1. Dual Energy Dinosaurs
Nikki Newman
Queensland XRay
Greenslopes Private Hospital
From the above applications we have seen the importance of
Dual Energy CT in medicine, however Dual Energy CT has also
been found to be useful in non-medical applications.
Palaeontology is the scientific study of pre-historic life. It includes
the study of fossils to determine an organisms evolution and it’s
interaction with it’s environment [6]. Recently, at GPH, fossils
found in central western Queensland were scanned for
Palaeontologists (lead by Senior Curator of Geosciences, Scott
Hocknull) from the Queensland Museum. Many of these fossils
were embedded in iron ore sediment. On conventional single
source, single energy CT scanners, we would expect the image
quality of these fossils to be severely degraded, rendering the
images useless. However, the Dual Energy capabilities of the
Somatom Definition Flash allowed us to see fossils embedded
within the rock sediment, free from artefact. This was made
possible due to the metal artefact reduction application used with
DECT. Fossils scanned for the Queensland Museum included
dinosaur skulls, Horsetail plants, dinosaur track ways and
footprints.
What is Dual Energy CT?
Metal streaking artifacts result in a significant reduction in CT
image quality . There are two main causes for these artifacts:
1. Photon starvation due to full absorption of the x-ray quanta.
2. Beam hardening due to absorption of low energy photons [1].
Dual Energy CT (DECT) was first proposed by Sir Godfrey
Hounsfield in 1973 [2], but due to limitations in technology at the
time, further investigations were put on hold. It wasn’t until the
release of Dual Source CT (DSCT) in 2006, that Dual Energy
could be used in clinical applications. DECT uses two x-ray
tubes at different kV levels and mAs settings to acquire two data
sets simultaneously. With an extra x-ray source, we have the
advantage of exposing the patient to two different x-ray energies
without scanning the patient twice. Since the attenuation profiles
of different tissues are energy dependent, it makes it possible to
differentiate between fat, bone, soft tissue and iodinated contrast
material because we know how these materials behave at
particular energies [3,4]:
Figure 1: The x-ray tube’s kilovoltage (kV) determines the
average energy of the photons in the x-ray beam. Changing the
tube potential alters the photon energy, resulting in a difference in
the attenuation of the x-ray beam between the scanned
materials. Therefore, scanning an object at 80kV results in a
different attenuation than with 140kV and thus produces a
difference in Hounsfield Units. Image courtesy of Siemens AG,
Germany [3,4,5].
Figure 2: Bone and Iodine scanned at different energies. Notice
the difference in material specific attenuation (Hounsfield Units).
Image courtesy of Siemens AG, Germany [3,4].
Dual Energy Applications
Several post-processing applications for DECT are available [5].
These include:
1. Renal Calculi Characterisation
2. Gout Analysis
3. Myocardial Perfusion
4. Virtual Non Contrast
5. Lung Perfusion
6. Neuro Stroke Perfusion
7. Metal Artefact Reduction
8. Plaque Characterisation
Dual Energy and Palaeontology
Figure 5: Dual Energy Multiplanar Reformation of two
different dinosaur track ways and Horsetail plants
embedded in iron ore sediment. Notice the different layers
visible within the sediment. Also of importance, is the
disruption to the layers caused by the shallow track way
(to the right of the horsetail plant). These track ways and
plant fossils are approximately 93 million years old.
Figure 3: Volume Rendering Technique of a Diprotodon Optatum jaw. The
Diprotodon (often termed a “giant wombat” ) was the largest marsupial to
have ever lived and is part of Australia’s native Megafauna [6]. This particular
jaw was found on the Darling Downs, Queensland. The Queensland Museum
estimates the age of this fossil to be 60, 000 years old, however, it is hoped
that with CT imaging, a closer date can be achieved.
CT Imaging
CT Imaging
Figure 4: Volume Rendering Technique of a Euryzygoma Dunense skull.
Named for it’s “flaring cheekbones”, this marsupial was common in regions
of Queensland and New South Wales [7]. This fossil was found near
Chinchilla, Queensland and is estimated to be 3.5 million years old.
Figure 7: Volume
Rendering Technique
of a Muttaburrasaurus
footprint, estimated to
be 100 million years
old. The
Muttaburrasaurus was
a herbivore and
measured
approximately 7 m in
length.
1. Barrett, J F. Keat N K. Artefacts in CT: Recognition and Avoidance, Radiographics 2004; 24:1679-1691
2. Hounsfield, G N. Computerized transverse axial scanning (tomography): Part 1 Description of system, BJR 1973; 46 1016-1022
3. The Technical Background of Dual Energy CT <www.dsct.com/dualenergyimaging>, 2009 May 13
4. Riedel , M. An Introduction to Dual Energy Computed Tomography <http://ric.uthscsa.edu/personalpages/lancaster/DI2_Projects_2010/dual-energy_CT.pdf>
5. Dual Energy CT.pdf <www.siemens.com/healthcare>, Siemens AG 2008.
6. Megafauna <www.qm.qld.gov.au >
7. Euryzygoma dunense < http://australianmuseum.net.au/Euryzygoma-dunense>
Figure 6: Dual Energy Multiplanar Reformation of the above
images showing dinosaur track ways and Horsetail plant
fossils embedded within the sedimentary rock.
References
Figure 2: Volume Rendering
Technique of a Thylacine skull.
The Thylacine was once
widespread over mainland
Australia, however in recent times,
it was confined only to Tasmania.
The last known Thylacine died in
captivity in 1936. This Thylacine
skull is dated at 60, 000 years old.
Figure 1: Volume Rendering
Technique of an early Tetrapod
skull. A close relation to the
Salamander and Axolotl, this
fossil is estimated to be 214
million years old.
Figure 8: Curved Multiplanar Reformations
of Horsetail plant fossils. Notice the shift in
the sediment layers described in Figure 5.