Presented by Moderator
Sujan Poudel Mr. Mukesh Kr. Jha
B.Sc. MIT 2015 Demonstrator
Department of Radio-
diagnosis and imaging
B.P Koirala Institute of Health Sciences
Word tomography literally means ‘tomo’
meaning to cut, section , ‘graphy’ meaning
drawing. In case of CT a sophisticated
computerized method is used to obtain data and
transfer them into cross sectional slice of human
Earlier scanner produced only axial cuts so it was
called computerized axial tomography (CAT) scan.
CT combines rotating X radiation and radiation
sensitive detectors coupled with a computer to
create cross sectional image of any part of the
ADVANTAGE OF COMPUTED
To overcome superimposition of structures.
Ability to distinguish between two tissue with
similar density. i.e. it have higher contrast
For an object to be visible on image produced
from screen film radiography it must have a
least 5% difference in contrast from its
background material whereas for CT it is 0.5%
In CT 1% contrast difference correspond to
difference of 10 HU.
PRINCIPLE OF TOMOGRAPHY
The internal structure of the object can be reconstructed
from multiple projections of the object.
A source of ionizing radiation is transmitted
through an object to recreate an image of
the object based on its x-ray absorption.
Attenuation depend on Atomic number and density of
material so linear attenuation coefficient will be higher
for bone than soft tissue.
Difference in linear attenuation are responsible for
contrast of CT image.
To produce image, attenuation is expressed in HU,water
is given HU number zero and attenuation less than water
is given negative number and vice versa and different
shade of grey is assigned for each HU. There are more
than 2000 shade of grey.
Terminology used in CT
Generation of CT
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
FIRST GENERATION CT
x ray tube was mounted in a fixed relationship with
two detector (NaI)
80 parallel rays were acquired as assembly translated
across FOV then system rotated 1 degree and x ray
tube and detector translated in another direction. This
was continued until 180 degree of data were acquired.
Large number of projection are generated and image
It uses pencil beam.
Scan required 4 min and reconstruction ran overnight.
SECOND GENERATION OF CT
Narrow fan beam
Linear detector array(5 to30)
Translate-Rotate movements of Tube-
• Fewer linear movements are needed as there
are more detectors to gather the data.
Between linear movements, the gantry
Scan time~30secs(advantage over first
THIRD GENERATION OF CT
Pulsed wide fan beam. Generally 60 degree.
Arc of detectors(600-900)
Detectors are perfectly aligned with the X-Ray
Both Xenon and scintillation crystal detectors can
Scan time< 5secs
Disadvantage: Due to rigid alignment of x ray
source and detector array when CT detectors are
not properly calibrated with respect to each other
Ring artefacts arise.
FOURTH GENERATION OF CT
Complete circular array of about 1200 to 4800
Single x-ray tube rotates with in the circular array
of detectors due to which alignment of x ray tube
changes with respect to detector array and CT data
are computed from so called detector fan.
Wide fan beam to cover the entire patient
Scan time of newer scanners is about ½ s or, <2s.
Each detectors serves as its own reference
detection, changes in detector sensitivity are forced
out in computation of projection data sets thus
eliminating ring artefacts.
Disadvantage: High cost.
FIFTH GENERATION OF CT
Developed specifically for cardiac
No conventional x-ray tube; large arc of
encircles patient and lies directly opposite to
Electron beam steered around the patient to
strike the annular tungsten target
Capable of 50-msec scan times; can
produce fast frame- rate CT movies of the
SIXTH Generation CT
Sixth generation are dual energy source ( two x ray tube)
that have two sets of detector that are offset by 90 degree .
A typical approach would be to operate one
tube at 80 kV and the other tube at 140
The key advantage of dual-energy and spectral
CT techniques is that they can be used to
probe the attenuation arising from density and
atomic number separately by making two different
measurements of the same sample, object, or
The dual source CT scanners provide improved temporal
resolution needed for imaging moving such as heart
The most promising applications for dual-energy
and spectral CT capabilities are virtual
noncontrast (VNC) exams, iodine quantification,
and calcium quantification
COMPONENT OF HELICAL CT
Gantry : mechanical support for the X-
ray tube, tube collimator and data
measurement system (DMS) has to be
designed so as to withstand the high
gravitational forces associated with fast
gantry rotation (~17 g for 0.42 s
rotation time, ~33 g for 0.33-s rotation
X-ray source and generator : should
provide a peak power of 60–100 kW,
usually at various,user-selectable
voltages, e.g., 80 kV, 100 kV, 120 kV
and 140 kV.
Heat storage capacity: typically of 5
to 9 MHU, realized by thick graphite
layers attached to the backside of the
anode plate detector and detector
Data transmission systems (slip rings) :
contactless transmission technology is generally
used for data transfer, which is either laser
transmission or electro-magnetic trans- mission with
a coupling between a rotating trans-mission ring
antenna and a stationary receiving antenna .
computer system for image reconstruction and
manipulation : divergence of the fan beam
along the longitudinal axis (z-axis),creates a
cone-beam shape due to source-detector
geometry; there is a significant difference
between the distance from the x-ray focal
spot to the center detector row and the
distance from the focal spot to the outer
The most commonly used reconstruction algorithm
in MDCT systems is a modification of the filtered
back-projection method called the Feldkamp
Requirement for helical CT
X ray gantries with slip ring : for
continuous gantry rotation.
