Resultados en la categoría de AMBIENTE FÍSICO Y CULTURAL: Característica Positiva:

The imaging system utilized 15 ultrasound transducers (model V304, 1" diameter, 2.25 MHz with fractional bandwidth of 65%, Panametrics-NDT, Waltham,

Massachusetts) in a staring hemispherical arrangement. Transducers were mounted on 5 custom-built frames, each supporting 3 transducers at zenith angles of 22.5°, 45°, and 67.5°. The frames were designed such that the sensitivity of all 15 transducers were intended to overlap in a specified object space of approximately 25x25x25 mm3 near the geometric center of the array. Laser illumination ("Surelite OPO Plus", OPO-coupled Nd:YAG, Continuum, Santa Clara, California) was directed to a bifurcated fibre (400 μm

61

diameter) such that half of each laser pulse was guided to a photodiode (to measure pulse- to-pulse variation) and the other half to an optical fibre immersed in the liquid (where the photoacoustic signal was generated) for a total of 16 channels collecting data (15

transducers, 1 photodiode). The pulse duration was 6 ns at a repetition rate of 10 Hz with a maximum laser output of approximately 100 mJ/pulse. Note that only a small fraction of the pulse was accepted by the fibre due to its small core size relative to the beam diameter (~1.5 cm). All calibration scans were done at 675 nm. Each transducer was electrically connected to a dedicated channel on a preamplifier card (custom built). The analog signals were acquired in parallel, converted to digital signals, and sent to a personal computer for analysis. The custom built data acquisition system sampled with 14-bit resolution at a frequency of 50 MHz. The PA system (with PA point source and optical fibre) is shown in Fig. 3.1(a) while a representative PA time series acquired during an experiment from a single transducer is shown in Fig. 3.1(b).

Figure 3.1: (a) Isometric view of the hemispherical PA imaging array illustrating the transducer arrangement, placement of the liquid reservoir, and the optical fibre PA source. (b) Example of raw data acquired on a single acoustic transducer.

3.2.2 System calibration scan

In order to acquire calibration scans with step-size on the order of the system resolution, a number of improvements were made to the scan procedure in comparison to

62

our previous work [15]. First, the linear slides responsible for translating the optical fibre through the absorbing liquid were replaced with a SCARA robot (Model E2C351S - UL, Epson), which could translate the source between data points faster and more reliably to reduce translation time. Second, the data acquisition cards were updated with USB 2.0 connections to relay data to the PC in a much shorter interval. With the upgrades, each scan point required approximately 15 seconds to complete. Subsequently, two calibration scans were performed with different object spaces and step-sizes. The first scan was completed with a 16x16x16 mm3 object space and 2 mm step-size for a total of 512 data points. The second scan was performed on an object space of 30x30x30 mm3 and 3 mm step-size for a total of 1000 data points. At each test position in the scan, the PA signal was averaged over 10 pulses and recorded simultaneously on all 15 transducers. After the calibration scan, the time series data for each transducer and grid location was analyzed off-line to obtain the imaging operator corresponding to each scan. Analysis included extracting the time series of a particular transducer and grid point, rectifying the time series, and then smoothing the time series using a moving average with a bin size of 40 points. Each time series was copied to the matrix representative of the imaging

operator. Each row contained the concatenated time series for all 15 transducers corresponding to a position in object space. Therefore, the imaging operator had rows corresponding to the number of calibration grid points in object space and columns corresponding to the number of time points used to sample object space multiplied by the number of transducers used to collect the data. A de-noised imaging operator was

constructed by removing the noise from the transducer responses prior to analysis. De- noising was performed by estimating the peak size, peak width and time of flight from the smoothed time series data. The peak size was found by locating the maximum in the time series data and the peak width estimated by differencing the location of the time points, which corresponded to half the value of the peak size. The time of flight was recorded as the temporal location of the peak. The parameters were then used to compute a synthetic time series consisting of zeros everywhere, except for the points representing the peak width centered upon the time of flight. These specific points were filled with a scaled and inverted parabola representative of the smoothed experimental time series. The inverted parabola is representative of the basis function used in the back projection

63

model of the reconstruction algorithm from our previous work, which closely resembles the velocity potential of the bipolar pressure signal. In this approach, the characteristic information of the bipolar wave is retained (amplitude, FWHM, time-of-flight). In order to backproject accurately, the velocity potential is used in order to retain this information and not cancel signal placed in voxels because of the negative lobe in the bipolar wave. The interference effects potentially present in an imaging task are still accurately

measured in the forward model of the solution. The de-noised imaging operator was then constructed from these de-noised time series using the same method described above. The de-noised imaging operator then contained the same shift-variant response as the

experimental imaging operator but with greatly reduced noise.

3.2.3 Singular value decomposition and singular vector