我々は,International Atomic Energy Agency (IAEA)肝ファントムを用いて^<67>Gaプレイナ画像における種々のphotopeak収集が画像に及ぼす影響について,画像コントラスト及び画像周波数解析を応用して定量的評価を行った。ファントムはIAEA肝ファントムを使用し,ファントム内に^<67>Gaを封入して実験を行った。評価した^<67>Gaのphotopeakは,93keV,185keV,300keVの組み合わせで画像収集を行った。画像コントラストは,散乱体の有無により大きく変化し,散乱体無しで93peak収集画像のコントラストが高くなったが,高いエネルギーが加わるとコントラスト低下が生じた。一方,散乱体が加わるとその厚さが増すにつれてコントラストが低下し,特に93peakでのコントラストが他のphotopeakに比較して著しく低下した。画像周波数解析では散乱体がない場合,動径強度分布関数(Pr(n))は93peak>93+185peak>3peak≒93+300peak>185peak>185+300peak>300peakの順となり93peakのphotopeakが良い結果となった。しかし,散乱体厚を増加するにつれて信号強度は弱くなり3peak>185peak>93+185peak≒93+300peak≒300peak≒185+300peak>93peakの順で3peakが良い結果となった。また,93peakのPr(n)は,0.1～0.3cyles/cmの低周渉領域の信号強度の劣化が激しく散乱体がない場合のそれに比較して約20%も低下した。散乱体厚が10cm以上の場合には,93peakは深部の情報を検出できなく,従来の3peak収集が良い結果となった。臨床におけるクエン酸ガリウム(^<67>Ga)プレイナ画像での収集photo peakは,単検出器型シンチレーションカメラでは高いphotopeakを含む3peak収集が良く,2検出器型シンチレーションカメラでは93+185peak収集が最も良い収集方法であった。

The purpose of this study was to analyze quantitatively the influence of acquisition of various combinations of photo-peaks on gallium-67 planar images using frequency analysis. A liver phantom provided by the International Atomic Energy Agency was imaged using a GCA-901WB (Toshiba, Tokyo) ; 37MBq of gallium-67 was injected into the phantom, and its planar images were acquired with each of three photo-peaks and the following combinations ; one each of 93, 185 and 300keV peaks, two of the three peaks, i.e. 93 and 185, 93 and 300, and 185 and 300 keV peaks, and all of the three peaks. Image contrast was varied with and without scatter material. Without scatter material, the 93keV-peak image had the highest contrast, and the contrast degraded when adding a higher energy photo-peak or photo-peaks. With scatter material, the image contrast decreased when the scatter material thickness increased, and the contrast degradation of the 93keV-peak image was particularly remarkable. Radius distribution function (Pr(n)) was used for quantitative frequency analysis. Images were collected up to 500 kcounts, and tough water phantoms (thicknesses: 5, 10, 20cm) were used as scatter material. The highest Pr(n) was obtained with the 93keV-peak without scatter material ; however, with a 20cm-thick tough water phantom, the image with three peaks had the highest Pr(n). According to the increase in tough water phantom thickness, the intensity of Pr(n) for the 93keV-peak in the range from 0.1 to 0.3 [cycles/cm] decreased by 20% of that without the tough water plate. When the thickness of the scatter material was over 10cm, 93keV-photons from the deep portion of the phantom were not detected properly. The optimal energy photo-peaks were three peaks for a single-head camera and the combination of 93 and 185keV peaks for a dual-head camera.

原著

Original Article

国立情報学研究所で電子化