Dual energy subtraction angiography: A simulation study
using the three material approach
Poster No.:
C-783
Congress:
ECR 2009
Type:
Scientific Exhibit
Topic:
Physics in Radiology
Authors:
A. Toutountzis , G. Fountos , A. Samartzis , C. Michail , I.
1
2
2
1 1
2
1
2
Kandarakis , G. Nikiforidis ; Patras/GR, Athens/GR
Keywords:
angiography, dual energy, selective tissue imaging
DOI:
10.1594/ecr2009/C-783
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Page 1 of 12
Purpose
The purpose of the present study was to perform a simple simulation of X ray
angiographic imaging in order to use the resulting images for dual energy imaging.
Two dual energy imaging algoithms basedon the weighted logarithmic subtraction were
developed and evaluated with respecto to the visualization ability of the Iodine contrast
agent, expressed in terms of Signal to Noise Ratio. The firstalgorithm, considering
that two materials are present has been widely used for chest imaging
[1]
and also for
[2,3]
angiographic applications , but suffers from the anatomicalnoise due to the presence
of the bones in the angiographic image. The second algorithm was developed taking
into account that three materials are present in the area under examination: Soft
tissues,bones and Iodine. This prototype algorithm has not been evaluated before for
angiographic imaging and a comparison with the two materials algorithm is invesigated
for angiographic applications.
Methods and Materials
A theoretical phantom was considered consisting of spatially invariant 20 cm PMMA
[2,3]
thickness
in which both Iodine and bone structures are embedded. The Iodine
structures are three slabs of 0.01, 0.02 and 0.03 cm thickness (or 49.3, 98.6 and 147.9
2
[2]
mg/cm coating thickness ) placed horizontally. Four slabs consisting of Hydroxyapatite
(HAp) are also embedded inside the PMMA block. The thickness of the Hydroxyapatite
structures are 0.5, 1, 1.5 and 2 cm and are placed vertically with respect to the Iodine
structures.
In order to selectively enchance Iodine structures, the Beer-Lambert formalization
was considered and the mathematical investigation of attenuation across the phantom
materials led to two dual energy algorithms; one for two materials phantom and one
for three materials phantom. The two material algorithm is the mathematical solution of
a linear system formed by the Beer-Lambert defined absorbance of X rays across the
phantom for low and high energy spectrum. The linear system reslts to the thickness
[4]
of each material respectively . For the three material algorithm, a modified form of
absorbance was used, normalized on the corresponding attenuation through a region
of the phantom containing only PMMA. Iodine and bone structures are considered
embedded in the PMMA mass. The sollution of the linear system may result to the
thickness of the Iodine structures, thus suppressing the PMMA and bone visualization
in the final image.
The phantom was virtually irradiated using the exponential law of atenuation with
[5]
computer generated tungsten X ray spectra
data of linear attenuation coefficients
[6]
varying from 55 to 125 kVp, and tabulated
corresponding to software phantom materials.
Page 2 of 12
The low energy images were obtained for 55, 60, 65, 70 and 75 kvp with filtration of 4
mm Al and 0.1 mm Cu. For the formation of high energy images 115, 120 and 125 kVp
X ray spectra with 4 mm Al and 0.9 mm Cu filtration, normalized at 27.7uGy were used
[7]
in order to immitate conditions found in a commercial angiographic unit . The images
were formed considering an ideal X ray detector, so the pixel values corresponded to
2
the attenuated X ray spectrum integration, in units of photons/mm . Poisson distributed
noise was added in order to simulate the statistical nature of attenuation and detection
of X rays.
On the images produced by the irradiation process described above, either with the two,
or with the three materials algorithm was applied considering the proper combinations
based on the X ray spectrum used for the irradiation. For the evaluation of the dual
[7]
energy algorithms, both the Iodine SNR and the bone SNR on the final image were
measured[ducote]. The Iodine SNR was the image enchancement factor, and the bone
SNR served as a bone signal suppression factor.
Results
Page 3 of 12
Fig.: Single Energy image acquired at 115 kVp, with 4 mm Al and 0.9 mm Cu filtration
used as a high energy image.
