about ct tube and detectors

1. Presented by: Sandip Mandal
(B.Sc.MIT)
(BPKIHS,DHARAN,NEPAL)
CT TUBE AND DETECTOR

2. INTRODUCTION AND BRIEF HISTORY OF X-RAY TUBE
X-ray tube is a device in which energy
conversion 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

3. 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

4. BRIEF HISTORY AND ADVANCEMENTS IN CT X-RAY TUBE
CT 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

5. 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.
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6. METAL CERAMIC X-RAY TUBE USED IN SPIRAL CT

7. 8-Feb-17 7
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

9. Detector X-ray
beam
X-ray
tube

10. • 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.
10
Advantages Of Metal ceramic Tube

11. OTHER TYPES OF X-RAY TUBES USED IN CT
MRC(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.
FEATURES
o Higher output and longer tube life
o 200 mm graphite backed anode
o Anode heat storage capacity(8MHU)
o Tube voltage - 90 to 140 KV
o Tube current - 20 to 500 mA
o Anode angle - 7 degree
o Directly cooled anode

12. OTHER TYPES OF X-RAY TUBES USED IN CT
MRC X-RAY TUBE
Advantages
o 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.
USES
o Cardiovascular imaging
o MDCT

13. OTHER TYPES OF X-RAY TUBES USED IN CT
AQUILION X-RAY TUBE
o High capacity multi-slice CT tube
o Heat storage capacity 7.5 MHU
o Cooling rate 1.7 MHU/min
o Anode grounded
o Focal spot 1.4mm × 1.4 mm
o Air cooled
Aquilion vs conventional tube

14. TYPES OF X-RAY TUBE USED IN CT
NEW STRATON TUBE
o New construction
o Focused and deflected beam
of thermal electron
o whole tube and anode assembly rotates
o Bearing located out side
o Oil cooled

15. FEATURES
o Zero heat storage capacity
o Cooling rate 4.7 MHU/min
o Cooled down within 20 sec
o Enables gantry speed of 0.37 sec per rotation
o Tube current 500 mA
o Based on RET (rotating envelope tube) technology
o The electron beam in the tube is shaped and controlled
by magnetic deflection coil
o Focal spot – tungsten and rhenium

16. • 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.
16
RECENT DEVELOPMENT IN X-RAY TUBE

17. • 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.
17

18. 18
Conventional VS Straton

19. 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

20. • 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

21. o E . After glow:- refers to the persistence of the
image even after the radiation has been turned off
o TYPES OF DETECTOR
Two types:-
1. Gas ionization detector
Convert x-ray energy directly into electrical signal
2. Scintillation detectors
Convert x-ray energy into light

22. 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.

23. 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

25. Fig. Shows a typical gas-filled ionization detector
Charges
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26. 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

27. 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

28. 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

29. 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

30. X-ray beam X-ray photon
8-Feb-17 30
Isotropically
emitted 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.

31. 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

32. 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

33. MULTI-ROW/MULTI -SLICE DETECTOR
o The goal of MR/MS detector is to increase the
volume coverage speed
o 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 groups
o A . MATRIX ARRAY DETECTOR
o Referred to as a fixed array detectors

34. o Contains channels or cells, they are often referred
to isotropic, that are equal in all dimensions
o B . ADAPTIVE ARRAY DETECTORS
o The adaptive array detectors are anisotropic in
design
o The cells are not equal but rather they have diff.
sizes

36. • 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
R.R. YADAV (CT INSTRUMENTATION)

38. 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

39. 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

41. 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.
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Scintillation/Solid-state Detectors

42. Detector performance
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43. Detector Array configuration
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44. CROOK’S X-RAY TUBE

45. COOLIDGE X-RAY TUBE

46. EARLY LOW VOLTAGE THERAPY TUBE

47. STATIONARY ANODE X-RAY TUBE

48. HOODED ANODE X-RAY TUBE

49. Hooded Anode

50. ROTATING ANODE X-RAY TUBE

51. Rotating Anode X-ray Tube

52. Rotating Anode X-ray Tube

53. Anode Pitting

54. YOUARE SAFE NOTBECAUSEOF THE ABSENCEOF DANGER
BUT
BECAUSEOF THE PRESENCEOF RADIOLOGIC TECHNOLOGIST