 Hello everyone, Myself Dr. Mithusha Verma, I am a consultant radiologist reporting PEDCT CTN MRI at Department of Imaging Nanavati Super Speciality Hospital, Mumbai. First of all, I would like to thank Indian radiologist for giving me this wonderful opportunity and an excellent platform. The topic which we are going to discuss is arterial spin labeling. This is a relatively newer and novel perfusion technique. We are going to discuss this topic in two parts. The first part we will see the basics and second part we are going to see the applications. Let us start by understanding the concept of perfusion. As we all know that conventional MRI sequences including post-contrast imaging delineate anatomy, the various pattern of enhancement which we get is secondary to disruption of blood-brain barriers. Here we are discussing imaging in brain tumors. So, this makes this clear that enhancement does not correlate with tumor grade and therefore there was a need of shifting to physiologic imaging. As we all know that basic feature of any malignancy and specifically high-grade malignancy is neo-angiogenesis. That means formation of new leaky tortuous vessels. With these fragile vessels the tumor bed always has a very high blood flow and blood volume and with the perfusion techniques what we are assessing actually is the grade of neo-angiogenesis which correlates well with the tumor grade itself. Therefore, to conclude, perfusion is a better way to grade brain tumors as well as also to guide biopsy to avoid sampling bias. Even for post-treatment follow up this is a more sensitive technique. There are various ways in which perfusion imaging can be performed. The most commonly used techniques are DSE and DC perfusion which is dynamic susceptibility contrast and dynamic contrast enhanced. Both these techniques require contrast injection and that too in a particular way and contrast injection speed. Therefore, we understand that for a very high speed contrast injection as is required by DSE perfusion technique like around 10 ml of contrast in two seconds a large bore venous excess is required which might not be possible in all patients specifically in patients after chemotherapy and after radiation. Also there might be a subset of patients where contrast itself is contraindicated. Thirdly in even pediatric age group getting a good venous excess might be difficult. The second pitfall is that these imaging techniques of post-contrast perfusion are very much prone to artifacts secondary to lesions that alter magnetic field. In all practical scenarios the most common thing is hemorrhage. Third is air and bone can also cause artifact in evaluation of post-contrast perfusion images. Lastly these are not easily repeatable. Suppose for any reason we need to repeat it we need again to give contrast to the patient which might be difficult. Therefore arterial spin labeling the topic which we are going to discuss today becomes really important on of high utility. This is a novel gadolinium free MR perfusion technique. It uses magnetically label arterial blood water proton as an endogenous tracer. If we actually go into the depth of physics we'll find it very complicated but just to understand the basics here we are acquiring two sets of images number one we are going to acquire labeled images or tagged images where we are going to tag using radio frequency pulses the inflowing arterial blood by magnetic inversion. After allowing a specific period of delay same area is imaged and we acquire control image without tagging. By subtracting the two set of images what we acquire is ASL data set. So to summarize basically we are going to obtain two sets of images labeled image or tagged image and a control image. Next this is how we are doing is by using radio frequency pulses that saturate water protons. After this we are going to subtract the two and the signal which we are going to acquire is going to be directly proportional to the perfusion and cerebral blood flow. This is a eco planner imaging based sequence and always we have to take care that ASL has to be acquired before gadolinium administration since gadolinium will cause T1 shortening leading to a decrease in the measurable signals. There are various ways in which ASL can be performed one is pulsed ASL second is continuous ASL the one which we use currently is PCASL or pseudo continuous ASL the fourth is velocity selective ASL. This is how ASL grayscale map of a normal person looks like but after a little bit of post processing this is what we acquire on our workstations and this is a normal ASL color maps where in a rainbow spectrum the areas which are hyper perfused are red in color the areas which are hyper perfused are towards the blue part or the violet part of the spectrum. Coming to the potential applications we can actually use this non contrast perfusion technique which will just take an additional four minutes of an MRI sequence at various applications. The potential applications are one stroke imaging second in epilepsy imaging third in dementia fourth tumor imaging next in imaging CNS infections also in evaluating psychiatric disorders. We are going to see these applications in the details in the next section but in this section let us go through the role of ASL in stroke imaging. As we all know in stroke the most important concept is of perfusion diffusion mismatch which means that there is an area beyond the infected brain parenchyma which we call as the ischemic penumbra which can actually be saved by timely interventions. So the entire aim of stroke imaging as well as stroke treatment is to save this salvageable penumbra. We have been using the concept of perfusion diffusion mismatch to delineate this salvageable penumbra. So here we have a novel sequence by which we can give this useful information even without using contrast. So let us see few examples here are two patients. In the first patient there is a small wetship area of restricted diffusion delineating the infected brain parenchyma. On corresponding color ASL map a similar size area of hypoperfusion is also seen that means there is no significant perfusion diffusion mismatch. However in the second example which is example b we can see that though the area of restricted diffusion is similar small in size on ASL color maps a larger area beyond the restricted diffusion is hypoperfused which means that there is definite perfusion diffusion mismatch and this patient if treated timely will definitely come up with positive outcomes. A little bit about the concept of perfusion diffusion mismatch here we should understand that all the area which