Fig 1.
The components of the pneumatic turbines.
a) CAD schematic of turbine A. b) CAD schematic of turbine B. c) Photograph of turbine A. Compressed air is supplied to the turbine via the air inlet (top left). The inserted unbalance causes a dynamic harmonic excitation. The compressed air exits the turbine through the sound damper (top right). The frontal side housing of the pneumatic turbine is removed for a clearer representation.
Fig 2.
The configuration of the actuator and control unit.
Compressed air is supplied via a pressure hose, which is available in all scanner rooms in the clinic (left). All magnetic and active electronic parts are located in the control room (right) and comprise the active driver system. The compressed air is fed to the proportional pressure regulator. The output pressure is regulated by a control voltage. During start-up, the control voltage, i.e. the output pressure, is increased until the nominal frequency of the pneumatic turbine is reached. A fiber optic probe attached to the housing of the turbine provides feedback over the current frequency.
Table 1.
Complex shear modulus of tested silicone samples.
Fig 3.
The vibration frequency response of the pneumatic vibrator housing.
The vibration frequency response spectrum (in m/s2) for frequencies ranging from 20 Hz to 90 Hz with a step width of 10 Hz were evaluated. The acceleration increased with increasing frequency.
Fig 4.
Measured uniaxial acceleration of the pneumatic vibrator acquired by the accelerometer.
Unbalances with a weight of mA1 = 1.2 g, mA2 = 2.3 g, and mA3 = 4.5 g (turbine A) and mB1 = 3.0 g, mB2 = 4.3 g, and mB2 = 8.6 g (turbine B) at frequencies ranging from 30 Hz to 100 Hz were evaluated. The maximum measurable acceleration with our current set up was limited to amax < 14.95 m/s2 due to the range of the accelerometer and thus no greater values could be recorded.
Fig 5.
Complex shear modulus G* of selected silicone samples # 4 - # 11 at frequencies ranging from 30 Hz to 100 Hz measured with a strain controlled rheometer.
Samples # 1 - # 3 are not shown in this graph as their elasticity is more than three times higher than that of sample # 4. The standard deviation calculated from five re-tests of sample # 7 is also shown. Samples # 5, # 7 and # 9 (—) were chosen for the tissue-mimicking phantom, other samples are displayed as—- -.
Fig 6.
Results of 3 T MRE measurements.
Left: Schematic drawing of the phantom used for image acquisition at 3 T. Inclusion A, corresponding to the elasticity of a bladder, is shown in dark blue. Inclusion B, corresponding to the elasticity of a prostate, is depicted in light blue. The actuator was placed on top of the phantom. Middle: Magnitude and phase images. Some trapped air is visible between the inclusions and the background material yielding to artifacts. Phase images were obtained for frequencies ranging between 50 Hz and 80 Hz. Right: Elastograms reconstructed at 80 Hz. Top row is a transverse slice showing inclusion A, Bottom row displays a coronal slice with both inclusions.
Fig 7.
Line profiles of phase signal at 50 Hz, 60 Hz, 70 Hz, and 80 Hz in Inclusion A.
A 25-pixel-line was placed in inclusion A and compared at four actuation frequencies. The wave length shortened with increasing frequencies as indicated by arrows at the first maximum of each line profile.