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Background
Initial presentation of acute pulmonary embolism (PE) is highly variable, with the most common presenting signs and symptoms including dyspnea, tachycardia, and hypoxia. PE has also been shown to present as acute flank pain in some rare cases. This can easily be missed in a diagnostic workup for flank pain, which is mostly dedicated to renal abnormalities. Prior studies have demonstrated findings associated with PE on noncontrast CT of the chest, including the hyperdense lumen sign, which is not helpful in cases where the central pulmonary arteries are not included in the FOV. Additional studies have assessed contrast-enhanced CT of the abdomen and pelvis for evidence of missed PE, including artificial intelligence models trained to detect incidental PE on contrast-enhanced CT; less attention has been placed on signs and indicators of PE that can be detected on noncontrast CT of the abdomen. We seek to highlight key PE findings on noncontrast CT of the abdomen (flank pain/stone protocol studies) that may help address the cause of flank pain.

Educational Goals / Teaching Points
The goal of this educational exhibit is to demonstrate findings suggestive of PE on noncontrast CT of the abdomen and pelvis, and differentiate these findings from other mimicking findings, such as atelectasis or pulmonary consolidations. We reviewed cases of noncontrast CT of the abdomen and pelvis with subsequent CT pulmonary angiogram (CTPA) within the next 48 hours. Patients with positive CTPA studies were reviewed in detail for lung findings, yielding a total of 20 patients with positive findings.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Features of PE on noncontrast CT abdomen pelvis studies are typically secondary findings of PE that may be seen on CTPA, including peripheral, pleural-based, and wedge-shaped consolidations. Additional findings that may be seen include enlarged feeding vessels containing PE, adjacent pleural effusions, and groundglass consolidation or consolidation with internal air density surrounding infarcted lung parenchyma. These findings are contrasted with other common findings within the lung bases, including atelectasis, pneumonia, pulmonary nodules, and metastatic lesions, to highlight key differences in appearance relative to sequelae of PE.

Conclusion
This educational exhibit demonstrates key findings that indicate PE as a possible etiology for patients presenting with flank pain on noncontrast CT of the abdomen and pelvis. PE can be associated with high morbidity for patients if the diagnosis is missed, which may be especially difficult with atypical presentations. Although many of these findings are nonspecific, they may help to suggest PE as a possible etiology for flank pain in the setting of an otherwise negative CT of the abdomen and pelvis. This exhibit highlights the importance of careful assessment of the lung bases on noncontrast CT of the abdomen and pelvis, with attention paid to findings that may suggest a need for further evaluation with CTPA.