Diagnostic MRI Ch 5.6 - Ischaemic brain disease

Question 1

What does the term stroke refer to?

Answer 1

The clinical manifestation of cerebrovascular disease characterized by the acute onset of persistent neurologic deficits.

Stroke is defined as pathology or rupture of the blood vessels supplying the brain.

Question 2

What are the two main types of stroke?

Answer 2

Ischemic stroke and hemorrhagic stroke.

Ischemic stroke is due to obstruction of blood vessels, while hemorrhagic stroke results from vessel rupture.

Question 3

What is cerebral perfusion pressure (CPP)?

Answer 3

The difference between mean arterial pressure and intracranial pressure.

CPP determines cerebral perfusion, which is critical for brain function.

Question 4

How is cerebral blood flow (CBF) calculated?

Answer 4

CBF = CPP / CVR, where CVR is cerebrovascular resistance.

CBF is influenced by both CPP and the resistance of blood vessels in the brain.

Question 5

What happens to brain tissue when CBF falls below 15–20 mL/100 g/min?

Answer 5

Loss of cellular function occurs.

This threshold is critical for maintaining neuronal activity.

Question 6

What is cytotoxic edema?

Answer 6

Increased intracellular water due to sodium and calcium influx following cell membrane depolarization.

This occurs as a result of energy depletion in affected brain cells.

Question 7

What is the ischemic penumbra?

Answer 7

The area of ischemic tissue that is potentially salvageable with treatment.

It surrounds the core of irreversible infarction.

Question 8

What area has the highest susceptibility to ischemia?

Answer 8

Gray matter and areas with high metabolic activity.

Regions such as the occipital and parietal lobes are particularly vulnerable.

Question 9

What is the role of diffusion-weighted imaging (DWI) in stroke diagnosis?

Answer 9

DWI is used to detect cytotoxic edema and identify acute infarction.

It is more sensitive than conventional MRI techniques in the early stages of ischemia.

Question 10

What are the two main ways to measure brain perfusion using MRI?

Answer 10

Contrast-based techniques and non-contrast based techniques.

Contrast-based techniques include dynamic susceptibility contrast imaging, while non-contrast methods include arterial spin labeling.

Question 11

What does a reduction in the apparent diffusion coefficient (ADC) indicate?

Answer 11

It indicates reduced diffusion due to cytotoxic edema when CBF drops below critical levels.

ADC values can help estimate the age of ischemic infarcts.

Question 12

What classification scheme is used for strokes?

Answer 12

Strokes can be classified by clinical presentation, imaging results, underlying risk factors, and diagnostic tests.

Common classifications include hemorrhagic vs. ischemic and large vessel vs. small vessel strokes.

Question 13

What is the definition of relative cerebral blood flow (rCBF)?

Answer 13

The flow of blood within brain tissue measured relative to normal values.

Normal rCBF in gray matter is approximately 55–75 mL/100 g/min on CT.

Question 14

True or False: In small animals, naturally occurring atherosclerosis is common.

Answer 14

False.

Naturally occurring atherosclerosis is relatively rare in small animals.

Question 15

What is the main cause of large artery stroke in humans?

Answer 15

Thrombosis due to atherosclerosis.

This occurs either at the site of plaques or from embolism from upstream blood vessels.

Question 16

What does MTT stand for and what does it represent?

Answer 16

Mean transit time; it represents the average time for blood to pass through a given volume of brain tissue or voxel of interest.

MTT is calculated as CBV/CBF.

Question 17

What does CBV stand for and what is its approximate value in dogs?

Answer 17

CBV is the total blood volume within a voxel of interest, approximately 2.5–3.0 mL/100 g in dogs.

CBV stands for Cerebral Blood Volume.

Question 18

How does CBF vary in different brain regions of normal dogs?

Answer 18

CBF measured using MRI in normal dogs varies from approximately 145 to 280 mL/100 g/min, greater in gray matter compared to white matter.

CBF stands for Cerebral Blood Flow.

Question 19

What imaging techniques are recommended for diagnosing stroke in small animals?

