An analytical formalism to calculate phantom scatter factors for flattening filter free (FFF) mode photon beams

Evaluation of different respiratory gating schemes for cardiac SPECT

BACKGROUND

Respiratory gating reduces motion blurring in cardiac SPECT. Here we aim to evaluate the performance of three respiratory gating strategies using a population of digital phantoms with known truth and clinical data.

METHODS

We analytically simulated 60 projections for 10 XCAT phantoms with 99mTc-sestamibi distributions using three gating schemes: equal amplitude gating (AG), equal count gating (CG), and equal time gating (TG). Clinical list-mode data for 10 patients who underwent 99mTc-sestamibi scans were also processed using the 3 gating schemes. Reconstructed images in each gate were registered to a reference gate, averaged and reoriented to generate the polar plots. For simulations, image noise, relative difference (RD) of averaged count for each of the 17 segment, and relative defect size difference (RSD) were analyzed. For clinical data, image intensity profile and FWHM were measured across the left ventricle wall.

RESULTS

For simulations, AG and CG methods showed significantly lower RD and RSD compared to TG, while noise variation was more non-uniform through different gates for AG. In the clinical study, AG and CG had smaller FWHM than TG.

CONCLUSIONS

AG and CG methods show better performance for motion reduction and are recommended for clinical respiratory gating SPECT implementation.

An Analytical Method to Calculate Phantom Scatter Factor for Photon Beam Accelerators

Introduction

One of the important input factors in the commissioning of the radiotherapy treatment planning systems is the phantom scatter factor (Sp) which requires the same collimator opening for all radiation fields. In this study, we have proposed an analytical method to overcome this issue.

Methods

The measurements were performed using Siemens Primus Plus with photon energy 6 MV for field sizes from 5×5cm² to 40×40cm². Phantom scatter factor was measured through the division of total scatter output factors (Scp), and collimator scatter factor (Sc).

Results

The mean percent difference between the measured and calculated Sp was 1.00% and -3.11% for 5×5, 40×40 cm² field size respectively.

Conclusion

This method is applicable especially for small fields used in IMRT which, measuring collimator scatter factor is not reliable due to the lateral electron disequilibrium.