1. CT scanning has progressed through several generations from conventional to computed tomography. 2. First generation CT scanners used a translate-rotate motion and were slow, taking 5 minutes to complete an image. 3. Later generations incorporated faster rotating gantry systems and multi-detector arrays for faster acquisition times, with current scanners able to image in under 1 second. 4. The introduction of helical or spiral CT scanning allowed for true 3D imaging within a single breath hold.

1. GENERATIONS OF CT
Presented by :Prativa Khanal
BSc.MIT 2nd year
NMCTH

2. CONVENTIONAL RADIOGRAPHY
Limitation of conventional radiography
• Inefficient xray absorption
• High scatter to primary x-ray ratios
• superimposition
• Inconspicuity

3. CONVENTIONAL TOMOGRAPHY
• These issues were recognized and led
investigators to consider improvements
• One such innovation was conventional
tomography

4. COMPUTED TOMOGRAPHY
• In the last 30 years of X-ray Computed Tomography
development produced a great change in the role of
diagnostic imaging in medicine
• The basics of CT is to take a series of conventional cross-
sectional x-rays while the patient "is rotated" slightly around
an axis between each exposure
• A series of projection data is obtained, these data are used
to reconstruct cross-sectional images

5. COMPUTED TOMOGRAPHY
• 1917: The austrian mathematician Radon develop a way to
reconstruct the density distribution of an object if the line
integrals (for every direction)are available
• 1957-63:The physician Cormack develops a lot of theoretical
work on x-rays creating the basis of CT scanning (without
knowing Radon work)
• 1971: The first CT system has been created by Godfrey N.
Hounsfield, he received the Nobel prize in 1979 (together
with Cromack)
• 1972: The first clinical use of a CT system in London

6. COMPUTED TOMOGRAPHY
• 1973-74: The first II Generation CT system total-body
scanner has been realized in the U.S., the acquisition time for
1 tomogram was 18 sec
• 1976-77: III and IV Generations CT with acquisition time for 1
tomogram lower than 5 sec
• 1983: First electron beam CT, very expensive
• 1989: First helical CT, very low acquisition time (less than 1
sec) and able to explore a large body volume
• 2000: Multislice CT scanner, multiple arrays of detectors;
continuous development

8. INTRODUCTION
• Generation is the order in which CT scanner design has been
introduced,and each has a number associated with it
• Classification based on arrangement of components and
mechanical motion required to collect data
• Higher generation number doesn’t necessarily indicate
higher performance system

9. FIRST GENERATION CT
• It is functional marriage of diverse technology including
computer hardware,control systems,x-ray
detectors,sophisticated reconstruction algorithms,x-ray
tube,generator systems
• 1ST generation of CT scanners employed a rotate-translate,
pencil beam system
• Two x-ray detectors were used, and they measured the
transmission of x-rays through the patient for two different slices
• X-ray tube and detectors are rigidly linked and scan across
subject sweeping a narrow x-ray beam through the slice

10. FIRST GENERATION
• Starting at a particular angle, the x-ray tube and detector
system translated linearly across the FOV acquiring 160
parallel rays per view
• After end of translation,tube and detector assembly rotated
around the subject by 1 degree
• This procedure was repeated until 180 projection
• A total of 180x160=28,800 rays were measured
• This combination of linear translation followed by
incremental rotation is called translate-rotate motion

12. • As the system translated and measured rays from the thickest part
of the head to the area adjacent to the head, a huge change in x-
ray flux occurred
• Early detector system couldn’t accommodate large change in signal
so patient head was recessed via a rubber membrane into a water
filled box/water bath
• Acted to bolus the x-rays so that the intensity outside the head is
similar to the intensity inside head

13. • ADVANTAGE
• With regard to scatter rejection,pencil beam geometry used in 1st
generation scanners were best
• DISADVANTAGE
• 5minute was required to complete a single image
• Contrast resolution of internal structures was
unprecedented, images had poor spatial resolution
• NAI detectors was used which are hygroscopic in nature

