This patient has a forearm psuedoaneurysm of an interosseous artery. He underwent a CTA of the right forearm which shows nonvisualization of the ulnar artery within the proximal forearm. The CTA was unable to fully evaluate the pseudoaneursym (possible due to its size- there was a small blush of contrast within the region. This was thought to represent artifact vs. hematoma vs. pseudoanuerysm).
The patient ultimately underwent angiography with embolization of the pseudoaneurysm. After successful embolization, the patient's radial artery was widely patent, but the ulnar artery teminates at the site of the pseudoaneysm. Ulnar artery filling was perserved only by retrograde filling through the palmar arch.
Pseudoaneurysm
Pseudoaneurysm or false aneurysm, is a tear in the arterial wall allowing blood to leak from the vessel into the surrounding tissue. The resulting hematoma walls off the surrounding tissue, creating a cavity. Blood leaking from the artery then swirls within this cavity, creating the classic findings on ultrasound. It is a complication most commonly described after diagnostic cardiac catheterizaiton through the femoral artery. It is indeed a "false" aneursym, as the intimal layers of the arterial wall do not comprise the borders of the outpouching. On color flow doppler, you can clearly see blood swirling in the cavity. This is termed the "ying-yang" sign, which is blood swirling both toward the probe (seen in red) and away from the probe (in blue).
Note that the patent vessel deep to the pseudoaneurysm maintains its normal size and shape. Spectral doppler of the stalk of the psuedoanuerysm would demonstrate turbulent flow.