A new method for experimental characterisation of scattered radiation in 64-slice CT scanner
A Akbarzadeh1,2, MSc,
MR Ay*,1,2,3, PhD,
H Ghadiri2,4, MSc,
S Sarkar1,2, PhD,
H Zaidi5,6, PhD
1 Department of Medical Physics and Biomedical
Engineering, University of Medical Sciences, Tehran, Iran
2 Research Center for Science and Technology in
Medicine, Tehran University of Medical Sciences, Tehran, Iran
3 Research Institute for Nuclear Medicine, Tehran University of Medical Sciences, Tehran, Iran
4 Department of Medical Physics, Iran University of Medical Sciences, Tehran, Iran
5 Division of Nuclear Medicine, Geneva University Hospital, CH-1211 Geneva 4, Switzerland
6 Geneva Neuroscience Center, Geneva University, CH-1205 Geneva, Switzerland
Abstract
Purpose: The consummate 64-slice CT scanner that
spawns a new generation of non-invasive diagnostic tool, however revolutionary,
brings with it the incidental by-product that is scattered radiation. The
extended detector aperture capability in the 64-slcie CT scanner allows the
effects of scattered radiation to be more pronounced and therefore demands that
the magnitude and spatial distribution of scatter component be addressed during
the imaging process. To this end, corrective algorithms need to be formulated
on a basis of a precise understanding of scatter distribution. Relative to a
64-slice CT scanner, here now a unique solution is based upon dedicated
blockers operative within various detector rows, calculating scatter profiles
and scatter to primary ratios (SPR).
Materials and methods: A single dimension blocker
array was installed beneath the collimator, and the extrapolated shadow area on
the detectors revealed the scatter radiation after exposure. The experiment was
conducted using a 64-slice CT scanner manufactured by GE Healthcare
Technologies.
Results: Variables such as tube voltage, phantom
size and phantom-off centring on the scatter profile and the SPR was measured
using the dedicated blocker method introduced above. When tube voltage is
increased from 80kVp to 140kVp in a 21.5 cm water phantom, the SPR is found to
reduce from 219.9 to 39.9 respectively.
Conclusion: The method developed within this study
is applicable to any measurement and is direct with minimal complexity. � 2010
Biomedical Imaging and Intervention Journal. All rights reserved.
Keywords: Scatter, SPR, CT, tube voltage
Introduction
X-ray computed tomography (CT) scans are inherently
associated with unwanted by-products of error that have a deteriorating impact
on image quality. If the impairment of the image quality is substantial, a
corrective algorithm aimed at providing some redress may be called for or
alternatively, modification of the geometric schematics of the scanner may be
in order. One such side effect that is inherent to CT imaging is the noise
brought about by scatter radiation. Resulting artefacts may be disposed of
successfully with careful consideration of the scanner design, or it may be
corrected for during the imaging procedure.
As a result of scatter radiation in CT imaging, data is
corrupted and cupping errors thereby affect the reconstructed images. For state
of the art CT scanners incorporating cone-beam configurations with extended
detector aperture, the result is exaggerated. These multi-detector scanners are
far more prone to these artefacts than their cousins, the fan-beam CT scanners.
While scanner design remains at the heart of any available
solution, in order to resolve these anomalies, the magnitude and spatial
distribution of scattered radiation collected by a CT scanner needs to be
quantified accurately. From this premise scanner design can be modified to
reflect the optimal geometric configuration demanded by the task, and also
extensive corrective techniques can be applied to reduce remnant scatter
radiation.
The incidence of scatter radiation has attracted the
remedy of mathematical modelling, specifically Monte Carlo simulations for
fan-beam and cone-beam configurations. Indeed, the research to date on scatter
radiation distribution in fan-beam configurations uses single blockers [3,4,5]
or Monte Carlo simulations [6,7,8] to deduce a conclusion.
Here is put forward, a revolutionary method of
incorporating dedicated blockers to measure the scatter profile and the scatter-to-primary
ratio (SPR) with precision. This is applicable to all detector rows within a
64-slice CT scanner.
As this method measures scatter radiation in multiple
detector rows, it improves on the limitations of the single measurement points
in previous methods.
