W4+5 CT Systems PDF

The CT Imaging System W4+5 ILO’s On completion of this section, the student should be able to know the Component and Function of each part of the imaging system...

The CT Imaging System W4+5 ILO’s On completion of this section, the student should be able to know the Component and Function of each part of the imaging system BASIC COMPONENTS (GANTRY) OUTSIDE COMPONENTS X-RAY TUBE FILTRATION & COLLIMATION DETECTOR ASSEMBLY & SLIP RING PATIENT COUCH S. Jastaniah, J. Al-Ghamdi, A. Aref (KAU), A. Musa (KKU), K. Hassan (BMC) 3/25/24 1  CT SCANNER INSTRUMENTATION: THE BASIC COMPONENTS OF CT EQUIPMENT ARE THE:- 1. IMAGING SYSTEM. 2. COMPUTER SYSTEM. 3. IMAGE DISPLAY, RECORDING STORAGE AND COMMUNICATION SYSTEM.  IMAGING SYSTEM: Ø THE PURPOSE OF IMAGING SYSTEM IS TO PRODUCE X-RAYS, SHAPE AND FILTER X-RAY BEAM TO PASS THROUGH ONLY DEFINED CROSS-SECTION OF THE PATIENT AND CONVERT THE TRANSMITTED PHOTONS INTO DIGITAL INFORMATION. Ø THE MAJOR COMPONENTS OF THE IMAGING SYSTEM ARE THE X- RAY TUBE AND GENERATOR, COLLIMATORS, FILTER, DETECTORS AND DETECTOR ELECTRONICS. Gantry  GANTRY Ø THE GANTRY IS THE RING-SHAPED PART OF THE CT SCANNER. Ø IT HOUSES MANY OF THE COMPONENTS NECESSARY TO PRODUCE AND DETECT X-RAYS. Ø COMPONENTS ARE MOUNTED ON A ROTATING SCAN FRAME. Ø GANTRIES VARY IN TOTAL SIZE AS WELL AS IN THE DIAMETER OF THE OPENING. Ø IT MOUNTED IN THE SCANNING ROOM.  GANTRY — THE GANTRY INCLUDES : Ø INSIDE COMPONENTS: (a) THE X-RAY TUBE (b) FILTER. (c) SLIPS RING (d) THE HIGH VOLTAGE GENERATOR (e) DATA ACQUISITION SYSTEM (COLLIMATOR, DETECTORS) Gantry Outside view of modern CT system Inside view of modern CT system, showing the patient table and CT the x-ray tube is on the top position scanning patient aperture and the arc-shaped CT detector is on the bottom position. OUT SIDE COMPONENTS:  OUT SIDE COMPONENTS: A- DISPLAY PANEL: IT SHOWS THE READING OF THE GANTRY TILT IT ALSO SHOWS THE TABLE HEIGHT, AND POSITION OF LANDMARK. B- CONTROL PANEL: THE CONTROL PANEL INCORPORATES THE SEVERAL BUTTONS TO MAINLY CONTROL THE MOVEMENTS OF THE GANTRY AND TABLE. EACH FRONT AND REAR COVER HAS TWO CONTROL PANELS, LOCATED ON EITHER SIDE OF THE GANTRY OPENING C- EMERGENCY BUTTON: PRESSING THE EMERGENCY BUTTON STOPS EVERY MECHANICAL MOVEMENT AND THE X-RAY EMISSION.  CONTD… ØIN MOST SCANNERS, THESE FUNCTIONS MAY ALSO CONTROLLED VIA THE OPERATOR’S CONSOLE. ØA MICROPHONE IS EMBEDDED IN THE GANTRY TO ALLOW COMMUNICATION BETWEEN PATIENT AND TECHNOLOGIST THROUGHOUT THE SCAN PROCEDURE.  OUT SIDE COMPONENTS: ØD- POSITIONING LIGHTS: RED LASER LIGHTS ARE USUALLY MOUNTED ON THE GANTRY AND USED FOR POSITIONING. ØTHE BODY PART OF INTEREST MUST BE PROPERLY CENTERED TO THE APERTURE. ØTHERE ARE OFTEN THREE TO FOUR POSITIONING LIGHTS  OUT SIDE COMPONENTS: E- APERTURE: IS THE OPENING THROUGH WHICH THE PATIENT PASSES ØTHE RANGE OF APERTURE SIZE IS TYPICALLY 70 TO 90 CM.  