➨5❶㻌 ప⥺㔞CT⏬ീ䝅䝭䝳䝺䞊䝅䝵䞁䛾䝣䜯䞁䝖䝮䛻䜘䜛⏬㉁ホ౯
᧯᧲
➨5❶㻌 ప⥺㔞CT⏬ീ䝅䝭䝳䝺䞊䝅䝵䞁䛾䝣䜯䞁䝖䝮䛻䜘䜛⏬㉁ホ౯
᧯᧳
Fig. 5.3 Demonstration of low-dose simulation for Catphan and TOS phantoms: (a), (b) real high-dose CT images (300 mAs), (c), (d) real low-dose CT images (100 mAs), (e), (f) simulated low-dose images (100 mAs).
Fig. 4.4
㻌(a)
F
㻌(c)㻌
(b)㻌
(d)㻌
(f)㻌 (e)㻌
➨5❶㻌 ప⥺㔞CT⏬ീ䝅䝭䝳䝺䞊䝅䝵䞁䛾䝣䜯䞁䝖䝮䛻䜘䜛⏬㉁ホ౯
᧯᧴
Table 5.2 Comparison of noise in terms of standard deviation for Catphan and TOS phantoms. High-dose and low-dose correspond to 300 mAs and 100 mAs, respectively. The noise model and the reconstruction filter were determined from an ACR phantom.
Phantom
High-dose image (HU)
Low-dose image (HU)
Simulated low-dose image (HU)
Catphan 15.1 ± 0.2 26.7 ± 0.3 27.5 ± 0.2
TOS 50.7 ± 0.5 89.5 ± 0.7 89.8 ± 0.6
Fig. 5.4䛻䝅䝭䝳䝺䞊䝅䝵䞁ప⥺㔞CT⏬ീ䛸ᐇ㝿䛾ప⥺㔞CT⏬ീ䛾ゎീᗘ≉ᛶ䛾ẚ㍑䛾䛯
䜑䛻ィ 䛧䛯 MTF䜢♧䛩䠊Fig. 5.4a䠈5.4b 䛿䛭䜜䛮䜜䠈Catphan 䛸TOS䝣䜯䞁䝖䝮䛾MTF 䜢
♧䛩䠊ప⥺㔞CT䝅䝭䝳䝺䞊䝅䝵䞁⏬ീ䛿ᐇ⥺㔞300 mAs䛛䜙సᡂ䛧䛯100 mAs䠈200 mAs䠈ᐇ
⥺㔞 200 mAs 䛛䜙సᡂ䛧䛯 100 mAs 䛾 3 ✀䛷䠈䛭䜜䛮䜜䛻䛴䛔䛶䛾 MTF 䛜♧䛥䜜䛶䛔䜛䠊
TOS䝣䜯䞁䝖䝮䛾MTF䛿䜟䛪䛛䛻ኚື䛧䛶䛔䛯䛜䠈䝅䝭䝳䝺䞊䝅䝵䞁⏬ീ䛻䛚䛡䜛MTF䛿ᐇ㝿䛾 㧗⥺㔞⏬ീ䛾 MTF 䛸䜋䜌୍⮴䛧䛯䠊䛣䜜䜙䛾⤖ᯝ䛛䜙Catphan䠈TOS䝣䜯䞁䝖䝮䛻䛚䛔䛶䛿䠈ప
⥺㔞CT⏬ീ䝅䝭䝳䝺䞊䝅䝵䞁䛷䛿䠈CT⏬ീ䛾ゎീᗘ≉ᛶ䜈䛾ᙳ㡪䛜ᑡ䛺䛔䛣䛸䛜ド᫂䛥䜜䛯䠊
Fig. 5.5a䠈5.5b 䛻䛭䜜䛮䜜䠈Catphan 䛚䜘䜃 TOS 䝣䜯䞁䝖䝮䛻䛚䛡䜛 NPS 䜢♧䛩䠊ୖグ䛾
MTF ィ 䛸ྠᵝ䛾᮲௳䛷ప⥺㔞 CT ⏬ീ䝅䝭䝳䝺䞊䝅䝵䞁䜢⾜䛳䛯䠊200 mAs䠈300 mAs 䛾୧ CT⏬ീ䛛䜙ᚓ䜙䜜䛯100 mAs䛾ప⥺㔞⏬ീ䛾NPS䛿ᐇ㝿䛻100 mAs䛷ᙳ䛥䜜䛯⏬ീ䛾 NPS 䛸ྠ➼䛷䛒䛳䛯䠊䛣䜜䜙䛾⤖ᯝ䛛䜙䠈Catphan䠈TOS 䝣䜯䞁䝖䝮䛻䛚䛔䛶䛿䠈䝅䝭䝳䝺䞊䝅䝵䞁
⏬ീ䛿ᐇ㝿䛾CT⏬ീ䛸ྠ➼䛾䝜䜲䝈≉ᛶ䜢ᣢ䛳䛶䛔䜛䛣䛸䛜♧䛥䜜䛯䠊
ᇶ‽䝣䜯䞁䝖䝮䛾㐪䛔䛜䝅䝭䝳䝺䞊䝅䝵䞁⏬ീ䛻䛘䜛ᙳ㡪䜢᳨ウ䛩䜛䛯䜑䛻ồ䜑䛯䝇䝷䜲䝇ཌ 䛸䝟䝷䝯䞊䝍ܿ䛾㛵ಀ䜢䛭䜜䛮䜜Fig. 5.6a, 5.6b䛻♧䛩䠊ᮏ᳨ウ䛷䛿䠈䝅䝭䝳䝺䞊䝅䝵䞁䛻⏝䛔䜛㧗
⥺㔞䛸ప⥺㔞䛿䠈䛭䜜䛮䜜300 mAs䛸100mAs䛸䛧䛯䠊䛣䜜䜙䛾⤖ᯝ䛛䜙䠈ᇶ‽䝣䜯䞁䝖䝮䛾㐪䛔 䛸⥺㔞䛿䝟䝷䝯䞊䝍ܿ䛻䛭䜜䜋䛹䛝䛺ᙳ㡪䜢䛘䛺䛔䛜䠈䝟䝷䝯䞊䝍ܿ䛿䝇䝷䜲䝇ཌ䛻౫Ꮡ䛧㠀⥺
