown, MRI DWI-FLAIR mismatch can be used to identify patients who would still remain candidates for thrombolysis, which will be discussed in detail later [47].

There are a plethora of other MR sequences and techniques that can be used in the diagnosis and management of AIS. Susceptibility weighted imaging (SWI) is a sequence that is sensitive to products that especially distort the magnetic field like iron. Given blood breakdown products contain iron, SWI provides information about the existence of possible hemorrhagic transformation has occurred within the first 12 hours [25, 48]. MR angiography without contrast (time-of-flight) is an alternative to CT angiography [49]. And MR perfusion has obtained 90% concordance between CT-based and MR-based mismatch status [40], and is a highly reliable alternative for patients not amenable to NCCT and CTA.

### **2.5 Emergent bloodwork**

Emergent laboratory investigations should include complete blood count, electrolytes, aPTT, INR, creatinine, and glucose [7]. It is not necessary to wait for all the results before thrombolysis, as blood glucose can be reliably given by a finger stick test, and as mentioned, creatinine values are not needed prior to performing CTA. Other labs such as an INR > 1.7, platelets <100,000/mm3 , and blood glucose <50 mg/ dL, are also helpful as they are part of the relative contraindications for tPA, discussed in further detail later. Patients suspected of having ischemic strokes should have a 12-lead EKG and initiate telemetry. This may assess cardiac rhythm and uncover atrial fibrillation [50].
