Presented by: Sandip Mandal (B.Sc.MIT) (BPKIHS,DHARAN,NEPAL)

CT TUBE AND DETECTOR

INTRODUCTION AND BRIEF HISTORY OF X-RAY TUBEX-ray tube is a device in which energyconversion takes place i.e. kinetic energy of fast moving electrons is converted into x-ray (1%) and heat (99%).There has been a series of advancement in the X-ray tube during the course of time;

· 1895- Roentgen discovered x-rays (using crook’s type of tube)· 1913- The Coolidge hot cathode x-ray tube· 1929- Rotating anode tube· 1932- Grid controlled stationary anode tube · 1937- Grid controlled rotating anode tube· 1959- High speed tube· 1962-Rhenium alloyed tungsten composite anode tube· 1967-First dedicated mammography unit with molybdenum anode· 1971-Glass metal tube with molybdenum anode· 1973- Three layered anode (W-Re) + Mo or (W-Re) + (Ti-Zn-Mo)· 1979- Metal ceramic tube· 1981- Three focus tube· 1989- Direct anode cooling with noiseless rotor

X-RAY TUBES USED IN CT Since CT require longer continuous exposure time at higher KV and mA than needed for general radiography hence; the general radiographic X-ray tubes cannot be used for CT. It Should supply monochromatic X-ray beam for accurate reconstruction.

X-ray tubes for CT have been charged with heavy duty rotating anode tube with higher thermal capacity and smaller focal spot (up to 0.6mm). These tubes are air cooled with current value up to 600mA.

COMPARISION OF CT TUBE WITH VARIETY OF X-RAY TUBES AVAILABLE

BRIEF HISTORY AND ADVANCEMENTS IN CT X-RAY TUBECT X-ray tubes which are located in the heart of the gantry Provide radiation source for CT.• Early experimental models used radionuclide to supply such a radiation beam.• First and second generation scanner used fixed anode (relatively large (2x16mm)focal

spot operated at 120 kVp & 30 mA & heavily filtered) , oil-cooled x-ray tube but with the demand for increased output, gradually rotating anode tubes become common in CT scanner.

• The introduction of spiral/helical CT with the continuous rotation scanner has placed new demands on CT tubes.

Several technical advances in component design have been made to deal with the problem of heat generation, heat storage and heat dissipation. For example, i)anode assembly including anode rotation- Anode is of larger diameter with graphite backing, which allows the anode to absorb& dissipate large amounts of heat. ii)target design- Anode target angle is made 7-10 degrees to diminish heel effect. iii)cathode assembly- Bigger filament size, increased effective focal spot. Focal spot size smaller (0.6 mm). iv)the tube envelop-Although the borosilicate glass envelope in early CT tube provides good thermal and electrical insulation, electrical arching result from tungsten deposits on the glass caused by vaporization. To solve above problem, tubes with metal envelop and ceramic

insulator are now common e.g. Metal ceramic x-ray tube

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METAL CERAMIC X-RAY TUBE USED IN CT• Glass envelope has been replaced by metal casing and ceramic is used as insulation

of high voltage cable.• A more recent development in x ray tube construction is metal ceramic tube,it’s

construction includes alloy of chromium and iron cylinder brazed to alumina ceramic (aluminium oxide) insulators at each end. These insulators carry the anode and cathode assemblies .

• The metal ceramic tubes are smaller and more robust than their glass equivalents. They have another advantage, in that they enable more flexibility in the electrical circuitary associated with the tube.

• Metal envelope grounded offers no chance of arcing of x-ray tube and hence increase tube life.

• Anode rotates on an axle with bearing at each end providing greater stability and reduced stress on shaft permits massive anode approx-2kg.

• Ceramic insulator (Al oxide) are used to insulate high voltage parts of x-ray tube from metal envelope allowing more compact tube design.

• Metal ceramic tube offers Higher tube loading, Reduce off focus radiation, Allow high tube current.

METAL CERAMIC X-RAY TUBE USED IN SPIRAL CT

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Fig. Schematic diagram o f a Super Rotalix ceramic x-ray tube: 1.Metal casing, 2. Anode, 3/6. Ball bearings, 4/8. Ceramic insulators, 5. Cathode, 7. Stator windings, 9. Anode shaft, 10. Beryllium window

SUPER ROTALIX CERAMIC X-RAY TUBE IN CT

--It is also the type of metal ceramic X-ray tube

Detector X-ray beam

X-ray tube

• Higher Tube Loading– Allows higher tube currents to be used because of

larger heat storage capacity of anode• Longer Tube Life

– Deposition of tungsten on the glass wall acts as electrode causing arcing bet. Glass and filament shortening tube life. When metal enclosure is grounded, this deposition will not alter grounding thus increasing its life

• Reduced off Focus Radiation– Electrons back scattered from the anode may strike anode

again producing x-rays from areas other than focal spot. The metal enclosure decreases off focus radiation by attracting off focus electrons to the grounded metal wall relatively Positive as compared to electrons. Low atomic no. of metal may produce few and low energy x-rays.

