Atlas of Brain Perfusion SPECT

Stroke

TUTORIAL


Introduction

Characterized by acute onset of focal neurologic symptoms due to ischemic injury to the nerve cells, stroke is the most common neurologic disease, affecting 5% of the population over 65. Strokes can either be due to ischemic or hemorrhagic processes.

Ischemic strokes are caused by interruptions to the blood flow to any region in the brain either due to occlusion of blood flow or due to reduced perfusion. The extent of neurological deficit lies in proportion to the territory involved. Common causes of ischemic stroke are emboli, large or small artery disease states, and various systemic and hematologic disorders. Though stroke is usually seen in the elderly, in young adults ischemic stroke can occur due to substance abuse (cocaine, alcohol), HIV associated CNS conditions, cardiogenic emboli, hypercoagulable states, vasculitis and cancer, amongst other causes.

Transient ischemic attacks (TIA) are short-lived neurologic deficits lasting from minutes to hours, but not longer than a day. They usually arise due to ischemia in the carotid or the vertebrobasilar arterial distributions.

Hemorrhagic strokes are caused by intraparenchymal or subarachnoid hemorrhage. Chief underlying causes of intraparenchymal hemorrhage are chronic or severe hypertension and lipohyalinotic small vessel disease. Common causes for subarachnoid hemorrhage are rupture of berry aneurysm, trauma, coagulopathies, A-V malformations and vasculitis, amongst others.

Diagnostic imaging in stroke

CT scanning

CT scanning without contrast can virtually exclude the possibility of a hemorrhagic episode as the cause in focal stroke. It can detect lesions up to 0.5-1 cm in the supratentorial regions.
Pitfalls: CT (1) cannot detect most infarcts for at least the first 2 days, (2) is less reliable in the diagnosis of ischemic strokes, and (3) does not detect surface or brainstem lesions.

MRI

MRI is more useful than CT in the diagnosis of ischemic strokes. Surface and posterior fossa lesions can be appreciated, blood flow in many cranial arteries can be imaged, and lesions smaller than 0.5 cm can be detected. MRI can visualize infarcted areas within hours following the stroke.

SPECT

Brain SPECT is very useful in imaging acute strokes and can detect lesions hours to days before CT can. While CT is typically normal during the first hours to days after the ictus, regional CBF alterations occur instantaneously in patients with stroke. For example. 8 hrs post-infarction, 90% of SPECT rCBF scans emerge abnormal, while only 20% of CT scans show identifiable lesions. SPECT rCBF imaging is also useful to dilineate stroke subtypes. Since evolving therapeutic regimens are subtype-specific, brain SPECT helps in providing fast and accurate classification of the acute episodes.

MRI, however, also provides early assessment of perfusion, blood vessel anatomy, and other indices of functional status. SPECT appears better at dilineating ischemia in the first few hours after stroke.

The difference between functional and morphological imaging modalities disappears by about 72 hours after the episode. In addition, the sensitivity of SPECT is significantly reduced for lacunar infarctions.

Pitfalls: Although rCBF studies using SPECT are very useful for the diagnosis of stroke in the acute phase, its sensitivity decreases in the subacute period due to the phenomenon of luxury perfusion (described below), particularly with Tc-99m-HMPAO (exametazime).

Brain perfusion SPECT in stroke: some specific findings

Crossed cerebellar diaschisis frequently accompanies cortical strokes due to reduced stimulation of the cerebellum contralateral to the indicated cortex from cortico-pontine-cerebellar linkages. Reduced perfusion to the contralateral cerebellum commonly follows as a secondary phenomenon following cerebral ischemia, continuing even during luxury perfusion.

Luxury perfusion is a phenomenon that occurs due to decoupling of perfusion and metabolism during the period begining approximately 5 days after the ictus and continuing for as much as 20 days, contributing to false negative studies and a resultant decrease in the sensitivity of brain SPECT for the detection of stroke during the subacute phase. Perfusion is often normal or increased, particularly at the borders of the ischemic zone during the subacute phase of the stroke.

Radiotracers used for imaging

Iodine-123-IMP and Tc-99m HMPAO (Exametazime)

Vascular Dementias

References

  1. Masdeu JC, Brass LM, Holman BL, Kushner JM. Brain single-photon emission computed tomography (special review). Neurology 1994;44:1970-1977
  2. Holman BL, Moretti JL, Hill TC. SPECT perfusion imaging in cerebrovascular disease. Noninvasive imaging of cerebrovascular disease. 1989. pp 147-162.
  3. Holman BL, Hellman RS, Goldsmith SJ et al. Biodistribution, dosimetry, and clinical evaluation of technitium-99m ethyl cysteinate dimer in normal subjects and in patients with chronic cerebral infarction. J Nucl Med 30:1018-1024, 1989.
  4. Holman BL, Devous MD. Functional brain SPECT: The emergence of a powerful clinical method. J Nucl Med 1992; 33:1888-1904.
  5. Early and delayed brain SPECT with technitium-99m-ECD and iodine-123-IMP in subacute strokes. J Nucl Med 1994; 35:1444-1449.
  6. Harrison's Principles of Internal Medicine. 12th edition. McGraw-Hill. pp 1977-2002.
  7. Harrison's Principles of Internal Medicine (companion handbook). 12th edition. McGraw-Hill. pp 639-645.
  8. Medicine. 2nd ed. Harval publishing. pp 549-555.

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    Last updated: Aug 23, 1995 Redesigned June 1998