-CT scan technique for dose reduction -Iterative algorithm - Ct scan technology

1. Mohd Aiman bin Azmardi
Pegawai Pengimejan,
Jabatan Pengimejan Diagnostik
Hospital Kulim
11th August 2015

2.  Introduction
 Dose in CT Scan
 Dose Reduction Technique
 Equipment Design
 What is dose in CT Scan..?
 Acquisition Parameters
 mAs
 ATM
 kVp
 Image Reconstruction
 FBP
 Iterative Reconstruction
 Scan Technique
 Recommendation
 Conclusion
 References

3.  CT scan advancement in technology
 Near to isotropic pixel – high resolution
 Fast scan – cover ROI in short time
 Advance post processing (e.g.: MIP, 3D
reconstruction, VRT and many more)
 Weakness?

4.  CT scan lead high dose compared to other
imaging modalities
 50 times riskier from plain abdomen
radiograph
 MSCT compared to single slice
 Increase 10% and 34% of effective dose per
patient
 CT scan induce cancer arise rapidly
 ALARA

5.  The most powerful dose reduction tool in
radiation protection -justification of a study,
 However, where an examination is
undertaken, the emphasis must be on dose
optimization,
 achieving the required image quality at the
lowest possible dose level.
 This can be approached in two ways
 design of dose efficient equipment,
 through the optimization of scan protocols.

7.  Siemens intoduced in the SOMATOM Sensation
64 CT-system is equipped with the 0 MHU
STRATON x-ray tube.
 This newly developed X-ray tube offers
 significantly reduced cooling times for shorter
interscan delays
 increased power reserves.

10.  Double Z sampling is two overlapping
projections result
 doubling the scan (more data)
 without an increase in dose.
 Advantages
 Slice widht can be reduce e.g: it is possible to
reconstruct 0.6mm slices at any pitch less than
1.5) with best image quality
 Improve image quality
 decreasing slice width
 and removing windmill artifacts, without
increasing the radiation dose
Z axis direction

11.  Windmili artifact can be reduced which
commonly found in MSCT images in the vicinity of
sharp contrast in axial direction.

12.  Dual energy CT scans are a relatively new
form of CT scanning that use separate X-ray
energies to make images. Images can be
generated:
 by the simultaneous use of two X-ray tubes (‘dual
source’);
 by using an X-ray detector with separate layers
to detect two different energy ranges (‘dual
layer’); or
 by using a single scanner to scan twice using two
different energy levels (electronic kVp
switching).

13.  Advantages
 CT angiography. Dual energy scans can amplify the
iodine signal of contrast agents, improving the
delineation of arteries. They can also better
distinguish iodine from calcium, therefore allowing
better bone subtraction around vessels; for example,
at the skull base.
 CT of the kidney, ureter and bladder (CT KUB). Dual
energy CT KUB scans can reliably distinguish urate
from non-urate calculi.
 CT imaging around metal implants. Dual energy CT
can significantly reduce the streak artefact normally
associated with metal implants and allow better
visualisation; for example, around spinal rods or hip
replacements.

14.  With fast kV-switching, Dual
Energy data can be acquired
by rapidly switching the tube
voltage between CT
projections.
 Disadvantages
 lower number of projections are
available to create each image;
reduced image quality
 In addition, only the kVp can be
modulated between individual
projections. Resulting over-
exposure in the highkV
projections or under-exposure in
the low-kV projections

15.  Idealized dual layer detector
technology: In reality a certain
amount of high and low-energy
photons are registered in both
layers which significantly reduces
spectral separation
 Disadvantages
 Detector not able to distinguish
between high and low energy
photons. Both high and low energy
photons are absorbed in both layers
 The construction of this detector
requires two photodiodes, which
significantly increases electronic
noise. Leads to inferior image
quality for dual and single energy
images.

16.  Slow kV-switching: Both kV and
mA are switched between half
rotations of the gantry, either
in sequence or in spiral modes
 Disadvantages
 The time needed to switch from
80 kV to 140 kV and adjust the
mA is typically in the order of
100 ms.
 During this time, the patient is
exposed to radiation that does
not provide useful information.
 Thus, this method does not
follow the ALARA (“as low as
reasonably achievable”)
principle.

17.  Dual Energy imaging means that the system
uses two X-ray sources simultaneously at
different energy levels.
 This makes it possible to differentiate
 between fat, soft tissue, and bone,
 and also between calcifications and contrast
material (iodine) on the basis of their unique
energy-dependent attenuation profiles.

