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 # +400

CT #

+500

CT #+40

CT #+15

CT # - 200

CT # OF CYST0

CT # OF LIPOMA ( FATTY TUMOR)

                                                                        

                                                

-100

CT #

-100