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Advantages Of Metal ceramic Tube

OTHER TYPES OF X-RAY TUBES USED IN CTMRC(MAXIMUM ROTALIX CERAMIC) X-RAY TUBE

o In 1989 Phillips became the first company to introduce MRC.IT is based on the technology of spiral groove bearing using liquid metal alloy as lubricant. Focuses Significant improvement in rotating anode x-ray tube.

FEATURESo Higher output and longer tube lifeo 200 mm graphite backed anodeo Anode heat storage capacity(8MHU)o Tube voltage - 90 to 140 KVo Tube current - 20 to 500 mAo Anode angle - 7 degreeo Directly cooled anode

OTHER TYPES OF X-RAY TUBES USED IN CTMRC X-RAY TUBE

Advantageso Noiselessly rotating anode that could be switched on the

morning and switched off in the evening.o Avoid waiting time during and between examination.o Possible to achieve dose saving filter technique in

angiography.USESo Cardiovascular imagingo MDCT

OTHER TYPES OF X-RAY TUBES USED IN CTAQUILION X-RAY TUBE

o High capacity multi-slice CT tubeo Heat storage capacity 7.5 MHUo Cooling rate 1.7 MHU/mino Anode groundedo Focal spot 1.4mm × 1.4 mmo Air cooledAquilion vs conventional tube

TYPES OF X-RAY TUBE USED IN CTNEW STRATON TUBE

o New constructiono Focused and deflected beam of thermal electrono whole tube and anode assembly rotates o Bearing located out sideo Oil cooled

FEATURESo Zero heat storage capacityo Cooling rate 4.7 MHU/mino Cooled down within 20 seco Enables gantry speed of 0.37 sec per rotationo Tube current 500 mAo Based on RET (rotating envelope tube) technologyo The electron beam in the tube is shaped and controlled

by magnetic deflection coilo Focal spot – tungsten and rhenium

• One of the more interesting developments is the Siemens Straton x-ray tube, which is currently available as an option on Sensation 16 scanners (Fig ).

• The tube itself is a radical new design, where the entire tube body rotates, rather than just the anode, as is the case with conventional designs. This change allows all the bearings to be located outside the evacuated tube, and enables the anode to be cooled more efficiently.

• The Straton has a low inherent heat capacity of 0.8 MHU, but an extremely fast cooling rate of 5 MHU / min.

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RECENT DEVELOPMENT IN X-RAY TUBE

• This compares with typical figures of 7-8 MHU and up to 1.4 MHU / min for existing tubes.

• The heat capacity and cooling rate combine to produce a tube which Siemens claim is ‘0 MHU', implying that tube cooling considerations are a thing of the past. Sensation 16 scanners fitted with the Straton tube now have a fastest scan time of 0.37 seconds.

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Conventional VS Straton

CT DETECTOR

o CT detector capture the radiation beam from the patient and convert it into electrical signal, which subsequently converted into binary coded information

o Detector characteristic

o A. efficiency:- refers to the ability to capture, absorb and convert x-ray photons to electrical signal

Detectors measure the intensity of radiation transmitted through the patient

• B. Stability:- refers to the steadiness of the detector response, if not stable frequent calibrations are required to render the signals useful

• C. Response time:- refers to the speed with which the detector can detect an x-ray event and recover to detect another event (should be very short in micro second)

• D. Dynamic range:- refers to the ratio of the largest signal to be measured to the precision of the smallest signal to be discriminated

o E . After glow:- refers to the persistence of the image even after the radiation has been turned off

o TYPES OF DETECTORTwo types:- 1. Gas ionization detector Convert x-ray energy directly into electrical signal

2. Scintillation detectors Convert x-ray energy into light

1. GAS IONIZATION DETECTOR

o Use high pressure ( about 25 atm) nonradioactive xenon gas, in long thin cells between two metal plates

o Based on the principle of ionization

o Consists of a series of individual gas chamber, usually separated by tungsten plate carefully positioned to act as electron collection plates with voltage applied across it.

o When x-ray fall on the individual chamber, ionization of gas result and produces positive and negative ions. The positive ions migrate to the negatively charged plate, whereas the negative ions at positively charged plate

o The migration of ions causes a small signal current that varies directly with the number of photons absorbs

o QDE is only 50 – 60 percent

o Can be used for 3rd generation scanner only

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Fig. Shows a typical gas-filled ionization detector

