Accuracy of reference air kerma in estimating incident air kerma at skin in interventional neuroradiology: Effect of X-ray field size and alternative PERP corrections.
Fukuda Atsushi A, Ichikawa Nao N, Hayashi Takuma T, Lin Pei-Jan Paul PP et al.
Kerma-area product (KAP) meters monitor reference air kerma ( K a , r ) and KAP ( P K A ) . While K a , r K A P , H y b r i d is calculated using P K A and collimator positions, K a , r K A P , M e a s . is directly measured via a small central ionization chamber. Although K a , r is a common surrogate for incident air kerma at the skin ( K a , s k i n ), its clinical application to interventional neuroradiological procedures carries higher uncertainty than regular interventional radiology, with source-to-skin distance being the greatest factor due to the smaller head size. Additionally, K a , r may not perfectly represent actual K a , s k i n due to variations in beam quality, filtration, and X-ray field size. This study investigated (1) the accuracy of K a , r K A P , H y b r i d and K a , r K A P , M e a s . , and (2) the feasibility of employing them as surrogates for K a , s k i n during cerebral angiography and interventional neuroradiological procedures. A solid-state dosimeter (AGMS-DM+) was placed at the patient entrance reference point (PERP) on a polymethyl methacrylate phantom to measure K a , r A G M S - D M + . Accuracies of K a , r K A P , H y b r i d and K a , r K A P , M e a s . were evaluated against K a , r A G M S - D M + in two investigations. First, effects of tube voltage and spectral shaping filters (1.2 mmAl, 0.03 mmTa, and 0.2 mmCu) were evaluated with/without equalization filters inserted symmetrically by 2.5 cm from both sides at a fixed X-ray field size of 10 × 10 cm2. Second, the effect of varying the X-ray field size was investigated for each filter. Additionally, a fluoroscopic system and a head phantom were modeled using computer-aided design software. Three fields of view (FOVs) were evaluated to simulate different clinical stages: 30 × 30 cm2 (for whole angiography), 15 × 15 cm2, and 3.8 × 3.8 cm2 (for targeted interventional neuroradiology). Target centers were aligned with the brain center and precallosal segment of the anterior cerebral artery. Six typical clinical projections were used per FOV to determine source-to-skin distance and X-ray field size at the PERP, allowing calculation of the skin-level errors for both K a , r K A P , H y b r i d and K a , r K A P , M e a s . . The effects of the corrections were also investigated using the X-ray field size measured by P K A / K a , r K A P , M e a s . and an alternative PERP (aPERP) positioned 11 cm from the isocenter toward the X-ray focus. Relative errors for both methods as functions of tube voltage across the three filters were within ± 4.0%. With equalization filters, K a , r K A P , H y b r i d errors ranged from -49.0%--33.9%, exceeding the acceptable accuracy tolerance (± 35%), whereas K a , r K A P , M e a s . errors (-15.8%--10.8%) remained within the tolerance. The relative errors of K a , r K A P , H y b r i d and K a , r K A P , M e a s . as a function of the X-ray field size ranged from -9.7%-12.7% and -58.3%-26.1%, respectively. Although most skin-level errors for both K a , r K A P , H y b r i d and K a , r K A P , M e a s . remained within the ± 35% tolerance in clinical simulation, those for K a , r K A P , M e a s . at the 3.8 × 3.8 cm2 FOV exceeded it. However, with the X-ray field size and aPERP corrections, the errors for K a , r K A P , M e a s . were reduced to within ± 2.7%. K a , r K A P , H y b r i d and K a , r K A P , M e a s . should be strategically selected based on the specific clinical application, such as standard cerebral angiography versus targeted interventional neuroradiology. Particularly, K a , r K A P , M e a s . provides superior accuracy when equalization filters are used or when appropriate corrections for X-ray field size and aPERP are applied in small-field dosimetry.