Materials and Methods
Acquisition system
A 64-slice Light Speed VCT scanner, manufactured by GE
Healthcare Technologies (Waukesha, WI) equipped with Highlight (Y2Gd2O3:Eu)
ceramic scintillators was the CT scanner of choice for this study. The
source to isocentre distance on this CT scanner is 540mm, while the source to
detector distance is 950mm. Among 64 rows each with 888 active patient elements
and 24 reference elements, 58,368 individual elements of 0.625mm thickness at
isocentre are distributed. Complementing the scanner is a Performix Pro Anode
Grounded Metal-Ceramic Tube Unit using a 56� fan angle, a 7� target
angle, and a minimum inherent filtration of 3.25mm Al and 0.1mm Cu at 140kVp.
Phantoms
In order to measure the scattered radiation profile and
the SPR, a water-filled cylindrical phantom with 215mm internal diameter and a
6 mm thick Plexiglas wall was constructed, together with a uniform
polypropylene cylindrical phantom with a 300mm diameter. The purpose of the
polypropylene phantom is to recreate clinical conditions present when the
subject is obese.
Lead blocker array
While it has its limitations, the usual method of calculating
SPR is by placing a small lead blocker on top of the phantom. Measurement of
scattered radiation is possible by monitoring the x-ray intensity from behind
the phantom to the underside of the blocker, but this is only possible at one
point.
However, this study suggests placing a lead blocker array
after the collimator (Figure 1a). This array is constituted of 20 lead bars of
3mm thickness. The attenuation coefficient for lead is 3.32cm2/gr
and with its density being 11.3 gr/cm3, the linear attenuation
coefficient for lead is 37.52 cm-1. The transmission capability of
3mm of lead is approximately 1.24394x10-5.
When the array is placed under the second collimator it is
160mm under the source. With the economy of only one irradiation, not only are
the scattered and primary radiation levels in numerous detector channels able
to be measured, but also the SPR at various locations.
Methods
The following measurements were collected without the
usual bow-tie filter that normally resides within the collimator box.
The lead blocker is placed between the collimator and the
phantom during tube exposure with different voltage (kVp) and current (mA)
settings.
In order to eliminate the scatter produced by the lead
blocker itself, irradiation is performed twice; once to obtain the precise
measurement of the scatter that occurs without the phantom in position, and
once with the phantom in place. The scatter from the lead blocker array is then
compensated for.
The raw data from the CT scanner is transferred from the
scanner�s database to a PC for remote processing. The CT scanner has 64
detector rows and 912 detector channels in each row. The target file contains
the entire 912 x 64 detector readings.
Due to the fact that scanner software invariably applies
certain calibration factors, these factors must be identified and applied
inversely in order to obtain the untainted data. The scanner software used in
this study is the Light Speed VCT Scanner�s Data Acquisition System, which
reduces high value readings from the detectors to within a 16 digit range to
successfully compress data into a manageable file size.
To implement this processing of the data that is extracted
from the CT scanner, a program was written in Matlab 7.4, to read the binary
data file, apply the inverse of the calibration factors, and obtain the
untainted data output in a 2-dimensional matrix (912 x 64).
This matrix attributes values to reflect the amount of
radiation on each detector, and is divided into two sections; a shadowed area
reflecting the particular detector channels that were within the coverage of
the lead blocker array (invariably recording scatter radiation if any), and
also an unshadowed area that represented the detector channels that remained
exposed (recording total radiation both primary and scatter radiation).
The algorithm governing the conversion of these data first
interpolated the total radiation profiles and also the scatter radiation
profiles for all detector channels, and then found primary radiation by
deduction of the latter from the former. SPR ratios were then easily calculated
for each detector channel.
Results
Scatter profiles
Figure 2(a) shows the scatter profile for detector row 32,
which is the central row in the CT scanner, after the cylindrical water phantom
was irradiated for 3 seconds. Various x-ray tube voltages were applied, with a
constant tube current of 100mA.
Figure 2(b) shows the scatter profile, also for detector
row 32, after the cylindrical Polypropylene phantom with 300mm diameter was
irradiated for 3seconds. Various x-ray tube voltages were again applied, also
with a constant tube current of 100mA.
The scanner software has normalised these measurements.
Primary radiation
Figure 3(a) shows the primary radiation recorded on row
32, after irradiation of the cylindrical water phantom (diameter 215mm), again
at varying x-ray tube voltages and a constant tube current of 100mA.
