8th Interventional MRI Symposium Book of Abstracts - Otto-von ...

V-11
Kalman-filtered velocity navigator triggering for motion
compensated PRF thermometry
F. Maier 1 , A. J. Krafft 1 , J. W. Jenne 2,3 , R. Dillmann 4 , W. Semmler 1 , M. Bock 1
1
Medical Physics in Radiology and 2 Clinical Cooperation Unit Radiation Oncology,
German Cancer Research Center (DKFZ), Heidelberg, Germany
3
Mediri GmbH, Heidelberg, Germany
4
Institute of Anthropomatics, Karlsruhe Institute of Technology, Karlsruhe, Germany
Objective
For thermal therapies such as radio frequency, laser or focused ultrasound ablation
thermometry is required. The preferred thermometry methods are non-invasive MR
temperature measurements. In the most commonly applied proton resonance frequency (PRF)
shift thermometry, temperature changes are calculated based on phase differences with
respect to a reference. Since PRF thermometry is very sensitive to inter scan motion, this
method is prone to motion artifacts. In the treatment of abdominal organs such as liver or
kidney, breathing motion causes large tissue displacements. Different approaches have been
proposed to address this problem. Triggered pulse sequences synchronize image acquisition to
breathing motion [1-4], multi-baseline approaches acquire an atlas of reference images [3],
and referenceless methods estimate the reference phase without a reference acquisition [5].
Recently, a novel navigator technique for triggering MR thermometry image acquisition was
presented [6]. In this work, an improved navigator acquisition was proposed. Additionally, the
quality of the navigator signal was enhanced by Kalman filtering.
Materials and Methods
All experiments were carried out on a clinical 1.5 T whole body MR system
(Magnetom Symphony, Siemens, Erlangen, Germany). The filtering and trigger algorithm
was implemented directly on the image reconstruction system of the MR scanner.
Pulse Sequence - A segmented EPI sequence (parameters: TR/TE = 25/15 ms, =
18°, FOV: 298×216 mm, matrix: 128×92, slice thickness: 5 mm, EPI factor: 11, acquisition
time per slice: 225 ms) was modified for triggered temperature mapping (Fig. 1). A bipolar
gradient was inserted after slice selection and subsequently a navigator ADC without spatial
encoding was acquired prior to the start of the readout train and the phase encoding gradients.
Since the echo times of thermometry sequences are relatively long, the additional navigator
gradients did not lead to a lengthening of TR. Until a trigger event was generated to start the
acquisition of a complete slice (cf. trigger algorithm below), the first segment was acquired
repeatedly to maintain magnetization steady state for subsequent image acquisition. The
bipolar gradient was inverted for every second acquisition of the EPI echo train. Therefore, an
increased total velocity sensitivity of VENC = 0.15 m/s could be realized. Velocity encoding
can be applied in readout, phase encoding and slice selection direction. After a trigger event,
velocity encoding was automatically disabled during image acquisition.
Trigger Algorithm - Each navigator ADC measured eight complex signal values
within 20 s, which were averaged. The phase of the values was unwrapped over time.
Velocity was calculated by using the current velocity encoded navigator value and its
inversely encoded predecessor (temporal distance: 1 × TR = 25 ms). A Kalman filter (cf.
below) was applied to reduce noise and remove pulsation without a considerable temporal
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