Applied measurement techniques

Applied

Measurement

Techniques

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Course of Applied Measurement Techniques,

Prof. Lorenzo Scalise, a.a. 2016-2017.

Notes by students: L. A. Pettinari, A. Tigrini.

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INDEX

ULTRASOUND IMAGING ........................................................................................ 6

General concepts ............................................................................................... 6

Basic physics of sound ...................................................................................... 11

Transducers and probes ................................................................................... 21

Ultrasound modalities ...................................................................................... 33

Doppler ultrasound .......................................................................................... 41

Image artefacts ................................................................................................ 55

X-RAY IMAGING .................................................................................................. 58

General concepts ............................................................................................. 58

Emission mechanisms ...................................................................................... 61

X-ray system .................................................................................................... 68

Development of the image ............................................................................... 82

Fluoroscopy ..................................................................................................... 89

Digital X-ray imaging ........................................................................................ 95

COMPUTED TOMOGRAPHY ............................................................................... 102

General concepts ........................................................................................... 102

Principles of tomography ............................................................................... 103

Back-projection algorithm .............................................................................. 109

CT generations ............................................................................................... 111

Components of a modern CT .......................................................................... 116

NUCLEAR MEDICAL IMAGING ............................................................................ 122

General concepts ........................................................................................... 122

Basic physics of radioactivity .......................................................................... 124

Radiation detectors ........................................................................................ 128

Gamma camera .............................................................................................. 132

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Emission Computed Tomography ................................................................... 136

RADIOTHERAPY ................................................................................................. 143

General concepts ........................................................................................... 143

Internal radiotherapy ..................................................................................... 144

External Radiotherapy .................................................................................... 147

ABLATION ......................................................................................................... 168

General concepts ........................................................................................... 168

Ablation techniques ....................................................................................... 168

LASER ABLATION ............................................................................................... 181

General concepts ........................................................................................... 181

Properties and classification ........................................................................... 183

Interaction with tissues .................................................................................. 186

Applications and safety .................................................................................. 187

ENDOSCOPY ...................................................................................................... 193

General concepts ........................................................................................... 193

Endoscope ..................................................................................................... 193

Endoscopic ultrasounds .................................................................................. 197

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ULTRASOUND IMAGING

General concepts

Ultrasound imaging is one most diffused imaging techniques. Basic concepts of ultrasounds are based on the

branch of physics called acoustic. In imaging diagnostic, 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 any 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 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 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 consist 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,

-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

-rays

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 term 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

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can travel in empty very quickly (propagation velocity in empty is 10 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

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range that is 10 -10 Hz. The frequency range of acoustic wave is represented in the picture below:

While for EM waves, the frequency range is in a very much wider interval, starting from extremely low

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frequencies (1 Hz) to extremely high (-rays reach 10 Hz).

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For US, the fundamental two equations are:

= =

Thus, in 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 is the frequency, the smaller is 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 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. Lens are devices capable to reduce the traveling section

of wave, focusing the intensity being equal 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:

= 20 log

(12) 9: :

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= 10 log

(12) 9:

:

As already said, ionizing radiation are not very good to distinguish differences in soft tissues, such as exploring

the liver searching for stones or cancer, while they cannot be used on foetus at all. On the other hand, for

both latter purposes US are very suitable, being widely employed to detect different muscle structure;

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 foetus 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 by atoms, thus if the

wavelengths are not small enough, they are rebounded by the atomic structure, while small ones will

penetrate in 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 mean 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, 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 to elaborate 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 are 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 quantitative 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 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

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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 colour 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 wave 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 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, small wavelength is preferable because it

increase 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

=

wavelength is 0.44 mm, a good value for almost any tissue investigation and that is because it is a typical

resolution; when they are searching something very small it is possible to choose a 7.5 MHz probe, which has

0.2 mm resolution (double of the resolution of the latter case).

Notice from the above picture: the higher is the frequency, the higher is the spatial resolution and the

attenuation, hence the lower is the penetration, allowing only superficial exploration of inside tissues.

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Consider also that none of the emitting systems in 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 centred in . Probes with high frequency (6-15 MHz, 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, 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 is the reflection the higher is the scale

to white. In the picture, it is possible to see brightly the scalp of the foetus 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 foetus, 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 foetus during pregnancy, for their non-invasiveness and low risk.

The propagation velocity of US is different with respect to the mean. In air is more less 334 m/s while in water

is about three time 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 to generate 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 human body, US travel more less the same speed, but for example lungs

have low speed because they are filled of 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 comparts (like air and lungs) meet, they little reflect and there is no sensible imaging of

them. 10

Basic physics of sound

Basic physic of undulatory phenomena are needed to understand what happens to pressure waves travelling

inside the tissues. A wave travelling 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.

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• 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

phenomeno