Use of Ultrasound as an Alternative to CT

In light of concern over radiation dose in CT, it is helpful to remember that ultrasound is the imaging modality of choice for a number of abdominal, pelvic, and cardiovascular indications. Because ultrasound does not use ionizing radiation it is particularly useful in children and in women of child-bearing age when CT would otherwise expose the patient to pelvic radiation. This is particularly important in pregnancy (see The Pregnant Patient: Alternatives to CT and Dose-Saving Modifications to CT Technique). The following document illustrates scenarios for using ultrasound instead of CT.

Assessment of Pelvic Pain

Ultrasound is the imaging modality of choice for evaluating acute pelvic pain in women of reproductive age, as recommended by the American College of Radiology, since it effectively assesses common gynecologic causes such as ovarian cysts, hemorrhagic cysts, ovarian torsion, ectopic pregnancy, and pelvic inflammatory disease. Both transabdominal and transvaginal ultrasound provide complementary information in this setting [1].

In addition to evaluating the uterus and adnexa, ultrasound can also be used to assess nongynecologic causes of pelvic pain, such as appendicitis and urinary tract pathology. For suspected appendicitis, ultrasound has a pooled sensitivity of 83% and specificity of 93%, although computed tomography provides higher sensitivity and is preferred when ultrasound is inconclusive or clinical suspicion remains high [1]. For ureteral stones, the diagnostic utility of ultrasound is limited, and CT is generally favored for urinary tract evaluation [1].

In pregnant patients, ultrasound is especially valuable due to its safety profile and avoidance of ionizing radiation. Although operator experience and patient factors can influence accuracy, ultrasound remains the safest and most informative initial imaging test for acute pelvic pain in this population [2].

Assessment of Abdominal Pain

Ultrasound is the first-line imaging modality for acute abdominal pain, particularly in patients with right upper quadrant pain or suspected biliary disease. ACR and ACEP guidelines recommend ultrasound as the initial test for acute cholecystitis, given its high accuracy for gallstones and biliary obstruction while avoiding ionizing radiation [1,3-5].For right upper quadrant pain, ultrasound rapidly identifies gallstones, cholecystitis, and other hepatobiliary causes, and may also detect alternative diagnoses involving the liver, kidneys, or vasculature [3,7-9]. CT is reserved for cases where ultrasound is inconclusive or when nongallbladder pathology is suspected, with MRI used for problem-solving or in pregnancy [1,3,5].

In acute pancreatitis, ACR and NASPGHAN recommend transabdominal ultrasound as the initial study, especially in first episodes, to evaluate for gallstones and biliary etiology. Ultrasound also detects peripancreatic fluid collections and vascular complications, though CT remains superior for necrosis [6,10-11].

Assessment of Blunt Abdominal Trauma

In unstable trauma patients, Focused Assessment with Sonography for Trauma (FAST) or extended FAST (eFAST) is recommended to rapidly detect free intraperitoneal or pericardial fluid and guide immediate management [13-15]. A positive FAST in an unstable patient often prompts surgery, though its sensitivity is limited for solid organ and bowel injuries, and false negatives remain common [12-13,16].

In stable patients, CT remains the gold standard for comprehensive injury assessment, while in pediatrics FAST is less sensitive and should be interpreted alongside clinical and laboratory findings [15].

Assessment of Clinically Suspected Adnexal Mass

Transvaginal and transabdominal ultrasound is the imaging modality of choice in assessing clinically suspected pelvic masses [17]. Ultrasound offers high sensitivity for characterizing adnexal lesions by assessing features such as cystic versus solid composition, septations, internal echogenicity, vascularity, and solid elements. These characteristics help distinguish benign from malignant masses and guide management decisions, determining whether a lesion can be ignored, safely followed with imaging, or requires surgical intervention [18].

Cardiovascular Imaging

When an asymptomatic patient has a pulsatile abdominal mass and aortic aneurysm is suspected, ultrasound is the initial imaging modality of choice [19]. Population-based ultrasound screening studies have been recommended for male patients over the age of 65 [20]. Surveillance intervals are based on aneurysm size, with 3-year, annual, or 6-month follow-up recommended for AAAs measuring 3.0–3.9 cm, 4.0–4.9 cm, and ≥5.0 cm, respectively. CT or MRI is reserved for cases where ultrasound is inconclusive or for preoperative planning [19].

In patients with acute or chronic chest pain and in patients with dyspnea of suspected cardiac origin, ultrasound (with or without pharmacologic stress, with or without transesophageal echocardiography) can be used to assess abnormalities of ventricular wall motion, pericardial effusion, valve dysfunction, cardiac thrombus, and aortic pathology such as dissection [21,22,23].

In patients with acute chest pain and suspected pulmonary embolism, ultrasound of the lower extremity can be utilized to assess for deep venous thrombosis, to aid in triage of patients to therapy, no therapy, or additional imaging [24].

In patients with suspected bacterial endocarditis, transesophageal echocardiography is the clinical reference standard. It is more sensitive for detecting vegetations and complications, and should be performed when TTE is negative but clinical suspicion remains high. [25].

Assessment of the extracranial carotid arteries is typically performed with ultrasound rather than CT; it is the first-line imaging modality, with robust sensitivity and specificity for detecting clinically significant stenosis. The Society for Vascular Surgery recommends its use for both symptomatic and asymptomatic patients, reserving additional imaging for equivocal or technically limited studies [26].

Conclusion

The descriptions above are common examples of using ultrasound where CT might be considered as an alternative imaging modality. There are of course many other indications for ultrasound. The prudent use of ultrasound will allow for appropriate patient care and diminish unnecessary exposure of patients to ionizing radiation from CT.

References

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