Quantification of mechanical vibration during diffusion tensor imaging at 3 T
Introduction
Diffusion tensor imaging (DTI) by means of magnetic resonance imaging (MRI) enables the studies of tissue microstructure based on random movements of water molecules (Basser et al., 1994, Le Bihan, 1995). Besides diffusion, the DTI measurements are very sensitive to any movements within the imaging area and are therefore easily disturbed by, e.g., motion of the subject or mechanical vibration of the MRI system. During the scanning, the subject's movement can be minimized with appropriate fixation of the anatomical area to be imaged or by applying specifically designed DTI sequences that include motion compensation (Liu et al., 2004). Moreover, some effects of movement can be corrected with post-processing methods (Rohde et al., 2004), also in combination with correction for eddy current distortions (Andersson and Skare, 2002). Cardiac-gated DTI that reduces physiological motion caused by heart-cycle-related pulsation has been used for imaging brain, spine, and prostate (Andersson and Skare, 2002, Golay et al., 2002, Nunes et al., 2005, Sinha and Sinha, 2004, Wheeler-Kingshott et al., 2002).
Gradient magnetic fields, needed for spatial encoding of the MRI signal, are produced by directing strong currents into the gradient coils. These currents cause Lorentz forces to the gradient coils, which thus tend to move (vibrate) when currents are switched on and off. This mechanical vibration is also the cause of the acoustic noise during MRI scanning. The vibration can spread from the gradient coils to other solid structures nearby.
The MRI environment is very demanding for high-quality vibration measurements because of the presence of the stationary, gradient, and radiofrequency (RF) magnetic fields which easily interfere with measurement equipment and its cables close to the scanner inside the shielded room. Therefore, acceleration sensors, commonly used for measuring mechanical vibrations in non-MRI environments, may not suit for high-accuracy quantification of vibration in the MRI environment. Moreover, vibration can not be measured separately in several directions, since these sensors provide only a summation signal. Better suited for vibration measurements in the MRI environment is an optical system that enables the measurements separately in several directions and is not disturbed by gradient or RF fields.
Until now, the MRI vibrations have been of interest mainly in applications that aim to reduce the acoustic noise during the scanning. Acoustic studies naturally tend to focus on the higher frequency components, inherently ignoring the lower frequencies that, however, are the source of the large displacements. A piezoelectric sensor has been used to measure mechanical vibrations of gradient coils during a single-shot echo-planar imaging (EPI) sequence (Tomasi and Ernst, 2003). On the basis of the observed main resonance frequencies, the imaging sequence was then adjusted to minimize the acoustic sound pressure levels during scanning.
A laser-based optical vibrometer has been used to assure that the motion of a phantom was small enough to enable the actual measurements without additional movement-related magnetic field fluctuations (Wu et al., 2000). A similar laser-based device has been used for measuring the velocities of a gradient coil insert in three directions (Yao et al., 2004). These measurements were used for verification of a finite-element model and for defining its boundary conditions, ultimately used for characterizing the vibration and acoustic noise levels in MRI.
The Lorentz forces, and thus the level of mechanical vibration, depend on the strength of both the main and the gradient magnetic fields. The diffusion-sensitizing gradients used in DTI are typically stronger than the gradients used for spatial encoding of the MRI signal, and the subjects can clearly sense the vibrations during DT imaging. Despite their potentially image-quality-deteriorating effects, the DTI-related mechanical vibrations have not been previously studied. Instead, it has been commonly assumed that the mechanical vibration during DTI is of the same strength and phase in all parts of the MRI system so that it could be ignored.
Our goal was to quantify mechanical vibrations during DTI with various parameters of the imaging sequence. We used an optical laser-based interferometer to measure the vibrations from several parts of the MRI scanner and its surroundings in the shielded room. The vibrations were quantified separately in three directions along the main gradient axes, using a prism to reflect the laser light onto the desired target surfaces in horizontal and vertical directions.
Section snippets
Methods
Mechanical vibrations were measured from different parts of our AMI Centre's Signa VH/i 3.0 T MRI scanner (General Electric, Milwaukee, WI) and its surroundings in the shielded room. The scanner was equipped with a standard body RF coil and a quadrature head coil, and the software version VH3.291_M38_0230.a was used. The manufacturer-specified maximum amplitude of field gradients was 40 mT/m, with slew rate of 120 mT/m/s. During all measurements, a spherical gel-filled phantom (weight 3 kg) was
Vibration phenomenon
Fig. 3 shows three examples of vibration measurements: the velocity signals with (top) and without (middle) ECC, and a longer time period, now covering the time between two RF excitations and readout periods with ECC gradients on (bottom). Similar measurements were repeated for each measurement point with different combinations of b values, diffusion-sensitizing gradients, and with and without ECC gradients, as well as with and without the human subject on the bed.
The pink lines of Fig. 3 show
Discussion
In contrast to a general assumption, our results show that DTI-related mechanical vibrations differ in strength and phase in different parts of a MRI scanner and its surroundings. The largest movements occurred during the readout period, affecting therefore the image quality as well as the accuracy of the diffusion measurements. Thus, the vibrations cannot be ignored and should at least be considered when choosing the sequence parameters for DTI.
The MRI system vibrates more with than without
Acknowledgments
This study was financially supported by the Academy of Finland (National Center of Excellence Program), Louis-Jeantet Foundation (Switzerland), and Jenny and Antti Wihuri Foundation (Finland). We gratefully acknowledge GE Medical Systems (Finland) and GE Medical Systems Engineering (Milwaukee, WI, USA) for collaboration and VTT Technical Research Centre of Finland for the interferometer.
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