More efficient tube cooling
Higher X- ray output
Smoother table movement
More efficient detectors
Single Row Detector System
Contain many detector elements aligned in
Detector element is quite high in z direction
(approx 15mm) .
Opening and closing of collimator controls
Width of detectors in a single detector array
place upper limit on slice thickness.
insufficient volume coverage within one
breath-hold time of the patient or missing
spatial resolution in the z-axis due to wide
The ideal isotropic resolution, i.e., of equal
resolution in all three spatial axes, can only
be achieved for very limited scan ranges
Multi detector row system
Uses many detector elements in multiple
Single detector can produce multiple slices.
Slice thickness is determined by combination of
x ray beam width and detector configuration.
Each detector element consists of a radiation-
sensitive solid-state material (such as cadmium
tungstate, ceramics, CSI), which converts the
absorbed X-rays into visble light The light is
then detected by a Si photodiode.
The resulting electrical current is amplified and
converted into a digital signal.
To select different slice
combine several detector
rows electronically to a
smaller number of slices
according to the selected
beam collimation and
the desired slice width
All recently introduced
16-slice CT systems
employ adaptive array
16 (Siemens, Forchheim, Germany) as a
representative example uses 24 detector rows.
The16 central rows define 0.75-mm collimated
slice width at isocenter; the 4 outer rows on
both sides define 1.5-mm collimated slice
width. The total coverage in the transverse
direction is 24 mm at isocenter. By appropriate
combination of the signals of the individual
detector rows, either 12 or 16 slices with 0.75-
mm or 1.5-mm collimated slice width can be
PITCH IN SDCT
Pitch is defined as travel distance of CT
scan table per 360 rotation of x ray tube
divided by x ray beam collimation width
Pitch is inversely proportional to dose
Pitch les than 1 is overlapping pitch and
pitch greater than 1 is extended pitch.
Increasing pitch will result more anatomic
coverage and reduce radiation dose to
PITCH IN MDCT
In MDCT slice thickness is not controlled by
beam width but by detector configuration.
So for calculation of pitch in MDCT beam
width should be determined by multiplying the
number of slices by slice thickness.
For Example : In 4 slice scanner at 1.25 slice
thickness and table feed 0f 6mm per rotation
Beam width = 4 X 1.25 = 5 putting value in
formula we get
Pitch = 6/5 = 1.2
Advantage of helical CT over AXIAL CT
The ability to minimize motion artifacts
owing to faster/shorter acquisitions.
Decreased incidence of misregistration
between consecutive axial slices
Reduced patient dose
Improved spatial resolution in the z-axis
Enhanced multiplanar or 3D renderings
Advantage of Axial CT OVER
Highest image quality than helical methods
because of axial nature i.e. slices are
perpendicular not slanted and patient table
remains stationery during data acquisition.
Surface renderings : also known as shaded
surface display (SSD)
Image are created by comparing the intensity of
each voxel in data set to some predetermined
threshold CT value
Thus software will include or exclude the voxel
depending on whether its CT number is above
or below the threshold and uses this
information to create a surface of an object
Tubular structures like airways, colon , and
bone surface etc
It is 3D semitransparent representation of the imaged
structure in which all voxels contribute to the image.
VR images display multiple tissue and show their
relationship with one another
It sums the contributions of each voxel along line and
each voxel is assigned an opacity value based on its
Opacity value determine the degree to which each
voxel will contribute to final image.
Pixels in final image can be assigned a colour,
brightness and degree of opacity. Generally soft
tissue have high transparency bone strong
opaqueness due to difference of HU.
Virtual endoscopy, virtual bronchoscopy and virtual
colonoscopy is form of VR.
Curve planer reformations
Allows image to be created along centerline
of tubular organs like CBD, ureters.
Curved planar reformations were obtained
using a cursor to draw a curved line along a
special anatomic structure on a stack of
axial, sagittal, coronal section at
Maximum intensity projection
Voxels with higher HU number are displayed
Generally used in angiography to see blood
Depth information is minimal.
Increased in mean background intensity.
Artefacts in MDCT
Spiral Pitch Artifacts :
Spiral artifacts exhibit
stepping in reformatted
images, the steps
appear as a spiral
groove. It have a
unique appearance in
axial scans; a star
pattern is seen off of
sharp edges, where
the number of spokes
in the star is directly
related to the number
of multislice detector
row this is because
each row contributes
only a portion of its
Windmill artefacts :
that spin off of features
with high longitudinal
gradients. The number
of black/white pairs
matches the number of
slices (detector rows) in
the multi-slice detector.
Faint stripes may be
apparent in multiplanar
from helical data
because the helical
gives rise to a degree
the z axis