Principles and Components of CT PDF

# Principles and Components of CT ## Principles and Components of CT - **with a lossless compression scheme they retain the original image quality and often are large in size. [13] TIFF is preferred where high image quality is desired, for example, when the image contains illustrations and line di...

# Principles and Components of CT ## Principles and Components of CT - **with a lossless compression scheme they retain the original image quality and often are large in size. [13] TIFF is preferred where high image quality is desired, for example, when the image contains illustrations and line diagrams.** - **GIF (Graphics Interchange Format): GIF is an old file format that is compatible with older versions of internet browsers and other software. A major advantage of this format is its ability to save animations. It can store only a limited amount of color information and is becoming increasingly unpopular as a format for storing digital images.** - **PNG (Portable Networks Graphics): The PNG file format was developed to outperform and eventually replace the GIF format. It has better browser compatibility and supports greater color depth than the GIF format. Its lossless compression enables better image quality, though at the expense of large file sizes.** - **Field of View (FOV)** - **The field of view (FOV) is defined as the dimensions of the exact anatomic region included in a scan. In MR, the FOV may be square or asymmetric. Depending on the vendor, it is specified in millimeters or centimeters. The FOV is also the mathematical product of the acquisition matrix and the pixel dimensions. For example, if 512 readout and 256 phase-encoding steps are specified in a scan for which the pixel dimensions are chosen to be 0.45 x 0.9 mm, the FOV would be 512 x 0.45 mm 230 mm by 256 x 0.9 mm 230 mm (and thus in this instance a square FOV, despite the use of a rectangular pixel). Head imaging is typically performed today with a FOV of 230 mm or less, to achieve high in-plane spatial resolution. Depending on body habitus, the FOV for a scan of the upper abdomen may be as large as 400 mm** ## Pixel - **A pixel (or pel or picture element) may refer to either the smallest discrete element of the physical display or to the smallest element of the image. Voxel is its 3-dimensional equivalent, as employed in CT and other cross-sectional imaging modalities.** ## History and etymology - **The history of the term pixel is long and complex. A pixel is a portmanteau of the shortening of the word's 'picture' and 'element'. Picture element as a phrase is first seen in a publication in 1927, one of the first articles about the new technology of television 1!** - **The use of the word 'pix' as a shortening of picture was already in common usage in 1934, when it is found in Variety, the American film industry magazine.** - **Pixel does not appear in print until 1965, in a paper presented at SPIE (Society of Photo-Optical Instrumentation Engineers) by an engineer working at the Jet Propulsion Laboratory (JPL) called Fred Crockett Billingsley (1921-2002) 2,3. It's very first use in speech is not known, but it was clearly already being used in engineering circles in 1964 3.** - **The term 'pel' (sometimes PEL) as a contraction of 'picture element' has also been used previously as another synonym for pixel but outside specialized fields has never caught on and it is not used in medical imaging.** - **The term pixel was commonly appearing in the radiology literature in the 1970s with the advent of CT.** ## Voxel - **Voxel: is a portmanteau of contractions of the two words 'volume' and 'element' and was coined as a 3-D equivalent of a pixel. It is an individual point in space on a 3-dimensional, regular matrix. The location of each voxel is encoded by its relative relationship to other voxels.** ## Matrix: - **A matrix is an array of numbers in rows and columns. The horizontal lines in matrices are called rows and the vertical lines are called columns. A matrix with m rows and n columns is called an m-by-n matrix (or mon matrix) and m and n are called its dimensions. The matrix used in computed tomography determines the scan resolution.** - **Matrices are useful to record data that depend on two categories, and to keep track of the coefficients of systems of linear equations and linear transformations.** # Principles and Components of CT - **There are several situations where DICOM files find their way into radiology practice. One common example of this is when information from a radiological study is exported into an offline medium such as a compact disk (CD) for easy transport or archival. Such a CD usually contains several DICOM image files as well as other files that are necessary for display of these images. Even though the specific folder architecture varies from vendor to vendor, the CDs usually contain an autorun file, a DICOM viewer, a DICOM directory (DICOMDIR), and a folder containing the DICOM images** ## Principles and Components of CT - **The common tags that indicate the patient identity include the patient's name, age, sex, birth date, hospital identity number, ethnic group, occupation, referring physician, institution name, study date, and DICOM Unique Identifiers (UIDs). As described earlier, such demographic information of the patient and a host of other information about the imaging study is encoded within an image header. The data may or may not be displayed on the screen, but the information can be extracted from the header by anyone who has access to the DICOM file. Several educational resources using DICOM files are available for radiology students on the World Wide Web. Creating and accessing such electronic teaching files often involve transmission of DICOM data over the Internet. In the interest of patient confidentiality, all information identifying the patient should be removed from the DICOM header when a DICOM file is uploaded for such purposes.