Download methods for dose reduction in 128 slice multidetector ct

J. Calatayud Moscoso del Prado,
D. Castellón Plaza,
G. Tardáguila de la Fuente,
E. Santos Armentia
POVISA Hospital, Vigo (SPAIN)
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In recent years, the media has focused on the potential
danger of radiation exposure from CT, even though the
potential benefit of a medically indicated CT far
outweighs the potential risks.
This attention has reminded the radiology community
that doses must be as low as reasonably achievable
(ALARA) while maintaining diagnostic image quality.
The idea of an advanced 3rd generation system
concept with two set of tube detectors pairs has
already been proposed only a few years after CT
became clinical practice.
 To
describe system concept and design of a CT
scanner with two X-ray tubes operated at
different voltages and two detectors
 To
analyzed different methods for
reduction in 128 Dual Source MDCT
dose
 Radiation
exposure of patient during computed
tomography and the resulting radiation hazard
have recently gained increasing attention.
 One
of the most advantages of the new
generation in MDCT, besides the higher
temporal resolution, is the decreased radiation
dose en CT exams.

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Dual
Source
Computed
Tomography
(DSCT)
is
equipped with two X-Ray
tubes and two corresponding
detectors.
The two acquisition system
are mounted on the rotating
gantry with an angular offset
of 90º
One detector (detector A)
covers
the entire scan
field of view (about 50 cm
in diameter) while the
other is restricted to a
smaller, central field of
view (26 cm) in order to
maintain a compact system
geometry with a short
distance between
the
focal
spot
and
the
detectors
90º
Detector B

26cm
Detector A
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The main characteristic feature of DSCT is the possibility it offers
with respect to modes of operation and the possibility to combine
the resulting acquisition data. After detector B data with the
smaller scan field are extrapolated to a full-size detector using
detector A data at the same projection angle, dual source
acquisition data can be used in a multitude of ways .
Thus, DSCT scanners show promising potential for general
radiology applications, such as the use of dose accumulation to
examine obese patients or the use of dual energy acquisitions
(including
tissue characterization,
local blood volume
quantification in contrast enhanced scans and iodine / calcium
separation enabling)
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The main benefit of dual source for cardiac scanning is the improved
temporal resolution.
Temporal resolution of a multi-slice CT scanner with a single source is
equivalent to half the gantry rotation time using single segment half-scan
image reconstruction. A Dual-Source CT provides temporal resolution of
approximately a quarter of the gantry rotation time, indepent of the patient
heart rate and without the need for multi-segment reconstruction
techniques.
SINGLE SOURCE CT
DUALSOURCE CT
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In DSCT scanners, a complete data set of 180° of parallel-beam
projections can be generated from two acquisition system in the
same relative phase of the patient’s cardiac cycle and at the same
anatomical level due to the 90º angle between both detectors.
With this approach, a constant temporal resolution equivalent to
one quarter of the gantry rotation time Trot / 4 is achieved in a
centered region of the scan field of view. For Trot= 0.33 s, the
temporal resolution is T rot / 4 = 83 ms, independent of the
patient’s heart rate.

The
second-generation
Dual Source scanner, is
equipped
with
two
detectors and two X-ray
sources set at an angle of
approximately 94 degree
to one another.
With to gantry rotation
swindles of 0.28 s, the
scanner
boasts
to
temporary resolution of
just 75 ms.
94º
Detector B

33cm
Detector A
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Moreover, an innovation introduced with these equipments
eliminates the need for the patient table to slowly inch forward
during dates back acquisition. Instead, in low-dose Flash Spiral
mode, the scanner achieves gapless z-sampling even with the
wide-open spiral created by to pitch of 3.2 and to table speed of
more than 40 cm/s.
This is because the two detectors create two complementary data
spirals that together include all the information that would be
found in to single spiral acquired at to much slower table speed
but
without overlapping data and unnecessary
radiation
exposure.
 New
methods for dose reduction have been
developed:
1. HIGH PITCH - FAST SPIRAL SCANNING
2. SELECTIVE PHOTON SHIELD
3. ADAPTATIVE DOSE SHIELD
4. CARE DOSE 4D
5. ORGAN SENSITIVE DOSE PROTECTION
6. SINGLE DOSE DUAL ENERGY
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The high pitch spiral acquisition mode provides high
temporal resolution and fast image acquisition both at a very
low level of radiation dose.
For helical CT scanners, pitch is defined as the ratio of table
feed per gantry rotation to the nominal width of the x-ray
beam. An increase in the pitch decreases the duration of
radiation exposure to the anatomic part being scanned due
to a shorter exposure time.
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Although scanning at higher pitch is generally more dose
efficient, also tends to cause helical artifacts, degradation of
the section-sensitivity profile and decrease in spatial
resolution.
It is well known that gapless z-sampling with a single source
MDCT is limited to a maximum spiral pitch value of 1.5. With
a second detector (B), DSCT provides volume coverage
without gaps at much higher pitch values, depending on the
desired scan field of view.

The two detectors create two complementary data spirals
that, when put together, include all of the information that
would be found in a single spiral acquire at much lower
table speed.

