The document contains questions and answers about various topics related to CT scans. It includes definitions and explanations of ring artifacts, HRCT techniques, image reconstruction methods, CT numbers, scintillation detectors, pixels, radiation profile width in CT collimators, CT number, resolution types, mass attenuation coefficient, parallel multi-hole collimators, low dose CT scans, CT guided biopsies, and CT artifacts. The document consists of questions from several students on technical aspects of computed tomography imaging.

1. CT SCAN - QUESTION AND ANSWER 3 MARKS
Name of the student
Iniya sanjana.A
Nandha kumar.A
Nithya.M
Mano.L
Melhin Hebi.J
Simon.I
Bharath.v

2. 1.Ring artifact: by Iniya sanjana
 Ring artefacts are a CT phenomenon that occurs due to mis
calibration or failure of one or more detector elements in a CT
scanner.
 Less often, it can be caused by insufficient radiation dose or
contrast material contamination of the detector cover.
 They occur close to the iso centre of the scan and are usually
visible on multiple slices at the same location.
 They are a common problem in cranial CT.

3. 2.HRCT ct thorax (by,sanjana)
High-resolution computed tomography (HRCT) is a type of
computed tomography (CT) with specific techniques to
enhance image resolution. It is used in the diagnosis of
various health problems, though most commonly
for lung disease, by assessing the lung parenchyma.
H.R.C.T Technique
HRCT should be done in axial mode.
Selected slice thickness should be .5 to 1.5mm (thin cuts)
Slice gap should be 3 to 5 mm.
Scan field can be limited to include region of interest.
PNS, ORBITS, TEMPORAL BONE are regions meant for
HRCT.

4. Recent advance machine taking 5mm cuts in axial mode and
breaking into .625 mm cuts by using multi detector and
software.

5. 3.Imagereconstruction (by,iniya sanjana)
Image reconstruction in CT is a mathematical process that
generates tomographic images from multiple X-ray
projection data acquired at many different angles around the
patient. Two major categories of reconstruction methods
exist, analytical reconstruction and iterative reconstruction.

6. Reconstruction algorithms
There are various algorithms used in CT image
reconstruction, the following are some of the more common
algorithms utilised in commercially available CT today.
 iterative algorithm without statistical modelling
o used originally by Godfrey Hounsfield, however not
commercially used due to the inherent limitations of
microprocessors at that time
o will use an assumption and will compare to the
assumption with its measured data. Then will continue to
make iterations until the two data sets are in agreement.
 iterative algorithm with statistical modelling
o iterative reconstruction with statistical modelling that
takes into account
 optics (x-ray source, image voxels and detector)
 noise (photon statistics)
 physics (data acquisition)
 object (radiation attenuation)
 back projection
o not used in the clinical setting, as it is unable to produce
sharp images
o known for its distinctive artefact that resembles a star

7.  filtered back projection (convolution method)
o still widely used in CT today
4.CT Number(by,injya sanjana)
(Computed tomography number) The CT number is a
selectable scan factor based on the Hounsfield scale. Each
elemental region of the CTimage (pixel) is expressed in terms
of Hounsfield units (HU) corresponding to the x-ray
attenuation (or tissue density).

8. 5. Scintillation detector (by,iniya sanjana)
A scintillation detector or scintillation counter is obtained
when a scintillator is coupled to an electronic light sensor
such as a photomultiplier tube (PMT), photodiode, or silicon

9. photomultiplier. PMTs absorb the light emitted by
the scintillator and re-emit it in the form of electrons via the
photoelectric effect.
The difference being that with CT technology the screen-film
system is replaced by a detector. In CT imaging there are two
main types old detectors: xenon detectors and solid-
state detectors
6.Pixel : by A. Iniya sanjana
The image by the CT scanner is a digital image and consists of a
square matrix of elements (pixel), each of which represents a voxel
(volume element) of the tissue of the patient.
The voxel is represented in the image as a two dimensional
element called pixel ( picture element).