Answer 19

A standard brain protocol including T2-FLAIR, T1W post-contrast, T2*W gradient echo or SWI, and DWI if available is recommended.

Question 20

What is a common cause of ischemic stroke in dogs?

Answer 20

In-situ thrombosis of cerebral vessels is presumed to be the most common cause of ischemic stroke in dogs.

Question 21

List some predisposing medical conditions for stroke in small animals.

Answer 21

• Atherosclerosis due to hypothyroidism or hyperlipoproteinemia
• Renal disease
• Sepsis
• Neoplasia
• Coagulopathy
• Endocrinopathy
• Fibrocartilaginous embolism
• Aortic or cardiac embolism
• Leishmania
• Dirofilaria
• Cuterebra
• Granulomatous meningoencephalitis.

Question 22

What is the expected MRI appearance of ischemic infarcts in dogs?

Answer 22

Ischemic infarcts typically appear as hyperintense on T2W and T2-FLAIR images, often solitary and sharply marginated.

Question 23

What is the most common site of territorial infarcts in dogs?

Answer 23

The cerebellum is the most common site of territorial infarcts in dogs.

Question 24

Describe the enhancement pattern seen in ischemic infarcts on MRI.

Answer 24

The enhancement pattern is variable, often showing no enhancement, especially in small infarcts.

Question 25

What imaging features are indicative of acute ischemic infarction?

Answer 25

* Reduced CBF * Increased MTT * Increased Tmax on PWI * Restricted diffusion on DWI.

Question 26

What does the term 'T2 shine-through' refer to in MRI?

Answer 26

T2 shine-through refers to high signal on DWI due to T2 effects, which may mimic restricted diffusion.

Question 27

What usually causes chronic infarcts to appear smaller and more sharply marginated on MRI?

Answer 27

Resolution of edema and resorption of necrotic tissue, typically complete within 6 weeks.

Question 28

What are lacunar infarcts and how do they differ from large territorial infarcts?

Answer 28

Lacunar infarcts are small infarcts deep within the brain caused by small perforating arteries, while large territorial infarcts involve larger arteries.

Question 29

What is a common clinical sign associated with striate artery infarcts?

Answer 29

Ipsilateral circling and contralateral hemiparesis/ataxia.

Question 30

Fill in the blank: The primary arterial supply to the brain in dogs is via the _______ and _______ arteries.

Answer 30

[internal carotid] and [basilar] arteries.

Question 31

What does MRA stand for and what is its limitation in diagnosing stroke?

Answer 31

MRA stands for Magnetic Resonance Angiography, and its limitation is the low sensitivity for detecting occlusion of small cerebral arteries.

Question 32

What are the clinical signs associated with ventrolateral lesions?

Answer 32

Circling and contralateral proprioceptive deficits.

Question 33

What MRI changes are suggestive of vascular pathology?

Answer 33

Cerebral microbleeds and white matter hyperintensities.

Question 34

What are microbleeds?

Answer 34

Small foci of hemosiderin-containing macrophages in normal brain parenchyma.

Question 35

How do microbleeds appear on MRI?

Answer 35

They appear as round, hypointense foci measuring ≤4 mm on T2*W gradient recalled echo images.

Question 36

What is the common clinical outcome of lacunar infarcts in people?

Answer 36

Most are clinically silent but increase the risk of dementia and cognitive dysfunction.

Question 37

What are the MRI features of lacunar infarcts?

Answer 37

Small focal lesions that are irregular or angular in shape, located within the territory of small vessels.

Question 38

What is typically absent or mild in lacunar infarcts?

Answer 38

Mass effect.

Question 39

What can be a characteristic appearance of multiple infarcts?

Answer 39

Suggestive of an embolic cause.

Question 40

What are potential differential diagnoses for arterial infarcts?

Answer 40

Cytotoxic edema due to other disease processes.

Question 41

What percentage of observers may mistake infarcts for gliomas?

Answer 41

Up to 47%.

Question 42

What is a common symptom of cerebral venous thrombosis in people?

Answer 42

Headache.

Question 43

What is the primary underlying cause of cavernous sinus syndrome in dogs?

Answer 43

Neoplasia involving that sinus.