At low heart rates, the entire heart can be covered in the diastolic phase of the
cardiac cycle in one single heart beat. The only additional prerrequisite is
synchronization of the start of the scan with patient´s ECG, so it predict next 2
RR-cycles based on the ECG and then, plan the scan.
estimation
Table need time
to accelerate to
the final speed of
460mm/s for
pitch of 3.4
phase
60 61
60
60 70
tolerance band
Scan
acceleration time
table speed
time
trigger of table
acceleration by CPI
R-peak in tolerance
executed scan was
actually too late
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A selective photon shield pre-filters high kilovoltage Xray, removing low-energy photons that never reach the
detector and thus does not contribute to the image.
This has two beneficial effects.
1.Improves energy separation, therefore material
differentiation by 80%.
2.It markedly reduces unnecessary radiation dose
use in everyday routine.
This Selective Photon Shield makes Dual Energy as
dose-efficient as any single 120 kV scan.

If a filter is applied in front of the tube operating at 140 Kv, it
should have property to eliminate those low energy parts from the
X-ray spectrum that mainly absorbed by iodine.
This would lead to a
maximum suppression of
iodine enhancement in
140 Kv image.
80Kv
140Kv
Hence, the filter should
consist of iodine itself or
of metals like indium, tin,
antimony and tellurium
with similar absorption
characteristics
80 kV
140 kV
overlap
80 kV
140 kV with SPS
overlap
 Significant spectral overlap
Overlap in DS Multidetector-CT
(1st generation)
 Minimized spectral overlap
Overlap in 128 DS Multidetector-CT
(2nd generation)
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Conventional CT applies a certain amount of radiation in spiral
scans, that cannot be used for data reconstruction. This is due to
the fact, that at the beginning and end of every spiral scan a
reconstruction is no longer possible, when less than 180° of data
are available.
This pre- and post-spiral overradiation depends on the scan range
and detector width, with higher overradiation when using wider
detectors or scanning shorter scan ranges.
necessary radiation
unnecessary radiation

Adaptative dose shield consist in dynamic collimators that
blocks unnecessary patient dose. These blinds dynamically
opening quickly at the beginning and closing at the end of a spiral.

Therefore the amount of unnecessary patient radiation before and
after the scan range is eliminated.

Care dose 4D automatically adapts radiation dose to the size
and shape of the patient, achieving optimal tube current
modulation in two ways. First, tube current is varied on the basis of
a topogram, by comparing the actual patient to a “standard-size”
Reduced dose level
patient. As might to be expected,
tube current is increased for
based on topogram
larger patient and reduced for smaller patients. Differences in
attenuation in distinct body regions are taken in account.
X-ray dose

In addition, real-time
angular
dose
modulation measures
the actual attenuation in
the patient during the
scan and adjust tube
current
accordingly
different body regions
and also for different
angles during rotation.
1600 mAs
20 mAs
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Another technical development for keeping the patient´s radiation
exposure as low as possible is the X-care application.
This application selectively reduce the radiation exposure of dose
sensitive anatomical regions, such as the female breast, thiroyd
gland or eyes
This is done by switching the X-ray tube assemblies off during the
rotation phase in wich the anatomical region concerned are most
directly exposed to radiation.
In this way it is possible to reduce the radiation exposure of
individual anatomical regions by up to 40%
 X-Care application allows to
X-ray off
radiologist to turn off the x-ray
tube during the portion of the
gantry rotation that would
directly expose radiation
sensitive organs.
 Latest research demonstrates
that scans, not irradiating the
breasts directly, reliably reduced
dose by up to 40 % while
distribution of noise is
homogeneous and image quality
uniform.
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The most common CT scan require initial unenhancent
scanning. Dual Energy allows to obtain a virtual noncontrast scan, eliminating extra work-flow steps and
dose of additional unenhanced.
Therefore, Single Dose Dual Energy remove the need
for an initial non contrast scan owing to allows the
possibility of eliminate iodine content retrospectively.
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Dual Energy is based on the energy dependence of two
predominant absorption mechanisms at clinically relevant X-ray
wavelenghts, compton scattering an photo-absorption.
In particular, the energy-dependent ratio of the absorption crosssections of the two processes is different for each chemical
element. Hence, Dual energy is sensitive to the object´s chemical
composition.
If dual energy CT data with high quality regarding noise, spatial
and temporal resolution and without registration problems are
provide, this brings about material analysis capabilities with
several clinical applications, such as bone removal, plaque display,
virtual non contrast imaging, myocardium or pulmonary perfusion
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80Kv
140Kv
128 Dual Source Multidetector
CT configuration allows fast
simultaneous scanning with
different X-ray spectra and two
multi-row detector acquisition
system.
The
standard
voltage
combination is 140 kV on tube
A and 80 kV on tube B.
The data delivered by both
detectors
is reconstructed
separately.
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Ultimately, Dual Energy analysis based on a so- called “three
material decomposition” of the resulting images. To each voxel
inside the common scan field of the A and B detectors, two
different CT numbers can be assigned. Each pair of CT numbers
is represented by a point in a coordinate system which is defined
by the CT numbers generated at each of the particular tube
voltages.
Based on a material descomposition and a subsequent
segmentation it is possible to obtain different post-procesated
images with several clinical applications.
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There are a high additional dose-saving potential in a
consequent application of dose reduction techniques.
Therefore, it`s of utmost importance to individually
adapted scan protocols to obtain CT studies with
optimal diagnostic image quality and lowest possible
radiation dose.
Based on the understanding of the advantages of 128
MDCT, it is useful to know the physical bases of the
different techniques for dose reduction because of the
fact that its requires careful settings to optimize image
quality