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Introduction to Medical Imaging

1.1 Medical Imaging - An Insight

As per the World Health Organization, medical imaging is related to the various imaging modalities and connected processes to obtain images from inside the human body, which is extremely useful for diagnostic as well as therapeutic applications. Medical imaging is preferred during a clinical examination for various disorders. It also refers to the process and the protocols involved with the development of visual representation of the anatomical as well as the physiological aspects of the body often useful for diagnostic and therapeutic applications. Different compositions and structures of the muscles and bones are visualized so as to identify any plausible anomalies in the body. The most common aspect of medical imaging is the radiology and hence, medical imaging with respect to clinical applications is termed as radiographic imaging, and the related science of study is called radiology. X-rays, computed tomography, Catheter laboratories, Ultrasound, Positron emission tomography, single photon emission computed tomography, Magnetic resonance imaging, and Functional magnetic resonance imaging are a few examples of radiological imaging principles used for medical diagnosis in the field of medicine. Although there exist numerous approaches to diagnose different pathological aspects in the human body, this book shall be confined to radiological imaging modalities alone.

The medical imaging equipment are conventionally equipped with a source which would emanate the signals and a detector to detect the strength of these rays passing through the body or being reflected by the body. Based on predefined information about the nature of the different parts of the body, in terms of the ability to absorb or reflect these rays, an image of the corresponding body is developed. However, with the advent of technology, medical imaging systems have slowly evolved from being mere hardware setup, to a stage driven more by electronics and software-based technologies. Modern medicine hence prefers such approaches for better clinical diagnosis as well as treatment.

The main advantage of medical imaging is the ability of the equipment to provide a pictorial representation of the internal organs of the body without even accessing the organ, in a noninvasive approach. This makes the medical imaging techniques one of the most preferred approaches for the diagnostics of different anatomical as well as physiological disorders in the human body. Almost every clinician depends on these approaches for diagnostic applications. A large aspect of this is attributed to the mathematical approaches incorporated in the processing of the signals acquired. The observed signal is subjected to different reconstruction algorithms so as to arrive at a meaningful image of the region of interest. Such images provide a plethora of information to the clinician to help plan the line of treatment for different disorders. This makes medical imaging the most important aspect of diagnostic equipment and it is of great demand in the healthcare industry.

From a clinical perspective, Visible light-based photographic approaches are useful in the field of dermatology as well as wound care. However, certain equipment use the light from a spectrum which is not visible to the human eye and are often related to radiology, and the images obtained are interpreted by a radiologist, with a medical background. This is called as diagnostic radiology and is often known to be the field of medicine which uses noninvasive imaging approaches for the diagnosis of the health condition of an individual.

1.2 Types of Diagnostic Imaging Modalities

While numerous modalities are being used at different levels in the field of diagnostic radiology, this book shall be confined to a few important modalities which are very commonly used in every clinical facility for diagnostic applications. Figure 1.1 below provides a pictorial representation of the different types of medical imaging modalities.

1.2.1 Radiography

Radiography is an imaging technique using the properties of X-rays, gamma rays and similar ionizing/non-ionizing radiations so as to obtain a visual description of a given organ. Conventional radiography includes the generation of X-rays by a suitable X-ray source and projected towards the body. This was the first-ever approach in modern science to image the body part using noninvasive approaches. Some quantity of these X-rays are absorbed by the body, based on the structure and density of the body. The rest of the X-rays pass through the body and are detected by a detector placed behind the body. Detectors can either be photographic or digital in operation. In the case of photographic detectors, commonly called as films, these X-rays would create an image called the X-ray image highlighting the structural aspects of the region of interest. In the case of digital detectors, the X-rays detected are transformed into signals and processed for further applications.

Figure 1.1 Types of medical imaging modalities (Categories of medical image modalities based on the type of information that they provide about the organ being imaged Source: https://www.researchgate.net/publication/260124237_Developing_Advanced_Mathematical_Models_for_Detecting_Abnormalities_in_2D3D_Medical_Structures/figures?lo=1

There are two approaches in radiography, projection radiography and fluoroscopy. While projection-based approaches are used for conventional diagnostic imaging, fluoroscopy is more of a catheter guidance-based system. Of late, 3D volumetric reconstruction techniques have been successful in the provision of a better insight into the region of interest. However, 2D scans are still common due to cost effectiveness and resolution.

Conventional projection radiography, known as X-ray imaging, is used to visualize the hard structures of the human body, specifically, bone fractures. With the usage of contrast agents such as those of barium, X-ray imaging is also useful in applications such as the diagnosis of ulcer and certain types of cancers. Shown in Figure 1.2 below are the pictorial representations of the same.

Fluoroscopy is useful in the production of real-time images of the internal organs of the body, at a lower intensity of X-rays. Usage of contrast agents make it possible to obtain a visual cue of the organ being probed into. Hence this is extremely useful in image guides procedures which need a constant visual feedback while a medical process such as an angioblast is being performed.

1.2.2 Tomography

Tomography refers to the process of obtaining an image of an organ based on the cross sectioning of the same. This involves methods such as those of X-ray computed tomography (CT), Computed Axial Tomography (CAT) and Positron Emission Tomography (PET).

Figure 1.2 Mobile X-ray machine (Skanray technologies, SKANMOBILE). Source: https://www.skanray.com/?q=content/skanmobile

Figure 1.3 X-ray image The Antero Posterior X-Ray showing the magnification of heart and widening of the mediastinum. Source: https://iem-student.org/how-to-read-chest-x-rays/

Figure 1.4 Fluoroscopy system (Discovery IGS 730, GE healthcare). Source: https://www.medicalexpo.com/prod/ge-healthcare/product-70717-647375.html

Figure 1.5 Fluoroscopy procedure (X-ray Barium enema showing normal colon mucosa. Image Credit: Richman Photo/Shutterstock). Source: https://www.news-medical.net/health/Fluoroscopy-Procedure.aspx

CT involves an X-ray source which is used to obtain the X-ray images of the ROI at different angles of projection thereby arriving at multiple X-ray images of the ROI from different angles of view. These images are processed using different tomographic algorithms so as to arrive at a reconstructed imager, which provides a cross sectional view of the ROI. The entire process is software oriented and includes multiple mathematical approaches as well. Figure 1.6 below shows a CT machine and a CT image as well.

1.2.3 Ultrasound

Ultrasound imaging in clinical applications incorporates the usage of high-frequency broadband acoustic waves (in the range of Mega Hertz) which are incident on the ROI and are reflected/transmitted and are utilized for the imaging of the corresponding ROI. While the other medical imaging modalities discussed in this chapter are a part of the electromagnetic spectrum of light (light waves) such as X_rays, Gamma rays, etc., ultrasound is the only modality which is sound based and not light oriented. Real-time imaging functionality of ultrasound makes it more preferable for the imaging of the fetus, heart, arteries and veins. This does not involve any ionizing radiation and is hence considered to be the safest of all modalities. This makes it a preferable imaging for pregnant women as well.

Figure 1.6 CT machine (GE Optima CT 660) (128 slice full body tomography CT scanner). Source: https://www.medicalexpo.com/prod/ge-healthcare/product-70717-428667.html

Figure 1.7 CT Image (Normal chest CT - lung window). Source: https://radiopaedia.org/cases/normal-chest-ct-lung-window-1?lang=us

A predefined set of high-frequency sound waves are generated using an ultrasound generator, mostly a piezoelectric crystal and is incident on the ROI. This signal is attenuated as it passes through the ROI and returned at...