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Introduction to medical imaging Li SHAO DONG Department of Radiology, Affiliated Hospital of Xuzhou Medical College X-RAY IMAGING CONVENTIONAL RADIOGRAPHY (X-RAYS; PLAIN FILMS) X-rays are a form of electromagnetic radiation. The frequency and energy of X-rays are much greater than visible light. X-RAY IMAGING They are produced in an X-ray tube by focusing a beam of high-energy electrons on to a tungsten target. They are able to pass through the human body and on to X-ray film thus producing an image. X-ray film are held in cassettes of varying size depending on the part of the body to be examined. X-RAY IMAGING After an X-ray exposure is made the films are processed in a darkroom or more commonly in free-standing daylight processors. The resulting image is commonly known as an ‘X-ray’. The common terms ‘chest X-ray’ and ‘abdomen X-ray’ are widely accepted and commonly abbreviated to CXR and AXR, respectively. More correct terms for an X-ray image are ‘radiograph’ or ‘plain film’. 锁骨 肩胛骨 肋骨 椎体 diaphysis medullary cavity compact bone metaphysis Epiphysis epiphyseal plate periosteum X-RAY IMAGING As a beam of X-rays passes through the human body some of the X-rays are absorbed or scattered producing reduction or attenuation of the beam. The degree of X-ray beam attenuation is largely dependent on the density and atomic number of the various tissues. Tissues of high density cause more X- ray beam attenuation. X-rays turn X-ray film black. X-RAY IMAGING Therefore, less-dense tissues and structures appear darker than tissues of highter density. Similarly, materials of high atomic number cause more X-ray beam attenuation than those of low atomic number. Five principal densities are recognized on plain radiographs. They are listed here in order of increasing X-RAY IMAGING 1. Air/gas: black, e.g. lungs, bowel and stomach. 2. Fat: dark grey, e.g. subcutaneous tissue layer, retroperitoneal fat. 3. Soft tissues/water: light grey, e.g. solid organs, heart, blood vessels, muscle and fluid-filled organs such as bladder. 4. Bone: off-white. 5. Contrast material/metal: bright white. 锁骨 肩胛骨 肋骨 椎体 正常后前位胸片 (图) X-RAY IMAGING An object will be seen with conventional radiography if its borders lie beside tissue of different density. For example, the heart border is seen because it lies against aerated lung, which is less dense. When lung consolidation occurs, such as in pneumonia, the lung density approaches that of soft tissue. X-RAY IMAGING Consolidated lung lying against the heart border will therefore obscure that border. A good example is consolidation or collapse of the right middle lobe causing loss of definition of the right heart boder. These comments apply to all radiographically visible anatomical interfaces in the body. Right upper lobe consolidation Density in the projection of right upper lung field Upper lobe distribution No significant loss of lung volume Air bronchogram 大叶性肺炎 (图) 右下肺密度增高 影,水平叶间裂 显示清楚 Note forward movement of left oblique fissure in the lateral view. X-RAY IMAGING FLUOROSCOPY Fluoroscopy refers to the technique of examination of the anatomy and motion of internal structures by a constant stream of Xrays. The term ‘fluoroscopy’ is derived from the ability of X-ray to cause fluorescenc. 人工对比 X-RAY IMAGING The original fluoroscopes were rather primitive and consisted of an X-ray tube, fluorescent screen and X-ray table. The radiologist directly viewed the image on the fluorescent screen. The images were very faint; examinations were performed in a darkened room by a radiologist with darkadapted vision. Dark-adaptation was achieved by wearing red goggles for X-RAY IMAGING Fluoroscopy was revolutionized in the 1950s by the development of the image intensifier. The image intensifier converts X-ray into images that are usually viewed via a closed circuit television chain. Images may be recorded as X-ray spot films performed during screening or electronically from television cameras in digital format. X-RAY IMAGING Uses of fluoroscopy include: 1. Barium studies of the gastrointestinal tract. 2. Angiography and interventional radiology. 