ᙧ䛻ኚ䛩䜛䛣䛸䛜♧䛥䜜䛯䠊
➨5❶㻌 ప⥺㔞CT⏬ീ䝅䝭䝳䝺䞊䝅䝵䞁䛾䝣䜯䞁䝖䝮䛻䜘䜛⏬㉁ホ౯
᧯᧵
Fig. 5.4 Comparison of MTFs measured from real CT images and simulated images. (a) Catphan phantom, and (b) TOS phantom.
0.0 0.5 1.0
0.0 0.5 1.0 1.5
MTF
Spatial frequency [cycles/mm]
real 100 mAs CT image real 200 mAs CT image real 300 mAs CT image
simulated 100 mAs from 200 mAs simulated 100 mAs from 300 mAs simulated 200 mAs from 300 mAs
0.0 0.5 1.0
0.0 0.5 1.0 1.5
MTF
Spatial frequency [cycles/mm]
real 100 mAs CT image real 200 mAs CT image real 300 mAs CT image
simulated 100 mAs from 200 mAs simulated 100 mAs from 300 mAs simulated 200 mAs from 300 mAs (b)㻌 (a)㻌
➨5❶㻌 ప⥺㔞CT⏬ീ䝅䝭䝳䝺䞊䝅䝵䞁䛾䝣䜯䞁䝖䝮䛻䜘䜛⏬㉁ホ౯
᧰᧬
Fig. 5.5 Comparison of NPSs measured from real CT images and simulated images. (a) Catphan phantom, and (b) TOS phantom.
1 10 100 1000
0.0 0.2 0.4 0.6
NPS [mm2]
Spatial frequency [cycles/mm]
real 100 mAs CT image real 200 mAs CT image real 300 mAs CT image
simulated 100 mAs from 200 mAs simulated 100 mAs from 300 mAs simulated 200 mAs from 300 mAs
1 10 100 1000 10000
0.0 0.2 0.4 0.6
NPS [mm2]
Spatial frequency [cycles/mm]
real 100 mAs CT image real 200 mAs CT image real 300 mAs CT image
simulated 100 mAs from 200 mAs simulated 100 mAs from 300 mAs simulated 200 mAs from 300 mAs (a)㻌
(b)㻌
➨5❶㻌 ప⥺㔞CT⏬ീ䝅䝭䝳䝺䞊䝅䝵䞁䛾䝣䜯䞁䝖䝮䛻䜘䜛⏬㉁ホ౯
᧰᧭
Fig. 5.6 Relationship between slice thickness and parameter c of the noise model. (a) Effect of reference phantoms. (b) Effect of exposure doses when an ACR phantom was used as a reference phantom.