19.  Volume Computed Tomography Dose Index
(CTDIvol) is a standardized parameter to
measure Scanner Radiation Output
 CTDIvol is NOT patient dose
 CTDIvol is reported in units of mGy for either a 16-cm
(for head exams) or 32-cm (for body exams) diameter
acrylic phantom
 For the same technique settings, the CTDIvol reported for
the 16-cm phantom is about twice that of the 32-cm
phantom
 The reported CTDIvol is based on measurements made by
the manufacturer in a factory setting
 In these slides, the term "patient dose" is
used to describe the absorbed dose to a
patient, while the generic term "dose" refers
to CTDIvol

20. CTDIvol is not patient dose
 The relationship between the two depends on
many factors, including patient size and
composition
 AAPM Report 204 introduces a parameter
known as the Size Specific Dose Estimate
(SSDE) to allow estimation of patient dose
based on CTDIvol and patient size
 For the same CTDIvol, a smaller patient will
tend to have a higher patient dose than a
larger patient

21. Both patients scanned with the same CTDIvol
Patient dose will be higher for the smaller
patient
CTDIvol = 20 mGy CTDIvol = 20 mGy
120 kVp at 200
mAs
120 kVp at 200 mAs
32 cm
Phantom
32 cm
Phantom

22. Smaller patient scanned with a lower CTDIvol
Patient doses will be approximately equal
CTDIvol = 10 mGy CTDIvol = 20 mGy
120 kVp at 100
mAs
120 kVp at 200 mAs
32 cm
Phantom
32 cm
Phantom

23.  CTDIvol provides information about the amount of
radiation used to perform the study
 CTDIvol is a useful index to track across patients
and protocols for quality assurance purposes
 CTDIvol can be used as a metric to compare
protocols across different practices and scanners
when related variables, such as resultant image
quality, are also taken in account
 The ACR Dose Index Registry (DIR) allows
comparison across institutions of CTDIvol for
similar exam types (e.g., routine head exam)

24.  The Dose Length Product (DLP) is also
calculated by the scanner
 DLP is the product of the length of the
irradiated scan volume and the average
CTDIvol over that distance
 DLP has units of mGy*cm

25.  The relationships between acquisition parameters and
CTDIvol described in the following slides assume all other
parameters are held constant
 The relationship between a parameter and CTDIvol is often
described as proportional in some way
 The symbol  is used to indicate “proportional to”
 Directly proportional means that a change in the
parameter results in the same change in CTDIvol
 Example: Doubling the rotation time from 0.5 to 1.0 seconds
will double the CTDIvol
 Inversely proportional means that a change in a parameter
has the opposite effect on CTDIvol
 Example: Doubling the pitch from 1 to 2 will reduce the CTDIvol
by half

27.  Acquisition Parameters define the technique
that will be used and how the scan will
proceed
 Acquisition Parameters are set in the user
interface where scans are prescribed
 Changing a single Acquisition Parameter
while holding everything else constant will
typically affect the CTDIvol for that scan
 The following slides describe what that
affect is for each parameter

28. CT Scanners offer a variety of Scan Modes
which describe how the table moves during
an exam
 Scan Modes include
 Axial
 Helical or Spiral
 Dynamic
The Acquisition Parameters that affect
CTDIvol may change amongst different
Scan Modes

29.  In the Dynamic Scan Mode multiple
acquisitions covering the same body region
are acquired. Examples of these study types
include:
 Perfusion Studies
 Bolus Tracking Studies
 Test Bolus Studies
 Dynamic Scans often have large CTDIvol
values because the scanner reports the sum
of the CTDIvol values from each rotation
 The reported CTDIvol is NOT skin dose or
organ dose

30. Is the movement of the table through the bore
of the scanner over a full 360 degree rotation
Units: millimeters/rotation or
millimeters/second
The parameter is known both as Table Feed
(helical/spiral acquisition) & Table Increment
(axial acquisition)
Table Feed affects CTDIvol through its
inclusion in Pitch (discussed later)

31.  Is the combination of the number of data channels and the
width of the detector associated with each data channel
 The Detector Configuration determines the Beam Width or
Beam Collimation (nT), which is the number of channels
(n) times the detector width associated with each data
channel (T)
 For a selected detector width per data channel, a smaller
total Beam Collimation usually has a higher CTDIvol than a
larger Beam Collimation
 Example: On a 16 slice scanner with a detector width per
channel of 1.25 mm, a collimation of 4x1.25mm is generally less
dose efficient than a collimation of 16x1.25mm
Users should monitor CTDIvol values
when changing detector
configuration