14. SECOND GENERATION CT
• 1st waterless full body CT scanner was developed and
installed by Ledley Gorgetown University in February 1974
• It consist of narrow fan beam (3-10) degree and multiple
detectors (linear array of 30 detectors)
• With 10 degree rotation increment,only 18 translation would
be required for 180 degree image acquisition
• Shortest scan time with 2nd generation CT was 18 s per slice,
15 times faster than with 1st generation CT

16. Disadvantage
• Further speed improvements were limited by mechanical
complexity of translate – rotate geometry
• Low efficiency
• Scatter radiation increased due to narrow fan beam
• One disadvantage of fan beam is the increased radiation
intensity towards the edge. But it is compensated with the
use of bow-tie filter (limits the range of intensity reaching
detector and hardens beam)

17. Third generation
• The translation motion of 1st and 2nd generation CT scanner
was fundamental impediment to fast scanning
• 1st and 2nd generation had to be dynamically recalibrated at
the end of each translation
• Faster scans required the elimination of translational motion
and the use of simple and pure rotational motion
• This is accomplished by widening the x-ray beam
encompassing the entire patient width and using and array
of detectors to intercept the beam

18. • The design, characterized by linked tube - detector system
undergoing only rotational motion is called third generation of
CT
• Wide angle fan beam of 50-55° was used and no. of detectors
was increased to more than 800 detectors - limits spatial
resolution - 5 to 10 lp/cm.
• The early third generation CT scanners installed on late 1975
could scan less than 5 sec, current designs can scan as quickly
as one third of a sec for cardiac application

20. DISADVANTAGE
• It requires extremely high detector stability and matching of
the detector response
• Any error or drift in the calibration of detectors relative to
other detectors is back projected along these ray path and
reinforced along a ring where they cross-the result is the ring
artifact
• Another disadvantage is sampling - sample size and spacing
are fixed by detector design. Samples cannot be closure
together than distance between rays associated with
detector at the level of center of rotation

21. Contd…
• A solution was provided by Xenon detector arrange
• Xenon arrays were inherently stable and well matched
because factor affecting detector response were either
uniform for the entire array or constant over chamber
• Xenon were eventually replaced by solid state detectors

22. Contd..
• Ring artifacts are never completely eliminated, rather they
are minimized by high quality detectors design and frequent
calibration
• Residual ring artifacts are then removed by image processing
algorithms
• Despite these limitations, 3rd generation CT was highly
successful and remains the basic geometry of must CT
scanners manufactured today

23. FOURTH GENERATION CT
• By 1976, a design was incorporated with a large stationary
360 degrees ring of detectors with the x-ray tube alone
rotating round the patient
• This approach of wide angle fan beam and rotate/stationary
motion with sub second imaging time is referred to as fourth
generation of CT
• It uses about 4800 individual detectors and spatial resolution
of more than 20 lp/cm

24. • Xray beam angle = 50-55°
• Rotate(tube)-Fixed(detectors)
• Xray tube rotates in a circle inside the detector ring
• Was designed to overcome ring artifact

26. DRAWBACKS
• Size and geometric dose inefficiency
• Because tube rotated inside the detector ring large ring
diameter was needed. On the other hand acceptable spatial
resolution limited detector aperture to approx. 4mm.
• Scatter-The scatter absorbing septa used in 3rd generation
could not be used in 4th generation because septa could
necessarily be aimed at center of the ring which was the
source of scatter.

27. Fifth generation
• Cardiac imaging required ultra fast scan times(<50ms) which
was a hurdle with previous existed generation
• A novel CT scanner was developed specifically for cardiac
imaging which was capable of performing complete scans in
a little as 10-20ms
• The idea behind the ultrafast scanner is a large bell
shaped x- ray tube

28. Contd…
• It doesn't use conventional x-ray tube, instead a large arc of
tungsten encircles the patient and lies directly opposite to the
detector ring
• X-rays are produced from a focal track as a high energy
electron beam strikes the tungsten
• Electron beam is produced in cone like structures behind the
gantry and is electronically steered around the patient so that
it strikes the annular target
• Wherever it strikes - produces x-rays.The concept is known as
EBCT(Electron Beam CT)

