Variation of Imaging Dose and Image Quality with Tube Potential in Body Perfusion CT. An Analysis Using TG111 Formalism
M Axente*, D Hristov, Stanford University Cancer Center, Palo Alto, CAMO-B-Salon EF-6 Monday 10:00:00 AM - 12:00:00 PM Room: Salon EF
Purpose: To investigate the variation of imaging dose and image quality with tube potential in body perfusion protocols using the task group 111 dosimetric formalism.
Methods:TG111 recommendations were followed in choosing the phantom, dosimetric equipment, and methodology. Specifically, equilibrium doses were measured centrally and peripherally, and an average planar equilibrium dose was determined for each tube potential, for a reference set of exposure parameters (collimation, pitch, filtration) on a Siemens Somatom Definition AS+ scanner. This was compared to an equivalent dosimetric index, the weighted CT dose index. To analyze the image quality variation with tube voltage, inserts with different concentrations of iodinated contrast agent were placed in the phantom. The imaging was done using a 144 mm, 1.5 s body perfusion protocol in shuttle mode, while the imaging doses were kept constant.
Results:Equilibrium dose increases with tube potential following an approximate kVp cube function. Furthermore, there is a significant underestimation of the imaging dose as reported based on CTDI measurements, ranging from 51% at 70 kVp to 58% at 140 kVp. CNR values generally decrease with increasing tube potential. Acceptable contrast-to-noise ratios values (70 kVp: 1.7; 80 kVp: 1.3) for the lowest concentrations of agent used, indicating that lower patient doses could be utilized for imaging.
Conclusion:Significant dose underestimation by the console report was noted. When imaging different concentrations of imaging agents, it was observed that for the same imaging dose lower kVp values, and inherently lower imaging doses can produce acceptable CNR images for all investigated concentrations. Future work will include further investigations in the TG111 parameterized dose metrics and assumptions. A full dosimetric evaluation of current body perfusion protocols will determine the exposure parameters associated with their clinical implementation. Furthermore, the scanning speed, sampling frequency and tube current limits will also be investigated.
Research funded by Siemens Medical Solutions.