SIZE ESTIMATION AND MAGNIFICATION ERROR IN RADIOGRAPHIC IMAGING - IMPLICATIONS FOR CLASSIFICATION OF ARTERIOVENOUS-MALFORMATIONS

PURPOSE: To assess magnification error in digital subtraction angiogra phy as it pertains to arteriovenous malformation (AVM) size. METHODS: A rectangular grid phantom with equally spaced markers mounted in a st ereotactic frame was imaged with digital angiographic equipment. The l ocation and orientation of the grid was altered relative to the centra l plane of the phantom. Both linear and area measurements were made ac cording to the perceived location of phantom markers using a standard catheter calibration technique and compared with stereotactically deri ved estimates. Finally, a single case example of an angiographically i maged rolandic AVM was used to compare linear dimensions obtained with both described techniques. RESULTS: The determination of location and size with standard angiographic imaging is subject to error because o f the divergent geometry of the incident x-ray beam. The resulting non constant geometric magnification causes errors in linear measurements of 10% to 13% at depths of 7 cm from the calibration plane, Errors in area measurements at the same position increase by 20% to 25%, Measure ments of maximum diameter or cross-sectional area may have an addition al error when nonspherical objects are inclined to the viewing directi on (40% at 45 degrees inclination). These errors are reduced to less t han 1 mm using the stereotactic technique. Some commercial angiographi c systems have internal software to enable a spatial calibration based on known distances in the image or on the diameter of a catheter. The catheter technique was accurate in the calibration direction (perpend icular to the catheter axis) but had a 12% error in the direction para llel to the catheter because of a nonunity aspect ratio in the video s ystem. Measurement of the dimensions of a rolandic AVM using the cathe ter calibration technique had an error that ranged from -3% to +26% (s tandard error, 20%) with respect to the stereotactic technique. CONCLU SIONS: Numerous nonstereotactic referential systems for determining li near distances are inherently erroneous by varying degrees compared wi th the stereotactic technique. Area and volume determinations naturall y increase this error further. To the extent that no standardized meth od for determining linear distances exists, significant Variations in estimation of AVM size result. Classification schemes for AVMs have be en hampered by this technical error.