Download Real-Time Kymogram Detection from Cardiac Spiral CT Scans

Real-Time Kymogram Detection from Cardiac Spiral CT Scans
Dirk Ertel Dipl.-Ing., Marc Kachelrieß Ph.D., Dirk-Alexander Sennst Ph.D.,
Stephan Achenbach M.D., Willi A. Kalender Ph.D.
Institute of Medical Physics, University of Erlangen-Nuremberg, Erlangen, Germany, www.imp.uni-erlangen.de
Purpose
Results
To generate a rawdata-based synchronization signal for cardiac spiral CT
scans in real time. The signal may be used as an alternative to the ECG
acquired at scan time and as a basis for cardiac phase-dependent on-line
tube-current modulation. The already existing offline computed kymogram [1] is
taken as the base for these enhancements.
9 of 12 patients showed a high correlation with the ECG to the kymogram
where the phase lag value stayed below 25% for all three methods. The least
reliable results were generated by the derivation of the projected mass with
respect to the view angle. The prerequisite of being able to perform in real time
was satisfied by all three methods.
Methods and Materials
Fig. 6 shows examples of coronal CT images using phase-correlated
reconstruction. The quality of the different images point out the usability of the
different synchronization signals. In contrast Fig. 7 shows coronal CT images
with the usage of the predicted sync peaks derived from the different
kymograms. It has to be mentioned that the predicted signal will be used for a
phase-dependent tube current modulation and not directly for an image
reconstruction.
The synchronization signal –the motion or kymogram function– is generated by
analyzing the periodic temporal variation of the mass distribution m(t) of the
current section and correlates to the heart rate. Three different methods are
used to generate the kymogram: A two dimensional center-of-mass tracking of
the scanned object [1] (Fig. 1), the difference of the projected mass after a half
rotation [2] (Fig. 2) and a derivation of the projected mass with respect to the
view angle [3] (Fig. 3).
Fig. 4: ECG signal and kymogram signal (COM) from patient 310179
For all three methods the data were integrated over several detector rows. The
data of 12 patients scanned with a collimation of 12×0.75 mm, a table
increment of 2.8 mm/rotation, a rotation time of 0.42 s and a concurrent ECG
recording (Sensation 16, Siemens, Forchheim, Germany) was used for
validation. The signal extrapolation was performed by predicting the heart rate
according to the values observed so far with the normalized least-mean-square
algorithm. The sync peak prediction shall allow for real-time performance. The
degree of correlation is expressed by the value of the standard deviation σ of
the lag function L(t), which is the difference of the absolute phase of the ECG
and the kymogram (Fig. 5).
COM(t)
ξ c,2
ECG
COM
∆M π0,p
∆ϑ
ξ c,1
Fig. 6: Coronal CT images of patient 310179, phase-correlated
reconstruction, based on kymogram (C=0 HU/W=1000 HU)
Fig. 1: Two dimensional center-of-mass tracking of the scanned object: COM(t) [1]
0
π ,p
m(t1)
Heart area
predicted signal: COM
m(t 1+∆ t)
predicted signal: ∆M π0,p
Fig. 5: Lag functions and heart area from patient 310179
Fig. 2: Difference of the projected mass after half a rotation: ∆Mπ 0,p (t) [2]
m(t3)
m(t2 )
The phase lag value σ is computed for the ROI covering the heart area. A high
correlation with the ECG leads to small values of σ. The time needed for signal
processing was determined with a standard PC to verify the real-time capability
of the kymogram calculation. Table 1 shows averaged values of σ for 12
patients. Here the correlation of the different kymogram methods and of the
derived predicted sync peaks to the ECG can be seen.
m(t1 )
predicted signal: ∆ϑ
Fig. 7: Coronal CT images of patient 310179, phase-correlated
reconstruction, based on predicted sync peaks (C=0 HU/W=1000 HU)
Conclusion
σ (kymogram)
σ (prediction)
COM
24.76
31.52
0
∆M π ,p
32.66
40.57
∆ϑ
54.55
48.57
Method
Fig. 3: Derivation of the projected mass with respect to the view angle: ∆ϑ (t) [3]
Three different methods for generating a kymogram
Table 1: Averaged values of σ [in %] for L(t) of 12 patients
For most patients predicted sync peaks on the basis of a kymogram correlated
well with the ECG. We conclude that a kymogram-based tube current
modulation for dose reduction in cardiac CT appears possible.
References
[1] Marc Kachelrieß, Dirk-Alexander Sennst, Wolfgang Maxlmoser, and Willi A. Kalender (2002).
“Kymogram detection and kymogram-correlated image reconstruction from subsecond spiral
computed tomography scans of the heart”. Medical Physics, 29(7):1489-1503
[2] H. Bruder, E. Maguet, K. Stierstorfer, T. Flohr “Cardiac spiral imaging in Computed
Tomography without ECG using complementary projections for motion detection” Proc. SPIE,
5032:1798-1809, May 2003. Medical Imaging 2003: Image Processing
[3] Dirk Ertel (2004). “Real-Time Kymogram Detection from Spiral CT Scans of the Heart”
Diplomarbeit, Friedrich-Alexander Universität, Institut für Medizinische Physik and Lehrstuhl für
Multimediakommunikation und Signalverarbeitung