Stroke (Suspected)

Imaging in the setting of suspected stroke serves a number of purposes 

1. To distinguish between haemorrhagic and ischaemic stroke
2. To determine the vascular territory of the stroke and location and extent of intravascular clot
3. To determine the presence and extent of “infarct core” and “ischaemic penumbra”
4. To determine the aetiology of the stroke
5. To identify alternative causes of clinical symptoms

• Activation of the stroke emergency response team.
• Should be activated as soon as clinical suspicion of stroke arises (typically  within 6 hours of symptom onset)
• People with suspected stroke within 24 hours of symptom onset should be imaged as rapidly as possible to assess if mechanical thrombectomy is indicated/likely to be beneficial.

• Ischaemic stroke is a medical emergency and requires immediate assessment for determining eligibility for thrombolysis/thrombectomy  1, 2
• The role of unenhanced CT in the initial setting is to exclude intracranial haemorrhage, thus assessing for eligibility for intravenous thrombolysis 2, 3 . Unenhanced CT has excellent sensitivity for acute haemorrhage 4 .
• Additional helpful findings include evidence of cytotoxic oedema (established infarction), dense vessel sign (thrombus), or evidence of a stroke mimic.
• The majority of unenhanced CTs in acute stroke will demonstrate no abnormality 5 . 
• Unenhanced CT can also be used to calculate the Alberta Stroke Program Early CT Score (ASPECTS) which is used to quantify the volume of ischaemia in the middle cerebral artery territory 6 and pc-ASPECTS for the posterior circulation (the latter of which is more sensitive and specific when assessed on the CTA source images) 7 . 
• Disadvantages include lower sensitivity than MRI for acute ischaemia  4 , and use of ionising radiation. 
• On table review of the above findings is recommended prior to proceeding immediately with the next imaging steps.

• Ischaemic stroke is a medical emergency and requires immediate assessment for determining eligibility for thrombolysis/thrombectomy  1, 2
• The role of unenhanced CT in the initial setting is to exclude intracranial haemorrhage, thus assessing for eligibility for intravenous thrombolysis 2, 3 . Unenhanced CT has excellent sensitivity for acute haemorrhage 4 .
• Additional helpful findings include evidence of cytotoxic oedema (established infarction), dense vessel sign (thrombus), or evidence of a stroke mimic.
• The majority of unenhanced CTs in acute stroke will demonstrate no abnormality 5 . 
• Unenhanced CT can also be used to calculate the Alberta Stroke Program Early CT Score (ASPECTS) which is used to quantify the volume of ischaemia in the middle cerebral artery territory 6 and pc-ASPECTS for the posterior circulation (the latter of which is more sensitive and specific when assessed on the CTA source images) 7 . 
• Disadvantages include lower sensitivity than MRI for acute ischaemia  4 , and use of ionising radiation. 
• On table review of the above findings is recommended prior to proceeding immediately with the next imaging steps.

• CECT Head (Post contrast CT) is the first investigation modality when a stroke mimic is suspected to be the cause of clinical stroke like presentation
• Useful to rule out the following conditions
  • Arteriovenous Malformation (AVM)
  • Intracranial Abscess
  • Intracranial Tumours
• No added advantage to NCCT in Ischaemic Stroke
• Advantages:
  • Widely available
  • Less expensive than MRI
  • Can be used for patients not suitable for MRI
• Disadvantages:
  • Ionizing radiation – this may be of significance in younger patients
  • Less sensitive than MRI
  • Not suitable for patients allergic to iodine-based contrast and in renal impairment

Acute endovascular therapy: Mechanical Thrombectomy or IV thrombolysis

• Thrombectomy should be offered within 6 hours if there is occlusion of the proximal anterior circulation on CTA or MRA 1,2,30.
• Thrombectomy should be offered as soon as possible for stroke 6-24 hours (including wake up stroke) if there is a proximal occlusion and potential to salvage brain tissue (either anterior or posterior circulation) 2,8 and in some guidelines, mechanical thrombectomy may be considered beyond 24 hours (8).
• There is high level evidence for the use of intravenous thrombolysis (with either Alteplase or Tenecteplase) for eligible patients within 4.5hours of symptom onset 1.
• A patient’s clinical status (including comorbidities and pre-morbid function) and extent of the infarct should be considered in decision making, as well as the patient’s or family’s wishes 8,30.
• Neither mechanical thrombectomy nor intravenous thrombolysis should delay the other treatment, and both can be performed concurrently 8,9,31.

