Master's Thesis - Studierstube Augmented Reality Project - Graz ...

3.5 Cine Cardiac Fast Low-Angle Shot with Parallel-Imaging Technique
be measured at once. The images itself result from a combination of more than one
heartbeat with the assumption that the heart-cycle is perfectly periodical and that an
image taken at a certain time point of the cycle will always look the same.
To trigger the same point for some lines of a certain image’s k-space, the patients
ECG is recorded simultaneously for detection of the silhouetted R-wave. The images
are then attempted to be equal spaced between the R-waves. Depending on the heart
rate and the desired time-steps as many k-space lines as possible are measured for
the morphological and the flow-sensitive images. This sounds simple but can be very
difficult. Gathering a mV-signal of the human heart in the center of a KW transmitter
demands special ECG-electrodes and electrode arrangements so that on the one hand
the magneto-hydrodynamic effect 1 of the large vessels can be neglected and on the other
hand, dependent on the used wires and their length, as less interferences as possible are
absorbed. However, it is enough to have a detectable R-wave in the resulting signal.
The signal-conduction is done as shown in figure 3.5 2 and submitted via Bluetooth ®
to the pulse sequence control unit. Figure 3.9 underlines this measurement process.
Figure 3.9: Triggered retrospective data acquisition. The phase encoding steps are
incremented when the trigger signal is detected. Each line of data is timestamped and
retrospectively sorted to reconstruct a series of images covering the entire cardiac cycle.
Image taken from [Siemens2003].
As a result of the periodicity of the 2D Fourier k-space an image would be completed
if enough lines are gathered after some heart-cycles.
1 The magneto-hydrodynamic effect is is an additional electrical charge generated by ions in blood
(loaded particles) moving perpendicular to the magnetic field. see [MR-TIP]
2 Product (b): Invivo Research Inc., Quadtrode ® 3, MRI ECG Electrode Pad, REF 9369N
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