Utility of ASL in CNS Infections:

1. Viral Encephalitis: ASL offers a distinct advantage by identifying perfusion abnormalities in viral encephalitis, often revealing issues before conventional MR sequences show significant findings. Clinicians can link the perfusion patterns observed in ASL to the underlying pathophysiology of different viral infections and categorize these patterns into several groups. [3, 4]

Table 1: Comparison of ASL perfusion patterns and conventional MRI findings in various types of encephalitis, highlighting affected regions and clinical relevance for early and accurate diagnosis

1. Hyperperfusion with Conventional MR Abnormalities: Viral encephalitis, such as Japanese encephalitis and herpes simplex encephalitis, demonstrates hyperperfusion on ASL, often aligning with T2-weighted or FLAIR hyperintensities on conventional MRI sequences [5, 6]. This pattern reflects the inflammatory response and increased permeability associated with the disease. Depending on the viral aetiology, conventional abnormalities can present as focal or multifocal hyperintensities involving regions such as the temporal lobes, insular cortex, or thalamus [7, 8].
   

   Fig 1: A 43-year-old patient arrived at the emergency department unconscious and with high-grade fevers. FLAIR imaging showed a hyperintense signal (Red arrow), T1-weighted imaging revealed a hypointense signal (Blue arrow), and ASL imaging indicated significant hyperperfusion extending beyond the FLAIR findings(Black dashed line).
   

   Fig 2: A 41-year-old female presented with a fever and altered sensorium. DWI images reveal gyral restriction (Yellow arrow), with corresponding areas of FLAIR imaging showing hyperintense signals (Blue arrow). ASL imaging demonstrates a focal area of hyperperfusion (Red arrow).
   

   Fig 3: A 4-year-old child was brought in due to seizures. Axial T2W images show multifocal areas of hyperintensity in the bilateral fronto-parietal lobes (Red arrows), and ASL images reveal hyperperfusion in the corresponding areas (Yellow arrows).
2. Hyperperfusion Without Conventional MR Abnormalities:Early stages of viral encephalitis may present with Hyperperfusion in cortical or subcortical regions without any visible changes on T2 or FLAIR sequences [9]. This highlights ASL’s sensitivity in detecting perfusion changes indicative of inflammation at an early stage.
   

   Fig 4: A 26-year-old presented with high-grade fever and seizures. FLAIR and DWI images reveal no altered signal intensity. ASL image reveals hyperperfusion in the left hippocampus and amygdala (Yellow arrow).
3. Hypoperfusion in Dengue Encephalitis:Dengue encephalitis exhibits a contrasting pattern of hypoperfusion on ASL, reflecting endothelial damage and capillary dysfunction rather than inflammation [10]. This pattern differentiates dengue encephalitis from other viral encephalitides.
   

   Fig 5: A 52-year-old patient showed decreased responsiveness and left limb weakness, testing positive for Dengue NS1 antigen. T2-weighted imaging revealed hyperintensity in the right ganglion-capsular region and the splenium of the corpus callosum (yellow arrows). Post-contrast T1-weighted imaging showed mild enhancement (blue arrow), while ASL indicated hypoperfusion in these areas (red arrow).
   

   Fig 6: A 10-year-old boy showed sudden deterioration of consciousness, vomiting, and an unsteady gait after a 2-day fever. Dengue serology was positive for NS1 antigen. T2-weighted imaging displayed symmetric hyperintensities in the frontal, parietal, and occipital regions, along with the splenium of the corpus callosum (Red arrows). Diffusion-weighted imaging indicated restricted diffusion (Green arrows), and Arterial Spin Labeling confirmed significant hypoperfusion in these areas Yellow arrows).

2. Fungal Infections: Fungal encephalitis demonstrates distinct perfusion patterns on ASL, often reflecting the angioinvasive nature of certain fungi. Hyperperfusion in fungal encephalitis typically indicates inflammatory angiogenesis or vasculitis regions, while hypoperfusion may suggest vascular compromise due to thrombotic or ischemic events.

