Imaging dose in variable pitch body perfusion CT scans: an analysis using TG111 formalism

To investigate the variation of imaging dose with tube potential in variable pitch body CT perfusion (CTp) protocols using the TG111 dosimetric formalism.

TG111 recommendations were followed in choosing the phantom, dosimetric equipment, and methodology. Specifically, equilibrium doses (D(eq)) were measured centrally and peripherally in a long PMMA phantom. Reference planar average equilibrium doses were determined for each tube potential, for a reference set of exposure parameters (collimation, pitch, filtration) on a Siemens Definition CT scanner. These reference values were utilized to predict the imaging dose during perfusion scans using interpretations of the TG111 formalism. As a gold reference, the midscan average planar perfusion doses (D(CTp)) were obtained directly from central and peripheral D(eq) measurements for body CTp scans (144 and 271 mm) using variable pitch acquisition. Measurement-based D(CTp) values obtained using a thimble chamber were compared to the TG111-predicted values, and to CTDI(vol) reported at the console.

Reference planar average equilibrium dose values measured for reference uniform pitch helical scans were consistently higher than console-reported or measured values for CTDI(vol). The measurement-based perfusion dose D(CTp) was predicted accurately by the reported CTDI(vol) for the 144 mm scan. The 271 mm scans delivered systematically larger dose than reported. The TG111-based dose estimates were proven to be conservative, as they were systematically higher than both the measured and the reported imaging doses.

Upon successful implementation of TG111 formalism, standard imaging dose was measured for a body CTp protocol using the variable pitch helical acquisition. The TG111 formalism is not directly applicable to this type of acquisition. Measurement of dose for all variable pitch protocols is strongly suggested.