A PHD MEDICAL PHYSICS PRESENTATION COURSE ON UlTRASOUND SYSTEMS (2021) COLLEGE OF MEDICINE, IRAN UNIVERSITY OF MEDICAL SCIENCE

1. PHOTOACOUSTIC
IMAGING
By: Elmira Yazdani
A PHD MEDICAL PHYSICS PRESENTATION COURSE
ON UlTRASOUND SYSTEMS (2021) BY DR.SHIRAN
COLLEGE OF MEDICINE, IRAN UNIVERSITY OF MEDICAL SCIENCE
Yazdani.el@iums.ac.ir
1

2. PHOTOACOUSTIC
IMAGING
• Introduction
• Ultrasound and Optical Imaging
• Photoacoustic Imaging (PAI)
• History
• Different Techniques of PAI
• Principle of Photoacoustic Imaging
• Applications of Photoacoustic Imaging
• Advantages and Disadvantages
• Conclusion
• Reference
Outline

3. 3
• Conversion of photons to acoustic waves due
to absorption and localized thermal excitation.
• Pulses of light is absorbed, energy will be
radiated as heat.
• Heat causes detectable sound waves due to
pressure variation.
Introduction

4. 4
Introduction
Ultrasound
Imaging
Optical
Imaging
Photoacoustic
Imaging

5. Transducer emit ultrasound wave and get signals back from object.
Scan volume to get image.
Pros & cons: No side effects but low contrast.
High resolution ~ 100 𝜇𝑚
Ultrasound Imaging
5

6. • Optical imaging tools provide anatomical, functional, and molecular
images that drive large areas of biomedical research and modern
clinical practice. However, because of the strong photon-scattering
nature of biological tissues, the performance of optical imaging tools
drastically degrades at depths greater than a few hundred
micrometers, even though light in the near-infrared (NIR) region can
penetrate through several centimeters of tissue. Therefore tissues
must be excised and sliced to be imaged under the microscope;
otherwise the resolution of macroscopic optical imaging techniques is
restricted to a few millimeters in deep tissues.
• Disadvantages of optical imaging include low depth penetration and
limited clinical translation.
6
Optical Imaging

7. • PAI or Thermoacustic Imaging (TAI): Combine advantages of optical (high
contrast) and ultrasound (great imaging depth and high resolution).
7
Principle of Photoacoustic Imaging (PAI)
High optical
contrast images
Microscale
resolution
Reasonable
penetration depth
Optical fiber
Energy absorption layer
Laser
excitation
Acoustic signals

8. 1
2
3
Steps:
Photoacoustic Imaging
4

9. 9
History of PA

10. 10
Photoacoustic imaging systems consists of the following technologies:
• Variable wavelength laser
• Ultrasonic sensors
• Data acquisition system
• Image reconstruction
• Operation / display system”
• Connecting the processing / control system to a data server
Principle of Photoacoustic Imaging
Photoacoustic Imaging System

11. 11
Photoacoustic imaging occurs by taking
advantage of the photoacoustic effect, in which a
subject emits ultrasonic waves when irradiated
with light. The emitted ultrasonic waves can be
gathered with multiple ultrasonic sensors, and
the data can be used to construct an image.
Pulsed laser light from a variable wavelength
laser is irradiated on a subject, and the absorber
inside this subject absorbs light. The ultrasonic
waves are generated by volume expansion,
which is caused by a temperature rise in the
subject.
Principle of Photoacoustic Imaging
Fusion of Light and Ultrasound

12. 12
When generating ultrasonic waves with light, it
is possible to determine the characteristics of
the color of the absorber (i.e., its absorbing
properties) as image information, just as in
optical imaging. With the introduction of
ultrasound, which can propagate through living
bodies, a high spatial resolution is maintained
even at deeper parts, providing clear
visualization that was difficult to obtain with
conventional optical imaging.
Principle of Photoacoustic Imaging
Fusion of Light and Ultrasound

13. 13
Principle of Photoacoustic Imaging
Reconstruction / Three-dimensional imaging
The time required for the ultrasonic waves to
reach each sensor is calculated by the timing
between the emission of a laser pulse from the
variable wavelength laser and its reception at
the ultrasonic sensor array. Based on this
arrival time, the position information for each
ultrasonic sensor and the speed of wave
propagation in the subject, it is possible to form
a three-dimensional image of the absorber
using an image reconstruction method known
as the back-projection method.

14. 14
Principle of Photoacoustic Imaging
Reconstruction / Three-dimensional imaging
The shape of the absorber is reflected in the form of the ultrasonic wave generated. If the shape of
the absorber is small, the generated wave has a narrow time axis, and vice versa. It is possible to
use this kind of waveform information to measure the size of the absorber.