10. Images are typically matrix of 512×512 or 1024×1024 pixels.

11. 7. Radiation profile width in CT collimator ( by Sanjana)
This is the technique to remove the scatter radiation before reaching
detector in the CT imaging. One of the most important parameters in
x-ray CT imaging is the noise induced by detected scattered radiation.
The detected scattered radiation is completely dependent on the
scanner geometry as well as size, shape and material of the scanned
object. The magnitude and spatial distribution of the scattered
radiation in x-ray CT should be quantified for development of robust
scatter correction techniques. Empirical methods based on blocking

12. the primary photons in a small region are not able to extract scatter in
all elements of the detector array while the scatter profile is required
for a scatter correction procedure. This technical note demonstrates
computed tomography (CT) radiation profile measurement using
computed radiography (CR) imaging plate raw data showing it is
possible to perform the CT collimation width measurement using a
single scan without saturating the imaging plate.
1.CT NUMBER by ( Nandhakumar)
 CT is a mathematical or numerical representation of each pixel
 The CT number is a selectable scan factor based on the
Hounsfield scale.
 The CT numbers are calculated from the x-ray linear attenuation
coefficient values for each individual tissue voxel.
 It is the attenuation coefficient that is first calculated by the
reconstruction process and then used to calculate the CT
number values.
 CT scan on an arbitrary scale on which water has density 0, air
−1000, and compact bone +1000

13. 2.WHAT IS RESOLUTION AND ITS TYPES ( by
Nandhakumar)
 Resolution is the measure of how far apart two objects must be
before they can be seen as separate details in the image. For two
objects to be seen as separate the detectors must be able to
identify a gap between them.
There are three types of resolution in CT
 Spatial resolution
 Temporal resolution
 Contrast resolution

14. Spatial resolution :
Spatial resolution in CT is the ability to distinguish between
object or structures that differ in density. A high spatial
resolution is important for one to discriminate between
structures that are located within a small proximity to each
other.
Temporal resolution :
The concept of temporal resolution is fundamental to cardiac CT
and MRI, in which a rapidly beating heart is imaged over the
order of milliseconds into multiple frame-captures.
Contrast resolution:
Contrast resolution in radiology refers to the ability of any
imaging modality to distinguish between differences in image
intensity. The inherent contrast resolution of a digital image is
given by the number of possible pixel values, and is defined as
the number of bits per pixel value.

15. 2.MASS ATTENUATION COEFFICIENT ( by
Nandhakumar)
The mass attenuation coefficient is a measurement of how strongly
a chemical species or substance absorbs or scatters light at a given
wavelength, per unit mass.
or
The mass attenuation coefficient is a measure of the probability of
the interaction that occurs between incident photons and the
matter of the unit mass per unit area.
 Mass attenuation coefficient is the ratio between the linear
attenuation coefficient and density.
 The SI unit of mass attenuation coefficient is the square metre
per kilogram (m2/kg). Other common unit cm2/g (the most
common unit for X-ray mass attenuation coefficients)

16. 4.PARALLEL MULTI HOLE COLLIMATOR ( by
Nandhakumar)
 parallel-hole collimators routinely used for planar and
SPECT imaging, there are special focused collimator options
specifically designed for SPECT imaging the brain and the
heart.
 These typically are a type of converging collimator that
permits more of the camera crystal to be used for radiation
detection.
 These collimators cause magnification of the object and an
increase in sensitivity proportional to the level of
magnification.
 Collimators spatial resolution of multi hole collimators is
determined by geometry of the holes

17. 5. WHAT IS LOW DOSE CT SCAN ( by Nandhakumar)
 A CT Scan system that limits the radiation exposure to the
patient and produces high-resolution three-dimensional images
or pictures of the inside of the body.
 Low-dose CT or LDCT uses less ionizing radiation than a
conventional CT scan. In children and young adults, exposure to
CT radiation is of particular concern.
 Low-dose CT is recommended Low-dose Abdominal CT Protocols
With a Tube Voltage Setting of 100 kVp or 80 kVp: Performance
of Radiation Dose Reduction and Influence on Visual Contrast
 low-dose CT scan that is recommended for lung cancer and the
kidney , ureters, and bladder (CT KUB) is the investigation of
choice for renal colic screening is a newer form of CT scan
known as a low-dose spiral or helical CT scan.
For abdominal CT in young adults with suspected appendicitis and
renal calculi.
For pediatric head CT.
For CT screening of Lung and follow up cases.

18. ADVANTAGES :
 A low-dose CT scan is a quick, less than a minute.
 This type of CT scan uses no dyes, no injections, and requires
nothing to swallow by mouth.

