Causas Estructurales y Factores de Riesgo de la Violencia y Delincuencia

Before going into the clinic, the DCS module must be tested thoroughly for quality, in a fashion similar to the DOS module (see Section 3.1.3). After assembly and soldering, the wired connections must be checked. Then the device has to be tested for stability and accuracy.

Before turning the device on, it must be verified that the parts and power supplies are correctly connected. Then, each component must be powered up, i.e., after confirming that they are receiving the right voltages. The following list details this procedure step-by-step: 1. Make sure AC power is unplugged, and the power switch is OFF. Unplug the power to both sets of APDs. Also unplug power to both lasers, making sure that the lasers are switched OFF.

2. Get a multimeter and turn dial to the beeping thing. (Have circuit diagram alongside you as you check that all appropriate connections are made.)

3. Make sure no connections exist where there shouldn’t be any by using the voltmeter. 4. Now plug in AC power. Remember that power to APDs should still be disconnected.

5. First thing you should notice: fan starts spinning. If it is spinning, then you have wired the fan correctly.

6. Pick up your multimeter again, but this time turn the dial to measure AC voltage. Use this setting to test the voltage going to the lasers, as well as the fan (if it is not spinning). BE CAREFUL! You could easily shock yourself, or cause a short circuit. Watch where you put those test pins.

7. Check that the AC voltages coming into the power supplies is correct (check N, G and L pins).

8. Test that the AC voltages coming into both lasers is correct. If they are, turn off the power switch and plug the laser power cords in.

9. Turn ON the power switches of both lasers. Turn the key of Laser 1 and place a piece of white paper in front of the output. You should see a red spot. Repeat for Laser 2. 10. Switch your multimeter to measure DC voltage. Start with the power supplies,

putting one test pin on +V and the other on -V. You should get a value very close to 2 V, 5 V and 30 V for the respective power supplies.

11. Now turn your attention to the wires you disconnected from the APDs. Use jumper wires to test that the +V and +V return pins are receiving the appropriate voltages (2 V, 5 V or 30 V). Repeat for both sets of wires.

12. Congratulations! Now you can turn on the device with everything connected. Even close the lid if you want.

As with the DOS module, the laser stability of the DCS module must be confirmed. As stated in Section 3.2.1, stability for all lasers should be checked with a power meter overnight as soon as they arrive from the company. Then, once the DCS module is com- pletely wired up, and the connections are tested, then the stability of the device as a whole must be verified. Typically a probe is made to hold optical fibers at multiple source-detector separations. Then the probe is held so it is just touching the surface of a liquid Intralipid phantom, so as to mimic a semi-infinite medium geometry. Data is recorded overnight.

First we must check intensity (i.e. photon counts per second) over time, which is out- putted in kHz by the correlator. Figure 3.19, top, shows an example intensity plot from a successful stability test. Notice that percent standard deviation is small (<2%), and there are no large jumps in the data. Even though intensity values are not used explicitly in cal- culatingBF I, they can be an indication of good laser performance. A laser that has large jumps in intensity could be unstable in coherence as well, which would affect the stability ofBF I.

Next we observe the stability ofβ over time. Again, this is a feature of the correlation curve that may not affect values ofBF Idirectly, but can give insight into whether there are issues with laser coherence or other mishaps with experimental setup. Figure 3.19, bottom, shows an example plot ofβ from an overnight stability test. The values should be close to 0.5 with no abrupt changes in value.

Figure 3.20 shows a plot of relative BF I versus time for a DCS module during an overnight stability test. For analysis, we used the first 50 or so frames as baselineBF I,

Figure 3.19: Example result of intensity and β from an overnight stability test on a basic DCS module. Intensity values are returned by the correlator in photon counts per second, or kHz. Note that percent standard deviation over∼13 hours is<2%. βshould be close to 0.5 and with no large jumps, as seen in the second plot.

Figure 3.20: Example result of relative BF I from an overnight stability test. BF I was derived from fitting a correlation diffusion model tog2curves obtained from a liquid phan-

toms. The first 50 frames (integration time 3 s) were used as baseline.

then the remainder of the timeseries is normalized to this baseline to find relative BF I

in terms of percent change. This is to see how large the variations of relative BF I are (acceptable would be<2%), and to ensure that there are no abrupt jumps. Also, individual frames ofg2data and their fitted curves should be plotted. In some cases, such as when the

source-detector separation is small and we are on the edge of the diffusion approximation, or when a brain-injured patient has an injury that affects blood flow strangely, the model will produce badly fit curves. In these instances, corresponding data should most likely be left out of analysis.

A method to test whether the DCS module is able to distinguish changes in flow is to use a liquid phantom with a magnetic stirrer (see Figure 3.21). The speeds depend on what is available on the stirrer, but we went from having the stirrer completely off to the stirrer on at low speed. This scheme was used to make sure the same correlation curves were being produced for both 8-channel and 4-channel modes (see Figure 3.22). This test is carried out to confirm that the two modes are working correctly, retrieving the sameBF I values,

Figure 3.21: Diagram of stirring test. A liquid phantom is placed on top of a magnetic stirrer that is capable of different stirring speeds. Optical fibers go to the laser source and detectors (an array of APDs), which take data to the correlator and laptop. This test is to enable that the DCS box is capable of distinguishing changes in flow.

and that 4-channel mode has increased sensitivity and higher resolution.

As with the DOS module, we must advance our DCS device from testing on tissue phantoms to real human subjects. The arm cuff experiment described in Section 3.1.3 is typically performed after the DOS/DCS hybrid device has been fully compiled on the cart. Figure 3.23 is an example result from a basic DCS module during an arm cuff occlusion and release. After an initial stable baseline period, blood flow should sharply drop to about 80%of baseline during occlusion (typically at a pressure of 180 mmHg). Then there will be a rapid overshoot as the blood rushes back into the forearm. Lastly, the blood flow should slowly stabilize back to its baseline value.

Figure 3.22: Examples of g2(τ) during stirring test, directly comparing curves during 8- channel versus 4-channel modes. Notice howg2(τ)in 4-channel mode is noisier, because it is has a higher resolution and smaller bins to enable less smoothing of the correlation curve. Also the 4-channel g2(τ) is able to retrieve more of the initial part of the curve,

since the first delay time is smaller.

Figure 3.23: Relative blood flow (rBF) from blood pressure arm cuff testing. When arm is cuffed, rBF drops sharply to about 80% of initial baseline value. Then when cuff is released, an overshoot occurs initially, then rBF slowly returns to baseline level.