GS - Imaging of Stroke

Question 1

Where does the word stroke come from?

Answer 1

Biblical - “One is stuck as if hit by the hand of God”

• Also sometimes called cerebrovascular accident or CVA

Question 2

How many people in the UK have a stroke each year?

Answer 2

150,000 people have a stroke in the UK each year. About 10,000 under retirement age

Over 67,000 deaths from stroke each year

3rd most common cause of death in England and Wales

Question 3

What is an acute stroke?

Answer 3

Stroke is the clinical term for acute loss of perfusion to vascular territory of the brain, resulting in schema and a corresponding loss of neurological function. Classified as either haemorrhagic or ischemic, strokes typically manifest with sudden onset of focal neurologic deficits, such as weakness, sensory deficit, or difficulties with language.

Question 4

What is a Haemorrhagic stroke ?

Answer 4

Also knows as intracerebral haemorrhage, it accounts for 10 - 15% of all strokes and is associated with higher mortality rates than ischaemic infarctions. Patients will present with similar focal neurological defects but tend to be more ill than ischaemic stroke patients.

Patients with intracerebral bleeds are more likely to
have headache, altered mental status, seizures, nausea and vomiting, and/or marked
hypertension; however, none of these clinical findings distinguish reliably between
hemorrhagic and ischemic strokes.

A CT scan typically makes the diagnosis:
- acute blood is bright white on CT, and
Often has mass effect With distortion Of the normal Structures. A dark grey halo often surrounds
the haemorrhage — this is oedema and Often exacerbates the degree Of mass effect. This combined
effect may be so severe that intracranial pressure is raised to be fatal.

Question 5

What are Ischaemic Strokes ?

Answer 5

Ischemic strokes have a heterogeneous group of causes, including thrombosis,
embolism, and hypoperfusion.

Thrombotic strokes are generally the result of rupture of atherosclerotic plaque in the intracranial arteries, activating platelet adhesion and the
clotting cascade to form occlusive thrombus. Emboli typically originate from thrombus in the heart or break off from atherosclerotic plaques in the carotid arteries, lodging in and
occluding intracranial arteries.

Question 6

What is the Pathophysiology of Strokes?

Answer 6

The brain is the most metabolically active organ in the body. While representing only 2% of the body’s mass, it requires 15-20% Of the total resting cardiac output to provide the
necessary glucose and oxygen for its metabolism.

The processes involved in stroke injury at the cellular level are referred to as the ischaemic cascade. Within seconds to minutes of the loss of glucose and oxygen delivery to neurons, the cellular ischemic cascade begins and neurons cease to function. This is a complex process that begins with cessation of the normal electrophysiological function of the cells. The resultant neuronal and glial injury produces edema in the ensuing hours to days after stroke,
causing further injury to the surrounding tissues.

Question 7

What Clinical presentations would an anterior cerebral artery stroke show?

Answer 7

Anterior cerebral artery occlusions primarily affect frontal lobe function and can result in;

• Disinhibition and speech perseveration producing primitive reflexes (eg. grasping,
  sucking reflexes)
• Altered mental status
• Impaired judgment
• Contralateral weakness (greater in legs than arms)
• Contralateral cortical sensory deficits gait apraxia,
• Urinary incontinence.

This is the rarest type of stroke

Question 8

What Clinical presentations would an middle cerebral artery stroke show?

Answer 8

Middle cerebral artery occlusions commonly produce;

• Contralateral hemiparesis,
• Contralateral hyperaesthesia
• Ipsilateral hemianopia (blindness in one half Of the visual field)
• Gaze preference toward the side of the lesion
• Agnosia
• Receptive or expressive aphasia may result if the lesion occurs in the dominant hemisphere
• Neglect and inattention may occur non-dominant hemisphere lesions.

Since the MCA supplies the upper extremity motor strip. weakness of the arm and face is usually worse than that Of the lower limb.

This is the most cornmon type Of stroke

Question 9

What Clinical presentations would an posterior cerebral artery stroke show?

Answer 9

Posterior cerebral artery occlusions affect;

• Vision and thought
• Producing contralateral
  homonymous hemianopia
• Cortical blindness
• Visual agnosia
• Altered mental status
• Impaired memory

Question 10

What is the Ischaemic penumbra?

Answer 10

Unlike muscle, brain tissue is exquisitely sensitive to because of the absence
of neuronal energy stores. In the complete absence of blood flow, the available energy
can maintain neuronal viability for approximately 2—3 minutes. However, in acute stroke,
ischemia is more often incomplete, with the injured area of the brain receiving a
collateral blood supply from uninjured arterial and leptomeningeal territories. Therefore,
acute cerebral ischemia may result in a central irreversibly infarcted tissue core
surrounded by a peripheral region of stunned cells with a reduced blood supply called the penumbra.

Potentials in the peripheral region are abnormal and the cells have ceased to function, but this region is potentially salvageable with early
revascularization

Without early recanalization, the infarction gradually expands to
include the penumbra.

Question 11

What is a non-enhanced CT?

Answer 11

Imaging
Unenhanced CT is widely available and can be performed quickly — a key factor in the
emergency scenario. Its key role is the detection of hemorrhage or other possible mimics
Of stroke (eg, neoplasm, arteriovenous malformation) that could be the cause Of the
neurologic deficit. The second role of nonenhanced CT is the detection of ischemic signs
of established infarction. The main CT finding is an abnormal cortical-subconcal area
within a vascular territory — be wamed this may be very subtle! This area is typically
darker (termed HYPOattenuation) than normal brain due to oedema.
Figure 4 Typical appearance of an established right MCA territory stroke. The abnormal dark area
of oedematous brain involves both the white matter AND overlying cortex. Note the swelling — the
right Sided Suki are completely effaced — compare With normal sulCi on the

Question 12

What is a non-enhanced CT?

