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Three-dimensional CT pyelography for planning of percutaneous nephrostolithotomy: accuracy of stone measurement, stone depiction and pelvicalyceal reconstruction
Patel U, Walkden RM, Ghani KR, Anson K
Department of Radiology, St George’s Hospital and Medical School, London, UK
Eur Radiol. 2009; 19: 1280-8

• Retrospective evaluation of computed tomographic (CT) pyelography before percutaneous nephrostolithotomy (PCNL). Twenty patients with renal calculi underwent CT pyelography using a dedicated protocol. Calculus size, uniformity of contrast excretion and accuracy of calculus and pelvicalyceal (PC) system reconstructions were scored and compared on axial and coronal maximum intensity projections (MIP) and volume reconstructions (VRmovie loops). After contrast medium administration, the size of calculi is accurate on axial images, but underestimated on coronal studies: mean 14.7 mm vs. 14.4 mm (axial) and 17.2 mm vs. 16.1 mm (coronal) for measurements before and after enhancement, respectively (p = 0.11 and 0.03). Uniform contrast medium excretion (median 228 HU; 95% CI 209-266 HU) was sufficiently lower than calculus density (median 845 HU; 95% CI 457-1,193 HU) for precise calculus and pelvicalyceal reconstructions in 87% and 85%, respectively. Coronal MIP scans were rated best for calculus depiction (mean score 2.68 vs. 2.50 and 2.41 for coronal, axial and VRs, respectively; p = 0.14) and VR studies best for PC anatomy (mean score 4.4 vs. 3.73 and 2.89 for VR, coronal and axial studies, respectively; p = < 0.0001). Three-dimensional CT pyelography can accurately demonstrate calculus position and spatial relationships of the collecting system before PCNL.

• Editorial Comment
  Percutaneous nephrostolithotomy (PCNL) requires detailed imaging techniques to define stone burden and delineate the anatomy of the kidney and other adjacent organs and structures. Adequate safe percutaneous access can also be accomplished with preoperative imaging studies. As we know, non-contrast MDCT studies using multiplanar, curve and surface reconstruction are useful tools for the detection of renal stones but offers poor information regarding the pelvicalyceal anatomy. Coronal and sagital MIP and volume-rendered reconstructions obtained during routine CT-urography technique offers superb anatomic details of the pelvicalyceal system. However using this technique the visualization of low-density stones (pure acid uric with density ranging from 230-340 HU) is a difficult task since the contrast density within pelvicalyceal system in CT-urography ranges from 500-600 HU.
  For this reason, the authors developed a tailored protocol called CT-pyelography, using both furosemide and saline bolus after contrast medium, in attempt to decrease the contrast density within pelvicalyceal system. The median contrast medium density observed with CT-pyelogram was 228 HU (range 134-1498 HU). This technique allowed the detection of 87% of significant renal calculi and 85% of all upper pelvicalyceal system details. In our opinion, however, low-density matrix and small or low-density stones may still be missed by this new approach.

Dr. Adilson Prando
Chief, Department of Radiology and
Diagnostic Imaging, Vera Cruz Hospital
Campinas, São Paulo, Brazil
E-mail: adilson.prando@gmail.com