Principles of Radiography & Various Imaging techniques .
X-ray introduction .
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| X-ray showing frontal and maxillary air sinuses |
X-ray are a kind of electromagnetic waves which are used extensively in medicine for both diagnostic and therapeutic purposes. All electromagnetic waves (X-rays, ultraviolet rays, infrared rays and radio waves) are produced by acceleration of electrons.
Historical aspects of X-ray .
[1]. X-rays were discovered accidentally on November 8,1895, by Wilhelm Konrad Roentgen. Roentgen was a German physicist from the University of Wurzburg. He was engaged in studying the behaviour of an electron beam when passed through a vacuum to strike a tungsten plate.
[2]. To his surprise, he observed that, in addition to electrons, certain unknown rays were also produced, which could pass through the glass envelope of his apparatus and caused a glow on a distant fluorescent screen.
[3]. He was able to photograph the bones of his hand by placing the hand over a photographic plate and then shining the rays on it. For his unique discovery, Roentgen was awarded the first Nobel Prize in Physics in The discovery of X-rays provided a new dimension to the advancement of medical and other sciences.
[4]. The medical uses of X-rays are both diagnostic and therapeutic. As a diagnostic tool, radiography has proved of great value in detection of the early stages of deep-seated diseases, when the possibility of cure is greatest.
[5]. Therapeutically, X-rays (radiotherapy) are used in the treatment of cancer (selected cases) because, the rays can destroy cancer cells much more easily without destroying the adjacent normal cells.
Properties of X-rays .
1. Penetrating Power .
[1]. X-rays form a part of the spectrum of electromagnetic radiation. They closely resemble the visible light rays in having a similar photographic effect. But they differ from the light rays in being invisible and in having a shorter wavelength.
[2]. The wavelength of X-rays is 1/10,000 of that of the light rays, i.e. 7.5 x 10″6 – 1.71 x 10 9 cm. It is this property of shorter wavelength which gives them the power of penetration of different materials. When X-rays pass through the matter, the rays are absorbed to varying extents.
[3]. The degree of absorption depends on the density (atomic weight) of the matter. Radiography is based on the differential absorption of the X-rays. Dense tissues, like the bone, absorb X-rays far more readily than do the soft tissues of the body.
[4]. Structures which are easily penetrated by the X-rays are described as radiolucent, and the structures which are penetrated with difficulty or are not penetrated at all are described as radiopaque.
[5]. The various structures can be arranged in a scale of increasing radiopacity.
(a) Air, in the respiratory passages, stomach and intestines.
(b) Fat.
(c) Soft tissues, e.g. muscles, vessels, nerves, and viscera.
(d) Bones, due to their calcium content.
(e) Enamel of teeth, and
(f) Dense foreign bodies, e.g. metallic fillings in the teeth, and radiopaque contrast media.
2. Photographic Effect .
[1]. When X-rays strike a photosensitive film, the film gets photosensitized. When such a film is developed and fixed chemically, a radiography image is obtained. X-ray film is made up of cellulose acetate, which is coated on its both sides with silver bromide (photosensitive) emulsion 0.001 inch thick.
[2]. The film is blue tinted and transparent. An X-ray image (picture) is called skiagram (skia = shadow), radiograph, or roentgenogram. Radiolucent structures produce black shadows and the radiopaque structures produce white shadows in the usual negative film. It is useful to remember that gas shadows are black shadows and bone shadows are the white shadows.
3. Fluorescent Effect .
When X-rays strike certain metallic salts (phosphorous, zinc, cadmium, sulphide), the rays cause them to fluoresce, that is, light rays are produced. This property of X-rays is utilized in fluoroscopy.
4. Biological Effect .
X-rays can destroy abnormal cells (e.g. malignant cells) much more easily than the adjacent normal cells. This property of X-rays is utilized in the treatment of various cancers. However, X-rays are potentially dangerous. On repeated exposures, they can cause burns, tumors, and even mutations. Therefore, adequate protective measures must be taken against repeated exposures to X-rays.
Radiographic Views of image capture.
Radiographs of a part taken in more than one view give a more complete information about the entire structure by eliminating some particular overlapping shadows in particular views. The ‘view’ expresses the direction of flow of the X-rays. In AP (anteroposterior) view the rays pass from the anterior to the posterior surface, and the posterior surface faces the X-ray plate. In PA (posteroanterior) view the X-rays pass from the posterior to the anterior surface, and the anterior surface faces the X-ray plate. The part of the body facing the X-ray plate (i.e. near the X-ray plate) casts a sharper shadow than the part facing the X-ray tube. The chest skiagrams are usually taken in PA view, but for visualizing the thoracic spine, AP view is preferred. The ‘view’ can also be expressed by mentioning the surface facing the X-ray plate. Thus AP view can also be called as posterior view, and the PA view as the anterior view. Similarly, when right surface of the body faces the plate, it is called the right lateral view, and when the left surface of the body faces the plate it is called the left lateral view. Oblique and other special views are taken to visualize certain special structures.
