25. tia hình reû quaït vaø CT xoaén oác vaø coù theå laø 0.4 - 2 ñoái vôùi CBCT. Vì nhöõng lyù do naøy, ngöôøi
ta luoân mong muoán söû duïng FOV nhoû nhaát coù theå khi chuïp CBCT.
Caùc nguoàn nhieãu aûnh khaùc trong CBCT laø caùc bieán thieân thoáng keâ (statistical variations)
veà tính ñoàng nhaát cuûa chuøm tia tôùi (nhieãu löôïng töû [quantum mottle]) vaø nhieãu theâm vaøo cuûa
heä thoáng detector (nhieãu ñieän töû). Tính khoâng ñoàng nhaát cuûa photon tia X phuï thuoäc vaøo soá
löôïng tia X sô caáp vaø tia X taùn xaï bò haáp thuï, phoå tia X sô caáp vaø taùn xaï ñeán detector, vaø soá
löôïng hình chieáu cô baûn. Nhieãu ñieän töû laø do söï xuoáng caáp voán coù cuûa heä thoáng detector lieân
quan ñeán hieäu quaû haáp thuï naêng löôïng tia X taïi detector.
Ngoaøi ra, do söï phaân kyø taêng leân cuûa chuøm tia X treân caûm bieán vuøng, neân coù moät hieäu
öùng goùt chaân (heel effect) roõ reät. Hieäu öùng naøy taïo ra söï phaân boá khoâng ñoàng ñeàu cuûa
chuøm tia tôùi vaø daãn ñeán söï khoâng ñoàng ñeàu trong quaù trình haáp thuï vôùi hình aûnh coù tyû soá tín
hieäu/nhieãu (nhieãu) lôùn hôn ôû phía aâm cöïc (cathode) so vôùi phía döông cöïc (anode).
Ñoä töông phaûn cuûa moâ meàm keùm
Ñoä töông phaûn laø khaû naêng bieåu loä söï khaùc bieät tinh teá veà ñaäm ñoä tia (radiodensity) cuûa hình
aûnh. Söï khaùc nhau veà ñoä saùng laø keát quaû cuûa söï suy giaûm tia X khaùc nhau bôûi caùc moâ khaùc
nhau veà tyû troïng, soá hieäu nguyeân töû, vaø ñoä daøy. Coù hai yeáu toá chính laøm giaûm ñoä töông phaûn
cuûa hình aûnh CBCT. Thöù nhaát, böùc xaï taùn xaï laø moät yeáu toá quan troïng laøm giaûm ñoä töông
phaûn cuûa hình aûnh baèng caùch ñöa theâm vaøo caùc tín hieäu neàn (background signals) khoâng ñaïi
dieän cho giaûi phaãu, laøm giaûm chaát löôïng hình aûnh. CBCT coù ñoä töông phaûn moâ meàm keùm
hôn ñaùng keå so vôùi MDCT, vaø khoâng ñuû ñeå ñaùnh giaù chi tieát beänh lyù moâ meàm. Thöù hai, coù
raát nhieàu xaûo aûnh coá höõu do taám caûm bieán phaúng aûnh höôûng ñeán ñoä tuyeán tính (linearity) hay
ñoä nhaïy (response) ñoái vôùi tia X. Ñoä baõo hoøa (hieäu öùng pixel phi tuyeán treân moät ñoä phôi saùng
nhaát ñònh), doøng toái (dark current) (ñieän tích tích luõy theo thôøi gian khi coù hoaëc khoâng coù aùnh
saùng chieáu vaøo), vaø pixel xaáu (pixels khoâng phaûn öùng vôùi söï phôi saùng) goùp phaàn gaây ra hieän
töôïng phi tuyeán (nonlinearity). Ngoaøi ra, ñoä nhaïy cuûa caùc vuøng khaùc nhau cuûa taám caûm bieán
ñoái vôùi böùc xaï (pixel-to-pixel gain variation) coù theå khoâng ñoàng ñeàu treân toaøn boä vuøng.
KEÁT LUAÄN
CBCT laø moät coâng ngheä chaån ñoaùn hình aûnh daïng khoái hieäu quaû, taïo ra hình aûnh coù ñoä phaân
giaûi döôùi millimeter, xaùc ñaùng veà chaát löôïng chaån ñoaùn ôû caùc ñònh daïng, cho pheùp hieån thò
caùc caáu truùc xöông vuøng haøm maët ôû daïng khoái vôùi lieàu vaø chi phí thaáp hôn so vôùi MDCT.
Maëc duø deã thöïc hieän veà maët kyõ thuaät, hình aûnh CBCT neân ñöôïc xem laø moät phöông thöùc
chaån ñoaùn boå trôï cho beänh söû vaø khaùm laâm saøng. Chuïp aûnh chaån ñoaùn neân tuøy theo “nhieäm
vuï cuï theå” maø ñieàu chænh caùc giao thöùc chieáu, queùt vaø caùc tuøy choïn ñònh daïng hình aûnh ñeå toái
öu hoùa söï hieån thò hình aûnh vaø giaûm thieåu lieàu böùc xaï cho beänh nhaân.
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