What is the most appropriate diagnostic scan to use for a patient?

Diagnostic Imaging for Suspected Clinical Conditions

General Principles

The choice of diagnostic imaging depends critically on the anatomic location of symptoms, patient characteristics (particularly pregnancy status), and the specific clinical question being asked. 1

Initial Approach by Clinical Presentation

Right Upper Quadrant Pain

• Ultrasonography is the initial imaging test of choice for patients with right upper quadrant pain, particularly when biliary pathology is suspected 1, 2, 3
• This modality avoids ionizing radiation, is widely available, and has excellent sensitivity for gallstones and acute cholecystitis 1

Right or Left Lower Quadrant Pain

• CT of the abdomen and pelvis with intravenous contrast is the recommended initial imaging for suspected appendicitis or diverticulitis 1, 2
• For suspected appendicitis specifically, CT with IV contrast (oral or rectal contrast may not be needed) receives the highest appropriateness rating 1
• In children and women of childbearing age with suspected appendicitis, ultrasonography should be performed first, with CT reserved for equivocal cases 1, 3

Diffuse or Nonlocalized Abdominal Pain

• CT with intravenous contrast is typically the imaging modality of choice when serious pathology remains a concern after history, physical examination, and laboratory testing 1
• CT altered the leading diagnosis in 49% of patients and changed management in 42% in one prospective study 1
• Conventional radiography has limited diagnostic value for most causes of abdominal pain, though it retains utility for detecting bowel perforation (pneumoperitoneum) and bowel obstruction 1, 4

Special Populations

Pregnant Patients

Ultrasonography and MRI without contrast are the imaging studies of choice in pregnancy because they lack ionizing radiation 1

Specific recommendations by clinical scenario:

• For suspected appendicitis or acute abdominal pain: Ultrasonography is rated as the most appropriate initial test, followed by MRI without contrast if ultrasonography is inconclusive 1
• For suspected stress fractures of the pelvis or hip: MRI without contrast is the initial imaging test of choice 1
• For suspected stress fractures of long bones: Radiographs should be obtained first (fetal radiation dose is substantially lower for extremity imaging), followed by MRI if negative 1
• For suspected pyelonephritis: Ultrasound color Doppler of kidneys and bladder or MRI without contrast are appropriate alternatives 1

Important radiation considerations:

• The fetal absorbed dose from pelvis radiograph is approximately 1.1 mGy, from pelvis CT is 25 mGy, and from bone scintigraphy is 4.6 mGy early in pregnancy 1
• The theoretical risk is 1 additional cancer death per 1,700 exposures for each 10-mGy exposure 1
• If CT is necessary after ultrasonography and MRI are unavailable or inconclusive, it can be used as the risk of negative outcomes from a single CT is very low 1

Pediatric Patients

• Ultrasonography should be the first-line imaging modality whenever feasible to avoid ionizing radiation 1, 3
• For suspected appendicitis, ultrasonography is preferred initially, with CT reserved for equivocal cases 1
• Risk-stratification tools can identify children unlikely to have appendicitis who do not need imaging 1

Specific Clinical Conditions

Suspected Stress Fractures

• Radiographs should be the initial imaging evaluation despite low sensitivity, as findings may be conclusive 1
• MRI without contrast is the most sensitive complementary examination and should be performed when radiographs are negative but clinical suspicion remains high 1
• For suspected insufficiency fractures of the pelvis or hip with negative radiographs, MRI without contrast is recommended due to high risk of complications 1
• CT without contrast remains a reasonable but less sensitive alternative to MRI 1

Suspected Pyelonephritis

• Imaging is usually not appropriate for first-time presentation of uncomplicated acute pyelonephritis 1
• CT abdomen and pelvis with IV contrast is usually appropriate for complicated patients or those with history of renal stones/obstruction 1
• In pregnant patients, ultrasound color Doppler or MRI without contrast are appropriate alternatives 1

Suspected Bowel Perforation

• Upright chest radiograph is the initial test to detect pneumoperitoneum (free air), with a sensitivity of approximately 49% for bowel obstruction but maintaining utility for detecting free air 1, 4
• CT is the most sensitive imaging modality for detecting pneumoperitoneum (89% sensitivity) and can identify the perforation site and detect free air not visible on plain films 4

Critical Pitfalls to Avoid

• Never proceed with imaging studies involving ionizing radiation in women of reproductive age without first obtaining β-hCG testing 1, 2
• Conventional radiography should not be routinely ordered for abdominal pain as it has limited diagnostic value in most cases 1, 2
• Do not perform lumbar spine MRI for nonspecific chronic low back pain without red flag features, as many asymptomatic adults have abnormal findings 5
• Avoid gadolinium-based contrast agents in patients with stage 4 or 5 chronic kidney disease (eGFR <30 mL/min/1.73 m²) due to risk of nephrogenic systemic fibrosis 1, 5, 3

Algorithmic Decision-Making

Step 1: Determine anatomic location of symptoms and pregnancy status Step 2: Apply location-specific imaging recommendations:

• Right upper quadrant → Ultrasonography
• Right/left lower quadrant → CT with IV contrast (ultrasonography first in children/pregnant patients)
• Diffuse pain → CT with IV contrast if serious pathology suspected
• Pregnant patient → Ultrasonography or MRI without contrast

Step 3: If initial imaging is negative but clinical suspicion remains high, proceed to more sensitive modality (typically MRI for musculoskeletal, CT for abdominal pathology) 1