image generation in ct. ct exam protocol and parameters selection patient positioning scanning data...
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IMAGE GENERATION IN CT
CT EXAM
• PROTOCOL AND PARAMETERS SELECTION
• PATIENT POSITIONING
• SCANNING
• DATA RECONSTRUCTION
• IMAGE DISPLAY
PROTOCOL AND TECHNIQUE SELECTION
• PROTOCOL• PARAMETERSSCANNING MODEkVpmATimeRECONSTRUCTIONPATIENT POSITIONPATIENT ORIENTATIONSFOVDFOV
PROTOCOL
• PEDIATRIC OR ADULT
• HEAD, ABDOMEN, EXTREMITIES
• VASCULAR OR REGULAR
SCANNING MODE
• AXIAL (SLICE BY SLICE)
• SPIRAL (HELICAL)
TECHNIQUE
• kVp
• mA
• time
RECONSTRUCTION
• SECTION THICKNESS
• SECTION INDEX
• ALGORITHM
PATIENT POSITION
• SUPINE OR PRONE
• BODY PART UNDER SCRUTINY MUST BE PLACED IN THE ISOCENTER OF THE SCANNER
PATIENT ORIENTATION• HEAD FIRST • FEET FIRST
SFOV SCANNING FIELD OF VIEW
(CALLIBRATION FIELD OF VIEW)
• AREA WITHIN THE GANTRY’S ISOCENTER FROM WHICH THE RAW DATA IS ACQUIRED DURING THE SCAN. IT DEPENDS ON THE NUMBER OF DETECTORS ACTIVATED TO COLLECT DATA.
LARGE SFOV
SMALL SFOV
SMALL SFOV SFOV
LARGE SFOVSFOV
CHOOSE THE SMALLEST SFOV THAT WOULD
ACCOMMODATE BODY PART FOR BEST RESOLUTION
IF BODY PART LIES OUTSIDE SFOV NO DATA WILL BE
COLLECTED FOR THAT PART THAT EXTEND BEYOND. IT
ALSO CAUSES OUT OF FIELD ARTIFACT
OUT OF FIELD ARTIFACT
DFOV – DISPLAYED FIELD OF VIEW
(ZOOM OR TARGET)
• DETRMINES HOW MUCH RAW DATA FROM WITHIN SFOV WILL BE UTILIZED TO CREATE AN IMAGE.
SFOV
DFOV
DISPLAYED FOV vs
SCANNING FOV
• DFOV CAN BE EQUAL OR LESS OF SFOV
DFOV VS PIXEL SIZE
• SMALL DFOV
• LARGE FOV
• SMALL PIXEL
• LARGE PIXEL
LARGE DFOV
LOW SPATIAL RESOLUTIONLOW VISIBILITY OF DETAIL
LARGE DFOV
SMALL DFOV
DFOV
DFOV
PIXEL SIZE
PIXEL SIZE= DFOV (mm)/ MATRIX SIZE
SAMPLE
SCANNING
• TOPOGRAM
• REGULAR SCAN
TOPOGRAM (SCOUT)
• TUBE DOES NOT REVOLVE AROUND THE PATIENT
AP SCOUT
• TUBE SUSPENDED ABOVE PATIENT DURING SCOUT GENERATION
TUBE
LAT SCOUT
• TUBE AT THE 90º ANGLE TO PATIENT
REGULAR SCAN
I, II, AND III GENERATION
• CONTINUOUS DATA ACQUSITION GEOMETRY
IV GENERATION
• STATIONARY DATA ACQUSITION GEOMETRY
REGULAR SCAN
• AXIAL (CONVENTIONAL, SLICE-BY-SLICE)
• SPIRAL
AXIAL SCAN
• TABLE STOPS AT THE SCANNING POSITION AND THE TUBE ROTATES AROUND A PATIENT.
SPIRAL
• PATIENT CONTINUOUSLY MOVES IN THE Z-AXIS DIRECTION WHILE THE TUBE ROTATES AROUND.
TOTAL NUMBER OF TRANMISSION
MEASUREMENTEQUALS
NUMBER OF VIEWS X
NUMBER OF RAYS IN EACH VIEW
ACQUSITION TERMINOLOGY
• RAY
• VIEW
• PROFILE
RAY
PART OF THE X-RAY BEAM THAT FALLS ON ONE
DETECTOR
VIEW
COLLECTION OF THE RAYS FOR ONE TRANSLATION ACROSS
THE OBJECT
PROFILE
ANALOG SIGNAL GENERATED IN A VIEW
RAY
PROFILE
VIEW
RECONSTRUCTION
• DATA THAT HAVE BEEN BACKPROJECTED INTO THE IMAGE MATRIX TO CREATE CT IMAGES DISPLAYED ON THE MONITOR
RECONSTRUCTION TRANSMISSION MEASUREMENT
(LINEAR ATTENUATION COEFFICIENT) IS CONVERTED INTO CT NUMBER
(HOUNSFIELD UNIT)
Ц CT #RECONSTRUCTION
LINEAR ATTENUATION COEFFICIENT ( cm-1)
• BONE 0.528• BLOOD 0.208• G. MATTER 0.212• W. MATTER 0.213• CSF 0.207• WATER 0.206• FAT 0.185• AIR 0.0004
CT NUMBER CALCULATION
CT NUMBER SCALE
• CORTICAL BONE +1,000• MUSCLE +50 • WHITE MATTER +45• GRAY MATTER +40• BLOOD +20• CSF +15• FAT -100• LUNG -200
• AIR -1,000
• APPEARS WHITE
• GRAY
• LIGHT GRAY
• GRAY
• GRAY
• GRAY
• DARK GRAY TO BLACK
• DARK GRAY TO BLACK
• DARK GRAY TO BLACK
WATER – 0 BASELINE
CT # vs BRIGHTNESS LEVEL
+ 1000
-1000
CT #
+500
CT #+40
CT #+15
CT # OF CYST0
CT # OF LIPOMA ( FATTY TUMOR)
-100
CT #
-100