The impact of vessel orientation in space on densitometric measurements of cross sectional areas of coronary arteries.

Under ideal conditions, densitometric measurement of a coronary arterial cross section in biplane angiographic images should result in nearly equal cross sectional areas for both planes. However, quite appreciable discrepancies have been found by some authors in patients. In this study, the role of inadequate spatial orientation of the vessel axes relatively to the x-rays was assessed by use of a 3D technique applied to 60 stenoses (45 pre PTCA and 15 post PTCA) in simultaneously acquired digital biplane coronary angiograms of 27 CAD patients. The 3D technique yields two radius values per projection directly in mm at any arterial cross section of interest. This was used to determine the areas Ar(in mm2) of the reference cross sections. As with catheter calibration, these cross sections were thus assumed to be more or less circular, but out-of-plane effects and errors due to a catheter diameter determination in pixels were avoided. The areas of the stenotic sections were then determined densitometrically (in mm2) from the two projections (1 and 2) according to As1 = ArDs1/Dr1, resp. As2 = ArDs2/Dr2, where Dr1, Dr2, Ds1 and Ds2 are the conventional densitometric areas of the reference and stenotic cross sections measured in planes 1 and 2. As expected, the areas As1 and As2 correlated only moderately: As2 = 0.92 As1 + 0.7 mm2, r = 0.82, n = 60, SEE = 1.4 mm2. The 3D method also yielded the two spatial angles between the local vessel axis and the X-rays of both planes. These two angles were then used to correct each densitometric area for inadequate orientation. With the corrected densitometric areas As1c and As2c, the correlation improved to: As2c = 1.05 As1c + 0.03 mm2, r = 0.93, n = 60, SEE = 0.8 mm2. Inadequate orientation of the cross sections in space thus appears to be an important factor of inaccuracy in densitometric measurements of stenotic cross sections in patients.