Tc^99m HDP  may accidentally be injected or leak into the perivascular space surrounding the jugular vein.  A well seated needle and an adequate amount of time invested in holding off the area after withdraw of the needle usually prevents this artifact.  In some instances the horse may retract from the initial stick leaving a puncture wound in the jugular vein.  If a reattempt is made to soon and the wound is not sealed from the initial stick, isotope could leak from this area even though careful efforts were made to seal the area of the second injection site.  Isotope in the perivascular space has been known to trickle down to the proximal humeral area eliciting an artifact and potential false positive reading if an anterior posterior view is not acquired. Uptake of isotope by the lymph nodes is also known to create intense focal artifacts in the region.    Significant Tc^99m remaining in the perivascular space alters the target to background ratio in neck and shoulder images and may be non diagnostic.