30. • EBCT is also restricted to single slice acquisition for ECG-
triggered scan examination times may be still beyond a single
breathe hold
• Typical scan times are 30-40s for a 12 cm volume
• Although still available, EBCT was limited to cardiac screening
mostly because of image quality for general screening was lower
than that of conventional CT (because of low mAs values) and
higher equipment costs
• With progress being made cardiac scanning by multi slice CT, the
future of EBCT is uncertain

31. Advantages
• Produce high resolution image
• Faster than conventional CT scanners
• Uses electron beam instead of x-ray tube
Disadvantages
• Costly equipment
• Cmplicated electronics

32. Sixth generation CT
• Though 3rd and 4th generation CT scanners elimated the
transitional motion,the gantry had to be stopped after each
slice was acquired
• Cables are spooled onto a drum, released during rotation
and respooled during reversal
• Scanning,braking and reversal required at least 8-10 sec of
which only 1-2 sec were spent for data acquisition
• The result was poor temporal resolution and long procedure
time

33. • The development of helical or spiral CT was truly revoluntary
advancement in CT scanning that finally allowed true 3D image
acuistion within a single breath hold technique
• As the table is smoothly moved through the rotating gantry,
the resulting trajectory of the tube and detector relative to
patient traces out a helical or spiral path
• Three technological developments were required;
Slip ring technology
High power x-ray tubes
Interpolation algorithms

34. • The patient is continuously translated while multiple
rotations of scan data are acquired
• The path of x-ray tube and detector relative to the patient is
a helix
• An interpolation of the acquired
measurement data has to be performed
in the z-direction to estimate a complete
CT data set at the desired image position

35. Slip ring
• Electromechanical devices consisting of circular electrical
conductive rings and brushes that transmit electrical energy
across a rotating intereference
• Elimating interscan delays required continuous rotation,a
capability made possible by low voltage slip ring
• A slip ring passes electrical power to the rotating components
without fixed connections
• It allows the complete elimination of the interscan delays
expcept for the time required to move the table to next slice
position

36. • For example : if scanning and moving the table each take 1s
only 50% of time is spent acquiring the data
• Furthermore rapid table movement may introduce tissue
jiggle artifact
DESIGN
Disk design-corporates concentric conducting ring lying
parallel to plane of rotation
Cylindrical design-has conducting ring lying parallel to axis of
rotation forming a cylinder
The common brush designs are wire and composite brush

38. ADVANTAGES
• Less potential possible
• Less potential for motion
• Improves contrast protocals possible
• Greater accuracy for multiplanar and 3D images

39. HIGH POWER X-RAY TUBE
• Stationary tubes were used in 1st and 2nd generation CT
scanner - long scan time - allowed heat dissipation
• Shorter scan time required high power of x-ray tubes and use
of oil cooled rotating anodes for efficient thermal dissipation
• Largest heat capacities are achieved with thick graphite
backing of target disks, anode diameters of 200mm or more,
metal housing with ceramic insulator
• The working life of tubes ranges from 10,000-40,000 hours

40. Pitch
• It is defined as the table movement per rotation divided by beam
width
• If the beam widith is 10mm, table moves 10mm during one tube
rotation then pitch is 1 xray beam associated with consecutive
helical loops are contigious
• If beam width is 10mm and table moves 15mm per tube rotation
the pitch is 1.5-gap exits between x-ray beam edge of
consecutive loop
• If beam width is 10mm and table moves 7.5mm then pitch is
0.75-beams and consecutive loops overlap by 2.5mm (doubly
irradiating the underlying tissues)

41. Interpolation
• Helical CT scanning produces a data set in which the x-ray
source has travelled in helical trajectory around the patient
• Present day CT reconstruction algorithms assume that x-ray
source as negotiated a circular not a helical path around the
patient
• With helical scanning, CT images can be reconstructed at any
position along the length of scan
• It allows the production of additional overlapping images
with no additional dose to the patient

42. Adavantages
• Fast scan time and large volume of data collected
• Minimized motion artifacts
• Less mis-registration between consecutive slices
• Reduced patient dose
• Improved spatial resolution
• Enhaanced multiplaner of 3D renderings
• Improved temporal resolution

