在PET或SPECT系統裡，身體中間發出的光子其穿過的物質要比由身體邊緣的多，因此實際上投影數據中央會受到更多的衰減。重建出來的影像，中央的活性較實際值小，而邊緣的較高，此為衰減效應。為了修正此衰減效應，需要把投影數據的衰減部份補償回去。現今的衰減修正方法主要有二:其一是以附加射源(如Co-60)在體外旋轉，經由影像重建原理，求得體內衰減係數分布，本法需要硬體重整相關的配合；其二是將體內各點的衰減係數均假設為定值，僅修正距離部份。本研究方法是利用軟體計算由已知的CT標準體經由扭曲標準軸幾何轉換算出待測體的內部結構。如此病人既不用再接受額外的劑量即可推知體內的衰減係數分布，如此來作衰減修正，將可增加核醫影像的準確性。

In PET or SPECT system, the photon transited from the midst of the body passes through more materials than those from the edge of the body does. Therefore, the center of the projection will be attenuated more than the edge of the projection. As a result, in the reconstructed image, the activity in the center section will be less than actual situation, and the activity of the edge section is on the contrary. It’s called attenuation effect. In order to correct this effect, we must compensate the attenuation in the projection data.
Currently, there are two major methods for attenuation correction. First, use a radiation source (ex. Co-60 ) to rotate round the patient body. Second, using Chang’s method, it assume that total attenuation coefficient in the body is constant, which is usually equal to water. Accordingly, we just correct the error from the distance. If the attenuation coefficient is not uniform, the error might be large. So, we focus on the attenuation coefficient in the body and search for new method. How do we know the inner attenuation coefficient distribution? We can calculate unknown inner structure from known standard CT slice, using standard-axis distortion interpolation method. Then, obtaining the body’s attenuation coefficient distribution information, we can repeat Chang’s calculation with adding attenuation coefficient information. In this way, it could increase precision of nuclear medicine image, and also the patients may avoid receiving additional radiation dose.

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