1. Hyperperfusion in Angioinvasive Fungal Encephalitis: Commonly observed in mucormycosis and aspergillosis, which have a predilection for the basal ganglia, thalamus, and frontal lobes. ASL Hyperperfusion highlights active fungal invasion and vasculitis areas, often corresponding to oedema and enhancement on T2/FLAIR and post-contrast sequences. [11]
   

   Fig 7: A 67-year-old patient with uncontrolled diabetes presented with seizures. Axial T2-weighted image shows heterogeneous hypointense granulomas accompanied by disproportionate oedema (Red arrow). Post-contrast T1 fat-suppressed images reveal peripheral and nodular enhancement (Blue arrow), and ASL images indicate hyperperfusion (Yellow arrows).
2. Hypoperfusion in Ischemic or Infarcted Regions: Hypoperfusion can be seen in regions with extensive vascular occlusion caused by fungal invasion, particularly in mucormycosis. This dual perfusion pattern underscores the diagnostic value of ASL in identifying both active and necrotic areas. [12]
   

   Fig 8: A 51-year-old renal transplant patient presented with seizures and unconsciousness. An axial T2-weighted image showed a cystic area with a fluid-fluid level and a smooth hypointense rim (Red arrow). DWI indicated central restriction (Yellow arrow). Post-contrast T1 fat-suppressed images revealed peripheral and nodular enhancement (White arrows), and ASL demonstrated hyperperfusion (Red arrow). DTI indicated displaced tracts.

3. Tubercular Pathologies:

Tubercular infections in the CNS present with diverse imaging patterns, including tuberculomas, abscesses, and tuberculous meningitis. Tuberculomas often mimic high-grade aggressive neoplasms on conventional imaging due to their ring-enhancing appearance and perilesional oedema. However, ASL provides critical differentiation and typically demonstrates hypoperfusion [13,14]. This perfusion pattern helps distinguish tuberculomas from neoplastic lesions, where hyperperfusion is usually diffuse or central.

4. ASL in Meningeal Diseases:

ASL is valuable in meningeal diseases, showing hyperperfusion along the meninges in viral and tubercular meningitis. In viral meningitis, this reflects inflammation and increased vascular permeability [15]. These findings can be particularly valuable when conventional imaging is non-specific, aiding early diagnosis and management.

Fig 10: A 32-year-old patient presented with a fever, severe headache, and visual disturbances. The FLAIR image reveals subtle sulcal hyperintensity (Red arrow), and the post-contrast FLAIR images reveal leptomeningeal enhancement in the left occipital lobe and folia( Blue arrows). The ASL image shows hyperperfusion (Yellow arrow).

Fig 11: A 63-year-old woman with meningeal involvement due to unspecified viral infection. High intensity along the cerebral sulci is observed on fluid-attenuated inversion recovery (A: arrowheads). High perfusion is prominent in the affected leptomeningeal areas on arterial spin-labelling magnetic resonance imaging (B: arrowheads).

5. ASL in Immunocompromised Patients:

In immunocompromised patients, ASL helps differentiate CNS infections and tumours. Tuberculosis shows a hypoperfused core with a hyperperfused rim, toxoplasmosis shows hypoperfusion, and lymphoma demonstrates homogeneous hyperperfusion, reflecting its vascularity. These patterns aid accurate diagnosis when conventional imaging is inconclusive [16].

Fig 12: An immunocompromised patient with low CD4 counts and abdominal lymphoma presented with balance issues, raising suspicion of CNS involvement. Axial FLAIR images showed nodular hypointense lesions with surrounding oedema (red arrow). Post-contrast T1 fat-suppressed images revealed ring-enhancing lesions (blue arrow), and ASL images indicated significant hypoperfusion (yellow arrow). The biopsy turned out to be toxoplasmosis.

Advantages of ASL

1. Non-Invasive Nature: Does not require contrast agents, making it safer for patients with renal impairment or contrast allergies [1].
2. Quantitative Insights: Provides quantitative CBF data, enhancing diagnostic specificity [2].
3. Early Detection: Identifies perfusion abnormalities in early stages, even when conventional imaging is normal [3].
4. Differentiation of Pathologies: Differentiates infectious from neoplastic or vascular lesions, guiding appropriate treatment
   

   Fig 13: Role of ASL in Narrowing Differentials for Bilateral Thalamic Lesions: Conventional MRI (FLAIR) suggests a broad range of possibilities, including viral encephalitis and infarction, while ASL highlights hyperperfusion, pinpointing specific diagnoses like Bithalamic glioma and lymphoma