Medical Imaging Technology II HMI 2102 PDF

Medical Imaging Science Medical Imaging Technology II HMI 2102 2. CLO1 (Part II): Computed Radiography Thursday, September 26, 2024 Objectives At the completion of this lecture, you should be able to d...

Medical Imaging Science Medical Imaging Technology II HMI 2102 2. CLO1 (Part II): Computed Radiography Thursday, September 26, 2024 Objectives At the completion of this lecture, you should be able to do the following: 1. Describe several advantages of computed radiography over screen/film radiography. 2. Identify workflow changes when computed radiography replaces screen-film radiography. 3. Discuss the relevant features of a storage phosphor imaging plate. 4. Explain the operating characteristics of a computed radiography reader. 5. Discuss spatial resolution, contrast resolution, and noise related to computed radiography. 6. Identify opportunities for patient radiation dose reduction with computed radiography. 2 Outline The Computed Radiography Imaging Characteristics Image Receptor - Image Receptor Response - Photostimulable Luminescence Function - Imaging Plate - Image Noise - Light Stimulation–Emission Patient Characteristics The Computed Radiography - Patient Radiation Dose Reader - Workload - Mechanical Features - Optical Features - Computer Control 3 Introduction Digital imaging began with computed tomography (CT) and magnetic resonance imaging (MRI). Digital radiography was introduced in 1981 by Fuji with the first commercial computed radiography (CR) imaging system. Today medical imaging is complemented by multiple forms of digital radiography (DR) in addition to CR. At this time, CR is the most widely used DR modality. Although other DR systems are increasingly in use, it seems there will always be a need for CR because of its unique properties. Much of the information relevant to CR applies also to DR because CR is a form of DR. 4 Sequence of activity for screen-film radiography CR imaging eliminates some of these steps and can produce better medical images at lower patient dose. 5 The Computed Radiography Image Receptor Similarities between screen-film ”cassette” imaging and CR imaging. 1. Both use as the image receptor an x-ray–sensitive plate that is encased in a protective cassette. 2. The two techniques can be used interchangeably with any x-ray imaging system. 3. Both produce a latent image that must be made visible via processing. 6 Photostimulable Luminescence and Photostimulable Phosphors Some materials emit light promptly following x-ray exposure. - These are called photostimulable phosphors (PSP). - E.g., barium fluorohalide with europium (BaFBr:Eu or BaFI:Eu) PSP also emit light some time later when exposed to a different light source. - This process is called photostimulable luminescence (PSL). A note on Europium (Eu) - Present in very small amounts. - It is an activator and is responsible for the storage property of the PSL. - Without it there would be no latent image. 7 Cont’d Over time, the metastable electrons return to the ground state on their own. This can be accelerated or stimulated by exposing the phosphor to intense infrared light from a laser. 8 Storage Phosphor Screens The PSP, barium fluorohalide, is fashioned similarly to a radiographic intensifying screen. Because the latent image occurs in the form of metastable electrons, such screens are called storage phosphor screens (SPSs). SPSs are mechanically stable, electrostatically protected, and fashioned to optimize the intensity of stimulated light. 9 Cont’d PSP particles are either: - randomly positioned throughout a binder (3–10 μm), or - grown as linear filaments that enhance the absorption of x-rays and limit the spread of stimulated emission. 10 Imaging Plate The PSP screen is housed in a rugged cassette that appears similar to a screen-film cassette. In this form as an image receptor, the PSP screen- cassette is called an imaging plate (IP). The IP has lead backing that reduces backscatter x-rays. - This improves the contrast resolution of the image receptor. 11 Light Stimulation–Emission Light is emitted when an PSP crystal is illuminated. The sequence of events engaged in producing a PSL signal include: 1. Exposure. 2. Stimulation. 3. Reading. reader 4. Erasing. 12 1. Exposure When an x-ray beam exposes a PSP, the energy transfer results in excitation of electrons into a metastable state. Approximately 50% of these electrons return to their ground state immediately, resulting in prompt emission of light. The remaining metastable electrons return to the ground state over time. - This causes the latent image to fade. - Requires that the IP must be read soon after exposure. - CR signal loss is objectionable after approximately 8 hours. 13 2. Stimulation A finely focused beam (50 to 100 μm) of infrared light (laser) is directed at the PSP. - The diameter of the laser beam determines the spatial resolution of the CR imaging system. As laser beam intensity increases, so does the intensity of the emitted signal. Note that as the laser beam penetrates, it spreads. - The amount of spread increases with PSP thickness. 14 3. Reading The laser beam causes metastable electrons to return to the ground state with the emission of a shorter wavelength light in the blue region of the visible spectrum. Through this process, the latent image is made visible. Some signal is lost as the result of: 1. Scattering of the emitted light. 