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研究生:蕭雅慧
論文名稱:以統計方法比較JPEG壓縮與小波壓縮在低壓縮比時的醫學影像品質
論文名稱(外文):Evaluating medical image quality of JPEG compression and wavelet compression at low compression ratio by using statistic methods
指導教授:莊克士老師
學位類別:碩士
校院名稱:國立清華大學
系所名稱:原子科學系
學門:工程學門
學類:核子工程學類
論文種類:學術論文
論文出版年:2002
畢業學年度:91
語文別:中文
論文頁數:77
中文關鍵詞:有損壓縮小波壓縮JPEG壓縮統計方法
外文關鍵詞:lossy compressionstatistic methodwavelet compressionJPEG compressionMoran’s I test
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隨著數位影像廣泛應用,有損壓縮 (lossy compression) 將逐漸成為壓縮方法的主流,但其可能造成重要資訊損失,故有損壓縮在醫學影像上僅適合低倍壓縮,而欲如何提高壓縮比又不影響影像品質,是大家追求的目標,因此影像品質評估便扮演著重要的角色,目前以ROC (Receiver Operating Characteristic) 方法最常被使用,主要用來評估現在醫學影像上,廣被提及的小波壓縮 (wavelet compression) 與JPEG (Joint Photographic Experts Group) 壓縮的影像品質,但在低壓縮比時,影像品質變化甚少,難以肉眼評估,使得哪種壓縮方法好,至今尚未有一論。因此本研究主要是利用客觀的統計方法,比較小波與JPEG壓縮兩種壓縮影像品質。
本文中採用CR、CT和T1等三種醫學影像,並使用數種影像品質評估指標,作為小波與JPEG壓縮在三種醫學影像低壓縮比 (約小於25倍) 時,影像品質評估之較客觀依據。結果顯示10倍壓縮比以下,JPEG壓縮較小波壓縮好;而當壓縮比約10至25倍時,則顯示JPEG壓縮較小波壓縮好。未來希望能將這些統計方法應用在更多影像品質評估上。

With the widely applications of digital images, the lossy compression will gradually become the mainstream of compression methods. Because this method might cause the lost of important information, only compression with low ratio are considered on medical images. The goal that we are pursuing is to maximize compression ratio while keeping image quality unchanged. Therefore image quality assessment plays an important role for it. ROC (Receiver Operating Characteristic) is a well-known method for evaluating wavelet compression and JPEG (Joint Photographic Experts Group) compression. Human eyes are difficult to differentiate the nuances of compressed images at low compression ratio. Our research focuses on evaluating image quality by using statistic methods. We develop some image quality indexes using Moran’s I test, kurtosis, etc to evaluate the image quality of wavelet and JPEG compression at low compression ratio (about < 25 times). The results indicate that JPEG compression is better than wavelet compression when the ratio is below 10 times and the reverse is true at compression ratio from 10 to 25 times.

中文摘要…………………………………………………………Ⅰ
英文摘要…………………………………………………………Ⅱ
目錄………………………………………………………………Ⅲ
圖目錄……………………………………………………………VI
表目錄………………..……………………………………...…IX
第一章 緒論…………………………………………………….1
1.1 引言…………………………………………………1
1.2 研究動機……………………………………………2
1.3 文獻回顧……………………………………………5
1.4 論文架構……………………………………………7
第二章 影像壓縮………………………………..…..………….9
2.1 JPEG壓縮法..………………………………………11
2.2 小波壓縮法…………………………………………14
2.3 有損耗壓縮過程對影像品質的影響………………16
第三章 研究方法………………………………………………18
3.1 Moran’s I 測試……………………………..……….18
3.2 峰度……………………………………..…………..23
3.3 影像品質比較指標………………………………..23
3.3.1 Z-直方圖和尖峰比…….………………..23
3.3.2 平均平方差…………..…………………..25
3.3.3 平均平方Z值差………………………....26
3.3.4 Moran’s Z_rG…………………………..26
3.3.5 Kurtosis_rG…………………………….29
3.3.6 變異數……………………………………29
3.3.7 訊號雜訊比………………………………30
3.3.8 Quality Index……………………………..31
3.4 區塊效應…………………………………………....32
第四章 結果……….………………………………………..….35
4.1 影像資料及背景資訊……………………………..35
4.2 影像品質比較指標的結果………………………..37
4-2.1 Z直方圖和尖峰比的結果…………….… 37
4-2.2 平均平方差之結果………………………..38
4-2.3 平均平方Z值差之結果…………………..39
4-2.4 Moran’s Z_∆G之結果…………………….39
4-2.5 Kurtosis_∆G之結果……………..………..40
4-2.6 變異數之結果……………………….…….41
4-2.7 訊號雜訊比之結果……………………...…41
4-2.8 Quality Index之結果………………………42
4.3 區塊效應對結果的影響………………………...……42
4.3.1 區塊效應影響尖峰比的結果…………....…42
4.3.2 區塊效應影響MSZE的結果……….….….44
第五章 討論…………………………………………………..…63
5.1 影響影像品質評估之原因…………………………..63
5.1.1 影像對比…………….………..……………63
5.1.2 雜訊………………….……………………..65
5.1.3 區塊效應…………………….…………….65
5.2 CR、CT影像在壓縮影像品質評估方面之探討…...66
5.3 T1影像在壓縮影像品質評估方面之探討……….…68
5.4 在低倍壓縮比時的去雜訊現象……………….……70
5.5單點比較與視窗比較法……………………………..70
5.6 統計方法的優點………………….…..……………..71
第六章 結論……………………………………………………..73
參考文獻……………………………………..……………………75

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