Chapter 13: Photoacoustic imaging

Question 1

What are the principles of photoacoustic tomography?

Answer 1

1. The tissue is illuminated, by a wide-beam, short duration, laser pulse of a few nanoseconds duration.
2. The optical pulse spread out throughout the region of interest
3. The light is absorbed by chromophores within the tissue
4. Absorbed energy causes pressure rise
5. Pressure rise propagates out as acoustic waves
6. Acoustic waves detected by US sensor array

Question 2

The absorption of light occurs on what timescale

Answer 2

femtoscale 10^-15
thermalised heat 10^-12 - 10^-19
acoustic propagation 10^-6
thermal diffusion 10^-3

Question 3

Thermal confinement?

Answer 3

If the pulse duration is much shorter than the thermal relaxation time then heat diffusion can be neglected as everything happens too fast for the heat to have diffused any significant distance.

Question 4

what is the gruneisen parameter?

Answer 4

represents the efficiency of conversion of absorbed energy density into pressure

Question 5

pressure confinement

Answer 5

if the laser pulse duration is much shorter than the time taken for the acoustic wave to propagate across the heated region, then the pressure will build up furing the laser pulse. Tp < d/c0.

Question 6

What determines how broadband the signals are from the photoacoustic image?

Answer 6

the depth aa they will experience acoustic absorption which increses with frequency,.

Question 7

How can we solve the issue of further absorption deeper down?

Answer 7

time reversal which involved using the measured time series in reversed temporal order as sources in a numerical model of wave propagation. as a wave equation is time invariant these waves will recombine so that a t=0 the pressure field in the model will be the initial aocustic pressure distribution.

Question 8

Ultrasound imaging has disadvantages

Answer 8

low contrast

limited specificity:weak sensitivity to chemical differences

Question 9

how can optical imaging overcome the issues in ultrasound?

Answer 9

high intrinsic contrast based upon optical absorption and scattering
Spectroscopic specificity - chemical information
functional imaging of physiological parameters

Question 10

using light suffers from a major disadvantage

Answer 10

imaging depth/spatial resolution severely limited by strong optical scattering