An 18-year-old boy presented to the emergency department following a fall from his moped. He sustained an injury of the spleen and haemoperitoneum. The patient was discharged following successful selective arterial embolization of the spleen. Two weeks later he presented again with abdominal pain, low-grade fever, and raised inflammatory markers.
Initial Contrast-Enhanced Computed Tomography (CT) revealed a laceration of the spleen, extending to the hilum and >50% devascularisation of the parenchyma, in keeping with a grade IV injury (Fig. 1). Two weeks following selective splenic embolization, B-mode ultrasound revealed anechoic areas within the splenic parenchyma, the largest of which demonstrated a characteristic ‘Yin-yang sign’ on colour Doppler ultrasound (Fig. 2). Both CT and Contrast-enhanced Ultrasound (CEUS) demonstrated a 3.0 x 2.0 cm rounded area showing arterial enhancement within the spleen parenchyma, readily differentiating this from the adjacent multiple foci of heterogenous non-enhancing collections consistent with infarcts (Fig. 3-4) and thus accurately diagnosing a pseudoaneurysm.
A pseudoaneurysm is the result of a breach in the arterial wall resulting in extravasation of blood, which is contained by the adventitial layer or perivascular soft tissues. Unlike true aneurysms that involve all three layers of the arterial wall, pseudoaneurysms are more prone to rupture, therefore more likely requiring urgent management. 
Pseudoaneurysm formation is a well-known complication following blunt or penetrating trauma of the solid organs. On colour Doppler, the yin-yang sign may be seen indicating bidirectional flow due to swirling of blood within the aneurysm. This is a useful radiological sign that can be identified at bedside ultrasound in the setting of trauma. CEUS is highly sensitive and specific for diagnosis and follow-up of traumatic injuries of solid abdominal viscera (kidney, liver, and spleen) and pseudoaneurysm diagnosis, outperforming conventional US and considered comparable to CT. [2, 3] It is inexpensive, lacks ionising radiation, particularly important in children, no deployment of nephrotoxic contrast agents and can be performed everywhere including the bedside or the emergency department. The spatial and temporal resolution of CEUS within its field of view is superior to conventional imaging techniques, allowing for optimal flow detection and diagnosis and follow-up of a pseudoaneurysm. CEUS is also characterised by optimal contrast resolution compared to conventional flow detecting ultrasonographic techniques such as colour or power Doppler and can thus confidently demonstrate the presence of flow inside tissue and thus accurately diagnose a pseudoaneurysm within the otherwise unenhancing infarcted tissue. CEUS is also accurate in detecting other vascular complications such as active bleeding. The dependent pooling of microbubbles can indicate the nature of haemorrhage while the wall of a pseudoaneurysm maintains the shape of the structure.
Since the development of interventional techniques, embolization has become the preferred treatment for active bleeding in patients that do not have to undergo immediate surgery.  In this case, initial splenic artery embolization successfully treated the active bleeding but re-embolization was not possible on the second presentation with a pseudoaneurysm, therefore open splenectomy was opted for.
Pseudoaneurysm formation is a well-recognised complication following solid organ injury. Delayed rupture is potentially life-threatening, therefore routine follow-up imaging with an optimised imaging protocol is important to assess for complications.
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