19. 6. What is a CT guided biopsy of organ (by
Nandhakumar)
 A CT-guided biopsy is a procedure by which the radiologist uses
a very thin needle and a syringe to withdraw a tissue or fluid
specimen from an organ or suspected tumour mass for
pathological evaluation.
 The needle is guided while being viewed by the radiologist on a
computed tomography (CT) scan on console.
 CT-guided biopsy can be performed on almost all organs of the
body, for example on the lungs, liver, kidneys, adrenal glands,
pancreas, and pelvis.

20. 7.CT ARTEFACTS ( by Nandhakumar)
 An artefact is a structure or an appearance that is not normally
present on the radiograph or in CT images and is produced by
artificial means.
 CT artefacts are common and can occur for various reasons.
Artefacts are items, are any things made by or used by humans.
 Some examples would be whole pottery, metals, and stone
tools.
There are many different types of CT artefacts,
 Beam hardening artefact
 Scatter artefacts
 motion artefacts
 cone beam artefacts
 Spiral Pitch artefacts
 Partial volume artefacts
 Ring, and metal artefacts.

21. 1.Tomography by nithya
Tomography is a radiological technique for obtaining clear x-
ray images of deep internal structure by focusing on a specific
plane within the body.
It refers to slice view or sectional imaging and is usually
referred as body section radiography
There are two types of tomography, namely
 Linear tomography
 Computed tomography
Linear tomography- In Linear tomography, the image of the
overlying and underlying structures can be blurred by moving the x-
ray tube and film during exposure, about an axis through the
structure of interest.
It is an imaging technique that produces sectional view of
the patient in a plane, parallel to the table top.

22. Computer tomography- Is a special form of tomography in
which a computer is used to make a mathematical reconstruction
of a tomographic plane or slice.
It generates images in trans axial section, the special features
of CT images are
 Images are cross sectional
 Eliminates the superimposition of structures
 Not influenced by the properties of the neighboring region

23. 2.CTDI- computer tomography dose index: by nithya
o CTDI is measured in mGy
 CTDI is always stated for 100mas value.
 CTDI increases with tube voltage and CTDI of body scans lower than
head, due to greater attenuation in the body.
 The typical effective dose in CT scan is 1-2 mSv for head, 6 mSv for
chest and 4 mSv for abdomen.
 CT scanner needs to be tested for CTDI once in 2 years
 As a part of QA program.
 The CTDI of each CT unit is useful to calculate the effective dose, to
patient and workers.

24. 3.Pitch ( by nithya)
Pitch is a term used in helical CT.
It has two terminologies depending on whether single or
multislice CT scanners
Single slice CT:
The term detector pitch is used and is defined as the ratio
between the table distance traveled in one 360 degree gantry
rotation divided by beam collimation/slice thickness.
Pitch = Table movement per rot/ slice thickness.
for example, if the table traveled 5 mm in one rotation and the
beam collimation was 5 mm then pitch equals 5 mm / 5 mm = 1.0.
Choice of pitch affects both image quality and patient dose.

25. Multislice CT (MSCT) :
Beam pitch is defined as table distance traveled in one 360° gantry
rotation divided by the total thickness of all simultaneously
acquired slice
Pitch= Distance travelledperrot/ slice thicknessX no of slicesacquired.

26. 4.Cone beam CT ( by nithya)
Cone beam CT (CBCT) is a variant type of computed tomography (CT),
and is used particularly in dental and extremity imaging.
In CBCT, a divergent cone-shaped source of radiation is directed
through the target. The attenuated x-rays are detected on the
opposite side by an x-ray detector, which has multiple in the pixels x
and y-axis
Advantages :
Decreased examination time.

27. Decreased patient movement artifact’
Increased x-ray tube efficiency.
Disadvantages:
Increased scattered radiation.
Potential for cone beam artifact if an inappropriate reconstruction
algorithm is used.
5.CT guided biopsy ( by nithya)
CT guided procedure benefits
 Precise lesion targeting
 Clear image guidance for
 needle placement
 Immediate post procedure
diagnostic information
 Post procedure bleeding
 Pneumothorax
Types of CT guided procedures
 Drainage catheter placement
 Chest
 Abdomen / Pelvis
 Biopsy
 Lung
 Solid Organ
 Lymph nodes
 Bone

28.  Tumor ablation
 Vertebroplasty
 Embolization
5. CT NUMBER LINEARITY (by nithya)
In computer tomography it concerns the linear relationship
between calculated CT number and the linear attenuation coefficient
of each element of the object.

