Geometrical parameters and scattered radiation effects on the extrinsic sensitivity and counting efficiency of a rectangular gamma camera

Determination of Gamma Camera's Calibration Factors for Quantitation of Diagnostic Radionuclides in Simultaneous Scattering and Attenuation Correction

OBJECTIVE

The characterization of cancerous tissue and bone metastasis can be distinguished by accurate assessment of accumulated uptake and activity from different radioisotopes. The various parameters and phenomena such as calibration factor, Compton scattering, attenuation and penetration intrinsicallyinfluence calibration equation, and the qualification of images as well.

METHODS

The camera calibration factor (CF) translates reconstructed count map into absolute activity map, which is determined by both planar and tomographic scans using different phantom geometries. In this study, the CF for radionuclides of Tc-99m and Sm-153 in soft tissue and bone was simulated by the Monte Carlo method, and experimental results were obtained in equivalent tissue and bone phantoms. It may be employed for the simultaneous correction of the scattering and attenuation rays interacted with the camera, leading to corrected counts. Also, the target depth (d) may be estimated by a combination of scattering and photoelectric functions, which we have published before.

RESULTS

The calibrated equations for soft tissue phantom for the radionuclides were obtained by RTc = - 10d+ 300 and RSm = -8d + 100, and the relative errors between the simulated and experimental results were 4.5% and 3.1%, respectively. The equations for bone phantom were RTc = -30d + 300 and RSm = - 10d + 100, and the relative errors were 5.4% and 5.6%. The R and d are in terms of cpm/mCi and cm. Besides, the collimators' impact was evaluated on the camera response, and the relevant equations were obtained by the Monte Carlo method. The calibrated equations as a function of various radiation angles on the center of camera's cells without using collimator indicated that both sources have the same quadratic coefficient by -2E-08 and same vertical width from the origin by 8E-05.

CONCLUSION

The presented procedure may help determine the absorbed dose in the target and likewise optimize treatment planning.

A study on the extrinsic sensitivity and counting efficiency of a gamma camera for a cylindrical source and a rectangular detector

In this work, the extrinsic counting efficiency and extrinsic sensitivity of a cylindrical source and a gamma camera with a rectangular detector were determined using homogeneous Tc-99^m. Scattered radiation effects were evaluated by both analyzing the energy spectrum of ^99mTc for the scatter fraction and plotting the extrinsic counting efficiency and sensitivity. It is found that the scattered fraction values increased with increasing source thickness. Calculated extrinsic sensitivity shows a gradually decreasing trend with increasing source thickness. The extrinsic efficiency decreases with increasing source thickness. It is concluded that increasing source to detector distances results with increasing extrinsic counting efficiency but decreasing extrinsic sensitivity.