Radiographic Procedure .
1. Fluoroscopy .
[1]. Fluoroscopy is of special advantage in observing the movements of the organs (lungs, stomach, intestines, etc.), and in changing the position of the subject during the examination. Fluoroscopy is done in a dark room.
[2]. The fluoroscopic image is visualized directly on the fluorescent screen which is covered with a sheet of lead glass to absorb the X-rays and to protect the fluoroscopist. The sharpness of the fluoroscopic image is inferior to that of a radiograph.
[3]. Fluoroscopic image is photographed by a camera in mass miniature radiography (MMR) by which the masses can be surveyed for the detection of diseases such as tuberculosis.
2. Plain Radiography .
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| Plain radiography of abdomen |
A natural X-ray image, obtained directly without using any contrast medium, is called a plain skiagram or a plain radiograph. Plain radiography is particularly useful in the study of normal and abnormal bones, lungs, paranasal air sinuses and gaseous shadows in the abdomen .
3. Contrast Radiography .
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| The stomach and small intestine visualized by barium metal |
[1]. The various hollow viscera and body cavities cannot be visualized in plain radiographs due to their poor differential radiopacity. However, their contrast can be accentuated by filling such organs or cavities with either a radiopaque or a radiolucent substance.
[2]. Radiography done after artificial accentuation of the contrast is called contrast radiography . The radiopaque compounds used in contrast radiography are:
1. Barium sulphate suspension (emulsion) in water for gastrointestinal tract .
2. The aqueous solution of appropriate iodine compounds, for urinary and biliary passages and the vascular system.
Special Procedures of computerized tomography (CT scanning) .
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| CT scan of the thorax |
[1]. Computerized tomography is a major technological breakthrough in radiology, especially neuroradiology . It provides images comparable to anatomical slices (3-6 mm thick) of the brain, in which one can distinguish tissues with even slight differences in their radiodensity, viz. CSF, blood, white and grey matter, and the neoplasms.
[2]. Differentiation between vascular and avascular areas can be enhanced by simultaneous injection of a radiopaque medium in the vessels. Thus CT scanning helps in the diagnosis of the exact location and size of the tumour, haemorrhage, infarction and malformations, including hydrocephalus, cerebral atrophy, etc. This technique is also called as CAT (computerized axial tomography) scanning because it provides images in transverse, or axial plane.
Special Procedures of Ultrasonograph .
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| Ultrasound of the neck and thyroid gland |
[1]. Ultrasonic diagnostic echography is a safe procedure because instead of X-rays the high frequency sound waves are used. These sound waves are reflected by the acoustic interface (different tissues) back to their source and are recorded in a polarised camera.
[2]. The sound waves used are above the range of human hearing, i.e. above 20,000 cycles per second, or 20 kilohertz (hertz = cycles per second). As the technique is quite safe, it is especially valuable in obstetric and gynaecological problems. Figures 10.4 and 10.5 show the ultrasound of the neck and thyroid gland .
Magnetic resonance imaging .
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| Magnetic resonance imaging of brain |
[1]. The MRI uses a strong magnet and pulses of radiowaves. A pulse of radiowaves of the appropriate frequency displaces hydrogen nuclei from their new alignment. They return to their position immediately after the pulse ceases.
[2]. At the same time they release the energy absorbed as a radio signal of the same frequency which is detected. The signal returned is proportional to the concentration of the protons.
[3]. This is converted by computer into an analogue image presented on a screen. MRI can produce axial images resembling those of CT. It has the great advantage that images can also be produced in any other plane. This can also used for tissue characterization and blood flow imaging.
Interventional radiology .
Interventional radiology involves a wide variety of procedures. These include:
1. Percutaneous catheterization and embolization in the treatment of tumors is done to reduce tumour size and vascularity prior to operation in difficult cases.
2. Percutaneous transluminal dilatation and arterial stenosis for the treatment of localized stenosis in the arteries.
3. Needle biopsy under imaging control for lung tumors and abdominal masses.
4. Transhepatic catheterization of the bile ducts for draining in obstructive jaundice.
5. Needle puncture and drainage of cysts in the kidney or other organs using control by simple X-ray or ultrasound .
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