** - **Respecting the patient's privacy is important when images are used in presentations, teaching files, or publications. A simple and easy method of ensuring this is by converting and exporting the DICOM file into other image formats such as JPEG or TIFF. The header information is lost and patient identity cannot be obtained from the resultant image. Another method is "anonymization," whereby all patient information is removed from the DICOM header. This is achieved by using software like DicomWorks, ImageJ, and FP Image. Specifically, all tags contained in groups "0008" (study information) and "0010" (patient information) of the DICOM header should be removed and replaced during anonymization.** - **Although DICOM images have found wide acceptance in medical practice, they have two disadvantages: file sizes are large and special software is required for viewing them on personal computers. Outside the radiology department, most personal computers run on the Windows® operating system, which does not recognize the DICOM file structure. Thus, for incorporating images in PowerPoint presentations, for creating teaching files, or for publishing in Web pages, DICOM images need to be converted into image formats that can be recognized by Windows®.** - **There are over a hundred formats described to store images, most of them being proprietary. The more popular formats used in daily practice are the JPEG, JPEG 2000, TIFF, GIF, and PNG formats. In contrast to DICOM images, images saved in these formats can be viewed on any personal computer without the need for dedicated viewers. They can be easily incorporated into presentations and Web pages. Image files saved in these formats are devoid of bulky header information and usually contain 8-bit information. These files therefore require less storage space and demand less resource to transfer over a network or via the Internet. One big disadvantage of these file formats, compared to DICOM, is that they contain a user-determined window level and window width that is set at the time of creation of the image. Consequently, the contrast between structures within the image cannot be adjusted and postprocessing cannot be performed on these images.** - **JPEG (Joint Photographic Experts Group): The JPEG format is the most popular format and can be read by all computer platforms. Because JPEG files are small in size and extremely portable, they are the preferred format when transferring images over the Web. The advantage of the JPEG format is that it facilitates use of compression to reduce file size. Typically, the least noticeable bits of information are removed by complex mathematical algorithms, so that the image is represented with less information. When saving as a JPEG file, options are available for selecting the amount of compression that can be applied. The more the file is compressed (lossy compression), the more the original image information lost; such an image will not look good when reproduced. Lesser degrees of compression (lossless compression) retain high image quality, but this is achieved at the cost of a large file size.** - **TIFF (Tagged Image File Format): The TIFF format is versatile and supports the full range of image sizes, resolutions, and color depths. Since TIFF images are saved without compression or with a lossless compression scheme they retain the original image quality and often are large in size.** - **All modalities in radiology practice have become digital, and therefore deal with DICOM images. Image files that are compliant with part 10 of the DICOM standard are generally referred to as "DICOM format files" or simply "DICOM files" and are represented as ".dcm." DICOM differs from other image formats in that it groups information into data sets. A DICOM file consists of a header and image data sets packed into a single file. The information within the header is organized as a constant and standardized series of tags. By extracting data from these tags one can access important information regarding the patient demographics, study parameters, etc. In the interest of patient confidentiality, all information that can be used to identify the patient should be removed before DICOM images are transmitted over a network for educational or other purposes. In addition to the DICOM format, the radiologist routinely encounters images of several file formats such as JPEG, TIFF, GIF, and PNG. Each format has its own unique advantages and disadvantages, which must be taken into consideration when images are archived, used in teaching files, or submitted for publication. Knowledge about these formats and their attributes, such as image resolution, image compression, and image metadata, helps the radiologist in optimizing the archival, organization, and display of images. This article aims to increase the awareness among radiologists regarding DICOM and other image file formats encountered in clinical practice. It also suggests several tips and tricks that can be used by the radiologist so that the digital potential of these images can be fully utilized for maximization of workflow in the radiology practice** - **we deal with DICOM (digital imaging and communications in medicine) image files sourced from different modalities, either in a standalone or integrated manner. DICOM files have several unique features, the knowledge of which is important for the practicing radiologist. This article aims to increase the awareness of radiologists regarding DICOM and other image files so that all their features can be fully exploited.