**2. Main diagnosis and imaging tools**

Though AVM prevalence is estimated to reach up to 0.2% of the population, actual confirmed diagnosis rate is only ~0.02% [23, 37]. This significant gap is attributed to the fact that AVMs are congenital pathologies remaining generally mildly symptomatic until clinical presentation appears (unfortunately, this is typically hemorrhagic stroke) [23]. AVMs are mostly diagnosed and confirmed using radiology. The imaging findings also greatly influence treatment evaluations. Two strict AVM diagnostic criteria are: indication of a nidus and clear venous drainage [38]. Angiographic elements evaluated as indications for AVMs' clinical significance, and risk assessments include: arterial supply and venous drainage identification, nidus position, geometry, and size, presence of intracranial hemorrhage, and related pathologies [39]. The presence of aneurysms is associated with 26% of AVM patients [27].

#### **2.1 Digital subtraction angiography**

DSA is a vascular mapping technique based on conventional angiography. DSA involves an additional pre-processing step that removes non-vascular anatomical data [40]. Angiography is the gold standard methodology for the characterization and delineation of cerebral AVMs and in the evaluation of cerebrovascular diseases [41, 42]. The method dynamically displays vascular transitions and is uniquely able to delineate the size and number of feeding arteries and their origins, measure the nidus size and compactness, and evaluate venous drainage locations [42]. In addition to its significant diagnostic role, angiography is the main technique used by interventional radiologists to accurately navigate through the neurovascular environment and assess treatment progression and related events through interventional sessions [43, 44]. DSA is performed using selective (the catheter used for contrast administration is only advanced to the AVM vicinity) or super-selective (the catheter is further advanced to the AVM feeding arteries) approaches [23]. DSA is increasingly being supplanted by the less invasive computed tomography angiography (CTA) and the X-ray and nephrotoxic contrast-devoid MRA.

## **2.2 Computed tomography**

CT comprises a limited diagnostic tool for unruptured AVMs [45]. Lower density areas may (potentially) indicate the AVM in CT scans [46]. The great contribution of CT scanning in AVM diagnosis lies in its high sensitivity for acute bleeding detection, which makes it a front-end diagnostic tool when patients present acute symptoms related to the pathology [47]. CT is also used to assess efficacy and complications of embolization interventional treatments [47].

## **2.3 Computed tomography angiography**

CTA combines the CT imaging procedure with contrast agent injection. This enables real-time (RT) structural presentation of neurovascular angioarchitecture (e.g., nidus, arterial feeders, and draining veins) and identifying associated pathologies such as fistulas and aneurysms [38]. Gupta et al. demonstrated that it is even possible to obtain good quality intranidal angiograms using RT CTA for anatomic localization of a specific catheterized (embolized) AVM region [48].

### **2.4 Magnetic resonance imaging**

Smith et al. found MRI which is superior to CT and angiography in determining nidus size and location, detecting AVM effects on adjacent brain tissue, and showing the obliteration extent following embolization [49]. MRI also facilitates AVM classification into base categories (e.g., sulcal/gyral) [23]. Use of flow turbulence and velocities as benchmarks has been shown to increase MRI diagnosis efficacy [23, 50]. MRI is considered highly efficient in detecting and delineating hemorrhagic episodes [23]. In addition, it can detect damage and atrophies in surrounding brain tissue and related pathologies such as aneurysms and fistulas. The latter are considered challenging for early detection and feature high annual risks for hemorrhagic events (7.5%) [41].
