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Basics of musculoskeletal radiography and radiology

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Abstract

Radiography is an excellent tool to generate diagnostic information in cases where either a skeletal lesion or systemic disease with skeletal manifestation is suspected. The fact that radiographic equipment is available in almost all veterinary practices makes it the imaging method most often used initially to detect and characterize lesions of bones and joints. This chapter covers radiography techniques and the basics of musculoskeletal radiography.

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Figures

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1.2 CR image reading devices. Numerous types of systems are on the market. They differ in size, the number of stackers and other technical features such as matrix and scanning speed. (a) Single-cassette reader. (b) Four-cassette reader.
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1.3 Types of CR systems. (a) Single-side readout system. In ‘conventional’ systems, during the readout process, a laser beam scans the image on a single side of the plate pixel by pixel. The photostimulated luminescence is proportional to the absorbed radiograph intensity. The output of the photomultiplier is logarithmically amplified and subsequently digitized by an analogue–digital converter. The phosphor layer has a grainy structure. To avoid excessive light spread that results in increasing intrinsic ‘unsharpness’, the thickness of the layer is limited. (b) Dual-side reading system. In comparison with ‘conventional’ single-side readings, in dual-side reading systems a transparent support is used that allows light to pass through this layer of the image plate. The phosphor layer is thicker. This system results in improved quantum efficiency without a loss of resolution in comparison with single-side reading systems. (c) Line scanning system. In contrast to flying-spot scanners (a–b), in this system a line scanner is used. In this approach, an entire line is illuminated with a set of stimulation sources (e.g. a row of solid-state laser diodes). The light from this line is read by an array of photodetectors. The stimulation sources, light-collecting optics, photodetectors and other technical components are contained in a scan head that is as wide as the screen. Therefore, the screen surface can be scanned while moving the scan head along the image plate. Because of the needle structure of the image plate, a thicker phosphor layer can be used, resulting in a higher DQE and improved resolution. CCD = charge-coupled device. (Reproduced and modified from Clinical technique: digital radiography in exotic pets – important practical differences compared with traditional radiography. , 71–79, with permission from Elsevier)
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1.4 DR system. The wireless detector is stored in a drawer under the tabletop. The grid is positioned in a slot between the tabletop and the detector. If the grid is not required for a radiograph it can be removed. Alternatively, the detector can be positioned directly on the tabletop.
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1.5 Types of flat-panel detector. (a) A direct flat-panel detector uses a semiconductor material layered between two electrodes, and electron/hole pairs are directly produced as a result of local radiograph energy absorption. A high-voltage bias placed between the electrodes separates the charge pairs with little or no lateral spread, allowing for high intrinsic spatial resolution. (b) An indirect flat-panel detector has a scintillator to convert absorbed energy into visible light. Thus, sensitivity (DQE) is high. The photodiode layer/electrode on the surface of the array produces photo-induced charge within each detector element and the resultant charge is stored in the local TFT. aSe = amorphous selenium; aSi = amorphous silicon; CsI = caesium iodide; GdOS = gadolinium-oxide sulfide. (Reproduced from Clinical technique: digital radiography in exotic pets – important practical differences compared with traditional radiography. , 71–79, with permission from Elsevier)
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1.6 Image processing. Multiscale processing was used to create three different versions of the same image data. One out of several parameters of this filter, namely frequency enhancement, has been changed stepwise. The modification resulted in a changed appearance of image details such as bone contour and implant surface.
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1.7 Ideally, a workstation should be equipped with a minimum of two side-by-side display monitors and a typing monitor. Two monitors are required because it is necessary to display multiple radiographs simultaneously at an adequate size. If only one monitor is used the images can be displayed either consecutively or side-by-side in a smaller format.
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1.8 Characteristic curves. The characteristic curve of a digital detector is linear. The high dynamic range means that a wide range of attenuation differences are registered and subsequently can be displayed. The system can compensate for vast exposure differences. In contrast, screen–film systems have a sigmoid-shaped curve with ‘toe’ (too bright), linear and ‘shoulder’ (too dark) regions. The dynamic and dose ranges are narrow. (Reproduced from Clinical technique: digital radiography in exotic pets – important practical differences compared with traditional radiography. , 71–79, with permission from Elsevier)
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1.10 DQE of various detectors. The DQE describes how well a detector is able to transform radiation into image information. A detector that outputs the same signal-to-noise ratio as it receives as input will have a DQE of 100%. If the output has a higher signal-to-noise ratio than the input, the DQE is less than 100%. An ideal system has a DQE of 100% for structures of any size. The DQE of any real imaging system is always below 100% and DQE generally decreases with increasing spatial frequency. Hence it becomes more difficult to maintain the incident signal-to-noise ratio at higher spatial frequencies (i.e. in regions of fine image detail). (Data adapted from ; reproduced from Clinical technique: digital radiography in exotic pets – important practical differences compared with traditional radiography. , 71–79, with permission from Elsevier)
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1.11 MTF of various detectors for unprocessed digital images. Modulation is a measure of contrast. MTF communicates how well a system ‘transfers’ contrast at ever-increasing levels of detail. The usual trend will be for loss of contrast between object and background with increasing levels of image detail (decreasing modulation with increasing spatial frequency). Ideally, the curve is a horizontal line at a modulation of 1 (equivalent to 100% retention of contrast). In reality, because of technical factors, there is a loss of contrast between objects and background, which is greater for smaller structures (higher spatial frequencies, higher lp/mm) in comparison with larger structures (lower spatial frequencies, lower lp/mm). As a result, the MTF progressively decreases with increasing spatial frequency. The curves of digital systems end at the Nyquist frequency value determined by the detector element size. For comparison, MTF curves of screen–film systems of two speed classes (100, 400) are shown. (Data adapted from ; reproduced from Clinical technique: digital radiography in exotic pets – important practical differences compared with traditional radiography. , 71–79, with permission from Elsevier)
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1.12 Image quality in digital radiography. The pelvic radiographs of a young Pug were acquired by the use of two different CR systems. Identical exposure settings were applied. There are significant differences in image quality. (a) The image is of poor, non-diagnostic quality. The grainy appearance, caused by insufficient detector performance, inadequate signal processing or a combination of these factors, hampers the evaluation of bone. (b) This CR system has adequate performance, allowing the evaluation of subtle changes such as the bony structure of the femoral head and neck.
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1.13 (a–h) Common artefacts in digital radiography. CR = computed radiography; DR = direct radiography; LUT = look-up table.
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1.14 Positioning aids such as foam wedges, sandbags, tie-downs and troughs are essential prerequisites to achieve good image quality. For practical reasons it is advisable to keep these aids next to the X-ray machine. A mobile table can be used to store the objects.
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1.15 Influence of positioning on the visibility of a lesion. (a) This cat had been involved in an accident and was physically restrained for the radiograph. Because of improper positioning the fracture of L5 (arrowed) can be easily overlooked. (b) The image was retaken with the cat under general anaesthesia. The lesion then became obvious.
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1.16 Indirect lymphangiography of the pelvic limb in two large-breed dogs. The images were exposed 30 minutes after injection of non-ionic water-soluble contrast media. (a) There are numerous dilated lymph vessels. Contrast medium can be seen outside these vessels. The findings are compatible with lymphoedema. The images allow detection of lymphangiectasia and assessment of its degree. (b) Fine vessels with multiple branches and normal popliteal lymph node opacification characterize normal findings.

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