29. Deviations from linearity should be < +- 4HU
CT number linearity is assessed using a phantom containing inserts of
a number of different materials (material should cover a wide range
of CT number).

30. 7. Fan-beam collimation(by nithya)
An essential feature of SPECT imaging is collimation of gamma
photons, allowing detection of only those photons propagating in the
appropriate direction.
Parallel beam collimators are widely used in different applications,
because reconstruction is straightforward.
Fan-beam collimators focus on a focal line parallel to the axis of
rotation. For organs centered within the field of view this enhances
the sensitivity.
Fan-beam
i. Each row of septa have a focal point in one plain
ii. Adjacent septa are arranged in the same focal plain and in
any given row they are parallel to each other
iii. This allows each independent row to focus on plain within
3D object

31. iv. When processed these rows generate individual
tomographic slices
1.DISCRIBE WINDOW WIDTH. ( BY MANO)
The widow width is the measure of the range of ct number
that an image contains.
That is range of CT numbers displayed in the CT image .
A wider window width (2000 HU), THEREFIRE , Will display a
wider range of CT numbers.
2. TEST FOR RESOLUTIONS IN CT.( by mano)
The spatial resolution depends on the reconstruction algorithm,
so it is important that repeat measurements are made under identical
conditions. If the resolution is determined visually from a bar
phantom the display should be adjusted for optimum viewing.
Current CT scanners have a spatial resolution of 0.5–0.625 mm in the
z-axis, and approximately 0.5 mm in the x- to y-axes.

32. Visual assessment
A method is through visual assessment using a test device consisting
of a repeated pattern of holes, bars or lines. When measuring the
limiting high contrast resolution using a phantom for high contrast
resolution having sets of test objects of equal diameters and spacing,
the high contrast resolution must be 1 mm or less. (For example the
ACR phantom described elsewhere).

33. 3.ADVANTAGES OF PET CT .( by mano)
1. Superior lesion localisation from near-perfect
anatomy/functional registration with fewer motion artefacts.
2. Better distinction between physiological uptake and pathological
uptake.
3. Consolidation of patients imaging studies.
4. Shorter scan time ( average of 30 mints , vs 60 mints with
standard PET ) by using CT for attenuation correction. This acid in
patient comfort and minimises claustrophobia problems.
5. Follow up scan will help us to compare with previous images and
come to conclusion for treatment purpose.

34. 3. EXPLAIN ABOUT DYNAMIC CT STUDIES. ( BY
MANO)
1. In the dynamic scan mode multiple acquisition covering the
same body region are acquired. Explain of these study types
include:
1. Perfusion studies
2. Bolus tracking studies
3. Teat bolus studies
2. Dynamic scans often have large CTDI volume values because
the scanner reports the sum of the CTDI volume value from each
rotation.
5. IMAGE DISPLAY IN CT.( by mano)
1. The reconstructed image is displayed on the monitor. It is a
digital image.
2. It consists of 2D representation of 3D object in the form of
pixels.
3. Window width and window level is more important and it is
applied to get visual effect for desired visuals.
4. CT pixels size is determined by dividing the FOV by matrix
size which is generally 512 ×512.
5. PIXEL SIZE = FOV (mm)/MATRIX SIZE

35. 6.TUBE POTENTIAL USED IN CT. WHAT IS THE
RADIATION DOSE FROM CHEST CT. (by mano)
Tube potential may be defined as the electrical “potential” difference
between anode and cathode of the x-ray tube. Tube potential is
measured in voltage or kilovoltage ranges in CT and commonly
represented as peak kilovoltage or kVp.
The tube potential used in CT are 80kvp, 100kvp, 120kvp and 140kvp.
It will give effects on image quality and radiation dose for CT in
different body regions and clinical indications.
Tube potential is an important scanning parameter for radiation dose
optimization.
Reduction of tube potential results in increased image contrast of
iodine-enhanced CT as well as increased image noise.
Radiation dose for CT Chest
Routine CT Chest 7 msv
Low dose CT Chest 1.5 msv

36. 1.CT number verification.( J. Melhin)
It is most important to determine the spatial uniformity of
CT number in a given media .This test has been conducted using water
phantom and body phantom for measuring the CT number of water
,bone and air .In simulated clinical conditions, Caliberation scan has
been performed with narrowest slice width at all possible kVp
stations. Average CT number in an region of interest (ROI) of
approximately 1 cm square in the center of the phantom and at four
locations at the phantom periphery are obtained .The filed of view of
the CT is completely uniform. Similarly using a circular uniformity (
water) phantom the aspect ratio (horizontal Vs vertical diameter) has
been measured and it has been found that this is incomplete
congruence of the actual value .This testified the caliper accuracy and
reproducibility .