32. Acquisition Parameter Settings

33.  Is the Table Feed per gantry rotation divided by
the beam width/collimation
 Pitch is the ratio of two distances and therefore
has no units
 Users should monitor other parameters when
changing Pitch. The scanner may or may not
automatically compensate for changes in Pitch (for
example, by changing the tube current) to
maintain the planned CTDIvol.
CTDIvol 1/Pitch:
Hitachi, Toshiba (no AEC)
CTDIvol independent of Pitch:
GE, Siemens, Philips, Neusoft, Toshiba (AEC)

34.  CTDIvol may not change in the expected manner
if the scanner automatically adjust other
parameters when the pitch is changed
 The relationships between CTDIvol and pitch for
the different vendors are described below
 CTDIvol inversely proportional to change in pitch:
Hitachi, NeuroLogica
 CTDIvol constant when pitch is changed due to changes
to other parameters: GE, Neusoft, Philips and
Siemens
 The relationship between CTDIvol and pitch depends
on scan mode or Software version: Toshiba

35. Pitch < 1
Beam Width has
some overlap at
each view angle
from rotation to
rotation
Pitch = 1
No overlap of Beam
Width at each view
angle and no view
angles not covered at
certain table positions
Pitch > 1
Some view angles are
not covered by the
beam width at certain
table positions
Acquisition Parameter Settings

36. Determines the number of electrons
accelerated across the x-ray tube per unit
time
Units: milliAmperes (mA)
CTDIvol is directly proportional to Tube
Current
CTDIvol Tube Current

37.  Is the electrical potential applied across the
x-ray tube to accelerate electrons toward the
target material
 Units: kiloVolts (kV or kVp)
 CTDIvol is approximately proportional to the
square of the percentage change in Tube
Potential
n
old
new
kV
kV






volCTDI

38.  Research done by reducing kVp improved
enhancement of iodinated contrast media as a dye in
the vascular and no significance increase in image
noise.
 Contrast to noise ratio(CNR) will increase at lower
tube potentials, radiation dose may be reduced to
achieve similar or improved iodine CNR compared to
high kV. This will lead to reducing the dose to the
patient.
 With phantom 10cm the require dose at 80kV is only
35% of the amount required at 120kV and at 100kV
the required dose is 62% of the amount required at
120kV.
 With the 25cm phantom, the doses are 46% of the
120kV dose at 80kV and 63% of the 120-kV dose at
100 kV

39. Graph shows the relative radiation dose required at each tube potential to obtain
the same iodine CNR For all three phantoms. For the 10-cm phantom, 35% of the
120-kV dose is required at 80 kV to achieve the same iodine CNR as at 120 kV, and
62% of the 120kV Dose is required At 100 kV. For the 25-cm Phantom, 46% of the
120kV dose Is requiredAt 80 kV, and 63% of the 120-kV dose is required at 100 kV

40. Axial contrast material–enhanced multidetector CT images obtained
in a 58-year-old man during late hepatic arterial phase with (left)
protocol A (140 kVp), (right) protocol B (80 kVp)((Marin et al., 2010)

41.  However selecting the appropriate kVp is not
straightforward task
 Scanning speed, motion artifacts, patient
size and diagnostic task must be considered
and carefully evaluated before the patients
undergo CT scan examination

42. Is the product of Tube Current and the
Exposure Time per Rotation
Units: milliAmpere-seconds (mAs)
CTDIvol is directly proportional to Tube
Current Time Product
CTDIvol Tube Current Time Product

43.  dose ~ tube current(constant tube potential,
scanning time & slice thickness)
 Weight or size based – 50% dose reduction
 One research done reduction from 100mAs to
40mAs in sinus examination (findings –
chronic and acute sinusitis)
 Low dose (40mAs) can be clearly visualize;
no significant difference dspite increase in
noise (graininess)
 Reduction should be made without degrade
the image quality

44. Figure 1.Axial and Sagittal images of MDCT scans obtained at 100 mAs (A, B) and 40 mAs (C, D)
at the level of the maxillary sinuses show complete opacification of the right maxillary sinus (star)
and air-fluid levelin the left maxillary sinus (arrow). No significant difference in the diagnostic image
quality of these tworadiological findings of these two scans.

45. Figure 2.Coronal reformatted images of MDCT scans obtained at 100 mAs (A) and 40
mAs (B) at the level of osteomeatal complex showing a normal right osteomeatal
complex and a blocked left osteomeatal complex (arrow). This structure is clearly
identified and correctly assessed in these two scans of this patient.

