Single photon emission tomography SPECT

This technique also uses compounds labelled with gamma-emitting tracers (ligands), but unlike conventional scanning, acquires data from multiple sites around the head. Similar computing to CT scanning provides a two-dimensional image depicting the radioactivity emitted from each 'pixel'. This gives improved definition and localisation. Various ligands have been developed but a 99Tcm labelled derivative of propylamine oxime (MHPAO) is the most frequently used. This tracer represents cerebral blood flow since it rapidly diffuses across the blood brain barrier, becomes trapped within the cells, and remains long enough to allow time for scanning. Of the total injected dose, 5% is taken up by the brain and 86% of this activity remains in the brain at least 24 hours.

A rotating gamma camera is often used for detection, although new multidetector systems will produce higher quality images. Data are normally reconstructed to give axial images but coronal and sagittal can also be produced.

Multiple short focussing colimators

Ligands for SPECT scanning

Purpose

H MP AO 123I IBZM 123I Iomazenil -123I CNB l23I MK801 123I tyrosine

Cerebral blood flow Dopamine D2 receptors Benzodiazepine receptors Cholinergic receptors Glutamate receptors Amino-acid uptake (e.g. in tumours)

ROTATING

GAMMA

CAMERA

ROTATING

GAMMA

CAMERA

MULTIDETECTOR SYSTEM

Clinical application

Absence of blood flow corresponds with area of infarction and tissue loss seen on structural imaging.

- Assessment of blood flow changes in DEMENTIA

Blood flow is generally reduced, especially in temporal and parietal lobes

The plane of scan lies in the same axis as the temporal lobe

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Do Not Panic

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