MRI Diagnostic Tools in Acute Ischemia
Magnetic resonance imaging plays a crucial role in detecting infarctions and evaluating the vascular bed. Specialized pulse sequences are used for comprehensive analysis:
- FLAIR (fluid-attenuated inversion recovery) provides clear visualization of hyperintense ischemic lesion areas.
- DWI (diffusion-weighted imaging) demonstrates high signal intensity in damaged tissues, reliably confirming the acute nature of the infarction.
- TOF MRA (time-of-flight MR angiography) is used to assess vessel lumens without contrast administration. Using maximum intensity projection (MIP), one can rapidly detect occlusion of major branches, such as the M1 segment of the middle cerebral artery.
For detailed evaluation of intracranial stenoses, which frequently accompany intracranial vasculitis, targeted projections are employed — Subvolume MIP.
Search for the Source of Cerebral Embolism
The topography of the infarction directly points to the search zone for the embolic source. In lesions of the middle cerebral artery (MCA) territory, thrombotic masses most commonly detach from the proximal segments of the ipsilateral internal carotid artery, specifically from the region of the carotid bulb (carotid bulb).
If the infarction is localized in the vertebrobasilar system, the source should be sought in the vertebral arteries. An important anatomical feature is that both vertebral arteries merge into a single basilar artery. Consequently, an ulcerated atherosclerotic plaque in one vertebral artery can dispatch emboli into the branches of the basilar artery on both sides, causing both ipsilateral and contralateral lesions.
The main advantage of modern CT angiography (CTA) and MR angiography (MRA) lies in the ability to evaluate all vessels in the scan field simultaneously. If the acute stroke scan is extended caudally to the aortic arch, all major head and neck arteries can be examined in a single procedure without selective catheterization.
In situations where the carotid arteries, vertebral arteries, and aortic arch are intact, screening for a cardioembolic substrate is indicated. Using echocardiography (EchoCG) or cardiac MRI, the following are ruled out:
- Mural thrombi in the left ventricle.
- Vegetations on the mitral and aortic valves.
- Thrombi in the left atrial appendage.
The risk of paradoxical embolism must also be considered, in which a thrombus from the right side of the venous system enters the arterial tree through a patent foramen ovale (patent foramen ovale).
Subclavian Steal Syndrome
Subclavian steal syndrome (Subclavian Steal Syndrome) develops due to a hemodynamically significant stenosis or complete occlusion of the proximal segment of the subclavian artery.
Under physical exertion of the affected upper extremity, vascular resistance in the limb drops. To compensate for the blood flow deficit, blood begins to flow in the reverse (retrograde) direction through the ipsilateral vertebral artery. The basilar artery then functions as a shunt, rerouting blood from the posterior cranial fossa to the arm, leading to ischemia of brain structures.
On CT angiography or post-contrast MRA (CEMRA), structural occlusion of the subclavian artery is easily diagnosed. However, assessing the direction of blood flow requires an understanding of MR signal physics. On routine TOF MRA, the left vertebral artery with retrograde blood flow may appear completely closed (pseudo-occlusion).
Nature of the Artifact: In TOF sequences, a superior saturation pulse is used to suppress venous signals by nulling any craniocaudal (top-to-bottom) blood flow. Because arterial blood also flows top-to-bottom during a steal phenomenon, the pulse "erases" its signal from the image. To accurately assess retrograde flow, flow-sensitive techniques or ultra-fast time-resolved MRA are used.
Basilar Artery Occlusion and Venous Thrombosis
Basilar artery embolism is a life-threatening condition that frequently presents with acute vomiting and quadriplegia. On FLAIR MRI, extensive infarctions are visualized in the pons and cerebellum. MR angiography detects a sudden cutoff of the distal vertebral arteries, and multiplanar reconstructions clearly demonstrate a filling defect (embolus) within the lumen of the basilar artery.
When cerebral venous thrombosis is suspected (e.g., of the superior sagittal sinus), the gold standard is volumetric contrast-enhanced MR venography (MRV). Sagittal and coronal reconstructions reveal a massive filling defect in the sinus.
An important diagnostic nuance: when generating phase-contrast venography using maximum intensity projection (MIP), the thrombus itself may be obscured due to mathematical signal-averaging algorithms, even though an extensive network of venous collaterals remains visible.
Peripheral vascular diseases
Angiographic imaging methods are also widely used to evaluate peripheral blood flow. Perfusion restriction due to stenosis causes ischemic symptoms, the nature of which depends on the level of the lesion:
- Intermittent Claudication: muscle pain upon walking, typical of proximal arteriopathies.
- Rest Pain: a sign of severe tissue ischemia, usually associated with distal stenotic levels in the arteries of the foot and lower leg.
Without adequate treatment, ischemia rapidly progresses, leading to trophic ulcers and gangrene.
Briefly: answers to topical questions
Which dural venous sinuses are most frequently affected by cerebral venous thrombosis?
The sources describe thrombosis of the superior sagittal, transverse, and sigmoid sinuses. For the superior sagittal sinus, an extensive filling defect is noted, while hyperintensity of the transverse and sigmoid sinuses confirms their thrombosis.
What branches arise from the basilar artery?
The basilar artery gives rise to two pairs of cerebellar arteries, the posterior cerebral arteries, and perforating branches to the brainstem.
- Anterior inferior cerebellar artery (anterior inferior cerebellar artery, AICA) — the first major branch, supplies the flocculus and the anterior part of the cerebellar hemisphere, gives off the labyrinthine artery
- Superior cerebellar artery (superior cerebellar artery, SCA) — arises just below the bifurcation of the basilar artery, supplies the rostral part of the cerebellar hemisphere
- Posterior cerebral arteries (posterior cerebral arteries, PCA) — terminal branches of the basilar artery
- Perforating branches — paramedian, short, and long circumferential branches to the brainstem
For quick memory retention of sources: "Anterior circulation (MCA) — Check the carotid bulb; Posterior circulation (vertebrobasilar) — Look to the vertebral arteries."
Class and exam questions
Why is a thrombus in the sinus lumen sometimes not visible on phase-contrast MR venography (MIP)?
This is related to the signal-averaging effect applied by the mathematical algorithm during the creation of the maximum intensity projection (MIP). The filling defect itself becomes "blurred," although the collateral venous network is displayed clearly.
What is the main advantage of CT and MR angiography over conventional arteriography?
Cross-sectional imaging methods (CTA and MRA) allow the evaluation of all vessels within the scan zone simultaneously. Traditional angiography requires more complex and invasive selective catheterization of each individual artery.
What is paradoxical embolism?
This is a pathological process in which thrombotic masses from the right side of the circulatory system (e.g., from lower extremity veins) enter the left side of the heart and the arterial tree through a patent foramen ovale, causing a cerebral infarction.
What else to study on this topic
- Physical principles of fluid signal suppression in the FLAIR pulse sequence
- The role of diffusion-weighted imaging (DWI) and ADC maps in determining the age of an infarction
- Method of applying saturation pulses in time-of-flight (TOF) angiography
- X-ray endovascular embolic protection devices during stenting
- Specifics of performing ultrafast time-resolved contrast-enhanced MR angiography
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