WEBVTT 1 00:00:04.600 --> 00:00:10.099 X-Ray computed Tomography or CT-scan is a radiological technique. 2 00:00:10.100 --> 00:00:14.399 It is, just like radiography, based on the use of X-rays. 3 00:00:14.400 --> 00:00:19.765 It therefore rests on the principle of differential absorption of X-rays by tissues 4 00:00:19.766 --> 00:00:23.865 a principle that we made clear under the study of radiography. 5 00:00:23.866 --> 00:00:27.832 The term scanner is often used to refer to this technique. 6 00:00:27.833 --> 00:00:33.865 In fact, the word scanner indicates the apparatus used to carry out the examination. 7 00:00:33.866 --> 00:00:38.265 It’s made up of a mobile table on which the patient lies down 8 00:00:38.266 --> 00:00:42.265 and an X-ray emitting tube which turns around the patient. 9 00:00:42.266 --> 00:00:44.232 Here is how it works. 10 00:00:44.233 --> 00:00:49.065 Detectors measure the intensity of the rays emitted by the tube. 11 00:00:49.066 --> 00:00:53.465 On the other side, receivers, composed of digital sensors 12 00:00:53.466 --> 00:00:59.465 record the intensity received, after having passed through the patient’s body. 13 00:00:59.466 --> 00:01:04.732 This device is linked to a computer, which will process the digital data. 14 00:01:04.733 --> 00:01:08.899 An image in grayscale is obtained by the difference in intensity 15 00:01:08.900 --> 00:01:14.699 between incident and emergent rays, which are calculated for different angles. 16 00:01:14.700 --> 00:01:19.532 Given that the rays penetrate perpendicular to the main axis of the body 17 00:01:19.533 --> 00:01:22.699 the images obtained are horizontal sections. 18 00:01:26.633 --> 00:01:29.499 The patient is always lying down. 19 00:01:29.500 --> 00:01:33.365 To explore a whole region, the abdomen for example 20 00:01:33.366 --> 00:01:36.332 we produce a succession of sections. 21 00:01:36.333 --> 00:01:39.332 While the tube turns and emits radiation 22 00:01:39.333 --> 00:01:45.799 the table moves a constant speed over a tiny distance each time. 23 00:01:45.800 --> 00:01:49.865 The images are viewed in real time, on a computer console 24 00:01:49.866 --> 00:01:54.832 by the radiology technician. They are then interpreted by the radiologist. 25 00:01:57.866 --> 00:02:01.799 Remember that the radiographic image is a projected view 26 00:02:01.800 --> 00:02:06.299 where all of the organs are superimposed in the same plane. 27 00:02:06.300 --> 00:02:09.565 CT-scanning provides views in section. 28 00:02:09.566 --> 00:02:12.799 But computer processing of a succession of sections 29 00:02:12.800 --> 00:02:16.265 can reconstruct an image in 3 dimensions. 30 00:02:16.266 --> 00:02:18.799 The physician thus has a precious tool 31 00:02:18.800 --> 00:02:23.265 for locating and evaluating the depth of a lesion, for example. 32 00:02:23.266 --> 00:02:30.432 Furthermore, CT-scan image has better resolution where contrast is concerned. 33 00:02:30.433 --> 00:02:33.965 Some specific low density tissues cannot be observed 34 00:02:33.966 --> 00:02:37.199 other than by using a contrast medium. 35 00:02:37.200 --> 00:02:40.665 This reconstruction of the cerebral arteries, for instance 36 00:02:40.666 --> 00:02:45.265 could not have been produced except by injection of a contrast medium 37 00:02:45.266 --> 00:02:47.632 based on iodine. 38 00:02:47.633 --> 00:02:52.532 One has to recognize that the power of a scanner lies to a very great extent 39 00:02:52.533 --> 00:02:56.599 in the computer processing that is associated with it. 40 00:02:56.600 --> 00:03:00.399 Thanks to that, the physician can reconstruct planar sections 41 00:03:00.400 --> 00:03:02.199 superimpose the tissues 42 00:03:02.200 --> 00:03:06.532 and thus navigate inside the reconstructed body of the patient. 43 00:03:06.533 --> 00:03:09.699 Virtual endoscopy, as its name indicates 44 00:03:09.700 --> 00:03:13.899 is a non-invasive method for observing the interior of a conduit 45 00:03:13.900 --> 00:03:16.232 like the bronchi in this sequence. 46 00:03:16.233 --> 00:03:22.332 This relatively new technique requires a specific kind of CT-scan, called spiral mode 47 00:03:22.333 --> 00:03:26.532 and expensive software, but its use is developing. 48 00:03:26.533 --> 00:03:28.499 Since this technique uses X-rays 49 00:03:28.500 --> 00:03:31.665 we take the same precautions as we do with radiography 50 00:03:31.666 --> 00:03:34.532 that is limiting the number of examinations. 51 00:03:34.533 --> 00:03:37.699 The apparatus are becoming more and more high-performance 52 00:03:37.700 --> 00:03:41.765 and the amount of radiation the patient receives is not that much greater 53 00:03:41.766 --> 00:03:44.399 than in the case of radiography. 54 00:03:44.400 --> 00:03:50.665 There is also a risk of allergic reaction to the iodine contrast media but that can be limited. 55 00:03:50.666 --> 00:03:57.099 Finally, the use of a CT-scan is an examination that is costly in both material and time 56 00:03:57.100 --> 00:04:01.599 and it is necessary to keep a close watch to ensure that guidelines are being followed.