Six-point star sign | Radiology Signs

Answer:

The six-point star sign is a star-shaped pattern of activity radiating from an intense focus on a planar or SPECT image, and it is an artefact of septal penetration rather than a finding in the patient. It occurs when the emitted photon energy exceeds the energy for which the collimator was designed, most typically when a high-energy radionuclide is imaged with a low-energy collimator, for example iodine-131, indium-111, gallium-67 or the high-energy emissions of a therapeutic agent imaged on a low-energy high-resolution collimator, or when a very hot focus is imaged with a medium-energy collimator. The pattern reflects the hexagonal geometry of the collimator holes and the thin lead septa between them. Recognising it prevents two errors: the star must not be mistaken for true tracer uptake in surrounding tissue, and the intensity of the underlying focus must not be quantified from a study degraded in this way. The remedy is technical, that is, to use a collimator matched to the photon energy, high-energy for iodine-131 and medium-energy for indium-111 and gallium-67, to set the energy windows correctly, and to consider a scatter correction. Other recognised gamma camera artefacts that should be distinguished are photopenic attenuation from metal or a hand across the field, dose infiltration at the injection site, patient motion and non-uniformity from a failing photomultiplier tube.

Why is it called so?

The photons penetrating the collimator septa emerge along the six directions of the hexagonal hole array, producing a six-pointed star around the hot focus.

Pathophysiology

A parallel-hole collimator selects photons travelling perpendicular to the crystal by absorbing oblique photons in lead septa whose thickness is chosen for a particular energy. When the photon energy exceeds that for which the septa were designed, a proportion of the oblique photons pass through the lead instead of being absorbed. The probability of penetration is lowest where the path through lead is longest and highest where the path is shortest, and in a hexagonal array of holes the shortest paths lie along six symmetric directions. Photons therefore preferentially reach the crystal along those directions and are recorded as spokes emanating from the true source position, producing the star. The effect is worst around intense point sources because the number of penetrating photons scales with source activity.

Alternative names: Star artefact; septal penetration artefact

Other associated named signs: Stocking-glove sign of intra-arterial injection and the hurricane sign of patient motion