Applied measurement techniques

Applied measurement techniques

Course information

Course of Applied Measurement Techniques, Prof. Lorenzo Scalise, a.a. 2016-2017. Notes by students: L. A. Pettinari, A. Tigrini.

Index

• Ultrasound imaging
• General concepts ............................................................................................... 6
• Basic physics of sound ...................................................................................... 11
• Transducers and probes ................................................................................... 21
• Ultrasound modalities ...................................................................................... 33
• Doppler ultrasound .......................................................................................... 41
• Image artefacts ................................................................................................ 55
• X-ray imaging
• General concepts ............................................................................................. 58
• Emission mechanisms ...................................................................................... 61
• X-ray system .................................................................................................... 68
• Development of the image ............................................................................... 82
• Fluoroscopy ..................................................................................................... 89
• Digital X-ray imaging ........................................................................................ 95
• Computed tomography
• General concepts ........................................................................................... 102
• Principles of tomography ............................................................................... 103
• Back-projection algorithm .............................................................................. 109
• CT generations ............................................................................................... 111
• Components of a modern CT .......................................................................... 116
• Nuclear medical imaging
• General concepts ........................................................................................... 122
• Basic physics of radioactivity .......................................................................... 124
• Radiation detectors ........................................................................................ 128
• Gamma camera .............................................................................................. 132
• Emission Computed Tomography ................................................................... 136
• Radiotherapy
• General concepts ........................................................................................... 143
• Internal radiotherapy ..................................................................................... 144
• External Radiotherapy .................................................................................... 147
• Ablation
• General concepts ........................................................................................... 168
• Ablation techniques ....................................................................................... 168
• Laser ablation
• General concepts ........................................................................................... 181
• Properties and classification ........................................................................... 183
• Interaction with tissues .................................................................................. 186
• Applications and safety .................................................................................. 187
• Endoscopy
• General concepts ........................................................................................... 193
• Endoscope ..................................................................................................... 193
• Endoscopic ultrasounds .................................................................................. 197

Ultrasound imaging

General concepts

Ultrasound imaging is one of the most diffused imaging techniques. Basic concepts of ultrasounds are based on the branch of physics called acoustic. In imaging diagnostics, ultrasounds (US) are thought not to travel in air but directly in tissues, interacting with them. There are many different ultrasound techniques: echography, Doppler ultrasound, harmonic imaging, etc.

Only 150 years ago, there was no technology to investigate inside the body without directly cutting the outermost tissues: medical doctors at that time were forced to use surgery and other invasive methods to see inside the patient and speculate a diagnosis on that. Nowadays, imaging allows us to investigate inside the body without surgery, being considered thus a very useful and relatively new area. As a definition, medical imaging concerns everything that allows the collection of information from the insides of the human body. This diagnostic tool requires quite always a physical quantity with which the body interacts, modifying and changing characteristics of the source: the measurement process consists of the detection of these changes.

Sources (and their related imaging techniques) are: X-rays (radiography, CT), (PET, SPECT), radiofrequency EM waves (MRI, fMRI, much less damaging than the-rays previous). X-rays and are invasive techniques, because of the ionizing nature of these radiation rays, thus they represent a potential risk to the patient: it is possible to see inside, by paying the risk of damages to cell and DNA (this is the reason why X-rays and must be used only if strictly required). Medicine research is earmarking capitals and putting a lot of attention to seek for an accurate trade-off between the need of a given imaging technique and the cost related to it in terms of technology, maintenance of the machineries, specialized personnel and possible risks to patients.

To have an idea of this issue, not all the tissues can be seen well enough by every imaging techniques: for instance, X-rays do not discern fairly enough soft tissues, so that damages to the muscles are not well investigated with CT, while US are better suited for this purpose. US are getting always more attention from diagnostic applications and the techniques related are very largely diffused because they use acoustic waves. They are pressure waves, the same used to produce human voice, but at different frequencies: they have totally different features from EM waves, which can travel in empty very quickly (propagation velocity in empty is 10⁸ m/s), while US need to have oscillating particles to propagate. A very important characteristic of these acoustic waves is no damages for tissues, except the fact that high intensity ones can damage the hearing apparatus (intensity up 120 dB is painful for our ears). US frequency range is higher than 20 kHz, out of audible range (2-20 kHz), versus X-rays frequency range that is 10¹⁸-10²⁰ Hz.