SLIP RINGS ØEARLY CT SCANNERS USED RECOILING SYSTEM CABLES TO ROTATE THE GANTRY FRAME. ØTHIS DESIGN LIMITED THE SCAN METHOD TO THE STEP-AND- SHOOT MODE AND CONSIDERABLY LIMITED THE GANTRY ROTATION TIMES. ØALTERNATIVE TO CABLING SYSTEM = SLIP-RING  SLIP RINGS ØCURRENT SYSTEMS USE ELECTROMECHANICAL DEVICES CALLED SLIP RINGS. SLIP RINGS USE A BRUSH TO PROVIDE CONTINUOUS ELECTRICAL POWER AND ELECTRONIC COMMUNICATION ACROSS A ROTATING SURFACE. ØTHEY PERMIT THE GANTRY FRAME TO ROTATE CONTINUOUSLY, ELIMINATING THE NEED TO STRAIGHTEN TWISTED SYSTEM CABLES  CONTD… vAND IT SERVE THREE FUNCTION :- (1) SUPPLY OF HIGH-VOLTAGE ELECTRICITY TO THE X-RAY TUBE. (2) SUPPLY OF LOW-VOLTAGE ELECTRICITY TO VARIOUS CONTROL MECHANISM. (3) ENABLE THE TRANSMISSION DATA FROM DETECTOR TO COMPUTER SYSTEM.  X-RAY GENERATOR: Ø CT SCANNERS NOW USE HIGH FREQUENCY GENERATORS, WHICH ARE SMALL, COMPACT, AND MORE EFFICIENT THAN CONVENTIONAL GENERATORS. Ø THESE GENERATORS ARE LOCATED INSIDE THE GANTRY. IN SOME SCANNERS HIGH-FREQUENCY GENERATOR IS MOUNTED ON THE ROTATING FRAME WITH X-RAY TUBE , IN OTHERS IT IS LOCATED IN A CORNER OF THE GANTRY AND DOSE NOT ROTATE WITH TUBE.  X-RAY GENERATOR: Ø CT GENERATORS PRODUCE HIGH KV (GENERALLY 120–140 KV) TO INCREASE THE INTENSITY OF THE BEAM, WHICH WILL INCREASE THE PENETRATING ABILITY  COOLING SYSTEMS Ø COOLING MECHANISMS ARE INCLUDED IN THE GANTRY. Ø COOLING MECHANISMS ARE IMPORTANT BECAUSE MANY IMAGING COMPONENTS CAN BE AFFECTED BY TEMPERATURE FLUCTUATION.  X-RAY TUBE Ø THE X-RAY TUBE IS RESPONSIBLE OF X-RAY PRODUCTION. Ø FIRST AND SECOND GENERATION SCANNERS USED FIXED ANODE, OIL-COOLED X- RAY TUBES, BUT ROTATING ANODE X-RAY TUBES HAVE BECOME COMMON IN CT BECAUSE OF THE DEMAND FOR INCREASE OUTPUT. Ø THESE ROTATING ANODE X-RAY TUBES PRODUCE HETEROGENEOUS X-RAY BEAM.  FILTRATION: Ø RADIATION FROM X-RAY TUBES CONSISTS OF LONG AND SHORT WAVE-LENGTHS. Ø IN CT, FILTRATION SERVES A DUAL PURPOSE:- (1) REMOVE LONG WAVELENGTH X-RAYS BECAUSE THEY DO NOT PLAY A ROLE IN CT IMAGE FORMATION, BUT INSTEAD CONTRIBUTE TO PATIENT DOSE. Ø (2) CREATING A MORE UNIFORM BEAM INTENSITY IMPROVES THE CT IMAGE BY REDUCING ARTIFACTS  CONTD… ØTHE FILTER IS POSITIONED BETWEEN THE X-RAY TUBE AND THE PATIENT. Ø TWO COMMONLY USED FILTERS ARE THE a) FLAT FILTER b) BOWTIE FILTER CT Filter  COLLIMATION IN CT: COLLIMATION IS PRE-PATIENT COLLIMATORS AND PRE-DETECTOR COLLIMATORS. ü PRE-PATIENT COLLIMATORS RESTRICT THE BEAM TO THE SLICE THICKNESS AND REDUCE SCATTER RADIATION (TO THE PATIENT). ü PRE-DETECTOR COLLIMATORS, CONTROL THE TRANSMITTED BEAM IMPROVE IMAGE QUALITY BY LIMITING SCATTER RADIATION BEFORE REACHING THE DETECTOR. CONTD…  CT DETECTOR CHARACTERISTICS Ø THE DETECTOR MUST EXHIBIT SEVERAL CHARACTERISTICS ESSENTIAL FOR CT IMAGE PRODUCTION:- Ø (1) EFFICIENCY, REFERS TO THE ABILITY TO CAPTURE, ABSORB AND CONVERT X-RAY PHOTONS TO ELECTRICAL SIGNALS.  