Page 17 - Imaging of Osteoarthritis and Rheumatoid Arthritis in Hand Joints
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 the patient during scanning. The detected signals from all the different angles 1 are then processed by the computer to construct multiple thin image slices
through the scanned human body. These slices can also be recalculated in any
desired direction. In human hand joint scanning these 2d images in all planes
eliminate the problem of overprojection from which conventional radiographs suffer, so the true 3d bony anatomy is visualized (fig3). This makes CT very good for detection of small ossal pathology like early erosions in RA patients and subtle osteophytes and subchondral cysts in OA patients, especially in areas of complex anatomy like the wrist. This increase in detail comes at the cost of increased monetary costs for a CT examination.
Figure 3. Single slice of a CT examination of the wrist. Notice, in comparison with figure 2, that there is no overprojection of bones with CT.
Magnetic resonance imaging does not use ionizing radiation, but uses strong magnetic fields and radiofrequency pulses to influence the magnetic spin of protons in the imaged subject. These spins create a small signal which is detected by specific antenna called coils. The received signals are then processed to images. Normal clinical MRI scanners are used to induce and measure differences in spins between hydrogen atoms. Hydrogen atoms are abundant in the human body, mostly in fat and water, and the proton spins behave differently depending on the molecule that they are in. These differences are the reason that MRI is very good in differentiating between soft
General introduction

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