0.0E+00 5.0E-05 1.0E-04 1.5E-04 2.0E-04 2.5E-04 3.0E-04 3.5E-04
0.00 1.25 2.50 3.75 5.00
parameter c[mAs]
Slice Thickness [mm]
ACR Catphan TOS
0.0E+00 5.0E-05 1.0E-04 1.5E-04 2.0E-04 2.5E-04 3.0E-04 3.5E-04
0.00 1.25 2.50 3.75 5.00
parameter c [mAs]
Slice Thickness [mm]
simulated 100 mAs from 200 mAs simulated 100 mAs from 300 mAs simulated 200 mAs from 300 mAs (a)㻌
(b)㻌
➨5❶㻌 ప⥺㔞CT⏬ീ䝅䝭䝳䝺䞊䝅䝵䞁䛾䝣䜯䞁䝖䝮䛻䜘䜛⏬㉁ホ౯
᧰᧮
䛥䜎䛦䜎䛺ᅉᏊ䛜ᵓᡂ䝣䜱䝹䝍䛻䛘䜛ᙳ㡪䛻䛴䛔䛶ồ䜑䛯⤖ᯝ䜢Fig. 5.7䛻♧䛩䠊ᇶ‽䝣 䜯䞁䝖䝮䜢ACR䝣䜯䞁䝖䝮䛸䛧䛶䠈ᐇ⥺㔞300 mAs䛛䜙100 mAs䜢䝅䝭䝳䝺䞊䝅䝵䞁䛧䛯䛸䛝䛾䝇䝷 䜲䝇ཌ䛾ᙳ㡪䜢Fig. 5.7a䠈ᇶ‽䝣䜯䞁䝖䝮䜢ACR䝣䜯䞁䝖䝮䛸䛧䛶䠈䝇䝷䜲䝇ཌ0.625 mm䛷ồ䜑䛯
⥺㔞䛾ᙳ㡪䜢Fig. 5.7b䛻♧䛩䠊䛥䜙䛻䠈ᐇ⥺㔞300 mAs䛛䜙100 mAs䜢䝅䝭䝳䝺䞊䝅䝵䞁䛩䜛 㝿䛻䠈䝇䝷䜲䝇ཌ䜢 0.625 mm ୍ᐃ䛸䛧䛶䠈ᇶ‽䝣䜯䞁䝖䝮䛰䛡䜢ኚ䛥䛫䛯ሙྜ䛾ᙳ㡪䜢 Fig.
5.7c 䛻♧䛩䠊䛣䜜䜙䛾⤖ᯝ䜘䜚䠈ᵓᡂ䝣䜱䝹䝍䛿䝇䝷䜲䝇ཌ䠈䝅䝭䝳䝺䞊䝅䝵䞁䛾⥺㔞䠈ᇶ‽䝣䜯䞁 䝖䝮䛻ᙳ㡪䜢ཷ䛡䛺䛔䛣䛸䛜♧䛥䜜䛯䠊
0.0 0.2 0.4 0.6
0.0 0.2 0.4 0.6
Enhancement Factor
Spatial Frequency [cycles/mm]
0.625 mm 1.250 mm 2.500 mm 5.000 mm RL Filter
㻌 (a)㻌
➨5❶㻌 ప⥺㔞CT⏬ീ䝅䝭䝳䝺䞊䝅䝵䞁䛾䝣䜯䞁䝖䝮䛻䜘䜛⏬㉁ホ౯
᧰᧯
Fig. 5.7 Effects of various factors on the reconstruction filter. (a) Effect of slice thickness for low-dose simulation of 100 mAs from 300 mAs with the reference ACR phantom. (b) Effect of exposure doses with slice thickness of 0.625 mm and the reference ACR phantom. (c) Effect of reference phantoms for low-dose simulation of 100 mAs from 300 mAs with the slice thickness of 0.625 mm.
0.0 0.2 0.4 0.6
0.0 0.2 0.4 0.6
Enhancement Factor
Spatial Frequency [cycles/mm]
200 mAs → 100 mAs 300 mAs → 100 mAs 300 mAs → 200 mAs RL Filter
0.0 0.2 0.4 0.6
0.0 0.2 0.4 0.6
Enhancement Factor
Spatial Frequency [cycles/mm]
ACR Catphan TOS RL Filter (b)㻌
(c)㻌
➨5❶㻌 ప⥺㔞CT⏬ീ䝅䝭䝳䝺䞊䝅䝵䞁䛾䝣䜯䞁䝖䝮䛻䜘䜛⏬㉁ホ౯
᧰᧰