15. 15
In photoacoustic imaging, it is possible to visualize material
properties based on the color of the absorber (i.e., light
absorption properties) by irradiating it with multiple laser
pulses with wavelengths adjusted to the optical characteristics
of the absorber. For example, the light absorption spectrum of
hemoglobin varies with oxygen saturation. Focusing on the
light absorbance difference of two wavelengths of hemoglobin
(e.g., 755 nm and 795 nm) dependent on the hemoglobin
oxygen saturation value, laser pulses with these different
wavelengths generate photoacoustic waves with different
intensities. By visualizing the intensity ratio of the
photoacoustic waves, it is possible to map oxygen saturation
in hemoglobin.
Principle of Photoacoustic Imaging
Visualization of color

16. 16
General Photoacoustic Wave Equation
Time Domain Photoacoustic
There are two important timescales associated with pulsed laser heating:
• Thermal relaxation time
• Stress relaxation time
The excitation is defined as being thermally or stress confined if the laser pulse
duration is much shorter than the corresponding relaxation time

17. 17
General Photoacoustic Wave Equation
Time Domain Photoacoustic
Short-pulsed Laser-induced Initial Photoacoustic Pressure

18. 18
General Photoacoustic Wave Equation
Frequency Domain Photoacoustic
The analytical description of frequency-domain PAI is most conveniently
formulated by utilizing the Fourier transformation.
The equation in time domain is Fourier transformed into an
inhomogeneous Helmholtz equation.

19. 19
Signal-to-Noise Ratios in the Time and
Frequency Domains
The SNR in PAI is defined as
𝑉𝑡𝑟 𝑡 : the detected PA response voltage from a transducer
< 𝑉𝑁
2
> : Thermal noise
𝑘𝐵: the Boltzmann constant
T : the absolute temperature
R: the loading resistor
𝛥𝑓: the detection bandwidth

20. 20
Signal-to-Noise Ratios in the Time and
Frequency Domains
Assuming that in the frequency domain the PA signals are detected by a phase-sensitive
detector with an equivalent noise bandwidth as narrow as 1 Hz (0.125-s time constant, 12
dB/octave roll-off) and that in the time domain they are detected by a broadband detector
with 100-MHz bandwidth, we have:
with typical excitation and detection parameters, the SNR in the time domain is about four
orders of magnitude (40 dB) higher than that in the frequency domain despite the different
data acquisition times.

21. Photoacoustic Imaging Modalities
21
Acoustic Resolution- Photoacoustic Microscopy (AR-PAM)
Optical Resolution- Photoacoustic Microscopy (OR-PAM)
Photoacoustic computed tomography (PACT)
Photoacoustic endoscopy (PAE)

22. 22
Photoacoustic microscopy (PAM)
The method involving the microscope-type device is a method in
which the living body is irradiated with pulsed laser light and, while the
body surface is scanned, ultrasonic waves generated near the focal
point of an acoustic lens are captured by ultrasonic sensors and the
measured signal is used to form images. It is possible to obtain high-
resolution images by measuring high-frequency ultrasonic waves. The
imaging becomes shallow with a depth of just a few millimeters. In this
program, we use the microscope-type device to image skin tissue.

23. By using a single focused ultrasonic transducer, usually placed confocally
with the irradiation laser beam, PAM forms a 1D image at each position,
where the flight time of the ultrasound signal provides depth information. A
3D image is then generated by piecing together the 1D images obtained
from raster scanning, and thus no inverse reconstruction algorithm is
needed. PAM has two forms, based on its focusing mechanism:
Photoacoustic Microscopy (PAM)
23
Uses:
- Brain Lesion detection
- Hemodynamics monitoring
- Prostate cancer detection
- Gastrointestinal tract endoscopy

24. To release the dependence of resolution on
acoustic frequency, OR-PAM utilizes focused
light to spatially confine the excitation,
resulting in an optical diffraction-limited
resolution in the lateral direction.
Unlike X-ray imaging, OR-PAM can work in:
• Back-reflection mode
• Transmission mode
In both cases, the excitation laser is focused
on an object to excite acoustic waves. To
maximize detection sensitivity, a focused
ultrasonic transducer is adopted and aligned
confocally with the optical lens. .
24
Optical‐Resolution Photoacoustic Microscopy
(OR‐PAM)

25. 25
The lateral resolution of OR-PAM is optically determined by the laser wavelength in
vacuo and the numerical aperture (NA) of the lens:
The axial resolution of OR-PAM is still acoustically determined by the bandwidth B of
an ultrasonic transducer:
Ultimately, the choice of the acoustic bandwidth is based on the desired imaging
depths.
Optical‐Resolution Photoacoustic Microscopy
(OR‐PAM)

26. Compared with conventional optical microscopy
technologies, OR-PAM has an advantage in
providing high contrasts for endogenous
chromophores without staining, enabling label-free
imaging of biological tissue or cells in vivo.
Representative images in Figure show the imaging
capability of OR-PAM at different scales: vasculature
structure in a nude mouse ear (a), melanin in
melanoma cells (b), and cell nuclei (c).
Optical‐Resolution Photoacoustic Microscopy
(OR‐PAM)
26

27. In AR-PAM, rather than light being focused on an optically diffraction-limited spot, a relatively large
area is illuminated. As a result, more laser energy is allowed by the ANSI laser safety standards in
AR-PAM than in OR-PAM, boosting the chance of photons reaching a much greater depth.
The illumination in AR-PAM can be:
• Bright- field (Figure a)
• Dark- field (Figure b)
Acustic‐Resolution Photoacoustic Microscopy
(AR‐PAM)
27
Although the bright-field approach can
deliver higher fluence to a targeted
volume (39), the dark-field method
has an edge in reducing surface
interference to deeper PA signals.