2. The collection efficiency of the photodetector. Photodiodes (PDs) are the light detectors of choice for CR. 15 4. Erasing The stimulation cycle does not completely transition all metastable electrons to the ground state. If residual latent image remained, ghosting could appear on subsequent use of the IP. Any residual latent image is removed by flooding the phosphor with very intense white light from a bank of specially designed fluorescent lamps. Optical filters are necessary to allow only emitted light to reach the photodetector while blocking the intense stimulating light. 16 The Computed Radiography Reader Commercial CR readers. 17 Mechanical Features When the CR cassette is inserted into the CR reader, the IP is removed and is fitted to a precision drive mechanism. - This drive mechanism moves the IP constantly yet slowly (“slow scan”) along the long axis of the IP. - Small fluctuations in velocity can result in banding artifacts. 18 Banding artifact Mechanical Features While the IP is being transported in the slow scan direction, a deflection device (a rotating polygon or an oscillating mirror) deflects the laser beam back and forth across the IP. - This is the fast scan mode. 19 Optical Features Components of the optical subsystem include: 1. Laser. 2. Beam-shaping optics. 3. Light-collecting optics. 4. Optical filters. 5. Photodetector. 20 Optical Features The laser is the source of stimulating light. The laser spreads as it travels to the deflection device. Focused by a lens system. 21 Cont’d As the laser beam is deflected across the IP, it changes size and shape. Special beam-shaping optics keeps the beam size, shape, speed, and intensity constant. Emitted light from the IP is channeled into a funnel-like fiber-optic collection assembly and is directed at the photodetector. 22 Cont’d Before photodetection occurs, the light is filtered so that none of the long-wavelength stimulation light reaches the photodetector. - Improves the signal-to-noise ratio. 23 Computer Control The output of the photodetector is a time-varying analog signal. The signal is transmitted to a computer system. The analog signal is processed for amplitude, scale, and compression. - This shapes the signal. Then the analog signal is digitized. - Sampling. - Quantization. An image buffer (hard disc) stores the completed image temporarily until it is transferred to a workstation or to an archival computer. 24 Imaging Characteristics The four principal characteristics of any medical image are: 1. Spatial resolution. 2. Contrast resolution. 3. Noise. 4. Artifacts. These are different for all DR, including CR from screen-film imaging (discussed in greater depth later). Other imaging characteristics include: 5. Image receptor response function. 6. Image noise. 25 Image Receptor Response Function Characteristic curve in S/F = Image receptor response function in DR and CR. Radiographic technique is so critical in S/F imaging. - The response of S/F extends through an optical density (OD) range from 0 to 3 (i.e., 1000 gray levels). - However, the S/F image can display only approximately 30 shades of gray on a viewbox. - Most S/F imaging techniques aim for radiation exposure on the toe side of the characteristic curve. 26 Cont’d CR imaging is characterized by extremely wide latitude. Five decades of radiation exposure results in almost 100,000 gray levels. - Each gray level can be evaluated visually by postprocessing. - A 14-bit CR image has 16,384 gray levels. 27 Cont’d Proper radiographic technique and exposure are essential for S/F radiography. - Overexposure and underexposure. With CR, radiographic technique is not as critical because contrast does not change over five decades of radiation exposure. The conventional approach that “kVp controls contrast” and “mAs controls OD” does not hold for CR. 28 Image Noise 29 Cont’d Fortunately, CR noise sources are bothersome only at very low image receptor radiation exposure. Newer CR systems have lower noise levels and therefore additional patient radiation dose reduction is possible. 30 Patient Characteristics Include: A. Patient radiation dose B. Workload 31 Patient Radiation Dose At low radiation exposure CR is a faster image receptor than S/F system. - i.e., lower patient radiation dose should be possible with CR. - However, noise is an issue at lower radiographic technique (discussed later). Because CR image contrast is constant regardless of radiation exposure, images can be made at higher kVp and lower mAs, resulting in additional reduction in patient radiation dose. 32 Cont’d A useful rule of thumb is that current “average” S/F exposure factors represent the absolute maximum factors for the body part in CR. 33 Workload The transition from S/F radiography to CR brings several significant changes. 1. Wide exposure latitude  Fewer repeat examinations. 2. Improved contrast resolution. 3. Reduced patient radiation dose. 4. No need to reload the cassette  lower workload. 34 Summary The first applications of DR appeared in the early 1980s as CR. CR is based on the phenomenon of PSL. X-rays interact with an SPS and form a latent image by exciting electrons to a higher energy metastable state. In the CR reader the latent image is made visible by releasing the metastable electrons with a stimulating laser light beam. On returning to the ground state, electrons emit shorter wavelength light in proportion to the intensity of the x-ray beam. The emitted light signal is digitized and reconstructed into a medical image. The value of each CR pixel describes a linear characteristic curve over five decades of radiation exposure and a 100,000 grayscale. This wide latitude can result in reduced patient radiation dose and improved contrast resolution. 35 Abbreviations 36  Thank You 800 MyHCT (800 www.hct.ac.ae 69428)