Medical imaging lecture notes PDF

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INTRODUCTION TO MEDICAL IMAGING 

Introduction

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Image quality

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Imaging modalities

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Processing & analysis of medical images

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The future of Medical Imaging

By the end of this subject student should be able to: 1. Explain the basic principles of the major medical imaging techniques; 2. Explain the mode of operation and medical applications of the major medical imaging techniques; 3. Understand the advantages and disadvantages of the major imaging techniques, including potential hazards for patients;

Introduction Medical Imaging is the technique and process of creating visual representations (images) of the interior of a body, i.e., of internal structures that are hidden by the skin and bones. It is used for sake of clinical analysis (e.g., to diagnose a disease) and medical intervention (e.g., to treat a disease). It is used to establish a database of normal anatomy and physiology to facilitate the identification of abnormalities.

Types of Medical Imaging 1) Functional Imaging

Functional imaging (or physiological imaging), is a medical imaging technique of detecting or measuring changes in metabolism, blood flow, regional chemical composition, and absorption. As opposed to structural imaging, functional imaging centers on revealing physiological activities within a certain tissue or organ by employing medical image modalities that very often use tracers or probes to reflect spatial distribution of them within the body. These tracers are often analogous to some chemical compounds, like glucose, within the body. To achieve this, isotopes are used because they have similar chemical and biological characteristics. By appropriate proportionality, the nuclear medicine physicians can determine the real intensity of certain substance within the body to evaluate the risk or danger of developing some diseases.

Fig 1: Functional Imaging in movement disorders of the brain

2) Structural Imaging Structural imaging refers to approaches that are specialized for the visualization and analysis of anatomical properties of the organ of interest. To reconstruct the 3-D shape of an internal organ (e.g., to capture tumors, lesions, etc.)

Fig 2: 3D image reconstruct of the heart

Imaging Modalities Modalities can be grouped according to the type of signal used to probe: Signal Electromagnetic Waves

Imaging Modality Radiography Thermography Computerized Tomography (CT) Magnetic Resonance Imaging (MRI) Positron Emission Tomography (PET) Single Photon Emission CT (SPECT)

Ultrasound waves

Doppler Echography Elastography

Image Formation Imaging can be regrouped according to how the image is formed. There are two types namely: 1. Reflection Mode The image is recreated based on the amount of signal reflected by the tissue/organ of interest.

2. Transmission Mode

The image is recreated based on the amount of signal passing through the tissue/organ of interest.

Image Quality Different imaging modalities produce images of different quality. One aspect that affects the choice of one modality over the others is the required image quality. Three parameters define the quality of a medical image: 1. Contrast ⇒ It is the difference between the intensity (or color) of the image and its surrounding.

Fig 3: chest image contrast

2. Resolution ⇒ it is the ability to resolve any two adjacent points of an image. It defines the smallest possible dimensions of a point that can be detected.

Fig 4: Image resolution

3. Signal to Noise ratio ⇒ between the image signal and the noise from the instrument. An estimation of the SNR is the ratio between the mean (µ𝑠𝑖𝑔𝑛𝑎𝑙 ) and standard deviation (σ𝑠𝑖𝑔𝑛𝑎𝑙 ) of the signal. 𝑆𝑁𝑅 =

µ𝑠𝑖𝑔𝑛𝑎𝑙 𝜎𝑠𝑖𝑔𝑛𝑎𝑙

In general, the SNR is expressed in decibels (dB), i.e., it is computed as: µ𝑠𝑖𝑔𝑛𝑎𝑙 𝑆𝑁𝑅 = 20 log 10 ( )𝑑𝐵 𝜎𝑠𝑖𝑔𝑛𝑎𝑙 What is a wave? In physics, mathematics, and related fields, a wave is a disturbance of a field in which a physical attribute oscillates repeatedly at each point or propagates from each point to neighboring points, or seems to move through space. The waves most commonly studied in physics are mechanical and electromagnetic. An oscillation is the repetitive variation, typically in time, of some measure about a central value or between two different states.

Fig: a sinusoidal wave

The time needed to complete one full circle is called a Period(𝑇). Two parameters characterize a wave: 1) Wavelength (ƛ), i.e., the distance between two sequential crests or troughs.

Fig: wavelength

2) Frequency(𝑓) i.e. the number of full cycles completed in one second. Period denoted with (Τ) the period of a full oscillation of a generic particle, it is: 𝑓=

1 𝑇

Frequency is measured in Hz. The speed with which a wave propagates through a medium is: 𝑣 =ƛ×𝑓

Ultrasound waves Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. Ultrasound is used in many different fields. Ultrasonic devices are used to detect objects and measure distances. Ultrasound imaging or sonography is often used in medicine.

Electromagnetic waves

Also EM waves, refer to waves that are created as a result of vibrations between an electric field and a magnetic field. In other words, EM waves are composed of oscillating magnetic and electric fields....