37. 2.CT Gantry…( J. Melhin)
It is the major part the CT scan system. The gantry is a
ring shaped structure; containing X-ray tube, detector, collimators,
filters, data acquisition system (DAS) ,associated electronics such as
gantry angulation motors ,rotational components, including slip ring
systems and the detector array .where we are positioning with help of
the patient with help of control.
The X-ray tube and detector spin rapidly in the gantry.
The gantry can rotate 360° around its axis.

38. 3.Dose reduction in CT.( J. Melhin)
 Pitch –– Increasing the pitch decreases the dose ,increases
the image noise ,increases the effective section thickness
and ,reduces the scanning time .
 Bowtie filters –– Bowtie beam shaping filters attenuate
off–axis rays to minimize dose and reduce X-ray scattering
effects.
 Shielding –– Bismuth shielding has been used to reduce
dose from CT to superficial radiosensitive organs like lens
and thyroid.
 mA modulation –– mA is adapted to body parts not patient
weight. Thinner parts need less radiation.
 Avoid repetition, Explain instructions properly.
 Tube voltage kV–– Decreasing the tube voltage significantly
reduces the dose.
 Scan length –– Scan length is directly related to CT
radiation dose. A shorter scan length means lower dose.
 AEC systems have a number of potential advantages
,including better control of patient radiation dose,
avoidance of photon starvation artifacts ,reduced load on
the X-ray tube, and the maintenance of image quality in
spite of different attenuation values on CT scans.

39. 4.Gantry tilt assessmentin CT. ( J. Melhin)
This test is performed to determine accuracy of tilt indicators
and to ensure that specified tilt can be accomplished under clinical
conditions. This test can be performed using a therapy localization
film similar to the procedure described in scan localization light
accuracy test. Exposures are given without tilt and with tilt on either
sides. On processed film the tilt angles between two density lines are
measured and compared with the tilt angle indicated on computer
console and gantry. Tilt accuracy should be within 3° and this can be
repeated for different angulations. According to present AERB +or – 2
° error is allowable.

40. 5.Techniques of CT cisternography.(J. Melhin)
CT cisternography is an imaging technique used to diagnose
CSF rhinorrhea or CSF otorrhea ( CSF leaks) ,as CT allows the
assessment of the bones of the base of skull.
Technique:
 Pre – contrast CT is performed with thin slices.
 3–10 ml of an iodinated non – ionic low– osmolar contrast
agent is installed into the thecal Sac after lumbar puncture.
 The patient is then tilted with foot end evaluation and a CT
scan is performed with thin slices; maneuvers that provoke
an active leak ,such as head hanging or sneezing, are
performed to visualize intermittent or occult leaks.
 Post–contrast images are then compared with the Pre–
contrast image to see where the CSF and the contrast are
leaking out.

41. 6.Volume rendering technique in CT. ( J. Melhin)
 Volume rendering is a type of data visualization technique which
creates a three dimensional representation of data. CT and MRI
data are frequently visualized with volume rendering in addition
to other reconstructions and slices.
 Volume rendering techniques can also be applied to
tomosynthesis data.
Clinical applications :
 It is very useful for CT angiography ,
 Virtual colonoscopy ,
 Virtual bronchoscopy ,

42.  Bony details.
7.Low pass and high pass filters in CT. ( J. Melhin)
High– pass (sharp) filters:
 Provide definitive borders, and edges.
 Used for high contrast areas, musculoskeletal
(increases image noise).
Low– pass ( soft) filters :
 Do not define borders and edges to the same extent
as high –pass filters.
 Used for low contrast areas such as brain, abdomen,
etc.

43. 1.CT DOSE INDEX : by bharath
* CT dose index (CTDI) (measured in mGy) is a standardised measure
of radiation dose output of a CT scanner which allows the user to
compare radiation output of different CT scanners
* The computed tomography dose index (CTDI) is a commonly used
radiation exposure index in X-ray computed tomography (CT), first
defined in 1981.
* The unit of CTDI is the gray (Gy) and it can be used in conjunction
with patient size to estimate the absorbed dose.