Figure 3(b) shows the primary radiation recorded on row
32, after irradiation of the cylindrical Polypropylene phantom (diameter
300mm), under the same conditions.
These values have had the calibration coefficients
inversely applied and as such produce the raw detector readings without
normalisation.
Scatter to primary ratio
This ratio is a useful qualitative measurement to
investigate the extent that scatter photons pollute projection data. The SPR is
a function of phantom size, phantom material, tube voltage and phantom off-centring.
Figure 4 shows the SPR for the cylindrical water phantom
at various tube voltages and a tube current of 100mA.
Figure 5 shows the SPR for the Polypropylene phantom, also
at various tube voltages and yet a 200 mA tube current. Again, this phantom is
used to recreate clinical conditions present when the subject is obese.
Integrated SPR
The integrated SPR is the sum of the SPR with respect to
all the detectors in a particular row.
Figure 6 shows the integrated SPR for the 64 detector rows
of the scanner when the cylindrical water phantom is irradiated at varying tube
voltages.
The central row (no. 32) has integrated SPR values of
219.5, 92.5, 55.2 and 39.9 for tube voltages of 80, 100, 120 and 140 kVp
respectively.
Figure 7 shows the resultant SPR with respect to a change
in distance between the phantom and the particular detector (here the 32nd
detector row). The positive and negative values indicate the phantom being
moved towards, and away from the detector respectively.
It is conclusive that the greater the distance between the
phantom and the detector, the lower the SPR. Both the tube voltage and the tube
current were constant at 120 kVp and 100mA respectively.
Discussion
The results of this experiment using a lead blocker array
are congruent with current research on the subject, and as such the method
introduced is able to be applied to measure scatter radiation distribution and
SPR�s.
The two peaks displayed in Figure 2(a) and 2(b) are more
likely a result of the Compton effect (that occurs when photons interact with
matter), and to a less extent due to the larger attenuation length of photons
scattered from the phantom. Since the phantom covers this area, the lower amount
of scatter radiation in the centre of the row is due to photons either being
absorbed before the Compton Effect or due to attenuation after the Compton
Effect.
Figures 4 and 5 show the highest SPR for the lowest tube
voltage applied which is 80kVp.
While the chance of a Compton effect is increased with an
increase in tube voltage (as Figures 2 and 3 indicate), primary radiation i.e.
primary photons increase at a greater rate than the number of scattered
photons, when tube voltage is increased. For this reason, the SPR decreases
when tube voltage is increased.
Arguably, SPR decreases proportionally to an increase in
the distance between the phantom and the detector [1,5,6,8]. Known as the
air-gap effect, this is obviously attributable to the reduction in the number
of scattered photons that the detector records.
Conclusion
A lead block array was used to measure scatter radiation
distribution and SPR in a 64-slice CT scanner, but this revolutionary method is
able to be adapted to other multi-slice CT scanners.
The method introduced is practical and able to be applied
in the acquisition of any related measurement, and can be used to accurately
extrapolate data to produce a scatter profile.
Designers of CT scanners are able to use the accurate
deduction of magnitude and spatial distribution of scattered radiation that
this method provides, and in doing so improve the geometry of scanners to
achieve the optimal corrective algorithms to reduce scatter radiation.
Considering the commercial introduction of 256-slice [9]
and 320-slice CT scanners, advances in this respect are timely will be highly
appreciated.
Acknowledgment
This work was supported by Tehran University of Medical
Sciences, Tehran, Iran under grant No. 6743-30-02-86 and the Swiss National
Foundation under grant No. 3152A0-102143.
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Received 1 May 2009; received in revised form 13 October
2009, accepted 15 October 2009
Correspondence: Department of Medical Physics and Biomedical Engineering, Tehran University of Medical Sciences, Tehran, Iran. E-mail: mohammadreza_ay@tums.ac.ir (Mohammad Reza Ay).
Please cite as: Akbarzadeh A, Ay MR, Ghadiri H, Sarkar S, Zaidi H,
A new method for experimental characterisation of scattered radiation in 64-slice CT scanner, Biomed Imaging Interv J 2010; 6(1):e3
<URL: http://www.biij.org/2010/1/e3/>
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