The frequency range of acoustic waves is represented in the picture below:

While for EM waves, the frequency range is in a very much wider interval, starting from extremely low frequencies (1 Hz) to extremely high (γ-rays reach 10²⁴ Hz).

For US, the fundamental two equations are:

• c = λf
• v = fλ

Thus, in a given medium, frequency and wavelength are inversely proportional by the propagation velocity which is constant if the medium in which the wave is traveling does not change. As a rule of thumb, the higher the frequency, the smaller the wavelength. The amplitude is the maximum value of the physical quantity involved in the undulatory phenomenon (in acoustic waves this quantity is pressure, in EM waves is the electromagnetic field, and so on). Intensity is directly proportional to the power of a wave source by the surface in which the wave front diffuses in each instant of its propagation: this entails that the intensity depends on the property of propagation of a wave. Lenses are devices capable of reducing the traveling section of the wave, focusing the intensity being equal to the power of the source.

Referring to amplitude, intensity, and power of a generic wave, these are frequently expressed using the decibel (dB) unit, which relates how these quantities increase or attenuate with respect of a fixed value:

• Intensity (dB) = 10 log(I/I₀)
• Amplitude (dB) = 20 log(A/A₀)

As already noted, ionizing radiation is not very good at distinguishing differences in soft tissues, such as exploring the liver searching for stones or cancer, while they cannot be used on the fetus at all. On the other hand, for both latter purposes, US are very suitable, being widely employed to detect different muscle structures; besides ultrasound imaging techniques have the great quality to provide imaging even if they are in motion. In fact, US are used to measure flows of interest inside the fetus through the Doppler effect. US have a low penetration, so they are not suitable to explore profoundly the body, while on the other hand X-rays cover every distance due to their extremely high frequency.

Tissues are composed of atoms, thus if the wavelengths are not small enough, they are rebounded by the atomic structure, while small ones will penetrate deep without reflection. Another aspect is that the US intensity is very low: this fact diminishes again the (already low) risks of exposing. As a consequence of the fact that US have a relatively large wavelength, they undergo significant reflection phenomena: that means that part of the wave will travel inside the tissues, and in part will be reflected. This is a positive aspect because it is possible to yield observations just from one side, by launching a pulse wave and waiting for the pulse back, caused by partial reflection from tissues, coming back from the target. This same aspect does not hold as well for X-rays, where the radiations are launched and then detected (along with their changes) on the opposite side.

This property of US is also known as back reflection: the same probe works in transmitting and receiving, while in X-rays it is necessary to launch the pulse and have the receiver on the other part. A general outline of the ultrasound imaging system is the following:

From this, it is possible to distinguish at least two objects: the probe, which is transmitting and receiving back US to the tissues, and a processing unit, capable of elaborating the signals into diagnostic images. High voltage generator block is the circuitry where the electronic signal is shaped to drive the transducer in generating US, which in turn is transmitted to the tissues. Some energy of the pulse will be reflected as echo waves, bouncing back with different characteristics with respect to the generated ones. The same transducer is then "hearing" what is echoed from the target (as a microphone, which transduces the mechanical wave into voltage), sending the detected signals to the DSP unit able to build up from it the diagnostic image: this is the echographic information back from the target.

As a general principle, almost used in other devices like radars or sonars, there is the launching of a quantity of energy and then the measure of the time taken to be rebounded. If the velocity is known, then from the time interval measurement is known also the distance of the obstacle. Nevertheless, it is not just a matter of calculating the distance of the obstacle: It is also possible to get information about the size of the obstacle with more transmitters, and from reflection properties, it is possible to get information also on the material or density of the obstacle.

Going back to the problem of liver inspection, with this technique it is possible to assess how dense is the tissue and understand if this condition is normal or pathological. Thus, US provide tissue information about their location, size, and density, making it explicit in the image with pixel location and color value, ranging in a greyscale where at lower densities is assigned black while at higher ones is assigned white. This three information can be collected by this imaging technique, and this unique characteristic is barely shared with other imaging techniques.