46.  Many CT scanners automatically adjust the
technique parameters (and as a result the
CTDIvol) to achieve a desired level of image
quality and/or to reduce dose
 Dose Modulation and Reduction techniques
vary by scanner manufacturer, model and
software version

47.  Automatically adapts the Tube Current or Tube Potential
according to patient attenuation to achieve a specified image
quality
 Automatic adjustment of Tube Current may not occur when Tube Potential
is changed
 Centering the patient in the gantry is VITAL for most AEC systems
 AEC aims to deliver a specified image quality across a range of
patient sizes. It tends to increase CTDIvol for large patients and
decrease it for small patients relative to a reference patient
size
The use of Automatic Exposure Control
may decrease or increase CTDIvol
depending on the patient size and body
area imaged and image quality
requested

48. Is the AEC parameter that is set by the
user to define the desired level of image
quality
Changing the Image Quality Reference
Parameter will affect the CTDIvol
The effect on CTDIvol when changing the
Image Quality Reference Parameter is
vendor dependent
Dose Modulation and

49.  A change in the Image Quality Reference
Parameter will affect the CTDIvol
 Setting the parameter for “increased” image
quality (e.g., lower noise) will result in more
dose
 Setting the parameter for “decreased” image
quality (e.g., more noise) will result in less
dose
Dose Modulation and

50.  Is an AEC feature that adjusts the Tube
Current as the x-ray tube rotates around the
patient to compensate for attenuation
changes with view angle
 Angular Tube Current Modulation is used to
adjust the Tube Current to attempt to deliver
similar dose to the detector at all view
anglesThe use of Angular Tube Current
Modulation may decrease or increase
CTDIvol depending on the patient size
and body area imaged
and image quality requested
Dose Modulation and

51.  Angular Tube Current Modulation uses
information from one or two view localizers

52. Is an AEC feature that adjusts the Tube
Current as patient attenuation changes in
the longitudinal direction
The CT Localizer Radiograph is used to
estimate patient attenuation
The use of Longitudinal Tube Current
Modulation may decrease or increase
CTDIvol depending on the patient size and
body area imaged and image quality
requested
Dose Modulation and

53.  Longitudinal Tube Current Modulation uses
information from one or two view localizers
Dose Modulation and

54.  Is an AEC feature that incorporates the
properties of both Angular and Longitudinal
Tube Current Modulation to
 Adjust the Tube Current based on the patient’s overall
attenuation
 Modulate the Tube Current in the angular (X-Y) and
longitudinal (Z) dimensions to adapt to the patient’s shape
The use of Angular and Longitudinal Tube
Current Modulation may decrease or
increase CTDIvol depending on the patient
size and body area imaged and image
quality requested
Dose Modulation and

55. Dose Modulation and

56.  AEC is available in modern CT scan machine
 Reduction dose 40 – 50% dose
 The penetration adjust according to patient
specific attenuation an all three planes
 Technique; depend on patient size,
attenuation profile and scanning parameter

57. Figure. Graph of tube current (mA) superimposed on a CT projection radiograph shows the
variation in tube current as a function of time (and, hence, table position along the z-axis) at
spiral CT in a 6-year-old child. An adult scanning protocol and an AEC system.

58. Is an AEC feature that allows for the tube
current to be decreased or turned off over
radiosensitive organs on the patient
periphery, such as the breasts or eye
lenses
To maintain image quality, tube current
may need to be increased at other view
angles
The use of Organ-Based Tube Current
Modulation may reduce the absorbed dose
to organs at the surface of the body but
may increase the absorbed dose to other
organs
Dose Modulation and

59. Gantry Gantry
Conventional Organ-Based Modulation
Dose Modulation and

60. Is an AEC feature that selects the tube
potential according to the diagnostic task
and patient size in order to achieve the
desired image quality at a lower CTDIvol
The use of Automatic Tube Potential
Selection is intended to decrease CTDIvol
while achieving the image quality required
for a specific diagnostic task and patient
attenuation
Dose Modulation and

61.  Tube Potential is not modulated in the same
fashion as Tube Current
 It does not change with different tube
positions (view angles) around the patient
 The Tube Potential for a specific patient,
anatomic region and diagnostic tasks is
selected and held constant for that
acquisition, though it may be changed to a
different tube potential for a different
diagnostic task
Dose Modulation and

62. 62
Reconstruct an image from tomographic scan data
Schematic of Projection and Reconstruction

63. 63
 How are we going to solve this problem ?
 Filtered Back Projection

64. 64
 How does BP work ?