3. General surgery (operative cholangiography, colonoscopy, etc.) 4. Orthopaedic surgery: reduction and fixation of fractures, joint replacements, etc. 5. Airway screening in children for tracheomalacia, and diaphragm screening. X-RAY IMAGING DIGITAL SUBTRACTION IMAGING Digital subtraction imaging (DSI) is a process whereby a computer removes unwanted information from a radiographic image. It is particularly useful for angiography, referred to as DSA. X-RAY IMAGING COMPUTED AND DIGITAL RADIOGRAPHY Diagnostic imaging is currently undergoing a digital revolution. Radiographic images may now be produced digitally using one of two processes, computed radiography (CR) and digital radiography (DR). Both methods use an X-ray tube, as described above. Instead of using X-ray film, CR employs cassettes that contain a photostimulable phosphor. X-RAY IMAGING After the X-ray exposure is performed the cassette is inserted into a laser reader. A fine laser beam passes across the phosphor in the cassette dislodging light photons. The number of photons dislodged is in proportion to the amount of X-rays that have hit the phosphor. X-RAY IMAGING An analogue-digital converter (ADC) produces a digital image. Digital radiography uses a detector screen containing silicon detectors. There detectors produce an electrical signal when exposed to X-rays. This signal is analysed to produce a digital image. X-RAY IMAGING Computed radiography is generally more portable and versatile than DR. The latter is most widely used in mammography and dental radiography. Both methods remove the need for the chemicals used in processing X-ray films. More important are the many inherent advantages of digital imaging. X-RAY IMAGING There include the ability to perform various manipulations on the images after they have been taken, including magnification of areas of interest, alteration of density and accurate measurements of distances and angles. X-RAY IMAGING PICTURE ARCHIVING AND COMMUNICATION SYSTEMS Many hospital X-ray departments now employ large computer storage facilities and networks known as picture archiving and communication systems (PACS). Images obtained by CR and DR may be stored digitally, removing the need for bulky X-ray packets and large X-ray storage rooms in hospital. The PACS also allow instant recall and display of a patient’s radiographs and scans. These can be displayed on monitors in the wards or theatre as required. X-RAY IMAGING CONTRAST MATERIALS The ability of conventional radiography and fluoroscopy to display a range of organs and structures may be enhanced by the use of various contrast materials. The most common contrast materials are based on barium or iodine. Barium and iodine are high atomic number materials that strongly absorb X-rays and are therefore seen as dense white on radiography. 人工对比 X-RAY IMAGING GASTROINTESTINAL CONTRAST MATERIALS Contrast materials may be swallowed or injected via nasogastric tube to outline the upper gastrointestinal tract and small bowel, or may be introduced via an enema tube to demonstrate the large bowel. Gastrointestinal contrast materials are usually based on barium, which is non-water-soluble. X-RAY IMAGING Occasionally a water-soluble contrast material based on iodine is used for imaging of the gastrointestinal tract. A single contrast barium study is one where a hollow viscus such as the stomach or bowel is filled with barium. The outline of the organ can be appreciated, although not its mucosal surfaces. If gas is then used to dilate the organ, the mucosal surfaces can be seen coated with barium. This is ‘double contrast’. 胃 • 分部(胃底fundus、胃体body、胃窦antrum、大弯 greater curvature、小弯lesser curvature) X-RAY IMAGING The majority of barium meals and enemas are performed in double contrast as it provides much better mucosal detail than single contrast. For double contrast barium meals gas-forming compounds are swallowed along with the barium. In double contrast barium enemas air is pumped into the bowel after coating of the mucosal surfaces with barium. Single contrast studies using barium only may be performed in children, and occasionally in the very elderly. X-RAY IMAGING IODINATED CONTRAST MATERIALS Water-soluble contrast materials may be injected into veins, arteries, and various body cavities and systems. The radiographic contrast of these water-soluble contrast materials is based on the high atomic number of iodine. These compounds are therefore known as iodinated contrast materials. Iodinated contrast materials are used in radiography to visualize various body systems and organs as follows: X-RAY IMAGING 1. Arteries: injection into arterial system – arteriography or angiography. 2. Kidneys, ureters and bladder: intravenous injection followd by renal excretion – intravenous pyelography (IVP). 3. Joints: injection into various joints including shoulder, hip and knee - arthrography. 4. Outline of nerve roots and spinal cord: injection into thecal sac – myelography. 5. Salivary glands: injection into salivary gland duct – sialography. IVP