Validation of optimal cut-off frequency using a Butterworth filter in single photon emission computed tomography reconstruction for the target organ: Spatial domain and frequency domain

In single photon emission computed tomography (SPECT) images, we evaluated cut-off frequency using two methods: spatial domain method (normalized mean square error: NMSE) and frequency domain method (radius direction distribution function in the power spectrum: Pr (n)) and we calculated the optimal cut-off frequency of the Butterworth filter according to the nuclide and collimator used, and the target organ. The optimized cut-off frequencies (Fc) were determined for nuclides of 99m-Tc, 123-I, 201-Tl, and LEHR and LEGP collimators, and compared. The Pr (n) was used to evaluate the SPECT images for frequency domain analysis, and the NMSE method was used for the assessment of images in spatial domain. In the brain phantom for both of these methods of analysis, the optimal Fc varies depending on the nuclide and collimator. Fc in use for 99m-Tc is 0.802 [cycles/cm] with LEHR and 0.656 [cycles/cm] with LEGP. However, those in use for 123-I are 0.656 [cycles/cm] with LEHR. In the myocardial phantom, the appropriate Fc are 0.516 [cycles/cm] with LEHR, and 0.469 [cycles/cm] with LEGP in use of 99m-Tc. We concluded that the cut-off frequency of the Butterworth filter should be changed in reconstructing SPECT images according to the collimator, nuclide and target organs.我々は，脳ファントム及び心臓ファントムで周波数空間と実空間の評価でSPECT 画像における使用核種，コリメータ及び標的臓器のButterworthフィルタの最適遮断周波数の算出を試みた。周波数空間での評価は動径強度分布関数（Pr（n））を用い，実空間での評価はNMSE法を用いた。脳ファントムでは核種として99m-Tcと123-Iを用い，心臓ファントムでは99m-Tc及び201-Tlを使用した。また，コリメータはLEHR及びLEGPを使用した。脳ファントムでは99m-Tcにおける最適遮断周波数は，LEHRで0.802 [cycles/cm], LEGPで0.656 [cycles/cm] と変化した。しかし123-IではLEHRで0.656 [cycles/cm]であった。心臓ファントムでは99m-TcでLEHRは0.516 [cycles/cm], LEGPで0.469 [cycles/cm]と変化した。また，同様に201-Tlでも異なった遮断周波数が算出された。この結果から，SPECT画像再構成時でのButterworthフィルタの遮断周波数は，使用核種，コリメータ，標的臓器により変化させなければならない。原著Original Article