Xenon (in interspace material)

Charges

Tung

sten

pla

te

Ceramic

2. SCINTILLATION DETECTORS

o Solid state detectors that consist of a scintillation crystal coupled to a photodiode tube

o When x-ray falls onto the crystals, flashes of light are produced. The light is then directed to PM tubes. Which then releases electrons and these electrons cascade through a series of dynodes that are carefully arranged and maintained at diff. potential to result in small out put signal

o Early scanner used sodium iodide crystals coupled to PM tube. Due to afterglow problem and the limited dynamic range of sodium iodide, other crystals such as calcium fluoride and bismuth germinate used in later scanner

o Now a days, solid state photodiode multiplier scintillation crystal detectors are used

o The photodiode is a semiconductor whose p-n junction allows current flow when exposed to light

o Photodiodes are normally used with amplifier because of the low output from the diode

o Response time of photodiode is extremely fast (about 0.5 -250 nanosecond)

o Scintillation materials currently used with photodiodes are cadmium tungsten and a ceramic material made of high purity, rare earth oxides based on doped rare earth compounds such as yttrium and gadolinium oxysulphide, these crystals are optically bonded to the photodiode

o The conversion efficiency and photon capture efficiency of cadmium tungsten are 25-30% and 45% respectively and the dynamic range is 1 million to 1

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X-ray beam X-ray photon

Isotropicallyemitted lightPhotodiode

Electrical signals

Fig. Solid-state detector. The photodiode is coated with a scintillator. When the remnant x-ray photons activate the scintillator, light photons are emitted and detected by the photodiode. The photodiode then gives off an electrical signal.

DETECTOR CONFIGURATION

o One major problem with single slice, single row detector is related to the length of time needed to acquire data

o CT scanner now use multi-row detector to image multi-slice during a 360 degree rotation

o DUAL ROW/DUAL SLICE DETECTOR In 1992, Elscint introduced the first dual slice

volume CT scanner

o This technology uses a dual-row, solid state detector array coupled with a special x-ray tube based on a double dynamic focus system

o The dynamic focal spot is where the position of focal spot is switched by a computer controlled electron-optic system during each scan to double the sampling density and total no. of measurement

o Twin beam technology results in the simultaneous scan of two contiguous slices with excellent resolution

MULTI-ROW/MULTI -SLICE DETECTORo The goal of MR/MS detector is to increase the

volume coverage speedo MR/MS detector are solid state detectors that

can acquire 4 to 64 to 320 slices per 360 degree rotation

Multi-row detector falls into three groupso A . MATRIX ARRAY DETECTORo Referred to as a fixed array detectors

o Contains channels or cells, they are often referred to isotropic, that are equal in all dimensions

o B . ADAPTIVE ARRAY DETECTORSo The adaptive array detectors are anisotropic in

designo The cells are not equal but rather they have diff.

sizes

R.R. YADAV (CT INSTRUMENTATION)

• C . HYBRID DETECTORS o Has a no. of narrow detector elements in the

centre of the detector and different no. of wider detector (usually double the width of the narrow detector) on both sides of the span of narrow detector

o The no. of narrow and wider detector can vary

AREA DETECTORS

o Currently undergoing testing

o Two such CT scanners based on area detector technology are the 256 slice ct scanner prototype(Toshiba Acquillion) and the flat panel CT scanner prototypes ( one from siemens and another from koning corporation)

o The 256-slice CT detector is a wide area multi-row array detector that has 912 channels * 256 segments and a beam width of 128 mm

o Possible to scan entire heart in a single rotation

o Flat panel detectors are similar to the ones used in digital radiography are being investigated for the use in CT imaging

o The detector is a flat panel type and is based on the CsI indirect conversion digital radiography detectors

o Recently, flat panel detectors are being investigated for use in breast CT

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Ultra Fast Ceramic UFC) Detectors• The afterglow of the UFC detector material developed in

decays 400 times faster than yttrium gadolinium oxide.

• Aside from the rare earth element gadolinium, the UFC detectors also includes sulfur and other additives.

Scintillation/Solid-state Detectors

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Detector performance

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Detector Array configuration

CROOK’S X-RAY TUBE

COOLIDGE X-RAY TUBE

EARLY LOW VOLTAGE THERAPY TUBE

STATIONARY ANODE X-RAY TUBE

HOODED ANODE X-RAY TUBE

Hooded Anode

ROTATING ANODE X-RAY TUBE

Rotating Anode X-ray Tube

Rotating Anode X-ray Tube

Anode Pitting

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