Question 44

What are the typical MRI features of cerebral venous thrombosis?

Answer 44

Abnormal signal within a sinus and corresponding absence of flow on MRV.

Question 45

What causes hypertensive encephalopathy?

Answer 45

Neurologic dysfunction secondary to systemic hypertension.

Question 46

What MRI findings are reported with hypertensive encephalopathy?

Answer 46

T2 hyperintensities within the white matter due to vasogenic edema.

Question 47

What types of vascular anomalies involve the brain?

Answer 47

* Absence or hypoplasia of normal vessels * Persistent transient embryonic vessels * Abnormal morphology of vessels * Arteriovenous malformations * Dural arteriovenous fistulas * Aneurysms * Cavernous malformations

Question 48

What is a common MRI finding in global brain ischemia?

Answer 48

T2W and T2-FLAIR hyperintensity within the cortical gray matter.

Question 49

What is a transient ischemic attack (TIA)?

Answer 49

Brief episodes of neurologic dysfunction resulting from focal cerebral ischemia not associated with permanent cerebral infarction.

Question 50

What is the diagnostic approach for TIAs?

Answer 50

Demonstration of absence of infarction.

Question 51

What can cause global brain ischemia?

Answer 51

Transient period of complete ischemia of the whole brain, followed by reperfusion.

Question 52

What may indicate the presence of cerebral venous thrombosis?

Answer 52

Demonstrating the presence of thrombi within the cerebral veins/sinuses.

Question 53

What is a characteristic feature of venous infarction MRI findings?

Answer 53

Cortical/subcortical location not following arterial vascular territories.

Question 54

What can be a consequence of hypertensive encephalopathy?

Answer 54

Vasogenic and interstitial edema due to failure of auto-regulation of the cerebral vasculature.

Question 55

What are vascular abnormalities that are hemodynamically compensated likely to be?

Answer 55

Clinically silent and may be found incidentally on MRI ## Footnote This means they do not show symptoms but can be detected through imaging.

Question 56

In most cases of congenital arteriovenous vascular malformation, what are the MRI changes associated with?

Answer 56

Secondary brain hemorrhage ## Footnote This is especially true except for capillary telangiectasis.

Question 57

What is required for a definitive diagnosis and differentiation of vascular malformations?

Answer 57

Angiography ## Footnote This procedure helps in detecting arteriovenous malformations, dural arteriovenous fistulas, and aneurysms.

Question 58

What risk is associated with arteriovenous malformations (AVMs) and similar conditions?

Answer 58

Risk of recurrent intracranial hemorrhage ## Footnote These conditions may also be amenable to treatment.

Question 59

What components do arteriovenous malformations (AVMs) of the brain comprise?

Answer 59

Feeding arteries, draining veins, and a nidus of abnormal vessels ## Footnote The nidus functions as an arteriovenous shunt.

Question 60

How do most symptomatic AVMs present in people?

Answer 60

As spontaneous intraparenchymal hemorrhage ## Footnote They account for 15% of people presenting with spontaneous bleeds.

Question 61

What diagnostic method is typically used to confirm AVMs?

Answer 61

Intra-arterial digital subtraction angiography ## Footnote This method demonstrates the tangle of dilated arteries and veins.

Question 62

What indirect MRI features suggestive of AVMs in people include?

Answer 62

* Flow voids on T1W and T2W images * Hemorrhagic foci * Perilesional T2 hyperintensity * Lack of a mass effect but localized cerebral distortion ## Footnote These features help in the identification of AVMs.

Question 63

What are cavernous malformations characterized by?

Answer 63

Closely packed, thin-walled vessels without normal interposed brain parenchyma ## Footnote They can occur anywhere in the brain but are most commonly found in the subcortical white matter.

Question 64

What is the MRI appearance of cavernous malformations?

Answer 64

Focal hemorrhages without associated edema ## Footnote They typically have a complete rim of hemosiderin.

Question 65

Has the MRI appearance of confirmed AVMs and other vascular anomalies in small animals been reported?

Answer 65

No ## Footnote This indicates a gap in the existing literature regarding imaging in small animals.