Page 4 of 12
Fig.: Single Energy image acquired at 60 kVp, with 4 mm Al and 0.1 mm Cu filtration
used as a low energy image.
As it can be seen, in the single energy images, both the horizontal Iodine structures and
the vertical Hydroxyapatite structures are visible, and in the areas that the structures are
overlapped, are darker. In more complex images like medical images consisting of soft
tissues, Iodinated vessels and bone tissues, it is difficult to discriminate the the shape
and the type of the examined area, thus making difficult the diagnosis.
Page 5 of 12
Fig.: Dual Energy Image. 60 kVp and 115 kVp were used as low and high energy
respectively.
The image above shows a dual energy image using the three materials algorithm. The
energies used were 60kVp and 115kVp for low and high energy respectively. The bone
signal seems to be suppressed, but it is not completely eliminated from the image.
Page 6 of 12
Fig.: Dual Energy Image. 75 kVp and 115 kVp were used as low and high energy
respectively.
The last image shows the three material algorithm dual energy image, obtained with
75kVp and 115kVp for low and high energy respectively. As it can be seen, the bone
structures, have been almost completely eliminated. In comparison the previous image,
the Iodine signal has been significantly enchanced.
The two following figures represent the SNR for Iodine and bone for the embedded
structures of the software phantom used in this study.
Page 7 of 12
Fig.: Iodine Signal to Noise Ratio versus Low energy for Iodine thicknesses 0.01, 0.02
and 0.03 cm. The High energy was set to 115 kVp.
Page 8 of 12
Fig.: Bone Signal to Noise Ratio versus Low energy for Iodine thicknesses 0.01, 0.02
and 0.03 cm. The High energy was set to 115 kVp.
For comparison purposes, dual energy reconstruction was performed not only applying
the three materials algorithm, but also the two materials algorithm, which was solved for
PMMA elimination, and for Hydroxyapatite (bone) elimination.
Page 9 of 12
Fig.: The figure shows the variation of Iodine SNR with the Iodine thickness for the
three material algorithm, and for the two material algorithm, set for PMMA elimination
and Hydroxyapatite elimination
The figure above represents the SNR of the Iodinated structures for the three algorithms
used. The highest Iodine SNR values were oobtained with the PMMA elimination-two
materials algorithm. The HAp elimination dual energy algorithm led to relatively low Iodine
SNR values.
Page 10 of 12
Fig.: The figure shows the variation of Iodine SNR with the Iodine thickness for the
three material algorithm, and for the two material algorithm, set for PMMA elimination
and Hydroxyapatite elimination
In the last figure, it is obvious that the applied two materials algorithms failed to eliminate
the bone signal from the final image. On the other hand, the three materials algorithm
seems to completely eliminate the bone signal, maintaining near-zero values.
Conclusion
The results showed that the three materials algorithm not only led to high Iodine Signal to
Noise Ratio values, thus enchansing the visualization ability of the Iodined structures, but
also led to relatively low Hydroxyapatite Signal to Noise Ratio Values, thus eliminating
almost completely the bone structures. The optimal x ray energy spectra combination
was found at 75 kVp for low energy and 115 kVp for high energy, where both the highest
Iodine SNR and the lowest bone SNR values were obtained. At the optimal spectral
pair, the Iodine SNR was found 0.876, 1.7148 and 2.5938 for 0.01, 0.02 and 0.03 cm
Iodine thickness respectively, whereas the bone SNR maintained values lower than 0.06
expressing mainly the fluctuations of the statistical noise. These results are of interest
[7]
for medical imaging .
Page 11 of 12
The dissociation between three materials and the enchancement of each one separately
cannot be performed with two materials algorithms as results showed. This can be
explained by the fact that the two materials algorithm, is the solution of a linear system
of two equations, thus only two unknown parameters can be defined. On the other hand
the three materials algorithm is in fact the solution of a linear system consisting of three
equations with three unknown variables.
The simultaneous Iodine enchancement and bone elimination from the final image,
obtained by the prototye dual energy three materials algorithm is a desired characteristic
in dual energy angiography, solving the problem of the anatomical noise created by the
[8]
overlying bone structures .
Page 12 of 12