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Rawdata-Based Generation of a Heart Motion
Function (Kymogram) using Multi-Threaded CT Scans
of the Heart
Dirk Ertel, Marc Kachelrieß, Willi A.
Kalender Institute of Medical Physics
(IMP) University of Erlangen-Nürnberg http//www
.imp.uni-erlangen.de
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Cardiac Imaging
C 0, W 1000
C 0, W 1000
Standard reconstruction
Phase-correlated reconstruction
The consideration of the hearts motion phase in
cardiac imaging is necessary to reduce motion
artifacts.
M. Kachelrieß, W. A. Kalender. Med. Phys, 25(12)
2417-2431, December 1998
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Aim

• Generate a purely rawdata-based synchronization
  signal as an alternative to the commonly used ECG
  Kymogram function
• Modify current implementation of the kymogram
  generation to allow for its application in
  multi-threaded CT
• Validate the modified kymogram function for
  multi-threaded CT using a cardiac simulation
  environment
• Show the improvements for a kymogram-based
  phase-correlated image reconstruction in cardiac
  CT

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General concept
Cardiac Imaging The insufficient temporal
resolution of standard CT imaging demands a
phase-correlated reconstruction synchronized by
an ECG or kymogram. Multi-Threaded CT
(MT-CT) CT systems with multiple tube-detector
systems offer a higher temporal resolution than
single tube CT imaging systems.
Applying multi-threaded CT to cardiac CT improves
image quality1.
1M. Kachelrieß, M. Knaup, W. A. Kalender IEEE
TMI special issue, submitted, 2006
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Cardiac Imaging
Phase-correlated reconstruction For image
reconstruction only rawdata acquired at a
selected heart phase with respect to a
synchronization signal (e.g. ECG) are used.
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Kymogram Function
The kymogram function is generated by a
center-of-mass tracking of the scanned object
x parallel projection axis p monochromatic
projection data
However, multiple projections covering a time
span Dt are necessary to triangulate the COM.
Kachelrieß et al., Med. Phys., 29 (7)1489-1503,
July 2002
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Proposed Configurations for Multi-Threaded CT
The configurations of multiple tube-detector
systems (with NT tubes) for MT-CT to achieve
optimal temporal resolution have been identified
as
M. Kachelrieß, M. Knaup, W.A. Kalender Proc. of
the 2005 Int. Meeting on Fully 3D Image
Reconstruction, July 2005. www.fully3d.org
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Advantages in MT-CT
Compared to single tube CT scanners
multi-threaded CT scanners offer a higher
temporal resolution in cardiac imaging.
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Multi-Threaded Kymogram
Multiple tube-detector systems are reducing the
time span Dt for triangulating the COM by
offering NT projections at same time. Therewith,
the COM signal shows a higher temporal resolution.
NT 2
NT 3
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Multi-Threaded Kymogram
The kymogram coordinates xc and yc of the COM can
be computed by minimizing the following
expression
Ji angular position of thread i
In contrast to standard CT systems, in MT-CT a
number of NT integrals contribute to the
minimization problem.
The integration range of 2fp can be reduced by a
factor of NT without reducing the accuracy of the
computation. Instead, the temporal resolution of
the kymogram signal is increased by the same
factor.
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Validation
CT rawdata were generated using mathematical
phantoms1 and the ImpactSim2 software package.
dHeart(t)

• Simulations
• Cardio motion phantom with simulated ECG
• Added quantum noise
• Polychromatic spectrum
• Variable heart rate
• 64-slice CT (Somatom Sensation 64)
• NT1,2,3

calcifications
heart
C 0, W 1000
Cardio motion phantom
1M. Kachelrieß, S. Ulzheimer, W.A. Kalender. Med.
Phys., 27(8)1881-1902, August 2000. 2ImpactSim,
VAMP GmbH Möhrendorf, Germany
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Validation

• Quality measure
• Variation in the total projected mass caused by
  beam hardening and scatter should be minimal
• Mean difference of the heart rate detected by the
  kymogram and the simulated ECG should be minimal

p monochromatic projection data w(E)
energy-dependent detected spectrum y(E) energy
dependence of the attenuation coefficient
m total projected mass q polychromatic
projection data
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Results
The variation in the total projected mass can be
minimized by averaging over multiple tubes (i
1,2,3).
i1
i2
i3
average value
For this simulation only beam hardening effects
were taken into account.
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Results
The mean heart rate difference decreases with the
number of threads indicating a higher correlation
of the kymogram with the ECG.
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Conclusions

• The variation of the projected mass due to beam
  hardening and scatter can be reduced by using
  multi-threaded CT systems.
• Synchronization signals on the basis of a
  kymogram signal correlated well with the ECG
  using a multi-threaded CT scanner.
• The temporal resolution of the kymogram signal
  can be improved just the same as the temporal
  resolution of the image by increasing the number
  of threads NT.
• With an increased number of threads the
  correlation between the kymogram and the ECG
  improves. Therewith, the image quality of a
  kymogram-based phase-correlated image
  reconstruction can be improved in cardiac CT.

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• Thank you for your attention!

An updated version of this presentation is
available at www.imp.uni-erlangen.de