Answer 12

Unenhanced CT is widely available and can be performed quickly — a key factor in the emergency scenario. Its key role is the detection of hemorrhage or other possible mimics of stroke (eg, neoplasm, arteriovenous malformation) that could be the cause Of the neurologic deficit.

The second role of nonenhanced CT is the detection of ischemic signs
of established infarction. The main CT finding is an abnormal cortical-subconcal area
within a vascular territory — be warned this may be very subtle! This area is typically darker (termed HYPOattenuation) than normal brain due to oedema.

Question 13

What can you see in this CT?

Answer 13

Typical appearance of an established right MCA territory stroke. The abnormal dark area of oedematous brain involves both the white matter and overlying cortex. Note the swelling - the right sided sulci are completely effaced - compare with normal sulci on left

Question 14

Why is it important to pay careful attention to the extend of the hypo attenuating area?

Answer 14

Careful attention to the extent of the hypoattenuating area is crucial: the presence of
established hypoattenuation affecting more than one-third Of the MCA territory is a
contraindication for revascularization because of hemorrhagic complications.

(The CT attenuation value is defined as the radiodensity of each material)

It is, however, well-known that nonenhanced CT has a relatively low sensitivity the first 24
hours, especially within the limited (3—6-hour) time window for thrombolytic treatment.

Although the scan may not show an obvious abnormality, subtle signs that may aid stroke detection include

Question 15

What is abnormal about this scan?

Answer 15

Hyperdense vessel sign - high density thrombus occluding middle cerebral artery

Question 16

What is abnormal about this scan?

Answer 16

Inability to visualise the left lentifom (white arrows) due to cytotoxic oedema - this may be seen within the first 2 hours or onset or symptoms.

The normal right lentilorm nucleus is clearly seen in the red circle.

Question 17

How does MRI work?

Answer 17

Rather than using x-rays, MRI works by inputting pulses Of energy into the body whilst it lies in a very strong magnetic field.

After each pulse the scanner “listens” as the energy
is re-emitted by different tissues at different rates. The types of pulses can be altered in
a multitude of ways in order to detect different pathological findings. A single MRI
examination may use several different types of sequence, the most commonly used
include:

• T1 weighted image (best for anatomy)
• T2 weighted - fluid (e.g CSF) is bright white; oedema if present will also be bright

Question 18

What is abnormal about this scan?

Answer 18

FLAIR (fluid attenuated) - by signal manipulation, free fluid (ie CSF) is
dark whilst oedema may remain bright (note slightly increased signal left parietal lobe)

Question 19

This image is a gradient echo, what is this good at showing?

Answer 19

Gradient echo - makes blood products Very dark

Question 20

What is Diffusion-weighted MR Imaging ?

Answer 20

Diffusion-weighted MR Imaging;

Although conventional MR imaging more sensitive than CT in the first few hours after a stroke, it may still result in false-negative findings.

Newer MRI sequences such as
diffusion weighted imaging (DWI) have been proven to be exquisitely sensitive to
hyperacute strokes so that conventional MR imaging sequences now play only a
relatively minor role in acute stroke imaging, mainly excluding bleeds and tumours.

Underlying principles — The normal motion Of water molecules within living tissues is
random (Brownian motion). In acute stroke there is an alteration of the homeostasis
which normally maintains steady-state proportions of intracellular and extracellular water.

Acute stroke causes excess intracellular water accumulation, or cytotoxic edema. with an
overall decreased rate Of extracellular diffusion Within the affected tissue. On diffusion-
weighted images from patients With hyper-acute stroke, ischemic tissue appears brÉhtin
comparison With normal brain tissue.

Question 21

What are the features of conventional MRI?

Answer 21

Conventional MR imaging more sensitive and more specific than CT for the detection of acute cerebral ischemia

Within the first few hours after the onset of a stroke. It has the
additional benefit of depicting the pathologic entity (stroke and its mimics) in multiple planes.

Typical MR imaging findings in patients with hyperacute cerebral ischemia
include hyperintense (hyperintense = brighter/ whiter than normal) signal in white matter
on T2-weighted images and fluid-attenuated inversion recovery images, with a resultant loss of gray matter—white matter differentiation on T1 weighted imaging analogous to the loss at CT

Question 22

When can you see restricted diffusion ?

Answer 22

Restricted diffusion is typically seen within 30 minutes of onset of symptoms. In contrast
to unenhanced CT or conventional MR imaging, which have low sensitivities (<50%) for
acute ischemia detection, diffusion-weighted imaging was reported to have both high sensitivity and specificity and respectively).

Question 23

What does this MRI show?

Answer 23

Acute stroke of the posterior circulation in a man.

Diffusion weighted MR
image shows bilateral areas of increased Signal intensity (arrows) in the thalamic and occipital lobes

Question 24

Why do we use CT and MRI?

Answer 24

In summary, MRI is more sensitive than CT in the detection Of acute stroke. However, CT is faster, readily available at any acute hospital 24 hours a day and relatively easy to
interpret for any radiologist/ neurologist.

MRI a more specialized investigation that is
usually available only during daytime hours — even in major teaching hospitals.

For this
reason CT is currently the initial investigation of choice whilst MRI serves a more
problem solving role.