Question 66

What MRI sequences are used to estimate age of ischaemic infarcts?

Answer 66

ADC ## Footnote ADC values continue to drop for several days after the onset of stroke, and remain reduced for 4–5 days before pseudo-normalizing between 4 and 10 days.

Question 67

How quickly will changes on DWI be evident after a reduction in CBF?

Answer 67

Within minutes

Question 68

Describe the main ways to classify stroke

Answer 68

* Hemorrhagic versus ischemic stroke. * Arterial versus venous stroke. * The vessel(s) affected. * Type of stroke: Large vessel, small vessel, cardioembolic, other causes, cryptogenic

Question 69

Define ischaemic penumbra

Answer 69

The area of ischemic tissue within the lesion that is potentially salvageable with treatment

Question 70

What is ADC useful for?

Answer 70

Shows low signal confirming restricted diffusion and excluding T2 shine-through effects.

Question 71

What are the two main ways that relative brain perfusion can be measured on MRI?

Answer 71

1. Contrast-based techniques, such as T2 star echo 2. Non-contrast based techniques, such as arterial spin labelling ## Footnote Signal loss (hypointensity) may be evident in blocked arteries and parenchyma

Question 72

In how many veterinary cases is an underlying cause of stroke identified?

Answer 72

50% ## Footnote The most common predisposing factors are renal disease and hyperadrenocorticism

Question 73

Describe the concept of restricted diffusion

Answer 73

On DWI in the acute stages of infarction, there is restricted diffusion, which appears hyperintense on the DWI and of low signal on ADC maps. As DWI images usually have some T2-weighting, lesions that are hyperintense on T2W images may also appear hyper- intense on DWI; this high signal on DWI due to T2 effects is known as ‘T2 shine-through’, and may mimic restricted diffusion. This underscores the importance of a comparison with the ADC map to prove restricted diffusion.

Question 74

List the main feastures of ischaemic arterial infarcts

Answer 74

* Most commonly solitary * Hyperintense of T2W and T2-FLAIR (unless haemorrhagic transformation * Often sharply marginated * Confined to specific vascular areas * Cerebellum most common site * Preferentially affecting gray matter * Only mild mass effect * Often no enhancement initially * Contrast enhancement may occur at 1-8w * Restricted diffusion on DWI in acute phases (low ADC signal) * Occlusion of supplying artery on MRA

Question 75

Describe the four lacunar syndromes in dogs

Answer 75

* Striate arteries: ipsilateral circling, head and neck turn, contralateral menace deficit with normal pupillary light reflex, contralateral postural reaction deficit, contralateral hemiparesis/ataxia. * Paramedial lesions: signs of vestibular dysfunction. * Extensive dorsal lesion: vestibular ataxia, circling, and contralateral menace response deficit. * Ventrolateral lesions: circling and contralateral proprioceptive deficits.

Question 76

Describe the MRI features of lacunar infarcts

Answer 76

* Small focal lesions that are irregular or angular in shape * Located within territory of small vessels (thalamus, basal ganglia, internal capsule, brainstem). * Mass effect usually absent or mild. * Hyperintense on T2W images. * Commonly solitary but may be multiple. Multiple infarcts in differing vascular territories are suggestive of an embolic cause. * Usually contrast enhancement is absent or mild. * DWI pulse sequences may show restricted diffusion in the early stages, but due to small size lesions may not be visible * In chronic stages, cavitation and loss of volume are common * Hemorrhagic transformation is uncommon.

Question 77

Outline features that may help differentiate infarcts from gliomas

Answer 77

- Lesion location (gliomas are not confined to vascular territories). – Size (gliomas tend to be larger). – Mass effect (more pronounced with gliomas). – Perilesional edema (more pronounced with gliomas). – Differences on DWI images with gliomas being more often bright on ADC maps while infarcts are more often dark. – Wedge-shaped lesions are more commonly seen with infarcts.

Question 78

Define TIA

Answer 78

Transient ischemic attacks (TIAs) are brief episodes of neurologic dysfunction resulting from focal cerebral ischemia not associated with permanent cerebral infarction