Photoacoustic imaging

Photoacoustic imaging is a scanning technique that has been under increasing development in the past decade. It is based on the photoacoustic effect, which is where sound waves are formed following the absorption of light in a material. [1]

How the photoacoustic effect works

The photoacoustic effect can be broken down into four steps:

An object absorbs light, which is a form of energy
The absorbed energy is converted into heat (another form of energy) which causes the material to heat up
The material expands as a result of the heating
The expansion produces sound, specifically ultrasound (sound of a frequency too high for the human ear to hear)

Energy transfers of photoacoustic effect:

Light energy absorbed by tissue

Heat energy, heats up tissue

Kinetic energy, causes tissue to expand

Sound energy, emitted from the tissue

Figure 1: Flowchart showing the energy transfers that occur during the photoacoustic effect

Depending on what material the object is, it will produce ultrasounds at different frequencies. This is because different materials have different optical properties, i.e. they absorb light at different wavelengths. Certain wavelengths correspond to certain energies this will lead to a certain sound being emitted (according to Figure 1).

Why does heating an object cause it to expand?

All objects are made up of particles such as atoms or molecules. When we heat up an object, the particles within that object start moving around more, and so they take up more space. Therefore, the object expands.

In a similar way, if we cool down an object, it can become smaller!

What is the frequency of a sound?

The frequency of a sound tells us how quickly the soundwaves are moving. Slow-moving soundwaves produce low-pitched sounds, fast-moving soundwaves product high-pitched sounds.

How photoacoustic imaging is carried out

A laser pulse is delivered into the body. Laser if a form of light that is very focused.

The photoacoustic effect occurs when the laser is absorbed by the human tissue. The ultrasound that is emitted from the tissue is picked up by a device known as an ultrasonic transducer. The transducer changes the sound energy to electrical energy, and allows the frequency of the ultrasound to be obtained. We can use just one, or many ultrasonic transducers. Different numbers of transducers are used for different applications. [2] [3]

For more information about how ultrasonic transducers work, visit Ultrasound - Advanced:

Once the frequency of the photoacoustic emission is known, the material of the tissue can be determined.

Types of photoacoustic imaging

There are two main types of photoacoustic imaging: photoacoustic microscopy (PAM) and photoacoustic tomography (PAT).

Photoacoustic microscopy (PAM)

A single-element ultrasound transducer is used. This is a transducer that converts one source of sound energy to electrical energy at a time.

This is a direct image formation: a mapping of the varying frequencies within the scanned region is produced very simply.

Photoacoustic tomography (PAT)

A multi-element ultrasound transducer is used. This is a transducer that converts multiple sources of sound energy to electrical energy.

This is a reconstructive image formation: the signals received from all the transducer elements are merged in order to reconstruct an image.

What is tomography?

This is the process of obtaining a 2D image of a slice (tomos means 'to slice' in Greek) within a 3D object.

Figure 1: Photoacoustic computed tomography scan showing the vascular system of a mouse

Source: ncbi.nlm.nih.gov

Applications of photoacoustic imaging

Brain lesion detection

Damaged tissue within the brain will have different optical properties to healthy brain tissue. As a result, emitted ultrasound frequencies vary between the tissues and the lesions can be clearly identified by photoacoustic imaging.

Haemodynamics monitoring

Haemoglobin (Hb) is the chemical in red blood cells that is responsible for holding oxygen. When haemoglobin is carrying oxygen, it is referred to as oxy-haemoglobin; when haemoglobin doesn't contain any oxygen, it is referred to as deoxy-haemoglobin. [4]

Since oxy- and deoxy-haemoglobin are chemicals which absorb light within the visible range, multiple wavelength measurements can be used to determine the relative concentration of these using photoacoustic imaging.

From this, the following can be determined:

Total concentration of haemoglobin
Haemoglobin oxygen saturation (the proportion of oxygen in haemoglobin)
Changes in blood flow to/within the brain associated with brain function

What is a brain lesion?

A lesion is an area of tissue that has been damaged by injury / disease. A brain lesion is a lesion in the brain.

How much haemoglobin do we have in our bodies?

The average adult human has between 20 to 30 trillion red blood cells in their body.

Each red blood cell contains approximately 270 trillion haemoglobin molecules.

So in the average adult human there are

6750000000000000000000000000 haemoglobin molecules!

If you'd like to learn more about the physics behind how photoacoustic imaging works, try the advanced-level description by clicking this button:

Photoacoustic Imaging - Advanced

References:

[1] Xia, J. et al., 'Photoacoustic tomography: principles and advances' Electromagnetic Waves Cambridge, 2014; 147: 1–22.

[2] https://en.wikipedia.org/wiki/Photoacoustic_effect#Photothermal_mechanism [Accessed: 26/02/19]

[3] Rosencwaig, A. 'Photoacoustics and photoacoustic spectroscopy', New York: John Wiley & Sons, 1980

[4] https://www.verywellhealth.com/importance-of-hemoglobin-2249107 [Accessed: 27/02/19]