28. Acustic‐Resolution Photoacoustic Microscopy
(AR‐PAM)
28
The spatial resolution of AR-PAM is acoustically limited along all axes. For a focused ultrasonic
transducer with an aperture of diameter D and focal length l, the lateral resolution is computed as:
λ0: the acoustic wavelength.
The axial resolution can still be calculated by the same equation as in OR-PAM:

29. Despite its high spatial resolution and improved imaging speed,
PAM usually has a limited focal depth and is not yet capable of
video‐rate imaging. In contrast, PACT is typically implemented
using full‐field illumination and a multi‐element ultrasound array
system to improve penetration depth and imaging speed,
although some PACT systems use a single‐element transducer
with circular scanning.
PACT uses an array of ultrasonic transducers to detect PA waves
emitted from an object at multiple view angles simultaneously,
allowing a much faster cross-sectional or volumetric imaging
speed at the expense of system and computational costs.
To accurately render an object’s boundaries in 2D PACT, a
detector array must cover at least a π-arc directional view.
Ultrasonic transducer arrays with various populating patterns,
such as line, half ring, full ring, and hemisphere, have been
employed and demonstrated in both animal and clinical
applications.
Photoacoustic Computed Tomography (PACT)
29

30. 30
PAM vs. PACT

31. Even though the penetration depth of PACT can reach several centimeters, internal organs such
as the cardiovascular system and gastrointestinal tract are still not reachable.
Noninvasive tomographic imaging of these internal organs is extremely useful. in clinical practice.
Besides pure optical and ultrasound endoscopy, photoacoustic endoscopy (PAE) is another
promising solution for this clinical need. The key specifications of PAE are:
the probe dimensions and imaging speed.
PAT of internal organs through endoscopy is referred to as PAE. Compared with routinely
used clinical ultrasound endoscopy, PAE offers the same strength in spatial resolution while
providing additional functional information at physiological sites.
PAE features miniaturized optical illumination and acoustic detection components that are
assembled in a compact package.
Photoacoustic Endoscopy (PAE)
31

32. Photoacoustic Endoscopy (PAE)
32
Figure shows a typical setup of a PAE probe,
which has an outside diameter of 3.8 mm. This
probe integrates ultrasonography with PAE and
is designed for gastrointestinal tract imaging.
Circumferential sector scanning is accomplished
by rotating a scanning mirror, which reflects both
ultrasonic waves and illumination laser pulses to
a physiological site. The ultrasonic wave and
pulsed laser are fired sequentially, and the
ensuing ultrasound echoes and PA signals are
collected by the same ultrasonic transducer after
being reflected by the mirror

33. Photoacoustic Resolution
33

34. 34
Photoacoustic imaging is capable of imaging finer blood vessels than other existing
techniques. Compared to other imaging techniques such as ultrasound, MRI, CT, and X-ray
(angiography), a contrast agent is not required. It is a noninvasive technique that performs
measurements using a variable wavelength laser with an intensity that does not harm the
skin. Because photoacoustic imaging irradiates specimens and detects ultrasonic waves
with no X-ray exposure involved, it is possible to repeat the measurements multiple times.
This makes it applicable to a wide range of subjects, ranging from children to the elderly. In
addition, unlike MRI and CT, it does not require shielding against radiation or magnetism,
nor does it require restricted areas, which makes it easier to introduce devices in a clinical
setting.
Advantages of Photoacoustic Imaging
Advantages of the blood vessel imaging technique
Noninvasive / No X-ray exposure

35. 35
Advantages of Photoacoustic Imaging
Photoacoustic Imaging Capabilities

36. 36
Application of Photoacoustic Imaging
.

37. ADVANTAGES
1. Ability to detect deeply situated tumor and its
vasculature
2. Monitors angiogenesis
3. High resolution
4. Compatible to Ultra Sound
5. High penetration depth
6. Non-ionizing/Non-radioactivity
7. Small size
8. Easy to clean and maintenance
9. No acoustic noise
10. No facilities that shield radiation and magnetism
11. No require restricted areas
Photoacoustic Imaging
DISADVANTAGES:
1. Limited path length
2. Temperature dependence
3. Weak absorption at short
wavelengths

38. 1- Hebden JC, Arridge SR, Delpy DT. Optical imaging in medicine: I.
Experimental techniques. Physics in Medicine & Biology. 1997 May;42(5):825.
2- Lüscher E. Photoacoustic Effect Principles and Applications: Proceedings of
the First International Conference on the Photoacoustic Effect in Germany.
Springer Science & Business Media; 2013 Apr 17.
3- De Montigny E. Photoacoustic tomography: principles and applications.
Department of Physics Engineering, polytechnic school Montreal. 2011.
4- Mb-shiran, Ultrasound Waves In Diagnostic And Therapy.
5- Xu M, Wang LV. Photoacoustic imaging in biomedicine. Review of scientific
instruments. 2006 Apr 17;77(4):041101.
38
References

39. 39