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Phantom equivalents of different human anatomical parts are routinely used for imaging system evaluation or dose
calculations. The various recommendations on the generic phantom structure given by organizations such as the AAPM,
are not always accurate when evaluating a very specific task. When we compared the AAPM head phantom containing 3
mm of aluminum to actual neuro-endovascular image guided interventions (neuro-EIGI) occurring in the Circle of Willis,
we found that the system automatic exposure rate control (AERC) significantly underestimated the x-ray parameter
selection. To build a more accurate phantom for neuro-EIGI, we reevaluated the amount of aluminum which must be
included in the phantom. Human skulls were imaged at different angles, using various angiographic exposures, at kV's
relevant to neuro-angiography. An aluminum step wedge was also imaged under identical conditions, and a correlation
between the gray values of the imaged skulls and those of the aluminum step thicknesses was established. The average
equivalent aluminum thickness for the skull samples for frontal projections in the Circle of Willis region was found to be
about 13 mm. The results showed no significant changes in the average equivalent aluminum thickness with kV or mAs
variation. When a uniform phantom using 13 mm aluminum and 15 cm acrylic was compared with an anthropomorphic
head phantom the x-ray parameters selected by the AERC system were practically identical. These new findings indicate
that for this specific task, the amount of aluminum included in the head equivalent must be increased substantially from
3 mm to a value of 13 mm.
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