CNS Scintigraphy

Breakdown of BBB detected with Tc-99m-DTPA (diethylenetriaminepentaacetic acid) or Tc-99m-GH (glucoheptonate). Flow images obtained every 3sec for 60sec anterior to detect blood pool abnormality. Static images anterior, posterior, lateral and vertex. Increased or asymetric activity on static images indicates breakdown of the BBB (infarction, tumour, infection). Lenticular photo-enhanced (or occasionally photopenic) rim from subdural haematoma. Assymmetric radionuclide angiogram indicates occlusive disease. Flip-flop sign – reduced arterial activity and increased venous from carotid occlusion. Increased flow from vascular malformation, high-grade or vascular tumours (GBM, meningioma), inflammation. Absence of any brain activity in presence of carotid and scalp flow indicates brain death.

Cisternogram – Injection of intrathecal In-111-DTPA via spinal tap. Initial images to ensure intrathecal injection. Subsequent images at 4-6hrs and 24hrs ± 48 and 72hrs. Normally activity seen in basal cistern at 4hrs and convexity by 24hrs.

  • Normal pressure hydrocephalus – Early activity in lateral ventricles (abnormal) persisting beyond 24hrs, delayed clearance over convexities.
  • CSF leak – High sensitivity. Imaging at 1-3hrs, 24hrs ± 48hrs. Lateral views for rhinorrhea and anterior for otorrhea. Cotton pledgets in nostrils, with well counter detecting activity at 4-6hrs; pledget activity >1.5x serum sample indicates CSF rhinorrhea.

CSF shunt and reservoir study – Injection of small volume of tracer into the shunt, with dynamic imaging for 10min in supine position. Abdominal image in VP shunts. Half-life of reservior clearance should be <8min. If opening pressure is low blockage is proximal; if high (>20 cm H2O) blockage is distal. Partial obstruction allows a small amount of radionuclie clearance. Peritoneal loculations causes stagnation of activity, with absent diffusion in the peritoneum. Normal ventricular reflux excludes proximal obstruction, absent reflux may be normal or indicate proximal obstruction.

Functional Tomographic Brain Scan

Perfusion agents that cross the intact BBB include:

  • Xe-133 – Rapid perfusion and diffusion. Lipophilic and retained in cells. Has now been superseeded due to difficulties handling (gas) and imaging.
  • Iodoamphetamines (I-131) – Uptake and BBB diffusion reversible, slowy redistributes over time (hence scan shows integration of all brain activity over time). Is sequestered and released from the lung, requiring slow intra-arterial injection. Now superceeded.
  • Tc-99m-HMPAO (hexamethylpropyleneamine oaxime) – Crosses BBB irreversibly, trapped in brain substance ?due to change in ionic state, binding to glutathione or chemical decomposition. Images are a snapshot of activity over the ~10min (peak 1min) after injection. Stable for 4hrs after preparation.
  • Tc-99m-ECD (ethyl cysteinate dimer) – Does not localise in areas of luxury perfusion (increased blood flow but reduced O2 uptake). Stable for 6hrs. High cost.
  • FDG-PET may also be used as glucose consumption and blood flow are linked normally, and in most pathology. F-18-FDG-PET used for epilepsy, dementia and glioma.

SPECT brain scan performed with multi-detection rotating camera. Injection under controlled resting state: supine, eyes closed, quiet room (or white noise), indirect lighting; remaining in this state for at least 5min after injection. Sedation (if required) ideally given after the radioisotope. Injection can also be done during seizure ictus or acute stroke. Medications (presence and withdrawal) may filter biodistribution. Acetazolamide (Diamox) is a cerebrovascular-specific vasodilator to assess reserve, given 5-20min prior to tracer.

Infarct/ischaemia – Tc99m-ECD has high sensitivity (86%) and specificity (98%) for acute stroke. Particularly useful in vasospasm after SAH. Prolonged deficit on functional scan after TIA suggest high risk of subsequent ischaemic stroke. In early compromise autoregulation increases blood volume; once exceeded blood flow falls as oxygen demand remains (increased oxygen extraction fraction); once extraction fraction maximal further decline causes ischaemia. Prolonged compromise causes infarct with reduced oxygen demand (but persisting vasodilatation) causing drop in extraction fraction and luxury perfusion (persisting for days-weeks). Stress testing can be done with and without acetazolamide; areas of impaired vasodilatory reserve show relatively reduced perfusion on stress scan. Cross-circulation of the COW can be tested with carotid artery occlusion, showing areas of reduced perfusion. Diaschisis occurs from reduced metabolism due to reduced neural communication from a distant infarcted area.

In dementia FDG-PET has superceeded SPECT. A negative PET suggests cognitive impairment in 3yrs is unlikely. Z-score maps make assessment easier.

  • Alzeimer disease (AD) – Magnitude and extent of hypometabolism correlates with severity of symptoms. Early minor reduction in temporoparietal lobes; later significant in left midfrontal, bilateral parietal and superior temporal; when severe marked hypometabolism with sparing of sensorimotor, visual and subcortical areas. Temporoparietal defect also seen in CO poisoning, hypoglycaemia, mitochondrial encephalomyelopathy, severe PD, diffuse Lewy body dementias.
  • Diffuse lewy body dementia (DLBD) – Similar hypometabolism as AD, but also shows occipital hypometabolism, might not have medial temporal atrophy cf AD.
  • Frontotemporal lobe defects in frontotemporal degeneration (Pick disease), depression, alcohol, schizophrenia, severe AD, progressive supranuclear palsy.
  • Defects can also occur in vascular dementias (may coexist with AD) and elderly (sulcal enlargement, atrophy); hence correlation with anatomical imaging required.
  • Parkinson disease – Activity in the lentiform nuclei and thalami from lack of dopaminergic inhibition, sparing of the caudate nuclei. F-11-fluorodopamine study may show dopamine deficiency in the putamen (substantia nigra) and response to therapy.

Seizures – Aim of study is to detect side of seizure focus. Tends to cause increased activity during the ictus, reduced interictally (?from interruptions with adjacent neurons reducing activity). During the ictus injection should be given as early as possible due to secondary activiation and spread of seizure foci. Ictal SPECT more accurate than interictal, both are complementary. Mesial temporal sclerosis can be surgically cured in 85% if abnormal brain is limited to a single temporal lobe. FDG cannot be held to perform scanning in the post-ictal period.

High-grade tumours are hypermetabolic, low-grade hypometabolic (except high in juvenile pulocytic astrocytoma). High sensitivity and moderate specificity of recurrent tumour vs radiation necrosis.

Brain death – Perfusion agents (HMPAO, ECD) cf other agents (GH, DTPA) are less dependent on quality of injection, easier to interpret and allow evaluation of the posterior fossa. It is not affected by drugs, hypothermia or hypovolaemia. Activity stops at the base of skull due to raised ICP with no intracerebral arterial flow, no venous sinuses activity on subsequent images. Injection should be repeated if there is no activity seen in the CCA. Hot nose sign – increasd collateral flow to nasal area.