** - **The DICOM standard is useful for integrating all modern imaging equipment's, accessories, networking servers, workstations, printers, and picture archiving and communication systems (PACS) that may have been installed by multiple manufacturers. [1] Because of its ease of integration and continuous evolution this communication standard has over the years achieved a nearly universal level of acceptance among vendors of radiological equipment.** - **A DICOM image file is an outcome of the Digital Imaging and Communications in Medicine standard. Specifically, image files that are compliant with part 10 of the DICOM standard are generally referred to as "DICOM format files" or simply "DICOM files" and are represented as ".dcm".** - **The window width determines the number of Hounsfield units represented on a specific image. The software assigns shades of gray to CT numbers that fall within the range selected. All values higher than the selected range appear white, and any value lower than the range appears black. By increasing the window width, usually referred to as "widening the width," more numbers are assigned to each shade of gray.** - **Using a simplified scenario to demonstrate gray scale and window width, assume that we have 10 shades of gray available. We have selected 300 as our window width. Therefore, only 300 (of the more than 2,000 possible den sity values in our scale) will be represented on the image as a shade of gray. All others will be either black or white. In this example, 30 different Hounsfield units will be grouped together and represented by each shade of gray in the image (Fig. 4-2).** - **If the window width is set at 300, which 300 Hounsfield values, from all those possible, will be shown? Now that we have selected the quantity of Hounsfield units to be displayed by selecting the window width, we now need to determine the range of values to display.** ## Window Level - **The window level selects the center CT value of the window width (Fig. 4-3). The terms window level and win- dow center are often used interchangeably. The window level selects which Hounsfield numbers are displayed on the image. Answering the question posed in the previous para graph, the particular Hounsfield units to be included in our image are entirely dependent on the window level selected. If O is chosen as the window level, the Hounsfield values that are represented as a shade of gray on this image will range from-150 to 150 (Fig. 4- 4). Now assume the width stays unchanged at 300, but the center is moved to 200. Determining the range of Hounsfield values requires only simple arithmetic.** - **The software assigns shades of gray to CT numbers that fall within the range selected. All values higher than the selected range (in the current example, 350) will appear white, and any value lower than 50 will appear black (Fig. 4-6). If we increase the window width, a wider range of values will be included in the grayscale range; more values will be assigned to each shade of gray (Fig. 4-7).** - **The window level should be set at a point that is roughly the same value as the average attenuation number of the tissue of interest. For example, a window level setting that is intended to display lung parenchyma will be approximately 600 because air-filled lung tissue measures around -600 HU. The manipulation of window width and window level to optimize image contrast is referred to as windowing.** ## 2.6 Gray Scale - **In an ideal world, the image would be displayed with a different shade of gray for each Hounsfield unit represented. However, although there are more than 2,000 different Hounsfield values, the monitor can display only 256 shades of gray. Even more limiting, the human eye can differentiate only a fraction of those shades-typically fewer than 40. As a general rule, the human eye cannot appreciate contrast differences of less than about 10, whereas CT scanners can easily demonstrate differences of less than 1. To overcome these inherent limitations, a gray scale is used in image display. In this system a display processor assigns a certain number of Hounsfield units (HU) to each level of gray. The number of Hounsfield units assigned to each level of gray is determined by the window width.** - **As was explained in Chapter 1, the Hounsfield scale assigns 0 to the density of water. Correspondingly.** ## Format of the CT image: - **All modalities in radiology practice have become digital, and therefore deal with DICOM images. Image files that are compliant with part 10 of the DICOM standard are generally referred to as "DICOM format files" or simply "DICOM files" and are represented as ".dcm." DICOM differs from other image formats in that it groups information into data sets. A DICOM file consists of a header and image data sets packed into a single file. The information within the header is organized as a constant and standardized series of tags. By extracting data from these tags one can access important information regarding the patient demographics, study parameters, etc. In the interest of patient confidentiality, all information that can be used to identify the patient should be removed before DICOM images are transmitted over a network for educational or other purposes. In addition to the DICOM format, the radiologist routinely encounters images of several file formats such as JPEG, TIFF, GIF, and PNG. Each format has its own unique advantages and disadvantages, which must be taken into consideration when images are archived, used in teaching files, or submitted for