Subtraction Angiography: Comparative Performance And Cost Of Digital Vs. Film Subtraction Methods

Subtraction angiography: Comparative performance and cost ofdigital vs. film subtraction methodsFlynn MJ, Patel S, Zerwekh J, Clark J, Wilderman SDepartment of Diagnostic Radiology, Henry Ford Hospital2799 W. Grand Blvd., Detroit, MI 48202AbstractThe radiation dose, image signal -to -noise ratio, image resolution and cost of film /screenand digital subtraction angiography have been investigated. Theoretical analysis indicatesthat the dose normalized signal -to -noise ratio for DSA is 50% better than film /screenmethods. Iodinated vessel phantoms have confirmed this result. An evaluation of 55patients indicates improved small vessel detection, better delineation of diffuse contraststains, and fewer artifacts with DSA when compared with film /screen results on the samepatients. Substantial reduction in cost for DSA methods is attributed to film utilizationreduction as well as reduced examining times.IntroductionDigital Subtraction Angiography, when introduced over five years ago, was touted forit's ability to diagnose vascular disorders with easily performed, venous injections.However, poor image quality associated with the dilution of contrast has left IV -DSA withonly a limited role, while conventional film angiography with arterial contrast injectionshas continued to be employed at most institutions. Recently, digital subtractionangiography performed with contrast material selectively injected into a specific arteryhas been recommended) as a preferred procedure relative to conventional film angiography;however, skeptism amongst angiography specialists and surgeons has remained.Over the last year we have been studying the relative attributes of digital and filmbased subtraction techniques for arterial angiography. Our method has involved the theore-tical modeling of the two image receptors, DSA and film /screen experimental phantom com-parisons, an absolute procedural cost determination, and the clinical evaluation of over 50patient studies on whom both digital and film subtraction angiography was performed usingequivalent arterial injections. This paper reports the essential conclusions for eachmethod of evaluation.TheoryUsing a numeric model, we have calculated the sensitivity of a DSA system and a rareearth screen /film system for detecting iodine contrast within tissue. The numeric modelpredicts the Bremsstrahlung and characteristic photon spectrum using semiemperical rela-tionships2,3. The accuracy of the x -ray spectrum predictions has been verified by com-parison with published data4'5. The effects of inherent tube filtration, added filtration,tissue filtration, iodine filtration and grid filtration were numerically modeled usingMcMaster's interaction coefficients6. The DSA and film /screen system responses were calcu-lated by determining the average absorbed energy deposited in the DSA systems cesium iodideinput phosphor or the screen of the film /screen system. The absorbed energy was calculatedfrom a monte carlo radiation transport code7 which accounts for all secondary charac-teristic radiations which may escape from the scintillation phosphor. Using similarmethods, the average absorbed dose to a 10 cm. thick tissue sample was calculated.