CT Generation (Generation of CT)

GENERATIONS OF CT
UPAKAR PAUDEL
B.SC.MIT
ROLL NO:6
UCMS BHAIRAHAWA

CONVENTIONAL RADIOGRAPHY
• Limitation of conventional radiography
 Inefficient x-ray absorption
 High scatter to primary x-ray ratios
 Superimposition
 Inconspicuity

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

COMPUTED TOMOGRPHY
• In the last 30 years 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

• 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.
• 1971: The first CT system has been created by Godfrey N.
Hounsfield, he received the Nobel prize in 1979 (together
with Cromack)
• 1957–63: The physician Cormack develops a lot of theoretical
work on x-rays creating the basis of CT scanning (without
knowing Radon work)
• 1972: The first clinical use of a CT system in London, the
acquisition time of a couple of tomogram was 5 mins.
COMPUTED TOMOGRPHY

• 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 (scarce diffusion)
• 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.
COMPUTED TOMOGRPHY

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.

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 system etc .
• X-ray beam was collimated down to a narrow (pencil width)
beam .
• X-ray tube and scintillation detectors are rigidly linked and
scan across subject sweeping a narrow x-ray beam through
the slice.

• 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 180 x 160 = 28,800 rays were measured.
• This combination of linear translation followed by incremental
rotation is called translate – rotate motion .
FIRST GENERATION CT

• 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 .
• It acted to bolus the x-rays so that the intensity outside the
head is similar to the intensity inside head .
• Though water bath cannot be used for body scanning , it was
used because it allowed Hounsfield to maximize accuracy of
attenuation coefficient measurement (limitation of dynamic
range , beam hardening correction)
FIRST GENERATION CT

• ADVANTAGE
- With regard to scatter rejection , pencil beam geometry used
in 1st generation scanners were best .
• DISADVANTAGE
-The major drawback was nearly 5 minutes was required to
complete a single image.
-Contrast resolution of internal structures
was unprecedented , images had poor
spatial resolution .

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

• DISADVANTAGE
• However further speed improvements were limited by
mechanical complexity of translate – rotate geometry .
• Even small deviations(because of vibration or other
misalignment)of scanner hardware position relative to
reconstruction voxels would cause data to be back projected
through wrong voxels creating severe artifacts.
• 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)
SECOND GENERATION CT

THIRD GENERATION CT
• 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.

THIRD GENERATION CT
• 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 degrees 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 .

• 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.
THIRD GENERATION CT

• A solution was provided by a 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.
THIRD GENERATION CT

• 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 .
THIRD GENERATION CT

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.

• 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.
FOURTH GENERATION CT

3rd generation fan beam geometry has the x-ray tube as the apex
of the fan; 4th generation has the individual detector as the apex.
3rd generation-detectors near the edge of the detector array
measure the reference x-ray beam
4th generation-the reference beam is measured by the same
detector used for transmission measurement
3rd Vs 4th GENERATION

FIFTH GENERATION CT
• 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.

• 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.
• There are no moving parts in the gantry.
• 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)
FIFTH GENERATION CT

• 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 .
FIFTH GENERATION CT

SIXTH GENERATION CT
• Though 3rd and 4th generation CT scanners eliminated the
translational 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.

• The development of helical or spiral CT was truly
revolutionary advancement in CT scanning that finally allowed
true 3D image acquisition 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
SIXTH GENERATION CT

• 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.
SIXTH GENERATION CT

• Slip ring
• Eliminating 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 interscan delays except
for the time required to move the table to next slice position.
• For eg : if scanning and moving the table each take 1s , only
50% of the time is spent acquiring the data .
• Furthermore rapid table movement may introduce tissue
jiggle artifact.
SIXTH GENERATION CT

• High power x-ray tubes
• 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
SIXTH GENERATION CT

• Pitch
• It is defined as the table movement per rotation divided by
beam width.
• If beam width is 10mm , table moves 10mm during one tube
rotation then pitch is 1 – x-ray beam associated with
consecutive helical loops are contiguous.
• If beam width is 10mm and table moves 15mm per tube
rotation then pitch is 1.5 – gap exists 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)
SIXTH GENERATION CT

• 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 has 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.
SIXTH GENERATION CT

• Advantages
• Fast scan times and large volume of data collected.
• Minimizes motion artifacts.
• Less mis-registration between consecutive slices.
• Reduced patient dose.
• Improved spatial resolution.
• Enhanced multiplaner or 3D renderings.
• Improved temporal resolution.
SIXTH GENERATION CT

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.

• 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 up
the collimator increases slice thickness which is good for
utilization of x-ray but reduces spatial resolution in the slice
thickness dimension.
• With introduction of multiple detector arrays , the slice
thickness is determined by the detector size and not by the
collimator.

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

16x0.75mm4 x 1.5mm 4 x 1.5mm
24mm
16x0.75mm
16x1.5mm
8x3mm
4x3mm2 x 3mm 2 x 3mm
24mm
8x1.5mm4 x 1.5mm 4 x 1.5mm
24mm

• 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.

Advancement.
• Dual energy CT.
• Flat panel detector CT.

Advancement.
• 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.

Graph of mass-attenuation coefficients for iodine (blue), calcium (green), and
water (red) on CT images obtained at two different energies (vertical dashed
lines) shows that these materials can be characterized by comparing their
attenuation at the lower energy with that at the higher energy. When dual-energy
images reconstructed for 50 and 80 keV are compared, iodine demonstrates a
greater decrease in attenuation than calcium does at the higher energy, whereas
the attenuation of water remains more or less constant

Types of Dual Energy CT Scanners
• 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.

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)

AQUILION ONE CT
• In 2007 Aquilion One CT systems were tested at Fujita Health
University and the National Cancer Center in Japan AND Johns
Hopkins University in U.S.
• Aquilion CT can see not only a 3D depiction of an organ but
also shows how the organ and blood flow are dynamically
functioning or moving within and around the organ.
• It can perform functional studies – reduces the amount of
contrast media and radiation dose.
• It is fast enough to image entire heart in less than heart beat –
0.35 sec.
• Detector rows – 128x0.626 mm.
• Z-axis coverage – 80mm.

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.

Discovery CT750 HD
• It is 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.

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.

• 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 setups-
– 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.
Micro-CT

REFERENCES
• Radiologic sciences for technologists – Stewart Carlyle
Bushong
• The essential physics of medical imaging – Jerrold T. Bushberg
• Computed tomography for Technologist ,Loise E. Romans
• Various websites.

QUESTIONS
• Features of 1st generation CT?
• Features of 2nd generation CT?
• Features of 3rd generation CT?
• Features of 4th generation CT?
• Features of EBCT?
• Advantages of helical CT over conventional axial scanning?
• Advantages of 7th generation CT?
• Principle of dual source energy CT?

CT Generation (Generation of CT)

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