Interobserver Discrepancies in Distance Measurements from Lumbar Spine CT Scans

Lumbar spine computed tomographic (CT) scans of 10 patients were examined independently at two levels by five experienced radiologists. At each level the minimum midline sagittal diameter was measured, and at each intervertebral space the left foramen was measured for its minimum diameter. Statistically significant differences were found between the measurements of different observers, differences that in a number of cases could have led to disagreement over whether or not stenosis was present. There were reasonably strong correlations between different observers' readings of midline sagittal diameters but generally not of foraminal diameters. Reasons for discrepancies between observers in spine CT measurements are reviewed briefly. Our clinicians have requested that when we interpret spinal computed tomographic (CT) scans, we state exactly how large the foramina are, rather than reporting that they are "ample," "moderately to severely narrowed," etc. In fact, various authors have attempted to make the use of spinal CT as objective as possible in the diagnosis or treatment of spinal stenosis [1-6], lateral recess stenosis Materials and Methods Ten consecutive CT scans from the top of L4 to S1 obtained at University Hospital were selected from patients without previous surgery or residual contrast material. All scans were obtained on a 2002 Elscint scanner at Boston University Hospital using the "A" filter-function , a scan speed of 17 sec, the absorber (a beam hardener) out, "standard" collimation , and "normal" sample density. As recommended by the manufacturer for all scanning situations, 140 kVp and 43 mAs were used. The translate-rotate mode was used with a 140 mm reconstruction circle. Images were zoomed by a factor of 1 .10. To obtain cuts approximately parallel to disks, typically a series of cuts was obtained from the top of L4 to the top of L5 and another from the top of L5 to the top of S1. No reformatting in other planes was performed in the 10 cases. Five radiologists then measured the following distances on each scan: minimum midline sagittal diameter (MSD) at L4, MSD at L5 , minimum width in the axial plane across the left L4-L5 foramen at a level where the root can be seen to traverse it (FD), and FD at L5-S1 . To avoid being influenced by the window settings at which the technologists transferred the images to the floppy disks, all radiolog ists began by viewing the studies at window widths and centers of zero, and throughout the study had the "keep window" button pressed so that they saw only the window settings they chose themselves. Apart from very specific instructions about what was to constitute MSD and FD for the purpose of the study , the radiologists were given no instructions on how to make measurements. Among other things, they were given no specific guidelines about window settings, whether to use black or white cursors, or how much attention to pay to the density reading