A new technique for MR elastography of the supraspinatus muscle: A gradient-echo type multi-echo sequence.

Magnetic resonance elastography (MRE) can measure tissue stiffness quantitatively and noninvasively. Supraspinatus muscle injury is a significant problem among throwing athletes. The purpose of this study was to develop an MRE technique for application to the supraspinatus muscle by using a conventional magnetic resonance imaging (MRI). MRE acquisitions were performed with a gradient-echo type multi-echo MR sequence at 100 Hz pneumatic vibration. A custom-designed vibration pad was used as a pneumatic transducer in order to adapt to individual shoulder shapes. In a gradient-echo type multi-echo MR sequence, without motion encoding gradient (MEG) that synchronizes with vibrations, bipolar readout gradient lobes achieved a similar function to MEG (MEG-like effect). In other words, a dedicated MRE sequence (built-in MEG) is not always necessary for MRE. In this study, 7 healthy volunteers underwent MRE. We investigated the effects of direction of the MEG-like effect and selected imaging planes on the patterns of wave propagation (wave image). The results indicated that wave images showed clear wave propagation on a condition that the direction of the MEG-like effect was nearly perpendicular to the long axis of the supraspinatus muscle, and that the imaging plane was superior to the proximal supraspinatus muscle. This limited condition might be ascribed to specific features of fibers in the supraspinatus muscle and wave reflection from the boundaries of the supraspinous fossa. The mean stiffness of the supraspinatus muscle was 10.6 ± 3.17 kPa. Our results demonstrated that using MRE, our method can be applied to the supraspinatus muscle by using conventional MRI

A Simple Method for MR Elastography: A Gradient-Echo Type Multi-Echo Sequence

To demonstrate the feasibility of a novel MR elastography (MRE) technique based on a conventional gradient-echo type multi-echo MR sequence which does not need additional bipolar magnetic field gradients (motion encoding gradient: MEG), yet is sensitive to vibration. In a gradient-echo type multi-echo MR sequence, several images are produced from each echo of the train with different echo times (TEs). If these echoes are synchronized with the vibration, each readout\u27s gradient lobes achieve a MEG-like effect, and the later generated echo causes a greater MEG-like effect. The sequence was tested for the tissue-mimicking agarose gel phantoms and the psoas major muscles of healthy volunteers. It was confirmed that the readout gradient lobes caused an MEG-like effect and the later TE images had higher sensitivity to vibrations. The magnitude image of later generated echo suffered the T2 decay and the susceptibility artifacts, but the wave image and elastogram of later generated echo were unaffected by these effects. In in vivo experiments, this method was able to measure the mean shear modulus of the psoas major muscle. From the results of phantom experiments and volunteer studies, it was shown that this method has clinical application potential

Magnetic Resonance Elastography using an air ball-actuator

The purpose of this study was to develop a new technique for a powerful compact MR Elastography (MRE) actuator based on a pneumatic ball-vibrator. This is a compact actuator that generates powerful centrifugal force vibrations via high speed revolutions of an internal ball using compressed air. This equipment is easy to handle due to its simple principles and structure. Vibration frequency and centrifugal force are freely adjustable via air pressure changes (air flow volume), and replacement of the internal ball. In order to achieve MRI compatibility, all parts were constructed from non-ferromagnetic materials. Vibration amplitudes (displacements) were measured optically by a laser displacement sensor. From a bench test of displacement, even though the vibration frequency increased, the amount of displacement did not decrease. An essential step in MRE is the generationof mechanical waves within tissue via an actuator, and MRE sequences are synchronized to several phase offsets of vibration. In this system, the phase offset was detected by a four-channel optical-fiber sensor, and it was used as an MRI trigger signal. In an agarose gel phantom experiment, this actuator was used to make an MR elastogram. This study shows that the use of a ball actuator for MRE is feasible

Measurement of Tip and Friction Force Acting on a Needle During Penetration

We present the tip and friction forces acting on a needle during penetration into a canine prostate, independently measured by a 7-axis load cell newly developed for this purpose. This experimental apparatus clarifies the mechanics of needle penetration, potentially improving the development of surgical simulations. The behavior of both tip and friction forces can be used to determine the mechanical characteristics of the prostate tissue upon penetration, and the detection of the surface puncture, which appears in the friction force, makes it possible to estimate the hue insertion depth of the needle in the tissue