A term given to clinical features of non-vascular conditions that resemble a stroke like manifestation. Conditions include:

• Post-ictal State
• Intra-cranial Tumour
• Systemic Infection
• Toxic Metabolic conditions
• Demyelinating Disease

Embolic stroke of uncertain source necessitates carotid artery review, ECG monitoring 8,32, and consideration for echocardiography8,33.

Imaging with unenhanced CT should be performed within 24 hours to guide management and secondary prevention 30.

• The purpose of vascular imaging in the setting of stroke is to assess for vascular occlusions as targets for thrombectomy and to assess proximal access 8, 2, 9, 10
• In addition, the carotid arteries can be assessed for potential revascularisation in the setting of carotid territory symptoms 8. 
• Vascular imaging can be performed with CT angiogram (CTA) or MR angiogram (MRA) and the decision to image with CT or MRI is dependent on the most immediately available 1. Carotid doppler is also reasonable to assess carotid arteries in the setting of carotid territory symptoms 8.
• Note CT angiogram should not be delayed by concerns for impaired renal function8, 11.
• CT angiogram should not delay the decision to start thrombolysis 2.

CT Perfusion (CTP)

• The goal of CTP is to assess the ratio of infarct core (irreversibly damaged brain tissue) to penumbra, thus identifying the “tissue at risk”. CTP should not delay the treatment if eligible for Thrombolysis +/- Endovascular Treatment
• Useful in evaluation of tissue viability (ischaemic core versus penumbra)
• CTP can be performed on any standard helical CT Scanner with a bolus tracking technique in which a contrast agent is injected rapidly (5-7 cc/sec) into a peripheral vein and images of the brain are acquired repeatedly as the contrast agent passes through the brain. Images are then converted to contrast agent concentration versus time curves 12 .
• CT perfusion is not required in hyperacute stroke (<6 hrs) and should not delay thrombolysis or mechanical thrombectomy 2  but may be beneficial in cases of diagnostic uncertainty or advanced decision making.
• In the case of suspected stroke >6 hours after symptom onset, perfusion imaging may be helpful in decision making surrounding mechanical thrombectomy 13-15 . The DAWN and DEFUSE 3 trial have significantly changed the management of stroke due to large vessel occlusions in this extended time window 14, 15
• Benefits:
• High sensitivity of 80% and very high specificity of 95% for detecting infarcts 12
• Quick to perform (1 minute or less) 16
• If performed in hyperacute stroke, can reduce numbers of patients who require thrombolysis without affecting overall outcome 17, 18 .
• Disadvantages:
  • Can over or underestimate size of the core infarct 19-21 .
  • Correct interpretation requires appropriate training 22 .
  • Less accurate than DWI MRI for acute ischaemia 23

MR Perfusion and Diffusion imaging

• MRI with DWI is more sensitive to small infarctions. Penumbra can be inferred by perfusion-diffusion mismatch or clinical-diffusion mismatch.
• Penumbra can be inferred on MRI either by diffusion-perfusion mismatch or a mismatch between the volume of infarct on DWI and the clinical deficit 15 .
• Benefits: 
  • MRI DWI is more accurate for ischaemia than CT perfusion (accuracy 0.99 and negative predictive value of 0.98 compared to CT perfusion which is 0.81 and 0.74 respectively) 23 .
  • Can predict haemorrhagic transformation of ischaemic stroke, however it is not clear that this has a clinical benefit 24 .
  • Helpful for specific clinical questions such as confirming diagnosis, stroke localisation and prognosis 25 .
• Disadvantages: 
  • May not benefit clinical outcome compared to CT 26 However, MRI can be helpful for specific clinical questions. 
  • Can be time consuming 13 .
  • Difficult to establishing a time effective workflow 13, 27, 28 , although is possible 29 .