is hypoperfused is not the salvageable penumbra because in the periphery of it that is the blue area is going to be benign oligemia. So only the yellow rim in this particular graphic beyond the red so red is already dead or impacted brain parenchyma. The yellow rim is the rim which is actual salvageable penumbra so that was the first application that is determination of area of perfusion diffusion mismatch second is localization of arterial thrombus. This is a relatively newer application and is based upon a phenomena called bright vessel sign which happens due to accumulation of protons and label arterial blood immediately upstream from the arterial occlusion. This is very important and useful in determining the site of intra arterial thrombus in cases of stroke and as per the study is coming up this sign might be even more sensitive as compared to the area of blooming which we pick up on so on images. Third is actual depiction of hyper perfusion in these patients post therapy. Sometimes we do see it secondary to autolysis when we follow on these patients most of the time post recanalization or post thrombosis we can actually show reperfusion in the previously seen areas of hyper perfusion. Coming to various examples of whatever we have discussed till now in this patient small patchy areas of restricted diffusion are seen in the right corona area also into the insular cortex subcortical right matter. On ASL colour maps a larger area beyond the area of restricted diffusion is hyper perfused signifying a perfusion diffusion mismatch even on the angio we can see that beyond few centimeters there is near complete cutoff in the right MCA territory. Next example small patchy areas of restricted diffusion and on ASL colour maps larger area of hyper perfusion is seen post thrombosis the hyper perfused area shows successful restoration of perfusion and stabilization of the areas of restricted diffusion so there is no actual increase in the size of restricted diffusion no significant increase in the size of impacted brain parenchyma and we have saved a lot of salvageable penumbra. Next example again that area of restricted diffusion which is seen on the first image a larger area beyond that is seen on ASL as blue which is hyper perfused post thrombosis actual reperfusion can be depicted on the ASL colour maps which is seen as red. Next example here there is near complete cutoff in the left MCA large area of restricted diffusion and a large area of hyper perfusion this patient underwent thrombectomy successful recanalization of the MCA as is seen in the post thrombectomy angio images and on the follow-up CT only pethical hemorrhages were seen there was no significant mass effect no midline shift. Next example a small area of restricted diffusion and a large area of hyper perfusion so perfusion diffusion mismatch band like area of blooming is seen in the area of right MCA suggestive of intraderial thrombus this patient underwent tending as well on the contralateral side and we can see that there was a restoration of perfusion in the previously seen hyper perfused area and there was no significant increase in the size of the impact. Another example with positive perfusion diffusion mismatch areas of restricted diffusion on the first set of images on the left upper corner a larger area of hyper perfusion and after thrombectomy we can actually see the thrombus in one of the images and the thrombus was removed the artery was recanalized and even the perfusion was restored. Now this is a different patient here there is definite perfusion diffusion mismatch in the first set of images on the left but due to some reasons this patient was not taken for any active intervention and we can actually see on follow that this entire hyper perfused area developed into an actual infarct. Next application of this particular technique in patients with suspected stroke is to confidently diagnose stroke mimics we do see cases where we have an area of restricted diffusion but on ASL we see that there is hyper perfusion so we understand that in patients with hyper acute infarcts or within a particular time limit we do not expect luxury perfusion so almost all the hyper acute and acute infarcts are hypo perfused but in this case we see that there is an area of restricted diffusion with a different kind of distribution and on ASL it is hyper perfused therefore it was taken as a post rectal change rather than an infarct. Another example where we have diagnosed this as migraine with aura so there is a idea of hypo perfusion a very large area of hyper perfusion which is not limited to single vascular territory and it may be predominantly posterior in distribution and you can see in 24 hours follow-up the images which are below there is all restoration of the perfusion. So many a times we do see torts periperesis or post rectal status where patient may present with stroke like symptoms but actually it is just a stroke mimic and ASL definitely help us to confidently diagnose these mimics. To end we will just conclude with this list of hyper perfusion and hyper perfusion and we will see the rest of the application in the second part of this series why we are discussing this list because these are the areas where actually we can use ASL which is a very novel non-contrast perfusion technique. So wherever we get hyper perfusion like luxury perfusion we saw the example thrombolitic induced recanalization again we saw the example post rectal changes, high-grade tumor, vascular malformation, migraine, inflammation, encephalitis mainly viral definitely ASL will help us. Hyper perfusion also can be depicted in ischemic core, encephalomalacia, post rectal status, hematomas, infections mostly non-viral, vasculitis, demyelination, atrophy and bring dead situations. There are pitfall to this technique as well it is very much prone to movement artifact you have to take care of that and patient positioning is important. Physiologic areas of hyper perfusion may overestimate penumbra as we saw the banana oligemia plus the white matter which is usually hyper perfused. Low signal to noise ratio it's better with 3 tesla scanners, minimum acquisition time of 3 minutes is required, spatial resolution is limited, quantification of blood volumes is not possible what we get is CVF and because of difference in the manufacturer techniques comparison might not be very robust and accurate. Thank you for your patient listening this was about the arterial spin labelling technique basics and little bit about the applications we are going to see details of rest of the applications in the second session. Thanks.