Kazuyuki Mizuhara

Measurement of the 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 true insertion depth of the needle in the tissue. The friction model caused by the clamping force on the needle can also be determined from the measured friction forces.

A Model for Relations between Needle Deflection, Force, and Thickness on Needle Penetration

A force-deflection model of needle penetration is proposed and evaluated experimentally. The force at the fixed end of the needle and the needle deflection were measured using a force sensor and a bi-plane X-ray imaging system, and the model was evaluated with the data. We define a physical quantity ?, which we call infinitesimal force per length, analogous to traction (force per surface area). The model predicts ? to be constant over the length of the inserted portion of the needle. However the results indicate that this assumption does not fully account for the real deflection. It is strongly suggested that there is an additional degree of freedom: a moment or a rotational force acting on the needle.

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 readouts 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.

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 100Hz 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±3.17kPa. Our results demonstrated that using MRE, our method can be applied to the supraspinatus muscle by using conventional MRI.

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 generation of 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.

MRE Imaging for Elasticity and Viscosity of Biological Tissues

This paper discusses an MR Elastography (MRE) method for noninvasive measurement of elasticity and viscosity of biological tissues. In this imaging method, bipolar gradients put into any pulse sequence for MRI. The phase contrast of acquired image is changed by the number of bipolar gradients and their amplitude. From acquired signal (gradient echo and/or spin echo), changing of the phase of magnetized spin is calculated, and analyzed frequency response of applied mechanical vibration from the outside of the biological tissue. The frequency and amplitude of applied mechanical vibration should be changed by elasticity and viscosity of the tissues. The frequency of applied mechanical vibration is from around 100 Hz by mechanical vibrator to high around 1MHz by ultrasound device. In the post processing, elasticity and viscosity is calculated and the phase shift of magnetized spin is analyzed in two or three dimensions. An image is reconstructed as MRE image which is extracted and enhanced elasticity and viscosity of biological tissues.

The Development of Vibration System for Applying Magnetic Resonance Elastography (MRE) to the Supraspinatus Muscle

Palpation is a standard clinical tool to diagnose abnormal stiffness changes in soft tissues. However, it is difficult to palpate the supraspinatus muscle because it locates under the trapezius muscle. The magnetic resonance elastography (MRE) uses harmonic mechanical excitation to quantitatively measure the stiffness (shear modulus) of both the superficial and deep tissues. The purpose of this study was to build a vibration system for applying the MRE to the supraspinatus muscle. In this study, a power amplifier and a pneumatic pressure generator were used to supply vibrations to a vibration pad. Six healthy volunteers underwent MRE. We investigated the effects of position (the head of the humerus and the trapezius muscle) of the vibration pad on the patterns of wave propagation (wave image). When the vibration pad was placed in the trapezius muscle, the wave images represented clear wave propagation. On the other hand, when the vibration pad was placed in the head of the humerus, the wave images represented unclear wave propagation. This result might be caused by wave interferences resulting from the vibrations from bones and an intramuscular tendon of the supraspinatus muscle. The mean shear modulus also was 8.12 ± 1.83 (mean ± SD) kPa, when the vibration pad was placed in the trapezius muscle. Our results demonstrated that the vibration pad should be placed in the trapezius muscle in the MRE of the supraspinatus muscle.