65. 65
 Another
diagram
that
illustrates
this
concept

66. 66
 Without filter:
• Point object
becomes
widely spread
out

67. 67
 With Filter: • Cancellations in
the vicinity of
the object
• The result is a
sharper
reconstructed
image

68. 68
 Process is linear
- if it works for a point object (at any location),
then it will work for any image

69. Is a feature that uses the information
acquired during the scan and repeated
reconstruction steps to produce an image
with less “noise” or better image quality
(e.g., higher spatial resolution or
decreased artifacts) than is achievable
using standard reconstruction techniques
The use of Iterative Reconstruction by itself
may not decrease CTDIvol; with use of Iterative
Reconstruction, image quality will change and
this may allow a reduction in the CTDIvol by
adjusting the acquisition parameters used for
the exam
Dose Modulation and

70.  Iterative Reconstruction may be completed using
data in Image Space, Sinogram Space or a Model
Based Approach
 Changing/Turning On the %/Level of the iterative
reconstruction used may or may not affect the
CTDIvol of the scan and will affect the image
quality of the final set of images
 In consultation, the Radiologists and Medical
Physicists at an institution may adjust the
acquisition parameters for studies reconstructed
using iterative reconstruction based on the
imaging task, the patient population, the desired
image quality, dose concerns and the needs of
the interpreting Radiologist
Dose Modulation and

71.  According to the Dictionary.com the iteration is define as
problem solving or computational method in which a the
most close to the approximation
 First an initial estimate of the x-ray photon from the tube
is made. Second an estimate is made of the x-ray detector
counts that would be acquired in each projection with the
use of forward projection (Nelson et al., 2011). Then the
estimated forward projection before will be compared
with the actual measured projection acquired by the CT
system detector array (Nelson et al., 2011). The
comparison between the estimate and the actual
measured will be use to update the original estimate
(Nelson et al., 2011). Then the whole process will be
continuously repeated that would result from the revised
x-ray photon distribution. The process done will results in
the estimation of x-ray photon closer to the actual photon
distribution

72.  Siemens
 IRIS: Iterative Reconstruction in Image Space
 SAFIRE: Sinogram-Affirmed Iterative
Reconstruction
 Thosiba
 AIDR 3D: Adaptive Iterative Dose Reduction 3D
 GE
 ASIR: Adaptive Statistical Iterative
Reconstruction
 MBIR: Model-Based Iterative Reconstruction
 Philips
 iDose4

74. (A) FBP image of the right shoulder in the coronal plane reconstructed with a soft tissue
algorithm. Note the horizontal streak artifacts because of aliasing from low x-ray photon
counts through the bones. (B) FBP image of the right shoulder in the coronal plane
reconstructed with a bone algorithm. Note the horizontal streak artifacts because of aliasing
are even more noticeable with this reconstruction. (C) MBIR image of the right shoulder in
the coronal plane depicted with a soft tissue window and level. Note excellent depiction of
the soft tissue about the shoulder and significantly fewer streak artifacts. (D) MBIR image of
the right shoulder in the coronal plane depicted with a bone window and level. Note
excellent depiction of the comminuted fracture of the coronoid process (arrow)

75. Filtered back projection (FBP) (a) and SAFIRE (b) reconstructions at
the same level of the ascending aorta. Image noise expressed as the
standard deviation of the attenuation (HU) in the region of interest
was significantly lower in images reconstructed using iterative
reconstruction (circle in B) compared with those reconstructed using
FBP (circle in A).

76.  Information about the CTDIvol planned for each
scan is typically displayed before the exam on
the user console
 Information about the CTDIvol delivered by
each scan is typically reported in a data page
or DICOM structured dose report
 Dose information provided after the exam
typically also includes the DLP and the CTDI
phantom size. These may also be included in
information displayed before the scan.

77.  CTDIvol is displayed before a study is
performed based on the selected technique
parameters
 It is important to check CTDIvol before a
study is performed to ensure that the output
of the scanner is appropriate for the specific
patient and diagnostic task
CTDIvol is displayed for each planned
acquisition
Dose Display

78.  Development of technology in CT scan make
essential for radiographer cope with new
skills and techniques.
 Training should be provided
 Increase knowledge
 Know your THINGS. Avoid becoming PUSH
BUTTON RADIOGRAPHER..!!

79.  Radiographers are integral part of a
multidisciplinary team whose responsibility is to
treat and manage patient came to the imaging
department.
 The role of the radiographer is to take care of
the patients’ imaging before proceed to other
treatment or investigation.
 Current advances in technology have made it
essential for nowadays radiographers to
constantly learn and cope with new skills.
 The increasing usage of CT scan facilities
increases radiation doses to the staff and the
population and this create a full awareness for
continuous efforts in reducing the dose level.

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83. Mohd Aiman bin Azmardi
Peg. Pengimejan
Hospital Kulim