Medical Imaging

Medical imaging is the visualization of the interior of a body. It is the technique and process which seeks to reveal internal structures hidden by the skin and bones for clinical diagnosis, treatment and disease monitoring. There are many types of medical imaging, the main types of imaging used in modern medicine are radiography, magnetic resonance imaging (MRI), nuclear medicine (such as PET and SPECT), and ultrasound.

Radiography
Radiography uses electromagnetic radiation to take images of the internal parts of the body. The most well-known and common form of radiography is x-ray which use ionizing radiation to produce images of the internal structure by sending beams through the body. The X-rays are absorbed by the material they pass through in differing amounts depending on the density and composition of the material (soft tissues do not absorb these waves while objects such as bones do).

Computed Tomography
Computed Tomography (CT) is an imaging technique that combines a series of X-ray images taken from different angles to create cross-sectional internal images, resulting in more detailed images compared to regular X-rays.

Magnetic Resonance Imaging
Magnetic Resonance Imaging (MRI) involves radio waves and magnetic fields to look at the internal structures of the body. The MRI scanner uses powerful magnets to polarize and excite hydrogen nuclei of water molecules in human tissue, producing a detectable signal resulting in images of the body.

Nuclear Medicine Scan
Nuclear Medicine Scan (such as PET and SPECT) involves in the use of radioactive tracers, which are radioactive materials that are injected or swallowed to travel through the digestive or the circulatory system. The radiation produced by the material can then be detected by the use of a special camera (gamma) to take pictures of tissues and organs in the body to observe its activity and function.

Ultrasound Imaging
Ultrasound Imaging uses high frequency sound waves which are reflected off tissue to create images of organs, muscles, joints, and other soft tissues. The light travels through the skin layers which can be viewed by using electronic sensors.

Digital Image
A digital image is a numeric representation of (normally binary) a two-dimensional image. Depending on whether the image resolution is fixed, it may be of vector or raster type. By itself, the term "digital image" usually refers to raster images or bitmapped images. Digital images are made of picture elements called Pixels. Typically, pixels are organized in an ordered rectangular array. The size of an image is determined by the dimensions of this pixel array: The image width is the number of columns, and the image height is the number of rows in the array, thus the pixel array is a matrix of M columns x N rows.

Image Digitization
Digitization is the process used to convert analogue data (such as text, image) into digital form that is represented by Binary Code, through two steps:

Sampling
Analog signal is read at specific frequencies to create sampling of those values at each specific point. The frequency of sampling is referred to as resolution of the image, and sampling turns continuous data (analog) into discrete (digital) data.

Quantization
Each sample is quantified (i.e. assigned a numerical value drawn from a defined range such as 0-255 in the case of an 8-bit grayscale image). By the quantization activity of Digitization, the brightness levels are determined. Any image or audio, like color, projects a signal of its wavelength. The signals are measured through a y=sin(x) graph. It is a mathematical representation that becomes digitized when sampled by a computer.

Image Contrast
Contrast can be defined as the differences in intensities of colors between the pixels in an image. If the differences in intensities are high then that translates to an image having a high contrast (image appears to “pop”), but if the differences in intensities were low then the image would have a low contrast (image appears to be “washed out”).

Function
The histogram is a graph showing the number of pixels in an image that refers to each different pixel intensity value. For an 8-bit grayscale image there are 256 different possible intensities, and so the histogram will graphically display 256 numbers showing the distribution of pixels among those grayscale values. The x-axis of the histogram shows the range of pixel values. Since it’s an 8 bpp image that means it has 256 levels of gray or shades of gray in it. That’s why the range of x-axis starts from 0 and end at 255 with a gap of 50. Whereas on the y axis, is the count of these intensities.

Applications
•	Image processing

•	Brightness adjustments

•	Equalization of an image

•	Thresholding (mostly used in computer vision)

Image Formats
Some of the more common examples of digital image formats are JPEG, TIFF, and PNG. JPEG Supports the 8-bit grayscale and 24-bit color images. TIFF is an adaptable file format that can save 8 or 16-bits per color for 24 and 48-bit totals. PNG which supports 8-bit paletted images and 24 and 48-bit TrueColor.

Most of the Digital Image Formats can be divided by their compression method:

Loseless
No information is lost from the digital image file when a compression algorithm is applied to it. Includes: RAW, TIFF, PNG and BMP. Those image formats store the full RGB digital image without any loss of information. This comes with the advantage of permitting high quality reproductions but at the price of requiring a lot of memory to save those files.

Lossy
Results in the loss of some image data to achieve a smaller file size. Includes: JPEG, GIF. JPEG achieves compression by removing some color and image details. GIF compression is achieved by limiting the total number of colors allowed to be present in the image file.

PACS
PACS (Picture Archiving and Communication System) is a medical imaging technology which provides economical storage and convenient access to images from multiple modalities. Electronic images and reports are transmitted digitally via PACS, and eliminates the need to manually file, retrieve, or transport film jackets. The universal format for PACS image storage and transfer is DICOM.

DICOM
DICOM (Digital Imaging and Communications in Medicine) is a file format definition and network communications protocol for handling, storing, printing, and transmitting information in medical imaging. The communication protocol uses TCP/IP to communicate between systems. DICOM files can be exchanged between two entities that are capable of receiving image and patient data in DICOM format. DICOM enables the integration of medical imaging devices (like scanners, servers, workstations, printers, network hardware, and PACS) from multiple manufacturers. The different devices come with DICOM Conformance Statements which clearly state which DICOM classes they support. DICOM has been widely adopted by hospitals and is making inroads in smaller applications like dentists' and doctors' offices.

Components
PACS consists of four major components:

• Imaging modalities (such as X-ray, CT, MRI)

• A secured network for the transmission of patient information

• Workstations for interpreting and reviewing images

• Archives for the storage and retrieval of images and reports. Combined with available and emerging web technology, PACS has the ability to deliver timely and efficient access to images, interpretations, and related data. PACS breaks down the physical and time barriers associated with traditional film-based image retrieval, distribution, and display.

Applications
PACS has four main uses:

• Hard Copy Replacement - PACS replaces hard-copy based means of managing medical images, such as film archives. With the decreasing price of digital storage, PACS provide a growing cost and space advantage over film archives in addition to the instant access to prior images at the same institution.

• Remote Access - It expands on the possibilities of conventional systems by providing capabilities of off-site viewing and reporting, and enables practitioners in different physical locations to access the same information simultaneously.

• Electronic Image Integration Platform - PACS provides the electronic platform for radiology images interfacing with other medical automation systems such as Hospital Information System (HIS), Electronic Medical Record (EMR), Practice Management Software, and Radiology Information System (RIS).

• Radiology Workflow Management - PACS is used by radiology personnel to manage the workflow of patient exams.