What is the recommended imaging modality for pregnant patients with suspected kidney stones, considering the risks of computed tomography (CT) scans?

Imaging for Suspected Kidney Stones in Pregnant Patients

Ultrasound of the kidneys and bladder should be the initial imaging modality for pregnant patients with suspected kidney stones, as it avoids ionizing radiation to the fetus while effectively detecting hydronephrosis and larger stones. 1

Initial Imaging Approach

• Start with renal ultrasound as the first-line screening examination because it is sensitive and specific for diagnosing hydronephrosis and does not expose the patient or fetus to ionizing radiation 1
• Ultrasound has nearly 100% sensitivity for detecting large stones (>5 mm) and hydronephrosis, though accuracy is poor for small stones (<3 mm) 1
• This modality is widely recommended despite being highly nonspecific and sometimes unable to differentiate between ureteral obstruction from calculi versus physiologic hydronephrosis of pregnancy 2

When Ultrasound is Nondiagnostic

If ultrasound fails to establish a diagnosis and symptoms persist despite conservative management, MR urography (MRU) without contrast should be the second-line imaging modality. 1

MRU Advantages and Limitations:

• MRU avoids fetal radiation exposure and can detect hydronephrosis and causes of renal obstruction 1
• MRI can differentiate physiologic from pathologic dilatation by identifying renal enlargement and perinephric edema in true obstruction 2
• Critical limitation: MRU has only 69% sensitivity for identifying the site of stone impaction compared to 100% sensitivity with CT 1
• MRI has poor accuracy for detecting small urothelial calculi 1
• Do not use gadolinium-based contrast agents unless the indication is critical, as potential risks to the fetus are unknown 1, 3

Role of CT Scanning

Noncontrast CT abdomen and pelvis can be used when diagnosis remains uncertain after ultrasound and MRI, but only when the clinical benefit clearly outweighs radiation risks. 1, 3

CT Considerations:

• CT without contrast is sensitive and specific for diagnosing stones in pregnant patients 1
• The key principle is mitigating radiation dose by optimizing each study and reducing the total number of imaging studies to the absolute minimum necessary 1
• Most diagnostic CT studies deliver far less than 20 mGy to the uterus, well below the 50-100 mGy threshold for detectable fetal harm 3
• Low-dose CT protocols should be used when CT is deemed necessary 4, 5
• Radiologists are more likely to use CT than MR in the second trimester (35% vs 20%) and third trimester (48% vs 18%) when clinical urgency demands definitive diagnosis 1

Practical Algorithm

1. First-line: Renal ultrasound with bladder evaluation 1
2. Second-line (if ultrasound nondiagnostic and symptoms persist): MRU without contrast 1, 4
3. Third-line (if diagnosis still uncertain and clinical urgency exists): Low-dose noncontrast CT 1, 3, 4, 5

Common Pitfalls to Avoid

• Do not dismiss physiologic hydronephrosis: This occurs in >80% of pregnant patients, more commonly on the right, typically beginning in the second trimester 1
• Avoid contrast-enhanced CT protocols: There is no documented additional benefit of IV contrast for stone detection in pregnancy, and it adds unnecessary complexity 1
• Do not use KUB radiography: There is no relevant literature supporting its benefit in pregnant patients with suspected stones 1
• Recognize MRI limitations: While radiation-free, MRI misses approximately one-third of stone locations that CT would identify 1
• Balance maternal and fetal risks: A delayed or missed diagnosis can be more harmful to both mother and fetus than appropriately indicated imaging with ionizing radiation 6, 5