43. Seventh generation ct
MS/MD CT
• It is introduced in 1998
• Allows acquisition of multiple slice in single row
• A body section can be scanned faster with a multiple row of
detectors system with multiple fan beams scanning
simultaneously
• Crucial for covering a large body section with thin beams for
producing thin, high-detail slice images or 3-D images

44. • An approach to overcoming x-ray tube output limitation is it
make better use of x-rays that are produced by x-ray tube
• When multiple detector is used, the collimation spacing is wider
therefore more of x-rays that are produced by x-ray tubes are
used in producing image data.
• With conventional single detector array scanners, opening upthe
collimator increases slice thickness which is good for utilization
of x-ray but reduces spatial resolution in the slicethickness
dimension
• With introduction of multiple detector arrays, the slice thickness
is determined by the detector size and not by the collimator

45. Pitch
• With the introduction of multiple – row detector CT scanner
of the definition of pitch has changed
• Pitch is equal to the table rotation per
gantry rotation divided by width of
detector

46. •ADVANTAGES
•It is based on 3rd generation geometry
•Has better Z-axis resolution
•Provides larger coverage
•Faster scan times.Less motion artifacts
•8,16, 64, 256 slice CT machine are available

47. ADVANCEMENT
• Dual energy ct
• Flat panel detector CT

48. DUAL SOURCE CT
• Dual source increase the temporal resolution by reducing the
rotation angle required to acquire a complete image
• It permits cardiac imaging without the use of heart rate
lowering medication and imaging heart in systole
• The use of dual x-ray source makes possible the use of dual
energy imaging which allows an estimate of the average
atomic number in a voxel, as well as the total attenuation.
• Principle - materials show different attenuation at
different mean energies

50. TYPES OF DUAL ENERGY CT
SCANNER
• Three types of dual-energy CT scanners are available that
differ in the technique used to acquire high- and low-energy
CT datasets:
a dual-source dual-energy scanner
a single-source dual-energy scanner with fast kilovoltage
switching (ie, rapid alternation between high and low
kilovoltage settings)
a single-source dual-energy scanner with dual
detector layers

52. NEW GENERATION CARDIAC CT
• New generation CT scanners are recommended as the first line
imaging of coronary artery diseases in whom imaging is difficult
with earlier generation CT scanners
• These enhancement include better temporal resolution, better
spatial resolution and shorter acquisition times
These are;
• Acquilion One (Toshiba)
• Brilliance ICT (Philips)
• Discovery CT750 (GE Health Care)
• Samatom Defination Flash (Siemens)

53. Brilliance ICT
• The Brilliance ICT scanner has 128x0.625 mm detector rows
• It provides total z-axis coverage of 80mm
• Each detector row is double sampled to
increase spatial resolution
• It is claimed that it can capture an image
of the entire heart in two heart beats

54. Discovery CT750 HD
• 64x0.625 mm detector dual - energy CT scanner
• Contains single x-ray source that switches between two
energy levels allowing two data sets - high and low energy
• It uses Gemstone detector that contributes to high image
quality and "snap shot" pulse - allows a complete picture of
the heart to be captured in 3-4 sec
• Snapshots are taken at precise table position and timed to
correspond to a specific phase of cardiac cycle

56. Samatom Defination Flash
• It is dual source CT scanner of detector 64x0.6mm
• It provides high resolution image at a fast scanning speed
with low-radiation dose
• Maximum scan speed of 458mm/s
• Two x-ray tubes and detector
arrays are mounted at 95 degrees
to each other

57. MICRO-CT
• commonly known as Industrial CT Scanner
• The term micro is used to indicate that the pixel sizes of the
cross-sections are in the micrometer range.
• The machine is much smaller in design compared to the human
version and is used to model smaller objects
There are two types of scanner
• one setup, the X-ray source and detector are typically stationary
during the scan while the sample/animal rotates
• second setup, much more like a clinical CT scanner, is gantry
based where the animal/specimen is stationary in space while
the X-ray tube and detector rotate around.

59. Mobile ct

60. REFERENCES
• The essential physics of medical imaging-Jerrold T.Bushberg
• Radiologic sciences for technologists-Stewart Caryle Bushong
• Various websites

61. • Thank you