The azygos venous system is a crucial conduit of the posterior thorax and potentially vital collateral pathway. However, it is often overlooked clinically and radiologically. This pictorial essay reviews the normal azygos venous anatomy and CT findings of congenital variations and structural changes associated with acquired pathologies.
기정맥계는 후방 흉부의 중요한 부속 정맥이며 측부순환으로서 중대한 역할을 한다. 그러나, 그 중요성에도 불구하고 임상적 혹은 영상의학적으로 종종 간과된다. 본 임상화보에서는, 기정맥계의 정상 해부학에 대해 알아보고, 기정맥계에서 볼 수 있는 다양한 선천 변이와 후천적 질환에 따른 구조 변화의 CT 소견에 대하여 검토하고자 한다.

Identification of the Adamkiewicz\'s artery (AKA) prior to the operation is one of the spinal cord ischaemia preventive measures. A 75-year-old man presented with the rapid expansion of thoracic aortic aneurysm. Collateral vessels from the right common femoral artery to the AKA were observed on preoperative computed tomography angiography. The stent graft was successfully deployed through the contralateral side via a pararectal laparotomy to avoid collateral vessel injury supplying the AKA. This case highlights the significance of preoperative identification of collateral vessels to the AKA.

Common carotid artery occlusion (CCAO) is a rare phenomenon associated with the development of neurological symptoms. In CCAO, diversion of blood flow from the external carotid artery (ECA) to the internal carotid artery (ICA) via the carotid bulb (CB) may occur. This pathway activation has been called \"carotid steal\". Starting from a particular case we describe the ECD finding of a complete occlusion of CCA with patency of ICA and ECA. In case of occlusion of CCA, inversion of the ECA flow may occur, towards the ICA, and it can be damped by a significant stenosis crossed in retrograde direction that may concur to maintain the pressure balance between the two circulations. Usually, this particular compensation can guarantee normal flow velocities in middle cerebral arteries without signs of activation of anterior collateral pathways. In this review we underline the protective role of ECA and we propose a new definition for this phenomenon. The ECA may provide blood to the cerebral circulation through several anastomotic secondary channels. Finally, only with an extensive knowledge of hemodynamic information of all intracranial and extracranial arteries, including ECA, we can estimate cerebral ischemic risk of the patient and choose the correct management of this occlusion.

BACKGROUND: The collateral pathways between the superior mesenteric artery (SMA) and inferior mesenteric artery (IMA) play an important role in colonic surgery. The most well-known are the Drummond marginal artery and Riolan\'s arch. The Moskowitz artery, also known as the meandering mesenteric artery, is a lesser-known collateral pathway and represents another link between SMA and IMA. The Moskowitz artery runs along the colonic mesentery floor and represents the link between the proximal segment of the middle colic artery and the ascending branch of the left colic artery.
OBJECTIVE: To comprehend the presence and importance of the Moskowitz artery (meandering mesenteric artery) in preoperative patients by using computed tomography (CT) studies.
METHODS: We retrospectively reviewed all abdominal CT images performed using intravenous contrast for any reason at the Radiology Department of Yeditepe University Hospital between April 2015 and September 2018. Patients older than 18 years with intravenous contrast (arterial and venous phases with a cross-section thickness of 0.625 mm), who underwent abdominal CT scan, and patients without any abdominal surgery were included in the study. As a result of the screening, 109 CT scans with eligibility criteria were included in this study.
RESULTS: There were 109 cases in this study; 50 (45.9%) of them were male and 59 (54.1%) were female. Moskowitz artery was found in 18 (16.5%) cases; 8 were males and 10 were females. Riolan\'s arch was present in 30 cases, of whom 15 were male and 15 were female. In our study Moskowitz artery and Riolan\'s arch were monitored as separate vascular structures, and in all cases with Moskowitz artery, Riolan\'s arch was also present. There was a statistically significant difference (p < 0.05) between the combined MCA emerging type and the presence of MA, and 12 cases with combined branch and 6 cases with single branch had Moskowitz artery. There was a statistically significant difference (p < 0.05) between LCA types and the presence of MA, and the Moskowitz artery was the most common seen at type 1 LCA.
CONCLUSIONS: Knowledge of the relationship between Riolan\'s arch and the Moskowitz artery is valuable, and preoperative evaluation of this artery may be beneficial in the presence of Riolan\'s arch. In addition, preoperative radiological evaluation and its importance are prominent in minimising intraoperative bleeding during splenic flexure mobilisation with a medial laparoscopic approach and reducing the risk of colorectal anastomosis leakage.

The pelvic collateral system is a robust network of communicating vessels that provide the functional reserve to withstand chronic aorto-iliac occlusive disease. For establishment of collateral circulation, the afferent vessel must originate proximal to the occlusion and anastomose with vessel/s distal to the occlusion. These collateral pathways can be classified as viscero-systemic, systemic-systemic, and visceral-visceral. CT angiography (CTA) is often the initial modality for evaluating patients with atherosclerotic vascular disease, because it is non-invasive and has been shown to be comparable to conventional angiography. Most collateral pathways are well demonstrated on CTA, which therefore is a useful tool for preoperative planning and regional interventional procedures.