Besides, due to their properties of propagation, acoustic waves have wave fronts not only in the longitudinal direction of the source but often also in the transversal ones, so there is the possibility to investigate tissues also laterally, even if in these directions waves are rapidly attenuated, so it is preferable to use always longitudinal wave fronts from the source with respect to the target to yield the measurements.

Acoustic waves are a rarefaction and compression of air particles, which cause a chain effect, allowing the acoustic wave to travel through a medium, and their intensity decays with distance due to the spreading of the traveling section as the wave diffuses. The transmission of US is not made in a continuous fashion but much rather modulated as a set of pulses transmitted and then received back attenuated, measuring the time delay (then the distance if the velocity is known). As a rule of thumb, a small wavelength is preferable because it increases the spatial resolution of the imaging system. Ultrasound frequency is greater than the audible threshold (20kHz) and typically for diagnostic purposes the range is 1-20 MHz. For example, at 3.5 MHz, the wavelength is 0.44 mm, a good value for almost any tissue investigation and that is because it is a typical resolution; when searching for something very small it is possible to choose a 7.5 MHz probe, which has 0.2 mm resolution (double the resolution of the latter case).

Notice from the above picture: the higher the frequency, the higher the spatial resolution and the attenuation, hence the lower the penetration, allowing only superficial exploration of inside tissues.

Consider also that none of the emitting systems in the market are working at one precise frequency, the so-called nominal frequency. The probe rather works in a bandwidth, producing signals with frequency contents within a frequency interval almost centered in the nominal frequency. Probes with high frequency (6-15 MHz, for superficial exams) are used in dermatology and endoscopy, while lower ones are suitable for investigation in cardiac muscles, skeletal muscles or foetal echography (1-3.5 MHz, 3.6-6 MHz, for abdominal or cardiac exams).

In this latter case, transducers are placed on the mother's belly and the US are launched: when these waves are passing through the tissues, they are partially reflected. The higher the reflection, the higher the scale to white. In the picture, it is possible to see brightly the scalp of the fetus that is a very high-density tissue. Underneath the scalp, the image is quite confused because all the energy is locally back-reflected by the superior layer (the scalp). The area close to the probe is receiving and scattering the highest intensity, so if the goal of the imaging session is the fetus, it must be calculated a proper intensity to reach it. Observe that the downstream area has the worst SNR, due to multiple undulatory phenomena, confusing or interfering with a proper back reflection. It is important to underline that ultrasound imaging techniques are the only recommended for checking the health of a fetus during pregnancy, for their non-invasiveness and low risk.

The propagation velocity of US is different with respect to the mean. In air, it is more or less 334 m/s while in water it is about three times more. Notice the propagation velocity of a PTZ, piezoelectric crystal: 3791 m/s, almost ten times the air value. This crystal has a special property: when compressed, it creates electron charges free to move and, on the reverse, when a voltage is applied to its ends, then it compresses itself, changing dimension. Thus, this material is capable of generating a pressure wave every time it changes its dimensions, resulting in a temporary compression and rarefaction of the surroundings at a frequency instructed by a specific electric circuit which dictates the oscillating voltage; therefore, all ultrasound probes have a set of piezo crystals. In every soft tissue of the human body, US travel more or less at the same speed, but for example, lungs have low speed because they are filled with air. Since ultrasound imaging system is based on back reflection, and this physical phenomenon in turn relies on the change of velocity of propagation of the interfacing layers, when two similar comparators (like air and lungs) meet, they little reflect and there is no sensible imaging of them.

Basic physics of sound

Basic physics of undulatory phenomena are needed to understand what happens to pressure waves traveling inside the tissues. A wave traveling into a medium has interactions with this one, many of them resulting in a loss of energy it carries; in particular, US waves tend to spread over the propagation front, causing additional attenuation. In the most general scenario, to any kind of wave is associated a partition of its intensity by three main phenomena:

• Reflection. It is the bouncing of a wave front at an interface between two different media so that the wave front returns into the medium from which it comes. The law of reflection says that for specular reflection the angle at which the wave is incident on the surface equals the angle at which it is reflected. Reflection is fundamental for US because the rebounded pulse is the object of the measurement system.
• Transmission. It is the property of a substance to permit the passage of a wave, with some or none of the incident wave being absorbed in the process. For this reason, it is considered the opposite phenomenon...