When should patients get a computed tomography (CT) scan?

When Should Patients Get a CT Scan?

CT scanning should be obtained when there is clinical suspicion of specific pathology that requires anatomic imaging for diagnosis or management decisions, particularly for acute abdominal pain, suspected complications of infections, staging of known malignancies, or when other imaging modalities are contraindicated or non-diagnostic.

Emergency and Acute Care Indications

Acute Abdominal Pain

• CT of the abdomen and pelvis with IV contrast is the primary imaging modality for nonlocalized acute abdominal pain, as it changes diagnosis in 49-54% of patients and alters management in 42-53% of cases 1
• CT improves diagnostic certainty from 70.5% pre-scan to 92.2% post-scan in emergency department patients with abdominal pain 1
• CT angiography is preferred when mesenteric ischemia is suspected, with reduced segmental bowel-wall enhancement being 100% specific for segmental bowel infarction 1
• Avoid CT in patients with abdominal pain plus diarrhea, as it changes management in only 11% versus 53% for abdominal pain alone 1

Suspected CNS Infections

• CT with contrast or MRI is indicated when acute rhinosinusitis persists despite treatment or when complications are suspected 1
• CT is not routinely recommended for viral encephalitis unless there are contraindications to lumbar puncture (focal neurologic signs, papilledema, altered consciousness, seizures) 1
• CT may show alternative diagnoses (cerebrovascular accident) that eliminate the need for lumbar puncture in approximately 10% of suspected encephalitis cases 1

Cancer Staging and Surveillance

Prostate Cancer

• CT is indicated for initial staging in patients with T3/T4 disease 1
• CT may be considered for T1/T2 disease when nomogram-indicated probability of lymph node involvement exceeds 10%, though evidence level is low 1
• CT detects gross extracapsular disease, nodal metastases, and visceral metastases but is generally insufficient to evaluate the prostate gland itself 1

Bladder Cancer

• CT of abdomen and pelvis is recommended before TURBT only if the cystoscopic appearance suggests solid (sessile), high-grade tumor, or muscle invasion 1
• CT is not recommended for purely papillary tumors or suspected carcinoma in situ 1

Chronic Rhinosinusitis

• CT without contrast is the gold standard for radiologic evaluation of chronic rhinosinusitis 1
• CT is essential after failure of appropriate medical or surgical treatment in secondary care with continued symptoms and abnormal endoscopy 1
• A Lund-Mackay score ≥3 or complete obstruction of one sinus (score of 2) is clinically relevant 1
• CT is mandatory prior to sinus surgery to confirm disease extent and identify anatomical features predisposing to complications 1
• Repeat CT imaging is not essential if no surgical procedures occurred in the interim period 1

When CT Should NOT Be Obtained

Sudden Sensorineural Hearing Loss

• Routine head CT is not recommended for isolated sudden hearing loss with no etiology found on history or physical examination 1
• The American College of Radiology rates head CT for acute hearing loss and vertigo as only 3 out of 10 for appropriateness, meaning it is unlikely to be indicated 1
• Head CT has a radiation dose of 1-10 mSv (RRL of 3) and should be considered unnecessarily harmful in SSNHL evaluation 1
• MRI is the preferred imaging modality when central pathology or cerebellopontine angle tumor is suspected 1

Frequently Admitted Medical Patients

• In patients with multiple medical hospitalizations, only 4% of head CT scans show clinically significant findings 2
• Head CT scans should be ordered less frequently in this population to avoid unnecessary radiation exposure 2

Alternative Imaging Considerations

When CT Cannot Be Obtained

• CT may be used when MRI is contraindicated (pacemakers, severe claustrophobia) or unavailable due to financial constraints 1
• CT can be considered for patients with known bone disease (Paget disease, fibrous dysplasia, temporal bone metastasis) 1

Foreign Body Detection

• CT without IV contrast has 90-100% sensitivity for foreign body detection compared to only 32% for plain radiographs 3
• CT is essential when plain films are negative despite witnessed ingestion, findings are equivocal, or there is suspicion of complications 4, 3
• CT should be performed in symptomatic patients with suspected foreign body ingestion despite negative X-ray 4

Radiation Safety Considerations

• CT accounts for only 9% of all diagnostic studies involving ionizing radiation but contributes over 60% of the collective effective dose from medical radiation 5
• A CT study of head and trunk (e.g., polytrauma) delivers approximately 20 millisieverts, corresponding to an additional lifetime cancer mortality risk of approximately 0.1% 5
• The additional individual relative lifetime cancer mortality risk is estimated at 5% per sievert of whole-body exposure 5

Key Clinical Pitfalls to Avoid

• Do not order repeat abdominal CT after initially negative scans without strong clinical indication, as diagnostic yield drops from 22% on initial presentation to 5.9% on the fourth CT or greater 1
• Leukocytosis and APACHE-II scores may predict higher diagnostic yield on repeat CT 1
• Nontargeted head/brain CT has very low yield and should be avoided when specific pathology is not suspected based on history and physical examination 1
• CT has a negative predictive value of only 64% for nonspecific upper abdominal pain, commonly missing pancreaticobiliary inflammatory processes, gastritis, and duodenitis 1