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Emergency Radiology

E2784. Acoustic Shadowing: Can We Use This Limitation in Arriving at a Diagnosis?

Lim K. Pennsylvania Hospital, Philadelphia, PA

Address correspondence to K. Lim (khengll@yahoo.com)

Background Information: Ultrasound (US) is frequently ordered as the first cross-sectional imaging test when clinical diagnosis is unclear. US is a widely available modality in both emergent and nonemergent settings, is relatively low cost, lacks ionizing radiation, can be performed in patients allergic to iodinated and gadolinium contrast, and has the benefit of no IV catheter placement. However, one perceived limitation of US is the acoustic impedance encountered on a highly attenuating material-tissue interface, causing acoustic shadowing. This perceived limitation often results in more imaging using CT and MRI, which in turn drives up cost and creates potential for more follow-up studies of indeterminate incidental findings. The purpose of this exhibit is to explore the various composition of materials that attenuate acoustic beam within the human bodies and to illustrate the utility of acoustic shadowing in aiding imaging diagnosis.

Educational Goals/Teaching Points: The goal of this exhibit is to improve the diagnostic confidence of radiologists and other imagers when encountering echogenic material-tissue interfaces on daily clinical practice. Elements that are important in achieving high diagnostic confidence at US are reviewed as teaching points in case examples. These elements include: diligence in reviewing prior imaging studies when encountering echogenic interfaces with acoustic shadowing, reviewing electronic medical records, obtaining relevant history by the sonographer when meeting the patient prior to performing the scan, and obtaining cine clip of an ROI for physician’s review. In addition, we aim to increase awareness of etiologic mechanisms of acoustic shadowing (loss of technical information) while scanning such as proper selection of scanning angles, time gain compensation, transducer frequency, and sector versus linear array scanners.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques: Attenuation of different tissues in the human body varies greatly, with water at 0, liver at 0.9, bone at 5, and air at 12 dB/cm/MHz. The production of acoustic shadowing relies on this wide variation of tissue attenuation and the interaction with an abnormality at the interface. Case examples include calcifications such as dystrophic calcifications in the bladder, ureteral calculus, foreign bodies such as surgical clip, stent, or pessary, tumors of fibrous composition such as uterine leiomyoma, or ovarian fibroma, and the presence of gas such as in an abscess, Fournier gangrene, or pneumobilia.

Conclusion: US inability to penetrate highly attenuated material such as gas and bone in human tissue is well known. In many cases, this is indeed a limitation in the diagnostic prowess of US; however, by combining clinical history and US findings as illustrated in this exhibit, it is possible to reach the correct diagnosis without further imaging (i.e., CT or MRI).