44. * The CTDI and absorbed dose may differ by more than a factor of two
for small patients such as children.
* In the past CTDI100 (measured over a 100 mm long ionisation
chamber) and CTDIw (weighted average of dose across a single slice)
were used; for helical scanners in current use, the parameter CTDIvol
is the more commonly used index.
2 ARTIFACTS OF CT SCAN: (Bharath)
Beam Hardening:
An x-ray beam is composed of individual photons with a range of
energies. As the beam passes through an object, it becomes “harder,”
that is to say its mean energy increases, because the lower-energy
photons are absorbed more rapidly than the higher-energy photons .
Two types of artifact can result from this effect: so-called cupping
artifacts and the appearance of dark bands or streaks between dense
objects in the image.

45. Cupping Artifacts.—
X rays passing through the middle portion of a uniform cylindrical
phantom are hardened more than those passing though the edges
because they are passing though more material. As the beam
becomes harder, the rate at which it is attenuated decreases, so the
beam is more intense when it reaches the detectors than would be
expected if it had not been hardened. Therefore, the resultant
attenuation profile differs from the ideal profile that would be
obtained without beam hardening . A profile of the CT numbers across
the phantom displays a characteristic cupped shape.

46. Streaks and Dark Bands.—
In very heterogeneous cross sections, dark bands or streaks can
appear between two dense objects in an image. They occur because
the portion of the beam that passes through one of the objects at
certain tube positions is hardened less than when it passes through
both objects at other tube positions. This type of artifact can occur
both in bony regions of the body and in scans where a contrast
medium has been used. In the chest scan shown in , the contrast
medium has caused artifacts that might be mistaken for disease in
nearby anatomy.
3.USES OF CT SCAN: (bharath)
* Examine internal and bone injuries from vehicle accidents or other
trauma
* Diagnose spinal problems and skeletal injuries
* Detect osteoporosis

47. * Detect many different types of cancers and determine the extent
(spread) of the tumors
* Locate infections
* Look for injuries, stroke-causing clots, hemorrhaging, and other
issues in the head
* Image the lungs to reveal blood clots in the lungs' vessels, excess
fluid, pneumonia, and chronic pulmonary obstructive disease (COPD)
* Determine the cause of chest or abdominal pain, difficulty
breathing, and other symptoms
* Diagnose dangerous vascular diseases that can cause stroke, kidney
failure, and death
* Additionally, CT scans are used to assist with biopsies and other
medical procedures, and help with treatment planning for organ
transplants, gastric bypass, and cancer, among other things.
4.CT:COMPUTED TOMOGRAPHY ( bharath)
CT scans, also called CAT scans, use a rotating X-ray machine to create
cross-sectional, or 3D, images of any body part, They provide a
painless, noninvasive and fast way for doctors to examine bones,
organs and other internal tissues.

48. WORKING OF CT SCAN:
* During a CT scan, the patient lies on a table that moves through a
doughnut-like ring known as a gantry
* The gantry has an X-ray tube that rotates around the patient while
shooting narrow beams of X-rays through the body.
* The X-rays are picked up by digital detectors directly opposite the
source.
* After the X-ray source completes a full rotation, a sophisticated
computer creates a 2D image of that slice of the body, which typically
ranges from 0.04 to 0.4 inches (1 to 10 millimeters) thick.
* The computer then combines several 2D slices to create a 3D image
of the body, making it easier for a doctor to pinpoint where the
patient's problem exists.
* The scan itself typically takes less than 15 minutes depending on the
area of the body being imaged.
* To make it easier to identify abnormalities, the patient may be given
a contrast material. Solutions containing contrast materials, such as
iodine or barium, are introduced into the body orally, rectally or
injected directly into the bloodstream, depending on the target tissue.
* The materials in the solution work by temporarily altering how X-
rays interact with certain body tissues, which makes those tissues
appear different in the resulting image. The contrast helps doctors
distinguish between normal and abnormal tissue.