CONTD… Ø CAPTURE EFFICIENCY :THE ABILITY WITH WHICH THE DETECTOR OBTAINS PHOTONS THAT HAVE PASSED THROUGH THE PATIENT. Ø ABSORPTION EFFICIENCY REFERS TO THE NUMBER OF PHOTONS ABSORBED BY THE DETECTOR. Ø CONVERSION EFFICIENCY: IS DETERMINED BY HOW WELL THE DETECTOR CONVERTS THE ABSORBED PHOTON INFORMATION TO A DIGITAL SIGNAL FOR THE COMPUTER.  CONTD… (2)STABILITY IS THE STEADINESS OF DETECTOR RESPONSE (3) THE RESPONSE TIME OF THE DETECTOR REFERS TO THE SPEED WITH WHICH THE DETECTOR CAN DETECT AN X-RAY EVENT AND RECOVER TO DETECT ANOTHER EVENT. (4)DYNAMIC RANGE : DETECTORS MUST BE SENSITIVE ENOUGH TO MEASURE A BROAD RANG OF X-RAY TRANSMISSION DATA  TYPES OF DETECTOR SCINTILLATION DETECTOR:- CONVERT X-RAY TO LIGHT AND THEN LIGHT INTO ELECTRICAL SIGNAL. GAS IONIZATION DETECTOR:- CONVERT X-RAY ENERGY DIRECTLY TO ELECTRICAL ENERGY.  SCINTILLATION DETECTOR Ø SOLID-STATE DETECTORS MADE OF A SCINTILLATION CRYSTAL AND A PHOTOMULTIPLIER TUBE. Ø WHEN X-RAY FALLS ONTO CRYSTAL, FLASHES OF LIGHT OR SCINTILLATION ARE PRODUCED. Ø LIGHT FROM THE CRYSTAL STRIKES THE PHOTOCATHODE OF THE PHOTOMULTIPLIER TUBE WHICH THEN RELEASES ELECTRONS. Ø ELECTRICAL SIGNAL SENT TO COMPUTER TO DETERMINE HOW MUCH OF THE X RAY BEAM ATTENUATED BY THE BODY AND GENERATE AN IMAGE  GAS DETECTORS Ø WHEN THE X-RAYS FALL ON THE XENON GAS, IONIZATION OF THE GAS RESULTS AND PRODUCES POSITIVE AND NEGATIVE IONS. Ø POSITIVE IONS MIGRATE TO NEGATIVELY CHARGED PLATE, WHEREAS NEGATIVE IONS ARE ATTRACTED TO THE POSITIVELY CHARGED PLATE. Ø THIS MIGRATION OF IONS CAUSES A SMALL SIGNAL CURRENT THAT RELATED DIRECTLY WITH NUMBER OF PHOTONS ABSORBED. CHARACTERISTICS OF DETECTORS Solid-State Crystal Pressurized Xenon Gas High photon absorption Moderate photon absorption Sensitive to temperature, Highly stable Moisture Solid material Low-density material (gas) Detection efficiency 90%. Detection efficiency 45%  SINGLES(SSCT) AND MULTIPLE(MSCT) DETECTOR Ø THE MAIN DIFFERENCE BETWEEN SSCT AND MSCT IS THE DETECTOR ARRAYS. Ø SSCT USES DETECTORS THAT FORM A ONE-DIMENSIONAL ARRAY. ( USES MANY INDIVIDUAL DETECTOR ARRAY) Ø MSCT IS DIVIDED INTO SEVERAL SMALLER DETECTORS ALONG THE Z-AXIS, FORMING A TWO-DIMENSIONAL ARRAY.( EACH INDIVIDUAL DETECTOR IN EACH RAW DIVIDED INTO MULTIPLE DETECTOR ARRAY) Ø THE CT SCANNERS CURRENTLY USED IN HOSPITALS HAVE AT LEAST 64 ROWS OF DETECTORS THAT PROVIDE A VERY HIGH RESOLUTION. SSCT arrays of single, long MSCT arrays with several rows elements along z-axis. of small detector elements.