publication. Knowledge about these formats and their attributes, such as image resolution, image compression, and image metadata, helps the radiologist in optimizing the archival, organization, and display of images. This article aims to increase the awareness among radiologists regarding DICOM and other image file formats encountered in clinical practice. It also suggests several tips and tricks that can be used by the radiologist so that the digital potential of these images can be fully utilized for maximization of workflow in the radiology practice** - **we deal with DICOM (digital imaging and communications in medicine) image files sourced from different modalities, either in a standalone or integrated manner. DICOM files have several unique features, the knowledge of which is important for the practicing radiologist. This article aims to increase the awareness of radiologists regarding DICOM and other image files so that all their features can be fully exploited.** - **The DICOM standard is useful for integrating all modern imaging equipment's, accessories, networking servers, workstations, printers, and picture archiving and communication systems (PACS) that may have been installed by multiple manufacturers. [1] Because of its ease of integration and continuous evolution this communication standard has over the years achieved a nearly universal level of acceptance among vendors of radiological equipment.** - **A DICOM image file is an outcome of the Digital Imaging and Communications in Medicine standard. Specifically, image files that are compliant with part 10 of the DICOM standard are generally referred to as "DICOM format files" or simply "DICOM files" and are represented as ".dcm".** - **-1,000 HU represents air and 1,000 HU represents a dense material such as bone. Values higher than 2,000 HU repre- sent very dense materials, such as metallic dental fillings. By convention, the gray scale assigns higher HU values lighter shades of gray, whereas lower values are repre- sented by darker shades.** - **FIGURE 4-2 The display processor assigns a group of Hounsfield unit to each shade of gray. In this simplified illustration, 10 different shades are available to display the 300 Hounsfield units in the window width.** - **FIGURE 4-1 The effect of window settings on image appearance. A. This lung window provides good lung detail, but the mediastinum is completely white. B. The same slice displayed in a soft-tissue window provides good mediastinal detail, but the lungs are completely black.** - **In an ideal world, the image would be displayed with a different shade of gray for each Hounsfield unit represented. However, although there are more than 2,000 different Hounsfield values, the monitor can display only 256 shades of gray. Even more limiting, the human eye can differentiate only a fraction of those shades-typically fewer than 40. As a general rule, the human eye cannot appreciate contrast differences of less than about 10, whereas CT scanners can easily demonstrate differences of less than 1. To overcome these inherent limitations, a gray scale is used in image display. In this system a display processor assigns a certain number of Hounsfield units (HU) to each level of gray. The number of Hounsfield units assigned to each level of gray is determined by the window width.** - **As was explained in Chapter 1, the Hounsfield scale assigns 0 to the density of water. Correspondingly.** - **-1,000 HU represents air and 1,000 HU represents a dense material such as bone. Values higher than 2,000 HU repre- sent very dense materials, such as metallic dental fillings. By convention, the gray scale assigns higher HU values lighter shades of gray, whereas lower values are repre- sented by darker shades.** - **FIGURE 4-2 The display processor assigns a group of Hounsfield unit to each shade of gray. In this simplified illustration, 10 different shades are available to display the 300 Hounsfield units in the window width.** - **FIGURE 4-1 The effect of window settings on image appearance. A. This lung window provides good lung detail, but the mediastinum is completely white. B. The same slice displayed in a soft-tissue window provides good mediastinal detail, but the lungs are completely black.** - **In an ideal world, the image would be displayed with a different shade of gray for each Hounsfield unit represented. However, although there are more than 2,000 different Hounsfield values, the monitor can display only 256 shades of gray. Even more limiting, the human eye can differentiate only a fraction of those shades-typically fewer than 40. As a general rule, the human eye cannot appreciate contrast differences of less than about 10, whereas CT scanners can easily demonstrate differences of less than 1. To overcome these inherent limitations, a gray scale is used in image display. In this system a display processor assigns a certain number of Hounsfield units (HU) to each level of gray. The number of Hounsfield units assigned to each level of gray is determined by the window width.** - **As was explained in Chapter 1, the Hounsfield scale assigns 0 to the density of water. Correspondingly.** - **-1,000 HU represents air and 1,000 HU represents a dense material such as bone. Values higher than 2,000 HU repre- sent very dense materials, such as metallic dental fillings. By convention, the gray scale assigns higher HU values lighter shades of gray, whereas lower values are repre- sented by darker shades.** - **FIGURE 4-2 The display processor assigns a group of Hounsfield unit to each shade of gray. In this simplified illustration, 10 different shades are available to display the 300 Hounsfield units in the window width.**