49. 5 3D CT DEFINITION: ( bharath)
3D Computed Tomography (CT) is a nondestructive scanning
technology that allows you to view and inspect the external and
internal structures of an object in 3D space. Computed Tomography
works by taking hundreds or thousands of 2D Digital Radiography
projections around a 360 degree rotation of an object.
DIFFERENT TYPES OF CT SCAN:( BASED ON GENERATION)
* First Generation: Rotate/Translate, Pencil Beam.
* Second Generation: rotate/Translate, Narrow Fan Beam.
* Third Generation: Rotate/Rotate, Wide Fan Beam.
* Fourth Generation: Rotate/Stationary.
* Fifth Generation: Stationary/Stationary.
* Sixth Generation: Helical.
* Seventh generation: multiple detector.

50. 6.VIEWING WINDOW IN CT SCAN: ( by bharath)
HOUSNFIED UNITS:
* Numeric information in each pixel of the CT image
* Related to composition and nature of tissue
* Represent the density of tissue
* Also called as CT number.

51. 1 BOWTIE FILTER BY SIMON
Bowtie filter is a metallic filter and it is shape like man's tie, that
shape in a bow.
The term “bowtie” applies to a class of filter shapes featuring
bilateral symmetry with a thickness that increases with the distance
from the centre.
Filtering shapes the x-ray beam intensity. Removing low-energy x-rays
minimizes patient exposure and produces a more uniform beam.
Bowtie filters are commonly employed in CT scanners to
minimize radiation dose, by reducing intensity variations across
detector elements in the presence of patient anatomy.
This filtration modifies a number of x‐ray beam properties
(effective energy, flux, first and second order statistics), making
them non‐uniform across the fan beam field of view.

52. 2.Types of image reconstructionby Simon
Reconstruction algorithms
There are various algorithms used in CT image reconstruction, the
following
are some of the more common algorithms utilised in commercially
available CT
today.
• iterative algorithm without statistical modelling
o used originally by Godfrey Hounsfield, however not commercially
used
due to the inherent limitations of microprocessors at that time
o will use an assumption and will compare to the assumption with its
measured data. Then will continue to make iterations until the two
data
sets are in agreement.
• iterative algorithm with statistical modelling
o iterative reconstruction with statistical modelling that takes into
account
▪ optics (x-ray source, image voxels and detector)
▪ noise (photon statistics)
▪ physics (data acquisition)
▪ object (radiation attenuation)

53. • back projection
o not used in the clinical setting, as it is unable to produce sharp
images
o known for its distinctive artefact that resembles a star
• filtered back projection (convolution method)
o still widely used in CT today
o utilises a convolution filter to alleviate the blurring associated with
back
projection
o fast, however, has several limitations including noise and artefact
creation

54. 3. CT CHOLANGIOGRAPHY: by simon
• CT cholangiography is a technique of imaging
the biliary tree with the usage of hepatobiliary
excreted contrast. It is useful in delineating
biliary anatomy, identifying a bile leak or looking
for retained gallstones within the biliary system.
• A CT Cholangiogram is the injection of ‘Contrast’
(once called X-ray dye) into the bloodstream to
look at the bile ducts. This Contrast is called
Biliscopin®. When Biliscopin® is injected into
your body it is removed by your liver into the
bile.
• Ct cholangiopraphy has been used to describe
two techniques negative and positive contrast
media.
• The negative contrast makes use of bile as a
negative contrast agent to display the bilary tree
by using a various reformating techniques.
• The positive contrast agents are either IV or oral
Contrast agens that are excreted preferentially
by the liver to opacity the bile ducts.

55. 4.SPIRAL/HELICAL CT by simon.I
 With helical CT, the patient is moved through a rotating x-ray
beam and detector set. From the perspective of the patient, the
x-ray beam from the CT traces a helical path. The helical path
results in a three-dimensional data set, which can then be
reconstructed into sequential images for a stack.
 Helical CT allows a scan to be performed in a single breath-hold.
 A technique where by the patient is transported continuously
through the gantry while data are aquired continuously during
several 360 degree scans.

56. Advantages of spiral/ helical CT
 Increased speed a study
 Less motion artifact
 Exact continuity of images.

57. 5.Principles of CT: by simon
 The basic principle behind CT is that the internal structure of an
object can be reconstructed from multiple projections of the
object.
 The ray projections are formed by scanning a thin cross section
of the body with a narrow x-ray beam and measuring the
transmitted radiation with a sensitive radiation detector.
 CT scanning is a systematic collection and representation of
projection data.
The main parts of the CT system are Gantry, Table and console.
Gantry
Table
Console.