What is Helical CT in the Context of Diagnosing Nephrolithiasis?
Helical CT (also called spiral CT) is a computed tomography technique where the patient moves continuously through the scanner while the X-ray tube rotates around them, acquiring volumetric data in a continuous spiral pattern rather than individual slices—this technology has become the gold standard for detecting kidney stones with 95-97% sensitivity and specificity. 1, 2
Technical Principles
Helical CT differs fundamentally from conventional CT through several key features:
- Continuous volumetric acquisition occurs as the patient moves through the gantry while the X-ray tube rotates continuously, creating a spiral or helical data acquisition pattern 3, 4
- Slip-ring gantry technology enables continuous tube rotation without cables, making the helical acquisition possible 3
- Retrospective image reconstruction allows images to be created at any point along the acquired volume after scanning is complete 4, 5
Advantages for Kidney Stone Detection
Superior Diagnostic Performance
For suspected nephrolithiasis, non-contrast helical CT of the abdomen and pelvis has been the heavily relied-upon imaging modality for over two decades, with 95% or higher sensitivity and specificity. 1
- Precise stone measurement and clear depiction of exactly where in the ureter a stone has lodged are possible with helical CT 1
- Virtually all renal calculi appear radiopaque on CT, allowing accurate detection even of small stones without IV contrast 2
- Secondary signs of urolithiasis including ureteral dilatation, perinephric stranding, periureteral inflammation, and perinephric inflammation can be visualized 1, 2
Speed and Motion Reduction
- Dramatically shortened examination times compared to conventional CT reduce motion artifacts, which is particularly valuable in patients with acute pain who may have difficulty remaining still 4, 5
- Faster acquisition reduces the number of repeat sections needed and ultimately decreases radiation exposure 5
Clinical Implementation
Low-dose helical CT protocols (<3 mSv) should be used in place of conventional dosing when evaluating for renal or ureteral stones to minimize radiation exposure while maintaining excellent diagnostic performance (pooled sensitivity 97%, specificity 95%). 1, 2, 6
- Stone size and location are essential for determining the level of intervention required, as smaller, more proximal stones are likely to pass spontaneously 1
- Thin (1-1.5 mm) axial slice images are preferred for optimal stone detection 2
- Multiplanar reformations improve lesion detection and characterization 1, 4
Common Pitfalls to Avoid
- Do not use contrast-enhanced CT as first-line imaging for suspected kidney stones, as the enhancing renal parenchyma may obscure stones within the collecting system 2, 6
- Be aware that helical CT requires greater attention to the relationship between contrast administration and scanning timing when contrast is used, as the optimal temporal window for detection can be missed 4
- Recognize that artifacts unique to helical technology can occur, particularly those relating to vascular or parenchymal enhancement 4
Special Populations
- In pregnant patients, ultrasonography is the imaging tool of choice due to radiation concerns, despite helical CT's superior diagnostic accuracy 1, 2, 6
- For patients with recurrent stone disease, limiting the helical CT scan to the area of interest or using ultra-low-dose protocols helps reduce cumulative radiation exposure 2, 6